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Investigation of planar arrays of superheated superconducting spheres Meagher, Gail Aileen 1991

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INVESTIGATION OF PLANAR ARRAYS OF SUPERHEATED SUPERCONDUCTING SPHERES By GAIL AILEEN MEAGHER B . S c , Dalhousie U n i v e r s i t y , 1989 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE i n THE FACULTY OF GRADUATE STUDIES DEPARTMENT OF PHYSICS We accept t h i s t h e s i s as conforming t o the r e q u i r e d standard THE UNIVERSITY OF BRITISH COLUMBIA October 1991 © G a i l A i l e e n Meagher, 1991 72 In presenting this thesis in partial fulfilment of the requirements for an advanced degree at the University of British Columbia, I agree that the Library shall make it freely available for reference and study. I further agree that permission for extensive copying of this thesis for scholarly purposes may be granted by the head of my department or by his or her representatives. It is understood that copying or publication of this thesis for financial gain shall not be allowed without my written permission. Department of Physics  The University of British Columbia Vancouver, Canada Date October 8, 1991  DE-6 (2/88) A b s t r a c t The P l a n a r A r r a y of Superheated Spheres (PASS) has been put forward as a p o s s i b l e n e u t r i n o and dark matter d e t e c t o r . T h i s work has c o n c e n t r a t e d on the c h a r a c t e r i s t i c s of a 100x100 a r r a y of spheres, of indium or t i n . The superheated-supercooled h y s t e r e s i s curves, f o r magnetic f i e l d s r a n g i n g from the e a r t h ' s t o 16 mT, have been measured i n a pumped 4He c r y o s t a t u s i n g an RF-SQUID readout system. I n d i v i d u a l s u perconducting t o normal g r a i n f l i p s were r e c o r d e d w i t h the indium sample due t o exposure t o 60 keV gammas. The e f f e c t of g r a v i t y on the shape and s t r u c t u r e of the spheres was t e s t e d by r e m e l t i n g a r r a y s i n a m i c r o g r a v i t y environment. The h y s t e r e s i s curves o b t a i n e d f o r these a r r a y s showed a decreased t r a n s i t i o n width, i . e . the temperature d i f f e r e n c e from where a l l the spheres are superconducting t o where they are a l l normal. i i Table of Contents A b s t r a c t i i T a b l e of Contents i i i L i s t o f F i g u r e s v Acknowledgement v i i i 1 I n t r o d u c t i o n 1 1.1 Superconducting C o l l o i d Devices 1 1.2 H i s t o r i c a l P e r s p e c t i v e 4 1.3 Pr e s e n t Work 5 1.4 T h e s i s O u t l i n e 5 2 Theory 7 2.1 G r a i n Deformation Due t o G r a v i t y 7 3 Apparatus and Sample P r e p a r a t i o n 11 3.1 C r y o s t a t 11 3.2 Data A c q u i s i t i o n 18 3.3 Sample P r e p a r a t i o n 19 3.3.1 Indium A r r a y s 19 3.3.2 T i n A r r a y s 23 3.4 S i z e D i s t r i b u t i o n 25 i i i 4 Experiments-Indium A r r a y s 28 4.1 F a s t E l e c t r o n i c s 28 4.1.1 M u l t i c h a n n e l A n a l y z e r 29 4.1.2 D i g i t a l O s c i l l o s c o p e 31 4.1.3 F l i p S t r u c t u r e 33 4.2 SQUID Read Out 35 4.3 R a d i a t i o n T e s t 37 5 Experiments - M i c r o g r a v i t y M e l t i n g 40 5.1 P r e p a r a t i o n f o r F l i g h t 40 5. 2 Apparatus 41 5.3 M i c r o g r a v i t y F l i g h t 47 5.4 H y s t e r e s i s Curves 48 6 Experiments - T i n A r r a y s 51 6.1 H y s t e r e s i s 51 7 C o n c l u s i o n s 55 B i b l i o g r a p h y 58 i v L i s t o f F i g u r e s 1 . 1 P h a s e d i a g r a m f o r a T y p e I S u p e r c o n d u c t o r : b u l k t r a n s i t i o n - d o t t e d l i n e , s u p e r h e a t e d t r a n s i t i o n -s o l i d l i n e , s u p e r c o o l e d t r a n s i t i o n - d a s h e d l i n e . . . 2 2 . 2 F o r c e s o n a s e s s i l e d r o p 8 3 . 3 I n n e r c r y o s t a t a s s e m b l y 13 3 . 4 SQUID h o u s i n g 16 3 . 5 S o u r c e a n d s h u t t e r a s s e m b l y 17 3 . 6 I n d i u m a r r a y s , (a) a n u n m e l t e d a r r a y o f 40x40 Jim s q u a r e s , 50x (b) m e l t e d i n d i u m s p h e r e s o f r a d i u s 17 jum s e p a r a t e d b y 70 /urn, 200x 22 3 . 7 T i n a r r a y s , ( a ) a n u n m e l t e d a r r a y o f 4 0x40 (im s q u a r e s , 200x . (b) m e l t e d t i n s p h e r e s o f 9 /urn r a d i u s s e p a r a t e d b y 70 /um, 500x 24 3 . 8 I n d i u m a r r a y i m a g e s , 200x , g r a i n r a d i u s 11 jim (a) p i c t u r e f e d i n t o f r a m e g r a b b e r (b) i m a g e a f t e r p r o c e s s i n g 26 4 . 9 M u l t i c h a n n e l a n a l y z e r t r a c e , Y a x i s 1024 c o u n t s . . 30 4 . 1 0 D i g i t a l o s c i l l o s c o p e s e t u p 32 4 . 1 1 T r a c e s f r o m two d i f f e r e n t sweeps o n d i g i t a l o s c i l l o s c o p e . B o t h c h a n n e l s : X i s 100 / u s / d i v , Y i s 1 v / d i v 33 v 4.12 P o s s i b l e s t r u c t u r e of i n d i v i d u a l g r a i n f l i p s seen by f a s t e l e c t r o n i c s 34 4.13 S u p e r c o n d u c t i n g t o normal t r a n s i t i o n f o r 12 fim r a d i u s indium g r a i n s i n an e x t e r n a l f i e l d B = 4 mT. 36 4.14 I n d i v i d u a l superconducting t o normal f l i p s i n 12 ,um r a d i u s indium g r a i n s caused by gamma r a d i a t i o n . . . 38 4.15 Number of occurrences vs ste p s i z e f o r 100x100 a r r a y of indium g r a i n s f l i p p e d by r a d i a t i o n 39 5.16 Base p l a t e and f l i g h t apparatus 43 5.17 H e a t i n g and c o o l i n g curve f o r sample i n oven prepared f o r jug f l i g h t 44 5.18 Temperature c o n t r o l box 45 5.19 Rods i n sample h o l d e r (a) samples b e f o r e r e m e l t i n g (b) samples a f t e r r e m e l t i n g 4 6 5.20 Sample h o l d e r (a) top view (b) s i d e view 47 5.21 S u p e r c o n d u c t i n g t o normal t r a n s i t i o n f o r 12 jum r a d i u s indium g r a i n s prepared i n m i c r o g r a v i t y , i n an e x t e r n a l f i e l d B = 12 mT 49 5.22 S u p e r c o n d u c t i n g t o normal t r a n s i t i o n f o r 12 ,um r a d i u s indium g r a i n s prepared i n e a r t h ' s g r a v i t y , i n an e x t e r n a l f i e l d B = 12 mT 50 6.23 S u p e r c o n d u c t i n g t o normal t r a n s i t i o n f o r 18 nm diameter t i n g r a i n s i n an e x t e r n a l f i e l d of B = 4 mT 52 v i F i e l d v s t r a n s i t i o n temperature f o r ( a ) t i n a r r a y of r a d i u s 9 urn spheres, squares (b) t i n c o l l o i d o f 7 urn r a d i u s spheres, diamonds v i i Acknowledgement I would l i k e thank my s u p e r v i s o r , Dr. B r i a n T u r r e l l , f o r h i s h e l p f u l s u g g e s t i o n s and f o r h i s understanding. I would a l s o l i k e t o thank Dr. Andrzej K o t l i c k i f o r h i s a s s i s t a n c e , day t o day t o l e r a n c e , and h e l p i n t y i n g a l l those l i t t l e k n ots. I a l s o a p p r e c i a t e d h i s comments d u r i n g the w r i t i n g of my t h e s i s . I wish t o express my a p p r e c i a t i o n t o Dr Mark Le Gros, without whom I would never have found anything, o r been a b l e t o make i t work. I would a l s o l i k e t o acknowledge h i s c o n t r i b u t i o n t o my o p i n i o n o f t h e s i s w r i t i n g . My thanks go out t o Dr. Georg Eska, from whom I l e a r n e d a g r e a t d e a l i n a s h o r t time. I would a l s o l i k e t o thank the t e c h n i c a l s t a f f i n the p h y s i c s department, who he l p e d me through my ignorance, and NSERC,for t h e i r f i n a n c i a l support. T h i s t h e s i s i s brought t o by the l e t t e r R , the number 4 6 2 , and a s m a l l p i e c e o f s t r i n g . v i i i Chapter 1 I n t r o d u c t i o n 1.1 Superconducting C o l l o i d D e t e c t o r s Superconducting c o l l o i d d e t e c t o r s (SSCD) c o n s i s t of many s p h e r i c a l g r a i n s , of a s u i t a b l e metal, a l l n e a r l y a s - p o s s i b l e of the same diameter, which can range from one t o one hundred micrometers. The g r a i n s are embedded i n a s u i t a b l e d i e l e c t r i c m a t e r i a l , and are made from a type I superconductor. The g r a i n s are h e l d i n a superheated, superconducting s t a t e near the phase l i n e (B ,T ) . X SH' SH' Small samples of type I superconductors can e x h i b i t s u p e r h e a t i n g and s u p e r c o o l i n g . Superheating o c c u r s when the sample e x h i b i t s s u p e r c o n d u c t i v i t y a t a temperature h i g h e r than T c, the thermodynamic t r a n s i t i o n temperature f o r the bulk m a t e r i a l . S i m i l a r l y , a sup e r c o o l e d m a t e r i a l i s i n the normal s t a t e below T , u n t i l T i s reached. F i g u r e 1.1 shows the c' sc d i f f e r e n t phases. The 'x' i n f i g u r e 1.1 shows the s t a t e of the g r a i n s i n the SSCD, s l i g h t l y below the superheated t r a n s i t i o n , but w e l l above s u p e r c o o l i n g . S i n c e the g r a i n s i n the d e t e c t o r are superconducting they exclude a magnetic f i e l d v i a the Meissner E f f e c t . When a 1 Temperature F i g u r e 1.1: Phase diagram f o r a Type I Superconductor: b u l k t r a n s i t i o n - d o t t e d l i n e , superheated t r a n s i t i o n -s o l i d l i n e , s u p e r c o o l e d t r a n s i t i o n - d a s h e d l i n e . c o l l i s i o n event occurs i n the g r a i n (with a n e u t r i n o , alpha p a r t i c l e , gamma ray, WIMP etc.) the energy d e p o s i t e d i s mostly c o n v e r t e d t o heat' i n the g r a i n . I f i t i s s i t t i n g c l o s e enough t o t h e t r a n s i t i o n , t he heat can ' f l i p ' t he g r a i n from superconducting t o normal. When the g r a i n i s f l i p p e d , i t no long e r excludes the magnetic f i e l d and a change i n t h e magnetic f l u x r e s u l t s . T h i s can be 'read-out' by a SQUID (Superconducting Quantum I n t e r f e r e n c e Device) magnetometer. 2 Because the g r a i n i s superheated, i t remains normal a f t e r the f l i p . P r e v i o u s d e t e c t o r s were of the ' c o l l o i d ' type which c o n s i s t e d of a random suspension of g r a i n s i n a s u i t a b l e d i e l e c t r i c , e.g. p a r a f f i n or epoxy. Each s u p e r c o n d u c t i n g g r a i n a c t s as a d i p o l e i n the magnetic f i e l d , and t h e r e f o r e the f i e l d f e l t by each g r a i n i s d i f f e r e n t as the p o s i t i o n , s i z e and shape of i t neighbours i s d i f f e r e n t . The v a r i a t i o n i n l o c a l magnetic f i e l d f e l t by the g r a i n s causes them t o f l i p from s u p e r c o n d u c t i n g t o normal a t s l i g h t l y d i f f e r e n t temperatures. T h i s r e s u l t s i n a spread of t r a n s i t i o n temperatures, AT g H, f o r a g i v e n a p p l i e d f i e l d . T h i s problem has been addressed by the p l a n a r a r r a y of superconducting spheres (PASS) developed and t e s t e d a t UBC. The g r a i n s are arranged i n a r e g u l a r a r r a y , w i t h a s e t s p a c i n g between them so t h a t the l o c a l f i e l d s are the same. Because of the f a b r i c a t i o n p r o c e s s , the s i z e and shape of the g r a i n s are more uniform. The spread i n AT can be measured by f i r s t p u t t i n g the SH spheres i n the superheated s t a t e i n an a p p l i e d magnetic f i e l d . T h i s i s done by f i r s t c o o l i n g the a r r a y t o on the order of 1.5 K. The magnetic f i e l d i s then swept up from the background e a r t h ' s f i e l d , t o the d e s i r e d v a l u e . The temperature i s then r a i s e d , w h i l e m o n i t o r i n g both the temperature of t h e sample and the change i n magnetic f l u x , v i a the SQUID magnetometer. 3 1.2 H i s t o r i c a l P e r s p e c t i v e The s u g g e s t i o n of u s i n g superheated superconducting g r a n u l e s as a p a r t i c l e d e t e c t o r o r i g i n a t e d w i t h a group from O r s a y 1 almost twenty f i v e y e a r s ago. They proposed u s i n g type I s u p e r c o n d u c t i n g g r a n u l e s embedded i n a d i e l e c t r i c m a t e r i a l . S i g n i f i c a n t s u p e r h e a t i n g was f i r s t observed i n indium spheres by 2 . . Faber e t . a l . . The superconducting t o normal t r a n s i t i o n of 3 g r a n u l e s due t o an e l e c t r o n beam was observed by B l o t e t . a l . i n 1974. Many oth e r experiments have been done u s i n g a c o l l o i d of superheated superconducting g r a n u l e s (SSG). These have v a r i e d , as have the suggested uses of a d e t e c t o r made of t h e s e g r a n u l e s . 4 D r u k i e r and V a l e t t e suggested t h a t a c o l l o i d o f SSG c o u l d be 5 used t o d e t e c t charged p a r t i c l e s . D r u k i e r e t . a l . then suggested i t s use as a t r a n s i t i o n r a d i a t i o n d e t e c t o r . As w e l l , the SSC has been proposed as a d e t e c t o r f o r neutrons , 7 8 9 10 n e u t r i n o s , magnetic monopoles , and c o l d dark matter ' , i n p a r t i c u l a r , weakly i n t e r a c t i n g massive p a r t i c l e s (WIMPs) 1 1. R e s u l t s were f i r s t r e p o r t e d on work done by Le Gros 12 e t . a l . i n 1990 on PASS (Planar a r r a y o f superheated s u p e r c o n d u c t o r s ) , a new type of arrangement. By f a b r i c a t i n g the g r a n u l e s i n a p l a n a r a r r a y , u s i n g p h o t o l i t h o g r a p h y , the spread i n t r a n s i t i o n temperatures was an order of magnitude s m a l l e r than f o r the c o l l o i d d e v i c e s . I n d i v i d u a l g r a i n f l i p s were 4 observed, and the d i s t r i b u t i o n of f l i p s t e p s i z e s shows a s t r o n g 13 peak a t the p o i n t of the c a l c u l a t e d s t e p s i z e 1.3 P r e s e n t Work The main work of t h i s t h e s i s i n v o l v e d the f a b r i c a t i o n and t e s t i n g o f p l a n a r a r r a y s o f indium and t i n superheated superconducting spheres. A new mask was used t o produce the a r r a y s , making them l a r g e r and more uniform than p r e v i o u s l y 14 4 . made . These a r r a y s were t e s t e d i n the pumped He c r y o s t a t with 15 16 a ' f a s t e l e c t r o n i c s ' system and our RF SQUID readout and e l e c t r o n i c s . P i c t u r e s of the a r r a y s were a l s o s t u d i e d t o determine the s i z e and placement d i s t r i b u t i o n . In an e f f o r t t o improve the a r r a y s , some were remelted i n a m i c r o g r a v i t y environment. The t r a n s i t i o n widths of these samples were then found. 1.4 T h e s i s O u t l i n e In chapter two, the deformation of a g r a i n due t o g r a v i t y , and m i c r o g r a v i t y , when s o l i d i f y i n g i s c a l c u l a t e d . In chapter t h r e e , the apparatus used t o r e c o r d data i n the l a b i s d e s c r i b e d . The procedure t o f a b r i c a t e the a r r a y s i s a l s o o u t l i n e d . In chapter f o u r , the experiments performed on the indium a r r a y s are d i s c u s s e d . Chapter f i v e d e a l s w i t h the 5 preparation, apparatus, and r e s u l t s obtained from the microgravity f l i g h t . Chapter s i x gives the experiments performed on the t i n arrays, while chapter seven gives the r e s u l t s and conclusions. 6 Chapter 2 Theory 2.1 G r a i n Deformation Due t o G r a v i t y P r e v i o u s c a l c u l a t i o n s have been done assuming the g r a i n s i n t h e a r r a y s are s p h e r i c a l . However m e l t i n g i n the e a r t h ' s g r a v i t y w i l l induce a deformation which we need t o c a l c u l a t e . To do t h i s , the g r a i n w i l l be approximated as a drop of l i q u i d on a f l a t s u r f a c e , w i t h a l i g h t e r d e n s i t y f l u i d , the f l u x , c o v e r i n g i t . The d e n s i t y of the f l u x used i s much l e s s than the d e n s i t y of the molten metal. I t i s assumed t h a t the l i q u i d metal w i l l s o l i d i f y i n the shape c a l c u l a t e d f o r the l i q u i d . A s e s s i l e drop of f l u i d on a h o r i z o n t a l s u r f a c e , covered by 17 . . . a l i q u i d of l e s s e r d e n s i t y i s shown i n f i g u r e 2.2. The drop has a d e n s i t y p , and a volume v. The l i g h t e r l i q u i d has a d e n s i t y , h P i, and surrounds the drop except where i t i s pushed a g a i n s t the h o r i z o n t a l s u r f a c e . The r a d i a l d i s t a n c e from the a x i s of symmetry i s g i v e n by x. The c o n t a c t angle i s g i v e n by 8, w h i l e g i s the a c c e l e r a t i o n due t o g r a v i t y . The s u r f a c e energy per u n i t area, or i n t e r f a c i a l t e n s i o n i s g i v e n by cr . The excess 7 c u r v a t u r e 1/b p r e s s u r e a t a depth z i s g i v e n by: Ap = 2cr/b + ( P ^ P J ) gz = ( ± + | 1 cr (2.1) 1 2' where r i and are the p r i n c i p l e r a d i i o f c u r v a t u r e , g i v e n by: l / r x = d0/ds (2.2) l / r 2 = s i n e / x (2.3) where s i s the a r c l e n g t h . S i n c e the drop i s i n e q u i l i b r i u m , the sum of the v e r t i c a l f o r c e s must be zero. vp g + 27xxcrsin0 = vp g + Trx 2Ap (2.4) h 1 T h i s can be reduced t o : V (Ph-P,) g/cr = Tlx2 ( i + i 1 (2.5) ^ 1 2'' The equations can be s i m p l i f i e d f u r t h e r by i n t r o d u c i n g the f a c t o r : 8 which can make the v a l u e s d i m e n s i o n l e s s . V 3/2 = V C (2.7) X 1/2 = X C (2.8) z 1/2 = Z C (2.9) R l 1/2 = r c I (2.10) R 2 1/2 = r c 2 (2.11) R e p l a c i n g t he va l u e s i n equation 2 .5 w i t h equations 2 .7 through 2.11 g i v e s : (2.12) C a l c u l a t i o n s have been done f o r many d i f f e r e n t forms of 1/3 . t h i s e q u a t i o n . A p l o t of l o g X vs l o g V f o r v a r i o u s v a l u e s 18 of 6 can be used t o estimate the va l u e o f x, a measure of how much the g r a i n i s f l a t t e n e d . For t h i s i t i s necessary t o know the volume of the g r a i n , d e n s i t i e s o f the metal and f l u x , and the i n t e r f a c i a l t e n s i o n between the two. The d e n s i t y of indium a t 231 °C i s 6.99 g/cm3 and the 319 d e n s i t y o f t i n a t i t s m e l t i n g p o i n t o f 232 °C i s 6.98 g/cm The d e n s i t y o f a b i e t i c a c i d was determined i n the l a b t o be 0.79 g/cm3. The volume of the g r a i n i s taken t o be 4TT/3 times the cubed r a d i u s . The i n t e r f a c i a l t e n s i o n i s equal t o the sum of the two s u r f a c e t e n s i o n s , minus the adhesion f o r c e of a t t r a c t i o n . U n f o r t u n a t e l y , the p r o p e r t i e s of a b i e t i c a c i d have not been s t u d i e d i n g r e a t d e t a i l . The s u r f a c e t e n s i o n of 20 99.995% pure indium i s 556 dynes/cm a t i t s m e l t i n g p o i n t . The s u r f a c e t e n s i o n of 99.99% pure t i n i s 537 dynes/cm a t i t m e l t i n g 9 p o i n t . The i n t e r f a c i a l t e n s i o n w i l l then be estimated by 451 dynes/cm or 0.46 g/cm. Taking the indium g r a i n r a d i u s as 10 jLim, t h i s g i v e s a v a l u e o f : c = ( 6.99 - 0.79 ) 981 / 0.46 = 1.32x10* cm"2 (2.13) T h i s a l l o w s t he c a l c u l a t i o n o f : V = 6 .37xl0" 3 (2.14) l o g V 1 / 3 = -0 .732 (2.15) I t was not p o s s i b l e t o determine the c o n t a c t angle e x a c t l y . With a l i g h t microscope i t was estimated t o be approximately 1 8 0 ° . With t h i s the v a l u e of l o g X = - 2 . 0 5 was ob t a i n e d from th e graph. C o n v e r t i n g back: x = 0.77 /nm (2.16) For a g r a i n o f r a d i u s 10 nm, t h i s i s a s m a l l d i s t o r t i o n . The g r a v i t y d u r i n g the m i c r o g r a v i t y experiment were a t most 10~2 g. U s i n g t h i s i n equa t i o n 2.13 g i v e s t he much s m a l l e r v a l u e of c = 1 . 3 2 x l 0 2 cm - 2. T h i s g i v e s V = 6 . 3 7 x l 0 - 6 , w h i l e 1/3 • l o g V = - 1 . 7 3 . Again from the graph w v a l u e of l o g X = - 3 . 9 5 i s o b t a i n e d . So f o r the sphere i n m i c r o g r a v i t y : x = 0.098 jLim (2.18) 10 Chapter 3 Apparatus and Sample P r e p a r a t i o n The samples were t e s t e d i n a pumped 4He c r y o s t a t . T h i s was p r e c o o l e d w i t h l i q u i d n i t r o g e n , and then f u r t h e r c o o l e d w i t h l i q u i d helium. A copper p l a t e oven was f u r t h e r c o o l e d through a thermal l i n k t o a pot of pumped 4He. The sample was connected t o t he oven by a copper rod, a l l o w i n g the temperature of the sample t o be s e t from 1.7 K, the o p e r a t i n g temperature of the pumped 4He pot, t o 4.2 K and above, w i t h the oven h e a t i n g the sample. The sample s a t i n an a d j u s t a b l e magnetic f i e l d , produced by a superconducting s o l e n o i d s u r r o u n d i n g the sample. A d i f f e r e n t i a l p ickup c o i l was p l a c e d next t o the sample, so changes i n the magnetic f l u x c o u l d be measured w i t h a very a c c u r a t e magnetometer. 3.1 C r y o s t a t The continuous, pumped 4He c r y o s t a t used t o t e s t the a r r a y s 22 . . . had been c o n s t r u c t e d p r e v i o u s l y . Few m o d i f i c a t i o n s were needed. The placement, housing, and t u n i n g o f the SQUID were changed, which r e s u l t e d i n more than a t h r e e - f o l d i n c r e a s e i n the RF s i g n a l from the SQUID, and a t h r e e - f o l d decrease i n the 11 n o i s e . The s h u t t e r assembly was r e p l a c e d both t o reduce the v i b r a t i o n a l n o i s e and t o improve the r e l i a b i l i t y o f the opening and c l o s i n g mechanism. The i n n e r p o r t i o n of the c r y o s t a t i s shown i n f i g u r e 3.3. The sample was a t t a c h e d t o a copper c o l d f i n g e r which was screwed i n t o the oven, p r o v i d i n g a good thermal l i n k . The oven c o n s i s t e d of a copper p l a t e anchored t o the 1 K pot by t h r e e t e f l o n screws and washers. They were a l s o t h e r m a l l y connected by a thermal r e s i s t a n c e p r o v i d e d by a s h o r t p i e c e of w i r e . The oven was heated by a t h e r m a l l y anchored r e s i s t o r , f e d by the e x t e r n a l temperature c o n t r o l l e r . The sample temperature was assumed t o be the same as t h a t of the oven, which was measured by a c a l i b r a t e d germanium r e s i s t o r . The oven was c o o l e d by the 1 K pot. T h i s was a copper pot of i n n e r volume 15.3 cm3 which was pumped by a r o t a r y pump. I t was c o n t i n u o u s l y f i l l e d w i t h l i q u i d 4He by a copper n i c k e l c a p i l l a r y of 47 cm l e n g t h and 0.1 mm i n n e r diameter. A s c i n t e r e d s i l v e r f i l t e r was p l a c e d on the end i n the 4He bath t o p revent c l o g g i n g of the c a p i l l a r y . The magnetic f i e l d on the sample was produced by a s o l e n o i d 23 made of 443 t u r n s of Nb-Ti wire wound on a 2.5 cm diameter former c o n s i s t i n g of a c y l i n d e r of copper f o i l coated with epoxy. T h i s gave a f i e l d t h a t was uniform t o w i t h i n 0.1% over the area of the a r r a y . The former was a c y l i n d e r of copper f o i l c o ated w i t h epoxy. The c o n n e c t i o n t o the s o l e n o i d l e a d s was 12 pumping l i n e c a l i b r a t e d germanium thermometer pick-up c o i l sample 4-w f i l l l i n e oven c o l d f i n g e r SQUID housing s o l e n o i d source and s h u t t e r F i g u r e 3.3: Inner c r y o s t a t assembly. 13 made by screwing them i n t o niobium p l a t e s mounted a t one end of the s o l e n o i d . These l e a d s were s h o r t e d by a l e n g t h of superconducting niobium w i r e . T h i s allowed the o p e r a t i o n of the magnet i n p e r s i s t e n t c u r r e n t mode. T h i s Nb-Ti wi r e , of a few c e n t i m e t e r s l e n g t h , was t h e r m a l l y connected t o a 1 kQ r e s i s t o r . When about 1 mA was sent through the r e s i s t o r , the wire was d r i v e n normal, which allowed the p e r s i s t e n t c u r r e n t i n the superconducting s o l e n o i d t o be changed. The s o l e n o i d produced a f i e l d o f 38 Gauss/Amp i n the r e g i o n where the sample was l o c a t e d . The sample s a t i n the middle of the s o l e n o i d , i n the most homogeneous p a r t of the f i e l d . Located next t o the sample was the d i f f e r e n t i a l c o i l , which p i c k e d up changes i n the l o c a l f l u x . A d i f f e r e n t i a l c o i l has two o p p o s i t e l y wound loops s e p a r a t e d by a s m a l l d i s t a n c e . T h i s arrangement i s s e n s i t i v e o n l y t o changes i n f l u x t h a t occur v e r y c l o s e t o the loops, because a t l a r g e d i s t a n c e s the change through one i s n e a r l y c a n c e l e d by the o p p o s i t e change through the o t h e r . The sample was 2 mm below the t e f l o n p ick-up c o i l former. Two o p p o s i t e l y wound c o i l s of 6 windings each, were sepa r a t e d by 2.2 cm, t o form the p i c k - u p loop. The diameter of the c o i l s was 1.2 cm, so the sample s a t i n s i d e the loop, y e t above i t by 5 mm. The wires from the p i c k - u p loop r a n out above the top of t h e s o l e n o i d t o the SQUID. The SQUID, a v e r y a c c u r a t e magnetometer, was used t o 14 d e t e c t the changes i n f i e l d f e l t by the pickup c o i l . The SQUID was encased i n a Niobium c y l i n d e r which makes i t a good s h i e l d a g a i n s t e x t e r n a l magnetic f i e l d s s i n c e niobium i s super c o n d u c t i n g below 9 K. A 1 mm diameter h o l e was d r i l l e d i n t o one end of the s h i e l d , a l l o w i n g the wi r e s t o pass through. T h i s end, and the wire s , were then wrapped i n l e a d f o i l t o s h i e l d the c a b l e s e n t e r i n g the SQUID. A p i e c e o f t e f l o n was screwed i n t o each end of the tube, and the SQUID was screwed i n t o these, as shown i n f i g u r e 3.4. The diameter of the SQUID was l e s s than the i n n e r diameter o f the t u b i n g , a l l o w i n g s h i e l d e d w i r e s t o pass between the two. The double s h i e l d e d SQUID c a b l e was at t a c h e d o u t s i d e the Niobium t u b i n g . T h i s was surrounded by l e a d f o i l which was s o l d e r e d t o the o u t s i d e s h i e l d o f the double s h i e l d e d c a b l e running t o room temperature. The SQUID was h e l d i n p l a c e by a p i e c e of brass which e n c i r c l e d the Niobium tube and was screwed onto one of the t h r e e s u p p o r t i n g rods o u t s i d e the magnet, which s u p p l i e d h e a t s i n k i n g f o r the SQUID. Our t e s t s i n c l u d e d exposing the sample t o gamma r a y s i n order t o f l i p t he g r a i n s by r a d i a t i o n r a t h e r than by changing temperature. The source used was a s m a l l c i r c u l a r d i s k of 241 • • • • • Am, gl u e d t o a p i e c e o f s i l v e r wire f o r heat s i n k i n g . The 241 • . . . • • Am a l s o had t i n f o i l f a s t e n e d around i t . With the s h i e l d i n g , the sample was exposed t o 18 keV and 60 keV gammas when the s h u t t e r was opened, w h i l e the c c - p a r t i c l e s were absorbed i n the 15 t e f l o n SQUID t e f l o n niobium F i g u r e 3.4: SQUID housing. f o i l . I t was necessary f o r the y source t o be bl o c k e d when not needed. The source and s h u t t e r assembly was p l a c e d below the sample and i n s i d e the lower p a r t o f the s o l e n o i d . A c y l i n d e r was machined out of copper, w i t h a s l i t through the middle, as shown i n f i g u r e 3.5. A t e f l o n r o d s a t i n t h i s s l i t and r o t a t e d around a b r a s s screw. The source s a t on the end of the t e f l o n rod . When v e r t i c a l , t he sample was exposed t o the source through a h o l e i n the bottom of the h o l d e r . A p i e c e o f copper f o i l over the h o l e b l o c k e d alpha p a r t i c l e s e m i t t e d by the source. The t e f l o n r o d c o u l d a l s o be t i l t e d from o u t s i d e the c r y o s t a t , u s i n g t h i n n y l o n t h r e a d , t o b l o c k the source e f f e c t i v e l y . Two e l e c t r i c a l c o n t a c t s were made a t the bottom of the r od, f o r the 'source open' and 'source c l o s e d ' p o s i t i o n s . T h i s allowed knowledge of the p o s i t i o n o f the r o d wh i l e o p e r a t i n g the handle connected t o the n y l o n t h r e a d . The ends of 16 the n y l o n t h r e a d were wound on t h i s handle, one c l o c k w i s e , the o t h e r c o u n t e r c l o c k w i s e . T u r n i n g the handle would p u l l the r o d t o e i t h e r the open or c l o s e d p o s i t i o n , depending on the d i r e c t i o n t u r n e d . copper f o i l copper h o l d e r h attachment f o r d e n t a l f l o s s l e a d s F i g u r e 3.5: Source and s h u t t e r assembly. A second s o l e n o i d was c o n s t r u c t e d f o r the t i n a r r a y t e s t s when a few w i r e windings were broken i n the middle of the f i r s t s o l e n o i d . I t was c o n s t r u c t e d on a b r a s s former of 1 i n c h i n n e r diameter an 11/8 i n c h outer diameter. In order t o screw s e c u r e l y the s o l e n o i d t o the apparatus, t h r e e e q u a l l y spaced tapped h o l e s were made 0.2 inches from one end. The s o l e n o i d 24 was c o n s t r u c t e d w i t h t h r e e l a y e r s of superconducting w i r e and S t y c a s t 1266 epoxy, each w i t h an average of 331 t u r n s . One l a y e r of 1.4 cm was added t o each end, t o make the f i e l d more uni f o r m by r e d u c i n g the edge e f f e c t s . T h i s brought the t o t a l t e f l o n r o d 17 t u r n s t o 1114, and gave a magnet which was uniform t o g r e a t e r than 10~5 over the area of the a r r a y . The s o l e n o i d produced a f i e l d o f 9.33 mT/A, over t w i c e as much as our p r e v i o u s s o l e n o i d . The l e a d s were secured as b e f o r e . 3.2 Data A c q u i s i t i o n The data was c o l l e c t e d u t i l i z i n g a 16 b i t A/D I/O board t o a 25 IBM XT c l o n e computer employing the Labtech Notebook program The computer c o l l e c t e d two channels of data: the d i g i t i z e d SQUID s i g n a l , and the d i g i t i z e d v a l u e of the r e s i s t a n c e of the c a l i b r a t e d germanium r e s i s t o r . A t h i r d channel was devoted t o c o n v e r t i n g the r e s i s t a n c e v a l u e i n t o the temperature i n K e l v i n . A f i l e c o n t a i n i n g the SQUID r e a d i n g , temperature i n K e l v i n , and el a p s e d time s i n c e the s t a r t of the run was c r e a t e d f o r each run. From the s e f i l e s , p l o t s c o u l d e a s i l y be made. 3.3 Sample P r e p a r a t i o n 3.3.1 Indium A r r a y s The f i r s t s t e p i n making the a r r a y s was the vacuum d e p o s i t i o n o f indium. The s u b s t r a t e used was 0.025 urn t h i c k mylar. T h i s was cl e a n e d w i t h a l c o h o l and s t r e t c h e d a c r o s s a 20.5 cm diameter copper p l a t e . A r i n g was screwed i n t o the top 18 of the p l a t e t o h o l d the mylar f l a t . Tear shaped, 9 9 . 9 % pure indium shot was evaporated a t an average r a t e o f 2 0 - 3 0 k/sec, from a s t a r t i n g vacuum of 3 x l 0 ~ 5 , t o produce c i r c u l a r l a y e r s of indium 6 cm i n r a d i u s and r a n g i n g from 1.9 /am t o 4 .8 p i n t h i c k n e s s . The next s t e p r e q u i r e d a t h i n l a y e r o f p h o t o r e s i s t t o be p l a c e d on top of the indium. To make each sample, the mylar backed indium was c u t i n t o squares of about 1.5 cm on a s i d e . T h i s was c l e a n e d w i t h i s o p r o p y l a l c o h o l and allowed t o d r y under a 25 W halogen lamp. The sample was then p l a c e d on the p l a t f o r m of t he s p i n n e r , where i t was h e l d by a s m a l l p i e c e of Scotch double s t i c k tape under one c o r n e r . A mixture o f one p a r t 2 6 KPR t h i n f i l m r e s i s t t o one p a r t KPR t h i n f i l m r e s i s t t h i n n e r was used as the r e s i s t . A t h i c k l a y e r o f r e s i s t was a p p l i e d t o the indium, then spun f o r 1.3 seconds, p r o d u c i n g a t h i n , uniform l a y e r . The sample was then heated f o r a few minutes on a 65°C hot p l a t e t o dry the r e s i s t . The sample was covered w i t h a box of aluminum f o i l t o prevent the r e s i s t from d e v e l o p i n g due t o the ambient l i g h t i n the room, and t o decrease the amount of dust s t i c k i n g t o the r e s i s t l a y e r on the sample. The next s t e p was the exposing o f the p h o t o r e s i s t t o form squares. To form the a r r a y o f spheres, i t was f i r s t necessary t o make an a r r a y of squares. The s p a c i n g and s i z e of the squares was determined by a mask made of a p i e c e o f qua r t z w i t h chrome on the lower s i d e . An a r r a y of 100x100 squares of s i z e 19 40x40 am s e p a r a t e d by 30 am. was used. As n e g a t i v e p h o t o r e s i s t was used, the squares are c l e a r , w h i l e the r e s t o f the mask i s covered i n chrome. Other s i z e s an spacin g s o f squares were g i v e n on d i f f e r e n t p a r t s of the mask. When exposed t o u l t r a v i o l e t r a d i a t i o n , bonds form i n the n e g a t i v e p h o t o r e s i s t , changing i t s s t r u c t u r e . T h i s a l l o w s a p a t t e r n t o be c r e a t e d on the indium. 27 The sample was exposed t o a UV lamp w i t h a q u a r t z d i f f u s e r j u s t below the bu l b t o d i s t r i b u t e the l i g h t e v e nly. The sample was p l a c e d on a stage covered by a p i e c e o f foam-wipe which help e d p r e s s the sample evenly t o the mask, l e a v i n g no space between the two. I f space was l e f t , t he l i g h t would develop some of the r e s i s t under the edges of the mask, l e a v i n g a l e s s s h a r p l y d e f i n e d p a t t e r n . A t o r o i d o f metal was p l a c e d on the mask t o ensure t h a t the mask was p r e s s e d i n t o the sample. The sample was exposed f o r f o u r minutes. A s h o r t e r exposure r e s u l t e d i n a p a t t e r n of s m a l l c i r c l e s , o f r a d i u s l e s s than 20 urn b e i n g formed. A lon g e r exposure i n c r e a s e d the s i z e of the squares, and made t h e i r edges l e s s w e l l d e f i n e d . I f the sample was exposed f o r more than e i g h t minutes, a 7 mm by 7 mm square was formed, w i t h no i n d i v i d u a l , s m a l l e r squares i n evidence. A f t e r exposure, the sample had t o be developed, by p l a c i n g i t i n the d e v e l o p i n g f l u i d . T h i s loosened and r i n s e d away the r e s i s t which had not been exposed t o the UV l i g h t , l e a v i n g the p a t t e r n o f squares p l a i n l y v i s i b l e on the indium. 20 The sample was p l a c e d i n the developer f o r n i n e t y seconds. The developer was s t i r r e d d u r i n g t h i s time by the f r e q u e n t squeezing of a p i p e t t e f u l l o f developer over the sample. The sample was then removed from the developer and r i n s e d w i t h a l c o h o l , s i n c e water was found t o l e a v e too much dust on the sample. The squares of r e s i s t l e f t on the indium a r e r e s i s t a n t t o m i l d a c i d s . T h i s allowed the indium not covered by r e s i s t t o be etched away, l e a v i n g the squares behind. The e t c h used was one p a r t c o n c e n t r a t e d HCl, t e n p a r t s e t h a n o l . The e t c h was heated on the h o t p l a t e t o speed the e t c h i n g p r o c e s s . I t was a l s o s t i r r e d by r e p e a t e d l y squeezing a p i p e t t e f u l l o f e t c h over the sample. The samples were etched u n t i l the indium on the edges had disappeared, making i t p o s s i b l e t o see through t o the h o t p l a t e . The sample was then removed q u i c k l y and r i n s e d w i t h water. The e t c h i n g process took between f o u r and t e n minutes, depending on the temperature and age of the e t c h . With t h i s procedure an a r r a y of squares was produced, and an example i s shown i n f i g u r e 3.6a. The area between the squares was r e l a t i v e l y c l e a r of indium. The tops of the squares were etched under the r e s i s t l a y e r due t o the f a c t t h a t chemical e t c h i n g works evenly i n a l l d i r e c t i o n s through the metal. To melt the squares t o form spheres, the sample was glued (superglue) t o a copper boat. The boat was made of 0.1 mm t h i c k copper f o i l which was stamped t o make a 3/8 i n c h square i n d e n t a t i o n . T h i s gave approximately a 1 mm c l e a r a n c e between 21 © © > 0 o © © © • • i o © e © o © © e © o o © ©• O • 0 © © © o 9 0 9 9 nj © • © O O © © ©v O o o 9 © © • o • '#£ • • © Lb m-. (a) 750jim (b) 75/um F i g u r e 3.6: I n d i u m a r r a y s , (a) a n u n m e l t e d a r r a y o f 40x40 am s q u a r e s , 50x (b) m e l t e d i n d i u m s p h e r e s o f r a d i u s 17 urn. s e p a r a t e d b y 70 iim, 20Ox. t h e a r r a y a n d t h e e d g e s o f t h e b o a t . The b o a t s w e r e u s e d t o c o n t a i n t h e f l u x a d d e d t o t h e s a m p l e t o p r o d u c e s p h e r e s o n 2 8 m e l t i n g . The f l u x u s e d was a b i e t i c a c i d , w h i c h a c t e d a s a w e t t i n g a g e n t . W i t h o u t t h e f l u x , t h e i n d i u m m e l t e d , b u t w o u l d n o t f o r m s p h e r e s . The y e l l o w a c i d c r y s t a l s w e r e d i s s o l v e d i n i s o p r o p y l a l c o h o l a n d t h e n a f e w d r o p s w e r e p l a c e d o n t h e s a m p l e . A b i e t i c a c i d h a s a m e l t i n g t e m p e r a t u r e o f a b o u t 14 0°C, s i g n i f i c a n t l y b e l o w 157°C m e l t i n g p o i n t o f i n d i u m . 22 The a r r a y s were melted i n an oven c o n s i s t i n g o f a copper p l a t e heated by a r e s i s t o r . The temperature was monitored by an i n t e g r a t e d c i r c u i t sensor. T h i s formed a temperature c o n t r o l l e d c u r r e n t source w i t h a 1 mA/K s e n s i t i v i t y . A 1 kfi l o a d r e s i s t o r was p l a c e d i n t h i s c i r c u i t , a c r o s s which t h e v o l t a g e was read. To melt an a r r a y , the temperature was s l o w l y r a i s e d u n t i l 165°C. The a r r a y was watched u n t i l the squares began t o shimmer and melt i n t o spheres. When the e n t i r e a r r a y had melted, the heat was s l o w l y lowered. F i g u r e 3.6b shows some of the spheres of indium a f t e r m e l t i n g . The a r r a y s c o u l d e a s i l y be removed from t h e boats a f t e r m e l t i n g by d i p p i n g them i n l i q u i d n i t r o g e n . 3.3.2 T i n A r r a y s Some a r r a y s u s i n g t i n i n s t e a d o f indium were made by Leanne Graham, a summer student i n the l a b . T h i s r e q u i r e d a d i f f e r e n t s u b s t r a t e s i n c e mylar warps below 232 °C the m e l t i n g p o i n t of ®29 t i n . Kapton , a dark orange polymer, was chosen as i t has an upper working temperature of 250-320 °C. When t i n was ® evaporated onto the kapton the t i n f i l m tended t o l i f t o f f the s u b s t r a t e d u r i n g e t c h i n g o f the f i l m t o form squares. S p u t t e r i n g the t i n reduced t h i s problem and a number of t i n f i l m s of 5 am t h i c k n e s s were f a b r i c a t e d . The p h o t o l i t h o g r a p h i c f a b r i c a t i o n of the t i n a r r a y s was v e r y s i m i l a r t o t h a t f o r indium. S i n c e the m e l t i n g temperature 23 o f t i n was f a r a b o v e t h e w o r k i n g r a n g e o f t h e i n t e g r a t e d c i r c u i t p r e v i o u s l y u s e d t o m o n i t o r t h e t e m p e r a t u r e , a new h e a t e r was c o n s t r u c t e d . T h e t e m p e r a t u r e was m e a s u r e d w i t h a c r o m e l - a l u m e l t h e r m o c o u p l e . T h e t i n s eemed t o e t c h m o r e e v e n l y , f o r m i n g b e t t e r s q u a r e s h a p e s , a s shown i n f i g u r e 3 . 7 a . A s a m p l e o f a m e l t e d a r r a y i s shown i n f i g u r e 3 . 7 b . S i n c e t h e t i n e t c h e d m o r e e v e n l y , a s m a l l e r mask was a l s o t r i e d . A r r a y s o f 10x10 fim s q u a r e s w e r e m a d e , b u t h a v e n o t b e e n s u c c e s s f u l l y m e l t e d y e t . T h e p r o b l e m i s t h a t g r a i n s o f v e r y s m a l l s i z e a r e d i f f i c u l t t o • B B « « B BI m m m.mm-ffi ® s i eB.Bmmm9 i B'B'BB m e m\ G a a B B.B B i I S . i B RI'G i l l t l . B S.B I B I S S R I •i *j 'is m m ^ «® I 1 1 m i — i — i — i 0 100 200jnm 0 7511m F i g u r e 3 . 7 : T i n a r r a y s , (a) a n u n m e l t e d a r r a y o f 40x40 um s q u a r e s , 200x . (b) m e l t e d t i n s p h e r e s o f 9 fim r a d i u s s e p a r a t e d b y 70 /lm, 500x . 24 r e s o l v e u s i n g our microscope and w i t h medium m a g n i f i c a t i o n (125x) . T h i s make i t d i f f i c u l t t o judge when the t i n was melted and had formed spheres. Overheating even f o r o n l y a few seconds a f t e r the spheres were formed causes the spheres t o move around, d e s t r o y i n g the r e g u l a r a r r a y . 3.4 S i z e D i s t r i b u t i o n Once th e a r r a y s were melted, we needed t o measure the s i z e and s p a c i n g of the s p h e r i c a l g r a i n s . Using a Canon s t i l l v i d e o camera a t t a c h e d t o a microscope, i t was p o s s i b l e t o take p i c t u r e s of the a r r a y s a t v a r i o u s m a g n i f i c a t i o n s . The p i c t u r e s were t r a n s f e r r e d t o our PC u s i n g the Data T r a n s l a t i o n DT2853 frame grabber. There they were m o d i f i e d w i t h the h e l p of an Image Pro software package. The p i c t u r e s were c u t t o a seven by e i g h t g r i d of g r a i n s , then the t h r e s h o l d f u n c t i o n was used t o g i v e white as the background and b l a c k as the c o l o u r of the g r a i n s . An unprocessed p i c t u r e i s shown i n f i g u r e 3.8a, w h i l e the f i n a l r e s u l t of image p r o c e s s i n g i s shown i n f i g u r e 3.8b. The frame grabber r e c o r d e d o n l y every second l i n e of the p i c t u r e , l e a v i n g the r e s t completely white. I t was t h e r e f o r e n e c e s s a r y t o copy the image l i n e s on t o the white l i n e s . T h i s l e f t an image which was l e s s rounded than the o r i g i n a l , but which was now p o s s i b l e t o a n a l y z e . 25 • • • • • •B • • • • • • • • • • 1 t • # • , :::::::: • o « (a) I 1 1 (b) | 1 1 0 100 200/jm 0 100 200fim F i g u r e 3.8: I n d i u m a r r a y i mages, 200x, g r a i n r a d i u s 11 )Lim (a) p i c t u r e f e d i n t o f r a m e g r a b b e r (b) image a f t e r p r o c e s s i n g . N i n e p i c t u r e s were l o o k e d a t f r o m random p a r t s o f t h e 100x100 a r r a y . E a c h c o n t a i n e d f i f t y - s i x g r a i n s , f o r a t o t a l o f f i v e p e r c e n t o f t h e a r r a y . The images were r u n t h r o u g h a . 30 p r o g r a m d e v e l o p e d by A. K o t l i c k i t o f i t t h e d a t a t o a c i r c u l a r s h a p e . T h i s g a v e u s t h e r a d i u s and d i s t a n c e b e t w e e n g r a i n s i n e a c h d i r e c t i o n . The a v e r a g e r a d i u s <R> and t h e a v e r a g e d i s t a n c e b e tween n e a r e s t n e i g h b o u r s <d> was t h e n c a l c u l a t e d f o r e a c h p i c t u r e . The r e s u l t s g a v e <R> = 11.7 ± 0.4 /urn and 26 <d> = 70.2 ± 2.2 lira. These are much b e t t e r than the v a l u e s 31 <2R> = 23.2 ± 2.0 nm and <d> = 81.4 ± 6 |Lim o b t a i n e d w i t h an e a r l i e r method of f a b r i c a t i n g the a r r a y s . 27 Chapter 4 Experiments - Indium A r r a y s 4.1 F a s t E l e c t r o n i c s The measurements done w i t h the f a s t e l e c t r o n i c s r e a d out system were based on d i f f e r e n t p r i n c i p l e s than the experiments which used the SQUID read out. An r f p u l s e i s induced when a f l u x change A$ occurs due t o a g r a i n f l i p . T h i s s m a l l p u l s e i s then a m p l i f i e d and shaped o u t s i d e of the c r y o s t a t . T h i s read out system has two major advantages over the SQUID system. The p u l s e l e n g t h s are on the order of microseconds, g i v i n g much b e t t e r t i m i n g whereas the SQUID data i s taken i n the order of 0.1 seconds, g i v i n g poor r e s o l u t i o n f o r the f l i p s . With the f a s t e l e c t r o n i c s , i t i s a l s o p o s s i b l e t o sweep the magnetic f i e l d which can not be done w i t h the SQUID. In p r e v i o u s experiments u s i n g f a s t e l e c t r o n i c s two major problems had been encountered. The spread i n t r a n s i t i o n widths was a p p a r e n t l y v e r y wide and the number of p u l s e s g r e a t l y exceeded the number of gr a n u l e s p r e s e n t . I t was hoped t h a t these problems might be s o l v e d by i n v e s t i g a t i o n s u s i n g PASS. The a r r a y s had a t r a n s i t i o n width a t l e a s t an o r d e r o f magnitude narrower than the t e s t e d c o l l o i d s , which should presumably make 28 the spread of t r a n s i t i o n s narrower i n t h i s case. The a r r a y s a l s o have a known number of g r a n u l e s , u n l i k e the c o l l o i d s where i t i s much more d i f f i c u l t t o know the number, making a comparison t o the number of p u l s e s exact. 4.1.1 M u l t i c h a n n e l A n a l y z e r A s m a l l e r pickup c o i l was used than i n the SQUID s e t up, c o n s i s t i n g of 20 t u r n s on a 2 mm diameter g l a s s r o d . S i n c e the i n d u c t i o n loop read out produced much s m a l l e r s i g n a l s than the SQUID system, i t was necessary t o p l a c e the a r r a y d i r e c t l y i n the c o i l . A f u l l 100x100 indium a r r a y was c u t w i t h a s c a l p e l t o produce a p i e c e approximately 13x27 a f t e r p u t t i n g 'crazy g l u e ' over the f l u x t o s o f t e n i t . Without the c r a z y g l u e , the f l u x c r a cked and p e e l e d o f f the mylar when c u t t a k i n g the g r a i n s w i t h i t . One s e t of experiments i n v o l v e d u s i n g the computer as an 32 m u l t i c h a n n e l a n a l y z e r . The temperature was kept c o n s t a n t w h i l e the magnetic f i e l d was swept up and down w i t h r a t e s between 0.1 and 1.0 Hz. Runs were a l s o performed i n which t h e magnetic f i e l d was q u i c k l y swept down and up b e f o r e the temperature was ramped up. In each case, twenty or more sweeps were made per run. I t was expected t h a t a l a r g e , r e a s o n a b l y narrow peak would be o b t a i n e d i n t h i s manner. T h i s was a c t u a l l y not the case as shown i n f i g u r e 4.9. The t r a c e o b t a i n e d c o n t a i n e d a l a r g e 29 amount of n o i s e i n the low energy r e g i o n , a broad hump t h a t was p a r t i a l l y b u r i e d i n t h i s n o i s e , and two v e r y s m a l l , broad peaks i n the h i g h e r energy r e g i o n . Most of the f l i p s seemed t o occur i n the r e g i o n w i t h the n o i s e , making i t v e r y d i f f i c u l t t o d i s t i n g u i s h them. The two peaks w e l l above the n o i s e had a t o t a l o f e i g h t counts per sweep, f i v e counts i n one peak, and t h r e e counts i n the h i g h e r peak. I t was thought t h a t these }ffm l&t: i 1885 LI: ieee I H i i • A . l I M H i 258 = lineal it), 57 Cntsj m 1624 | ffl 512| F i g u r e 4.9: M u l t i c h a n n e l a n a l y z e r t r a c e , Y a x i s 1024 counts 30 peaks c o u l d be due t o g r a i n s which were a few microns i n diameter l a r g e r than the o t h e r s due t o e t c h i n g t h a t was not completely c o n s i s t e n t . In an attempt t o reduce t o low energy n o i s e , runs of 5000 sweeps were done a t 3.45 K, above the c r i t i c a l temperature of indium. These counts were then s u b t r a c t e d from s i m i l a r runs done a t lower temperatures. The r e s i d u a l peak a t low energy showed between 20 and 30 counts per sweep. I t was expected t h a t t h i s s u b t r a c t i o n procedure would not have as a c c u r a t e a count of the number of f l i p s . However, the r e s u l t was much lower than expected. An average of l e s s than 40 counts per sweep were reco r d e d , w h i l e the sample c o n t a i n e d over 340 g r a i n s . 4.1.2 D i g i t a l O s c i l l o s c o p e The o t h e r method of c o l l e c t i n g the data u t i l i z e d a d i g i t a l 3 3 . o s c i l l o s c o p e . The setup i s shown i n f i g u r e 4.10. I t was expected t o o b t a i n a l a r g e number of f l i p s i n a s h o r t time s c a l e , g i v i n g a narrow, h i g h peak. The temperature was kept c o n s t a n t a t a v a l u e r a n g i n g from 1.7 t o 3.45 K. The a p p l i e d magnetic f i e l d was then swept up from 0 t o 24.3 mT. R e p e t i t i v e sweeps showed t h a t the g r a i n f l i p s o c c u r r e d a t almost e x a c t l y the same times. A sample of two d i f f e r e n t t r a c e s showing t h i s i s g i v e n i n f i g u r e 4.11. However, the time spread of a l l the p u l s e s was much more than expected. The p u l s e s a l s o had v a r y i n g 31 e n e r g i e s , and tended t o occur i n groups. PICKUP COIL PREAMP AMP 3.25 lis DELAY GATE COMPUTER (MCA) COMPUTER LABTECH F i g u r e 4.10: D i g i t a l o s c i l l o s c o p e setup. 32 F i g u r e 4.11: Traces from two d i f f e r e n t sweeps on d i g i t a l o s c i l l o s c o p e . Both channels: X i s 100 / i s / d i v , Y i s 1 v / d i v . 4.1.3 F l i p S t r u c t u r e One p o s s i b l e e x p l a n a t i o n f o r the above r e s u l t s i s t h a t each g r a i n does not f l i p a t once. The time s c a l e w i t h the SQUID read out i s v e r y l a r g e compared t o the f a s t e l e c t r o n i c s . I t i s p o s s i b l e t h a t each f l i p t h a t i s seen w i t h the SQUID i s made up 33 of many s m a l l e r f l i p s . T h i s would occur i f a g r a i n becomes normal i n st a g e s . A p o s s i b l e g r a i n f l i p i s shown i n f i g u r e 4.12. The SQUID reads out data every 0.1 second, and t h e r e f o r e would i n t e g r a t e over these s t e p s . T h i s would e x p l a i n why p u l s e s o f lower energy are recor d e d i n the i n d u c t i o n loop read out. The m a j o r i t y o f the p u l s e s would then be b u r i e d i n the low energy n o i s e . T h i s a l s o would e x p l a i n the grouping of d i f f e r e n t s i z e d p u l s e s on the d i g i t a l o s c i l l o s c o p e t r a c e . 100 150 TIME [usee] 250 F i g u r e 4.12: P o s s i b l e s t r u c t u r e of i n d i v i d u a l g r a i n f l i p s seen by f a s t e l e c t r o n i c s . 34 4.2 SQUID Read Out A 100x100 a r r a y of Indium squares was prepared, w i t h a g r a i n r a d i u s of 12 microns. Superconducting t o Normal curves were o b t a i n e d f o r s e v e r a l d i f f e r e n t magnetic f i e l d s , r a n g i n g from 0.05 mT, the e a r t h ' s f i e l d , t o 12 mT. Indium has a 34 c r i t i c a l f i e l d of 28.3 mT . To o b t a i n these curves, the f o l l o w i n g s t e p s were taken: 1) the h e a t e r was s e t t o zero, a l l o w i n g the sample t o c o o l t o 1.7 K, the temperature of the 1 K pot, 2) the magnet was s e t t o zero c u r r e n t , making sure a l l the g r a i n s are i n the superconducting s t a t e . 3) the c u r r e n t i n the magnet was i n c r e a s e d t o supply the d e s i r e d f i e l d . 4) the temperature was r a i s e d by the a p p l i c a t i o n of a slow ramp t o the i n p u t of the temperature c o n t r o l l e r . The SQUID s i g n a l , temperature, and time were r e c o r d e d f o r the superheated t o normal t r a n s i t i o n . 5) upon r e a c h i n g a p r e s e t temperature, a t which the SQUID s i g n a l was constant, the ramp was r e v e r s e d , s l o w l y l o w e r i n g the sample through the normal t o s u p e r c o o l e d t r a n s i t i o n f o r low f i e l d s . The superheated t o normal t r a n s i t i o n o c c u r r e d between 2.6 and 3.4 K, The lower, and broader spread i n t r a n s i t i o n temperatures, AT , o c c u r r e d f o r h i g h e r f i e l d v a l u e s . An 35 example of the h y s t e r e s i s curve f o r B = 4 mT i s shown i n f i g u r e 4.13. The normal t o sup e r c o o l e d t r a n s i t i o n s were observed f o r f i e l d s l e s s than 4.0 mT, a t which p o i n t the t r a n s i t i o n o c c u r r e d w i t h i n o p e r a t i n g temperature range of the 1 K pot. A t h i g h e r magnetic f i e l d s the sup e r c o n d u c t i n g s t a t e can not be reached by sweeping the temperature, as can be seen i n f i g u r e 1.1. The spread i n t r a n s i t i o n widths f o r the sample prepared w i t h the new mask were found t o be narrower than 35 p r e v i o u s l y found w i t h o l d samples. In t h a t case the spread i n 1 . . -8 I 1 1 1 1 2 2.5 3 3.5 4 TEMPERATURE [K] F i g u r e 4.13: Superconducting t o normal t r a n s i t i o n f o r 12 ura r a d i u s indium g r a i n s i n an e x t e r n a l f i e l d B = 4 mT. 36 AT f o r a f i e l d o f 4 mT was 20 mK, whereas the new sample had SH a spread AT of 15 mK, which was the n o i s e l e v e l i n the SH temperature r e a d i n g . 4.3 R a d i a t i o n T e s t T e s t s were a l s o made t o f l i p t he g r a i n s by r a d i a t i o n . The magnet was a d j u s t e d t o g i v e 15.2 mT a t low temperature. The temperature was then r a i s e d t o j u s t below the t r a n s i t i o n p o i n t , 241 when t h e s h u t t e r was opened, exposing the Am source. The n o i s e l e v e l was approximately 100 mV, below the expected step s i z e of 260 mV. The run l a s t e d t h r e e and a h a l f hours. T h i s l o n g p e r i o d of time was necessary i n order t o observe i n d i v i d u a l g r a i n f l i p s . A sample of these f l i p s i s shown i n f i g u r e 4.14. The s t e p f u r t h e s t l e f t i s one g r a i n f l i p p i n g , the l a r g e r steps correspond t o more than one g r a i n f l i p p i n g a t n e a r l y the same time. The d a t a c o l l e c t e d i n the thermal sweep was analyzed u s i n g a program which counted the number of s t e p s of d i f f e r e n t step s i z e s . As shown i n f i g u r e 4.15, t h i s showed a l a r g e peak a t 0.25 V, i n d i c a t i n g a l a r g e number of s i n g l e g r a i n s f l i p p i n g . P r o g r e s s i v e l y s m a l l e r peaks are found a t m u l t i p l e s of t h i s v a l u e , i n d i c a t i n g 2, 3, 4, and 5 g r a i n s f l i p p i n g s i m u l t a n e o u s l y . When the area under each peak was t o t a l e d , 9,015 g r a i n s f l i p p e d i n 5,886 s t e p s . Some of the m i s s i n g 985 g r a i n s may have f l i p p e d 37 d u r i n g the SQUID r e s e t s , every 10 or so v o l t s . T h i s would not be a problem i n a p p l i c a t i o n s such as d e t e c t i n g WIMPs s i n c e much lower c o u n t i n g r a t e s a re expected. 38 F i g u r e 4.15: Number of occurrences vs s t e p s i z e f o r 100x100 a r r a y of indium g r a i n s f l i p p e d by r a d i a t i o n . Chapter 5 M i c r o g r a v i t y M e l t i n g 5.1 P r e p a r a t i o n f o r F l i g h t A s u c c e s s f u l a p p l i c a t i o n was made t o the Canadian Space Agency t o conduct an experiment t o f a b r i c a t e a r r a y s i n low g r a v i t y . In t h i s environment the e f f e c t s o f g r a v i t y on the shape of the g r a i n s and t h e i r m e t a l l u r g y would be reduced. The Canadian Space Agency arranged f o r a f l i g h t on NASA's KC 135 a i r p l a n e which i s a m o d i f i e d Boeing 707 which f l i e s p a r a b o l a s i n the a i r t o s i m u l a t e m i c r o g r a v i t y c o n d i t i o n s . The plane i s used t o t r a i n a s t r o n a u t s as w e l l as run s c i e n t i f i c experiments. On each f l i g h t t he plane f l i e s 10 p a r a b o l a s , f o l l o w e d by a f i v e minute break. T h i s p a t t e r n i s repeated u n t i l 40 para b o l a s i n a l l have been completed. Each p a r a b o l a c o n s i s t s of approximately 20 seconds i n ' m i c r o g r a v i t y ' , a t an a c c e l e r a t i o n -2 . . of around 10 g, f o l l o w e d by about 65 seconds i n approximately 1.8 g. Negative g i s sometimes experienced d u r i n g the p a r a b o l a . Each p a r a b o l a i s s l i g h t l y d i f f e r e n t and depends on t h e s k i l l of the p i l o t s (who are the same ones who f l y the p l a n e which pig g y backs the space s h u t t l e ) . The experiment must be designed around these c o n d i t i o n s . 40 5.2 Apparatus The aim of the experiment was t o r e m e l t and s o l i d i f y 100x100 a r r a y s of indium spheres, as w e l l as s i n g l e l a r g e r indium spheres, i n m i c r o g r a v i t y . To accomplish t h i s , i t was n ecessary t o have an oven, t o heat and c o o l the samples q u i c k l y , some way of c o n t r o l l i n g and m o n i t o r i n g the temperature,and a supply of power f o r the above. Some way of c o n t a i n i n g the samples i n an o r d e r l y f a s h i o n was a l s o r e q u i r e d , s i n c e 20 a r r a y s would be heated on each f l i g h t . Because o n l y two f l i g h t s were planned, i t was necessary t o make the experiment as t r o u b l e f r e e as p o s s i b l e . A l l equipment was backed up w i t h a second p i e c e . Wherever p o s s i b l e , the apparatus was designed so i t c o u l d be changed d u r i n g the f i v e minute l e v e l f l i g h t breaks between the s e t s of p a r a b o l a s . The experiment was a l s o kept v e r y simple. The a d v i c e of the Canadian Space Agency was sought i n p i n p o i n t i n g p o t e n t i a l t r o u b l e s p o t s t h a t might occur i n m i c r o g r a v i t y or 1.8 g. I t was a l s o n e c e s s a r y t o pass NASA's ve r y s t r i c t s a f e t y r e g u l a t i o n s b e f o r e the apparatus c o u l d be used on the KC-135. The experiment was designed t o f i t on a 26x26x1/2 i n c h aluminum p l a t e . T h i s was s e c u r e l y b o l t e d t o the 20 i n c h g r i d on t h e f l o o r of the a i r p l a n e ' s c a b i n . The aluminum gave a secure base t o a t t a c h the equipment t o , as w e l l as a l a r g e mass t o heat s i n k some e l e c t r i c a l components. Large handles were p l a c e d on 41 t h r e e s i d e s t o a i d the o p e r a t o r d u r i n g the f l i g h t . These handles a l s o gave a measure of p r o t e c t i o n t o the equipment d u r i n g the t r a n s i t i o n from m i c r o g r a v i t y t o h y p e r g r a v i t y . The apparatus c o n s i s t e d of f o u r major p a r t s : the oven, the 3 6 temperature c o n t r o l l e r , the b a t t e r y pack , and the sample h o l d e r . These are shown on the base p l a t e i n f i g u r e 5.16. The c a b l e c o n n e c t i o n s t o each box were supported underneath by s m a l l p i e c e s o f metal t o minimize damage i f they were stepped on. The t o o l c o n t a i n e r was made of heavy c l o t h and v e l c r o . I t h e l d a l i e n keys, s c r e w d r i v e r s , and wrenches, which might be necessary t o make r e p a i r s and adjustments d u r i n g the f l i g h t . The two 37 d i g i t a l thermometers monitored a type K thermocouple on the oven p l a t e . Only one meter was used, the second b e i n g a backup. I t was a l s o p o s s i b l e t o run the apparatus without these meters, u s i n g o n l y the l i g h t s on the temperature c o n t r o l l e r . The oven was heated by 0.3 mm diameter N i C r w i r e coated i n ceramic, and c o o l e d by P e l t i e r elements. The oven was made of a 1 square i n c h copper p l a t e . The NiCr wire was f o l d e d under the copper p l a t e , and h e l d i n p l a c e by ceramic. Along the edges of the copper p l a t e , f o u r P e l t i e r elements were p l a c e d , t o c o o l the hot p l a t e . On top of the p l a t e were p l a c e d a type K thermocouple and an IC t o monitor the temperature. C l a s p s were a l s o p l a c e d on the s i d e s of the p l a t e t o h o l d the sample i n p l a c e . The oven was encased i n an aluminum box. A h o l e was c u t 42 I 1 e d i g i t a l thermometer Ih © b a t t e r y pack • o o • o c o n t r o l l e r oven sample h o l d e r © F i g u r e 5.16: Base p l a t e and f l i g h t apparatus. i n the top of the box, so t h a t the sample c o u l d be observed. T h i s h o l e was covered w i t h a p l e x i g l a s s sheet t o minimize danger from the hot oven. The sheet and box were b o l t e d t o the base p l a t e . A narrow s l i t was machined out of the s i d e of the box next t o the window, through which the samples c o u l d be i n s e r t e d . The apparatus allowed the sample t o be heated and c o o l e d 43 r a p i d l y , a n d f i g u r e 5.17 shows the temperature r e c o r d e d by a thermocouple on a copper boat i n a h e a t - c o o l c y c l e . 200 i . 0 I 1 1 1 1 i i i i i i 0 10 20 30 40 50 60 70 80 90 100 110 TIME Tsl F i g u r e 5.17: Heating and c o o l i n g curve f o r sample i n oven prepared f o r jLig f l i g h t . The box f o r the temperature c o n t r o l l e r i s shown i n f i g u r e 5.18. The h e a t e r and c o o l e r c u r r e n t s e t were a d j u s t e d b e f o r e the f l i g h t so the upper temperature was 160°C and the lower temperature was 35°C. F i n e adjustments of the s e were made w i t h the h e a t e r and c o o l e r o f f d i a l s . A h e a t / o f f / c o o l switch, on the l e f t o f the box, c o n t r o l l e d the temperature. 44 HEAT o HEATER OFF TEMP.SET OFF COOLER OFF TEMP.SET COOL o HEATER CURRENT SET COOLER CURRENT SET F i g u r e 5.18: Temperature c o n t r o l box. The b a t t e r y c o n t a i n e r h e l d f o u r r e c h a r g a b l e b a t t e r i e s and was l i n e d w i t h t h i n sheets of cork w i t h foam p l a c e d between the b a t t e r i e s t o o b t a i n a t i g h t f i t . B a t t e r i e s were used r a t h e r than the l i m i t e d and somewhat u n r e l i a b l e power source on the p l a n e . The b a t t e r i e s were wired so t h a t each one c o u l d be used s e p a r a t e l y . A green LED i n d i c a t e d t h a t a b a t t e r y was connected and pr o d u c i n g over 11 V. Only one b a t t e r y was used a t a time and each one had a 10 Amp f u s e i n i t s c i r c u i t . The sample box was designed t o h o l d the samples i n zero g r a v i t y y e t a l s o a l l o w f o r easy access i n the h y p e r g r a v i t y p a r t of the p a r a b o l a . The box had two wide s l i t s c u t i n t o the top and f r o n t s i d e . A s t r i p of metal was l e f t i n the middle t o p r o t e c t the samples from being trodden on a c c i d e n t a l l y . Two rods were screwed i n t o the back of the box, one f o r h o l d i n g the samples b e f o r e r e m e l t i n g , the other f o r a f t e r w a r d s . The c l a s p s were used t o h o l d the samples on the rod, w h i l e a l l o w i n g easy ac c e s s , are shown i n f i g u r e 5.19. The green r o d h e l d the 45 unmelted samples and i t s c l a s p was designed t o prevent i n s e r t i o n of a sample w h i l e a l l o w i n g i t s easy removal. A s p r i n g and washer were p l a c e d on the back of t h i s r o d t o push the samples forward, a l l o w i n g the l a s t few t o be e a s i l y removed. The r e d r o d s t o r e d the samples a f t e r r e m e l t i n g and i t s c l a s p allowed samples t o be put on i t r e a d i l y , but not e a s i l y removed. The samples were gl u e d t o copper boats which were r i v e t e d t o 0.65 mm t h i c k p l a s t i c handles. The sample h o l d e r s a r e shown i n f i g u r e 5.20. The samples were pushed i n t o the oven as f a r as the t e f l o n screw p l a c e d i n the handle. A h o l e of a s l i g h t l y l a r g e r diameter than t h a t of the r o d i n t h e sample box was d r i l l e d i n t o the handles of h o l d e r s f o r easy s t o r a g e . (a) (b) F i g u r e 5.19: Rods i n sample h o l d e r (a) samples b e f o r e r e m e l t i n g (b) samples a f t e r r e m e l t i n g . 46 PLASTIC COPPER TEFLON NUT AND SCREW I 1 (a) (P~ < (b) T INDENTATION TO HOLD SAMPLE HOLE RIVETS F i g u r e 5.20: Sample h o l d e r (a) top view (b) s i d e view 5.3 M i c r o g r a v i t y F l i g h t The experiment was flown from E l l i n g t o n A i r Force Base, near the Johnson Space Center i n Houston, Texas. I t flew two days, f o r a t o t a l of f o r t y p a r a b o l a s , o r twenty samples. The procedure was as f o l l o w s . When l e v e l f l i g h t was obtained, the b a t t e r i e s were connected and the oven was t e s t e d t o make sure i t heated and c o o l e d p r o p e r l y . The f i r s t sample was p l a c e d i n the oven i n p r e p a r a t i o n . Approximately f i v e seconds b e f o r e the s t a r t of m i c r o g r a v i t y , the he a t e r was tu r n e d on. T h i s p o i n t was f i r s t e s t i m a t e d by the NASA t e s t d i r e c t o r , then by the f l y e r / experimenter l i s t e n i n g t o the sound of the engines. The oven was tu r n e d t o c o o l about t e n seconds i n t o the m i c r o g r a v i t y . A f t e r e n t e r i n g h y p e r g r a v i t y , the oven was t u r n e d o f f , the sample was removed t o the r e c e p t a c l e and a f r e s h sample was p l a c e d i n 47 the oven. T h i s procedure had t o be f o l l o w e d without head movement! I t had been planned t o complete one sample every two p a r a b o l a s , but i n the event r e q u i r e d o n l y one p a r a b o l a f o r each sample. The s m a l l mass of the copper boat allowed i t t o c o o l v e r y r a p i d l y once removed from the oven, w i t h no danger t o the o p e r a t o r . T h i s l e f t time t o enjoy the unique e x p e r i e n c e of m i c r o g r a v i t y ! 5.4 H y s t e r e s i s Curves Once the samples were r e t u r n e d from Houston, i t was necessary t o do c r y o g e n i c t e s t s on them t o d i s c o v e r i f the r e m e l t i n g had made any d i f f e r e n c e . H y s t e r e s i s curves were measured a t f i x e d f i e l d s f o r t h r e e samples. They showed a s m a l l e r t r a n s i t i o n width than p r e v i o u s l y t e s t e d a r r a y s . The d i f f e r e n c e i n the widths can be e a s i l y seen by comparing f i g u r e 5.21, t h e sample melted i n aq, and f i g u r e 5.22, a sample prepared i n the l a b . Both a r r a y s have g r a i n r a d i u s of 12 am w i t h the g r a i n c e n t e r s separated by 70 /am, and were measured a t 12 mT. 48 —20 ' 1 1 1 1 1 1 1 1 1  2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8 2.9 3 3.1 T E M P E R A T U R E [K] F i g u r e 5.21: Superconducting t o normal t r a n s i t i o n f o r 12 nm r a d i u s indium g r a i n s prepared i n m i c r o g r a v i t y , i n an e x t e r n a l f i e l d B = 12 mT. I t i s not completely understood why such a change i n the t r a n s i t i o n width occurs when the a r r a y i s melted i n a l e s s e r g r a v i t y . A p o s s i b l e reason i s t h a t the p o l y c r y s t a l l i n e . . 39 s t r u c t u r e changes, g i v i n g a much more homogeneous s t r u c t u r e over the volume of the g r a i n . T h i s would produce fewer d e f e c t s which n u c l e a t e the superconducting t o normal t r a n s i t i o n . 49 5 _25 ' 1 1 1 1 1 1 1 1 1  2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8 2.9 3 3.1 T E M P E R A T U R E [K] F i g u r e 5.22: Superconducting t o normal t r a n s i t i o n f o r 12 urn r a d i u s indium g r a i n s prepared i n e a r t h ' s g r a v i t y , i n an e x t e r n a l f i e l d B = 12 mT. 50 Chapter 6 Experiments - T i n A r r a y s I t was hoped t h a t the t i n a r r a y s produced would be as good i f not b e t t e r than the indium ones. T i n has two p r o p e r t i e s which make i t b e t t e r than indium f o r use as a d e t e c t o r . I t i s a much harder metal than indium, so the spheres are l e s s l i k e l y t o deform under p r e s s u r e when many a r r a y s are s t a c k e d on top of each o t h e r , i n the f u t u r e , f o r the f i n a l d e t e c t o r . Indium i s s o f t enough than l i g h t p r e s s u r e from f i n g e r t i p s can e a s i l y f l a t t e n i t . T i n a l s o has a h i g h e r c r i t i c a l temperature and magnetic f i e l d , 3.72 K and 30.3 mT f o r t i n compared t o 3.41 K and 28.3 mT f o r indium. In the temperature range i n which we w i l l work, t h i s r e s u l t s i n a s t e e p e r B vs T curve r e s u l t i n g i n a lower spread i n the t r a n s i t i o n temperatures. 6.1 H y s t e r e s i s A 100x100 a r r a y of 18 /nm diameter t i n spheres was t e s t e d f o l l o w i n g the same procedures used t o i n v e s t i g a t e indium a r r a y s . A v i s u a l examination w i t h the microscope a t 125 power showed t h a t the q u a l i t y of t h i s a r r a y was not as good as t h a t o b t a i n e d f o r the l a t e r indium a r r a y s , t h e r e being a f a i r amount of 51 s c a t t e r i n the placement of the spheres w i t h some completely m i s s i n g . H y s t e r e s i s curves were measured f o r magnetic f i e l d s r a n g i n g from 0.05 mT ( E a r t h ' s f i e l d ) up t o 16 mT. An example of one of the curves o b t a i n e d i s shown i n f i g u r e 6.23 f o r a f i e l d o f 4 mT. The c r y o g e n i c q u a l i t y of the sample i s r e f l e c t e d i n the spread of t r a n s i t i o n temperatures which i s about 30 mK, a v a l u e l a r g e r than t h a t of the indium samples prepared p r e v i o u s l y 4 0 . 3  0.5 1 1 1 1 1 1 1 1 1 1 3.41 3.42 3.43 3.44 3.45 3.46 3.47 3.48 3.49 3.5 T E M P E R A T U R E [K] F i g u r e 6.23: Superconducting t o normal t r a n s i t i o n f o r 18 jim diameter t i n g r a i n s i n an e x t e r n a l f i e l d of B = 4 mT. 52 Although the t i n sample had a much broader spread i n t r a n s i t i o n temperatures, the midpoint of the t r a n s i t i o n was found t o s c a l e w i t h magnetic f i e l d as expected. T h i s i s shown i n f i g u r e 6.24, i n which the magnetic f i e l d i s s c a l e d by 3/2 t o take i n t o c o n s i d e r a t i o n the h i g h e r f i e l d f e l t a t the equator of the sphere due t o the h i g h e r d e n s i t y of magnetic f l u x found T E M P E R A T U R E [K] F i g u r e 6.24: F i e l d vs t r a n s i t i o n temperature, squares: t i n a r r a y of r a d i u s 9 jam spheres, diamonds: t i n c o l l o i d of 7 /am r a d i u s spheres. 53 t h e r e . A l s o shown are data p o i n t s f o r a c o l l o i d of t i n spheres 41 of average r a d i u s 7 jim . The p o i n t s appear t o f a l l on the same curve. Note t h a t the t r a n s i t i o n i n h i g h e r f i e l d s has a g r e a t e r u n c e r t a i n t y than t h a t i n lower f i e l d s , due t o t h e l a r g e r spread i n t r a n s i t i o n temperatures. An attempt was made t o r e c o r d i n d i v i d u a l g r a i n f l i p s w i t h the t i n sample. T h i s proved u n s u c c e s s f u l as the f l i p s were c a l c u l a t e d t o be around 0.1 V a t B = 12 mT, w h i l e the n o i s e was s l i g h t l y g r e a t e r than 0.1 V. 54 Chapter 7 Co n c l u s i o n s The pumped 4He c r y o s t a t underwent minor m o d i f i c a t i o n s which s i g n i f i c a n t l y reduced the v i b r a t i o n a l n o i s e and g r e a t l y improved the SQUID s i g n a l . A new mask was used i n the p r o d u c t i o n of a r r a y s , which gave both b e t t e r s i z e and placement d i s t r i b u t i o n s , and a l o w e r i n g i n the spread of t r a n s i t i o n temperatures f o r the a r r a y . U s i n g t h i s c r y o s t a t and a f a s t e l e c t r o n i c r e a d out system, an a r r a y of 13x27 indium g r a i n s was s t u d i e d . The w e l l d e f i n e d energy peak expected i n the MCA spectrum d i d not appear, most of the p u l s e s appearing t o have a lower energy than p r e d i c t e d . Observing a sweep of the f i e l d on the d i g i t a l o s c i l l o s c o p e showed a s e r i e s of s m a l l p u l s e s , many i n groups and spread out i n time. T h i s suggests t h a t the s t e p s seen w i t h the SQUID may a c t u a l l y be composed of many s m a l l e r s t e p s a s s o c i a t e d w i t h the g r a i n becoming normal i n st a g e s . A f u l l 100x100 a r r a y of 12 u.m r a d i u s indium spheres was s t u d i e d w i t h the c r y o s t a t and SQUID e l e c t r o n i c s . S u p e r c o n d u c t i n g - s u p e r c o o l i n g h y s t e r e s i s curves were o b t a i n e d f o r magnetic f i e l d s r a n g i n g from 0.05 mT t o 12 mT. The s u p e r c o o l i n g t r a n s i t i o n c o u l d not be observed f o r f i e l d s of 4 mT or g r e a t e r 55 s i n c e they o c c u r r e d a t a temperature below 1.7 K, the o p e r a t i n g temperature o f the 1 K pot. As expected, the spreads i n t r a n s i t i o n temperatures i n c r e a s e d w i t h a p p l i e d f i e l d . S i n g l e g r a i n f l i p s due t o gamma ra y s were observed. These s t e p s were counted over an extended time p e r i o d . I t was found t h a t by ob s e r v i n g t he ste p s i z e and the number o f occur r e n c e s o f steps of t h a t s i z e i t was p o s s i b l e t o d i s c r i m i n a t e between s i n g l e and simultaneous m u l t i p l e events. Indium g r a i n a r r a y s were melted and s o l i d i f i e d i n a m i c r o g r a v i t y environment aboard NASA's KC-135 a i r p l a n e . H y s t e r e s i s curves f o r a sample w i t h 12 lira r a d i u s spheres w i t h c e n t e r s s e p a r a t e d by 70 /am i n d i c a t e d t he spread i n t r a n s i t i o n temperatures was narrower by a f a c t o r o f two than a s i m i l a r a r r a y melted i n the l a b . The l e s s e r g r a v i t y has o n l y a s l i g h t a f f e c t on the shape of the g r a i n s i n c e they are v e r y s m a l l , but presumably i t does a f f e c t the p o l y c r y s t a l l i n e s t r u c t u r e , making the i n d i v i d u a l c r y s t a l s l a r g e r and more r e g u l a r . Fewer d e f e c t s i n t he c r y s t a l s would a l s o g i v e fewer n u c l e a t i o n c e n t e r s , which would a l s o decrease the spread i n t r a n s i t i o n temperatures. T i n a r r a y s of 100x100 spheres, w i t h g r a i n r a d i i of between 8 and 13 /nm, wi t h c e n t e r s separated by 70 lira, were a l s o made. More work needs t o be done t o improve the q u a l i t y o f the a r r a y s s i n c e some spheres are m i s s i n g w h i l e o t h e r s had moved d u r i n g the m e l t i n g p r o c e s s . An a r r a y of 9 urn. r a d i u s spheres was t e s t e d i n the c r y o s t a t w i t h the SQUID setup. H y s t e r e s i s curves were 56 o b t a i n e d f o r v a r i o u s f i e l d s . The width of the t r a n s i t i o n of a l l the g r a i n s s c a l e s w i t h f i e l d , as expected. However, the spread i n t r a n s i t i o n temperatures are wider than those measured f o r the indium a r r a y s , a r e f l e c t i o n of the poor q u a l i t y o f the a r r a y . The B-T phase diagram was mapped from 0.05 t o 16 mT. I t matched t h a t p r e v i o u s l y o b t a i n e d f o r a t i n c o l l o i d . I t was not p o s s i b l e t o r e c o r d d i s c r e t e t r a n s i t i o n s of i n d i v i d u a l g r a i n s because the estimated s t e p s i z e was of the same order of s i z e as the n o i s e . 57 B i b l i o g r a p h y H. Bernas, J.P. Burger, G. Deutscher, C. V a l e t t e , and S.J. W i l l i a m s o n , Phys. L e t t . 2 4 A , 721 (1967). 2 J . Feder, S.R. R i s e r , and F. Rothwarf, Phys. Rev. L e t t e r s 17, 87 (1966). J . B l o t , Y. P e l l a n , J.C. Pineau, and J . R o s e n b l a t t , J . Appl. Phys. 45, 1429 (1974). 4A.K. D r u k i e r and C. V a l e t t e , N.I.M, 105, 285 (1972). 5 A.K. D r u k i e r , C. V a l e t t e , G. Waysand, L.C.L. Yuan and F. P e t e r s L e t t , a l Nuovo Cimento, 14, 300 (1975). 6A.K. D r u k i e r , J . I g a l s o n and L. Sniadower, N.I.M., 154, 91 (1978) . 7A.K. D r u k i e r and L. Stodolsky, Phys. Rev. D, 30, 2295 (1984). Q L. Gonzaalez-Mestres and D. P e r r e t - G a l l i x , Nuovo Cimento, C9, 573 (1986). 9M.W. Goodman and E. Witten, Phys. Rev. D, 31, 3059 (1985). 1 0A.K. D r u k i e r , K. Freese and D. S p e r g e l , Phys. Rev. D, 33, 3495 (1986). 1 1 S e e , e.g., L. Gonzales and D. P e r r e t - G a l l i x , i n Loiv Temperature Detectors for Neutrinos and Dark Matter II, e d i t e d by L. Gonzales and D. P e r r e t - G a l l i x ( E d i t i o n s F r o n t i e r e s , G i f - s u r -Y v e t t e , 1989), pp. 1-34. 12 . . M. Le Gros, A. Da S i l v a , B.G. T u r r e l l , A. K o t l i c k i , and A.K. D r u k i e r , i n Low Temperature Detectors for Neutrinos and Dark Matter III, ed. L. Gonzales-Mestres and D. P e r r e t - G a l l i x ( E d i t i o n s F r o n t i e r e s , G i t - s u r - Y v e t t e , 1990) page 91. 58 13 . . M . L e G r o s , A . Da S i l v a , B . G . T u r r e l l , A . K o t l i c k i a n d A . K . D r u k i e r , A p p l . P h y s . L e t t . 5 6 , 2234 ( 1 9 9 0 ) . 14 . . M . L e G r o s , A . Da S i l v a , B . G . T u r r e l l , A . K o t l i c k i a n d A . K . D r u k i e r , i n Low Temperature Detectors for Neutrinos and Dark Matter III, e d . L . G o n z a l e z - M e s t r e s a n d D . P e r r e t - G a l l i x , E d i t i o n s F r o n t i e r s , G i t - s u r - Y v e t t e ( 1 9 9 0 ) , p a g e 9 1 . 15 . . . . S u p p l i e d b y M . C h m i e l o w s k i , A p p l i e d R e s e a r c h C o r p . , L a n d o v e r , MD, U . S . A . 16 B . G . T u r r e l l , M . L e G r o s , A . Da S i l v a , A . K o t l i c k i a n d A . K . D r u k i e r , N . I . M . A , 289 ( 1 9 9 0 ) , p a g e 5 1 2 . 17 S . H a r t l a n d a n d R . W. H a r t l e y , Axisymmetric Fluid-Liquid Interfaces, E l s e v i e r S c i e n t i f i c P u b l i s h i n g C o m p a n y , A m s t e r d a m , T h e N e t h e r l a n d s (1976) p a g e 9 . 18 i b i d , p a g e 1 5 7 . 19 th CRC Handbook of Chemistry and Physics, 66 e d i t i o n , CRC P r e s s , F l o r i d a ( 1 9 8 6 ) , p a g e s B - 2 2 5 , 6 . 2 0 . . i b i d p a g e F - 2 3 . 2 1 . i b i d p a g e F - 2 5 . A . Da S i l v a , M . L e G r o s , B . G . T u r r e l l , A . K o t l i c k i a n d A . K . D r u k i e r i n Lour Temperature Detectors for Neutrinos and Dark Matter, e d s . L . G o n z a l e z - M e s t r e s a n d D . P e r r e t - G a l l i x , E d i t i o n s F r o n t i e r e s , G i f - s u r - Y v e t t e , (1989) p . 4 1 7 . 23 . S u p e r c o n , I n c . s u p e r c o n d u c t i n g w i r e , t y p e T 4 8 B , d i a m e t e r s c o r e / c o p p e r : . 0 0 5 " / . 0 0 6 5 " , i n s u l a t i o n : F o r m v a r . 2 4 s e e #7. 25 L a b t e c h N o t e b o o k V e r s i o n 4 . 1 , L a b o r a t o r y T e c h n o l o g i e s C o r p o r a t i o n , 255 B a l l a r d v a l e S t r e e t , W i l m i n g t o n , MA 0 1 8 8 7 . 59 2 6 KPR t h i n f i l m r e s i s t (catalogue number 705), KPR t h i n f i l m r e s i s t t h i n n e r (catalogue number 715), and KTFR dev e l o p e r (catalogue number 749) from M.G. Chemicals. 27 B l a k - r a y l o n g wavelength u l t r a v i o l e t lamp from UVP Inc. u s i n g a 100 W longwave mercury spot b u l b . 2 8 M. Le Gros, A. Da S i l v a , B.G. T u r r e l l , A. K o t l i c k i and A.K. D r u k i e r , i n Low Temperature Detectors for Neutrinos and Dark Matter III, ed. L. B r o g i a t o , D.V. Camin and E. F i o r i n i ( E d i t i o n s F r o n t i e r e s , G i f - s u r - Y v e t t e , 1990) page 91. 29 . K i n d l y s u p p l i e d by DuPont Canada. 30 . M. Le Gros, G. Meagher, A. K o t l i c k i , B.G. T u r r e l l , and A.K. D r u k i e r , Proceedings of the 2nd London Conference on Position-Sensitive Detectors, Sept. 4-7 1990, I m p e r i a l C o l l e g e , London. 31 M. Le Gros, A. Da S i l v a , B. G. T u r r e l l , A. K o t l i c k i , and A. K. D r u k i e r , A p p l . Phys. L e t t . 56, 2234 (1990). 32 ACE M u l t i c h a n n e l A n a l y z e r , v e r s i o n 4.03, EG&G O r t e c . 33 SCC-1220 D i g i t a l Storage O s c i l l o s c o p e , S o l t e c C o r p o r a t i o n , San Fernando, C a l i f o r n i a , 91340-1597. 34 C. J . S m i t h e l l s , Metals Reference Book Volume III, Chaucer Press, S u f f o l k , Great B r i t a i n (1967) F o u r t h E d i t i o n , page 747. 35 . . M. Le Gros, A. Da S i l v a , B.G. T u r r e l l A. K o t l i c k i and A.K. D r u k i e r , i n Low temperature Detectors for Neutrinos and Dark Matter, eds. L. Gonzalez-Mestres and D. P e r r e t - G a l l i x , E d i t i o n s F r o n t i e r e s , G i f - s u r - Y v e t t e , (1990) p. 91. 3 6 The temperature c o n t r o l l e r and b a t t e r y pack were designed and c o n s t r u c t e d by the t a l e n t e d t e c h n i c i a n s i n the UBC p h y s i c s e l e c t r i c a l and machine shops. 37 . . D i g i - s e n s e , model no. 9528-40, type K thermocouple, from Cole parmer. 3 8 Rechargable b a t t e r y NP6-12, from Yuasa, 12 v o l t s , 6.0 Ah. 60 J . J . F a v i e r , J . B e r t h i e r , Ph. Arragon, Y. Male j a c , V.T. Khryapov, and I.V. Barmin, A c t a A s t r o n a u t i c a , 9, 255 (1982). 40 M. Le Gros, A. Da S i l v a , B.G. T u r r e l l A. K o t l i c k i and A.K. D r u k i e r , i n Low Temperature Detectors for Neutrinos and Dark Matter, eds. L. Gonzalez-Mestres and D. P e r r e t - G a l l i x , E d i t i o n s F r o n t i e r e s , G i f - s u r - Y v e t t e , (1990) p. 91. 41 A. Da S i l v a , I n v e s t i g a t i o n of a Superheated Superconducting Colloid, M.A.Sc. t h e s i s , U n i v e r s i t y of B r i t i s h Columbia, 1988. 61 

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