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The decay scheme of Zn65 Daykin, Philip Norman 1949

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( H I ftSf 6* THE DECAY SCHEME OF ZN by PHILIP NORMAN DAYKIN A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE HEQUIHEMEHTS FOR THE DEGREE OF MASTER OF ARTS IN THE: DEPARTMENT OF PHYSICS; 1 fHEL UNXVERSITr OF BRITISH COLOMBIA \ APRIL, I 9 4 9 ABSTRACT 65 Radiations from Zn have been studied by means of a t h i n lens beta ray spectrometer* A s p i r a l b a f f l e waa used to separate positrons from negatrons* The gamma ray spectrum i n the energy range above 100 ker was found to consist of one gamma ray at 1.11 mev and an n i h i l a t i o n r a d i a t i o n at 0.^1 mer. One positron group was found with maximum energy at 0*327 mev. No i n t e r n a l conversion electrons were found. 65 A decay scheme has been proposed i n whieh Zn decays either 65 by K-capture to a 1.11 mev excited state of Gu or by 65 positron emission to the ground state of Cu • ACKCTOWIEIIGEMEMT This researah was carried out with the aid of National Research Council Sc3u?larahlp> (1) Introduction 65 Radiations from the Zn nucleus hare been studied 65 by several methods. The samples of Zn used have been (1) produced by the following reactions * 64 65 Zn (d,p) Zn 6? 65 Cu (d,2n) Zn 65 65 Cu (p,n) Zn 64 65 Zn (n, Y ) Zn 65 65 Ga (K-decay) Zn (2) Barnes and Valley investigated radiations from copper bombarded by- protons, using absorption and cloud chamber techniques* They reported ah a c t i v i t y with a h a l f l i f e of about f months* consisting of both positron and negatron emission i n the r a t i o of 2:1 and gamma rad i a t i o n with a gamma to positron r a t i o ef about 6 0 , Absorption measurements i n aluminum indicated an end point of 0*7 mev A A f o r the positron group* Through the Cu Cprn) Zn reaction 63 65 both Zn and Zn could be produced from the two stable 63 65 isotopes Ca ^and Cu by proton bombardment* Livingood and Seahorg , however, found a s i m i l a r a c t i v i t y i n an isotope of zinc produced by deuteron bombardment of z i n c . 64 65 They i d e n t i f i e d the reaction as Zn (d,p) Zn • Since 62 63 there i s no stable isotope Zn , from which Zn could r 6'5 s i m i l a r l y be produced, they assigned the a c t i v i t y to 2 n (2) ( 4 ) Doleasao et a l also Investigated the radiations from copper bombarded by protons, by absorption measurements in aluminum* The absorption curve was separated into three components which were identified as: one positron group;, one internal conversion electron and one gamma ray. The The lowest end point only waa reported at 0»55 mev. This value could be assigned to either the positron group or the internal converson electron. Fairly intense X-radiation found by them was attributed to both the K-capture and internal conversion processes. Livingood and Seaborgf^ reported X-ray a appropriate approximately to the CuK^ line* Previously Alvarez had showed that there was ho large difference in the absorption of these X-rays in nickel and copper. Livingood and Seaborg concluded from this that the X-rays were from an element of atomic number lese than that of zinc, and assigned them to the GuK^ line. ThevK-eapture process for the decay Zn -^Cu waa therefore postulated. They used magnetic separation of the particles and gamma raye and confirmed the high ratio of gamma rays to particles. Their absorption measurement a In aluminum indic-ated one -ray at 1*0 mev and a weak annihilation radiation at 6 .5 mev. The half l i f e waa given aa 250*5 days* Perrier Santangelo and Segre^^ reported a half l i f e of 245 days for a Zn isotope obtained from copper which had been bombarded by both protons and deuterohs. (3) Since the GuE^ X-rays could arise from either K-eapture process:,or i n t e r n a l conversion of gamma rays from the excited C u ^ i coincidence studies are required. Good and peacock investigated X-ray Jf-ray and @"V-»ray coincidences*. They con-cluded that $4% of the K-capture process leads to the ground state of Gu^, while 4-6% leads to the 1.14 mev^ 1 1 ^excited s t a t e , and 2*2% of the disintegrations go by positron emission '*'" (8 ) d i r e c t l y to the ground s t a t e . Watase, Itoh and Takeda also found some evidence by X-ray Jf-ray coincidence that some part of the K-capture process goes to the ground state. The Table of I s o t o p e s ^ l i s t s G.4 mev f o r the fl* end (9) point from cloud chamber measurements and 0*32 mev, using a beta ray spectrometer • The existence of i n t e r n a l conversi version electrons i s also l i s t e d from the work of Livingood (a) and Seaborg , but no energies are given. For the gamma ray energies, 1.14mev - 3^ w s a reported by Deutsch, Hoberts and E l l i o t t and l.llmev ± O.J% by (12) Jensen, Laslett and Pratt- . , both from spectrometer studies;* The l a t t e r found a weak an n i h i l a t i o n radiation at O.51 mev* v* The existence of both K-eapture and positron emission with the former process highly favored i s reasonably c e r t a i n * At least one y-ray has been found at 1.11 mev and at least one (3 f-group with end point at O.32 mev, both by spectrometer methods* In view of the work* of Good and Peacock, there should be a weaker ^ -group w i t h higher end point energy* The presence of i n t e r n a l conversion electrons has not been (4) confirmed "by spectrometer methods, and the energies are there-fore not Known with any certainty. In view of these uncertainties i t was deemed advisable to repeat the spectrometer study of the gamma ray spectrum. and further, te investigate bath positron and negatron spectra 64 65 separately. The isotope used was obtained by the Zn (n.y)Zn reaction from the Chalk River Laboratories of the National Research Council. Equipment  Spectrometer The spectrometer la of the thin lens type whieh has (13) been used in previous researches: in this laboratory and elsewhere^ 14 f l5» l6 )^ A diagram of the spectrometer i s shown In figure I • Electrons from the source are selected by the baffle B and f'ocussed by the magnetic f ield on the thin window cf the beta ray counter. The baffles A , C, D, and E are added to reduce scattered, radiation and prevent direct radiation from reaching the counter* For study of positron spectra an additional spiral baffle was inserted between C and the f ield (14) coils* Deutsch et a l have calculated the pitch of the focuseed electrons in their spiral path through the spectro-meter. The baffle shown in figure 2 is based on these cal-culations. Since the pitch Is almost a oonstant for paths (5) Figure 1* Diagram of the spectrometer* I 311 "H H II W II 1 % CMS. Figure 2. Spiral baffle Used with the spectrometer for studying positron spectra* having different r a d i i , radial baffle plates of constant pitch may be used. Electrons of either sign may be selected by choosing the direction of the magnet c o i l current; this baffle transmitted 75% o f the focussed electrons with one direction of c o i l current while i t reduced the count to background with the opposite direction. The loss of 25% transmission caused by insertion of the baffle must be attrib-uted to a difference between the pitch of the baffle plates and that of the electrons, since the geometric cross section of the plates is much less than 25% of the spectrometer cross section* Plate I. Assembled spectrometer. (7) Plate I shows: the assembled spectrometer* The axis of the instrument i s aligned with the magnetic meridian and the vertical component of the earth*s magnetic f i e l d i s effectively cancelled by a current through the pair of com-pensating c o i l s surrounding the instrument* The effect of the remaining axial component may he found by reversing the ( 1 1 ) magnet c o i l current (with the spiral baffle removed) • It was found that, with the present resolution of 3 to 4% obtained with this spectrometer, no effect could be observed* By use of non-ferromagnetic materials throughout, proportion-a l i t y between the momentum of the focussed electrons and c o l l current i s therefore preserved* (1*) Deutsch et a l have shown the relation between spherical aberration and mean c o i l radius* Spherical aberration may be minimized by operating with the largest radius permitted by the momentum of the electrons being studied* The magnet c o i l Is therefore wound in four layers having separate terminals, so that the inner layers may be disconnected* Counters The thin window Geiger counter, sketched in figure 3, was designed to use a minimum of wax seals. Unstable oper-ation peculiar to wax sealed counters has been experienced in this laboratory* Anode wire and f i l l i n g tube are brought through the brass envelope with Kovar seals, whioh are (8) F L A N G E S * B R A S S E N V E L O P E • K O . V A R S E A L S T U N O S T t N A N 0 O 6 . C O S ' f l A . H ' H A H O 5 0 L D E R S - S O F T S O L 0 C R F i g u r e 3. Counter c o n s t r u c t i o n s o f t s o l d e r e d t o the envelope. A l l other s o l d e r e d j o i n t s are hard s o l d e r e d and coated w i t h s o f t oiwolopo s o l d e r . The u s u a l f i l l i n g tube tap was omitted; the tube was s e a l e d o f f a f t e r a s a t i s f a c t o r y f i l l i n g w a a o b t a i n e d . 2 The mica window, 2»8mg / c m >$hick, was s e a l e d between the f l a n g e s u s i n g Cenco P l i c e n e , a wax i n s o l u b l e i n a l c o h o l , w i t h the f o l l o w i n g t e c h n i q u e . P l i c e n e , d i s s o l v e d i n turpen-t i n e was p a i n t e d smoothly on bot h f l a n g e s and allowed t o d r y . these were then heated to melt the wax and the mica dropped on the counter; a i r bubbles were p r e s s e d out w i t h a rubber tube and the outer f l a n g e b o l t e d to the f i r s t . The counter d e s c r i b e d has operated s a t i s f a c t o r i l y d u r i n g the present work. ( 9 ) A m p l i f i e r s The l a b o r a t o r y a r rangement r e q u i r e d t h e use o f a t e n f o o t p u l s e c a b l e t o c a r r y c o u n t e r p u l s e s t o t h e s c a l a r . To a v o i d d i r e c t l o a d i n g o f t h e c o u n t e r by t h e c a b l e , a ca thode f o l l o w e r was c o n n e c t e d t o t h e c o u n t e r w i t h s h o r t l e a d s , as shown i n f i g u r e 4. The c a b l e c o n n e c t e d t o t h e c a t h o d e i s a p p r o x i m a t e l y matched a t t i t s o u t p u t end b y t h e 100 ohm r e s i s t o r * The f o l l o w i n g p r e a m p l i f i e r i s a two s t a g e g rounded g r i d t r i o d e a m p l i f i e r , , e a c h s t a g e o f w h i c h i s p r e c e d e d b y a ca thode f o l l o w e r . The t h r e e v o l t p u l s e s o b t a i n e d f r o m t h e ca thode f o l l o w e r a r e s u f f i c i e n t t o s a t u r a t e t h e p r e a m p l i f i e r , whose ou tpu t c o n s i s t s o f s i x t y v o l t p u l s e s o f e q u a l a m p l i t u d e . The s c a l a r d i s c r i m i n a t o r was s e t a t 15 v o l t s t o e l i m i n a t e s t r a y ; p i c k u p and n o i s e , and t h e n t h e c o u n t i n g r a t e was i n d e -penden t o f d i s c r i m i n a t o r b i a s f l u c t u a t i o n s . C o u n t e r p l a t e a u s o b t a i n e d w i t h t h i s c o u n t e r and c i r c u i t a r e shown i n f i g u r e 5* Cio) Figure 4. Schematic of cathode follower and preamplifier. I I O O I I S O I I O O 1 2 5 0 C O U N T E R V O L T A G E Figure Counting rate of pulses from preamplifier of amplitude greater than 1$ v o l t s . (11) Regulators Magnet Current Regulator The regulator i s the same as used i n previous research-(13a) es except f o r the f o l l o w i n g modifications. The D. C. power i s taken from the "building supply, whose negative terminal i s grounded, instead of from t h e M f l o a t i n g " generator supply* The A. C. error voltage ±a then taken between the standard r e s i s t o r and ground. This modification required snth add i t i o n a l stage of A * C. amplification to obtain both error s i g n a l inversion and increased voltage gain. The d r i v e r stage was modified so that, i t operated as a tetrode w i t h normal bi a s f o r a l l b i a s settings of the type 6AS7 c o n t r o l tubes. Bias control f o r the l a t t e r was obtained from a 100,000 ohm: potientiometer Connected across the d r i v e r B supply, w i t h the movable arm grounded; the negative b i a s supply was then not required* Magnet current was determined as before by s e t t i n g the d i a l box potentiometer. S t a b i l i t y was 0.01^ at, 10 amperes and 0.1^ at 1 ampere with t h i s arrangement. Compensating C o i l Current Regulator The current carried by the p a i r of compensating f i e l d efifils i s regulated against l i n e voltage variations by the use of two b a l l a s t tubes (type CRC876") i n aeries w i t h the 10 ohm C12) f i e l d c o i l s . Since these operate normally with .1.7 amperes, whereas only 1 ampere is required for f i e l d compensation, the excess current was shunted "by the c o i l s through a rheostat. Current regulation: of 0+2$% par volt was obtained, which value Is sufficient for normal hourly line voltage variations. Experimental Technique Arrangement of Sources A diagram of the source arrangement i s shown: i n figure 6. For gamma ray spectra the active material i s inserted into the brass cup from' the outside. A screw cap holds th i s firmly in place. Sufficient brass Is left between the active material and the radiator to absorb the beta rays. The radiator, a thin disc of lead or uranium oxide Is cemented to the front face. For beta ray/ spectra, f i l i n g s fwom the active material wewe cemented with collodion to a thin disc of mica, which f i t s ilnto the brass cup. The brass i s removed from Immed-iatel y behind the mica to> reduce reflections of beta rays. Owing to the low specific activity, of the Zn , the deposit could not be made as thin as was desired} sufficients was added to produce roughly/ twice background count In the spectrometer* (1%) Figure 6 * Arrangement of sources* A, Gamma ray source* B, Beta ray source* Calibration The spectrometer was calibrated directly In terms of di a l box potentiometer reading, which i s proportional to the momentum of the fo cussed electrons. Phot ©electrons ejected from the E shell of lead by the 0 . 6 0 7 mev/ gamma ray of radium (13c) were used* This energy value was obtained by Ozeroff from a similar spectrometer calibrated in terms of the * line of thorium B* The momentum of the photo electrons was obtained by subtracting the binding energy fo.0875mev) of the K shell . The calibration curves in figure 7 show that both resolution and transmission are improved by using- only the magnet c o i l s of large r a d i i . Below/ are l i s t e d the calibration correspond-(14) Ing to peak values and the resolution for each c o i l combination used* Ciiils in series 4 coils 3 outer 2 outer 1 outer Width at half maximum 4 % 3-6* 3-5# Calibration in gauss-em* per volt 9600 6600 4090 1915 2.000 UJ r— D 1800 UJ a . ^ 1 6 0 0 ZD O U 1+00 OUTER COIL 1-56 VOLTS ^ Ic^SOMA 0-730 VOLTS 140 MA 0 4-55 VOLTS I,= <H0M A 0-3)1 VOLTS h-wo M A R A D I A T O R PB * 0 M ^ / M Z J L J_ O-IG 0<>? I-0O I J-*?^ -Pigure 7. Calibration curves. (15) The compensating current, I c , had to be adjusted for maximum peak intensity with each c o i l combination. Pre-sumably, the difference i s due to slight misalignment of the separate c o i l axes In the horizontal plane. The resolu-t i o n possibly could be improved for the outer c o i l alone by realignment of the spectrometer axis with the outer c o i l (14) axis, in the ve r t i c a l plane( , but this was not attempted because the outer c o l l alone was insufficient for most energies. Experimental Results  Gamma Ray Spectrum The gamma ray spectrum i s shown i n figure 8 . Compt on background, obtained with the radiator removed, i s dotted under the main curve; the difference gives the photoeleetrona ejected from the radiator. Several radiators and magnet coils were t r i e d both to obtain high peak intensity and resolution and to eliminate spurious peaks. The two peaks obtained with lead radiators show that l i t t l e i s gained by using a radiator thicker than 50 mg/cm • Several small peaks appear on the mala curve obtained by using; the lead radiator. These could be interpreted either as spurious peaks arising from unusually large s t a t i s t i c a l deviations or as photoelectron peaks from weak gamma rays. To remove the ambiguity, the region con-taining these peaks was repeated with the uranium radiator. 020 040 0-60 POTENTIOMETER VOLTAGE 65 F i g u r e 8. Gamma ray spectrum of Zn (16) Since the binding: energy of the uranium E-shell i s 2 7 . 5 kev higher than that of the lead K-shell, a photoeloctron peak obtained with the lead radiator must reappear when the uranium radiator i s used, but art 2 7 • 5 kev lower energy* Only one such peak satisfied this condition. This peak is indicated in figure 8 at 0 . 2 6 volts. The Compton background seemed rather excessive* It was shown that the high intensity was due to the large source area required by the low specific a c t i v i t y . A lead' cylinder 2 cm. long and 2 cm. i n diameter, with a conical hole d r i l l e d to f i t over the radiator, reduced the Compton background to £ but l e f t the photoelectron peak unchanged. The work however was not repeated since repetition was not considered worthwhile f o r a factor of 2 . It i s therefore recommended that fungi! sources of high specific activity, when available, be placed directly behind the radiator; and the lead cylinder baffle be used only when necessary, since additional scattering i s undoubtedly produced by i t s use. Gamma Ray Energies The gamma ray energies were obtained by adding the K sh e l l binding energies, l l j f kev for uranium and 8 7 . 5 kev U 7 ) for lead( , to the photo electron peaks. These are tabulated below. The center of the photoelectron peak was chosen generally, except i n the case of the 100 mg/cm lead radiator* Since this had a definite f l a t top, the high energy end of ( 1 2 ) the f l a t top/ was chosen . Cl7). Gamma Ray Radiator P o l l s By i n Mev (1) U, 80mg/cm2 4 c o i l s 0.$1 U, 80 2 outer 0.?08 (2) Pb r 50 4 c o i l s 1.107 Pb,100 4 c o i l s 1.104 U, 80 4 c o l l s 1.109 U, 80 2. outer 1.109 Positron Spectrum The positron spectrum, shown i n figure 9 , was obtained from the beta ray source with the magnet current reversedi the s p i r a l b a f f l e e f f e c t i v e l y removed negative p a r t i c l e s . The counter was shielded fsrom other sources (i n c l u d i n g a £00 m i l l i c u r i e radium source i n a second spec-trometer i n an adjoining room) with Vy cm. of lead. Back-ground was reduced to 10 counts per minute. The source thickness required to obtain twice background count was 130 mg/cm . The Fermi plot of the positron spectrum, shown i n figure 10, was obtained "by use of the fo l l o w i n g approxim-ations. The Fermi r e l a t i o n i s given by F = Ff ~ Kmax " E (17a) 15 OX 0-5 0 4 OS P O T E N T / 0 M E T E R V O L T A G E 6 ? Figure 9 . Positron spectrum of Zn 0 0-1 • 0-1 . Ori 0-+ r. ENERfiy IN MEV Figure 10. Fermi plot of positron spectrum ( I S ) where T\ ~ momentum of electron in units of n^e H - relative number of electrons with momentum 7 ^ 2S try l n end t{Zj\) = r\ e j] ( 1 + S + iy) where S = J l - Wl$7Y T z Z Jl f Tj2 - 1 1 3 7 71 The approximation discussed concerns the expansion of the gamma function. This was expanded In a Taylor series, to the f i r s t power of S only* By a second approximation to the f i r s t power, the expression The expression i n brackets was expanded i n series, using a well known expansion for PC'Z.) • The series involved, of the 00 ("HZ) . 0 0 form y 1 , was approximated by / dn • ^ n T n ^ T p T j n U * + y*J result used in the calculations i s The IPCl f S f Iy)| 2 TT v (l 28^ 4 8 log(l • y 2)} I . 1 sinhTTy ( j A commonly used approximate expansion for th i s gamma function i s TT y fl t 0 . 4(p(z) 2 ) . sinhTjy ( { This formula can be obtained from ours by two further approximations. (19) The Fermi plot of the positron spectrum indicates one postron group with end point at 0«32? Bier** with a standard deviation of 0.0037 mev for the 14 points used. Internal Conversion Electrons A negatron spectrum was attempted, using the beta ray source. Particular attention was paid to the low energy end in a search for internal conversion l i n e s . Bone were found* hut a distribution was obtained which was identified as a Compton distribution. The existence of Compton electrons i s attributed to the high surface density of the source Cl30 mg/cm2) and to the relatively Intense 1.11 mev gamma radiation. Conclusions The average values of the gamma ray energies, taken to the number of valid significant figures, are? O.51 mev and 1.11 mev. The 0*51 mev radiation i s identified with annihilation radiation. These results are similar, within (12) 1%, to those reported by Jensen et a l • The gamma ray energy is 3$ lower than that reported by Deutsch, Boberts (11) and E l l i o t t The end point of the positron group i s 0.327 mev. (20) ( 1 0 ) , This value i s 2% higher than that reported by Peacock from spectrometer measurements, and Is considerably l e s s than a l l values reported from cloud chamber and absorption ( 2 , 4 , 9 1 measurements • Decay Scheme The following decay scheme based on these r e s u l t s i s proposed. 65 65 Cu Zn The K-capture process i s energetically possible i f the energy difference between the i n i t i a l and f i n a l states I s l e s s than the rest energy of the electron. This condition i s s a t i s f i e d by the decay scheme. The statement made i n the i n t r o d u c t i o n — that a second (J^-group of higher end point ( 7 ) energy was required by the result s of Good and Peacock — i s therefore incorrect. According to t h e i r r e s u l t s there i s a second K-capture process, approximately equally favored, 65 leading to the ground state of Cu • They further concluded that. 2% of a l l disintegrations go by positron emission d i r e c t l y t o the ground state. Therefore, i f only one ^ -(21) group e x i s t s , the r a t i o of gamma to positron emission would he approximately 25- Barnes and vTalleyt , however, reported a r a t i o of 60. Further, the reported presence of i n t e r n a l ( 1 . 2 , 3 . ) conversion electrons has not "been confirmed. 65 I t would he possible to estimate the mass of Zn , assuming that the positron emission leads to the ground state. However, owing to the c o n f l i c t i n g r e s u l t s , further work w i t h a source of much higher s p e c i f i c a c t i v i t y i s recommended, to determine f i r s t the decay scheme with greater cert a i n t y * (22) ACKNOWI3Ja36EMENTS Acknowledgement i s g r a t e f u l l y made to Dr. K. C. Mann under whose d i r e c t i o n the project was established and carried out. The National Research Council made t h i s work possible by a Grant-in - A i d to Dr. K. G* Mann i n addition to a Student-ship granted to the author* The author wishes to thank Dr." J * B. Warren f o r making available the zinc isotope used and Dr. A* van der Z i e l f o r h i s valuable discussions on the current regulator* The author extends h i s gratitude to Mr* A* J . Fraser. f o r h i s useful advice oh the machine work connected with the counter and b a f f l e construction, and to Mr. A . Win. Eye f o r construsting the counter f i l l i n g systems* (23) References 1. G. T. Seahorg and I . Perlman Rev. Mod. Physics, 20, 597, 1948 2. S. W. Barnes and G. Valley • Phys. Rev., 53, 946(A), 1938 3. J. J . Livingood and 6. IT. Seahorg Phys. Rev. 5i, 459, 1939 4. L. A. Delsasso, L. N. Ridenour, R. Shear and M. G. $h i t e . Phys. Rev., i i , 113, 1939 5. L. W. Alvarez Phys. ReV., £4, 486, 1938 6 . 0 . P e r r i e r , M. Santangelo and E. Segre Phys. Rev., 53, 104, 1938 7. W. ffi. Good and W. C. peacock Phys. Rev., 6 2 , 680, 1946 8 . Y. Watase, Z. Itoh and E. Take da Proa. Physico-Math. S o c , Japan, 22, 90, 1940 9 . L. A. Dubridge Private communication to^ G. T. Seahorg 10. ¥ . C . Peacock Plutonium Project Report, Hon. ir-432, 56, Dec. 1947. (probably r e s t r i c t e d c i r c u l a t i o n ) 11. II. Deutsch, A. Roberts and L. G. E l l i o t t Phys. Rev., 6l> 389(A), 1942 12. E. N. Jensen, 1. J . La s l e t t and W. wT. Pratt Phys. Rev., 7£, 458, 1949 13. (a) P . Lindenfeld H. A. Thesis, University of B r i t i s h GoIambia,1948 Cb) P. Mathews M. A. t h e s i s , University of B r i t i s h Columbia, 1948 (c) M. J . Ozeroff M. A. Thesis, University of B r i t i s h Columbia,1948 14. M. Deutsch, L. G. E l l i o t t and R. D. Evans R. S. I . , i i , 178, 1944 (24) 1^. ¥• Rail and R. G. Wilkinson Phys. Rev., 2i» 321. 1947 16. L. C. Miller and L. P. Curtis J . Research, Hat. Bur. Stds., 38, 359, 1947 17. International C r i t i c a l Tables, 6> 35, i929 


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