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The gamma-rays of radium 1948

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THE GAMMA-RATS OF RADIUM by Michael John Ozeroff A thesis submitted in partial fulfilment of the requirements for the degree of' MASTER OF ARTS in the Department of PHYSIOS The University of British Columbia Apr i l , 1948 ACKNOWLEDGEMENT The present study has been made possible by a Grant-in-Aid of Research to Professor K. C. Mann from the National Research Council of Canada. The author wishes to acknowledge a g i f t of 4060 pounds of lead from the Consoli- dated Mining and Smelting Company of T r a i l , B. C. for the construction of spectrometer baffles and protective screens. He i s indebted to the Cancer Research Group of Vancouver, B.C. for providing a Radium source, and to Mrs. E. Speers of the Chemistry Department for preparing a Thorium B source. The award to the author of a Studentship from the National Re- search Council has greatly f a c i l i t a t e d this work. Finally, the author wishes to express his deep indebtedness to Professor K. C. Mann, whose unfailing interest and helpfulness assisted greatly i n the successful completion of the study. V TABLE OP CONTENTS Page I. INTRODUCTION 1 II. EXPERIMENTAL METHOD 1. Spectrometer Types 3 2. The Thin-Lens Spectrometer 6 3. Source Arrangement 8 4. The Geiger Counter 10 5. Counter Power Supply 11 -6. Magnet Current Supply 12 1. Earth's Field Compensator 14 8. Alignment 14 9. Resolution 17 10. Calibration 17 11. Calculation of Gamma-Ray Energies lo III. EXPERIMENTAL RESULTS 1. Reduction of Primary Beta-Ray Radiation . . 19 2. The Radium Gamma-Ray Spectrum . 21 3. S t a t i s t i c a l Accuracy 21 4. Error in Energy Determination 23 5. Comparative Results 24 IV. CONCLUSION . 26 V. BIBLIOGRAPHY 29 ILLUSTRATIONS Figure Page 1. -fT-Type Spectrometer 4 2. Electrostatic Spectrometer 5 J>, Electron Lens Spectrometer . . J> 4. Thin-Lens Spectrometer . . . 7 5 . Source Arrangement . . . . . 8 6 . Geiger Counter . 10 7. Current Regulator 12 8. Compensator Coil Efficiency 1 5 9. Effect of Compensator Current on Peak,Shape , . 1 6 10. Thorium B Source, and E-line . . . 18 11. Reduction of Primary Beta-Radiation 20 12. Radium Gamma-Ray Spectrum .22 Table 1. Comparative Results 24 Plate Facing Page I. Thin-Lens Spectrometer 7 ABSTRACT Th© thin-lens beta-ray spectrometer is described, together with i t s associated equipment. The. energies of gamma-rays, emitted by Radium in equilibrium with i t s disin- tegration products have been determined by measuring, in such a spectrometer, the energies of photoelectrons ejected from lead. These energies agree reasonably well with those re- ported by E l l i s and Skinner, although several values reported by Alichanov and Latyshev have not been found. The energy calculations were based on a calibration using the E line of Thorium B; (H o = 1385.6 gauss-cm.). An indication was found of a gamma energy not previously reported. 1 THE GAMMA-RAYS OF RADIUM I. INTRODUCTION Previous measurements of Radium gamma-ray energies nave been made by several investigators. E l l i s and- Skinner^ 1), measuring internal conversion and photoelectric line energies in a "IT-type spectrometer reported twenty-one gamma-rays of Radium B, C and D. Alichanov and Latyshev^) measured the energies of positrons formed by pair-production in lead with a f t-type spectrometer, and from these measure- ments reported eleven gamma-rays of Radium C, of energies greater than 2 moc2 (i.e. 1.02 Mev). T s i e n ^ ) , using selective absorption and crystal diffraction i n the range 25-50 Kev, and the cloud chamber in the range 7-25 Kev re- ported six gamma-rays of Radium D. While i n the high energy (Dc.D. E l l i s and H.W.B. Skinner, Proc.Roy.Soc, 105A, 165 (1924). (2) A. Alichanov and G. Latyshev, C.R.Acad.Sci. (U.R.S.S.), 20, 429 (1938). (3) s.T. Tsien, Phys.Rev., 69, 38 (1946). region at least, a comparison of results shows f a i r agreement, there are some discrepancies and i t seemed advisable to re- peat this work with the thin-lens spectrometer at our dis- posal. 2. II. EXPERIMENTAL METHOD 1. SPECTROMETER TYPES The negative beta-rays from radioactive nuclei con- si s t of electrons whose energy varies continuously from a certain maximum value down to zero* Gamma-rays may also be emitted from such nuclei, and since they represent transi- tions between excited nuclear energy states, they possess discrete energies. To observe beta-ray distributions, beta spectrometers of various designs have been developed. Under the proper conditions the beta spectrometer may be used equally well to Investigate gamma-ray energies, either by measuring the energies of photoelectrons expelled by these gamma-rays from thin high atomic number lamina, by measuring Compton recoi l electron distributions ejected from thick absorbers of low atomic number, or by measuring the energies of positrons or negatrons created by pair production in high atomic number absorbers. Pour types of instruments are in general use. (a) The Magnetic Semicircular Focussing Spectrometer (1^-type), shown in Figure 1 , was devised by Danysz^ in 1 9 1 2 . It was later improved by Robinson and Rutherford^) ( 4 ) j . Danysz,Le Radium, 9 , 1 ( 1 9 1 2 ) ; 1 0 , 4 ( 1 9 1 3 ) . Robinson and E. Rutherford, Phil.Mag., 2 6 , 717 ( 1 9 1 3 ) . 4. and has since been very widely used. A uniform magnetic f i e l d is applied perpendicular to the plane of the figure. Beta-rays in a small momentum interval describe circles of approximately equal radii in the f i e l d and are therefore focussed at the same point on the photographic plate. A Geiger tube may be used in place of the photographic plate, in conjunction with a magnetic f i e l d which can be varied. (b) The Electrostatic Focussing Spectrometer, shown in. Figure 2, was suggested by Hughes and Rojanskyt6). This instrument uses a radial, inverse first-power, electrostatic f i e l d to focus a bundle of electrons of the same energy in a manner similar to that of a magnetic f i e l d . An angle of deviation of 1 2 7 ° 1 7 ' i s found to give the correct focussing condition. The instrument is particularly useful for low (^A.L. Hughes and V. Rojansky, Phys.Rev., 34, 284 (1925). -Figure 1. energy particles, and lias been used successfully by Backus(7) to measure the low energy negatron distribution of Cu^. Figure 2. (c) The Electron Lens type of spectrometer i s shown i h Figure 3. 1̂ Source Figure 3. (7) J. Backus, Phys.Rev., 68, 59 ( W J , 6.. This arrangement was f i r s t used by Trick e r ^ ) in 1 9 2 4 . The evacuated cylinder is surrounded for i t s entire length by a solenoidal wound conductor. For a given current through the solenoid, electrons of a certain energy w i l l be focussed on the detector. (d) A variation of this type of instrument i s the thin- lens spectrometer, as introduced by Deutsch, E l l i o t t and Evans ( 9 ) . This i s the type of spectrometer used in the pre- sent study. It is described in detail in the sections which follow. 2 . THE THIN-LENS SPECTROMETER The thin-lens spectrometer is shown in section i n Figure 4 and i n a photograph in Plate I. It consists essen- t i a l l y of an evacuated cylindrical brass tube 8 inches i n diameter and 4 0 inches long, surrounded at i t s centre by a short magnet c o i l of heavy wire. The c o i l i s water cooled in order to reduce temperature fluctuations. The tube con- tains five lead baffles which perform several functions. Baffle A transmits a conical beam of electrons from the radiator into the focussing f i e l d of the magnet. Baffle B prevents high-energy radiation from passing directly from source to counter. C i s a.masking baffle and together with D and E serves to absorb much of the scattered radiation which might otherwise reach the counter and thus increase the ( 8 ) R . A . Tricker, Proc.Camb.Phil.Soc, 2 2 , 4 5 4 ( 1 9 2 4 ) . ( ? ) M . Deutsch, L. E l l i o t t and R. Evans, Rev.Sci.Instr., 1 5 , 7 ( 1 9 4 4 ) .   8 - normal background. A Genco Megavac pump i s used to evacuate' the system, with an o i l diffusion pump included for lower pressures when necessary. The vacuum indicator is a thermo- couple gauge. The cone of electrons passing through the defining baffle A is focussed by the action of the magnetic f i e l d of the c o i l . For a given c o i l current, electrons of the appro- priate energy w i l l pass through baffle C, and be focussed on the "window" of the Geiger counter. Electrons of other energies would, in the absence of baffles, be focussed at other points along the axis of the spectrometer tube. Since the c o i l contains no iron, the f i e l d and hence the momentum of the focussed electrons w i l l be linear with current. 3. SPURGE ARRANGEMENT Figure 5 shows the source arrangement used in this study. Figure 5. The Radium used was enclosed in a silver capsule 1 inch long and 1/8 inch in diameter. This was placed in a small Rhole d r i l l e d through a solid brass cylinder as shown. The cylin- der was sealed to the end of the spectrometer tube. On the end of the cylinder facing into the spectrometer was cemented a circular lamina of lead, 3 millimetres in diameter and 0.044 millimetres thick, of surface density 50 milligrams per square centimetre. This w i l l be referred to as the lead radiator. The thickness of brass, between the Radium and the lead was made sufficient to absorb a l l the primary beta-rays from the source, calculation for this minimum thickness being made on the basis of the well known Feather formula(10), R(gms/cm2) = 0.543 E (Mev) *- 0.16. Gamma-rays emitted from the source pass through the brass and eject photoelectrons from the lead. In addition, Compton electrons in a continuous distribution are ejected from the brass absorber. Both photoelectrons and Compton electrons are detected and counted in the spectrometer with the result that a plot of electron intensity versus electron momentum i s a composite curve, showing a series of mono- energetic photoelectric peaks superimposed upon the continuous Compton distribution. In order to correct the curve for Compton background, the lead radiator is removed and a back- ground curve i s plotted over the same momentum range. This curve, sometimes normalized to f i t the composite curve, is (•L°)j".M. Cork, "Radioactivity and Nuclear Physics", (Van -Nostrand) P. 121. 10. subtracted from the latter, and the resulting plot gives the line spectrum due to photoelectrons ejected by gamma-rays from the lead. 4. THE GEIGER COUNTER The counter, shown in Figure 6, i s of the bell type, > s~ W i n d 1̂ Figure 6, having a diameter of 0.75 inches, and a central anode of 0.005 inch tungsten wire. It i s f i l l e d with a mixture of Argon and Ethyl Alcohol vapor, 9.3 cm. (Hg) of Argon with 0.7 cm. of Alcohol vapor having been found to give a good pulse shape and a usable.plateau. A sample plateau rises from 750 counts per minute at 975 volts to 1000 counts per minute at 1070 volts, a rate of increase of 0.3 percent in counts per minute per volt. With a lead shield around the counter, normal background (with source in place, no current through the magnet coil) i s of the order of 60 counts per minute. A mica window of surface 11. density 0.89 milligrams per square centimetre was used. This window was found to absorb a l l energies below 50 Kev, and this automatically sets a lower limit to the energies which may be measured. Considerable care must be exercised i n order to avoid subjecting such a thin window to di f f e r e n t i a l pressures much greater than 10 centimeters of mercury, since i t s strength i s not great. A brass mask with a central circular hole in i t is f i t t e d over the counter window. The diameter of the hole is made about 1 millimetre greater than the diameter of the source. The mask is intended to improve the resolving power of the spectrometer by eliminating from the counter electrons not.properly focussed. A removable flange on the counter permits replacement of the window and easy sealing of the counter to the spectrometer tube. Pulses are counted by a scale-of-64 scaling unit which actuates a mechanical register. 5. C01INTER POWER SUPPLY The counter power supply consists of a high voltage battery pack with a switching arrangement which gives steps of 15 volts over the range from 840 to 1400 volts. A stable supply voltage i s a necessity since changes in voltage w i l l cause changes i n counting rate and w i l l thus distort the re- sults. In the absence of an accurate voltmeter, reproduci- b i l i t y of points on a curve is the most reliable test of the supply voltage. 12 6. MAGNET CURRENT SUPPLY A D.C. generator supplies current for the magnet c o i l . This current is regulated to within 1 part in 1000 by means of a photocell control ci r c u i t , shown in Figure 7« A B C D E F D. C. Generator G Generator Field Circuit H Generator Field Supply J Load Circuit F i l t e r K Magnet Coil L Standard Resistance Figure 7. Galvanometer Potentiometer Photocells Amplifier 8 Parallel 6L6 Tubes The operation of the regulator i s as follows. The potentio- meter, used as a reference voltage, i s standardized by means of a Weston Standard c e l l . The voltage across a standard resistance i n the load circuit of the generator i s then balanced by the required potentiometer voltage. When the .. system is in balance the galvanometer reads zero current, and the galvanometer light beam takes up a position midway between the two photocells. This i s the desired operating condition. 13-. In this condition, the two photocell output voltages are balanced against each other and no signal voltage reaches the next stage of the amplifier. If now the magnet current begins to change, the voltage across the standard resistance also begins to change, and this deflects the galvanometer light. The resulting off-balance photocell signal i s ampli- fied and applied to the grids of the 6L6 tubes in such a way that the generator f i e l d current is altered to compensate for the original change in magnet current. As shown i n the diagram, the generator f i e l d i s separately excited, from batteries of large current capacity. Such an arrangement adds to the st a b i l i t y of the regulator. Because of the re- latively slow response of the galvanometer and the long time- constant of the generator f i e l d , this system i s useful in controlling only, slow variations of current (greater than 0.5 seconds). Hence considerable extra f i l t e r i n g on the generator output as well as on the magnet load was found necessary. The importance of a high degree of regulation for the magnet current cannot be too firmly stre'ssed. A varying current has the effect of reducing peak height and increasing peak width, thereby reduc'ing both the resolving power and the sensitivity of the spectrometer. Since many of the gamma- rays are only weakly converted, their resultant photoelectric peaks are very small, and an instrument with poor sensitivity w i l l not detect them. At the same time i t must be admitted that this 14. control circuit which holds the current constant to 0.1 per- cent is hotter than is actually needed when we consider the relatively low resolution of the spectrometer. ?. EARTH'S FIELD COMPENSATOR Two rectangular coils connected as Helmholtz coils - were arranged in horizontal planes, one above and one below - the spectrometer tube and placed symmetrically with respect to i t s axis. Their function is to compensate for the effect of the vertical component of the earth's f i e l d , which could cause defocussing of beta particles .over their long path. Current for the coils i s supplied from .batteries and must be held as nearly constant as possible. Further remarks re- garding the importance of the compensator w i l l be made in the following section. 8. ALIGNMENT .Four major factors must be considered in the align- ment of the thin-lens spectrometer. (a) The spectrometer tube axis should l i e in the plane of the earth's magnetic meridian. The earth's f i e l d strength (vertical component} and direction (horizontal component) are plotted over the area available.in the laboratory. An optimum position is, then chosen for the spectrometer, taking into account the rate of variation of vertical f i e l d strength with distance along the tube axis. (b) The current through the compensator coils must be 15. adjusted to counteract the effect of the vertical component of the earth's f i e l d . If this f i e l d strength is not sensibly- constant throughout the length of the spectrometer tube, then obviously some compromise must be made in the current value chosen for the c o i l s . A plot of the resultant f i e l d , with compensating coils i n operation at an optimum current i s shown i n Figure 8. r VevV;«i\ F \ e U ] <L«HI art- Figure 8. "1 3*. to (c) The spectrometer tube was placed symmetrically with respect to the f i e l d of the magnet. First the tube was aligned visually so that i t s axis and centre point coincided as nearly as possible with those of the magnet c o i l . Then as a f i n a l adjustment, sample counts were taken with a source in place and a constant current through the magnet, for different positions of the tube. The position of each end of the tube was changed (vertically or hori- zontally only) in turn, and the f i n a l position chosen was 16. that for which the counting rate was a maximum. The tube was then clamped i n this position. (d) The chosen value of compensator c o i l current should give good peak shape, which implies maximum peak height com- bined with minimum width and least distortion. As a f i n a l criterion for this current value, a strong photoelectron peak was located in the spectrum of the Radium source, and this peak was plotted using several different values of com- pensator current. A sample plot i s shown in Figure 9, with the jvarious compensator currents indicated thereon. I t is seen from this that l i t t l e doubt arises as to the required compensator current value. Such a current value is then used in the earth's f i e l d compensator coils for a l l subsequent work. Figure 9. 1 7 . 9 . RESOLUTION The resolving power of the instrument, whioh is defined as the peak width (expressed as a percentage) at half-maximum intensity, was found to be approximately 4 per- cent. 1 0 . CALIBRATION As was mentioned previously, the f i e l d of the magnet is linear with current, because of the absence of iron. Therefore only single-point calibration i s required. The instrument was calibrated with the very strong (conver- sion) F line of Thorium B (Hp = 1 3 8 j ? . 6 gauss-cm)^11K Using a very thin source in order to obtain as sharp a line as possible, and mounted on a thin sheet of mica to reduce back-scattering* the Thorium F line was plotted as shown in Figure 1 0 . The Thorium source arrangement i s also shown i n the same figure. The potentiometer reading which corres- ponds to the H f> value of 1 3 8 5 . 6 gauss-cms for the F line was found to be 0 . 2 2 8 volts. From this a l l the required H p values are found. C.D. E l l i s , Proc.Roy.Soc, 1 3 8 , 318 ( 1 9 3 2 ) , and K.C. Wang, Zeits.f.Phys., 8 7 , 633 ( 1 9 3 4 ) . 18. r W ISO \00 So Counts | M V "T"V>oriuwr> B F O . t 0 . 3 L 0.3 Potentiometer S e t t i n g Source f \ r range merit" B w > « m a s k Figure 10, 1 1 . CALCULATION OF GAM.iIA.-RAY ENERGIES Using the well known equation Hp - i 2 l | T(T + 1 . 0 2 ) where H^ represents the electron momentum in gauss-cm, and T the kinetic energy in Mev, the latter can be determined. For a photoelectron peak, hV (gamma-ray energy) = T + Efc where Efc i s the electron binding energy, and hence the energy of the gamma-ray can be found. For lead, the value of E D for the K shell i s 8 7 . 6 Kev^ 1 2), and for the L shell 1 5 . 8 Kev, their difference being 7 1 . 8 Kev. ( 1 2 JJ.M. Cork, l o o . c i t . p. 3 0 1 . 1 9 . III. EXPERIMENTAL RESULTS 1. REDUCTION OP PRIMARY BETA BACKGROUND An attempt was made to improve the sensitivity of the spectrometer i n the following way. The brass absorber over the source has one function only, and that i s to prevent the intense primary beta radiation from the souroe from ar- riving at the counter. This i t does, but a Compton back- ground is introduced in i t s place, though much less intense than the primary beta radiation i t replaces. Nevertheless this Compton background s t i l l imposes a limit upon the photo- electron line intensity that can be observed because of the unavoidable s t a t i s t i c a l fluctuations of intensity of both background and photoelectric peaks. Therefore an attempt was made to remove the primary beta radiation by replacing the brass absorber with a strong magnetic f i e l d , which could not of course give rise to Compton secondaries. The gamma-rays would be unaffected and this beam would then eject photoelectrons from the lead with l i t t l e or no background. The experimental arrangement is shown in Figure 11. 20* Figure 11. The d i f f i c u l t i e s proved to be as follows: (a) .With a primary beta energy of the order of 2.5 Mev, strength of fields available about 7000 gauss, and the geo- metry employed, minimum source-to-radiator distances of the order of 1.5 centimetres were required to divert the most energetic beta-rays from the spectrometer beam. (b) Such a source-to-radiator distance proved to be so great that with the source available (10 millicuries) the photoelectron peaks were too small to detect, even without any appreciable background. (c) It was necessary to have the deflecting magnetic f i e l d cut off sharply short of the lead radiator i n order to avoid interfering with the focussing properties of the spectrometer magnet. Various arrangements of source, f i e l d and radiator were tested. Because of the d i f f i c u l t i e s noted above, and 21. the limitations imposed by'the geometry of the source, which were unavoidable as this was the only source available, this method was not found to be feasible. Indications are, how- ever, that i t would be useful for a source of greater i n - tensity, and perhaps even with a source of the strength used but with a more suitable shape. As was noted before, the source used was not a point source but a cylinder 1 inch long and 1/8 inch thick, and this shape complicated the problem considerably. 2. THE RADIUM GAMMA-PvAY SPECTRUM A graph of the photoelectron peaks over the entire momentum range covered in this study is shown in Figure 12. The upper curve is the composite curve referred to earlier. The dotted line indicates the Compton background, and the lowest curve represents the difference between the other two. The horizontal scale is such that the momentum interval at any point is a constant percentage of the total momentum at that point. (Electron momentum i s linearly proportional to the Potentiometer voltage shown.) 3. STATISTICAL ACCURACY The average intensity per point (on peak outline) is approximately 640 counts per minute. For the average counting time of 12 minutes this gives a total count per point of about 7700. On the Compton background curve the average intensity per point is about 600 counts per minute, \000| 8001 Gramma - Roys i of Radium. 600 +5 c I 00© 4 0 O Cut-otff of o.osK •I* .n 3 P .3? .50 Potentiometer Voltage "~ F i g u r e .60 .7* . .86 \J> \ .9L \.S 23. which leads to a total count of 3&00, for the counting time of 6 minutes. The s t a t i s t i c a l accuracies of these two mea- surements are 1.1 and 1.7 percent respectively. The resul- tant s t a t i s t i c a l accuracy u of the points which give the peak outline is given by the formula where x and y are the errors in each of the two independent measurements. This leads to an average s t a t i s t i c a l accuracy of ± 2 percent. 4. ERROR IN ENERGY DETERMINATION The accuracy of the energy determination is of course an important factor. The error in potentiometer standardization is small enough to be neglected. The pro- bable maximum error in determining the "calibration point" is estimated to be less than 1 percent. Similarly the maxi- mum error in reading the highest point of a given photo- electron line i s estimated to be also less than 1 percent. These are considered to be the major sources of error. They lead to a probable maximum error i n calculated gamma-ray energy of ± 1.5 percent. An indication of the accuracy o f the experiment i s given by the binding energy difference which was found between the K and L conversion lines of the 0,598 Mev gamma-ray. This difference was found to be 73 Kev, a value which agrees reasonably well with the quoted value of 71.8 Kev, noted earlier. 24. 5. COMPARATIVE RESULTS Table 1 snows a comparison between the values found i n this study and those of earlier investigators. TABLE 1 E l l i s and Alichanov and Mann and Skinner Latyshev Ozeroff Gamma-ray Gamma-ray Relative Gamma-ray Relative Energy s Energy Intensity Energy Intensity . 0 4 7 2 . 0 5 3 6 . 0 5 8 9 .243 . 2 6 0 . 2 7 5 . 2 9 7 . 3 3 2 . 3 5 4 .429 . 4 7 1 . 5 0 3 . 6 1 2 . 7 7 3 .941 1 . 1 3 1.248 1*39 1.43 1 . 7 8 2.22 1.21 1.29 1.39 1.52 1 . 6 2 1.69 1.75 1.82 2.09 2.20 2.42 18 49 29 22 17 100 1 7 15 41 21 . 2 3 7 .289 .428 . 4 4 8 . . 4 7 8 .59® . 7 6 8 i : i 2 1.22 1.40 1 . 7 ? 2.1? (2.4) 0.6s* 11** 2 83BE 6 2.2 11 53 11 78 33 22 100 22 Relative intensities not given. Not corrected for photoelectric cross-section. 25. Relative intensities of the gamma-rays are included also. The intensities shown here have been corrected to take into account the decreasing cross-section for the photoelectric effect with increasing energy, using published^ 1^ cross- section curves. (^C.D. Coryell, M. Deutsch, R.D. Evans, W.J. Ozeroff et a l - The Science and Engineering of Nuclear Power, (Addison-Wesley) p. 40. 26. IV. CONCLUSION An examination of the graph in Figure 12 shows that in the lower energy portion of the spectrum the photoelectron peaks are very prominent, while in the high energy section they become very weak. This condition i s due in part to the fact that the photoelectric cross-section decreases very rapidly with increasing photon energy. For gamma-rays in lead, the absorption coefficient decreases from a value of 1.6 cm"1 at an energy of 0.4 Mev to 0.03 cm"1 at 2.5 Mev. This means that we must expect the photoelectron peaks to be- come weaker.and weaker as we pass to higher gamma energies. From the Compton background end-point at the upper limit we can find an approximate value for the energy of the gamma-ray which is responsible for the Compton background in that region, but which is apparently too weak to show as a photoelectron line. This value i s lis t e d i n brackets i n Table 1. It was calculated from the equation ' , . -.51 T (Mev) h V0(Mev) = T _ ^ T ( T + 1.o2) cos 0 which i s developed from the Compton Scattering Formula. h VQ represents the energy of the incident gamma-ray, 0 the angle between the direction of the incident gamma-ray and that of the recoil electron and T the maximum recoil electron 27. energy, in this case 2.14 Mev; In the experimental arrangement, because of the relatively large size of the source as compared to that of the radiator, 0 may have values from 0° to about 60° depending upon which portion of the source i s considered. For 0=0° we get h V 0 = 2.4 Mev (approx.). A different value of 0*, say 15°, leads to a higher gamma energy which in turn would give rise to a maximum recoil electron energy greater than 2.14 Mev. Since the-maximum re c o i l electron energy detected ' was 2.14 Mev, i t was concluded that the gamma-ray responsible for i t was that at 2.4 Mev. As noted previously, cut-off at the lower end of the spectrum occurs at 50 Kev, because of window thickness. Therefore the spectrometer i s not efficient in the detection of gamma-rays whose energies are below about 138 Kev (50 Kev plus the lead K-shell binding energy of 88 Kev). L-shell photoelectrons might s t i l l be ejected but the fact that the probability of their ejection is far less than that for the K-shell effectively rules out the possibility of detecting them. The comparative chart in Table 1 shows fourteen gamma-ray energies found in this study. One of these, that at 2.4 Mev i s quoted only approximately since i t i s calcu- lated from the Compton end-point. Of the fourteen, a l l but one correspond reasonably well to values found by earlier investigators. The remaining one, at 0.45 Mev is a very weak line, as may be seen from Figure 12, and occurs between 28. two relatively strong lines. Because of i t s low intensity, much time was spent in making observations on i t and raising i t s s t a t i s t i c a l accuracy to a figure comparable to that of the more intense lines. Should such a line actually exist, i t is certain that i t s intensity is near the limit of detec- tion of the spectrometer used. Many gamma-ray energies, reported by other workers were not observed here. This might be due to their low in - tensity or perhaps to the fact that they are highly converted and hence have l i t t l e intensity l e f t for photoelectron emis- sion. It may be noted that in the region of the spectrum above 1.1 Mev, according to the present study the picture i s similar to that given by E l l i s and Skinner. Of the several other energies given by Alichanov and Latyshev i n this region no trace could be found, in spite of the fact that they are quoted as being of relatively high intensities. 29. V. BIBLIOGRAPHY A.. Alichanov and G. Latyshev J. Backus J. M. Cork C.R.Acad.Sci. (U.R.S.S.), 20, 429 ( 1 9 3 8 ) . Phys.Rev., 68, 59 (194-5). "Radioactivity and Nuclear Physics", (Van Nostrand). C. D. Coryell, M. Deutsch, R. D. Evans, W. J. Ozeroff et a l "The Science and Engineering of Nuclear Power", (Addison-Wesley). I. Danysz M. Deutsch, L. E l l i o t t and R. D. Evans C. D. E l l i s C. D. E l l i s and H. W. B. Skinner A. L. Hughes and V. Rojansky H. Robinson and E. Rutherford R. A. Tricker S. T. Tsien K. C. Wang Le Radium, 9 , 1 ( 1 9 1 2 ) ; 1 0 , 4 ( 1 9 1 3 ) . Rev.Sci.Instr., 1 5 , 7 (1944). ProcRoy.Soc, 138, 318 (1932). ProcRoy.Soc, 1 0 5 A , 165 (1924). Phys.Rev., 3 4 , 284 ( 1 9 2 5 ) . Phil.Mag., 2 6 , 717 ( 1 9 1 3 ) . Proc.Camb.Phil.Soc, 2 2 , 454 (1924). Phys.Rev., 6 9 , 38 ( 1 9 4 6 ) . Zeits.fur Phys., 8 7 , 633 ( 1 9 3 4 ) .

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