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Performance investigation of a modified thin lens spectrometer Morgan, Frederick John 1960

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PERFORMANCE INVESTIGATION OF A MODIFIED THIN LENS SPECTROMETER by FREDERICK JOHN MORGAN B.Sc, University of British Columbia, 1958 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE in the Department of PHYSICS We accept this thesis as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA October, I960 In presenting t h i s t h e s i s i n p a r t i a l f u l f i l m e n t of the requirements f o r an advanced degree at the U n i v e r s i t y of B r i t i s h Columbia, I agree th a t the L i b r a r y s h a l l make i t f r e e l y a v a i l a b l e f o r reference and study. I f u r t h e r agree that permission f o r extensive copying of t h i s t h e s i s f o r s c h o l a r l y purposes may be granted by the Head of my Department or by h i s r e p r e s e n t a t i v e s . I t i s understood that copying or p u b l i c a t i o n of t h i s t h e s i s f o r f i n a n c i a l gain s h a l l not be allowed without my w r i t t e n permission. Department of P sir.c,  The U n i v e r s i t y of B r i t i s h Columbia, Vancouver 8 , Canada. ABSTRACT The performance of a modified thin lens spectrometer was investigated over the energy ranges provided by and Cs^"^ radioactive sources. The resultant spectra and Kurie plots provided evidence of a satisfactory performance. TABLE OF CONTENTS Page I THEORY OF BETA DECAY 1 II THE SPECTROMETER 10 III EXPERIMENTAL INVESTIGATION 13 IV CONCLUSION 17 REFERENCES TABLE OF ILLUSTRATIONS Figure Following Page 1. Low Energy Beta Decay of Cs 2 2. Decay Scheme of C s 1 3 7 2 3. Spectrometer 10 4. Current Regulator 12 5. Beta Spectrum of Pm 1 4 7 13 6. Kurie Plot of Pm 1 4 7 13 7. Beta Spectra of C s 1 5 7 15 8. Kurie Plot of 0.51 MeV Beta Group of C s 1 5 7 15 9 . Modified Kurie Plot of Cs ' Low Energy Group 15 10. Spectrum Calculated from Modified Kurie Plot 16 ACKNOWLEDGMENTS The work described in this thesis was supported by a Grant-in-Aid-of-Research allotted to Dr. K.C. Mann by the National Research Council. I am indebted to Dr. Mann for the advice and assistance given throughout the course of this work. I would also like to thank F.A. Payne and H.R. Schneider for their assistance. I am grateful to the British Columbia Telephone Company for the award of a scholarship. I THEORY OF BETA DECAY Introduction Radioactivity was discovered near the end of the nineteenth century by Becquerel. Early investigators grouped the observed radiations into three classes, based on increasing penetration powers. These classes were labelled as alpha, beta, and gamma rays. A study of the second type, beta rays, revealed that these rays were in fact particles, identical to electrons. Further detailed investigations of the beta rays emitted in the process of radioactive decay showed that three different decay processes existed, namely, negatron emission, positron emission (these two processes are the emission of negative and positive electrons respectively), and orbital electron capture (the capture of an orbital electron by the nucleus). Furthermore, mass studies on the parent and daughter atoms involved in the decay showed that the daughter nuclei had the smaller mass. Moreover, in every beta decay process, the atomic number Z changes by plus or minus one (the Displacement Law of beta decay), while the mass number A remains unchanged, according to the following rules: negatron emission (Z,A) — ( Z + - 1 , A ) +• e~ , positron emission (Z,A) —> (Z-1,A) +• © , electron capture (Z,A) + e~ —•* (Z-1,A). This gives the following mass requirements on the parent and daughter nuclei for the beta process to occur: negatron emission M(Z,A) greater than M(Z + 1,A), 2. positron emission M(Z,A) greater than M(Z-1,A)+- 2mo , electron capture M(Z,A) greater than M(Z-1,A) , where DIQ is the electron rest mass. Neutrino Hypothesis Careful examination of the beta decay process has shown that only one beta particle is emitted per disintegration and that this particle comes from the nucleus. From these facts, and from the law of conservation of charge, i t is assumed that the mechanism of beta decay is the transformation of one nucleon into another, that i s , a neutron transforms into a proton, or vice versa, and this transformation is accompanied by the emission of a beta particle. The momentum spectron of this emitted beta particle is a charac-teristic continuum as exemplified by the beta spectrum of Cs-'-'7 shown in Figure 1. This momentum continuum raised two conservation law problems. Firstly, the end point (or maximum energy) of the spectrum has been shown experimentally to correspond to the energy difference between parent and daughter nuclei, that i s , to the energy of the emitted beta particle5 yet the average energy per beta particle is only one-third of this maximum energy. This is ah apparent violation of the law of conservation of energy unless i t is postulated that nuclei of any one kind may have a continuum of masses in contradiction to the findings of mass spectrometry. Secondly, since the number of nucleons is the same for both the parent and daughter nuclei, the angular momentum of the two nuclei can only differ by an integral multiple of ti . This i s so because i t has been demonstrated experimentally that a l l odd A nuclei have odd half integral spins, while even A nuclei have integral spins. However, the beta particle, which obeys Fermi-Dirac statistics removes a half integral unit of angular momentum, in contradiction to the law of Momnentuim 137 FIGURE 1 LOW ENERGY BETA SPECTRUM OF Cs rat- e/ri t Nucleus (z , k) F\GrURE Z DECAiY SCHEME OF OS 3. conservation of angular momentum. Both these difficulties were resolved by the Pauli neutrino hypothesis which assumed that the emission of an uncharged, massless particle with a half integral spin value accompanied each beta decay. This particle was called a neutrino. The neutrino postulate successfully satisfied the conservation law requirements by supplying the missing energy and half integral spin, and so i t was generally accepted. Fermi Theory While the neutrino hypothesis satisfied the experimental facts, there s t i l l remained the question of a mathematical expression to account for the observed shape of the beta ray spectrum. Fermi, utilizing both the neutrino and nuclear transformation hypotheses, proposed a theory which yielded a mathematical expression for the spectral shape. He accomplished this in the following formal mathematical manner, analogous to the quantum mechanical treatment of electromagnetic radiation. Fermi accounted for the nucleon interaction by a beta-heutrino field (analogous to the electromagnetic field in gamma radiation), and this resulted in another term being added to the nuclear Hamiltonian expression. Since the added term had the effect of a small perturbation, he computed a probability per unit tine of decay into a unit momentum (or energy) interval on the basis of perturbation theory. Then, using simple, straightforward assumptions, Fermi evaluated the quantum mechanical operators in terms of measurable quantities. His resultant expression for the decay probability wass P(p)dp = G2 1KL2 F(Z,p) p 2 (E 0 - E) 2 dp , where G is a universal constant governing the strength of the nuclear inter-action, 1M1 is a matrix element whose square is proportional to the 4. probability of the transition, F(Z,p) represents the effect of the nuclear Coulomb field on the emitted beta particle, Z is the atomic number of the daughter nucleus, E is the beta particle energy, E 0 is the maximum energy of the transition, and p is the momentum of the beta particle. Thus, i f $here are N0 total disintegrations, their distribution will be given by N(p)dp = N0 G2 1KL2 F(Z,p)p2 (E 0 - E) 2 dp , where N(p) is the number of particles in the momentum interval from p to p+dp. Kurie, Richardson, and others noted that i f this expression is re-written as N(P) = N0 G2 1ML2(E0 - E) 2, then a graphical plot of the p2F(Z,p) square root of the left hand side against energy should give a straight line graph provided lKL 2 i s energy independent, and the energy-intercept should be the end point energy. The fact that these graphs, termed Kurie plots, actually give straight lines in the majority of cases substantiates the statistical assumptions in the Fermi theory of beta decay. Theoretical considerations show that a deviation from linearity near the end point energy in a Kurie plot would indicate a non-zero rest mass for the neutrino. That such is not so strengthens the assumption of a zero rest mass for the neutrino. A non-linearity at the low energy end of the plot can generally be traced to instrument distortion, usually scattering of the low energy beta particles. Matrix Element and Interaction Forms The expression for the spectral shape was the probability per unit time of a decay into a certain momentum interval. Thus, integration over the entire momentum range should give the probability of a decay occurring, and l this should be identical to the decay constant >- -~ where X is the mean 5. l i f e . Hence •\= X = J G 2 lKL 2 F(Z,p)p 2 (E 0 - E ) 2 dp , where p Q i s the end point momentum. It i s usual to express both the momentum and energy as dimensionless quantities by the following relations: for the momentum \ = —P.— n^c » 2 for the energy W = E * , mo c^ and W r 7 + 1 i s the new relation between energy and momentum. In terms of these new quantities, the expression for the decay constant becomes where A ~ m0->c . If ML. i s independent of energy, and i f the momentum i s expressed i n terms of the energy, then X s = G 2 M L 2 f(Z,W 0), where f(Z,W0) = J F(Z,W)(W2 - 1)(W0 - W)^ WdW. This expression for the decay constant can be rewritten as f i- •- x , c ^ i m 2 from which i t i s clear that the quantity f X depends on the matrix element 1M1. This nuclear matrix element 1M1 can be expanded into an i n f i n i t e series of terms whose magnitudes successively decrease by a factor of approximately one hundred for beta particles whose energy i s about one MeV. Hence the magnitude of 1M1 i s essentially determined by the f i r s t non-zero term of the expansion. Whether or not a given term vanishes i s determined by the angular momentum change and by the parity change of the nucleus undergoing the transition. Since 1KL, as mentioned above, represents the relative probability of decay, the f t value must indicate the inverse probability of decay, that i s , the larger this quantity i s , the less l i k e l y Is a transition. 6. A particular transition i s labelled according to the f i r s t non-vanishing terra of the matrix expansion; as an "allowed" transition i f the f i r s t term i s non-zero, as a " f i r s t forbidden" transition i f the f i r s t term vanishes but the second does not, as a "second forbidden" transition i f the f i r s t two terms vanish but the third does not, and so forth. The precise angular momentum and parity changes which cause the terms of the matrix element to vanish depend completely oh the mathematical form assumed for the nuclear interaction. There are five possible forms for this interaction: scalar, vector, tensor, axial vector, and pseudoscalar. It turns out that there are two different sets of angular momentum changes and parity changes (which are called selection rules) that are of practical interest. These are the Fermi and the Gamow-Teller (called G-T) selection rules. The Fermi selection rules are characteristic of a scalar or vector form of interaction, while the G-T rules (which explain most, but not a l l , known decays) represent a tensor or axial-vector type of interaction. In practise, the f t values are rather large and unwieldy, so i t i s usual to express the comparative half l i f e , as this quantity is generally called, as a common logarithm. On this basis an allowed transition has a log fX value i n the range three to six, a f i r s t forbidden six to ten, and a second forbidden greater than ten. These ranges are only crudely defined. Forbidden Shape Correction Factors The Kurie plot, which was mentioned i n the section on Fermi Theory, i s affected by the degree of forbiddenness of the beta transition which i n turn depends on the type of interaction assumed, and hence on the selection rules used. The dependence of the plot shape on the type of transition i s affected by the evaluation of the nuclear matrix element. For an allowed transition, the dominant terms of the matrix element expansion are independent of energy 7. and so they are also independent of the interaction form. In a forbidden transition, however, the parity and total angular momentum changes cause energy dependent terms of the matrix expansion to become important, and and Uhlenbeck considered the effect of the different interaction forms on the shape of the beta ray spectrum, and they have expressed their results as shape correction factors. The allowed shape formula must be multiplied by these factors in order to obtain the correct expression for the shape of the forbidden spectrum, and hence to obtain a straight line Kurie plot. In general, the mathematical form of the correction factors is complicated and the complexity increases with the degree of forbiddenness. However, certain special cases deal with parity and total angular momentum changes which give particularly simple correction factors, for example, those transitions whose degree of forbiddenness is one less than the total angular momentum change. The evaluation of the correction factor requires a knowledge of the end point energy. The correction factor depends on the particular radioactive decay under consideration, as well as on the type of transition. Examples of correction factors arej Parent Nucleus Type of Transition Correction Factor hence the interaction form assumed also becomes important. Konopinski Pm" ,147 fir s t forbidden Cs15'(low energy) f i r s t forbidden C s ^ h i g h energy) second forbidden (W2-1) + (W0-W)2 5 0.03(W2-1) 1- (W0-W) 2 6 Internal Conversion An additional phenomenon which sometimes occurs along with beta decay is internal conversion. This causes a line, or peak, to appear in the beta spectrum, as in Figure 1 depicting the C s ^ low energy spectrum. This 8. phenomenon, and it s companion process, gamma ray emission, occur when the product nucleus of a beta decay i s not in its ground state, but in an excited state. It reaches the ground state by the emission of a gamma ray, or by the ejection of an orbital electron through a direct interaction of the nucleus with the electron. In this case, the orbital electron absorbs the excitation directly i n a sufficient amount to overcome its shell binding energy. A schematic diagram of this process i s shown in Figure 2. The energy of the emitted electron i s the difference between the energy states involved, less the electron binding energy. Thus the electrons are mono-energetic and so have a line spectrum. Because of its proximity to the nucleus an electron in the K-shell is more likely to be ejected than one from a different shell, provided that the excitation energy is greater than the K-shell binding energy; although, for large angular momentum changes, this may not hold. X-ray radiation characteristic of the particular shell involved accompanies internal conversion. As a measure of the relative occurrence rate of internal conversion electrons and gamma rays, a conversion coeffi-cient Is defined as: the ratio of the number of electrons emitted to the number of gamma rays emitted. Its value tends to Increase rapidly as Z increases, but to decrease as the transition energy increases. Generally, the greater the spin change, the larger is the conversion coefficient. Radiative Transitions The companion process which competes with internal electron conversion i s gamma ray emission. The mathematical description of the radiative transition (gamma ray) probability is the same as for a classical oscillating electric or magnetic moment. Thus the resultant radiation can be classified as either an electric or magnetic multipole. The former occurs through periodic variations in the nuclear charge density, while the latter arises 9. from periodic fluctuations in the nuclear current density. The actual multipole value of the transition is determined by the angular momentum change between the nuclear energy levels involved. Then, since the electric and magnetic multipoles have opposite parity for a given multipole order, both the parity and the multipole order determine the type of radiation emitted. Moreover, as the multipole order increases, the emission probability, and hence the intensity of the radiation, decreases. Thus the lowest order multipole radiation, which is consistent with the parity and angular momentum changes, will usually dominate, although higher orders may also be present. However, for a given multipole order, the electric multipole is much more intense than the magnetic multipole of the same orderj so an electric multipole of an order one greater than a magnetic multipole may have an intensity comparable to the lower order magnetic multipole. Hence both may be present in sufficient intensities to be observed in a given transition. Intensity considerations eliminate the possibility of a magnetic multipole competing with an electric multipole of one order lower. 10. II THE SPECTROMETER In the experimental investigation of beta decay, the most useful instrument has been the magnetic focussing spectrometer. This is a momentum analyzing device which records the momentum distribution of the emitted beta particles. The particular type of magnetic spectrometer used in this laboratory features a narrow, inhomogerieous magnetic field region. Its effect on the trajectory of an electron is roughly similar to the effect on the path of a light ray produced by a thin lens In geometric optics. Hence i t is called a thin lens spectrometer. Although this type of spectrometer suffers from a large inherent spherical aberration, it s economy of construction, its low power consumption, and its location of source and detector in field free regions more than compensate for this defect. The result of the spherical aberration i s to shift the optimum focus from an axial point to an annular ring in front of this axial point. In Figure 3 the main parts of the spectrometer are outlined. A complete description is contained in an unpublished thesis by E.D. Earle. The main parts are: a cylindrical brass tube, a source, a detector, two sets of baffles, arid a magnet. The detector consists of a ring of anthracene crystals mounted in an annular groove In the face of a lucite light cone. The cone i s optically coupled to a five inch photomultiplier tube by means of Silicone gum. There are two sets of baffles used to improve the definition of the electron focus; one at the source end to fix the emission angle and limit the beam width of the electrons, and the other at the detector end to govern the width of the ring focus. The magnet consists of three concentric copper-wire wound coils. A fourth coil is F \ GUR.E 3 SPECTROMETER. 11. inoperative at the present time. The radioactive source, deposited on a thin film, is mounted at one end. In addition to the spectrometer proper, there are several pieces of associated equipment required for its operation. A D.C. High Voltage supply is needed to provide the correct dynode stage potentials in the photo-multiplier tube. The spectrometer tube vacuum is maintained by a fore pump and an o i l diffusion pump. The magnet coils and diffusion pump are provided with water cooled copper piping. The detector feeds the photo-multiplier signal Into a variable gain amplifier by means of a cathode follower preamplifier. A discriminator in the amplifier selects the noise level and then a scalar-counter records the signal. In addition to these, there are also: centering mechanisms to align the geometric axis of the spectrometer tube with the magnetic axis of the magnet coils, and a com-pensating coil above and below the spectrometer tube to counteract the effect of the vertical component of the earth's magnetic field. The effect of the horizontal component is nullified by aligning the tube axis parallel to this component. The magnet current is controlled in the following manner. The current causes a potential drop across a standard resistor which is placed In series with the magnet. A Brown converter is used to compare the D.C. level of this voltage with the desired voltage from an accurate potentiometer. Any difference is then amplified by three stages whose overall gain exceeds f i f t y thousand. The amplified difference signal then passes through a phase sensitive detector and a rectifier. The rectified signal Is fed onto the grid of a beam power amplifier tube whose load voltage supplies part of the grid bias to the current supply tubes which consists of a bank of thirty eight 6AS7 tubes. The remainder of the grid bias is supplied by a manually 12. adjusted potential divider. An A.C. voltage fluctuation across the standard resistor is fed directly into a two stage amplifier whose overall gain exceeds ten thousand. The amplified fluctuation is fed onto the grid of the aforementioned beam power amplifier through a capacitor. This system controls the magnet current to better than one part in ten thousand. The schematic outline of the circuit is shown in Figure 4. The instrument used in this laboratory had a resolution of 0.95/6 at a transmission of Q.24$. A discussion of the thin lens spectrometer in general has been given Q by Deutsch, Elliot, and Evans ; while a more complete discussion of the .... Q instrument used in this laboratory has been given by F.A. Payne , and by E.D. Earle. F \ G U R E 4 C U R R E N T R E G U L A T O R 13 III EXPERIMENTAL INVESTIGATION 147 Pm ' Beta Spectrum In order to check the performance of the instrument at relatively low energies, that i s , energies less than 300 KeV, the beta spectrum of -i AH 147 Pm 1 was measured. The source had been prepared by evaporating Pm ' onto a thin aluminum f o i l backing with an average thickness of 250 micrograms per square centimeter. From the data obtained with this source a momentum plot of the beta spectrum as shown in Figure 5 was made. Although the beta decay of Pm^47 is a fir s t forbidden transition 2' previous work has shown that the spectrum has an allowed shape, that i s , a momentum plot will be symmetric, and so the Kurie plot will be a straight line. Thus the instrument's performance can be determined by the degree of linearity of this Kurie plot, and also by comparison of the observed end point energy with the accepted value of 227 KeV. The Kurie plot is shown in Figure 6. This plot deviates from linearity below an energy of approximately 100 KeV. There are two possible instrument causes for low energy non linearity. These are the high noise background at which i t i s necessary to operate, and poor compensation for the earth's magnetic field which becomes more important at low energies. The former would cause a statistical scattering of the low energy experimental points, while the latter would result in defocussing of the low energy electrons and hence experimental points that were too low. Neither occurs. The Kurie plot curves upward, not downward, and the points show no statistical scattering. This indicates the non linearity is caused by scattering from the aluminum f o i l source backing. Previous work in this laboratory and elsewhere has i.eo FIGURE 5 B E T A S P E C T R U M OF PTTV 200O 14. indicated such a scattering effect can be expected. Extrapolation of the linear portion of the Kurie plot yielded an end point energy of 229 KeV, in good agreement with the results of other workers. Cs^7 kow Energy Beta Spectrum A further check on the instrument's performance was provided by a study 1 V T 5,10,11 m . , of the low energy beta spectrum of Cs ±:>< . Thxs decay covers a greater energy range than that of The method whereby the performance of the instrument was investigated can best be understood by reference to Figure 2. The decay process from the radioactive Cs1-^ to the Ba"^ ground state can occur in two ways. The parent Cs1-^ nucleus can decay directly to the Ba1-^ ground state by emission of a beta particle group of end point energy 1.17 MsVj or the parent nucleus may fir s t decay to an isomeric state of the daughter nucleus by emission of a 0.51 KeV beta particle group, followed by a transition to the ground state either through emission of a conversion electron or of a gamma ray with an energy of 0.66 MeV. The high energy beta transition is known to be second forbidden, whereas the low 5 6 energy decay i s f i r s t forbidden. * A l l nuclei which decay by the second method must eventually reach the ground state either by the emission of a conversion electron or by emission of a gamma ray. A measurement of the total number of beta decays, and a measurement of the total number of conversion electrons will enable the number of gamma rays tb be found by a simple subtraction. The internal conversion coefficient can then be 137 determined. For a l l these measurements a Cs source mounted on an aluminum f o i l backing of thickness 1.25 milligrams per square centimeter was used. Since C s 1 ^ emits both high and low energy beta groups, i t i s 15. necessary to correct the experimental points for the contribution from the higher energy continuum in the overlap region of these two spectra. Unfortunately, the spectrometer magnet had one of its four coils inoperative because of an electrical short to the magnet case. The resultant weaker field made i t impossible to measure energies beyond 900 KeV with the current regulator used. Thus, a complete high energy spectrum could not be obtained for correction purposes. Rather, from a few experimental points beyond the low energy end point, and from correction factors used by Langer 6 and Moffat for the second forbidden transition, a Kurie plot matched to the available data was drawn. From this plot, a theoretical spectrum for the high energy decay was constructed, and then used as a correction spectrum. The results are shown in Figure 7. From the difference spectrum obtained by subtraction, a Kurie plot of the low energy spectrum was drawn, as in Figure 8, which indicated a forbidden transition. Other workers have identified i t as a first forbidden transition, and have calculated the required correction factor. This factor has the form C = (W0-W)2 + A(W2-1)5, where V is the dimensionless energy given by E(in MeV) + 1, WQ is the 0.51 dimensionless end point energy, and A is a complicated function of the fine structure constant , and the atomic number, Z, of the daughter nucleus. For higher energies A is almost unity, and'so C is easily calculated. At lower energies C was obtained from a graph appearing in a review article by 5 J.So Osaba . Figure 9 shows the modified Kurie plot drawn with the use of this correction factor. The upward curvature of the plot below about 250 KeV can be attributed to scattering by the thick aluminum f o i l used in the source backing. The deviation from linearity near the end point energy is caused by the poor statistics in this region, and by error in the correction spectrum. The extrapolated end point energy of 515 KeV is close to the FIGURE 9 MODIFIED KURIE PLOT 16. accepted value of 518 KeV obtained by others. This modified Kurie plot was used to draw a momentum spectrum of the low energy decay. Then the total number of beta decays occurring in one minute was calculated by measuring the area under this momentum continuum. From this calculation, and from the previously measured rate of internal conversion, the value of the internal conversion coefficient was determined. The experimental value for the K conversion coefficient was found to be 0.091, in close agreement with the recent value given by 12 S. Wapstra , 0.092. Similarly the overall conversion coefficient was found to be 0.109, also in close agreement with the value 0.110 found by S. Wapstra12. It is of interest to calculate the log(fT) value of the lower energy beta group of Cs 1^. This may be done in the following way, using the method 13 proposed by S.A. Moszkowski . He has divided the calculation of the log(fT) value into three parts which are then summed to give the final value. These parts are: log(f 0T) which ignores the Coulomb effect, log(C) which corrects for the Coulomb field, and A log(f X) which covers any branching of the decay process. The actual calculations are done by means of graphs and nomographs contained in the review article. The calculations require a knowledge of the branching ratio, of the end point energy, of the atomic 137 number of the parent nucleus, and of the half l i f e . For the Cs low energy beta group these are respectively: 0.95, 0.51 MeV, 55, and 34.7 years. From these, a value of 9.5 was obtained for the log ( f t ) value of the low 137 energy beta group of Cs . This is a value to be expected for a second, or possibly a f i r s t , forbidden transition. FIGURE 1 0 S P E C T R U M C A L C U L A T E D F R O M 4 0 0 0 y\a { <jaoss - c m ) • CONCLUSION 147 On the basis of the experimental investigation of the spectra of Pm and Cs^"'7, i t can be concluded that the spectrometer performance is satisfactory over the energy range considered. There is no evidence of any major low energy defocussing caused by incomplete compensation of the earth's magnetic field. Some defocussing may, of course, occur but i t i s not detectable because of source backing scattering below 100 KeV. Similarly, there is no appreciable defocussing of the higher energy electrons caused by a residual magnetic field affecting the photomultiplier tube. The lower energy performance could be improved in the following three ways. Firstly, use of a thinner source backing will reduce the back scatter-ing observed in both sources. Secondly, a more permanent coupling of the anthracene crystals to the light cone, and of the light cone to the photomultiplier tube will increase the low energy signal pulse by reducing the photon reflection losses at the interfaces between the crystals and the lucite. The present system uses Silicone gum, rather than a permanent -cement. Thirdly, a better selection of the photomultiplier tube will improve the operating characteristics. Investigation of the five inch Dumont 6364 photomultiplier carried out in the laboratory shows a significant difference among individual tubes in their signal-to-noise ratio at a given interstage voltage. The overall performance of the spectrometer could be improved by use of a more reliable centering mechanism. . The present system of centering suffers from a lack of stability and a lack of reproducibility. This imposes an unnecessary limitation on the accuracy with which the magnetic and geometric axes of the spectrometer can be aligned, arid hence imposes a 18. limitation on the resolution and transmission which can be obtained. Modifications which will counteract this are presently under consideration. REFERENCES 1. E.J. Konopinski and G.E. Uhlenbecks Phys. Rev. 60, 308 (1941) 2. Lo Lidofsky, P. Macklin, and S. Vfut Phys. Rev. 76, 1888 (1949) 3. H.M. Agnew: Phys. Rev. 77, 655 (1950) 4. L.M. Langer, J.W. Motz, and H.C. Price: Phys. Rev„ 77, 798 (1950) 5o J.S„ Osaba: Phys. Rev. 76, 345 (1949) 6. L.M. Langer and R.J.D. Moffat: Phys. Rev. 82, 635 (1951) 7. E.D. Earle: Master Thesis, University of British Columbia (I960) 8. M. Deutsch, L.G. E l l i o t , and R.D. Evans: R.S.I. 15,..178 (1944) 9. F.A. Payne: Master Thesis, University of British Columbia (1957) 10. C.L. Peacock and A.G.C Mitchell: Phys. Rev. 76, 1272 (1949) 11. L.M. Langer and H.C. Price: Phys. Rev. 76, 641 (1949) 12. S. Wapstra: Arkiv for Physik 7, 239 (1954) 13. S„A. Moszkowski: Phys. Rev. 82, 35 (1951) 


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