An empirical method for determining the relative efficiency of a Ge(Li) gamma-ray detector

NUCLEAR

INSTRUMENTS

AND METHODS

56 (1967) I89-I96;

,© N O R T H - H O L L A N D

PUBLISHING

CO.

AN EMPIRICAL M E T H O D FOR DETERMINING THE RELATIVE EFFICIENCY OF A Ge(Li) GAMMA-RAY DETECTOR* W. R. K A N E and M. A. M A R I S C O T T I

Brookhaven National Laboratory, Upton, New York, U.S.A. Received 29 June 1967

An empirical method for determining the full-energy peak efficiency between 0.2 and 3.0 MeV and the two-escape peak efficiency between 2 and 9 MeV of a Ge(Li) gamma-ray detector is described. The full-energy peak efficiency is determined with the use of several nuclides which emit gamma rays with accurately

known intensity ratios and a linear least-squares fitting procedure. With the full-energy peak efficiency determined, a number of high and low energy thermal neutron capture gamma ray cascades with known relative intensities are then utilized in the determination of the two-escape peak efficiency.

1. Introduction While the energies of gamma rays have been measured with Ge(Li) detectors with high precision for some time 1-3) the situation with regard to intensity measurements made with these devices is far less satisfactory. This is particularly true for the high energy gamma rays emitted in nuclear reactions, where the two-escape peak, which arises from the production of an electron-positron pair in the detector with the subsequent emission of two annihilation quanta, is used for intensity measurements. Consequently, many measurements made over the past several years have scarcely more than qualitative significance. Measurements of the full-energy and two-escape peak efficiences of a Ge(Li) detector have been made by Ewan and Tavendale+). They determined the fullenergy peak efficiency with the use of standardized sources emitting gamma rays of various energies, and the two-escape peak efficiency by two independent methods: 1. By comparing the efficiency of the Ge(Li) detector for high energy gamma rays with that of a 3" x 3" NaI detector; 2. By utilizing thermal neutron capture gamma rays whose intensities had been measured previously with the aid of a magnetic spectrometer. For the first method the stated errors are in the range 15-30%; for the second method the calibration error of the original instrument was estimated to range from ,-, 10°/0 at 2.75 MeV to ~ 30°/0 at 10 MeV. Several Monte Carlo calculations of both full-energy and two-escape peak efficiencies have been made s-v). The calculated full-energy peak efficiencies agree well with the measurements of Ewan and Tavendale. For two of the three calculations, however, the calculated two-escape peak efficiencies deviate considerably from the measured values, being in rough agreement at low

energies and approaching approximately twice the experimental value at 8 to 10 MeV s' 6). The calculations are approximate, both in the sense that there exist uncertainties in certain of the input parameters, for example, in the energy and angular distributions of some of the secondary radiations produced in the detector, and in the sense that approximations have been made to some of the input parameters even where these are well established, and certain effects in the detector neglected. On the other hand, a calculation of the two-escape peak efficiency of a 30 cm 3 coaxial detector by Orphan and Rasmussen 7) is in reasonable agreement with an experimental determination of the efficiency. In this case the agreement appears to result largely from a sizable correction in the calculation for bremsstrahlung escape, reaching, for example, ,-~ 50% at 8 MeV. However, as stated earlier, the uncertainties in the intensities of the thermal neutron capture gamma rays customarily used for determining detector efficiencies range from 10-30%, depending on energy. Even if completely reliable efficiency calculations were available, a straightforward empirical method for determining detector efficiencies would still be useful. Simple geometrical shapes and standard dimensions for detectors are ordinarily achieved only at a considerable sacrifice in detector volume, and most detectors currently in use do not have the regular shapes for which the efficiency calculations have been made. Furthermore, the determination of the true sensitive volume of a detector is not a completely straightforward problem. In view of these considerations, a widely applicable method for the determination of the relative efficiency of a Ge(Li) detector between 0.2 and 9 MeV has been developed. * Work performed under the auspices of the U.S. Atomic Energy Commission.

189

190

W. R. KANE A N D M. A. MARISCOTTI

2. Full-energy peak efficiency Instead of the widely used method of employing a number of standardized gamma ray emitting isotopes for the determination of the full-energy peak efficiency4), isotopes which emit gamma rays with accurately known relative intensities were employed. This method has the advantage of convenience as most of the isotopes used are readily obtainable and the accuracy with which the relative gamma ray intensities are known is equal to or better than the accuracy to which the disintegration rates of standardized sources are usually determined (usually 1 or 2%) in a number of cases. This method has been utilized previously to determine the efficiency of a beta-ray spectrometer for photoelectric conversion of

(o)

(b) 1775

gamma raysS), and by Freeman and Jenkin9), for the determination of the efficiency of a Ge(Li) detector between 0.5 and 1.5 MeV. In the present work, the isotopes lS°Hfm, l°SAgm, 22Na, 24Na and 228Th were employed, spanning an energy region from 0.22 to 2.75 MeV. Their gamma-ray energies and the relative intensity values adopted are listed in table 1', and relevant features of their level schemes are shown in fig. 1. Pertinent considerations are as follows: 1.18°Hfm. The first four excited states of 18°Hf constitute a well known example of a ground state rotational band, with 2 +, 4 ~, 6 ÷, and 8 + spin and parity1°-13). Cross-over transitions in competition with the 93, 215, 332, and 443 keV E2 transitions in this

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DETERMINING

THE R E L A T I V E E F F I C I E N C Y

TABLE 1 Energies and relative intensities of g a m m a rays utilized in the determination of the detector full-energy peak efficiency. Isotope

Ey (keV)

Iv (rel.)

Error

18°Hfm

215 332 443 432 618 727 511 1274 1368 2574 583 2614

1.00 1.16 1.02 1.000 1.006 1.007 1.000 0.563 1.000 0.999 1.000 1.174

0.01 0.015 < 0.002 < 0.002 0.003 < 0.002 0.01

l°8Agm

22Na 24Na 228Th

band are not expected to appear with appreciable intensity. In fact, none have been observed. The 8isomeric state, however, is deexcited by two competing modes, the 57 keV E1 transition to the 8 + state, and the 501 keV E 3 + ( M 2 ) transition to the 6 + state. Very low upper limits have been set on the intensity of a transition from the 8 - state to the 4 + state TM 13). Thus the transition intensities obey the relationship 193 = 1215 = 1332 = 1 4 4 3 + 1 5 0 1 = 1 5 7 + 1 5 0 1 .

In the present work only the 215, 332 and 443 keV transitions are of interest, as the 501 keV transition is weak, and the 57 and 93 keV transitions lie below the energy region considered. For the g a m m a rays corresponding to the 215, 332 and 443 keV transitions to be used as intensity standards, the relative branching of the 443 and 501 keV transitions and the internal conversion coefficients of all four transitions must be known. The value 14.8 ___0.8% was adopted '3) for the branching of the 501 keV transition. Since this transition is weak, it is seen that a measurement of its intensity to __+5% gives the intensity of the 443 keV transition to 1°/0. The internal conversion coefficients of the four transitions are comparatively small. (The theoretical value for ~k, for example, is 0.131, 0.040, 0.020 and 0.038 respectively for the 215, 332, 443 and 501 keV transitions.) Since, on the basis of a large body of experimental evidence, theoretical and experimental E2 internal conversion coefficients seldom disagree by more than 5°/0, the g a m m a ray intensities were calculated with the use of theoretical internal conversion coefficients. A 5% error in these would give rise to an error of ~ 1°/0 in the intensity of the 215 keV g a m m a ray and less than 0.5% in the intensities of the remain-

191

ing three g a m m a rays. These relative intensities of the 215, 332, and 443 keV g a m m a rays were computed to be 1.00, 1.16, and 1.02 respectively. 2. '°8Agm. The decay of the long-lived isomer of '°8Ag proceeds with a branching of 91.5% to a state in '°8pd at 1775 keV with probable spin and parity 6 +. This state, in turn, is deexcited by a triple cascade of E2 transitions with energies of 727, 618 and 432 keV to the 0 + ground state'4). No crossover transitions are expected or observed, so that the three transitions from the 1775 keV state should have equal intensities. In addition, however, the isomeric transition of longlived '°8Agm to the 2.4 min ground state of '°8Ag, with a branching of 8.5%, is followed by a 617 keV transition and the same 432 keV transition as above. These, however, are quite weak ( ~ 0.26% and ~ 0.45%, respectively, of all decays of 2.4 min '°8Ag), so that their contribution to the 727-618~32 keV cascade is less than 1:1000. If the 432, 618 and 727 keV transitions are taken to have equal intensities, and internal conversion is taken into account, with the use of theoretical internal conversion coefficients, then the relative g a m m a ray intensities are 1.000, 1.006 and 1.007 respectively, with probable uncertainties less than 0.2%. 3.22Na. Except for an extremely weak positron decay to the ground state of 22Ne (0.06%), the decay of 22Na proceeds entirely to the 1274 keV state of 22Ne. The electron capture to positron branching ratio has been measured with high accuracy in a number of experiments. The weighted average of these results given in the Nuclear Data Sheets'5), e/fl + =0.113 + 0.004 is adopted here, giving a positron branching ratio of 0.898. Corrections of 0.27% for three quantum annihilation and 0.99% for annihilation in flight, for positrons stopping in copper, were made to the yield of 511 keV annihilation quanta. The calculated relative intensities of the 511 and 1274 keV gamma rays are then 1.000 and 0.563 ___0.003. 4. 24Na. The decay of 24Na proceeds almost entirely to a 4122 keV 4 + state in 24Mg which is deexcited by successive 2754 and 1368 keV transitions. The 1368 keV state is also populated by a very weak (0.003%) direct transition from 24Na, and indirectly, by a transition from a 5.22 MeV state of 24Mg weakly populated in the decay of 24Na (0.04%). The intensities of the 2754 and 1368 keV transitions are thus equal within 1 : 1000. With account taken of internal oair formation - the coefficients are 0.6 x 10 -4 and 7.1 × 10 -4, respectively, for the 1368 and 2754 keV transitions '6) - the relative intensities of the 1368 and 2754 keV g a m m a rays are 1.000 and 0.999 respectively. The uncertainty in the intensity ratio should be less than 0.2%.

192

W. R. K A N E A N D M. A. M A R I S C O T T I

5.22STh. We are concerned with the 583 and 2614 keV g a m m a rays o f / ° 8 p b , the end product of the disintegration of 22STh. 2°Spb has two parent nuclei, 212po and 2°8T1. The decay of 2~Zpo is entirely to the ground state of 2°spb, so that it is of no consequence here. The decay of 2°STl populates several excited states of 2°spb. The direct population of the 2614 keV state, however, is negligibly weak ( ~ 0.03%)17). Except for this weak branch, the 2614 keV state is populated entirely by two electromagnetic transitions: 583 keV (M1), and 860 keV (E2). The 860 keV transition is comparatively weak. Its gamma-ray intensity has been determined by photoelectric conversion in a beta ray spectrometer to be 11.4 4- 1.2% of the intensity of the 2614 keV g a m m a rayS). A new value of 12.8 4-0.6% for the intensity of this g a m m a ray has been determined in the present work. With the adoption of this value and the use of theoretical internal conversion coefficients, the relative intensities of the 583 and 2614 keV g a m m a rays are 1.000 and 1.174 ___0.01 respectively. The seven intensity ratios listed in table 1 were utilized in determining the relative efficiency of a coaxial Ge(Li) detector with ~ 9 cm 3 active volumelS). The line breadth (fwhm) of the detector ranged from 3.6 keV at 370 keV to 7.2 keV at 6.5 MeV. The areas of peaks were determined with the use of a computer program which performed a least-squares fit of a Gaussian function to the peaks~9). Two or more runs were made for each isotope. In each spectrum the area of the weakest full-energy peak utilized was at least 1 0 4 counts. For each isotope the ratios of full-energy peak areas were reproducible to better than 2%. While a curve of detector efficiency vs gamma-ray energy can readily be constructed from the empirically determined ratios of efficiencies at different energies, it is preferable to express the efficiency as a function of gamma-ray energy and carry out a least-squares fitting procedure so that an unbiased estimate of the errors in the efficiency determination can be obtained. For this purpose use was made of the well-known fact that the full-energy peak efficiency of a Ge(Li) detector very nearly obeys a power law in its dependence on gammaray energy over the energy region of interest in the present work, from 0.2 to 3.0 MeV. For the 9 cm 3 coaxial detector, the efficiency e very nearly obeyed the relation

where x = ln(E.,.o/g7).

Casting the energy dependence of the efficiency in this form has the advantage that the problem of determining the efficiency function from intensity ratios reduces to a straightforward linear least-squares fitting procedure. thus, if R m , is the empirical ratio of efficiencies at two different energies, then In

Rmn = 111e,,,- In e,

=

2 2 b(x,.-x.)+c(x,,,-x.).

In the present instance there is a set of seven such linear equations giving the two coefficients, b and c, of the efficiency function. The values obtained for the 9 cm 3 coaxial detector from the least-squares fitting procedure are b = 1.454 4- 0.026, c = 0.064 4- 0.025. Fig. 2 shows the fitted efficiency function. The seven experimental ratios have been indicated by adopting the calculated value of the detector efficiency for the lowest energy g a m m a ray of each isotope and then

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=

bx-Fcx 2,

Fig. 2. C u r v e o f the full-energy peak efficiency of the 9 c m a coaxial detector as a function o f energy. T h e seven experimental efficiency ratios have been indicated by a d o p t i n g the calculated value of the detector efficiency for the lowest energy g a m m a ray o f each isotope a n d then utilizing the experimental ratios o f peak areas to calculate efficiencles for the higher energy g a m m a rays.

D E T E R M I N I N G THE RELATIVE E F F I C I E N C Y

utilizing the experimental ratios of peak areas to calculate efficiencies for the higher energy g a m m a rays.

3. Two-escape peak ettieieney The method employed above, utilizing gamma-ray cascades with known intensity ratios, has been extended to the high energy region. For this purpose the thermal neutron capture g a m m a rays of several medium-weight elements were used. The capture g a m m a rays of many nuclei in the region A ~ 40 to A ~ 80 are characterized by rather strong primary capture g a m m a rays to lowlying levels of the product nucleus (typically with intensities of 5 to 50 per hundred neutrons captured). The level spacing in the even-even or odd-A product nuclei is usually large up to excitation energies of several MeV and the decay schemes are usually comparatively simple. Moreover, the positions of energy levels have usually been well established in d,p reaction studies. Thus most transitions with appreciable intensities are placed in the level scheme and intensity relationships are well established. In each case two intense g a m m a rays were chosen with energies such that the efficiency of the detector for the lower energy g a m m a ray could be determined from the already established full-energy peak efficiency curve. A comparison of relative peak areas then gave the detector efficiency for the higher energy g a m m a ray. Capture gamma-ray cascades in the product nuclei 49Ti, 53Cr and 54Cr were utilized to determine the efficiency of the 9 cm 3 coaxial detector at a number of points. The cascades utilized are listed in table 2. Pertinent features of the decay schemes are as follows: 1.49Ti. The capture g a m m a rays of titanium have been studied in detail by a number of workers 2°-24) in high resolution gamma-ray measurements with crystal diffraction and magnetic Compton recoil and pair spectrometers, and in coincidence and angular TABLE 2 Thermal neutron capture gamma ray cascades utilized in the determination of the detector two-escape peak efficiency. Product nucleus

Primary transition

4UTi 4UTi 5aCr a4Cr S4Cr 54Cr a4Cr

4876 6413 5610 5999 6642 7100 8883

Subsequent transition 1497 340.8 2319 3720 2239 1783 835

* Ref. a4) for details of the level scheme.

Assumed intensity ratio 1~/12 1.00 1.00 1.00

1.33" 1.00

0.87* 0.64*

193

correlation measurements. The 4876 1497 and 6413340.8 keV gamma-ray cascades utilized here were established in the coincidence measurements23'24). Competing transitions into or out of the intermediate level have not been observed. Moreover, the measured intensities of the g a m m a rays in each cascade are equal within the experimental errors, ~5°'o and ~30°'o, respectively, per 100 neutrons captured. Thus, all available experimental evidence indicates that the transitions in each cascade have equal intensities. However, the possibility that relatively weak transitions in or out of the intermediate state, so far unassigned, may exist, cannot be ruled out. In the most recent investigation24), the lower intensity limit for the detection of such possible transitions appears to be ~ 0.3 per 100 neutrons over most of the energy region of interest. 2. 53Cr. The capture g a m m a rays emitted from 5aCr have been studied both in high resolution g a m m a ray measurements and in coincidence experiments 2°, 2s - 27). The 5610 and 2319 keV transiti~ms form a two-step cascade from the capture state to the ground state. The 2319 keV intermediate level has also been established in studies of S2Cr (d,p)S3Cr reaction TM 2,~). Coincidence measurements show only the 2319 keV g a m m a ray in coincidence with the 5610 keV g a m m a ray, and their measured intensities are equal within experimental error, ~ 10 per 100 neutrons captured by 52Cr. The two transitions have thus been assumed to have equal intensities. 3. S4Cr. The capture g a m m a rays emitted from SaCr have been the subject of many investigations 2s-2v;3°-33) similar to those discussed above. In addition, a new investigation, utilizing Ge(Li) singles measurements and Ge(Li)-Nal coincidence measurements has been carried out. The details of this work will be reported elsewherea4). As a consequence of this work, the details of the capture g a m m a ray decay scheme of S4Cr are well established. This scheme is somewhat more complex than those cited above, but it is still characterized by strong primary transitions to low-lying levels. Of the four g a m m a ray cascades utilized, only the 6642 and 2239 keV transitions have a 1 : 1 intensity ratio. In the three remaining cases the relative intensities of competing transitions were determined and taken into account to give the intensity ratios shown in table 2. The decay scheme of 54Cr and the other isotopes listed in table 2 are sufficiently complete so that errors greater than ~ 5% in the intensity ratios listed there appear unlikely. The two-escape peak efficiency of the 9 cm a detector was determined by the method indicated for the seven primarly transitions listed in table 2. For the determi-

194

W. R. K A N E A N D M. A. M A R I S C O T T I

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Ey(MeV) Fig. 3. C u r v e o f t h e two-escape peak efficiency o f the 9 c m a coaxial detector as a function o f energy. T h e five lowest energy points were obtained f r o m t h e ratios o f two-escape to full-energy peak areas a n d t h e full-energy peak efficiency curve o f fig. 2. T h e r e m a i n i n g points were obtained f r o m the curve o f fig. 2 a n d the ratios o f peak areas o f c a p t u r e g a m m a ray cascades with k n o w n intensity ratios. T h e relative efficiencies s h o w n are n o r m a l i z e d to those o f fig. 2.

nation of the efficiency at 5999 keV, it was necessary to extrapolate the previously constructed full-energy efficiency curve from 2754 to 3720 keV. The two-escape peak efficiency at 1783, 2239, 2614, 2754 and 3720 keV was determined by comparing the respective two-escape and full-energy peak areas. The results obtained are shown in fig. 3. The relative efficiency is normalized to that of the full-energy peak efficiency of fig. 2. With a smooth curve drawn through the points, the maximum deviation of any point (that at 5999 keV) is 6%. With the possible exception of the 5999 keV point, the dispersion in the experimental points is approximately as large as would be expected from the estimated uncertainty in the full-energy peak efficiency curve and the measurements of peak areas. Since the analog-todigital converter used in the spectrum measurements had only 1024 channel capacity, it was necessary to obtain each spectrum in several overlapping segments and then normalize the results with the use of peaks of intermediate energy. The use of a 4096 channel converter would eliminate this source of error. A detailed quantitative comparison of the present results with previous determinations of detector efficiency is impossible because of the diversity in the shapes and volumes of the detectors utilized. However, the general trend of the two-escape peak efficiency, increasing to a maximum at ,-,5 MeV, and then decreasing with further increase in energy, bears out

the results of Ewan and Tavendale 4) and Orphan and Rasmussen 7) and agrees with the calculations of Orphan and Rasmussen 7) but disagrees with other Monte Carlo efficiency calculations s'6). In the present work the dispersion of the experimental points is considerably smaller than in earlier determinations of detector efficiency4'7), where capture gamma rays of iron, chlorine, nickel and other elements were employed and the results of earlier magnetic pair and Compton spectrometer intensity measurements adopteda°'2s). Moreover, as indicated, the probable uncertainties in the intensity ratios listed in table 2 are 5% or less, while the estimated uncertainties in the magnetic spectrometer efficiencies are in the range of 10-30%, depending upon energy. The gamma rays from the S3Cr(n,7)S4Cr reaction afford a convenient means for determining the efficiency of a Ge(Li) detector over a wide energy range. There are a convenient number of strong lines, at intervals of 1 MeV, yet the spectrum is comparatively simple. The capture cross section of S3Cr for thermal neutrons is 18 barn, so that it may be used in external beam experiments with low neutron intensities. Since S3Cr accounts for about 60% of the neutrons captured by normal chromium, it is not necessary to use the separated isotope in high resolution studies. The relative intensities of the strong capture gamma rays of Sacr are listed in table 3. The intensities given are the most

D E T E R M I N I N G THE R E L A T I V E E F F I C I E N C Y TABLE 3 Relative intensities of the strong gamma rays from the 5zCr(n,7)54Cr reaction. Energy (keV)

Relative intensity

835 1783 2239 3720 4847 4872 5999 6642* 7100 8883

100 (adopted) 11.9 + 0.7 12.8 _+ 0.7 4.0 + 0.3 1.9 + 0.2 1.0 _ 0.2 5.5 _+ 0.6 12.8 + 1.3 10.5 + 1.1 64 _+ 7

* Note that if normal chromium is used as a target that the full-energy peak of the strong 5.61 MeV gamma ray of 53Cr will be nearly superimposed on the two-escape peak of the 6.642 MeV gamma ray of 54Cr.

consistent set that can be derived from the efficiency curves of figs. 2 and 3 and the 54Cr decay scheme34). Since earlier magnetic spectrometer measurements are still in wide use as intensity standards, it is worthwhile to compare the results of various measurements of the capture g a m m a rays of S4Cr. Measurements made by Kinsey and Bartholomew 2°) with a magnetic pair spectrometer were first published in 1953. Subsequently some modifications were made to the spectrometer - most important, to the slit system - and a new determination was made of the efficiency of the instrument. Earlier results, including those of 54Cr were revised in light of the new efficiency determination35). The earlier results of Kinsey and Bartholomew are in reasonable agreement with the present results in the energy region 5 to 9 MeV, with the exception that their value for the intensity of the 8883 keV line is ~ 20°,'o greater than in the present results. Their value for the relative intensity of the lowest energy line measured, at 3720 keV, however, is a factor ~ 2 smaller than the present value. In a comparison of the revised intensities of Kinsey and Bartholomew with the present results the discrepancy at 8883 keV is no longer present, and with the exception of the 3720 keV line, agreement is good. In the revised results, however, the relative intensity of the 3720 keV line is increased by a factor ,-~4, so that it is then greater by ~ 2 than the value reported here. On the other hand measurements of the capture g a m m a rays of 49Ti between 3 and 10 MeV, made on the same instrument24), fit into a decay scheme with consistent intensity relationships, suggesting that the most recent efficiency determination is very nearly correct. The intensity

195

measurements of Groshev et al. 25) made with a Compton recoil spectrometer, are in good agreement with the present results in the high energy region. However, their values for the intensities of the 835, 1783, and 2239 keV lines, are only 50-60~o as large as the present values when normalized against the 8883 keV line. The more recent results of Rudak and Firsov32), obtained with a Compton recoil spectrometer at the I R T reactor of the Belorussian Academy of Sciences, are in excellent agreement with the present results over the entire energy range from 835 to 8883 keV. With the exception of one weak peak and one doublet unresolved in their work, and the 1783 keV transition, for which there is a 16% discrepancy, their relative intensities all agree with the values in table 3 within _+ 10%. We are indebted to G. T. Emery, H. Kraner, C. Chasman and R. Ristinen for many helpful discussions and valuable advice. We are also indebted to M. McKeown for the communication of new experimental results o n 18°Hfm. References

1) W. W. Black and R. L. Heath, Nucl. Physics A90 (1967) 650. 2) R. C. Greenwood and W. W. Black, Phys. Letters 21 (1966) 702. a) C. Chasman, K. W. Jones, R. A. Ristinen and D. E. Alburger, Phys. Rev. (in press). 4) G. T. Ewan and A. J. Tavendale, Can. J. Phys. 42 (1964) 2286. ~) K. M. Wainio and G. F. Knoll, Nucl. Instr. and Meth. 44 (1966) 213. 6) N. V. de Castro Faria and R. J. A. Levesque, Nucl. Instr. and Meth. 46 (1967) 325. 7) V. J. Orphan and N. C. Rasmussen, Thirteenth Nuclear Science Symposium - Instrumentation in Space and Laboratory, Boston (Oct., 1966) IEEE Trans. Nucl. Sci. NS-14 (1967) 544. s) G. T. Emery and W. R. Kane, Phys. Rev. 118 (1960) 755. 9) j. M. Freeman and J. G. Jenkin, Nucl. Instr. and Meth. 43 (1966) 269. 10) j. W. Mihelich, G. Scharff-Goldhaber and M. McKeown, Phys. Rev. 94 (1954) A794. 11) W. F. Edwards and F. Boehm, Phys. Rev. 121 (1961) 1499. 12) S. D. Koi(:ki, A. H. Kuko(:, M. P. Radojevi6 and J. M. Simi6, Bull. Inst. Nucl. Sci. "Boris Kidri6" 13, no. 3 (1962) 1. 13) G. S. Goldhaber and M. McKeown, Phys. Rev. (in press). 14) O. C. Kistner and A. W. Sunyar, Phys. Rev. 143 (1966) 918. 1~) Nuclear Data Sheets, compiled by K. Way et al. (Printing and publ. Off., National Academy of Sciences - National Research Council, Washington, D.C.) N R C 59-4-19. 16) S. D. Bloom, Phys. Rev. 88 (1952) 312. xT) L. G. Elliott et al., Private communication to Nuclear Data Sheets, 1958. 18) This device was fabricated by H. Kraner of the Brookhaven National Laboratory Instrumentation Department. 19) M. A. Mariscotti, Nucl. Instr. and Meth. 50 (1967) 309. 20) B. B. Kinsey and G. A. Bartholomew, Phys. Rev. 89 (1953) 375. 21) H. T. Motz, Phys. Rev. 93 (1954) 925.

196

W. R. KANE AND M. A. M A R I S C O T T I

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