Received October 7, 1963

Transcription

1 Journal of NUCLEAR SCIENCE and TECHNOLOGY, 1, No.3, 101~109 (1964). 101 Absolute Measurements of Americium-241 Semiconductor of the Disintegration Rates Sources by Using Detectors* Eiji SAKAI**, Hiroyuki TAMURA*** and Yoshifumi SAKURAI*** Received October 7, 1963 Low geometry counting a and a-particle/g-ray coincidence counting were made to determine the absolute disintegration rate of an 241Am source. Alpha particles were counted by silicon junction detectors and g-rays by an NaI (T1) crystalphotomultiplier system. The half life of the 60 kev level of 237Np was measured to check the effect of the coincidence resolving time in the absolute measurement of the americium source by a-g coincidence counting; the half life measured was 6.3x10-8 sec. The mean value of the absolute disintegration rate of a typical 241Am source measured by coincidence counting was 1.696x104 dps and the 90% confidence interval for the mean value was [ dps, 1.713xl04 dps]. Accurate absolute measurements by low geometry a particle counting x are very difficult especially for very small semiconductor detectors because of the difficulty of measuring the accurate geometries. I. INTRODUCTION The research on semiconductor detectors now in progress at different laboratories includes the development of elements and their application at high resolution in instrumentation. The present paper describes the work done on the application of semiconductors to a particle measurement and associated absolute measurements of the disintegration rate of Am sources with higher accuracy. 241 Semiconductor detectors give good resolution, long term stability, and fast rise-time for charged particles(1), so they are suitable for coincidence counting. The coincidence counting is free from uncertainties introduced in the measurement of solid angle, self-absorption of the sample, window effect of the detector and so on. Thus, from the accuracy point of view, the coincidence counting method is better than low geometry a counting(2)(3).. PRINCIPLE II 1. Low Geometry a Counting The distance between an a source and a detector z and the radius of the detector r are measured. The geometry is calculated from the total solid angle subtended by the disc detector as follows(4): G=1/2(1-z/(z2+r2)1/2), (1) where the source is assumed to be parallel to and coaxial with the detector, and is approximated as a point. The absolute disintegration rate Caabs, is calculated from the equation Caabs=Ca/G, (2) where Ca is the count rate of the source measured by the detector. 2. Alpha-Gamma Coincidence Counting The decay scheme of 241Am is shown in Fig. 1(5). The a particle spectrum of 241Am has 5 a particle groups which define 4 prominent excited states of 237Np. The most promi- * The experiments were carried out at Tokai Research Establishment, Japan Atomic Energy Research Institute. ** Instrumentation and Controls Laboratory, Japan Atomic Energy Research Institute, Tokai-mura, Ibaraki Pref. * Nuclear ** Engineering Department, Faculty of Engineering, Osaka University, Osaka-city. 23

2 102 J. Nucl. Sci. Tech. nent g-rays in 241Am decay are those at 60 kev. slit width of the single channel pulse height analyzer. The coincidence counting rate is expressed by Cag=Min(M,R)e1f1+Min(N,S)e2f2 + Min(0,T)e3f3+.., (5) where Min(M,R) is the value of the M or R which is smaller of the two. For 241Am, M=0.85(6), R=0.36(7), Min(M,R)=R. (6) Since f2=f3=..=0, it follows that Cag=Re1f1. (7) Then, Since e1=e2=e3=..=e, the total disintegration rate may be calculated from the equation (8) Fig. 1 Decay Scheme of 241Am Let M,N,O,... be the disintegration rates of the a particle groups going to the 60, 103, 159, kev, a60, a103, a159,..., and the a counting... rate is obtained from Ca=Me1+Ne2+0e3+.., (3) where ei,e2,e3,.. are the a detection efficiencies, and ei's may all be assumed to have the same value since the energies are very close to each other. Let R,S,T,... be the disintegration rates of -rays of the 60, 103, 159,... kev, and the g-ray g counting rate is obtained from Cg=Rf1+Sf2+Tf3+..., (4) where f1,f2,f3,... are the g-ray detection efficiencies; and f2,f3,... can be rendered equal to zero by selecting an appropriate base line and CaCg/Cag=M+N+O+..., (9) where Ca, Cg and Cag are all measurable quantities; of which Ca and Cg must be corrected for the backgrounds and Cag for the chance coincidence counting. The chance coincidence counting rate can be obtained from(2) Cch=2tCaCg, (10) where T is the resolving time of the coincidence counting system, and Ca and Cg are the uncorrected a counting and g counting rates, respectively.. III EXPERIMENTS AND THE RESULTS 1. Electronic Circuits of the a-particle/g-ray Coincidence Counting by Using a Semiconductor Detector as a particle Detector Figure 2 shows the block diagram of the Fig. 2 Block Diagram of the Electronic Circuits and the Vacuum Cell 24

3 Vol.1, No.3 (1964) 103 electronic circuits employed for the coincidence counting and the arrangement of the two detectors for a vacuum chamber. The a particle detector used was a silicon p-n junction type 1,000O-cm detector 2 mm in diameter (Hughes SD1-1). The g detector was an NaI scintillator 1.5 in. in diameter and 0.75 in. thick, which was mounted on a Du Mont 6292 photomultiplier. The a source and the semiconductor detector were placed in the vacuum chamber. Between the source and the NaI scintillator, a 2 mm thick beryllium window was mounted on the vacuum chamber wall; the window allowed the low energy g-rays to pass through with little absorption. The distances between the source and the detectors are preferably short, because this increases the counting rates of the detectors. An ORTEC and a Franklin A-8 linear amplifiers were used for amplifying the output of the two detectors. The time signals for coincidence counting were taken from the outputs of the cross over pick off circuits(8). A conventional fast-slow coincidence system was employed. 2. Measurement of the Resolving Time of the Fast Coincidence Circuit The resolving time of the fast coincidence circuit was checked by varying the delay line 1 (DL-1 in Fig. 2) and the delay line 2 (DL-2 in Fig. 2). Figure 3 shows the experimental Fig. 3 Coincidence Counting Rate as a Function of the Time Difference between Two Channels for Pulse Generator Test value of the coincidence counting rate vs. the time difference between the two channels for pulse generator test; the results showed good agreement with the pre-determined setting value (2t=0.70 msec). Fig. 4 Coincidence Counting Rate as a Function of the Time Difference between Two Channels for Semiconductor-detector/ NaI Coincidence Figure 4 shows the same relation for the semiconductor-detector/nai coincidence. The difference in slope between the two curves of Figs. 3 and 4 may be due to the life time of the 60 kev level of 237Np. The shift of the ordinate axis in Fig. 4 may be due to the difference in rise time between the two signals from the semiconductor and the NaI-crystal. In the semiconductor-detector/nai coincidence counting, the delay time of msec was added to the a channel in order to set the system at optimum coincidence counting. The resolving time of this coincidence counting which is defined as the full width at half maximum of the curve was 2t=0.70msec. 3. Determination of the Discriminator Levels for the a and g Channels Figure 5 shows the pulse height distribution of the a particle spectrum for 241Am measured by the semiconductor detector. For the a channel, the bias voltage of the post amplifier and the base line of the single channel pulse height analyzer were so set that all pulses with energies less than 4.8 MeV were rejected. Figure 6 shows the pulse height distribution of the g rays from 241Am measured by the NaI-photomultiplier system. For the g channel, the base line and the slit width of the 25

4 104 J. Nucl. Sci. Tech. single channel pulse height analyzer were so set that only the pulses in the energy range 48.2~70.2 kev were received. Fig. 5 Alpha Particle Spectrum of 241Am Fig. 6 Gamma-ray Spectrum of 241Am 4. Background Counting and Chance Coincidence Counting The background counting for the a channel was zero for 1 hr, and the average counting for the g channel was 5,806 counts in 1 hr. The chance coincidence counting rate is estimated by Eq. (10). An alternative method of determining of the chance coincidence count was also applied using measured values for the uncorrected a and g counts, for delay line conditions DL-1=0 and DL-2=0.9 msec. The chance coincidence counting rate measured was 1,080 counts in 67 hr, or Cch=16.1 counts/hr, with Ca=331,292 counts/hr and Cg=296,496 counts/ hr. The chance coincidence count calculated from the Eq.(10) was 19.1 counts/hr for a resolving time of 0.35 msec. The experimental data directly obtained should be used for the compensation in coincidence counting. 5. Measurement of the Life Time of the 60 kev Level of 237Np by the Delayed Coincidence Method* An 241Am source emits the 60 kevg-rays in coincidence with the 5,477 MeV a particles; the g-rays from the level whose life times are longer than the resolving time t(0.35 msec) of the coincidence circuit are not counted by the coincidence system employed. In order to correct for the uncounted counts, the half life (T1/2) of the 60 kev level was measured. The electronic circuit for measuring the half life of the excited level is shown in Fig. 7. The time-difference distribution between the pulses from the semiconductor detector and the NaI crystal is determined by a time-to-pulse height converter which converts the time difference of the two pulses into a signal whose amplitude is proportional to the difference. Thus the time spectrum may be displayed on a multi-channel pulse height analyzer. The experimental results obtained are shown in Fig. 8. The half life of the 60 kev level was determined to be 6.3x10-8sec from the slope of the curve, and this value is very close to those in other reports(9)(10). The coincidence counting rate for the absolute determination of the disintegration * This work is being refined and detailed data will be published elsewhere. 26

5 t Cag Vol.1, No.3 (1964) 105 Fig. 7 Block Diagram of the Delayed Coincidence System the disintegration rate obtained by low geometry a counting for a typical 241Am source; in the table, r is the radius of the aperture of the slit set in front of the detector (r=4.7 mm), n is sample size, Ca the average counting rate and Caab8 the absolute disintegration rate. Table 1 Disintegration Rate of 241Am Source by Low Geometry a Counting t n is the sampling size Fig. 8 Time Spectrum of 60 kev -ray from 241Am g rate of 241Am sources should be corrected as follows, (11) where Cag is the coincidence count corrected for chance coincidence but not for the effect of resolving time of the coincidence circuit (0.35 msec), T the mean life-time of the t60 kev level, T1/2/0.693=9.1x10-8sec, and Cag' the coincidence count also corrected for the effect of resolving time. 6. Disintegration Rate of a Typical 241Am Source The 241Am source was electro-deposited as a dot of 3 mm diameter on a 2 mm thick stainless steel disc of 1 in. diameter*. Table 1 shows the experimental results of Table 2 shows experimental results of the disintegration rate obtained by coincidence counting for the same a source. Table 2 Disintegration Rate of 241Am Source by a-g Coincidence Counting 's are corrected for background. tt Cag's are corrected both for chance coincidence and for the effect of the resolving time. * Prepared at AERE, England. 27

6 106 J. Nucl. Sci. Tech. IV. DISCUSSION 1. Low Geometry a Counting This simple method is useful in the primary calibration of a sources. But many problems are involved which tend to introduce errors: (1) If z/r is too small to permit simulation of the source to a point, calculated geometry by Eq. (1) is larger than the true geometry. (2) There is the effect of self-absorption of the a source. (3) After each registration, the counters do not operate during the dead time. Consequently, the disintegration rate of an source determined by this method is smaller a than the true disintegration rate. In addition, to measure distances and physical dimensions with sufficient accuracy is not easy especially in very small semiconductor detectors. In order to check the effect of (1), countings were made by varying the distance between the source and the detector, as shown in Table 1. "Student's test"(11) was used to examine the hypothesis that the mean values of each of the two cases are equal. The hypothesis at the 5% level of significance that there is no difference between the mean values at z/r=9.2 and z/r=13.6 was rejected. The hypothesis at the 5% level of significance that there is no difference between the mean values at z/r=13.6 and =17.8, and also z/r=17.8 and =22.0 were accepted. It followed that z/r =9.2 is too small to regard the source as a point, but z/r=13.6 is sufficiently large. Even when z/r is large enough, the relative errors for the geometry values may be about +-8%, if the relative errors for z and r are assumed to be 2%. This means that the accuracy for low +- geometry a 10%. counting may not be better than Alpha-Gamma Coincidence Counting The coincidence counting method is a comparative measurement. The only error introduced in this comparative method is that due to stochastic fluctuations. As coincidence counts at a rate of about 1,500 counts in 1 hr is too small, longer counting must be made to increase the accuracy of the measurement. The 241Am source was counted for 2 hr, and the sampling size of ten was adopted so that an accuracy better than 1% might be obtained. To describe the problem of confidence interval of the sample mean(11), let x be a normally distributed random variable with mean m and standard deviation s(unknown), n the sampling size, x the sample mean and s2 the average squared deviation from the mean, then (12) where the probability is denoted by Pr., and ta/2;n-1 the 100,a/2% point of the t-distribution with (n-1) degrees of freedom. Equation (12) is equivalent to (13) Thus the 100,(1-a)% confidence interval for is given by m From experimental results, x=1.696x104, s=0.0288x104 and n=10, and 90% confidence interval for m is thus [1.679x104, 1.713x104]. V. CONCLUSION Semiconductor detectors show good energy resolution with high detection efficiencies, good long term stability, and facility in a particle measurement. Fine accuracy in absolute measurement of the disintegration rates of a emitting isotopes is rather difficult to obtain with the low geometry a counting method, due to the uncertainty in measuring geometries. Although a long counting time is necessary for the coincidence counting, the method is better than the low geometry acounting in determination of the absolute disintegration rate of a sources from the view point of accuracy. However, it can be used only in sources which emit g-rays in coincidence with a particles. Acknowledgement The authors wish to express their thanks 28

7 Vol.1, No.3 (1964) 107 to the members of Electronics Shop and Machine Shop of Japan Atomic Energy Res. Inst. One of the authors (H.T.) is grateful to Mr. N. Amano who gave a chance to study at JAERI. Thanks are also due to Dr. T. Kusuda of Osaka Univ., Mr. N. Suda and Mr. K. Mori of JAERI for their checking the manuscript. REFERENCES (1) CHETHAM-STRODE, A., et al.: IRE Trans., NS-8, No. 1, 59 (1961); and also many papers in IRE Trans., NS-8, No. 1, (1961); NS-9, No. 3, (1962), etc. (2) SIEGBARN, K.: "Beta- and Gamma-Ray Spectroscopy", (1955), Intersci. Publ. Inc., N.Y. (3) OVERMAN, R. F.: AEC Rept., DP-780, (1962). (4) JAFFEY, A.H.: Rev. Sci. Instrument, 25, 349 (1954). (5) STROMINGER, D., HOLLANDER, J. M., SEABORG, G. T.: Table of Isotopes, Rev. Modern Phys., 30, 585 (1958). (6) ROSENBLUM, S., et al.: J. phys. et radium, 18, 609 (1957). (7) MAGNUSSON, L. B.: Phys. Rev., 107, 161 (1957). (8) FAIRSTEIN, E., et al.: AECL-804, Pt. 4, 117 (1959). (9) BELING, J. K., et al.: Phys. Rev., 87, 670 (1952); 86, 797 (1952). TURNER, (10) J. F.: Phil. Mag., 46, 687 (1955). (11) BOWKER, A. H., LIEBERMAN, G. J.: "Engineering Statistics", (1959), Prentice Hall, Inc., N. J. 29