Modulation Technique for Neutron Capture Gamma Ray Measurements in Activation Analysis T. L.
ISENHOUR' and G. H. MORRISON
Department of Chemistry, Cornell University, Ithaca,
Neutron capture gamma ray activation analysis offers a number of reactions not available to delayed gamma ray activation analysis. The development of a system for using a modulated neutron beam allows cancellation of the half life dependent delayed gamma ray reactions producing a pure capture gamma ray spectrum. Neutron capture gamma ray activation analysis has greater inherent sensitivity than conventional activation analysis for most elements, but is presently limited by the difference in obtainable flux.
I
CONVENTIONAL ACTIVATION analysis the radiations from the radioactive products of nuclear bombardment are used t o determine the elemental composition of a sample. The method can be used only if the product nucleus is radioactive, is formed in reasonable amounts, and has a half life which is neither too short for measurement nor too long to produce sufficient activity. Prompt gamma ray activation analysis, however, is independent of the products' nuclear characteristics. The capture of a neutron by a nucleus leads to the formation of an excited state and the immediate emission of one or more prompt particles or photons. In the case of thermal neutron capture, generally gamma rays are produced. These neutron capture gamma rays are usually emitted in less than 10-12 second and represent the binding energy of the neutron to the nucleus. The rate of production of prompt gamma rays (dN*/dt) is given in Equation 1 for the case where the neutron beam is not appreciably attenuated by the sample
x
dN* -= at
NFu
where
N F Q
= number of target nuclei = flux of bombarding particles =
cross section
A considerable amount of work has been done in the past for the determination of prompt gamma ray energies
162
ANALYTICAL CHEMISTRY
N. Y.
and distributions for application in nuclear physics (IS). Most of these measurements were made with magnetic pair spectrometers and magnetic Compton spectrometers to obtain high resolution. These, however, lack the sensitivity required for most analytical problems. Prompt gamma ray spectrometry using a sodium iodide crystal scintillation spectrometer has been employed, and Greenwood (4-12) has compiled gamma spectra for possible use in analysis. This approach results in greater sensitivity, but poorer resolution. Lussie and Brownlee (16) have employed coincidence measurements which give improved resolution but again cause decrease in sensitivity. A basic problem in the use of prompt gamma ray scintillation spectrometry in conjunction with thermal neutron irradiation is the necessity to differentiate the prompt gamma rays emitted by the sample from the following radiations: prompt gamma rays produced by the surrounding materials, gamma rays caused by the inelastic scattering of fast neutrons in both the sample and surroundings, fast neutrons scattered into the detector by both the sample and surroundings, and reactor gamma rays scattered by both the sample and the surroundings. Hammermesh and Hummel (1.4) solved this problem by taking four separate spectra using various combinations of the sample and a cadmium shutter in and out of the neutron beam. The cadmium shutter absorbs thermal neutrons while transmitting fast neu-
trons and reactor gamma rays. This approach, however, ignores the buildup of delayed gamma radiation. The delayed spectrum is completely cancelled out by the sequence of measurements employed only when the half-life of every delayed product is very long with respect to the counting interval. A second disadvantage of this procedure is that it requires stability of the neutron source over the full measurement which may not be obtainable when long times are required. In order to apply prompt gamma ray activation analysis for accurate, quantitative analyses, it was necessary to eliminate or minimize these sources of error. This was accomplished in this study by making a series of measurements of short duration, so that the delayed activities and background did not change appreciably between the reA modulaspective measurements. tion technique using a high speed thermal neutron chopper was employed. Signals generated by the chopper mechanism were used to switch the memory sections of the multichannel analyzer in synchronization with the modulated neutron beam, thereby allowing recording of separate spectra for the two half-cycles of the copper. Two series of measurements were made with the modulation technique, that of background and of sample. The twohalf-cycles were corrected for background and then subtracted to give the prompt gamma spectrum. The chopper was designed to operate a t a maximum of 1000 C.P.S. allowing better than 99% cancellation of delayed gamma rays from reactions with half lives longer than one millisecond. EXPERIMENTAL
The overall apparatus I is Apparatus. I shown in Figures 1 and 2. The A; collimated neutron beam from the reI actor passes through the rotating chopper and is cut into bursts giving an approximate square wave of neutrons, I detector multichannel analyzer A signal generated by the chopper mechanism is used to switch the memI ory sections of the multichannel analyzer in synchronization with the neutron bursts. The beam catcher beam catcher Present address, Department of ChemFigure 1. Prompt gamma ray appaistry, University of Washington, Seattle, ratus Wash.
U
0
Figure 3.
Figure 2.
Prompt gamma ray apparatus
terminates the neutron beam for safety purposes. A detailed description of the individual sections of the apparatus follows. Detector and Shield (See Figure 3). The detector is a Nuclear Chicago Scintillation Prohe Model DS-304 consisting of a 3 X 3 inch thallium activated sodium iodide crystal with a three inch photomultiplier and a preamplifier. An EM1 9578B photomultiplier tube replaces the standard DuMont 6393 tube to reduce gain shift (14). The graded shield, modeled after Greenwood's (4), consists of successive layers of horal, l e d , tin, and copper to absorb scattered neutrons and reactor gamma rays and induced X-rays. The lithium-6 fluoride screen absorbs neutrons scattered from the target without the production of gamma rays (Li@(n, 0)H.a). The screen is constructed of a sandwich of powdered LiF between two sheets of mylar held in place by The three pieces of aluminum. aluminum sheets each have a threeinch diameter hole. The entire unit is
Figure 4. Neutron chopper
mounted on a mobile hydraulic cart which may be raised or lowered for positioning the target in the neutron beam. Multichannel Analyzer. The multichannel analyzer is a n R I D L Model 34-12 with facilities for rapid storage on magnetic tape and print out on an IBM typewriter and a Model 2d-2 Mosely X-Y plotter. The multichannel analyzer is equipped with external programming features which allow the control of storage - by . the chopper .. electronics. Chopper (See Figure 4). The 18 slots in the cadmium sheet (a) of the chopper are aligned with the notches in the iron ring (6) to an accuracy of + 0.002 inch. Two Electro Laboratory Model No. 3055-A magnetic uickuw ( e ) are mounted perpeGdicular-to the chopper edge and spaced to correspond to one half-cycle. These are mounted on a moving track (d) to allow adjustment for phase alignment with the signal from the scintillation probe. The chopper motor (e) is a Bcdine Model NSH-34 with a Minarik Electric Co. Model SH-32 control unit which allows chopper frequencies from 27 to 1000 C.P.S. Chopper Electronics (Figure 5). The signal produced in the magnetic pickups by the change of magnetic flux as the notches of the chopper pass by consists of two pulses of opposite polarity. The negative portions are clipped and the signals are amplified by modified Fisher Model PR-6 preamplifiers. These pulses are then converted to sharp spikes by the pulse shapers and used to trigger each half of the multivibrator. The multivibrator produces two square waves of opposite polarity which are amplified and used to switch the multichannel analyze alternately between two sections of the memory. Target Holder. A target holder which automatically and reproduceahly changes targets is used. This is a simple rotating set of twelve holders which is actuated by a relay circuit with a microswitch to indicate the stopping
Detector and shield
position (Figure 2). This unit rides on a motor driven trolley which allows horizontal positioning with respect to the neutron beam. Neutron Beam. The Cornell University TRIGA Mark I1 Reactor (General Dvnamics Corn.) is used as a neutron "source for ihese measurements. The TRIGA is a critical, swimming pool reactor operating a t 100 kw. steady state power and d e livering a maximum of 2.5 X lo'* thermal n/cm.%ec. The collimating system is shown in Figure 6. Reactor port 4E which passes tangent to the reactor core was used. The aluminum contained polyethylene reflector is used to scatter thermal neutrons, sending a component down the port. (The thickness is limited to minimize the fast neutron scattering.) The graphite plug collimates the beam to 1.25 inches in diameter and eliminates neutrons which might he reflected from the sides of the final collimator. The final graphite collimator gives a beam which is 1.0 inch high and 0.125 inch wide. A maximum flux a t the target of 1.7 X 106 thermal n/cm.*-sec. is obtainable with this arrangement. RESULTS AND DISCUSSION
Figure 7a shows a spectrum of vanadium taken without modulation. A mixture of prompt and delayed = gamma rays of the V5' (n, y ) lrS* (TI/% 3.77 min.) reaction appears in agreement with those reported by Greenwood and others. Figure 7b (with modulation) shows the prompt gamma rays only. The 1.43 m.e.v. delayed gamma peak
Figure 5.
U Chopper electronics
VOL 38, NO. 2, FEBRUARY 1966
163
which dominates the higher energy portion of the spectrum is now absent. Removal of the delayed gamma ray spectrum allows better reproduction of peak intensities since half life is no longer a consideration. The difference in dead time between half cycles is rather small, on the order of 5% or less. Since the delayed gamma reactions are growing and decaying during the open and closed intervals of the chopper, it is necessary to have the period of the chopper sufficiently short with respect to the half life to give the effect of a steady state. During the open half of the n t h cycle of the chopper, the number of delayed gammas (A,) produced by a given reaction is:
A,
graphite
\
plyethylene in
0,03O"a1umim~rn
Figure 6.
>> 1
and when n
= n-1
R=l
where
C T
N.F*u
= =
the half period of the chopper.
The number of delayed gammas (B,) produced by the reaction in the closed half of the same cycle will be:
(1 - e-irj2)
wherex = AT, For a complete derivation of Equations 2-5 see Appendix l. Table I gives values of E, as a function of A T . It should be noted the only nuclear parameter remaining is half life. The chopper constructed was designed to operate a t a maximum of 1000 C.P.S. allowing better than 99% cancellation of delayed gamma rays from reactions with half lives longer than one millisecond.
(3)
The ratio of net delayed gammas to prompt gammas of the same reaction scheme (E,) is then:
graphite
ANALYTICAL POTENTIAL
Because of the decrease in flux (approximately six orders of magnitude) encountered in producing a collimated, modulated neutron beam, prompt
(4) 1500
r
Collimator gamma ray activation analysis cannot presently compete with the high sensitivity of conventional neutron activation analysis using delayed gamma ray spectrometry where a reaction exists which produces a radioactive species of high activity. However, a number of elements having very high cross section reactiom do not produce good delayed products. In addition, capture reactions proceed without the growth and decay phenomenon. Thus, although resolution on the basis of half life is not possible, these reactions produce saturation activity as soon as the beam is turned on. Many delayed reactions have long half lives which make irradiation to saturation impractical if not unobtainable. Finally, with better geometry and collimator design and larger reactors, much higher fluxes will inevitably become available. A comparision of delayed and prompt gamma ray sensitivities calculated for
.I25
Table 1.
x
= AT;
X
0.1 0.2
0.3 0.4 0.5 0.6 0. ...7 0.8
0.9 1.0 1.5 2.0 2.5 3.0 4.0 5.0 6.0
7.0
8.0
9.0 10.0 20.0 50.0 100.0 1000.0
10000.0 ~~
164
E, as a function of AT 1 0.693 Y
=
2;; Tu2
v . Tiiz 3.466 1.733 1,155 0.8664 0.6931 0.5776 0.4951 0.4332 0.3851 0.3466 0.2310 0.1733 0.1386 0.1156 0.08644 0.06931 0.05776 0.04951 0.04332 0.03851 0.03466 0.01733 0,006931 0.003466 0,0003466 0.00003466
= -
x
E, 0.000841 0.00333 0.00743 0.01313 0.02010 0.02896 0.03893 0.05013 0,06245 0.07577 0.15314 0.23841 0,32137 0.39657 0.51799 0.60535 0.66315 0.71481 0.75017 0.77783 0.80002 0.90000 0.96000 0.98000 0.99800 0.99980
~
ANALYTICAL CHEMISTRY
IOOC
CPM
500
i'A
0
Figure 7. Vanadium spectrum (a) without modulation a n d (b) with modulation Flux, 2 X 10' n / c m . h ~ . Sample, 250 mg. V Count time, 30 min. (live time)
63 interference free elements is presented in Table 11. The criteria for the sensitivities obtained by conventional thermal neutron activation analysis are based on those used by Buchanan (2). From the basic equation for an irradiation process where no decay has occured ( I ) , one obtains the following expression for W , the weight of the target element in grams
Table 11.
Element
2 As
Au B Ba Be Bi Br C Ca Cd Ce c1 co Cr cs cu DY Er Eu F Fe Ga Gd H Hf Hg Ho I In Ir K La
E: M O
Na Nb Nd Ni P Pr Pt Re Rh S Sb
sc
Se Si Sm Sn Sr Ta Te Ti T1 Tm
v
W Zn Zr b
Product
AW.D W = e.NAV.F.u(i - e-XTI) ' c . 7 (6) where
AW
of target element (gm./mole) D = count rate (cps) e = isotopic abundance NAv = Avogadro's number = atomic weight
flux of bombarding particles (n/cm.z-sec) = capture cross section (cm.2) u = decay constant TI = irradiation time e = detector efficiency y = number of gamma rays produced per disintegration. Defining a sensitivity factor S (where S is the minimum detectable weight of
F
=
Estimated Sensitivity Factors for Delayed and Prompt Gamma Ray Activation Analysis
Delayed gamma Ti12 Ey(m.e.v.) 24s 0.340 1.78 2.3m 0.561 27h 0.412 2.7d
S D
7x 4x 5x 6X
102 102 102 10'
85m
0.165
1 x 103
17.6m
0.62
4
8.8m 54h 32h 37m 10.3m 27d 2.9h 12.8h 1.3m 7.5h 9.3h 11s 45d 14h 18h
3.10 0.523 0.29 3.75 1.17 0.323 0.127 1.34 0.108 3.08 1.327 1.632 1.289 3.350 0.364
6 X lo2 3 x 106 4 x 104 4 x 103 1 x 10' 2 x 106 6 X 101 2 x 104 8 x 10-1 4 x 102 3 x 10' 3 x 104 1 x 107 5 x 102 1 x 104
19s 65h 27h 25m 54m 19h 12.5h 40h 9.5m 2.6h 67h 15h 6.6m 11.6d 2.6h
0.160 0.077 1.37 0.45 1.09 0.328 1.53 1.597 0.843 0.845 0.140 2.75 0.042 0.091 1.48
3 x 102 4 x 103 6 X 101 5 x 102
19h 31m 17h 4.4m 5.0m 2.8d 84d 12od 2.6h 47h 9.5m 2.8h 112d 25m 5.8m
1.57 0,540 0.155 0.077 3.09 0.564 1.19 0.402 1.264 0.1032 0.326 0.388 1.22 0.147 0.323
127d 3.8m 24h 14h 17h
0.084 1.433 0.686 0.438 0.747
x 102
8 x 18 1 x 102 1 x 104
x 102 x 104 1 x 101 3 x 104 3 x 102 3 x 102 7 x 106 2 x 104 3 x 103 3 x 103 1 x 102 2 x 10' 5 x 106 1 x 103 3 x 103 2 x 108 4 x 106 1 x 102 2 x 106 5 x 102 1 x 104 1 x 104 2 x 104 7 x 103 3 x 10' 7 x 102 2 x 104 4 x 106 3 3
Reaction
Prompt gamma Er(m.e.v.) SP 0.202 1 x 10' 7.73 1 x 102 7.05 3 x 102 0.2149 4 x 102 0.477 4 x 10-3 4.10 2 x 102 6.80 4 x 102 4.170 1 x 104 7.579 2 x 102 4.95 1 x 103 6.42 3 x 10' 0.56 2 x 10-2 0.120 3 x 102 1.165 8 x 10-1 0.562 3 x 100 8.88 3 x 10' 0.120 6 X 101 7.91 2 x 10' 0.104 9 x 10-2 0.828 8 x 10-1 0,090 5 x 10-2 1.36 1 x 103 7.639 2 x 10' 6.369 1 x 102 0.079 2 x 10-3 2.23 8 x 10-1 0.213 9 x 10-1 0.37 4 x 10-1 0.142 2 x 100 0.135 2 x 10' 0.558 3 x loo 5.68 8 x 100 0.770 2 x 10' 0.440 3 x 10' 3.92 2 x 102 7.26 1 x 10' 0.770 1 x 10' 0.475 2 x 10' 6.85 3 x 103 0.695 1 x 100 8.997 1 x 101 0.43 7 x 10' 5.67 1 x 102 0.35 3 x 10' 5.94 4 x 10' 6.20 1 x 10' 5.44 4 x 10' 6.50 4 x 102 8.18 7 x 100 6.586 5 x 10' 4.93 1 x 102 0.338 1 x 10-2 9.35 1 x 104 7.53 4 x 102 0.272 3 x loo 0.609 2 x 10' 1.387 3 x loo 5.63 1 x 102 0.150 5 x 18 6.51 2 x 10' 5.25 7 x 10' 0.45 1 x 102 6.30 9 x 102
log ( S D I S P ) ' $2
0 0 -1
$1 0
fl
+7 +2 +4
+1 $4 0
+3 +1
+3 +3
+1
+6 +1 +7
+2 +4 +1
$2
0
+1 +3
+1
+2
0
+3 +1 -1
+6 $3 +1
+2
$1 0
$5 +1
$3 +5
+a
+4
+1 0
+4 $3 +4 +3
0
+1
+2 3-3
Information not available. No usable delayed gamma reaction known. ~
VOL 38, NO. 2, FEBRUARY 1966
165
a n element a t unit flux and 100% counting efficiency)
Bi
of delayed gammas produced during the closed portion of cycle i
= Number
C =N*F*u T A = length of open portion of finally
S w=E.F Solving Equation 8 for W using a minimum value of count rate gives the minimum weight of a given element detectable by delayed gamma ray activation analysis. The same formula applies to prompt gamma ray activation analysis when the growth term ( l - e - X T I ) is omitted. The delayed gamma ray sensitivities in Table I1 are computed using Buchanan's criteria of a one-hour irradiation with a count rate of 1000 counts per minute for half lives less than one hour; 100 counts per minute for half lives from one minute to one hour; and 10 counts per minute for half lives greater than one hour. The prompt gamma ray sensitivities are computed using a count rate of 10 counts per minute. Data for calculations were taken from standard sources (S1i3,15,17). Actual sensitivity levels may be computed from the values of S using Equation 8. For example using a detector efficiency of 5% and a flus of 2 x 1012 n/cm.2-sec., the detection level of silver by delayed gamma ray activation analysis would be 7 X gm. The last column of the table gives the logarithm t o the base 10 of the ratio of the delayed gamma ray sensitivity factor to the prompt gamma ray sensitivity factor for the same element. A value of +2, such as for silver, means that the sensitivity for prompt gamma ray analysis is two orders of magnitude better than that for delayed gamma ray analysis a t the same flus. Thus is it seen that in almost every case prompt gamma ray activation analysis has greater inherent sensitivity. With the present apparatus, however, there is a factor of 106 less flux available for the prompt method. This would reduce the numbers in the last column by 6 meaning that 10 elements are as good or better by the prompt gamma ray method. If the difference in flux were decreased to three orders of magnitude, 29 elements would be equal or better, and if the fluxes were equal, only two elements would be better by delayed gamma ray activation analysis. APPENDIX I
For a reaction which produces one delayed and one prompt gamma ray per neutron capture: Let A , = number of delayed gammas produced during the open portion of cycle i 166
ANALYTICAL CHEMISTRY
chopper cycle T B = length of closed portion of chopper cycle
(1
- e - X T A ) ( l - e-XTB)
For a chopper with equal half cycles
TA-TB =
T
And
(1
-
n- 1 e-X1)2
x
R-1
Let D, = A , - B,, where D,is the net delayed gammas produced during cycle n. Also let E,, = D,/Cr where E, is the ratio of delayed to prompt gammas produced during cycle n.. Now (1
- e--Xr)2
n-1 )( R-1
or
+ e-" - I} and IC = n - 1 x
LTA(1
/r
where x = A T ; But e-.>
n-1
This is a geometric progression where
a
=
r
= ratio =
n
= number
1st term = e - 2 2 of terms = b
then, the sum of n terms = S = a(1
1+
-
e- x [ R T B + ( R 1 ) T A
ATA
- rn)
1-r
+ e--XTA thus n-1
(1 - e--hTA)
X
R-1
(1
- e-z)2 + x + e-= -
(11) Ibid., ARF 1193-23 (Quarterly Report), 1963. (12) Ibid., ARF 1193-26 (Quarterly Report), 1963. (13) Groshev, L. V., Lutsenko, V. N.,
which reduces to
and when k>> 1 (3) Chart of the Nuclides, 2nd ed., Fed-
eral Minister of Nuclear Energy, Federal
Republic of Germany, 1961. (4) Greenwood, R. C., Reed, J. H., “Scintillation S ectrometer Measurements of
ACKNOWLEDGMENTS
The authors thank A. J. Beardon for basic schematics for the pulse shapers and multivibrator and A. W. McReynolds for aid in designing collimators and also S. F. Peterson for designing the automatic sample changer. LITERATURE CITED
(1) Boyd, G. E., ANAL. CHEM.21, 335 (1949). (2) Buchanan, J. D., Proceedings of the 1961 International Conference on Mod-
ern Trends in Activation Analysis, Texas A & M College, 1961.
Capture &amma Rays from Natural Elements,” Proceedings of the 1961 International Conference on Modern Trends in Activation Analysis, Texas A & M College, 1961. (5) Greenwood, R. C., “Scintillation Spectrometry Measurements of Capture Gamma Rays from Natural Elements,” U. S. At. Energy Comm. ARF 1193-3 (Quarterly Report), 1961. (6) Ibid., ARF 1193-6 (Quarterly Report), 1962. (7) Greenwood, R. C., Reed, J. H., Ibid., ARF 1193-9 (Quarterly Report), 1962. (8) Ibid., ARF 1193-12 (Quarterly Report), 1962. (9) Greenwood, R. C., Reed, J. H., Kolar, R. D., Ibid., ARF 1193-17 (Quarterly Report), 1962. (10) Ibid., ARF 1193-20 (Quarterly Report), 1963.
Demidov, A.M., Pelekhov, P. I., ‘‘At!as of Gamma-Ray Spectra from Radiative Capture of Thermal Neutrons,” Pergamon Press, London, 1959 (14) Hammermesh, B Hummel, V., Phys. Rev. 88, 916 (1952). (15) Heath, R. L., “Scintillation Spectrometry Gamma-Ray Spectrum Catalogue,” 2nd ed., Atomic Energy Research and Development Report, IDO-
16880, 1964. (16) Lussie, W. G., Brownlee, J., I., Jr.,
“The Measurement and Utilization
of Capture Gamma Radiation,” Proceedings of the 1965 International Con-
ference on Modern Trends in Activation Analysis, Texas A & M University, 1965. (17) Radiological Health Handbook, U. S. Department of Health, Education, and Welfare, Public Health Service, Bureau of State Services, Division of Radiological Health, Washington, D. c. RECEIVED for review August 11, 1965. Accepted December 10, 1965. Research supported by the Advanced Research Projects Agency.
Determination of Boron by Thermal Neutron Activation Analysis Using a Modulation Technique T. L. ISENHOURl and G. H. MORRISON Department o f Chemistry, Cornell University, Ithaca,
An activation analysis method is provided for the quantitative determination of boron using the Blo(n,a)Li7m (Tl,2 = 5 X second) reaction. Gamma spectrometric measurement of the sample being irradiated by a modulated neutron beam permits the determination of the short-lived product. The use of an internal standard overcomes the problem of variations in reactor flux as well as the asymptotic relation between concentration and count rate for materials of high cross section. The approach is applicable to many short-lived reactions.
A
MODULATED neutron
beam technique in conjunction with gamma ray spectrometry can be applied in activation analysis for the measurement of short half-life species as well as for neutron capture reactions. When bombarded by thermal neutrons, B10 undergoes an (np) reaction producing 1 Present address, Department of Chemistry, University of Washington, Seattle, Wash.
N. Y.
Li7 of which 92.5% is in an excited state (1). The excited lithium then decays with the production of a 0.477m.e.v. gamma ray with a half-life of 5 x 10-14 second. Irradiation of the sample with a modulated neutron beam, coupled with gamma ray spectrometry (d), affords a means of applying this reaction to the direct analysis of boron. To obtain quantitative results, it is necessary to employ an internal standard technique because the 4 x 1 0 3 barns cross section of BIO for thermal neutron capture results in an asymptotic relation between concentration and gamma ray production. The addition of samarium to the sample results in a gamma ray spectrum where the ratio of the two peaks may be mathematically fitted as a function of the concentration ratio. Using this method, therefore, allows boron to be analyzed to within the accuracy of the counting techniques,
inch outside diameters. The outside vial had a wall thickness of 1 nim. Mixing was accomplished by shaking in a mixer mill with polystyrene beads in the container. Neither samarium nor boron is present to any appreciable extent in these materials. The resultant homogeneity seemed satisfactory because the calibration curve followed theoretical expectations. Each sample was irradiated and counted for 2 minutes a t a flux of 1.8 X lo6thermal neutrons/ sq. cm.-second. The RIDLmultichannel analyzer was calibrated for a gain of 27 k.e.v./channel. Figures 1, 2, and 3 give spectra of pure boron, pure samarium oxide, and a mixture of the two, respectively. Data were temporarily stored on an RIDL magnetic tape unit and later printed out on an IBM typewriter. A calibration curve was developed using five samples varying from 41 to 64% boron by weight. RESULTS AND DISCUSSION
EXPERIMENTAL
The experimental apparatus has been described ( 2 ) . Samples of boron were mixed with samarium oxide and mounted between the ends of two concentric polystyrene vials of l/2- and
The counting rate (R) of a prompt gamma measurement or any delayed reaction with a short half-life is given by :
R VOL. 38,
= NFoe
NO. 2, FEBRUARY 1966
(1)
167
