Beta particle spectrometer for measuring aggregate beta spectra following fission

Nuclear Instruments

ELSEVIER

and Methods

in Physics Research A 404 (1998) 173- 180

NUCLEAR INSTRUMENTS a METHODS IN PHYSICS RESEARCH SecllonA

Beta particle spectrometer for measuring aggregate beta spectra following fission’ W.A. Schier”, J.M. Campbell, G.P. Couchell, S. Li, H.V. Nguyen, D.J. Pullen. E.H. Seabury, S.V. Tipnis Received 1 March 1995; received in revised

form ?

I

July

1997

Abstract The beta spectrometer consists of a gated plastic scintillator suitable for measuring aggregate beta energy spectra following fission. This general utility spectrometer is very insensitive to the gamma rays accompanying the fission products. has good linearity, and adequate energy resolution for these continuous beta energy distributions. Response functions are measured below 1 MeV and trial sets of response functions are tested with known beta spectra extending to 5 MeV. ,(‘-I 1998 Elsevier Science B.V. All rights reserved.

1. Introduction This beta spectrometer was designed for the measurement of aggregate beta energy spectra as a function of decay time following neutron fission of 235 U, 238U and 239Pu for the decay time range 0.1-50 000 s. Such a comprehensive set of beta spectra measurements were performed to provide tests of fission product data incorporated in nuclear evaluations such as ENDF/B-VI [ 11. This file consists of data from individual fission products supplemented with statistical calculations which account for unmeasured contributors, incomplete or suspect data sets and missing contributors. In particular, approximately 30% of the contributions at

I Work supported by U.S. Department of Energy. * C’orrespdnding author. Tel.: + I 508 934 3756: fax:

+ 1

50X 934 3003.

016X-9002,198/$19.00 (’ 1998 Elsevier Science B.V. All rights P/f SO16X-900’(97101 103-O

reserved

short decay times rely on these calculations. Therefore. the short decay times were of particular importance and dictated the method of fission fragment transfer. These aggregate beta spectra are continuous and nearly structureless since they arise from hundreds of fission products decaying to the line of nuclear stability by beta-particle emission. In order to achieve very short decay times desired in this study, a helium-jet system is incorporated to rapidly transfer the fission fragments from a fission chamber to a low background counting area (see Fig. 1). The fission fragments are deposited onto a moving tape which carries them to the spectrometer. By varying the tape speed one varies the time after fission at which the beta spectrum is taken. An essential requirement of the helium-jet system is that it ideally transfers all fission products with equal probability. This uniformity was achieved by careful design of the fission chamber

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NIEU-I

Sow

Q

/ x

FISSION ‘1 CHAMBER

ECTROMETER

Fig. 1. A diagram of the helium jet and tape transport assembly and fission chamber are usually surrounded

system with the beta spectrometer viewing the activity on the tape. The target by paraffin to thermalize the neutrons and enhance the fission rate.

[2] which compensates for the greater escape probability from the fission foil by the lighter fragment and by choosing oil vapor as a nucleation medium. The uniformity of the transfer efficiency over the full mass range of fission products has been documented in another publication [3]. Gamma rays typically accompany the beta decay of a fission product nucleus so the beta spectra measurements are made in a gamma-ray flux comparable to that of the beta particles. Therefore, an essential property of this beta spectrometer must be a high degree of insensitivity to gamma rays. A full listing of the beta spectrometer features desirable in the present aggregate beta spectra study now follows.

2. Design features To achieve the stated goal, the spectrometer should have the following properties: 1. compatibility with the helium-jet/tape-transport system, 2. .high degree of insensitivity to gamma rays,

3. linearity out to beta energies of 10 MeV, 4. sufficient energy resolution for beta particles, 5. minimized spectrum distortion from edge effects, and 6. simplicity of the response function, if possible. With the exception of features (2) and (5) concerning gamma insensitivity and edge effects, a 3” x 3” plastic scintillator would appear to meet all the requirements assuming a thin window is provided on the scintillator face. To simultaneously achieve the remaining two features, the design shown in Fig. 2 was established. A thin scintillator disk of diameter 1.5” by thickness 0.020” is placed on the surface of the main 3” x 3” scintiilator which has been optically isolated with a thin aluminum foil. This scintillator disk is viewed by two 2” photomultipliers. Betas passing through the scintillator disk give rise to a signal that triggers the main scintillator. Gamma rays seldom interact with the disk and are therefore very effectively discriminated away. Also beta spectrum distortion is essentially eliminated. This distortion is due to some of the recoiling secondary electrons in a beta-particle stopping event escaping the main scintillator and

W.A. Schier et al. /Nucl. Instr. and Mrth. in Phys. Res. A 403 (199X) 173 -180

SCINTILIATOR

v

rBE

I i Fig. 2. Cross-sectional

view of the beta spectrometer

thus a scintillation pulse is produced that does not represent the full beta energy. By choosing a 1.5” diameter thin disk, most secondary electrons in a detection event are kept away from the cylindrical wall of the detector and therefore they deposit their full energy in the main scintillator. The only wall that the incident beta and secondary electrons remain near is the flat surface through which the beta enters. This proximity will produce a low energy tail in the response function. In an attempt to collect these secondary electrons escaping through the front face, we experimented with a well cut into the front face of the 3” x 3” scintillator. But this seriously distorted the previously simple response function. It is believed that the main problem with the well was one of inefficient light collection, so the well design was discarded and the flat surface geometry of Fig. 2 was chosen for the spectrometer. The quick-coupling “0” ring joint at the mouth of the housing readily accommodates attachment to vacuum systems, light-tight chambers, and sliding tape jaws.

175

source was produced in the University of Massachusetts Lowell research reactor by neutron irradiation and placed in a light-tight chamber coupled to the mouth of the detector. This source has a continuous beta spectrum of allowed shape with an end-point energy 1.392 MeV and two gamma rays of energy 1.369 and 1.154 MeV. An intense (100 mCi) encapsulated “Na gamma source (0.511 and 1.274 MeV) was placed on the outside of this small chamber. near the other source. The spectra measured with and without disk gating are presented in Fig. 3. Without disk gating, the spectrum is dominated by the Compton bands of the OSll- and 1.274-MeV gamma rays from the encapsulated “Na source. But when disk eating is active. such gamma pulses are essentially eliminated and the 24Na beta spectrum emerges from the “sea” of pulses. There are, however, valid beta-gamma coincidences from the two gammaray lines of “Na contributing weakly to the spectrum. Such coincidences are held to a very small probability by maintaining a sufficient distance (e.g. 5”) between source and the face of the scintillator. (see Section 8).

4. Edge effects The effect of electrons from a detection event “leaking” out the cylindrical walls can be demonstrated by gating with two thin disk scintillators, one with 1.5” diameter, the other with 3” diameter. Beta spectra taken with the 24Na source in coincidence with these two different disk scintillators are shown in Fig. 4. One observes that the measured spectrum gated by the large disk is softer than that gated by the small diameter disk. The large-disk distribution is distorted with the peak shifted to lower channels. This distortion is attributed mainly to recoiling electrons escaping the main scintillator through its cylinderical walls.

5. Energy calibration 3. Gamma-ray

insensitivity

To demonstrate the excellent gamma-ray discrimination of the beta spectrometer, a thin 2”Na

Two methods of beta energy calibration are routinely used with the beta spectrometer. One method relies on internal transition (IT) sources of

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4

,&

9 w t -\_

k

CHANNEL

Fig. 3. Demonstration of the gamma-ray insensitivity of the gated beta spectrometer. In (a) the ungated spectrum is dominated by gammas from the intense “Na source. In (b) the gated beta spectrometer rejects nearly all the gamma rays and the beta spectrum from the thin ‘“Na source emerges from the “sea” of counts.

n ii F

CHANNEL Fig. 4. Demonstration of the improvement in beta spectrum shape from bare “Na by eliminating secondary electron losses from the cylindrical face of the main scintillator. In (a) the disk scintillator covers the full face of the main scintillator. In (b) the disk scintillator has the smaller diameter shown in Fig. 2.

monoenergetic electrons such as r’%, 137Cs, and ‘07Bi and on beta end points from beta sources such as ‘4Na, 38C1, 4ZK and 56Mn. The other method relies on the Compton-edge energies of gamma sources when the main scintillator is run in an ungated mode. Convenient gamma-ray sources for this purpose were an encapsulated 22Na source, a metalic thorium neutron-scattering sample, and a neutron shielded plutonium-beryllium neutron source. The energy calibration by these two methods, shown in Fig. 5, gives rise to two straight lines that are basically parallel to one another. The upper line is the true beta energy calibration since

0

200

400

600

800

1000

1200

CHANNEL Fig. 5. Energy calibrations using (a) electron sources and (b) gamma-ray sources. The more convenient gamma-ray calibration can be used for electron energies by performing a simple translation.

electron sources were used. The shift between this calibration and that due to Compton edges is interpreted as mainly due to energy loss by the electrons in passing through the disk scintillator. Calibration with bare IT sources and thin sources of short-lived beta emitters is not very convenient since source preparation is involved and the high voltage must be removed from the spectrometer when the chamber is opened. Calibration with the gamma sources is much more convenient, having the advantage of not needing to access the interior of the beta spectrometer nor to turn down high voltage. In practice, the routine (daily) energy calibrations were made with gamma sources and the electron source calibration was typically performed weekly.

6. Response functions The measuring of response functions as opposed to calculating them with a Monte Carlo procedure is the preferred method for most spectrometers. Electrons from a monoenergetic source can deposit their full energy in the spectrometer, but they can also scatter from the walls prior to entering the

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scintillator or deposit only part of their energy in the scintillator if secondary electrons escape the front face. These non-ideal events produce a lowenergy tail on the response function. With a beta spectrometer. one finds only a limited number of IT sources are available and these are limited basically to energies up to 1 MeV. Furthermore, these IT sources are often accompanied by betas that obscure the low-energy tail of the response function (see Fig. 6 for example). Fortunately, in the case of the bare 13’Cs IT source. the beta spectrum can be removed by performing an X-ray coincidence. The 13’Ba 0.662-MeV level populated in beta decay of 13’Cs is an isomeric state with a 2.6 min half-life. When the state decays and the electron is emitted (i.e. ejected from an electron shell), an X-ray is emitted by the atom when an electron from a higher shell fills the vacancy. By gating on such X-rays (e.g. K, X-rays), the low-energy tail of the response function becomes exposed as shown in Fig. 6 (b). Unfortunately there are no such sources readily available giving monoenergetic electrons above 1 MeV. Yet reliance on experimental measurements still seemed to be the most satisfactory approach to producing an acceptable set of response functions. Beta spectra measurements having predictable shapes. (i.e. known energy distributions), were used to test trial sets of response functions. Once an acceptable response function set is found, it should give a satisfactory energy distribution solution to any known beta spectrum measurement. This allows numerous tests to be made on a trial response function set. Such beta spectra, particularly if they are quite energetic, mimic the continuous spectra that will be measured following fission. So if beta spectra from sources can be correctly unfolded with the trial set. then it follows that unfolding of the aggregate beta spectra to yield the aggregate beta energy distribution will also be successful. The trial set of response functions shown in Fig. 7 was established based on the response function measurements below 1 MeV. This trial set has a gaussian peak with FWHM given by AE = II + b.&

(I = 0.007,

b = 0.05,

(1)

I? w >

CHANNEL Fig. 6. Electron spectrum from a bare “‘Cs IT source. The 13’Ba level involved has a 1.6 min half-life. In spectrum (a) both the continuous betas and the superimposed unresolved conversion-electron lines appear. In spectrum(b) the betas are removed by gating on the K X-rays thus revealing the low-energy tail on the response function.

$

0.12

iz iz

0.08

% i= 2

004

K

0.00 0

1

2

3

4

5

ENERGY (MeV) Fig. 7. Representative members of a trial set ol response functions based on beta response measurements below I MeV.

with energies in MeV and a ramp function tail whose area compared to the peak area remains constant.

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Instr. and Meth. in Phys. Res. A 404 (1998) 173-180

7. Beta spectra tests of the trial response function set There are two approaches to test a response function set. A known beta energy distribution can be broadened by folding in the trial set of response functions giving rise to a “generated” beta spectrum which is then compared to the measured beta spectrum. This approach is demonstrated in Fig. 8 with a thin 38C1 beta source. This source gives rise to a known-shape energy distribution consisting of three beta-distributions superimposed with the highest end-point energy equal to 4.81 MeV. The fit between the measured and “generated” spectrum is quite acceptable except at very low energies. This falloff at the low-energy end was attributed to the loss of detection efficiency below 0.3 MeV due mainly to the passage of the betas through the disk scintillator. An energy-dependent efficiency correction would ultimately boost the energy distribution at these low energies. The second approach is to unfold a measured spectrum of known shape using the same set of response functions and then comparing the result with the known beta energy distribution. This approach is demonstrated in Fig. 9 using a thin 24Na source. The unfolding program FERDO [4] uses a least-squares fitting approach. Here a spacing of response functions given approximately by OSAE

014

ENERGY

(MeV)

Fig. 9. The measured “‘Na beta spectrum is unfolded with program FERDO [l] using the trial set of response functions. The resulting bold curves (a) are the energy distribution solution limits. Comparison is made to the known beta spectrum (b) slightly broadened by the resolution of the spectrometer. The divergence at low energies is interpreted as a loss of efficiency and would normally be corrected.

(see Eq. (1)) has resulted in a rather smooth, nonoscillatory energy distribution given as two limiting solution solid curves. Here again one sees good agreement between the unfolded spectrum using the same trial set of response functions and the known energy distribution represented by the solid curve. Once again, the falloff observed at low energies would be boosted finally with an efficiency correction.

8. Correction for beta-gamma

Fig. 8. Comparison between a measured WI beta spectrum and the known shape modified by having the trial set of response functions folded in. No correction has yet been made for loss of efficiency at low energies and for beta-gamma coincidences.

1.2

018

coincidence

Coincident gamma rays will occassionally be counted with the gated beta spectrometer. Although coincidence with correlated gamma rays is a small effect (typically < 2%) due to the small solid angle and less than 100% detection efficiency for gamma rays, one wishes to correct for this effect at least approximately. One method is to deduce the gamma-ray spectrum from the ungated spectrum by subtracting the gated spectrum weighted by the solid angle ratio of the cylinder/disk. This gamma spectrum is then folded into the beta spectrum to generate a coincidence spectrum. Although such a folding process is not truly valid, it is not an unreasonable approximation when one is considering aggregate fission products, i.e. several hundred

W.4. Schier et al. JNtrcl. Instr.

and Meth.

superimposed beta and gamma-ray contributors. In performing this folding procedure, the condition that the “sum” spectrum does not exceed the beta spectrum maximum is applied because this highenergy region consists of betas proceeding to ground states which do not give rise to gamma coincidences. This sum spectrum weighted by the AQ/47t for the cylinder is then subtracted from the gated spectrum. This subtraction procedure ordinarily hardens beta spectra. The average energy is increased by 24% for a typical aggregate beta spectrum.

9. Fission-product

beta spectra

Aggregate fission product beta spectra were collected in a continuous manner at a fixed delay-time interval until good counting statistics were achieved with the helium-jet and beta spectrometer system described in the introduction. The measured spectra are corrected for background (including chance coincidence), true beta-gamma correlations and detection efficiency (at low energies where beta energy loss in the gating disk is significant). The spectra are then corrected for discrete peaks resulting from internal-conversion electrons which compete with gamma emission in the decay of many excited states formed by beta decay. Finally, the resulting continuous beta spectrum is represented as a superposition of 70 spectrometer response functions using the unfolding program FERD-PC. The beta energy distribution is thus expressed as a histogram consising of 70 energy bins. A beta energy distribution determined for the fission products resulting from the thermal neutron-induced fission of 23sU is shown in Fig. 10. This distribution shown as a solid curve corresponds to a decay-time interval after fission of 1.7-1.2 s, with a mean decay time of 2.08 s. A logarithmic intensity scale was chosen to better display the high-energy region up to an energy slightly more than 7.5 MeV. The beta energy distribution is compared to that determined by Dickens et al. [S] (see dashed curve) at a similar decay time of 2.3 s. The agreement is excellent considering differing local structure is likely due to differences in counting statistics and the number of energy bins used in

174,

in PhJs. Rrs. .4 JO4 (199X) 17% 180

9

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1

ti a 0.1 0

2

4

ENERGY

6

8

(MeV)

Fig. 10. Our measured aggregate beta energy distribution (UML) following the thermal-neutron fission of ’ “U compared with an earlier measurement (ORNL) [S].

the unfolding. Such good agreement over the full energy range of the distribution provides confidence in the validity of the response-function library derived from the source measurements described earlier. Because the spectrometer has a low-energy cutoff at approximately 0. I5 MeV due to electron energy loss in traversing the disk scintillator. the low-energy portion ( < 0.25 MeV) of the spectrum was estimated using the programs SPEC 5 [6] and CINDER 10 [7] in conjunction with ENDF,/B-VI fission product data. Extrapolation of the beta energy distribution down to zero energy is necessary for determining the average beta energy at each decay time. The average beta energy multiplied by the beta activity yields the beta-decay heat, i.e. power, as a function of decay time. The spectrometer has been used to determine the beta decay heat as a function of decay time following the thermal-neutron fission of 235U and ‘“9Pu and the fast-neutron fission of 238U. Results of these studies are in preparation for publication.

References [l]

ENDF;B: Evaluated Nuclear Data F’iles. available from and maintained by the National Nuclear Data Center. Brookhaven National Laboratory, Upton, NY. LISA. [2] R.S. Tanczyn, G.P. Couchell. W.A. Schier. Comp. Phys. Commun. 38 (1985) 61. [3] P.R. Bennett. W.A. Schier, G.P. Couchell, ES. Jacobs, D.J. Pullen, M. Villani, Nucl. Inst. and Meth. A 369 (1996) 163.

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[4] B.W. Rust, D.T. Ingersoll, W.R. Burrus, A User’s Manual for the FERDO and FERD Unfolding Codes, ORNL/TM8720, 1983. [S] J.K. Dickens, T.A. Love, J.W. McConnell, J.F. Emery, K.J. Northcutt, R.W. Peelle. H. Weaver, Delayed Beta- and Gamma-Ray Production due to Thermal-Neutron Fission of 235U. Spectral Distributions for Times After Fission And Graphical Data, Between 2 and 14 000 s: Tabular

NUREG/CR-0162. ORNL/NUREG-39, Oak Ridge National Laboratory (1978). [6] M.G. Stamatelatos, T.R. England, Nucl. Tech. 45 (1979) 219. [7] T.R. England, An Investigation of Fission Product Behavior and Decay Heating in Nuclear Reactors, University of Wisconsin Thesis, micro&x order no. 70-12, 727 (1969).