The 236U Neutron capture cross section

Nuclear Physics A141 (1970) 577--594 ; ~ ) North.Holland Publishing Co., Amsterdam Not to be reproduced by photoprint or microfilm without written permission from the publisher

T H E 2S6U N E U T R O N C A P T U R E C R O S S S E C T I O N A. D. CARLSON, S. J. FKIESENHAHN, W. M. LOPEZ and M. P. FRICKE Gulf General Atomic Incorporated, San Diego, California, USA t

Received 27 October 1969 Abstract: Level parameters for unbound states in 237U have been obtained from capture and selfindication measurements on 236Ufor neutron energies up to 415 eV. An estimate of the position of a bound state near the neutron binding energy was obtained from capture cross-section measurements from 0.01 to 1 eV neutron energy. Neutron strength functions were obtained from the parameters of the resolved resonances and also from average capture cross-section measurements from 0.5 to 20 keV.

I E

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NUCLEAR REACTIONS 23'U(n, 7), E = 0.01 eV to 20 keV, measured O'nT. ~svu deduced resonances, resonance parameters. Enriched targets.

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1. Introduction

Measurements of radiative capture cross sections for low-energy neutrons yield information about the levels of the compound nucleus. This information is frequently much more difficult to obtain by other means. Measurements at neutron energies 1 eV often yield the energies, and in some cases, the width parameters of negativeenergy states. In the energy region where resonvmces are reasonably well resolved, neutron capture data combined with other measurements (such as scattering or selfindication) determine properties of individual compound levels such as their energies, angular momenta, and partial reaction widths. From an ensemble of these levels, statistical properties such as average level densities, strength functions and resonance integrals can be obtained. At higher neutron energies where resonances are not resolved, the energy-averaged capture cross section also determines some of these statistical quantities. In particular, the neutron strength functions for I > 0 can be obtained. For the case of 236U, very few cross sections have been measured, and no measurements of the capture cross section were available below 300 keV. This lack of data has been largely due to the difficulty in obtaining satisfactory samples, but recently highpurity 236U samples have become available. The present measurements were undertaken both to provide more information on the compound nucleus 2 3 7 U and also to obtain cross sections necessary for the calculation of the production of the radioisotopic heat source 23Spu. We present resonance parameters for levels up to a neutron energy of 415 eV obtained from multi-sample capture and self-indication resot Work supported by the U.S. Atomic Energy Commission.

577

578

et aL

A.D. C ~ N

n a n c e a r e a m e a s u r e m e n t s . A v e r a g e c a p t u r e cross sections f o r two n e u t r o n energy regions, 0.01 to 1 eV a n d 0.5 to 20 keV, a r e also presented. T h e average s-wave level spacing, n e u t r o n a n d r a d i a t i o n widths, the s- a n d p - w a v e strength functions, a n d the r e s o n a n c e c a p t u r e integral are r e p o r t e d . 2. E x p e r i m e n t a l

procedures

T h e e x p e r i m e n t a l facility used in these m e a s u r e m e n t s is described elsewhere 1,2) in detail. E l e c t r o n s f r o m the G u l f G e n e r a l A t o m i c linear a c c e l e r a t o r a r e i n c i d e n t o n a tungsten a l l o y target. T h e n e u t r o n s f r o m this p u l s e d source pass t h r o u g h a n 18.6 m e v a c u a t e d drift t u b e to the c a p t u r e s a m p l e l o c a t e d a t the center o f a 4000 1 liquid scintillator. S t a n d a r d time-of-flight techniques a r e e m p l o y e d to r e c o r d the c o u n t i n g r a t e as a f u n c t i o n o f n e u t r o n flight time. T h e p r e s e n t m e a s u r e m e n t s were m a d e with a n electron energy o f a b o u t 28 MeV, b u r s t widths f r o m 0.05 to 0.8/~sec, a n d r e p e t i t i o n rates f r o m 22.5 to 180 pulses p e r second. F o r all m e a s u r e m e n t s except those b e l o w 1 eV, a b o r o n n i t r i d e filter was placed in the b e a m to p r e v e n t o v e r l a p o f l o w - e n e r g y n e u t r o n s at the high repetition r a t e

(180 rtz). C a p t u r e a n d self-indication m e a s u r e m e n t s were o b t a i n e d c o n c u r r e n t l y b y cycling a 236U s a m p l e in a n d o u t o f the n e u t r o n b e a m b e t w e e n the n e u t r o n source a n d the c a p t u r e sample. T h e samples used are listed in table 1. TABLE 1 Samples Thickness (atoms/b) 3.9 × 1 0 - s 1.62 × 10 - 4 4.93 × 10 - 4

Application (see below) c) c) c)

Composition a) ~3e U 23e U 23e U

6.15 × 10 -4

c)

236U

6.57 × 10 -4

~,c,e)

23eU

9.47 × 10 -4 1.23 × 10 -4 3.63 × 10 -4

~,c,c) a) a)

23~U 23~U 23~U

7.46 × I0 -s 1.53 x I0 -s 0.485

t) s) h)

23sU high purity gold graphite, reactor grade

") In the nominal 23~U samples an impurity of ~ssU was present which amounts to 0.16 ~o for some of the samples and 0.172 ~o for the others. b) Used in low-energy capture cross-section measurements. c) Used to determine resonance parameters in the resolved resonance region (capture samples). d) Used to determine resonance parameters in the resolved resonance region (self-indication samples). e) Used in high-energy capture cross-section measurements. f) Used to determine the 2ssu contribution to the 2s6U data. s) Used to measure low-energy flux shape. h) Used to determine background resulting from neutrons scattered into the gamma-ray detector.

236U NEUTRON CAPTURE CROSS SECTION 2.1. N E U T R O N

579

FLUX

Above 1 eV the energy dependence of the neutron flux was determined from timeof-flight measurements taken with a thin BFa proportional counter. In addition to corrections for background and dead time, a self-shielding correction which amounted to about 4 % at 1 eV was applied to the data. The absolute flux was then determined by the saturated-resonance technique 3) using the 5.45 eV resonance in 236U. Small corrections for multiple scattering, transmission, and resolution effects were made to the saturated-resonance data. It was determined that the neutron flux above 1 eV (with the boron nitride filter) is given by

~(t) = KE ~ e x p ( - # / x / E ) ,

(1)

where $(t) is the neutron flux in units of neutrons per cm 2 • see between the neutron flight times t and t + d t ; E is the neutron energy corresponding to the flight time t, and the quantities ~,/~ and K are constants. A least-squares fit was made to the B F 3 data for 2 eV =< E =< 1 keV to determine the parameters ~ and #, and the resulting fit was excellent. Previous measurements 4) made with the neutron source used in this investigation have shown that the neutron flux is given by eq. (1) for neutron energies as high as 40 keV, and the flux for the present measurements from 2 eV to 20 keV was also obtained from eq. (1). Below 1 eV, the neutron flux was determined by measuring the time dependence of the capture rate for a gold sample. The counting rate (corrected for backgrounds) in a channel of the time analyser is

C(t) = As~(t)f(E, n).

(2)

The quantity A is the sample area normal to the beam, s is the detection efficiency for capture, and f ( E , n) is the probability that a neutron of energy E is captured in a sample of thickness n nuclei/barn. The relative neutron flux is thus given by the ratio of the measured gold capture rate to the calculated neutron capture probability. The latter was determined from the resonance parameters s) of the level at 4.906 eV and a 1/v contribution at thermal energy of 4.09 b, which give a value of 98.8 b for the thermal neutron capture cross section. Multiple scattering and self-shielding effects were included in the calculation, and the calculated gold capture probability agreed within about 1 ~o with that determined from capture measurements made at this laboratory 6). The quantity As~b(t) used to determine the absolute 236U capture cross section was then obtained from the relative neutron flux by taking into account the relative sample areas and detector efficiencies for neutron capture in 236U versus gold. The relative efficiencies were obtained by applying the saturated-resonance technique to the 4.906 eV level in gold and the 5.45 eV level in 236U. 2.2. D E T E C T I O N E F F I C I E N C Y

For all measurements the capture detector was optically decoupled into two separate halves and operated in a coincidence mode in order to reduce the background

580

A.D. CARLSONet aL

counting rates. An event was accepted only if the gamma-ray energy deposited in each half of the detector was greater than or equal to 1 MeV and if the total energy deposited in the two halves was between 3.5 and 10 MeV. In this mode of operation, the efficiency might vary somewhat from resonance to resonance due to differences in the de-excitation processes. To ensure that this was not an important effect, simultaneous 2 3 6 U capture measurements were made up to 125 eV in the coincidence mode and also in a mode in which the only requirement was that the total deposited gamma-ray energy be between 3.5 and 10 MeV. The ratios of capture areas obtained with the two modes were constant from level to level within the statistical uncertainties ( ~ 2 ?/o). Some capture gamma rays are attenuated and degraded in energy before leaving the sample. This effect could make the efficiency for detection dependent on the position in the sample where the capture process occurs. However, the calculated difference in transmission for a gamma ray of energy greater than 0.5 MeV created near the surface of the sample (which is the case for the saturated 5.45 MeV resonance used to determine the absolute neutron flux) and those created uniformly throughout the sample (as is the case at low neutron energies where the sample is very thin) is less than 1 ~ for the thickest sample used. It was assumed that neutron fission of 236U could be neglected for the entire energy region (0.01 eV-20 keV). This assumption is supported by the data of Lamphere 7) which indicate that the threshold for 236U neutron fission is about 0.5 MeV. Also, Leonard s) and McNaUy 9) have found that the subthreshold neutron fission cross section is negligible compared to the capture and scattering cross sections. At the higher neutron energies (shorter flight times), the bremsstrahlung pulse from the neutron source produces gain shifts in the detection system. This effect was reduced by placing a high-purity lead filter in the flight path. The time dependence of the induced gain shift was determined from capture measurements made with and without a 24Na source placed near the sample but out of the neutron beam. The difference between the counting rates from these two runs is the constant rate due to the 24Na source modified by the gain shifts. From these data and the pulse-height distributions for 7-rays from the 24Na source and from 236U capture, the change in detection efficiency due to gain shifts was found to be less than 2 ~o at neutron energies below 20 keV.

3. Experimental results 3.1. R E S O N A N C E PARAMETERS

For the resonances up to 415 eV, measured capture areas, self-indication areas and self-indication ratios were analysed with the Monte Carlo computer program TACASI [ref. lo)]. In this analysis, maximum likelihood estimates of F r and F° and their uncertainties were determined by iterative least-squares fitting. Doppler and resolution broadening, multiple scattering corrections, and the contributions from the nearest neighbor resonances were included in the calculations. The resonance parameters and their uncertainties (standard deviations) obtained from this analysis are shown

236U NEUTRON

CAPTURE CROSS SECTION

581

in table 2. In all cases, it was assumed that the resonances are excited by s-wave neutrons. Some of the data employed in the resonance-parameter analysis are shown in fig. 1. The quantity shown here, neutron capture probability divided by sample thickness, is the neutron capture cross section where multiple scattering, self-shielding and instrumental resolution effects are negligible. Plots of curves in the (Fn, F~) plane deTABLE 2 236U resonance parameters Eo(eV) ") --9.7-t-1.0 b) 5.45 29.7 34.0 43.7 63.1 71.1 86.0 101.7 120.2 124.2 133.7 137.0 163.7 192.6 194.0 212.0 229.0 243.0 272.4 288.2 302.5 320.0 334.4 356.0 367.8 371.0 379.3 415.0

/~,(meV) *)

24.5-4-1 20.94-3.5 22.04-1.5 23.35=0.9 24.34-1.3 24.04-1.5 21.04--3.5

18.04-6.0 24.85=1.3

21.54-2.7 25.85=1.8

24.84-3.0

-Pa(meV) 1.6 a) 2.16 4- 0.08 0.585-4- 0.03 2.35 4- 0.13 11.8 4- 0.6 0.0344- 0.005 19.0 4- 1.0 26.0 4- 2.0 0.8 4- 0.08 51.8 4- 4.0 15.9 -t- 1.9 1.08 4- 0.09 0.48 4- 0.1 2.09 4- 0.15 13.2 4- 1.3 ~ 52.0 4-13.0 / 98.2 4-10.0 2.34 4- 0.14 0.3 4- 0.15 55.0 4-15.0 13.5 q- 2.0 83.5 5=15.0 5.8 4- 0.6 6.3 -4- 0.4 0,64 4- 0.1 0.4 4- 0.3 13.8 4- 1.0 130.0 4-30.0 17.8 4- 2.0

P.(meV) h)

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_P,(meV) ~)

o) 1,7640.6142.6419.0 -4-

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40.0 4- 10.0 44.0 -4- 11.0 53.0 4- 19.0 8.0 -4- 4.0

94.0 4- 25.0 80.0 -4- 30.0

100.0 4- 50.0 130.0 4- 70.0

190.0 4-100.0

") Uncertainties in resonance energies are ~ 0.5 % for positive energy levels. There is a small difference in the resonance energies obtained by ref. xg) compared with the present measurements. b) The stated uncertainty does not contain the uncertainty associated with the assumed radiation and neutron widths. c) Where no radiation width is given the average value o f / ' r was assumed in the determination o f F , . a) The reduced neutron width shown here is the average value for the positive energy levels. e) By assuming ~ n° = 2.4 meV and F~ = 30 meV, the negative energy resonance was found to occur at --8 eV by ref 17). r) A radiation width of 294-7 meV was determined for this level, by ref aT). s) A radiation width of 33 meV was deduced for this level, by ref. ts). h) Ref. 19). J) Ref. aT). k) Ref. as).

582

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Fig. 1. Experimental data used in the resonance parameter analysis. The ordinate on these plots is equivalent to the neutron capture cross section where multiple scattering, self-shielding a n d instrumental resolution effects are negligible. Some o f the strong 235U resonances which were observed are indicated by arrows. The sample thickness was 6.15 × 10 - 4 atoms/b for the measurements made f r o m 25 to 100 eV neutron energy. Above 100 e V a sample thickness o f 9.47 x 10 - 4 a t o m s / b was employed. In these plots the ordinate is a linear scale up to 10 b and logarithmic above this value.

584

A.D. CARLSON

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fined by the measurements for the 5.45 eV resonance are shown in fig. 2. Also shown is the error ellipse which determines the uncertainties in the parameters. In fig. 3, the observed counting rate as a function of neutron energy near the 5.45 eV resonance for capture and self-indication data is compared with that calculated from the resonance parameters determined in the analysis of fig. 2. I

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In addition to the levels reported in table 2, a number of possible very weak levels were observed that were either very close to a much stronger 236U resonance or near a level in the 235U impurity ( ~ 0.16 ~o). In these cases it was not possible to obtain a value of F . or even an assurance that these resonances definitely belonged to 23~U. From the parameters of this set of (assumed) s-wave resonances, estimates were obtained of F--~ (average reduced neutron width), D--o (mean level spacing), and So (s-wave strength function). The method of Slavinskas and Kennett 11) was used to

=SSU

NEUTRON

CAPTURE

CROSS SECTION

585

determine these estimates and their biases. The results obtained are shown in table 3, and comparisons o f the observed and calculated width and spacing distributions are shown in figs. 4 and 5. For both the widths and spacings there is reasonable agreement between the observed and calculated quantities.

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~n : 2.156 m eV

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CAPTURE CURVE >r~
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Fig. 3. Comparison of observed and calculated shape of capture and self-indication data for the 5.45 eV resonance in 23¢U. Capture sample thickness: 4.93 × 10 -4 atoms/b. Filter sample thickness: 1.23 × 10 -4 atoms/b.

Resonance energies for levels observed (but not analysed for width parameters) between 415 eV and 1 keV are shown in table 4, and a level-density plot is shown in fig. 6. The dotted lines in this figure represent the 68 ~o confidence limits o f the level density as evaluated from 0 to 415 eV. It appears as though some levels have been missed at energies >~ 700 eV.

586

A. D. CARLSON e t

aL

TABLE 3 236U

average parameters

s-wave strength function × 104

Calculation method 1.35-t-0.3 1.3 4-0.4

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2.3 =[=0.6

b)

., 1 . 0 2 _+ oo.z

this work Harvey and Hughes 19) p-wave strength function × 104 Observed level spacing (eV) this work Harvey and Hughes 19) Average reduced neutron width (meV) this work Harvey and Hughes 19) Average radiation width (meV)

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a)

1.9 -4-0.6

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a) Calculated from resonance parameters obtained from capture and self-indication data. b) Calculated from measured average capture cross sections. c) Calculated from resonance parameters obtained from total cross section data up to 140 eV. A 14 ~o correction was applied to the level density for small resonances which were missed.

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236U NEUTRON CAPTURE CROSS~I~CTION

587

3.2. LOW-ENERGY CAPTURE CROSS SECTION (E = 0.01 to 1 eV)

The 236U neutron capture cross section f r o m 0.01 to 1 eV was measured in order to obtain information about negative energy levels. Here, it was necessary to make a I00

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564 577 607 618 638 648 656 674

691 706 721 747 771 790 807 822

828 848 866 889 902 931 949 956

969 996

correction to the m e a s u r e d c o u n t i n g rate to a c c o u n t for events resulting f r o m capture a n d fission i n 23 s U. A l t h o u g h the n o m i n a l 236 U foil c o n t a i n s only a small c o n t a m i n a n t o f 235U ( m 0.16 ~o), the t h e r m a l cross section o f 235U is a b o u t two orders o f m a g n i -

588

A.D.

CARLSON et

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tude larger than that of 2a6U and thus causes a significant contribution to the counting rate. (Contributions from other impurities in the 236U samples are negligible.) To provide data for an accurate subtraction of the 235U contribution, measurements were also made with a 23 s U sample (see table 1). The contribution from impurities in the 2aSu sample was negligible. 70

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(3)

where C6 is the observed counting rate for the 236U sample, and Cs is that for the 23SU sample. The remaining quantities not previously defined are as follows: N6 = the number of 236U atoms/area, M6 = the multiple scattering factor for the 236U sample, M 5 = the multiple scattering factor for the 23 s U sample and R = the ratio of the number of 235 U atoms/area in the 236U sample to the number of 23 s U atoms/area in the 23SU sample. The multiple scattering factor is defined as the total neutron absorption probability divided by the probability of absorption of an incident neutron on the first interaction (for these measurements 1 ~ M __< 1.02).

236U

NEUTRONCAPTURECROSSSECTION

589

The 236U capture cross section thus obtained is shown in fig. 7. The points shown are the result of averaging the data points within a selected energy mesh (,4 E/E = O. 1) and do not indicate the energy resolution used. The residual cross section that remains after subtracting the contributions from positive energy levels was ascribed to the presence of a negative energy level. It was assumed that one negative energy level having width parameters equal to the average reduced neutron width and the average radiation width of the positive energy levels I0

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would be sufficient. From the measured cross section at 0.0253 eV, it was determined that the negative energy level is at - 9 . 7 eV. The calculated capture cross section from 0.01 to 1 eV resulting from both the negative energy level and the positive energy levels is shown as the solid curve in fig. 7. 3.3. H I G H - E N E R G Y A V E R A G E CROSS SECTION (0.5-20 keV)

Average neutron capture probabilities at higher energies may also be obtained from eq. (2); however determining the average capture cross sections from these average probabilities is not entirely straightforward. This is because the underlying cross section is not smooth but rather consists of unresolved resonance peaks. Since

590

A.D.

et al.

CARLSON

the self-shielded capture probability is related non-linearly to the total cross section, this resonance structure precludes a simple relationship between the average capture probability and the average capture cross section unless the sample is exceedingly thin. Corrections for the effects of resonance self-shielding and multiple scattering were determined with the code SESH 12) which uses Monte Carlo techniques to I0

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Fig. 8. 23eU average capture cross section from 0.5 to 20 keV. The circles are the measured values. The curve is the calculated capture cross section for Do = 15.4 eV, P~, = 23.9 meV, So = 1.35 X 10-4 and Sx = 2.3 × 10-4. sample distributions of resonance widths and spacings generated from specified values of the neutron strength functions and average values of the level spacing and radiation width (those obtained from the resolved resonances). In this process the strength functions are deduced by iterating their values to produce an optimum fit to the data. The s-wave strength function was deduced from the data at low neutron energies where the l = 0 component predominates, and this value was then used in determining the p-wave strength function by fitting the high-energy data. The values obtained are

236U NEUTRON CAPTURE CRO..~.SSECTION

591

discussed below. The resultant correction for self-shielding and multiple scattering was generally small, exceeding 5 ~o only below 1 keV. The maximum correction, 9 ~ , was applied at 0.55 keV. In these calculations it is assumed that the average capture cross section is given by a sum of single-level, Doppler-broadened contributions. It was also assumed that the s- and p-wave radiation widths are the same and that the level spacing for p-wave resonances is given by D l ~ . r g ( J ) = D O (therefore D1 = ~Do). The contribution to the cross section from d-wave neutron interactions is very small. For the d-wave strength function reported by Seth 13) the l --- 2 contribution never exceeds 1 ~ of the capture cross section due to s and p waves. The high-energy 236U capture cross section (corrected for all sample-thickness effects) is shown in fig. 8. These data have also been corrected for the small contribution ( ~ 2 ~ ) from the 235U impurity. The uncertainties in the data resulting from the neutron flux uncertainty and counting statistics vary from about 5 ~o at the lowest neutron energies to about 10 ~ at the highest energies. The scatter of the low-energy points is primarily due to fluctuations in the number and the neutron widths of the levels contained within the energy averaging interval. The solid curve in this figure is the cross section calculated from the average parameters. The uncertainties in the strength functions obtained from these data are due mainly to uncertainties in the neutron flux, level spacing and radiation width, and to the interdependence of the strength functions (from the fact that the sum of the cross sections obtained from the two strength functions equals the measured capture cross section.) The uncertainty resulting from counting statistics is generally small since the fit involves many data points. The values of the strength functions and their uncertainties obtained from these average cross-section measurements are listed in table 3 along with the average parameters obtained from the resolved resonances. The infinite-dilution resonance capture integral from 0.5 eV, calculated with the resonance parameters up to 415 eV and the s- and p-wave strength functions above this energy, is 350+25 b. 4. Discussion

In the low-energy region (below 1 eV), information about the capture cross section had been obtained previously from activation measurements and a total cross-section measurement (see table 5). In addition to these determinations, Baumann 18) has recently inferred a cross section of 6.0_ 0.5 b from an intercomparison of resonance integral measurements using two techniques, one of which is dependent and the other independent of the thermal neutron capture cross section. In the present determination the thermal cross section was measured three times under optimum experimental conditions with two different sample thicknesses. In each case the determination agreed within +2 ~ with a value of 5.1 b. With the exception of the determination of McCallum, which may have been affected by uncertainties in the scattering cross

A . D . CARLSON et aL

592

section, there is agreement between each of the previous measurements and that of the present work, within the sum of the quoted uncertainties. The recent activation measurements of Baumann, which tend to support the 6 b value of Halperin, indicate however that there may be a systematic difference between the results of some activation measurements and the results of the present investigation. TAst~ 5 Thermal capture cross section for ~3~U ~ro~,(b) 5.4-4-1.5 5.5+0.3 6 -4-1 8.1 ± 1.8 5.1 ±0.25

Method activation activation activation total minus scattering capture

Reference Schuman x4) Cabell xs) Halperin 16) McCallum17) This work

TABLE 6

Resonance capture integral for ~seU R I (b)

417=t=25 / 419 q-70 j 381 q - 2 0 397 ± 34 450+30 340-t-40 350±25

Method activation activation activation activation calculatedby Harvey from resonance parameters and an estimate of the contribution above 384 eV calculatedfrom resonance parameters up to 415 eV and from the s- and p-wave strength functions above this energy

Reference Baumarm is) Schuman 14) Cabell as) I-lalperin t~) Harvey zo) this work

Each of the groups that employed activation techniques to determine the thermal capture cross section has also obtained the resonance capture integral. These measurements along with that obtained in the present experiment are shown in table 5. The results of the activation measurements are generally in agreement or near agreement with the present determination within the sum of the uncertainties, but it is clear that the activation measurements consistently yield somewhat higher values. It is interesting to note, however, that the ratio of the resonance integral to the thermal capture cross section is in better agreement in a comparison between the activation measurements and the present results, which suggest an error in normalization. It is difficult to understand how this could be the case for the present measurements since the normalization, or neutron flux determination, is performed with the highly accurate saturated-resonance method. The present determination of the resonance integral

236U NEUTRON CAPTURE CROSS SECTION

593

agrees well with the value of Harvey et al. 19,20) which was calculated from their resonance parameters and an estimated high-energy contribution. Very few determinations of resonance parameters have been made previously for 236U. Table 2 shows the parameters determined by Harvey and Hughes and by McCallum from total cross-section measurements and the results inferred by Baumann from self-shielding and boron shielding investigations. In view of the large difference between the thermal capture cross section obtained by McCallum and that determined from the present measurements, it is not reasonable to expect agreement on the parameters of the negative-energy level. There are large differences in the values of the neutron width of the 5.45 eV level obtained in the various experiments. Above 5.45 eV comparisons can only be made with the neutron widths of Harvey and Hughes, which in most cases are higher than the present results. Finally, many new levels were observed in our investigation. There are, at present, no high-energy capture cross-section measurements that can be compared with those obtained in this investigation. There are measurements in the hundreds-of-keV neutron-energy region 2t), but it is difficult to compare these with the present measurements which end at 20 keV. An indirect check of the measurements can be made by comparing the s-wave strength function obtained from the average cross section data with that determined from the resolved resonances both in this work and that of Harvey and Hughes 19). As may be seen from table 3, these values are all in good agreement. It is also possible to compare the s- and p-wave strength functions obtained from this work with those of 23Su assuming that the strength functions are approximately the same for the two uranium isotopes. The s-wave strength function obtained from the resolved resonances and that obtained from the unresolved resonance region are both in agreement with the 23SU values 22). The p-wave strength function obtained from the present measurements is in good agreement with the average of the reported values 22) for 23Su.

References 1) E. Haddad, S. J. Friesenhahn, F. H. Frtihner and W. M. Lopez, Phys. Rev. 140 (1965) B50 2) E. Fladdad, R. B. Walton, S. J. Friesenhahn and W. M. Lopez, Nucl. Instr. 31 (1964) 125 3) E. Haddad, F. H. Fr6hner, W. M. Lopez and S. J. Friesenhahn, Proc. of the national topical meeting of the American Nucl. Soc., Vol. II (MIT Press, Cambridge, 1966) p. 125 4) S. J. Friesenhahn, D. A. Gibbs, E. Haddad, F. H. Fr6hner and W. M. Lopez, J. Nucl. Energ. 22 (1968) 191 5) M. D. Goldberg, S. F. Mughabghab, S. N. Purohit, B. A. Magurno and V. M. May, BNL-325, Vol. IIC, Suppl. No. 2 (1966) 6) S. J. Friesenhahn, E. Fladdad, F. H. Fr0hner and W. M. Lopez, Nucl. Sci. Eng. 26 (1966) 487 7) R. W. Lamphere, Phys. Rev. 104 (1956) 1654 8) B. R, Leonard, Jr. and R. H. Odegaarden, Bull. Am. Phys. Soc. 6 (1961) 8 9) J. H. McNally, G. F. Auchampaugh and J. Cramer, private communication (1968) 10) F. H. Fr0hner, General Atomic Report GA-6906 (1966) 11) D. D. Slavinskas and T. J. Kennett, Nucl. Phys. 85 (1966) 641 12) F. H. Fr6hner, General Atomic Report GA-8380 (1968) 13) K. K. Seth, R. H. Tabony, E. G. Bilpuch and FI. W. Newson, Phys. Lett. 13 (1964) 70

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14) R. P. Schuman and J. R. Berreth, Idaho Nuclear Corp. Report IN-1296 (1969) 15) M. J. Cabell, T. A. Eastwood and P. J. Campion, 3. Nucl. Energ. 7 (1958) 81 16) J. Halperin and J. H. Oliver, private communication (1958) reported to J. Halperin and R. W. Stoughton in Proc. 2rid U. N. Int. Conf. on the peaceful uses of atomic energy, Vol. 16 (United Nations, N.Y., 1958) p. 64 17) (3. J. McCallum, J. Nucl. Energ. 6 (1958) 181 18) N. P. Baumann, J. D. Halford and D. J. Pellarin, Nucl. Sci. Eng. 32 (1968) 265 19) J. Harvey and D. Hughes, Phys. Rev. 109 (1958) 471 20) J. Harvey and R. Schwartz, Progress in nuclear energy, physics and mathematics, Vol. 2 (Pergamon Press, London, 1958) p. 51 21) D. Stupegia, R. Henrich and (3. McCloud, J. Nucl. Energ. 15 (1961) 200; J. Barry, J. Bunce and J. Perkin, Proc. Phys. Soc. 78 (1961) 801 22) H. W. Newson, Nuclear structure study with neutrons (North-Holland Publ. Co., Amsterdam, 1966) p. 195