Nuclear Physics A372 (1981) 125-140QNorth-Holland Publishing Company

Abstract : Thermal and resonance neutron capture y-ray measurements have been performed on anatural Se target . Accurate energies and absolute intensities of primary and secondary y-rays forthermal and resonance capture were measured for~a.~s'rr .~s.sose . Neutron separation energiesderived from thermal measurements for the following are 8027 .5 f 0.4 keV for ~3Se [~4Se+n];7418.8f0.2keV for "Se ['6Se+n]; 10497.8f0.3 keV for'aSe ['~Se+n] ; and 6701 .1f0.ó keVfor s1Se [s°Se+n]. Ten resonances were analyzed and their spins and parities were determinedor confinmed as z+ for the 27.1 eV and 271.5 eV resonances in '°Se, and for the 377.0 eV and862 eV resonances in '6Se ; 1 - for 112.0 eV, 0- for 211 .6 eV, 1- for 340.8 eV and (1 -) for864.0 eV resonances in ~~Se ; i+ for the 383.0 eV resonance in 'aSe and (i +) for the 1970 eVresonance in s°Se . From the resonance data assignments of possible spins and parities of 20 levelsin ~s .~s.~9Se were made. E1 strength functions for thermal capture in'4''6'"Se and for the 27 .1 eVresonance in ~°Se were calculated for both single-particle and giant dipole resonance models andseem to conform to the systematics in the Se mass region . Indications of non-statistical effects inthermal and resonance neutron capture were seen from the anomalous strength of the 7734 .0 keVtransition to the z- 293.1 keV state in ~SSe . This is supported by the single-particle nature of the293.1 keV state as is evident from a large (d, p) 1=1 spectroscopic factor to this state . The valencemodel for direct neutron capture mechanism does not, however, account for the effect .

E

THERMAL AND RESONANCE NEUTRON CAPTURE STUDIESIN Se TARGETS WTTH A =74, 76, 77, 78, 80

GIDEON ENGLER*, ROBERT E. CHRIEN and H.I . LIOU

Physics Department Brookhaoen National Laboratory, Upton, New York 11973, USA

Received 28 July 1981

NUCLEAR REACITONS'4''6'"''s .so~(n~ y) ; E=thermal; 27 .1, 271.5 eV'°Se ; 377.0,862 eV'6Se ; 112.0, 211 .6, 340.8, 864.0 eV "Se; 383.0 eV'sSe ; 1970 eV s°Se ; measuredE� I,. ; ~3'~~.~s''9"siSe levels deduced; ~s'~a'' 9Se 1, a of levels deduced ; resonannces J, ar

deduced; Q deduced; natural target .

1. Introduction

Experimental investigations of level schemes of '3~tSe have previously beencarried out by means of ß-decay and particle reactions t

_t6) andby thermal neutron

ca tore t'-zop

). These yielded rather complete information of energy levels up to afew MeV and partial assignments of spins and parities of the levels .Thermal capture provided primary and secondary y-ray energies and absolute

intensities as well as neutron separation energies . No measurements of resonanceneutron capture y-rays were, however, reported previously for any of the Seisotopes.

* Permanent address : NuclearChemistry Department, Soreq Nuclear Research Centre, Yavne, Israel .125

126

G. Engler et al. / Capture strengths

We have carried out a study of both thermal and resonance neutron capture ina target of natural selenium, viewing both primary and secondary y-rays . Thiscombined with previous thermal data on separated isotopes yields newinformation,i.e . spin-parities for all measured resonances and possible spin-parities for 20low-lying energy levels.The Se isotopes measured in this experiment are of particular interest because

with'SSe the neutrons start to fill the lg9iz orbit from the simple shell model andsimple shell effects in addition to collective effects can be expected . In the presentexperiment the negative-parity states, i- and i-, of the odd-mass Se nuclei areexpected to be populated because E1 primary y-transitions from s-wave resonanceslead to such states . These states are also populated via (d, p) 1= 1 reactions. It wastherefore of interest to look for possible correspondence between (n, y) and (d, p)strengths along the odd-mass Se nuclei and thus gain more insight into both thermal

and resonance nature of capture states .Throughout the paper the target isotope will be designated when referring to

resonance properties, while the residual nucleus isotope will be specified whenreferring to levels populated by the (n, y) reaction .

2. Eaperlmental methods and analysis

Two different facilities were used for thermal and resonance neutron capture

experiments, respectively . In both cases natural Se samples were irradiated at the

High Flux Beam Reactor at Brookhaven National Laboratory .

For the thermal capture experiment an 8.7 g sample, 3.17x3 .17cm2, was used

at tailored-beam facility. This facility uses a three-crystal-pair spectrometer with

excellent peak to background ratio Zl ). Gamma rays up to 11 MeV were measured .

The spectrum from a carbon target was used for background peak identification .

For the neutron resonance capture experiment, a 453 .1 g sample 12.7 x9.8 cm2

was bombarded at the 22m and 48 m stations of the time-of-flight fast chopper

facility . The rotor was operated at 15Krpm and the time-of-flight channel width

was 1 ~.s. A 40 cm3 coaxial Ge(Li) detector with resolution of 7 keV for 8.5 MeV

y-rays viewed the sample at 45° to the incident neutron beam . The detector was

coupled to an on-line computer (PDP 11/20) where both the neutron time-of-flight

and y-ray spectra were recorded in event mode on magnetic tape . The primary

and secondary y-rays were recorded in two separate runs of 3000-11 000keV and

100-2500 keV intervals, respectively .The y-rays were analyzed by using a least squares code with a gaussian response

function to fit the line intensities . This was applied directly to the thermal spectra.

The y-rays of the neutron resonances were, however, obtained by scanning the

tape through the desired neutron energy intervals, i.e ., the resonance areas as well

as off resonance areas for background subtraction . Spectra free of y-ray background

were then obtained . The y-rays of ten resonances were subsequently analyzed bythe least squares computer code .

In order to make an absolute energy calibration and to correct for the non-linearity in the detection system for the primary y-rays, the well-known capturelines in 36C1 [ref . 2t)] were used . The lines were obtained by irradiating a CCi4target . The uncertainty in the calibration was estimated to be 0.3 keV, whichoriginates from the uncertainty in the energy of the 36C1 lines . Theenergy calibrationfor secondary lines was carried out with a source of Z3°'I'h .Absolute intensities of primary capture y-rays for both thermal and resonance

neutron energies were obtained relative to t9'Au(n, y) forwhich the y-ray intensitiesof thermal energy and of the 4.91 eV resonance are well known 22)

. For thismeasurement a Se target and a gold foil with an identical area were combined andirradiated together .The absolute intensities for thermal data were determined for all isotopes by

direct comparison to t98Au. Asystematic error of 20% is estimated in the intensitiesdue to uncertainty from this comparison . For the resonance capture y-rays the27.1 eV resonance in'4Se was used as a calibration reference. The y-ray intensitiesof the 27 .1 eV resonance were calibrated relative to the 4.91 eV resonance capturein t9'Au. All the other Se resonance capture y-ray intensities were then calibratedrelative to the y-rays of the 27.1 eV resonance using the net neutron resonance

aoo

320

160

80

0

G. Engler et al. / Capture strengtks

127

3. Gamma-ray energies snd intensities

4320

4480

4640

4800

4960CHANNEL NUMBER

Fig . 1 . A portion of the primary y-ray spectrum for Se isotopes in thermal capture using the TailoredBeam facility . The spectrum consists of DE peaks only . The mass numbers in parentheses are of the

product nuclei .

128

G. Engler et al. / Capture strengths

4

~

__

"uj _

$ ~` = n~n~w

fvI °o °'

N O;

I

~~

l'. ~~ ~ ~

,~ ~~ ~~A^,hlj._"�,sg1Ji% I~YYV'~w4yV~~W~1,.,'~..l~ll ..;l

-

~

r

SE and DE peaks.

Fig. 2 . Time-of-flight spectrum showing the entire region of the resonance capture experiment for Seisotopes . The mass numbers in parentheses are of the target nuclei .

peak areas for normalization. This method introduces systematic uncertainties inthe intensity calibration and together with a systematic uncertainty in the detectorefficiency the uncertainty is estimated to be 30% for all resonances except for the27 .1 eV resonance, where it is estimated to be 20% .

Fig. 1 displays part of the primary y-rays of the thermal spectrum . Fig. 2 showsthe time-of-flight spectrum for the 48 m station. The 27.1 eV resonance in'4Se isparticularly well resolved while all the other resonances are less resolved and withpoorer statistics . Figs . 3~ show parts of the net primary y-ray spectra in resonancecapture, for each of the measured Se isotopes taken at the 48 m station. It may benoted that the most detailed spectrum was obtained for the 27.1 eV resonance in'4Se (fig . 3), which is the most prominent resonance in the present work . Secondaryy-rays were measured but are not presented here. They are, however, availableon request from the authors.

~~ ,--r-r- ., --rlß5e

27. I eV RESONANGE

~~

In

_

ó

ó__

~

~Á

w

U 1

I

I

I

1

I

I

I

I

I

I

I

I

I

I

I

I

1

I

I

I

I

I

'~.I

I

I1000 1160 320 1480 I

I

I

IGhUW1~EL NUMBER

Fig. 3. A portion of the net primary y-ray spectrum of 75 Se of the 27 .1 eV resonance. Shown are FE,

G. Engler et al. / Capture strengtlu

129

Fig . 4. A portion of the net primary y-ray spectra of TSe and 79Se of the 377.0 and 383.0 resonances,respectively. The time-of-flight spectrum of the two resonances is not resolved . Shown are FE, SE and

DE peaks.

loo

b 6p

N2

1970 eV RESONANGE

SE and DE peaks.

01

-L _- ":-_

_

-_.

L- .. -. I

. .

L ..-.440

560

680

~ 800

920

1040 ~

1160

~~128ÓCHANNEL NUMBER

Fig. 5 . A portion of the net primary y-ray spectrum of'BSe of the 211.6 eV resonance. Shown are FE,

I ~

^

o,

~

I

1ma

~

á

I I

Fig. 6 . Aportion of the net primary y-ray spectrum of s'Se of the 1970 eV resonance . Shown are FE,SE and DE peaks.

130

G. Engkr et al. / Capture strengths

4.1 . THERMAL CAPTURE

4. Results

Thermal capture measurements were performed on '4''6'"''8'soSe . A listing ofprimary y-ray energies corrected for non-linearity together with energies of theexcited levels and absolute intensities are given in tables 1-5 for capture in '4Se,'6Se, "Se, 'aSe and a°Se, respectively . Since a natural target was used, onlytransitions which could be identified from previous measurements are listed withcertainty . 46 weak transitions which were not found before were attributed to"'' gSe and are listed as well . Based on intensity considerations these could not beattributed to the other Se nuclei .The thermal data enabled the extraction of neutron separation energies by

averaging over several known y-ray cascades for each case . They are listed in table6 together with previously measured values for comparison. As may be seen fromtable 6 there is a difference of 6 .2 keV between the present value and that of ref . t ')for '4Se. However, the present value, 8027.5 t 1.5 keV is consistent with thecompiled value of 8027 .4f 1 .5 keV [ref . 6

)] . Moreover, the present value is con-sistent with a level scheme previously compiled by ref . ó), and shows that thestrongest transition of 7734.0 keV populates the 293 .1 keV level rather than the286.6 keV level as stated in ref . ") . The intensities in ref . ") are systematicallyhigher than the ones in the present work .Using a maximum likelihood method to estimate the mean values of radiative

widths, (IY)rT, and assuming a Porter-Thomas distribution, strength functions were

TAHLE 1

Primary y-rays from thermal capture in'°Se(n, y)'sSe

°) The errors are purely statistical ; a systematic error of 0.3 keV is believed to be associated witheach of them (see text).

n) The errors are purely statistical ; a systematic error of 20% is believed to be associated with eachof them (see text).

E,, (keV) "~ E, (keV) `) I,. (%)q E,. (keV) °) E~ (keV) °) I,, (%)q

7734.0 (1) 293.1 (4) 22 .0 (1 .0) 5430 .5 (5) 2596.8 (6) 0.6 (1)7440 .9 (1 .2) 586.2 (1 .3) 0.4 (1) 5396 .3 (5) 2630.9 (6) 0.6 (1)7131 .3 (1 .2) 895 .8 (1 .3) 0.2 (1) 5289 .8 (4) 2737.5 (5) 0.8 (1)7064 .4 (4) 962.8 (5) 0.7 (1) 5247 .2 (9) 2780.1 (1 .0) 0.6 (1)7006 .7 (2) 1020.4 (4) 1.3 (1) 5140 .9 (2) 2886.4 (4) 1.6 (2)6843 .2 (1 .2) 1184.0 (1 .3) 0.1 (1) 5086 .9 (4) 2940 .4 (5) 2.2 (4)6828 .3 (3) 1198 .8 (5) 0.7 (1) 4874 .4 (7) 3153 .0 (8) 0.9 (3)6437 .5 (9) 1589.6 (1 .0) 2.7 (4) 4844.4 (1 .2) 3182.9 (1 .3) 0.4 (2)6223.9 (1 .1) 1803.3 (1 .1) 1 .5 (5) 4816.9 (6) 3210 .4 (7) 0.5 (2)6084.6 (1 .0) 1942.6 (1 .1) 0.6 (2) 4692 .5 (1 .2) 3334.8 (1 .3) 1.1 (4)5861 .0 (9) 2166 .5 (1 .0) 0.4 (1) 4686.9 (1 .2) 3340 .4 (1 .3) 1.0 (3)5570.4 (1 .9) 2456.9 (1 .9) 1 .1 (2) 4407.4 (1 .0) 3619 .9 (1 .1) 1.1 (4)5462.3 (6) 2564.9 (7) 0.6 (2)

G. Engler et al. / Capturc smngths

131

TABLE 2Primary y-rays from thermal capture in ~6Se (n, y)~~Se

') See table 1 .b) See table 1 .`) This transition is either from '6Se(n, y)"Se or ~'Se(n, y)'eSe (see text) ; these transitions are

therefore listed twice, i .e ., in the present table and in table 3.

E,. (keV) `) Ex (keV)') I,. (%) n) E,. (keV)') Ex (keV) °) I, (%) q

7418 .5 (2) 0.0 5.1 (2) 5276 .4 (1) 2142.3 (2) 0.33 (2)7247 .2 (3) ~ 171 .3 (4) 0.07 (1) 5241 .8 (3) `) 2176 .8 (4) 0.12 (2)7241 .7 (2) 176.7 (3) 0.11 (1) 5228 .7 (8) ~ 2189 .9 (9) 0.04 (1)7179 .4 (2) 239.1 (3) 3 .6 (2) 5205 .3 (6) 2213 .4 (7) 0.79 (6)7057 .9 (2) ~ 360.5 (3) 0.11 (1) 5169 .8 (4) 2248 .8 (5) 0.55 (4)6966.3 (4) ~ 452.2 (5) 0.03 (1) 5154 .2 (6) 2264 .4 (6) 0.8 (2)6897 .7 (2) 520.8(3) 0.13 (1) 5098 .E (7) 2320 .0 (8) 0.55 (9)6762 .3 (1) ~ 656.1 (2) 0.18 (1) 5078 .8 (2) 2339 .9 (3) 0.47 (5)6732 .1 (4) `) 686.4 (5) 0.02E (5) 5058 .0 (6) `) 2360 .E (7) 0.0E (2)6658 .2 (3) ~ 760.3 (4) 0.05 (1) 5043 .8 (4) 2374 .8 (5) 0.11 (2)6600 .5 (2) 818.0 (3) 8.8 (3) 5025 .8 (2) 2392 .9 (3) 2.3 (1)6557.2(2) `) 861 .3 (3) 0.021 (3) 5018 .3 (5) `) 2400 .3 (6) 0.13 (3)6523 .2 (3) `) 895 .3 (4) 0.047 (6) 4961 .E (9) 2457 .0 (9) 0.57 (8)6506 .E (1) 911 .9 (2) 0.22 (1) 4943 .4 (7) `) 2475 .2 (8) 0.12 (4)6412 .5 (1) 1006 .0 (2) 2.6 (1) 4926.5 (4) 2492 .2 (5) 0.73 (7)6394 .3 (2) `) 1024 .2 (3) 0.085 (9) 4902.9 (1 .3) ~ 2515.7 (1 .3) 0.0E (6)6290.E (2) 1127 .9 (3) 0.08 (1) 4865 .3 (5) 2553 .3 (6) 0.11 (2)6231 .0 (1 .1) 1187 .5 (1 .2) 1.3 (2) 4834 .E (6) `) 2584 .0 (7) 0.0E (2)6215 .4 (6) ~ 1203 .1 (7) 0.09 (2) 4798 .1 (6) `) 2620 .5 (7) 0.04 (1)6200 .3 (6) ~ 1218.2 (7) 0.0E (2) 4788.1 (6) ~ 2630 .5 (7) 0.04 (1)6182 .4 (3) `) 1236 .2 (4) 0.042 (8) 4777 .9 (2) 2640 .8 (3) 0.25 (3)6173 .1 (6) `) 1245 .4 (7) 0.022 (6) 4770.0 (5) ~ 2648.E (6) 0.07 (2)6139 .2 (2) ~ 1279.4 (3) 0.058 (6) 4719 .0 (4) ~ 2699.E (5) 0.08 (2)6091 .5 (5) ~ 1327 .1 (6) 0.0E (2) 4701 .8 (9) 2716 .E (9) 0.51 (6)6055 .2 (4) 1363 .3 (5) 0.15 (2) 4680 .9 (6) ~ 2737.8 (7) 0.08 (2)6016.7 (1) 1401 .8 (2) 1 .4 (1) 4673 .7 (6) `) 2745 .0 (7) 0.07 (2)6006.7 (2) 1411 .9 (3) 4.3 (2) 4660 .0 (6) `) 2758 .E (7) 0.09 (2)5989.5 (1 .1) ~ 1429 .1 (1 .1) 0.04 (2) 4642 .0 (2) 2776 .7 (3) 0.42 (3)5930 .E (1) 1487.9 (2) 0.14 (1) 4629 .7 (6) ~ 2788 .9 (7) 0.08 (2)5907 .E (1) 1511 .2 (2) 0.27 (1) 4609 .E (2) 2809 .0 (3) 0.88 (6)5824 .7 (4) `) 1593 .9 (5) 0.04 (1) 4603 .1 (5) 2815 .E (6) 0.34 (4)5808 .3 (2) ~ 1610.3 (3) 0.22 (3) 4596 .5 (8) `) 2822 .2 (9) 0.07 (2)5795 .4 (2) 1623.2 (3) 1.91 (1) 4565 .0 (6) 2853 .E (7) 3.8 (3)5760 .9 (6) ~ 1657.E (7) 0.04 (1) 4545 .7 (5) 2873 .0 (6) 1.0 (1)5710 .5 (2) ~ 1708 .1 (3) 0.18 (2) 4526.9 (4) 2891 .8 (4) 2.0 (2)5704 .0 (1) 1714 .E (2) 0.53 (2) 4517.8 (7) `) 2900 .8 (8) 0.10 (3)5686 .3 (3) ~ 1732 .3 (4) 0.08 (1) 4503 .9 (8) 2914 .7 (8) 0.20 (4)5627 .8 (i .l) ~ 1790 .8 (1 .1) 0.02 (1) 4470 .9 (9) ~ 2947.7 (9) 0.09 (3)5601 .0 (3) 1817 .8 (4) 4.3 (2) 4460.8 (7) 2957 .8 (8) 0.17 (3)5588 .4 (2) `) 1830 .2 (3) 0.25 (3) 4435 .9 (5) 2982 .8 (6) 0.64 (6)5503 .5 (2 .0) 1915 .0 (2 .0) 0.19 (4) 4424 .0 (5) 2994 .E (6) 0.25 (4)5493 .5 (4) `) 1925 .1 (5) 0.09 (2) 4412 .8 (5) 3005 .9 (6) 0.28 (4)5411 .8 (5) `) 2006 .8 (6) 0.041 (9) 4378 .3 (4) 3040 .4 (4) 1.41 (1)5364.5 (4) `) 2054 .1 (5) 0.0E (1) 4367 .8 (3) 3050.9 (4) 0.50 (5)5344.7 (2) ~ 2073.9 (3) 0.08 (1) 4354 .7 (4) 3063 .9 (5) 0.67 (6)5317 .5 (3) `) 2101 .1 (4) 0.09 (2)

132

G. Engler et al. / Capture strengths

TABLE 3Primary y-rays from thermal capture in "Se(n, y)' BSe

E,. (keV) °) Ex (keV) ") I,, (% ) b) E, (keV) °) E° (keV) °) I,. (%) b)

10497.0 (3) 0.0 0.70 (4) 5824.7 (4) ~ 4672.8 (5) 0.11 (3)9883.2 (1) 614.0 (3) 7.3 (4) 5813 .9 (1) 4683 .7 (3) 0.85 (7)9188.5 (1) 1308 .7 (3) 4.8 (2) 5808.3 (2) `) 4689 .3 (4) 0.55 (6)8998 .2 (1) 1499 .0 (3) 0.19 (2) 5775 .3 (1) 4722 .3 (3) 0.54 (4)8738 .E (4) 1758 .E (5) 0.05 (1) 5760.9 (6) ~ 4736.E (7) 0.09 (3)8501 .3 (1) 1996 .0 (3) 1.6 (1) 5710.5 (2) `) 4787 .0 (4) 0.43 (4)8170 .2 (1) 2327 .4 (3) 1.6 (1) 5686.3 (3) `) 4811 .2 (4) 0.19 (3)8162 .E (1) 2334 .8 (3) 2.0 (1) 5680.0 (4) 4817 .E (5) 0.13 (3)7959 .E (1) 2537 .7 (3) 0.49 (2) 5627.8 (1 .1) `) 4869 .8 (1 .1) 0.05 (3)7815 .3 (3) 2682 .1 (4) 0.08 (1) 5615 .5 (9) 4882 .0 (1 .0) 0.11 (4)7743 .7 (2) 2753 .E (4) 0.57 (4) 5588.4 (2) ~ 4909.2 (4) 0.61 (8)7658 .5 (1) 2838 .9 (3) 0.5E (2) 5574.E (6) 4923 .0 (7) 0.18 (4)7598 .9 (1) 2898 .4 (3) 0.43 (2) 5500.4 (1 .9) 4997 .2 (1 .9) 0.5 (1)7491 .7 (1) 3005 .7 (3) 0.90 (3) 5493 .5 (4) ~ 5004 .1 (5) 0.22 (5)7310 .4 (1) 3187 .0 (3) 0.23 (2) 5436 .4 (5) 5061 .2 (6) 0.10 (3)7255 .3 (1) 3242 .1 (3) 0.3E (2) 5411 .8 (5) `) 5085 .7 (6) 0.10 (2)7247 .2 (3) ~ 3250.E (4) 0.17 (3) 5370 .7 (3) 5126 .9 (4) 0.52 (4)7208 .9 (1) 3288 .E (3) 1 .9 (2) 5364 .5 (4) ~ 5133 .1 (5) 0.1E (3)7113 .E (1) 3383 .8 (3) 1 .1 (1) 5344 .7 (2) `) 5152 .9 (4) 0.20 (3)7057 .9 (2) `) 3439 .9 (4) 0.2E (2) 5333 .8 (7) 5163 .8 (8) 0.3E (8)7046 .0 (1) 3451 .4 (3) 0.29 (2) 5317 .5 (3) `) 5180 .1 (4) 0.22 (4)6974 .1 (1) 3523 .4 (3) 0.2E (2) 5241 .8 (3) ~ 5255.8 (4) 0.30 (3)6966 .3 (4) ~ 3531 .1 (5) 0.07 (1) 5228 .7 (8) ~ 5268 .9 (9) 0.09 (3)6905 .7 (1) 3591 .8 (3) 0.74 (3) 5156 .1 (6) 5341 .5 (7) 0.2 (1)6873 .E (1) 3623 .9 (3) 0.43 (2) 5107 .E (4) 5390.0 (5) 0.31 (6)6810.E (1) 3686 .8 (3) 0.82 (3) 5058 .0 (6) `) . 5439.E (7) 0.14 (5)6762.3 (1) ~ 3735 .1 (3) 0.43 (2) 5018 .3 (5) ~ 5479 .3 (6) 0.31 (6)6732.1 (4) ~ 3765 .4 (5) 0.0E (1) 4982 .8 (1 .0) 5514 .8 (1 .0) 0.21 (7)6658.2 (3) ~ 3839 .3 (4) 0.12 (2) 4962 .3 (9) 5535 .3 (9) 0.21 (9)6557 .2 (2) `) 3940 .3 (4) 0.051 (6) 4943 .4 (7) `) 5554 .2 (8) 0.28 (9)6523.2 (3) ~ 3974 .3 (4) 0.11 (2) 4902 .9 (1 .3) `) 5594 .7 (1 .3) 0.13 (6)6498 .5 (1) 3999 .0 (3) 1.35 (4) 4856 .2 (6) 5641 .4 (7) 0.18 (4)6460 .3 (1) 4037 .2 (3) 0.42 (3) 4834.E (6) ~ 5663.0 (7) 0.15 (4)6394 .3 (2) `) 4103 .2 (4) 0.21 (3) 4810.7 (5) 5686 .9 (6) 0.23 (3)6344 .4 (2) 4153 .1 (4) 0.33 (2) 4798 .1 (6) ~ 5699 .5 (7) 0.09 (3)6315 .2 (1) 4182 .3 (3) 1.3 (1) 4788.1 (6) ~ 5709 .5 (7) 0.10 (3)6243 .9 (1) 4253 .E (3) 1.25 (7) 4770.0 (5) ~ 5727.E (6) 0.18 (4)6215 .4 (6) ~ 4282.1 (7) 0.21 (4) 4719.0 (4) ~ 5778.E (5) 0.18 (4)6200 .3 (6) ~ 4297 .2 (7) 0.1E (4) 4689.0 (6) 5808 .E (7) 0.20 (7)6182 .4 (3) ~ 4315.2 (4) 0.10 (2) 4680.9 (6) ~ 5816.7 (7) 0.18 (5)6173 .1 (6) ~ 4324.4 (7) 0.05 (1) 4673.7 (6) ~ 5824.0 (7) 0.18 (S)6156 .3 (1) 4341 .3 (3) 0.3E (2) 4660 .0 (6) ~ 5837.E (7) 0.21 (5)6139.2 (2) `) 4358.4 (4) 0.14 (2) 4629 .7 (6) `) 5867 .9 (7) 0.18 (5)6131 .1 (1) 4366.4 (3) 0.34 (2) 4623 .E (7) 5874.0 (8) 0.1E (4)6110.2 (3) 4387 .3 (4) 0.30 (4) 4596 .5 (8) `) 5901 .2 (9) 0.18 (5)6091 .5 (5) ~ 4406.0 (6) 0.1E (4) 4517 .8 (7) `) 5979 .8 (8) 0.24 (7)6048.8 (4) 4448 .9 (5) 1.2 (1) 4501 .7 (1 .3) 5996.0 (1 .3) 0.19 (5)6028.3 (2) 4469 .2 (4) 0.60 (6) 4470 .9 (9) ~ 6026 .7 (9) 0.21 (6)5989.5 (1 .1) ~ 4508 .1 (1 .1) 0.10 (4) 4455 .1 (6) 6042 .E (7) 0.57 (9)

°) See table 1.n) See table 1 .

4.2 . RESONANCE CAPTURE

G. Engler et al. / Capture strengths

133

TABLE 3 (continued)

°) See table 1.b) See table 1 .`) This transition is either from '6Se(n, y)"Se or "Se(n, y)'sSe (see text) ; these transitions are

therefore listed twice, i.e ., in the present table and in table 2.

TABLE 4Primary y-rays from thermal capture in'sSe(n, y)'9Se

`) See table 1.q See table 1 .~ The large errors are mainly due to the error in the Q-

value, i.e ., 696113 keV [ref. ra)] .

TABLE SPrimary y-rays from thermal capture in s°Se(n, y)srSe

calculated for both single-particle and giant dipole resonance models for electricdipole transitions. These are given in table 7. Not enough evidence is available todiscriminate between the two models but the reduced strengths fit clearly into thesystematics of the Se region za) .

Resonance capture measurements were performed on '4''6'"''a'8o3e. Spin andparity assignments of resonances together with the resonance capture y-ray energies

E,, (keV) `) E~ (keV) `) I,. (%)

6866 .2 (1) 94 .7 (3 .0) `) 19 .0 (1 .0)6435 .0 (1 .3) 525.7 (3 .3) ~ 1 .9 (6)5875 .1 (1 .5) 1085 .5 (3 .4) `) 5.5 (1 .7)5223 .1 (8) 1737 .7 (3 .0) ~ 3.1 (7)

E,. (keV) °) E~ (keV) °) I,. (%) °) E,. (keV) °) E, (keV) °) I,, (%)

5968 .7 (2) 4528 .8 (4) 0.26 (2) 4448 .8 (6) 6048 .9 (7) 0.39 (8)5944 .2 (1) 4553 .4 (3) 0.50 (3) 4390 .2 (5) 6107 .5 (6) 0.24 (5)5875 .3 (1 .5) 4622 .2 (1 .5) 0.14 (5) 4377 .7 (6) 6120.0 (7) 0.16 (5)5852 .6 (3) 4644.9 (4) 0.21 (3)

E,. (keV) `) Ex (keV) °) I,. (%) b)

6232.9 (1 .1) 467 .9 (1 .3) 29 .0 (3 .6)5467 .6 (8) 1233 .2 (1 .0) 2.0 (4)4975 .9 (4) 1725 .0 (7) 8.8 (1 .0)

134

G. Engltr et al. / Captun strengths

TABLE 6Neutron separation energies for Se isotopes

`) Ref. l') .

~ Ref. 6) .

~ Ref.23) .

TABLE 7El strength functions for single-particle (s .p .) and giant dipole

resonance (GDR) models for Se isotopes

°) From thermal capture .d) From 27 .1 eV resonance capture.

of the product nuclei corrected for non-linearity, as well as the energies of theexcited levels, absolute intensities and spin and parity assignment of energy levelsare given in table 8. Only y-rays which could be fully identified, i.e ., for which theFE, SE and DE peaks were seen, are listed in table 8.4.2.1. Spin-parity of resonances. The results of the resonance spin-parities are

summarized in the third column of table 8. In the resonance discussion below themasses of the target nuclei are referred to (rather than the product nuclei, whichare listed in the first column of table 8) .

(a) '4Se and '6Se targets . Since the targets are even~ven nuclei with strongtransitions to i- states the resonances 27.1 eV and 271.5 eV in'4Se and 377.0 eVand 862 eV in'6Se were assigned as s-wave resonances with positive parity, i.e .,z+ resonances .

(b) "Se target. Basedon ref. zs) the 112 .0 eV,211 .6 eV and 340.8 eV resonanceshave spin-parities of 1-, 0- and 1 -, respectively . The assignment of the 340.8 eVresonance is also supported by a strong assumed El transition to the 2+ 614.0 keVlevel. For the 864.0 eV resonance a spin-parity of (1-) was assigned based on the

Targetisotope

Compound nucleus(target

isotope+neutron)

Separation

present

energies (keV)

others

74 75 8027 .5 f0.4 8021 .3 t 1.0 `)8027 .4 t 1.5 b)

76 77 7418 .8 t 0.2 7418 .0 t0.8 `)77 78 10497.8 t 0.3 10497.0 t 1 .0 `)80 81 6701 .1 t 0.6 6701 .2 t 1.2 ~

Targetisotope

s.p .109k(El) (MeV-3 )

GDR10'sk(El) (MeV-S)

74`) 1.4t0.6 5.6t2.674~ 1.St0.7 6.7t3.176`) 1.2t0.4 7.1t2.377`) 1.7t0.5 6.6t2.1

G. Engler et al. / Capture strengths

135

TAHLE g

Spin-parities for resonances, primary y-rays and spin-parities for energy levels of Se isotopes fromneutron resonance capture

') From ref. ss) for the target nuclei.`~ The quoted energies are those of thermal capture; this is because very good agreement exists

between the transition energies in thermal and resonance capture.`) The errors are purely statistical; a systematic error of 20% is believed to be associated with the

27 .1 eV resonance transition strengths and 30% for the other resonances (see text).a) From ref. e) for ~sSe, ref. ")for "Se, ref.' 3) for'sSe, ref.' °) for'9Se and ref.'6) for e'Se.`) This transition was not seen in the present thermal capture experiment.~) This transition is seen for the first time.~ The large errors are due to the error in the Q-value, i .e., 6961 t3keV [ref .' s)] .

Productnucleus

Resonance

(eV)') l~present

E,n)(keV)

Ex(keV)

h `)(%)

1' of

others °)

levels

present

75 27 .1 i+ 7734.0 (1) 293.1 (4) 14 .2 (2) i-

7006 .7 (2) 1020.4 (4) 1.4 (2) i, i- z-, i-6828.3 (3) 1198 .8 (5) 1 .4 (2) z, s-6437 .5 (9) 1589.E (1 .0) 3.2 (4) i-, i_6223.9 (1 .1) 1803.3 (1 .1) 1.3 (1) z-, i-6041.2 (3) °) 1986.0 (5) 0.6 (1) z, z5861.0 (9) 2166 .5 (1 .0) 0.8 (1) z,5570.4 (1 .0) 2456.9 (1 .0) 1.3 (1) i-, z-5462 .3 (6) 2564.9 (7) 0.9 (1) z, z5430 .5 (5) 2596.8 (6) 1.4 (1) i-, z-5396.3 (5) 2630.9 (6) 0.7 (1) Z, z5289.8 (4) 2737 .5 (5) 0.7 (1) z~ i

271 .5 i` 7734.0 (1) 293.1 (4) 12 .9 (1 .4) i_

7131 .3 (1 .2) 895.8 (1 .3) 3.0 (3) i, i_ i-, z-7064.4 (4) 962.8 (5) 3 .3 (3) i, z- i-, i-

77 377.0 z+ 7418 .5 (2) 0.0 36.2 (4 .3) i_

5025.8 (2) 2392.9 (3) 3 .0 (4) z-

862 i+ 7418 .5 (2) 0.0 15 .5 (3 .0) i-

78 112.0 1 - 8501.3 (1) 1996 .0 (3) 1 .8 (2) (2 +) 1+, 2+211 .E 0- 7057.9 (2) 3439.9 (4) 2.8 (2) 0+, 1+

6344.4 (2) 4153 .1 (4) 1.1 (1) 0+, 1+6252.2 (3) r) 4245.3 (5) 1.2 (3) 0+, 1+6131 .1 (1) 4366.4 (3) 1.0 (1) 0+, 1+

340.8 1 - 9883.2 (1) 614.0 (3) 14 .2 (1 .0) 2+7491 .7 (1) 3005 .7 (3) 1.6 (2) (1, 2) 1+, 2+

864.0 (1 -) 9188 .3 (1) 1308 .7 (3) 3 .4 (7) (2+)79 383.0 z

+6866.2 (1) 94 .7 (3 .0) ~ 1 .9 (4) z

-

5875 .3 (2) 1085 .5 (3 .0) s) 7.8 (1 .4) z-, z-81 1970 (z +) 6232 .9 (1 .1) 467.9 (1 .3) 37 .2 (2 .4) ~-, i_

4975 .9 (4) 1725.0 (7) 4.4 (3) (i+, ~+)

136

G. Engler et al. / Capture strengths

transition to the (2+) 1308.7 keV level with the negative parity also being consistentwith the parity of the target nucleus.

(c) 'gSe target. The assignment for the 383.0 eV resonance is i+ based on ref. z3) .

(d) a°Se target. The 1970 eV resonance is assigned (z+) which is consistent withan assumedE1 transition to the i-, z_ 467.9 keVlevel andan assumedM1 transitionto the (z+, i+) 1725.0 keV Ievel.4.2.2 . Spin-parity of energy levels in the product nuclei . Spins and parities were

determined for levels in'S'' 8''9Se and are listed in the last column of table 8 .(a) 'SSe . For the 27.1 eV resonance the results of the analysis of 11 transitions

are presented. The excited levels are assigned spin's of z, z because for either E1or Ml transitions there is a possibility of dJ = 0, 1, respectively . All transitionscomparable in strength to the 7006 .7 keV transition feeding the z-, z 1020.4 keVlevel e) were assumed to be E1 transitions and consequently negative parities wereassigned to all of these levels . For other levels no parities were assigned becauseno Ml transitions could be ruled out. The levels fed by transitions from the 271 .5 eVresonance were assigned i-, z- assuming E1 transitions .

(b) '8Se and '9Se . The spin-parity assignments are based on assumedE1 transi-tions for all resonances .

5 .1 . 'SSe

S. Non-statistical effects in the odd-mass 'a.rt.~.aiSe product nuclei

Very striking features were observed in the 'SSe nucleus. Especially noticed isthe evidence of non-statistical behavior in neutron capture as is apparent from the7734.0 keV transition to the 'z- 293.1 keV state. This assumption is supported byseveral independent results:

(a) A strong transition of 14.2%, to the 293.1 keV state exists for the 27.1 eVresonance in "Se. This strength is out of the range of the Porter-Thomas fluctu-ations, as the average strength calculated for this resonance was found to be

(IY)rrr=1.24% .(b) A strong transition of 12.9% to the 293.1 keV state exists also for the

271.5 eV resonance in'°Se .(c) It is estimated that out of the 51 .8 b [ref . zs)] of the thermal capture cross

section only 38.3 b are contributed by the 27.1 eV resonance tail . Thus, the strongtransition observed to the 293.1 keV state (thermal intensity ~22%) must be dueto bound-state components .

(d) The existence of non-statistical effects in capture is also indicated by experi-ments which point to single-particle nature of the 293.1 keV state. This is apparentfrom the (d, p) 1=1, of which about 75% is concentrated in this state s. ' °) . Thereseems to be therefore a correspondence between the single-particle nature of the293 .1 keV state and the 7734.0 transition strength to this state. Moreover, the

5.2 . "Se

5.3 . ~ 9Se

G. Engler et al. / Capture shengths

137

(d, t) 1=1 reaction, which selects hole states, does not feed the 293 .1 keV state .Its strength is rather concentrated in the z 286 .6 keV state x) to which no primarytransition was seen in the present experiment,The above experimental evidence points to a strong direct capture component

in thermal andresonance capture states . Twocalculations were therefore performedfor the 27.1 eV resonance in order to evaluate the possible nature of the resonancecapture state, i.e ., a valence-model calculation zs) to determine the direct capturewidth, I'�~, and a calculation of B(El)/B�,r(E1) [ref. 2ó )], the ratio between theexperimental reduced transition probability and the Weisskopf single-particle esti-mate, for the 7734.0 keV transition to the 293.1 keV state. It was found thatl'�~,=125 meVwhereas using a radiative width of 210 meV [ref. z3)] the width forthe 293.1 keV state is l',, = 210 x 14.2% = 29.8 meV. Hence, a valence contributionaccounts for only 4% of capture width. The other calculation resulted inB(E1)/Bw(E1) =0.19 MeV- ' . When compared to an average of ~--004 MeV-1 forall nuclides Za), this indicates an enhanced single-particle component in the capturingstate.For the 271 .5 eV resonance in'4Se, B(E1)/B,v(El) =0.18 MeV-', again pointing

to an enhanced single-particle content.In the other odd-mass nuclei, i.e ., ".79.s'Se, very strong transitions were seen to

the g.s. in "Se (in resonance capture), to the 94.7 keV state in '9Se (in thermalcapture) and to the 467 .9 keV state in g'Se (in both thermal and resonance capture) .The data are too scanty, however, to point clearly to a non-statistical nature forcapture in these isotopes .

Even though most of the transition strength for both the 377 .0 and 862 eVresonances, i.e ., of 36.2% and 15 .5%,respectively are concentrated in the transitionto the g.s ., there is no other evidence to support this case as being of non-statisticalnature . The thermal (n, y) strength is rather fragmented as is also the (d, p) l =1strength 1°) . (It should be noted however that about 35% of the (d, p) l =1 strengthis nevertheless concentrated in the g.s .) The B(E1)/Bv,(El) = 0.25 MeV-1 and0.11 MeV-' for the above transitions.

A strong transition of 19% is seen to the 94.7 keV state in thermal capture . Inresonance capture (383 .0 eV resonance), however, the strongest transition, of 7.8%,is to the 1085.5 keV state to which no (d, p) l =1 spectroscopic factor was found lo).

A (d, p) l =1 strength of about 30% was, however, found to the i- 94.7 keV levelto which only a transition of 1 .9% was seen in the present resonance captureexperiment. The B(E1)/Bw,(E1) = 0.065 MeV-' and 0.016 MeV- ' for the transi-

138

G. Engler etal. / Capture strengths

tions to the 1085.5 keV and 94.7 keV states, respectively, indicating that thetransitions indicate no single-particle enhancement.

5 .4, e1Se

In s1Se most of the transition strength for thermal as well as for the 1970 eVresonance capture, i.e ., 29% and 37.2% respectively, are concentrated in thetransition to the z- 467.9 keV state. However, no transitions at all were seen tothe i-g.s . where about 60% of the (d, p) 1=1 strength is concentrated while only15% (d, p) l =1 strength is concentrated in the 467.9 keV state 1°) . This mayindicate transitions populating preferably the i- hole state rather than the z-single-particle g.s., however the B(El)/B�,r(E1)~0~25 MeV-' for the transition tothe 467.9 keV state, indicating an enhanced transition . s1Se, therefore, presents anopposite case to what wasobserved for (n, y) and (d, p) ! =1 correspondence in'SSe .

6. Conclaeion

The analysis of y-ray and time-of-flight spectra from thermal and resonanceneutron capture in 'a.'e."''8'BOSe has resulted in accurate y-transition energies,absolute intensities, accurate level excitation energies, neutron separation energies,spin-parities for the measured resonances and assignments of spin-parities of levelsin'S"~s.~9Se .The present study provides some interesting results concerning neutron capture

mechanism in Se isotopes . Evidence for non-statistical effect in 'SSe is apparentfrom the strong 7734.0 keV transition to the'z- 293.1 keV state. This state appearsto be of single-particle nature to a great degree as is indicated by a large (d, p)! =1 spectroscopic factor to the state . The direct capture component as calculatedfrom the valence model does not account for the enhanced transition strength. Thestrength for this transition implies a photon strength of ~-0.2 Weisskopf units perMeV, which is considerably above the normal dipole strength of 0.04W.u . MeV-1 .For the other odd-mass Se nuclei, i.e ., ".'9.s'Se, no such non-statistical effects areclearly evident especially because no clear correspondence exists between (n, y)and (d, p) l = 1 strengths for these nuclei. Moreover, the (d, p) ! = 1 strengths forthese nuclei are rather fragmented whereas in (n, y), especially in resonance capture,intense transitions contribute mostly to a single state. Thus, even though thenon-statistical nature of the capture states is not clear for "''9' g1Se, anomalouslystrong transitions were seen in resonance capture for "Se (7418.5 keV transitionto g.s .), in thermal capture for'9Se (6866 .2 keV transition to 94.7 keV state) andin both thermal and resonance capture for s1Se (6232.9 keV transition to 467.9 keVstate) . In summary, the correspondence between (n, y) and (d, p) l =1 strengthsfor'S'"'' 9 ' e1Se is therefore expressed by intense E1 transitions in (n, y) and a largespectroscopic factor in the (d, p) p-wave reaction to the z- 293.1 keV state in'SSe ;

G. Engler et al. / Capture strengths

139

some correspondence in thermal capture for "''9Se but with only vague correspon-dence for "Se ; none at all for '9Se in resonance capture. There is an indicationfor anticorrelation in the strengths for g'Se.

Since 'SSe has 41 neutrons it starts to fill the lg9iz subshell according to thesimple shell model. The particular (n, y) behavior for this nucleus could be attributedto the simple p-state of such a nucleus. In order to test this assumption (n, y)experiments on neighboring nuclei with 41 neutrons should be conducted.'°Zn(n, y)'tZn seems to be especially appropriate for such an experiment whereabout 70% of the (d, p) 1=1 strength is concentrated in the z g.s ., a situation verysimilar to the case of "Se. 'ZGe(n, y)'3Ge and the higher odd-mass Ge isotopesare also of interest even though the (d, p) l =1 strength to all these nuclei is ratherfragmented . However, those experiments will allow a systematic study of the (n, y)behavior of Ge nuclei starting with 41 neutrons, similarly to the study of Se nucleiin the present experiment .

It is expected that conclusions concerning the nature of the capture state can bemade on the basis of a proposed structure of the lower p-levels, to which intenseE1 transitions from the capture state are observed . It is believed that the systematicstudies proposed here, of which the studies of the Se isotopes are only a beginning,will therefore contribute towards a better understanding of the nature of thermaland resonance neutron capture states .

Research has been performed under contract DE-AC02-76CH00016 with theUnited States Department of Energy . One of us (G.E .) would like to express histhanks for the hospitality that was extended to him during his sabbatical stay in1979 at Brookhaven National Laboratory . The authors would like to acknowledgehelpful discussions with Leon Peker.

References

1) E.K . Lin, Phys . Rev. 139 (1965) B3402) N.E . Sanderson, Nucl . Phys . A216 (1973) 1733) Y.K . Agarval, C.V.K . Baba, S.M . Bharathi, S.K . Bhattacherjee, B. Lal and B. Sahai, Pramana 3

(1974) 2434) T. Sugimitsu, Nucl. Phys. A224 (1974) 1995) K.O. Zell, H.-G. Friederichs, B. Heits, P. von Brentano and C. Protop, Z. Phys. A242 (1975) 276) D.J . Horen and M.B . Lewis, Nucl . Data Sheets 16 (1975) 257) N.E . Sanderson and R. Summers-Gill, Nucl . Phys . A261 (1976) 938) A. Shibuya, M. Dojo, T. Sugimitsu and N. Kato, Nucl . Phys . A301 (1978) 159) K.O . Zell, H.-G. Friederichs, B. Heits, D. Hippe, H.W . Schuh, P. von Brentano and C. Protop,

Z. Phys . A276 (1976) 37110) L.A. Montestruque, M.C. Cobian-Rozak, G. Szeloky, J.D. Zumbro and S.E. Darden, Nucl . Phys .

Á30S (1978) 2911) B. Singh andD.A. Viggars, Nucl. Data Sheeta 29 (1980) 7512) R.M. Lieder and J.E . Draper, Phys . Rev. C2 (1970) 53113) P.P . Drone and F.E . Bertrand, Nucl . Data Sheets 15 (1975) 10714) P.P. Drone and D.C . Kocher, Nucl . Data Sheets 15 (1975) 257

140

G. Engkr et al. / Capture strengths

15) K.O . Zell, W. Gast, D. Hippe, W. Schuh and P. von Brentano, Z. Phys. A292 (1979) 13516) J.F . Lemming, Nucl. Data Sheets 1S (1975) 13717) M.R . Akhmed, A.M . Demidov, M.A. Khalil and F.A . Khussein, Bull . Acad . Sci . USSR (phys . ser.)

37 (1973) 7018) D. Rabenstein and H. Vonach, Z. Naturf. 260 (1971) 45819) M.R . Akhmed, A.M . Demidov, M.A . Khalil and S. Al-Nazar, Bull . Acad. Sci. USSR (phys. ser.)

37 (1973) 7920) LF. Barchuk, D.A . Bazavov, G.V. Belykh, V.I. Golyshkín, A.V . Murzin and A.F. Ogorodnik,

Bull. Acad. Sci . USSR (phys . ser.) 34 (1971) 157921) M.L. Steps and R.E. Chrien, Nucl. Instr . 15S (1978) 25322) W.R. Kane, private communication23) S.F. Mughabghab and D.P. Garber, Brookhaven National Laboratory Report, BNL-325, 3rd ed .

(1973)24) C.M..McCullagh, thesis (1979) unpublished25) J.E . Lynn, The theory of nuclear resonance reactions (Clarendon, O~áord, 1968) p. 33026) M.A. Lone, in Neutron capture gamma-ray spectroscopy (Plenum, NewYork, 1979) p. 161