.-

I

AllTiiOU(S): S. Plattard, G. F. Auchmpaugh, S. K. Hill,

C. de Sauwiurc, R. B. Perez and J. A. Harw::

SUB.M1’17’ED ’m: lntcrna~ional Confcrcncc on ::uclear cross

Sectionfi for Technology, October 22-26, 197’1

.-.

mos

Fam NILw R2k No. 1629l/7u DEPAR’N-OF ENERGY

CwT’RAc2’w740mNam W

!iI~;li-RESOLL710% FISSIOS CROSS SECTIOS OF ~31pa*

S. Plmttard,+ G. F. AuchmpaughLhlvcrsity of California, Los Almnm Scientific Labrmcory

Low Alamos, New Nexico 87545 HA

s. K. Iiill,G. de Saussure, R. 8. Perez aud J. A. HarveyOak Ridge XatSonal Laboratory

Oak RidSe, Tenneseee 37830 l%A

A high-resolution fia~ion crowi section for231

Pa was ncasured at

CJREIAfro= 0.1 to 12 YcV and between 0.4 A’ and lt’1kcl’. T!iu data show

uvidcncc for 1) fractio~;atcd vibrational structures in t!~e threshold

region of the fission cross section, and 2) narrow fl’~~’m r.~omnccs

ahovu 1 . 3 Q,’.-xitl;an av(’ragefissim: width .:7f> w ~ .cY.

-2311 !%(n,f) rr.Jss s~”~.tion (l.: ~.~; - ](I kuq: and [1.1 - 12 ?!u.V,mmsurcd

!’k.lolh’ 60 (2’:.‘f 1

~ntroduction

231Previous mwsurucwnts of the Pa(nOf) cross section a!mvu

,,,(, kc!:’;J show considcral~lc’structure in the

fissfun truss wrtion. T?:csc struccurcs !Iaw

-2-

Takin~ advantage of the high chemical purity of the 231Pa saqles

(less than 0.1 ppm of fissile elements) a preliminary measurement in

the eV region was also carried cut. For the first time, narrow fission

resonances were observed above 1.; eV.

Besides the importance of231

Pa for basic fission studies, it is

also of interest in the field of reactor design in that it plays a part

in the unuanted generation of232

U in the 232Th/233U breeder cycle.

EIKi&!zt:~2-!@c edure~“ll;,,,(,,~, ~r,,%% ~ucLiu,,

TIlc- , . .~Js

‘frehLiVc LCI Lile 1“ fission cross

section Kns mcnsurcd usirg neutrons produced hy the oak Ridge Electron

I.incarAccelerator (!-IRFT.A) used as a pulsed sour~.c. Fission fragments

wrrr dctcctvd by a g;:sscintillator located on i] fllglltpath perpendicular

to the u]ccLrG,l beam. Ta!l].,I summarizes thu experimental contllticnsfor

l’)(lLi,t],~,],~~]]en~~:;y [(J.] . I.n(!lcl’) ~ 12] and the low cmer~y

[()~ El,(kur).“10] mcasurcmcnts.

TatIlcI. Experimental Conditions

-J===

O-L < ~n(Mev) <12

Fruqucncy (ilz) 800 450

Pulse width (ns) 6 -8 40

Power (kW) e-9 26

Average flightpath (m) 41.68 18.30

Filters hone 0.5 mmof Cd.

Duration (h) 632 60

-3-

The fission detector consisted of a sealed cylindrical chamber

containing a mixture of 98X He and 2% lt2at STP. The chamber was

divided into 3 pairs of optically isolated cells. The (istance between

two consecutive pairs was 12.5 cm. Each cell was viewed through two

circular quartz windows by two ~p20~0/Q photomultiplier tubes-

Each of the five231

Pa cells contained 53

oxide painted on both sides of a semicircular

in diameter and 12.5 l;mthick) to a density of

The remaining CC1l contained 140 mg of uranium

Th: ur~,,l-umwas electroplated on both sides of

mg of protactinium

titanium foil (20 cm

approximately 0.3 mg/cm2.

235Lenriched to 932 in .

a semicircular stainless-

2steul disk (250 urnthick) to a density of 0.5 mg/cm .

To c!cfinca valid fission cwent and to reduce alpha background and

noisv wc require a coincidence between the pair of photomultiplier tubes

on cacllCC1l. The fission events from each cell ~ncc digitized and

routed by an IXLC TDC 100 clock were stored separately in the computer.

Both pulse-tlcightand thnc-of-fligh~ informationwere rcco~dcd to pro-

vide a continuous monitor as a fun(”tionof neutron energy of the fission

bias for each CC1l.

Data Reduction

En ~ 0.1 Mel’. After background corrections,which were assumed

231essentially constmt for the Pa and 235U time-of-flight (TOT)

spectra, the shape of the231

Pa(n,f) cross section was obta:ncd using

~;le235U(n,f) cross section derived from ENT)F/9-V. The 231Pa fisuion

cross section was normalized bctwccn 0.9 and 1.1 Nel’to the data of

Sicrc2. In this procedure it was assumed implicitly that the effi-

ciency of the detector was Indcpcndcnt of neutron energy.

-4-

% < 10 kev. Since the data recorded in this energy region can

provide only preliminary results, duc to the limited available beam

time for this measurement, the simplest posstble reduction procedure

was adoptcdm By assuming that the energy dependence of the neutron flux is

ufw~ M proportional to the231close tol/En, Pa TOF spectrum. The

proportionalityconstant was detcrmir?d by the ratios of 231Pa/235U

counts at 230 and 380 keV and from the known fission cross section at 13.26 eV

~r 235U. AS a check of the energy dependence of the neutron

235LTOF spectrum was compared to Uf235,~E~ The two sets

n

agree to within a few percent between 0.4 and 100 eV.

flux, the

of data

..

The ‘J’Pa(n,f) CK9SS section is plotted in Fig. 1 from 0.1 to

12 Me}’. The peaks and the strong fluctuationsobserved below 1 Met’

are superimposedon an underlying cross-section characteristicof a

neutron-inducedthreshold fission reaction whose threshold is located

around 750 keV, The second chance fission threshold occurs in the

vicinity of En = 6 MeV leading to a plateau valucforaf of % 2 b.

Figure 2 displays a more detailed picture of Uf between 0.12

and 0.42 MeV. For comparison we have plotted the data of Sicre2

which were taken with a resolution of 5 keV, except for the 160-keV

data point which was taken with a resolution of 2 keV. The Sicre

data are systematicallylower even though the present data were

normalized to his cross section in the 0.9- to l.1-MeV reuion. The

striking feature of this figure lies in the considerableamount of

fine structure in the cross section which has not been obsemed in

previous measurements. This iE due to the substantial improvement

in the resolution of the present measurement. For instance, at 160

keV our resolution is 0.4 keV. As a result, the resonance at 156.7

-5-

keV which haE a FWHM of 2.9 keV is now completely resolved. However,

the base of this resonance cannot be described by the wings of a Breit-

Wigner function. This suggests the presence of additional structure

centered around the 157-keV peak. In addition, the good energy reso-

lution reveals the presence of other sharp resonances (at least two in

the vicinity of En = 370 keV) and separates into several components

the broad peaks located around En = 180 and 330 keV. Table 11 gives

a preliminary list of the peak energies of of together with K= values

extracted from ref. 2.

TABLF II: Resonance Energies andCorrespondingKm Values,

En(keV) Kll En(keV) K~

156.7 3+ 304.5173.3 312.1182.3 319.5187..I 328.6 0-193.8 0+ 336.?281.9 371.2300.6 375.7

Below 100 eV (see Fig. 3), a number of fission resonances are

observed for the first time except for the 4 resonances below 1.3 eV

3already reported by Leonard et al. The resonance energies are in

very good agreement with those given by Simpson et al: in their total-—

cross-sectionmeasurement. However, we observe additional resonances

-6-

above 15 eV. Rather than being clustered around definite energies ag

it has bee~ obeerved in several subtkreshold cross sections,5-0

these

resonances seem to be vnifozmly distributed, at least below 100 eV.

Because of poor stati~tica at higher energies, it is not possible at

the present time to verify if this behavtor is also valid in the keV

region. From the grn values reported in ref. 9, the rf values have

been calculated and are llsted in Table 111 together with other

resonance parameters. A meaningful average fission width <Tf>can be

obtained only for those resonances below 15 eV because resonances

were ❑issed above this energy in the total cross section measurement.

The average fission width for those resonances below 15 eV is 8 PeV,

about 3 to 4 orders of magnitude lower than what iB generally measured

for fissile nuclei. In the energy region analyzed, [1.24 < En(eV) < 27.31

no significant enhancement of rf is noted. This is similar to the be-

havior of the fission widths for the fissile nuclei.

Discussion

Sicre2 concludes from his fission fragment angular distribution

231measurements on Pa th~t the resonance at 156.7 keV is a pure vibra-

tional state having Km = 3+. However, our datd suggest that this reso-

nance i~ fragmented. A possible explanation for this fragmentationmay

be due to a rotational band built on this vibrational state. The reso-

nances of this band would have J“ > 3+ and therefore be formed with

1 >3 neutrons leading to peak cr~es sections on the order of a few mb.

Thi6 16 consistentwith the magnitude of the structure observed in the

taile of the 156.7-kcV peak. Hcwever, the spacing of members of such

a relational band is on the order of 20 keV if we assume f12/2~ 2 to

-7-

3 keV ●u reported for240P”10 and 231,233Thll,12

. Thim epacing is

much larger than the energy span of the underlying structure.

of a few keV would require a moment of inertie~ incompatible

etable shape of the nucleus.

A npacing

with a

To interpret the particular ehape of thie resonance we propoee

the following picture in the framework of the calculationsof tiller

Ct a123’14 This res~,ance could represent a vibrational state trap-

ped in the Ishallowasymmetricallydeformed third minimum of the triple-

humped fission barrier of 232Pa. The additional observed structure

would come from coupling between this vibrational state (Class-IIIstate)

and several less deformed and more complex states (Clafis11 states)

that exhibit an average spacing <D1~of a few keV. The gross struc-

tures obse~ed by Sicre2 at approximately 180 and 330 keV were inter-

preted in terms of K“ = 0+ and 0- rotational band respectively.

We obsene mo~e structure then can be accounted for by just two stipie

rotational bands. As in the case of the 156.7-keV resonance, this

additional structure may be due to the fragmentationof the ❑embers

the rotatio,,”l bands.

Since we adopt the model of an asymmetrically deformed third

of

mintium, one should expect to find vibrational states of both parities

15as it can be verified by a W calmlatioa. We speculate that the

resonance located at 173.3 keV could be a possible candidate ._ora

Km = 3- vibrational state. Indeed, 1) the peak cross section has the

correct magnitude to be formed with & = 2 neutrons and 2) its separation

from the 3+ state (16.6 keV below) is comparable to

by Blons et al12 for 231Th and calculated by M611er

?egion.

what has been reported

et a114 for this mass

-8-

We alao would like to point out that the fission widths of the

narrow resonances observed in the eV region are consistent with the

14 for 232Pashape of the barrier calculated by Mtillerand Howard .

Including axial asymmetry (gama deformation)in their calculations

lowers the first maximum below

232Pa. In addition, since the

induced fission threshold is Q

the neutron separation energy Sn In

third minimum is shallow and the neutron-

750 keV above Sn, a nucleus undergoing

fission in the eV region only will be sensitive to the overall shape

of the outer two barriers. If we approximate the shape of the outer

barriers by a simple parabola with curvature ?luthen we calculated

an fIU= 0.5 MeV when using 8 peV for the average fission width ~f>.

This value of the curvature for the outer barrier is in fairly good

16agreement with thab reported by Back et aL for odd-odd actinides..—

Conclusion

This measurement has revealed a considerable amount of fine structure

in the threshold region of the neutron-induced fission cross section of

231pa. We speculate that some of the newly obgerved structure may be

due to coupling between vibrational states of the third well to more

complex states of the second well of the fission barrier. A more ac-

curate knowledge of the fission mechanism for this nucleus would require

a high-resolutionmeasurement of 1) the fission fragment anisotropies

between 0.1 and O.t 14eVand 2) the fission cross section in the keV

region to look for possible intermediate structure. In addition, other

experiments such as study of the y rays emitted from the 231Pa(d,py)

reaction below Sn would provide a sensitive probe for determf.ningthe

depth of the first and the second mini- of ~he fission barrier.

-9-

Acknowledgments

We are particularly indebted to P. M611er and M. Howard for

fruitful discussions and access to their recent results prior to publi-

cation. We are grateful to P. Kuehn from the Solid State Division of

Oak Ridge National Laboratory for the very successful preparation of the

231Pa samples. The staff of OREM should also be thanked for the smooth

operation of the Linac. One of us (S. P.) would like specially to thank

the Physics Division of the Los Alamos Scientific Laboratory for its

warm hospitality and constant support during this experiment.

-1o-

References.—

1. D. W. Muir and L. R. Veeser, ir Proceedings of the Conference on.——

Nuclear Cross Sections and Technology, Knoxville, TN, March 15-1?,

1971, p. 292.

2. A Sicre, thesis, University of Bordeaux (1976), Report CENBG 7603.

3. B. R. Leonard, Jr., and R. H. Odegaarden, Bull. Amer. Phys. Sot., ~,

8 (1961); Hanford Report 67219 (1960).

4.

5.

6.

7.

8.

9.

10,

11.

12.

13,

14.

F. B. Simpson, W. H. Burgus, J. E. Evans and H. W. Kirby, Nucl. Sci.

Eng., 12, 243 (1960). -.—

D. Paya, H. Derrien, A. Fubini, A. Michaudon and P. Ribon, in Fuclear.—

Data for Reactors, IAEA Vienna 1967, II, p. 128; A. Fubini, J. B1.ens,

A. Michaudon and D. Paya, Phys. Rev. Lett., 20, 1373 (1968).—

E. Migneco and J. l?.Theobald, Nucl. Phys., A112, 603 (1968).

G. D. James, J. “ T. Dabbs, J. A. Harvey. N. W. Hill and R. N.

Schindler, Phys. Rev., C15. 2083 (1977).

F. C. Difilippo, R. B. Perez, G. de Saussurc, D. K. Olsen, R. W.

Ingle, Nucl. Sci. Eng.n ~, 43 (1978).

BNL 325, ~, Third Edition (1973).

H. J. Specht, J. Weber, F. Konecny and D. Heunemann, Phys. Lett.,

41B, 43 (1972).—.

J. Blcns, C. Mazur and D. Paya, Phys. Rev. Lett., 35, 1749 (1975).-—

J. Blons, C. Mazur, D. Paya, M. Ribr?g and H. Weigmann, Phys. Rev.

Lett., 41, 1202 (1978).

P. Hbller and J. R. Nix, in Proceedings of Physics and Chemistry o~—.

Fh6iOn, IAEA Vienna 1914, ~ p. 103.

P. tiller and M. Howard, private communication (1979).

-11-

15. L. Xl.Landau and E. M. Lifshitz, in QuantlJmMechanics, Pcrgamon

Press (1958), p. 176.

16. B. B. Back, J. P. Bondorf, G. A. Ostroschenko, J. Pedersen and

B. Rasmusen, Nucl. Phys., A165, 449 (1971).

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