The study of the (α, α′f) reaction at 120 MeV on 232Th: (III). Total kinetic energies and mass distributions for excitation energies below 12 MeV

Nuclear Physics A380 (1982) 27-41© North-Holland Publishing Company

THE STUDY OF THE (a, a'f) REAMON AT 120MeV ON 232'11(III). Total kinetic energies and mass distributions for excitation energies below

12 MeV

E

P. DAVID, J . DEBRUS, H. JANSZEN and J. SCHULZE

Institut für Strahlen- undKernphysik, UniversitätBonn, Nußallee 14-16, D-5300 Bonn, W-Germany

andM.N . HARAKEH, J. VAN DER PLICHT* and A. VAN DER WOUDE

Kernfysisch Versneller Instituu4 Riiksuniversiteit Groningen, Groningen, The Netherlands

Received 10 November 1981

Abstract : The total kinetic energy release (TKE) of the fissioning nucleus 2szLh is measured as afunction of excitation energy and for various mass separations. A direct correlation of the TKEand of the prompt neutron yield excitation functions is observed. The mass yield is presented asfunction of the total kinetic energy of the fragments. Fine structure in the TKE and in the massyield distributions is observed in the barrier region. An excitation function for the symmetric masscomponent of 232M is presented .

NUCLEAR REACTIONS, Fission 232 Th(a, a'f) E =120 MeV; measured fission fragmentenergies, mass distributions, angular distributions, fission probabilities, excitation functions .

1. lfntroduction

In this paper we report on the total kinetic energy release (TKE) and the massdistributions of the fission fragments for fission of 23211 induced by inelasticscattering of 120 MeV or-particles as a function of the excitation energy E_ .

In the concept of the potential energy surface used for describing the fissionprocess in actinide nuclei one can distinguish three stages 1) : the excitation, thebarrier penetration and the descent from saddle to scission .Although a vast amount of information has been collected about the second

stage 2,3) very little is known about the last stage of the fission process.Some insight into what is actually happening during the descent from saddle to

scission can be obtained from a study of the total kinetic energy release and themass distributions of the fission fragments as a function of the excitation energyof the fissioning system, including spontaneous fission from the ground state andfission from the isomeric state.For instance, Nix et al. °) have been able to describe the TKE in terms of a

nuclear viscosity parameter . Odd-even effects in the mass distributions have been* Present address : Los Alamos Scientific Laboratory, Los Alamos, New Mexico 87545, USA .

27

28

P. David et al. / (a, a f) study (III)

used to estimate the importance of quasiparticle excitation during this stage of thefission process S) .The overall features of the TKEand the mass distributions are mainly determined

by the bulk properties of the fissioning nucleus. However it is known from, forinstance, detailed studies of the fission probabilities that rather drastic effects areoccurring around the fission barriers due to barrier penetration phenomena 2). Itis then an interesting question whether such penetration effects can also be observedin the TKE andmass distributions. Although many charged-particle-induced fissionreactions have been used to study the behaviour of TKE versus the excitationenergy Ex the energy resolution was always insufficient to observe such effects.Neutron induced fission of 233Th [ref. 6)] measured with good resolution doesindicate though that barrier penetration phenomena might reflect themselves inTKE(Ex).The aim of the present experiment was to find out whether for 232Th barrier

penetration effects might also show up in the total kinetic energies and massdistributions of the fission fragments. Therefore we measured these properties for232Th as a function of excitation energy by using the (a, a'f) reaction at E. =120 MeV with an energy resolution of 75 keV. As will be shown we find relativelysmall variations in TKE(E,,) (i .e . TKE(Ex) averaged over all masses of the fissionfragments) and in the mass distributions, which most likely originate from barrierpenetration phenomena.This paper is the third of a series of studies of the fission of 232Th induced by

inelastic scattering of 120MeVa-particles . Papers I and II have been published''').Of special importance for the present work are the barrier heights for 232Th asobtained from analysing the fission probability. They are summarised in table 1.Related studies on 236.238U have been published 9-11) .

In sect . 2 we give the experimental set-up, sect. 3 describes how the energy andmass distributions were obtained taking into account the corrections due to promptneutron emission and target thickness . The results are presented in sect . 4 anddiscussed in sect. 5 .

2. Experimental procedure

In order to investigate the mass and energy distributions of the fission fragmentsas a function of excitation energy of the fissioning system in the reaction 232 Th(a, a'f) triple coincidence measurements between the inelastically scattered a-particles and the two fission fragments were performed . In these measurementsthe 120MeV analysed a-beam from the K VI cyclotron was used to bombard aselfsupporting 232Th target of 950 Wg/cm2 thickness . The inelastically scattereda-particles corresponding to excitation energies between 4 and 14 MeV of thefissioning 232Th nucleus were detected using the QMG/2 magnetic spectrograph 12)and its position-sensitive focal plane detection system 13 ) . The spectrograph was

P. David et al. / (a, a'f) study (III)

29

set at Bi .b =18° with a full horizontal opening angle of 6°, corresponding to a solidangle of 10.3 msr. The total energy resolution was about 75 keV. Fission fragmentsin coincidence with the inelastically scattered a-particles were detected in two60 pan thermo-electrically cooled surface barrier detectors situated opposite toeach other and at an angle of -72°, which is the recoil axis of a 232Th nucleusexcited to 11 MeV. The fission detectors subtended a solid angle of 102 msr each .The experimental set-up is sketched in fig . 1 .

Fig . 1 . Sketch of the experimental detector set-up.

A triple coincidence event was generated by a fast coincidence between thefission detectors and a slow coincidence between the fission detectors anda spectro-graph event. A spectrograph event was generated by a triple coincidence betweentwo position-sensitive detectors anda scintillator which constitute the spectrographfocal plane detection system 13 ) . The scintillator of the focal plane detection systemfurnished as well fast time signals which were used to stop a time-to-pulse-heightconverter (TAC) started by the fast time coincidence signal between the two fissiondetectors. These TAC signals along with left and right pulse height signals fromthe two position sensitive detectors, a pulse height signal from the scintillator andtwo pulse height signals from the two fission detectors were written event by eventon magnetic tape for further analysis. Unambiguous identification of a-particlescould be obtained using the above information. The two pulse height signals fromthe two fission detectors were further used as described in the following section to

30


obtain information about masses and energies of the fission fragments . Moreover,using the TAC spectrum, an example of which is shown in ref. 7), it was possibleto subtract random events which constituted a small part of the prompt peak. Datawere taken with an average beam current of 30 nA. The beam current was restrictedby the count rate in the fission detectors, which was held below 6kHz.

with

3. Energy and mass determination, correction for prompt neutronsand for target thickness

Before and after each irradiation of the target the silicon surface-barrier detectorswere calibrated with a 252Cf spontaneous fission source . Following the iterationprocedure of Schmitt et al, 14) this calibration allowed the determination of massesand energy release before neutron emission .

In the analyses it is assumed that the energy carried away by one neutron remainsconstant as a function of excitation energy and that the neutrons are emittedisotropically . The neutron corrections are in themselves small. Nevertheless wetried to take into account also the excitation energy dependence of the vp values,i.e . the prompt neutrons averaged over all fragment masses . This is possible onlyby a model assumption since only very few measurements of vp(A, Ex) as functionof fragment mass and of excitation energy exist 15,16).For this purpose the experimental data of Bishop et al. 15) for vp(A, Ej were

taken as a basis of the following procedure to get appropriate vp(A, E.) values for232Th [ref . 17)] . The vp(A, EO data were fitted by a weighted superposition of asawtooth curve, as measured at low excitation energies (around the barrier) and alinear function, as measured at excitation energies around 25 MeV. The weightingparameters were excitation energy dependent . The sawtooth structure gets weakeras Ex increases, corresponding to the observed washing out of shell effects 18'19) .

For each excitation energy vp(A, E.) was normalized to the vp(E.) data of ref. 20).Fig. 2 shows that the procedure fits the data for the nuclei 234U, 234Np, 239Np and240Pu. For the nucleus 23zTh vp(A, Ex) data do not exist. For this reason the sawtooth data are taken to be similar for 232Th, 233Th and 233U .The energy loss of the fragments in the targets was corrected for by assuming

the relation 21 )

AE =Kv'Ek: ,

Ekin _Ekin 2K=aO+al A +a2( A

Values for AE(A, Ek;n) were taken from the table of Northcliffe and Shilling 22)and the least square fit was made to determine values for a0, a, and a2.

4

3

2

1

3

2

1


31

80 100 120 140 160 80 100 120 140 160A [amu 1

Fig. 2. Variation of vp(A) with excitation energy for various fissioning systems . Insert numbers referto E, . The data are taken from ref . 15)

.

4.1 . THE TOTAL KINETIC ENERGY RELEASE OF 232Tb

Fig. 3 showsTKE(E.) with the data summed over 0.5 MeV intervals of excitationenergy and averaged over all masses of the fission fragments. For comparison thePp data of ref. z°) are included . As can be seen dTKE(E.)/dE. behaves very

160

54

'yP321

4. Results

Br

Bnt _t t t t1t ~ t . t t d

01 234 56 7691071121314Ex [MeV1

232Th

Caldwtdl etal .

0 10]E x [MeV

Fig. 3. (a) TKE(E.) for 232.Lh averaged over 0.5 MeV. (b) ,P(EX) for 232.Lh from ref. 2a).

5

.U-P24.25 WV 2 1411V

:nU.P 2311U.P13.75f nnwV

233U.n. 2»Pu-nth

8l4MW ".631AW

n.

Y

32


32

10

-1

-2

-3-4

d7KEI A14dE x

Fig. 4. Slopes d TKE(AH, Ej/dE, for 232,n, in the E, intervals as indicated (region 1, 11) for severalmass bins .

differently for excitation energies below and above about 8MeV. This makes clearthat in discussing the behaviour of TKE(E.) it may not be correct to take theslope over the whole E,, interval from threshold Bf to thresholdBaf , as is done e.g.in refs . 34.35,37) .

Fig. 4 shows dTKE(AH , Ex)/dE. for 6.15 MeV,E,,,7.65 MeV, averaged overdifferent mass groups as indicated. The slope S = dTKE(AH, Ex)/dE,, for7.65MeV,Ex,12.6 MeV is consistent with zero, whereas for Ex =6.15 to7.65 MeV the value of the linear least square fit slope is dTKE(AH, Ex)/dE,, =1 .5 :1:0.4 MeV/MeV. The slopes are mass independent within the accuracy of themeasurement.

It is noteworthy mentioning that the Pp(E.) data for 232Th of ref. 20), which weapply in the evaluation indicate an excitation energy dependence different fromthe monotonically rising linear one observed for heavier actinide nuclei in the Ex

range above threshold Bf up to about 8 MeV. This is shown in fig. 3b .Fig. 5 shows TKE(E.,) data for 5.7 MeV<Ex,8 MeV in bins of 75 keV. The

surprising effect is that relatively strong fluctuations of TKE(E.) are observed for

175

2UPX~ 170

232Th(a,a If)O 6.154 E �4765

-1

07654 E,412 .60

110

120

130

140

150

AH

160

165

160

232Th ( OL,d)E a .120MoV

pif'y'

f 1

Ii

I'll

lil,

1

III'III6A 6 .5 7.0 75 fl0

E,[MeV]

Fig. 5. TKE(E,) for 232Th in bins of 75 keV of excitation energy.

654321

654

:t 302W15=

12 7Q8

54321

654321


33

654321

654321

6543

21

654321

4.2 . THE 232Th FRAGMENT MASS DISTRIBUITONS

70 90 110 130 150 170

70 90 110 130 150 170AM

Alul

lag. 6. (a) 232Th mass yield for all TKE and for E= from 5 to 6.7 MeV. (b) Same as fig . 6a, but for a100 keV wide interval around E,=6.15 MeV. (c) Same as fig. 6b, but for Ex=6.25 MeV. (d) Same asfig . 6b, but for E==6.35 MeV. The dashed curve represents the data of the first figure for Ez from 5to 6.7 MeV to guide the eye. (e) Same as fig . 6a, but for E, from 6.7 to 8MeV. (f) Same as fig . 6b,but for E,=6.45 MeV. (g) Same as fig . 6b, but for a 200 keV wide interval around E.=6.7 MeV. (h)

Same as fig . 6g, but for E,=7.0 MeV.

Ex between 6 and 8 MeV of excitation energy, an effect that has not been observedbefore in charged-particle-induced fission. A similar effect has been reported forthe TKE from the reaction Zs2.hh(n, f) in ref. 6) and to some extent also for thereaction 236U(n, f) in ref. 23) .

Since TKE(E.) shows such surprising results we also evaluated as function ofthe excitation energy the mass distribution as averaged over all TKE. In fig. 6a the

ZuTh(«fi

'f) a)Ea " MWE �" fi.7-a01AaV

f f

ifff ff i~

E,rO .«Sf*V ~ f)

1 1

E,~fYaCMW y)

r

E,t0.~7.1M~1h)

r1

2>2Thfga'flIQ-1201W

a)

E, h0-&711W

bl

I1r

f.41E, "623.3WV c)

41%1

E," 03-0.411W d)

i

34


mass distribution is displayed for Ex from 5 MeV up to 6.7 MeV and in fig. 6e forE,, from 6.7 MeV up to 8 MeV. Figs . 6b to 6d give the mass yield distributions fora 100 keV wide interval around Ex =6.15, 6.25 and 6.35 MeV respectively . Thedashed curve in the figures corresponds to a line connecting the data points in fig.6a . Figs . 6f to 6h show further mass spectra for Ex = 6.4 to 6 .5, 6.6 to 6.8 and 6.9to 7.1 MeV respectively. The dotted curve is the same as mentioned above. Anenhanced yield around AH=145 is clearly visible for the Ex interval from 6.2 to6.3 MeV; within the accuracy of the measurement it is the only irregularityoccurring.

Fig. 7 display mass distributions for variousTKEbins . These data are normalizedto 200% of fission fragment yields and are averaged for the E,, interval from 6 MeVup to 12 .6 MeV.

öW3--

Fig. 7 . Mass distributions for E, = 6 to 12.6 MeV of the fissioning 23211 nucleus and for various TKEbins of the fragments.

For 232 Th the asymmetric mass . bumps are separated well enough to make itpossible to analyse the excitation energy dependence of the symmetric mass yieldfrom 113.5 to 118.5 a.m.u. The results of such an analysis are displayed in fig. 8atogether with a linear least square fit to the data . The error analysis includes thestatistical error and the influence of the asymmetric masses due to an experimentalmass resolution of AA = 2.5 a.m.u . [ref . ")]. This resolution was obtained from thecalibration data of a thin 252Cf source in comparison with radiochemical data 2° ) .

For this purpose the radiochemical data of ref. 24) were completed by linearinterpolation and folded with the experimental resolution of 2.5 a.m.u . In determin-ing the random events the average over six random peaks was used in the evalu-ations . In fig. 8b the symmetric mass yield of 228Th as obtained in ref. 25 ) is displayedfor comparison. These data had been evaluated on the assumption of a gaussiandistribution. For comparison with our data they were divided by 1.4 . Again the

-2.0a<0wr 1 .0

15

10

5


35

232 ThiQG')EQ.120 MeV113UMO1&5

f0

5

5

7

OT19

10

nl

121

Ex [MOV]

-227Acl 3 Flsd? 2sTS*-fSpecht d al .A xym .108-123 U

L_ A

45676910111213Ex[MeV]

Fig. 8. (a) Excitation function of the mass yield of the symmetric fragment mare 113.5 to 118.5 a.m.u.of 232Th in 400keV bins (O). Comparison of the symmetric yield of 232Tb from this work with the

results of ref. 26) for 233.1.b (O). (b) Results from ref. 23) for 22sZ.h .

36

P. David et aL / (a, a f) study (III)

curve gives a linear least square fit to the data . The open squares in fig. 8a are thesymmetric mass yields as obtained from the radiochemical analysis for Z32 Th ofref. 26) . Here the symmetric mass yield for ",5Cd was taken as the basis for theevaluation of the mass interval from 113.5 to 118.5 a.m.u .

5. Discussion and conclusion

In fig. 9 we have summarised selected data on TKE(Ez) for several nuclei inthe excitation energy range up to B�t . For the sake of comparison our 232Th data

160

aK .i

6i~".#~}" féi

Fig. 9. Comparison of TKE(E,) from our data 232Th(a, a'f),E- 120 MeVand from published data :233'rb, ref . 30) (O); 233n, ref . 31 ) (") ; 233U(d, Pf), Ed =13.5 MeV, ref . 32) ; 235U(d, Pf), Ed =23MeV,ref. 9) ; 238U(a, a'f), Ea -120 MeV, ref . 33) ; 239Pu(d, pf), Ed =20MeV and 241Pu(-r, a'f), E(r)=33.9 MeV, ref. 34) ; 24°Pu(d, pf), Ed =13.5 MeV, ref . 3s) ; 24°Pu(n, f), 242Pu(n, f), ref . n; 241Am(d, pf),

Ed =15 MeV, ref . 37) .

I

a

WYF-

160

1661

236U

180

180

E 232Th

7240 Pu

Bf=EA242Am

L

1


37

-ATA

Bf =EA EB1

EBf .EA

Bnf1

Britl

Bnf1

Bnf1

5

10 E�IMeV1

EA

DEFORMATION

Fig . 10. Schematic representation of the TKE(E=) as shown in fig. 9 in relation with the results of thedouble-humped barrier model as obtained from ref . 2 ) .

have been averaged over 500 keV energy intervals. The data for 233U and heaviernuclei show a remarkable similar behaviour : TKE(E=) remains approximatelyconstant or even decreases slightly with increasing Ex. This is very different fromwhat is observed for 232Th where dTKE(E=)/dE. -1 .5 up to about 8MeV afterwhich TKE remains approximately constant . This difference is not understood . Itmay be related to the relative height of the inner and outer barrier as they aredetermined for instance from fission probabilities in a two-humped barrier potentialmodel. This is illustrated in fig . 10 where TKE(E=) and the relative barrier heightsare schematically shown for different nuclei . It suggests that for those nuclei wherethe inner barrier is higher or about the same as the outer barrier one might expectd TKE(E.)/dE.- 0 while for a higher outer barrier d TKE(EZ)/dE,, > 0 in theregion around or a little bit above the barrier. It is also important to note that for232Th the relative strong increase in TKE(E.) is not correlated with a specialfission fragment mass division . Thus the observed increase must be due to someproperty of the fissioning system 232Th and not to, for instance, shell effects in thenascent fragments.The most surprising result of our present study is probably the behaviour of

TKE(E.) in the excitation energy range 5.7 MeVsEx 1!r.8 MeV taken with aresolution of AE =75 keV as illustrated in fig . 5. It shows that superposed on thegradual increase of TKE(E,,) from 6 to 8 MeV excitation there are considerablelocal fluctuations . From the present existing data on charged-particle-induced fissionit is not clear whether this effect is typical for 232Th or whether it also exists forother nuclei since in all the other charged-particle experiments the energy resolutionwas such that structures as observed for 232Th would be washed out. There existshowever neutron-induced fission data for 5.8 -- E_ ,7 MeV with AE,--100 keVwhich show that TKE(E.) for 23311 behaves somewhat similar to what we haveobserved for 232Th [ref . 6)] although the fluctuations are not that large .

38


I w~

6.0 6.5 7A 7.5 &0E,[MW1

Fig. 11 . Comparison of TKE as obtained in this work in the reaction 232Th(a, al) at 120 MeV andas measured in the reaction 232Th(n, f) of ref. 6) .

This is illustrated in fig. 11 . For 233Th it is known that precisely around E,, =6.3 MeV the neutron-induced fission cross section strongly fluctuates 6'2''211). Thesefluctuations have been suggested to be due to resonance excitation of states locatedin a third shallow mass asymmetric potential well [see for a recent review on thismatter ref. 2)]. Since similar effects have been observed for 231Th it is reasonablethat also for 232Th the fission phenomena for 6 McV--E,,--7 MeV are stronglyaffected by resonance penetration of a mass asymmetric potential barrier. In thisconnection the behaviour of the fragment distributions as a function of excitationenergy as shown in fig. 6 is quite interesting. It shows that around Ex =6.3 MeVthere is an abundance of asymmetric masses which is probably due to resonantpenetration through a mass-asymmetric barrier, with the barrier mass division toa certain extent frozen during the descent from saddle to scission . Tempting as thismodel may be, there are many open questions . The main one is probably how toexplain the rather large fluctuations in TKE (Ex) which could be correlated withdifferent barrier penetration paths and mass divisions . In order to check this ameasurement of TKE(E, A) would be necessary, but such an experiment wouldrequire a prohibitive long running time . Also one would expect to find a stronglyfluctuating fission probability for 232Th for say 6.0 -- E,,, 6.5 MeV like has beenfound for 231.233.Lh. Some evidence for such effects have been recently obtainedt .Also there is evidence from previous measurements that in the excitation energyinterval 6.2 to 6.3 MeV the angular correlation pattern of the fission fragments issomewhat different from neighbouring intervals. This is illustrated in fig. 12, wherethe anisotropy is plotted as a function of E.. Using the data from ref. 11 ), a significantdecrease in anisotropy is observed around Ex= 6.3 MeVwhich might be correlatedwith the observed mass distribution effects, but again more precise data are needed.

Finally it should be noted that the maximum in TKE(EJ around E�=6.7 MeVcould be correlated with the height of 6.6 MeV of the outer fission barrier asdetermined from the fission probability Pf(E.) [ref . 8)], see table 1.

t From a 232Th(p, p'f), Ep = 30 MeVexperiment performed at KVI Groningen, to be published.

170

232Th f165

I

~y 233Th160 i

s0

40

8 30t

20

,o

P. David et aL / (a, a'f) study (L11)

6 aE�IMaYl

Fig. 12 . Anisotropy W(10°)/ W(90°) for the reaction 232Th (a, a'f) as function of E,. The data arefrom ref . s) .

TAHLE 1

Fission barrier parameters for 232.Lh as derived from the analysis of the fissionprobabilities obtained from the present (a, a'f) experiments s) ; all energies are in

MeV

K' EA f+WA EE, AWB

References

En

0+ 5.7 0.9 6.6 1.2 3.0 1 .50- 6.7 0.9 6.6 1.2 3.0 1 .35 6.05

39

In conclusion then we have found rather strong evidence that for 23211 atexcitation energies below or close to the fission barrier the TKE(E=) and the massdistribution of the fission fragments are effected by barrier penetration effects .Whereas the overall features of TKE and the mass distribution are mainly deter-mined by the (Z, A) of the nucleus both observables clearly show fine variationswhich probably can be used to obtain informations on the shape of the potentialbarrier. Especially the mass distribution at Ez=6.3 MeVmight point to a subbarrierresonance in 232Th which is associated with an asymmetric shape.

This work has been performed as part of the research program of the Stichtrogvoor Fundamenteel Onderzoek der Materie (FOM) with financial support of theNederlandse Organisatie voor Zuiver-Wetenschappelijk Onderzoek (ZWO).

This work was supported partly by the Bundesministerium für Forschung undTechnologie der Bundesrepublik Deutschland.

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Documents

The study of the (α, α′f) reaction at 120 MeV on 232Th: (III). Total kinetic energies and mass distributions for excitation energies below 12 MeV