A Review of the Fission Decay of the Giant Resonances in the Actinide Region M

A REVIEW OF THE FISSION DECAY OF THE GIANT RESONANCES IN THE ACTINIDE REGION M. Harakeh

To cite this version:

M. Harakeh. A REVIEW OF THE FISSION DECAY OF THE GIANT RESONANCES IN THE ACTINIDE REGION. Journal de Physique Colloques, 1984, 45 (C4), pp.C4-155-C4-184. 10.1051/jphyscol:1984413. jpa-00224078

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A REVIEW OF THE FISSION DECAY OF THE GIANT RESONANCES IN THE ACTINIDE REG I ON

M.N. Harakeh

Kernfysisch VersneZZer Instituut, 9747 AA Groningen, The Netherlands and NucZear Physics Lab., University of Washington, SeattZe, WA 98195, U.S.A.

Resume - La decroissance par fission des resonances geantes dans la region des actinides est passee en revue. Les resultats invariablement contradic- toires de diverses experiences sont discutes. Cel les-ci comprennent des reactions inclusives de fission induite par electron ou positron, et des exp6riences ob les fragments de fission sont detectes en coincidence avec les electrons ou hadrons diffuses inelastiquement. Nous nous concentrons sur une exp6rience (a,alf) recente oO 1 'on etudie la d6croissance par fission de la resonance geante monopolaire en detectant les fragments de fission en coincidence avec les a inelastiques autour de, et ?I 0' .

Abstract - The fission decay of giant resonances in the actinide reqion is reviewed. Results from various experiments which are invariably conflicting are discussed. These include inclusive electzun and wsitron induced fission, as well as experiments in which fission fragments were detected in coincidence with inelastically scattered electrons or hadrons. Attention is focussed on a recent (a,a'f) experiment in which the fission decay of the giant monopole resonance was investigated by measuring fission fragments in coincidence with inelastically scattered a-particles at and around 00.

I - INTRODUCTION

The study uf the fission decay of the isoscalar giant resonances in the actinide region has been marred by claims and counter-claims concerning the magnitude of the fission probability of the giant quadruple resonance (QR). This is especially true for the 238~nucleus, which is the most studied by ex~~,hnentalists,where the fission probab-ility for the GQR obtained using various probes at various bombiarcling energies, ranged from 40% /1,2/ down to 4% /3/, an order. of magnitude difference1

These differences were not Limited to the now classical bouridaty line between investigations with electromagnetic probes versus those Wewith ha&.orric prolx?s, but even the results obtained with electmmagnetic probes were in disagreement. The same was true for experiments with hadronic probes. At the heart of the issue, of course, is the question of whether the decay of the GQR into fission is dominated by statistical conskderations or by direct fission ifecay. The first point of view stems from the general belief that the fission process for moderate excitation energies and low angular momenta takes a very long time (perhaps orders of magnitude longer) compared to the transit time needed for a bound nucleon to cross the boundaries of the nucleus (which is of the order of a 2x10-~* sec). During this time the nucleus, which is initially excited into the collective lp-lh giant resonance mode by a

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1984413 JOURNAL DE PHYSIQUE direct reaction such as inelastic hadron or electron scattering, would have enough time to mix into the more complicated 2p2h states which in turn spread into the more complicated 3p3h states, and so on and so forth until the equilibrated compound nuclear stage is reached. At this stage the fission decay probability is completely determined by the density of states at the saddle point versus ttle density of states in the (A-1) nucleus because of competition with neutron decay (the other dominant decay channel). This point of view is supported by experimental observation of the fission decay of the isovector giant dipole resonance (GDR). For the nuclei 232T?1 and 238~it is found /4,5/ to have fission probabilities similar to the Fission probabilities obtained from the (n,f) compound nuclear reaction at 2 similar excitation energies, and also agrees with the Z /A Systematics from cornpourid nuclear fission probabilities as extrapolated from heavier nuclei /6/.

'l't~esecond pint of view stems from the idea that the small amplitude oscillation of the giant resonances, especially the isoscalar ones and in pazkicular the GQR, can couple strongly to the large amplitude oscillation of the Fission We. Therefore once the nucleus gets a small kick which starts it oscillating in a giant resonance mx3e it is easily driven in the direction of Fission. In this case the fission Will not take a considerable time to occur. This will have the following consequences on both the fission probability of the giant resonance and on the angular correlations of the Fission fragments: i) the fission probability of the giant resonance will be considerably larger than that of the compound nucleus, and ii) the angular correlation will be characteristic of the initial K-value (here K refers to the projection of the total angular momentum on the symmetry axis of the nucleus) of the excited giant resonance.

Interestingly enough the interpretations of tne experimentally obtained fission probabilities for the WR in 238~did not always conform with either of the above ideas. For instance, in one experiment /1,2/ where a lxrge fission probability was deduced for the GQR (about a factor of two larger. than that of the GDR) t?le authors tried to justify /2/ their results on the basis of a statistical morlel calculation. Unfortunately, tttis was based on the wrong assumption that the Ireigtlts of the fission barriers ae different for positive and negative parity states for all excitation energies. If this were true, it could indeed lead Co 1aZge+ differences in the fission probabilities of the GDR (J~.- 1-) and the GQR (J= = 2 ) since the fission probability is determined by the density of states above the barrier. However, fission probabilities could differ substantially only al: excitation energies slightly above the barrier because of the different excitation energies of low-lying positive and negative parity transition states above the barrier. At higher excitation energies above the barrier the densities of positive and negative parity states are expected to be the same, which should lead to similar fission probabilities for them.

In another experiment /3/, where conversely a very low fission probakxility was deduced for the GQR (about a factor of five less than that of the GDR), the authors conjectured /3/ that their data supported a strong coupling between the GQR mode and the fission mode because experimentally they observed a peak at the +location of the GQR which apparently had an angular distribution similar to a = 0 component.

I therefore have the difficult job of reviewing these conflicting reports on the fission decay of the isoscalax giant resonances in the actinide region. Alt?~ougll this seems like a formidable task, I will try to cover most of what ha8 Wen published on the subject up till now. However, some very interesting results on the fission probabilities of the various isoscalar giant resonances in 238~llave been published recently /3,7,8/ since my last review /9/ on this subject. These results include i) a coincidence electrofission 238~(e,e*f)experiment /7/ which measured the sum of E2 and EO strength from the first fission barrier up to 11.7 MeV, ii) a measurement /3/ of the fission probability in the region of the 3fiw hiyh energy octupole resonance (HEOR) by inelastic a-scattering at E 172 MeV and iii) the a - measurement /8/ of the fission decay of the monopole resonance investigated by studying fission in coincidence with inelastically scattered a-particles detected at and around 00.

I1 - FISSION INDUCED BY ELECTROMAGNETIC PROBES

Photoabsorption cross sections are completely dominated by electric dipole y-absorption. Thus for the past four decades photonuclear reactions have been used to investigate the various properties of the GDR and in particular its decay properties. In the actinide region both the (yen) and the (y,f) reactions have been studied for 232Th and 238~by two groups /4,5/ with reasonable agreement between the experimental results. For 238~the differential cross sections differ by 15% where the quoted systematic uncertainty for the Livermore data /5/ is s 7% and for the Saclay data /4/ is < 10%. The Livermore group /5/ has also studied the above photonuclear reactions on other nuclei in the actinide region. Their results Showed that the fission probabilities for 232Th and 238~were essentially flat Ccom 1 7 MeV up to the second chance fission thresholds with the deduced fission probabilities being in agreement with those obtained from the compound nuclear (n,f) reactions and 2 the Z /A systematics /6/. The fission probability of the GDR in the region of 7-12 MeV in 23e~,which will serve as a standard against which tt~efission probabilities of the iSoScalar giant resonances will be compared, was found from the photofission experiments /5/ to be Pf = 0.22i0.02.

Because of the dominance of electric dipole absorption in photonuclear reactions, these reactions can not generally be useful for studying multipolarities otter than El. A more sensible way to investigate the other multipolarities (i.e. M1, E2, etc..) is by studying electron-induced reactions. This is because the virtual photon spectra for inclusive electron induced reactions have higher intensities for E2 and M1 multipolarities than for El and are thus more sensitive to the excitation of these multipolarities than in photo-induced reactions in which the real photon spectra have equal intensities for all multipolarities. This is illustraLed in Fig. 1, where the virtual photon spectra /lo/ calculated in DWBA by Gargaro and Onley /11/ for electrons of energy Eo = 9.5 MeV incident upon a Uranium (2=92) nucleus are shown. It is clear from this figure that all the way up to 9.0 MeV the E2 and M1 multipole intensities are favoured by large factors as compared with that of the El.

The early results on the decay of the isoscalar GQR in 238~came from inclusive electron-induced (e,x) experiments, where x represents a neutron, an a-particle or a fission fragment. In these experiments the inclusive electron-induced cross sections u are measured. These are related to the photoabsorption cross sections by: e.x C4-158 JOURNAL DE PHYSIQUE

where ahL is the photoabsorption cross section for multipolarity AL (AL = El, Ul, Y,X AL E2, etc.. ) and N (E,EO) is the virtual photon spectrum of multipolarity At calculated in DWBA for an electron of energy E The contribution of the El 0' multipolarity to the electron-induced reaction cross section is deduced by folding the virtual photon spectrum N~,calculated in DWBA, into the experimentally measured photoabsorption cross section which is assumed to be totally due to electric dipole absorption into the GDR region. After subtracting the El contribution the remaining electron-induced cross section is disentangled into the vxious contributions from the other multipolarities by making certain assumptions concerning the locations, widths and strengths of the various giant resonances connected with these different multipolarities.

Fig. 1 - Virtual photon spectra (ref. 10) for El, MI and E2 multipoles calculated (ref. 11) in DWBA for electrons of 9.5 MeV Incident on a Uranium (2=92) nucleus.

Photon Energy (MeV)

Thus the above method requires not only a precise dete'~m'ination of the absoluie electron-induced and photoabsorption cross sections but also a precise Calculation of the virtual photon spectra for the various multiples. The only rigorous experimental test of the virtual photon spectra was perfvrmed by Doifye et al. /12/ recently. In this test, the electro- and photoexcitation of an isolated resonance, the 16.28 MeV 1- isobaric analog state (IAS) in "2r, and its proton decay wore investigated for electron bombarding energies in the range 17-105 MeV. Because of the almost discrete nature of the resonance and its well defined spin and parity (~c1-),Eq. (1) simplifies /12/ to: where the integral yields the photoabsorption cross section into this level leadinq to population of the "Y ground state by proton decay:

The quantity r r /r was deduced /12/ from the electron-induced proton decay cross P Y section using tge above equations and the virtual El photon spectrum calculated in DWBA. This spectrum was corrected for nuclear size effects which become increasinqly important fur high electron bombarding energies. Tt~e result was in very god agreement (on the 2-3% level) with the value obtained from the photoabsorption measurements performed by the same group /12/ under similar experimental conditions. This test increases our confidence in the calculated ~l virtual photon spectra more than previous tests /33/ which had intecjrated over the broad ~i GDR without accounting for possible excitation of other multipolarities. A test for the E2 virtual photon spectra similar to the test performed by Dodge et al. /12/ for the El virtual photon spectra is highly desirable.

In the earliest reported electron-induced measurement of the excitation and decay of the GQR in 238~,Wolynec et al. /14/ reported the observation of y-ray activity from 234Tt1 following a bombardment of 238~by electrons. The integrated 238~(e,a)cross section necessary to account for the observed activity amounted to about 50-100% of the E2 energy weighted sum rule (EWSR). This was totally unexpected since a-decay of the GQR in Z3B~ should be strongly inhibited because of the high Coulomb barrier. Several experimental groups later searched unsuccessfully /15-17/ for this a-decay channel either by a search for y-activity from the residual 234~hproducts or by direct observation of a-particles during the electron bombardment. These unsuccessful attempts indicated that the measurement by Wolynec et al. /14/ had some experimental problems. A subsequent aCtempt by Lees at al. /18/ to determine the possible source of the 234Th activity via a study of the (y,a) and the (n,nva) reactions on 238~yielded negative results. Although it is now generally accepteO that no a-emission occurs from the region of the GQR in 238~,the problem behind the observation of the 234Th activity by Wolynec et al. /14/ is still unresolved.

Another surprising result was obtained by Martins et al. /19/ for the electron- induced emission of neutrons in the giant resonance region. Tt~eyfound /19/ that the a obtained from their 238~(e,n)measurement could be satisfactorily explzined by e, n El excitation only, with at most 8% of the E2, EWSR in the (e,n) channel. ?I~-is indicated that almost all of the E2 strength goes into the only other available decay channel i.e. fission, although on general grounds one would have expected a statistical decay of the GQR similar to that of the GDR, thus for 238~r/rf a 3.5. A later (e,n) measurement by Shotter et al. /20/ corroborated this result, but with an upper limit of 16% of the E2, EWSR. Along with the earlier electrofission measurement on 238u by Shotter et al. /21/ this yielded a rn/rf = 0.6-0.8 for the GQR /20/ which is to be contrasted with the value of 3.5 for the GDR.

These early results for the decay of the GQR were already controversial because of the unexpected difference In fission probabilities for the GDR and the GQR. However. it was not until the extensive electrofisslon measurements of Arruda-Neto et al. /22/ on a number of Uranium isotopes, with an acclaimed high precision on the Percentages of E2, EWSR strength exhausted in the electrofission channel, that interest intensified in the electrofission of the GQR in the actinide nuclei. In their initial analysis Arruda-Neto et al. /22/ located about 100% of the E2, EWSR C4-160 JOURNAL DE PHYSIQUE

234,236.23EU. However and a significant M1 component in three isotopes of Uranium: in a later analysis /1,2,10,23/ of their data, the percentages of E2, EWSR were found to be about 20% lower than their original values for all three isotopes. Nevertheless, the amount of E2 strength deduced from the (e,f) measurements indicated that a large percentage of the GQR decays'by fission. The 234,236,238 resulting fission strength functions for the U isotopes /l/ are shown in Fig. 2. These are obtained from the electrofission cross sections following the procedure described above to deduce the cE2 cross section which is then related to Y. f the fission strength function by:

where dB/do represents the E2 strength function and Tf/T, which in principle is a function of excitation energy, is the fission probability of the GQR. These E2 strength functions peak at different energies than the absorption cross sections which were incorrectly quoted as the GQR peak energies in earlier publications /10,22,23/.

PHOTON ENERGY (M~v)

Fig. 2 - Fission strength (dB/dw) (Tg/T) for the GQR for the 23 9236 9 238~isotopes as determined (ref. 1) from (e,f) cross sections. For more details concerning the experiment and this figure see ref. 1. The results for the three Uranium isotopes are summarized /1/ in Table 1. A number of remarks can be made: i) the peak energies differ appreciably from those determined from inelastic hadron scattering singles measurements where a bump, which could be a combination of a number of multipolarities, was observed in 238~at around 10.5 MeV /24,25/ and at 11 MeV /26,27/ and in 232~h/28/ at 11 MeV. ii) l?~e widths (F'WHM) are also appreciably larger than those obtained Erom hadronic scattering experiments. iii) The percentages of the E2, EWSR strength exhausted in the fission channel are very large for all three isotopes and even the estimated /1/ fission probabilities at the of the E2 strength funatj.on are still large (a factor of 2 for 238~)compared to those u£ the GDR. At the fission barria+, the fission probability deduced for the E2 StrenqCh function is also very lazqe. For 238~, it is found /2/ to be Pf = 0.8 Conlpared to a fission probability of 0.4 obtained from other studies /29/. iv) It is not clear why only El an8 E2 multipultirities have been included in the calculation of the e1ecf;roflssion cross sections apart from the hand-waving argument by Arruda-Neto et al. /lo/. This stated that for the incident electron energies under consideration the intey~ated electron-induced cross sections are dominated by low nwmentum trarlsfcr events thus leading to vanishing contributions from EO, E3 and therefore also Erom isoscala+ El and higher multipolarities. M1 contribution was neglected because of lack of experimental evidence for giant M1 strength.

Table 1 Fission decay parameters of the GQR strength function in 238~ from ( e, f ) measurements /I/. NuCle~lS Peak energy FWHEf EWSR PE(E2 ) ( MeV ) (M~v) (%) (%) 23aU 8.2f0.4 4.8t1.0 87f14 70*15 236" 8.9f0.4 4.7f1.0 72f10 60*10 238" 8.3t0.4 5.0t1.0 55f10 40k10

The justification for the high fission probability of the CQR in cornpaxison to the GDR as proposed by Arruda-Neto and Berman /2/ was based on a wrorlg assumption as was discussed in the introduction. Therefore one should seek the reason for these high fission probabilities in other quarters. This has been discussed rather extctertsively by Aschenbach et al. /30/ and StrOher. et al. /31,32/.

In their critique of the reSul.ts of Ar~uda-Neto et al. /l,z/, Asc?lenbach et al. /30/ and StrOher et al. /31,32/ emphasized that the results of their acalysis depend sensitively [see Eq. (I)] on three factors. i) Firstly, the results depend on the reliability of the calculated virtual photon spectra, especially the El c0mponen.t. The El virtual pttoton spectra were conclusively tested within a few percent i~yDodge et al. /12/ as discussed earlier but only £or a medium-heavy nucleus (Z=40). The only test of the E2 virtual photon spectra is the one performed by Arruda-Neto et al. /13/ by comparing angular distributions for E2 excitation near the fission barrier, i.e. in the range of 6-7 MeV. Their quoted uncertainty is 20%. ii) Secondly, the analysis depends on the absolute photofission cross sections and their extrapolations to energies above the GDR where no experimental data are available. Since El absorption is still the dominant mode in electroexcitatlon, uncertainties in the photofission cross sections are reflected in the calculated a accoiding to Eq. (1). These will be reflected even more strongly in the e, f extracted E2 strength from the electrofission measurements. Available experimental photofission cross sections differ by 15%. Iicrwevsr, the extra&wlation to eneryies C4-162 JOURNAL DE PHYSIQUE

above ttte GDI? is not very critical, since experimentiil result-? Cur elcctron cr41.r(~cf?~; above 20 MeV p

The positron-induced fission data in Fig. 3 are those of Stroher et al. /31,32/. The hatched curves represent /31,32/ predicted electrofission and positrofission cross sections calculated according to Eq. (1). The El virtual photon spectra were

Fig. 3 - Comparison of absolute electrof ission cross sections. For more details see text and refs. 31 and 32.

f Arruda Neto et al. 1 Arruda Neto et.at. 3 Aschenbach et.al 0 Shotter et.al fi (th~cktarget) this work 1 (thin target $ Calculation calculated in DWBA without including nuclear size effects. Wle absolute photofission cross sections of Caldwell et al. /5/ with the stated uncertainties+ of 7% were used. These were extrapolated for y-energies 2 18 MeV. The e- and e data lie below the calculated El curves, except for the data of Arruda-Neto et al. /1,2/. This led Aschenbach et al. /30/ and stroher et al. /31,32/ to conclude that their data are consistent with vanishing E2 strength in the fission decay channel.

The major difficulty inherent in the method described above for deducing E2 strength from electrofission cross sections lies in the sensitivity to the precise determination of absolute electrofission and photofission cross sections. It was pointed out by Stroher et al. /31,32/ that a more sensible method would be to compare the cross section ratio of o-/u+ for electron- and positron-induced reactions, since in the calculated ratio the absolute scale of the photoinduced cross section nearly cancels out. Moreover, it is exper-imentally easier to obtain reliable relative cross sections than to measure absolute ones. The results of such a measurement /31,32/ for electron- and positron-induced fission of 238~are shown in Fig. 4. Various symbols represent /31/ the results of different experimental runs which are obviously consistent with each other. The dashed line is the result of PWEIA calculation where the virtual photon spectra for electrons and positrons are the same. The full curve represents /31/ the result of a calculation of the El virtual photon spectra in DWBA without including nuclear size effects. The C photofission data of Caldwell et al. /5/ were used in this calculation. The u-/u data lie well below the full El curve. Small changes in the scale of the virtual photon spectra would lead /31/ to good agreement between the data and the full El curve. Such a change can be obtained /32/ by inclusion of nuclear size effects. The hatched area represents a calculation where, in addition to the El strength, 65*7% of the E2, EWSR is included. It is obviously too high compared with the data. The comparison made in this figure supports the contention that in the fission channel there is little E2 strength, a conclusion which is in disagreement with the results of ~rruda-Neto et al. /1,2/.

Fig. 4 Ratio of electrofission to positrofission cross sections 0-/o+ as a function of incident energy for 238~. For more details see text and refs. 31 and 32.

The most interesting of the electron-induced fission measurements is a coincident electrofission experiment performed by Dowell et al. /7/. Illis has the advantage over the previously discussed electron induced experiments in that: i) it is free from the inclusive energy integrated measurement and therefore the exact shape of C4-164 JOURNAL DE PHYSIQUE the multipole strength can be determined. ii) The momentum transfer can be chosen to enhance various multipolarities thus making it easier to disentangle the contributions of these various multipolarities. iii) It is relatively model independent. Moreover. it has the advantage over hadron scattering experiments in that it is free from the huge nuclear continuum underlying the giant resonances in the actinide nuclei. In hadrwn scattering experiments assumptions have to be made concerning the shape and magnitude of this background.

In the coincident electrofission experiment /7/, effective momentum transters varied from 0.36 to 0.59 fm-' for which only low multipoles (AGZ) are excited. Moreover, because the EO and E2 form factors are indistinguishable at low q, the data analysis is limited to disentangling the El contribution from the combined EO/E2 contribution to the coincident electrofission cross section. In the actual analysis the (y,f) data of Caldwell et al. /5/ were included on an equal footing with the (e,elf) data. The results are shown in Fig. 5. The E2/EO strength has the usual characteristic behaviour near the fission barrier (see Fig. 5a). The most

w (MeV) Fig. 5 - The E2/EO strength and its measured angular asymmetry deduced from the (e,elf) experiment of Dowel1 et al. 171. interesting feature of the E2/EO strength, however, is its flatness from 7 to 11.7 MeV. No resonant behaviour is observed similar to the one extracted by Arruda Net0 et al. /1,2/, or similar to the giant resonance bump observed in hadron scattering /24-28/. The anisotropy, represented by the ratio of the 1800 to 900 fission yields, is large near the fission barrier indicating a dominance of E2 contribution. Above 7 MeV, however, the isotropy could be due either to a large EO contribution or to K-mixing in the decay of E2 strength.

In Fig. 6, the E2 photofission cross section deduced from the (t3,e.f) measurement of Dowel1 et al. is compared /7/ with the E2 photofission Cross Section derived by Arruda-Neto et al. /1,2/ from their inclusive (e, f) measurement. It is clear from this comparison and the foregoing discussion that Arruda-Neto et al. have overestimated the E2 strength above 7 MeV, which indeed casts doubt on their data.

5.0 7.5 10.0 12.5 15.0 17.5 20.0 w (MeV)

Fig. 6 - Comparison between the E2 photofission cross sections deduced from the coincident electrofission measurement 171 and the inclusive electrofission results of Arruda-Neto et al. /1,2/.

The integrated E2/EO strength observed by Dowel1 et al. between 7 and 11.7 MeV corresponds to 10%of the isoscalar E2 EWSR with an uncertainty of 25%. The authors argue that if E2 strength has the same fission probability as the GDR then there would be in total 45% of the E2 EWSR in this excitation energy region. However, one should remark that in fact this experiment can riot differentiate between E2 and EO strength or between isoscalar and isovector strength. Although from theoretical considerations one would not expect much isovector E2 or EO strength in this enerqy region, there is evidence /8/ for substantial isoscalar EO strength as will be discussed later. This would lead to about 7% E2, EWSR strength between 7 and 11.7 MeV in the fission channel and therefore a total of 32% E2, EWSR strength in this region if the fission probability of the qRis the same as that of the GDR.

Before ending this section, it is worth mentioning the result of Lhe muon-induced fission expriment by Johansson et al. /34/. In this experiment, fission induced through the radiationless 9.6 MeV quadrupole 3d-1s transition, which is in the C4-166 JOURNAL DE PHYSIQUE

region of the GQR, was investigated. A fission probability of 2.1*0.9% was obtained which is an order of magnitude lower than that of the GDR. However, the reduced fission probability for the GQR here is due to the effect of the negative muon in the 1s orbit on the height of the outer barrier. The height of the barrier could increase /34/ by about 0.8-1.2 MeV, which would have a drastic effect on the fission probability. In principle, one can calculate the augmentation of the barrier from fission induced by the radiationless 2pls El transition. However, the statistical accuracy of the data /34/ was not good enough to warrant such an evaluation.

I11 - FISSION INDUCED BY HADRONIC PROBES

Motivated by the results of Arruda-Neto et al. /22/ on the high fission probability of the GQR in the actinide nuclei, a preliminary measurement /9,35/ on the a-induced fission decay of the isoscalaz giant resonances (GR) was performed at the KVI on 232Th at Ea = 120 MeV. In this experiment, a prominent GQR peak was expected in the a-fission coincident spectra because of the reported high fission probability for the GQR /22/ as compared to the GDR. This was based on the assumption that the continuum underlying the giant resonances at 3 11 MeV would have a fission 232 probability similar to the GDR (i.e. Pp.05 for Th /5,6/) since the Continuum is mainly due to multistep processes and thus consists of complicated structure states which would decay statistically. The contribution of direct neutron knockout in this excitation energy region is not expected to be significant. Moreover, this process leads mainly to states below the fission barrier in the (A-1) nucleus and thus does not contribute to the fission yield. Coincident spectra were measured for a number of a-scattering angles between 100 and l8.5O. me a-particles were detected with a AE-E solid state counter telescope. The solid state fission detector was positioned along the recoil axis of the 232~hnucleus. It quickly became apparent that no significant bump corresponding to the GQR was present in the a-fission coincident spectra for resonant scattering angles, whereas in singles spectra the GR bump (which probably consists of L = 0, 2, 4 and higher multipolarities) was obvious /28/. Because of the low fission probability in 232~hand thus the low statistics the measurement was inconclusive concerning the upper limit of the fission probability of the GR. However, it was Clear from this measurement that the fission probability of the GQR is not higher than that of the underlying continuum.

Taking advantage of the new QMG/2 magnetic spectrograph /36/ at the KVI, which is better suited for detecting the inelastically scattered a-particles than a solid state counter telescope, a new measurement was undertaken. In this experiwnt, the spectrograph was set at 180, near the third maximum for a 5 =2 transition, with a fully open solid angle (M = 10.3 msr and A9 = 60). Alpha particles inelastically scattered from 232Th and 238~targets were detected with the spectrograph in coincidence with fission fragments detected along the recoil axis using a solid state detector. The results are shown in Fig. 7. The top section shows a singles spectrum for 232Th taken with the spectrograph under the same conditions as for the coincidence spectra except for focussing on the contaminant peaks. The inset shows a 232Th singles spectrum taken /28/ with solid state detectors at 17O. In both spectra, the GR bump is quite apparent above the nuclear continuum. In the middle and the lower sections of Fig. 7, a-fission coincident spectra for 232~hand 238~ are shown. They display typical behaviour with a peak neat 6 MeV which is due to the fact that the fission barrier is below the neutron threshold. Above 7 MeV, the a-fission coincident spectra are smooth and structureless up till the second chance fission threshold where there is a slight hint of an increase in the fission Fig. 7 - Comparison between 232~h singles spectra and fission-coincident spectra taken along the recoil axis for 232~hand 238~at 120 MeV.

I -4

1 0. 5 exc~tafion energy (MeV)

yield. This increase is more obvious in spectra (not shown here) taken with a larger energy bite. It is rather clear that no bump corresponding to the GQR or to one of its components is present. This led us /36/ to deduce preliminary upper limits on the fission probabilities for the GQR bump in 232~hand 238~of 2 20% of that of the nuclear continuum.

The above upper limit was derived from a-fission coincident spectra taken at only one angle along the recoil axis. Some assumptions had to be made about the angular correlations of the GQR and its components. To be completely independent of any such assumptions. an angular correlation measurement is necessary. This may have to include full in-plane and out-of-plane angular correlation measurements, although it has been the practice that only in-plane angular correlation measurements have been made and axial symmetry around the recoil axis has been assumed to obtain the fully integrated (over en) coincident cross sections. This assumption of axial symmetry around the recoil axis is not always valid. However, angular correlation C4-168 JOURNAL DE PHYSIQUE calculations performed with the program ANGCOR /37/, whic?~uses m-state population amplitudes obtained from DWBA calculations, indicate that at least for our experimental conditions /9,25/ (Ea = 120 MeV, ea = la0 and Ex = 6-14 MeV) axial symmetry around the recoil axis is a reasonable approximation. This can be seen from Fig. 8, where the results for the components of three possible Gtransfers are shown. The calculations were performed for an excitation energy of 6 MeV in 238~and for two azimuthal angles @I = 00 (in-plane) and q~ = 90° (out-of-plane) taking the recoil axis to be the polar axis. The solid curves represent the results of PWBA calculations which display axial symmetry around the recoil axis with the angular X correlations being given by: W cc ]P,(e)l2. The angular correlation curves JK( 8) obtained using m-state population amplitudes from DWBA calculations are shown as dashed curves. These are very similar to the ones calculated in PWBA, both in-plane and out-of-plane, with a very slight breaking of the axial symmetry. Therefore the fission probability can be calculated from the in-plane coincident spectra by: -r

where the 2 in the denominator is to account for the fission multiplicity.

Fig. 8 - In-plane ($=0°) and out-of-plane (+=90°) angular correlation calculations for L=l, 2 and 3 (K22) obtained using m-state population amplitudes from PWBA (solid curves) and DWBA (dashed curves) calculations for 238~(a,a'f). To investigate the decay of the isoscalar giant resonances further, we have measured /25/ in-plane angular correlations of a-induced fission of both 232Th and 238~to obtain the total fission probability as a function of excitation energy. Again, the inelastically scattered a-particles corresponding to excitation energies between 4-14 MeV were detected with the spectrograph which was set at 180 with a fully open solid angle. Solid state detectors were used to detect fission fragments for angles ercm = -50° to +900 in the recoil center of mass (ran). me a-fission coincident spectra (not shown here) have the usual peak between the fission barrier and the neutron threshold. No bump corresponding to the GR was observed but evidence for a rather weakly excited bump at Ex 19.5 MeV and for 8- 3 50° was found.

This is very similar to the results of a 238~(6~i,6~isf)experiment by Shotter et al. /24/. In this experiment /24/ a GR bump, centered at 10.5 MeV with a FWHM = 7 MeV, was clearly observed in singles spectra very similar in shape to the GR bump seen /25/ in the 238~(a,a1)singles spectra except for the slope of the underlying continuum. No corresponding bump was observed in '~i-fission coincident spectra for fission angles ef = 00 and 20° w.r.t. the recoil axis. For backward angles (ef = 65O and goo), however, a narrow bump located around 9.5 MeV was seen. The results are shown in Fig. 9. In Fig. 9a. the coincident spectrum (I) corresponds to the sum of all coincident spectra for ef 3 65O w.r. t. the recoil axis. The solid line (11) corresponds to the smoothed singles spectrum from which contaminant peaks have been subtracted /24/. Fig. 9b shows the ratio of the coincident to the singles spectra. The spectrum in Fig. 9c is derived from that of Fig. 9b by applying angular correlation corrections and by normalizing to the results of the (t,pf) data at 6.1 MsV. It is interesting to note that the fission probability decreases in the reqion of the GR bump before it starts rising again above the second chance fission threshold, in contrast to the flat behaviour for the (y,f) case. It is also much lower (about a factor of two) in absolute value. From inspection of this figure one would corlclude that the fission probability of the GR bump is much lower than that of the underlying continuum, i.e. assuming that the underlying continuum has a flat fission pmbabitlity similar to the GDR. The authors /24/ concluded, however, that the GR bump must have a fission probability greater than 0.5 that of the underlying continuum by making assumptions about this underlying continuum in the singles and

I - 1400 t - 1200- (1) I ar - 1 r, I000 - r - U $ 800- a I Co~nc~dencespectrum 600- II Smoothed s!nqles spectra Fig. 9 - Comparison between $ 400- t- singles and fission-coincident spectra of inelastic 6~i- 200 - scattering from 238~at I , 150 MeV (for more details see f text and ref. 24). E, '0- +L 0 8- %- e 6- H - E 2 4; + t U 0 2- cn" - 1

Excitation energy(MeV) C4-170 JOURNAL DE PHYSIQUE coincident spectra as indicated in the dashed lines in ~iq.9a. 'Cf the fission probability of the continuum is only 0.1 as suggested by Fiq. 9c, this would still suggest a very low fission probabflity for the GR bump as compared to that of +;he GDR even with the large quoted uncertainCy of 50% for the estimated total fission probability /24/.

TO emphasize the points already discussed above and the s.imilarity between the experimental results from tt~e238~( 6~i, 6~i* f) experiment /24/ and ttle 238~(a,a*f ) experiment /25/, the sum of the forward angles spectra (0, S 20°) and the sum of L%m the backward angles spectra (BrLm 2 500) from the 238~(a,or'f) experiment /25/ are shown in Fig. 10. The smooth behaviour of the fission yield between the first and second chance fission thresholds for the Small angles a-fission coincident spectzum is evident. Tt~ebackwud angles a-fission coincident sj?ectrum shows evidence for the weakly excited structure at about 9.5 MeV also observed in the 238~(6~,6Li1f) experiment /24/ (see Fig. 9a). The dashed curve represents the smoothed GL.L-fission coincident spectrum of Fig. 9a. The similarjty between the two Spctra (apart from a slight difference arising from the different slopes of the continuua underlying the GR bump in the two reactions) indicates that the structures in 238~observed in these two experiments /24,25/ are the same. The nature of this bump structure Is not

Fig. 10 - Summed a-fission coincident spectra in the indicated angular ran es for a-induced fission on '32Th and 238~l. The dashed curve is the the smoothed 6~i-fission coincident spectrum of Fig. 9a.

c t- N m m10 N? N? if) - if)

03 1 , 8 ti 1000 500 "-/&.38 6 .6 . r U( LI, LIL) I

excitation energy (MeV) understood. If it is assumed to be due to khe GR bump then a fission probalxility of 4 0.11 would be deduced for the GR from the (a,alf) experiment by Wing reasonable assumptions on the underlying continuum both in the singles and coincident spectra. This is less than half the fission p'robabili-ty of the GDR. A similar conclusion can be drawn £rum the 232~1(a,a'f)data shown in the lower section of Fig. 10. The upper limit on the fission probability of the GR bump in 232111 as deduced from this spectrum is < 0.025.

The angular corlelations near the fission barrier and the GR bump obtained from the (a,alf) experiments on 232Th and 238~are shown in Kig. 11. Both in 232~1 and 238~, the angular correlations for the peak new- the fission barrier has large an'isotxopies indicating large ContrDutions of low K-values near the fission barrier. For higher excitation enerqies this anisotropy diminishes due to the contribution of many K-values in the fissioning of comund nuclear states. All angular correlations indicate symmetry with respect to the recoil axis to within the accuracy of the experiment.

I I I I I I I I I

1000 232~h(a,a'f) 2000 238~(aa'f)

e,,,(degl Fig. 11 - Experimental fission angular correlations for 232~hand 238~in the barrier and GR regions. Triangles represent the reflection of data w.r.t. the recoil axis (ere, = OO). We also performed /38/ an out-of-plane angular correlation~measurementuncle2 similar experimental conditions to these of the in-plane measurement. In this exper.&nt, however, the excitation energy region from 5 to 15 MeV was covef'ed. The plane in wtrich the fission detector moved was perpendicular to the reaction plane and intersected it along the recoil axis. The measured angular correlation (not shown here) agreed very well /38/ with the in-plane measured angular correlation. Tl~ls constituted the experimental proof that the recoil axis is an axis of axial symetry for our expe'rimental conditions. Moreover, the measured a-Ciss-ion coincident spectra showed the same features as the ones measured in-plane wl~icti is to be expcted if axial symmetry arounil the recoil axis is valid. The integrated in-plane (triangles) and out-of-plane (dots) total fission probalxilities derived accoriiing to Eq. (5) are Shown as a function of excitation enerqy in the bottom section of Fig. 12. They are in god agreement within the experimental errors. It is interesting to canpare the fission pr.obabil.ity spectra with the 238~(a,a') singles spectrum shown at tile top of JOURNAL DE PHYSIQUE

Fig. 12 - Comparison between coincident o fission probability 10 5 spectra, obtained from exc~tationenerqy (MeV) in-plane and out-of- plane angular correlation measurements, with I 238~(a.a1f) Ea=120 MeV I singles for 238~.

EXCITATION ENERGY (MeV)

Fig. 12. While the singles spectrum stiows a distinctive GR Fak, ttie fission probability is a decreasing function of energy between 8 MeV an0 the second cttarice fission threshold. It is impztant to note that if ttie GR bump and the underlylnq cori+cirruum have the same fission probability as ttie GDR then these fiss'iori probability spectza would have sttown a flat behaviour between a and 12.5 MeV at a value of 0.22. The decreasing fission p'rwility tttus .in&icates that eik?it?r the continuum or- the GR bump rxc both have a decreasing fission probability in this energy region. IC, however, we assume that the fission probability of ktie cont.huum is similar. to that of the GDR as we have argued earlier, then ttie decreased fission probabilfty would correspond to the GR bump. Tt~isseems to be corrobo'cated by the absence of a peak from the a-fission coincident spectra wlriclt would correspond to the GR bump. An estimate of the fission probability of the GR bump can be Wde flat a comparison of the fission probability spectra with the singles s&xxtrum. Making reasonable assumptions on the underlying continuum in the singles spect2um we obtain a fission probability of sO.11 -in ac~reement with the value derived from the a-fission Coincident spectra. In Fig. 13, the average of our in-plane and out-of-plane fission probability data (solid line) is compared with the fission probability from photofission data (dotted line) /5/ and also from (a,alf) data obtained /27/ at Ea = 152 MeV. Bertrand et al. /27/ made two a-fission coincidence measurements: one at an angle where the GR bump was prominently observed in the singles spectrum (on-resonance) and the other at an angle where the GR bump was absent (off-resonance). The fission detector in both measurements was set along the recoil axis. However, the data /27/ had rather low statistics. The fission probability spectra obtained from these data after arbitrarily normalizing to our data at 7.5 MeV are shown as dashed (off-resonance) and dashed-dotted (on-resonance) curves in Fig. 13. It is interesting to note that khe on-resonance fission probability is lower than the off-resonance fission probability for all excitation energies above 9 MeV. This can be reconciled only by invoking a decreasing fission probability of the GR bump as compared to the continuum underlying it. This conclusion, however, is in disagreement with the conclusions of Bertrand et ill. /27/ drawn from the same data. These authors /27/ derived a fission probability of 0.25t0.10 for the GQR assuming K-Conservation during fission, or 0.19*0.09 assuming K-mixing during the fission process and isotropy of fission in the nuclear continuum. However the picture that emerges for the fission decay of the giant monopole and quadruple resonances and that of the underlying continuum is not consistent and it cerkainly cannot be reconciled with the behaviour depicted in Fig. 13.

KVI 2290 /1- -PRESENT EXP...... CALDWELL ET AL. ---- BERTRAND ET AL.OFF RES. - .- .- . - RERTRAND ET AL. ON RES

Fig. 13 - Comparison between fission probability spectra from various experiments on 238~(see text for more details).

0.0 14 12 10 8 6 EXCITATION ENERGY (MeV)

In an experiment designed to determine the fission probability of the GQR and the 3Rw isoscalar dipole and octupole giant resonances, Uorsch et dl. /3/ measured a-fission coincident spectra for 238~at Ea = 172 MeV. A peak observed in the C4-174 JOURNAL DE PHYSIQUE a-fission coincident spectra, in the recoil direction only, was associated /3/ with the decay of the GQR. A fission probability of 4% was obtained for this peak assuming a 1~: angular correlation. The authors /3/ claimed that the non-isotropic angular dependence observed for this peak indicated a strong coupling to the fission channel. However, such a strong coupling should lead, as discussed in the introduction, to a high fission probability of the GQR instead. The authors /3/ also fitted the bump "associated" with the isoscalar dipole and actupole giant resonances. They made certain assumptions on the location and widths of these giant resonances as well as on the shape of the nuclear continuum underlying them both in the singles and coincidence spectra. It has to be pointed out that the assumptions on the magnitude and shape of the fissioning underlying continuum are rather dubious in this case. This is because the fission probability associated with statistical fission decay has a very strong energy dependence in the region of the 3Tio resonances since it coincides with the region of the third chance fission threshold. This is indicated by the statistical model calculation /9,39/ for the total fission probability in 238~(see Fig. 14) which accounts for lst, 2nd, 3rd and

0.6 - Fig. 14 - Calcuiated total fission probability as a function of > + excitation energy for the compound .-- - n nucleus 238~(see text for more n0 details). 0.4- ?a

C .-0 - -.-"lU) 0.2-

10 20 30 excitation energy (MeV) 4th chance fission probabilities. In this calculation experimental fission probabilities for the A, A-1, A-2 and A-3 nuclei at the fission barriers are used since they are difficult to reproduce in the simple statistical model used here. Above the fission barriers the experimental fission probabilities were reproduced by a variation of the fission barrier heights and af, the level density parameter at the barrier. The uncertainty in the calculated fission probability is estimated to be equal to the uncertainty in the experimental fission proDabilities in the various isotopes up to the second chance fission threshold. Because of the Strong variation in the fission probability and because of the uncertainty in the direct neutron knockout contribution to the continuum underlying the 3T1w resonances, it is difficult to derive any fission angular correlations for resonance bumps in this excitation energy region. Nevertheless, Morsch et al. /3/ obtained angular correlations for two bumps at 17 and 21 MeV in 238~,which seemed to agree with lp;l2 and (P1l0 2 , respectively. This is, of course, in disagreement with the statistical fission decay of these resonances which would lead to isotropic angular distribution of fission fragments at these high excitation energies. It is also not clear from this experiment what happens to the other components of these giant resonances. Their results /3/ for the Lission probabilities of the continuum and the 3Rw resonances are shown in Fig. 15. The fission probability of the continuum is represented by the solid line with (thin curve) and without (thick curve) including the giant monopole resonance decay. The dashed curve represents the fission probability as estimated by Morsch et al. /3/ from a-fission coincident spectra from 232 a 238~(a,a'f) experiment /40/ and from singles spectra from another Th(a,al) experiment. The fission probabilities for the 3Rw resonances are shown as hatched areas. At any rate, the fission probability of the continuum is far below the predicted statistical fission probability at energies of 21 MeV, possibly indicating a large contribution to the singles spectra from direct neutron knockout reactions. The fission probability of the 3Rw bump seems to agree better with the statistically calculated one, but because of the arguments discussed above it is difficult to draw any conclusion from these data.

Fig. 15 - Comparison between the fission probabilities of the 3fiw giant resonances and the underlying continuum in 238~(see text and ref. 3 for more details).

None of the experiments described above can differentiate between E2 and EO strength. This is true for the electron-induced fission exeprjments because of the similarity of the E2 and EO form factors for low-momentum transfers. It is also true for the hadron-induced fission experiments because the observed GR bump at about 11 MeV may consist of L = 0 and 2 strength (probably higher multipole strength as well) with very little chance of disentangling their relative contributions at the measured angles.

In inelastic hadron scattering experiments, it is possible to assess the EO contribution to fission from measurements /8/ at very forward angles (close to OO). The principle of the method is based on the fact that the L = 0 angular distribution of inelastically scattered particles is very steep, compared to those of other L-transfers, at angles very near to OO. This is illustrated in Figs. 16 and C4-176 JOURNAL DE PHYSIQUE

17. In Fig 16, the curves are results of DWBA calculations for isoscalar giant resonances in 238~, assumed to be located at lo MeV and to exhaust 100% of their respective EWSR's. The resonances are excited in inelastic a-scattering at Ea=120 MeV. for L 3 2, usual collective form factors were used, while for L = 0 and 1 transfers those of Satchler /41/ and Harakeh and Dieperink /42/, respectively, were used. This figure indicates that the L = 0 angular distribution is by far the steepest (decreasing by two orders of magnitude from 00 to 3O) of all the angular distributions. This is also true in comparison with the isovector GDR angillar distribution which is shown in Fig. 17. Here the cross section due to Coulomb KVI 2658 loZL, 8 r 8 , , , m rn , -.

1 238~(a,a') 238~, GDR Excitotion 1 E,= 120 MeV -Coulomb ---- Nuclear

v I1 11 lo0: I1II I111 11: I1 I1 I - 1 II I - II 1'- 11 - Y I1 - ',' -

10~'~"""""""~5 10 15 8,,. (degrees) Fig. 17 - Coulomb and nuclear excitation of the GDR assuming it is located at 10 MeV.

I I I I 1 0° 3O 6O 9O 12"

--tlg. . 16 - DWBA calculations for the indicated isoscalar multipolarities at 10 MeV assuming full exhaustion of the EWSR. excitation of a presumed GDR at 10 MeV exhausting its EWSR is drawn aS a solid curve. Although the a-particle is an isoscalar probe, it can excite /43/ the GDR by the nuclear interaction due to the different neutron and proton densities. The dashed curve represents such an excitation using the form factor given by Satchler /43/. It is clear that the Coulomb excitation cross section is an order of magnitude smaller than that of the monopole and it has a gentler fall-off near 00. This is in agreement with previous calculations /44/ for 208~band lighter nuclei. However, one should in principle add the Coulomb and nuclear excitations coherently. Such a calculation (not shown here for 238~)was performed by Izumoto et al. /44/ for 208~b for a number of bombarding energies and resulted in a total differential cross section that was always smaller at 00 than that of the Coulomb excitation alone. The above discussion leads us to expect that the difference in inelastic a-scattering cross sections measured for angular ranges 00-1.50 and 1.50-30 in 238~would be almost solely die to monopole excitation. This method has been checked experimentally at the KVI for a number of nuclei in the sd- and pshells where a large number oE discrete monople states are known.

The experiment was performed using the WG/2 magnetic spectrograph /36/ with a fully open solid angle (Ar3=60). The experimental arrangement is shown in Fig. 18. The beaut

Fig. 18 - Experimental layout of the spectrograph and its detection system for the O0 measurement. An enlarged view of the scattering chamber indicating the positions of the fission detectors is also shown. FC '~bshield I \

was stopped in a Faraday cup in the focal. plane. The Faraday cup was well shielded from the focal plane detector /36/. Fission fragments in coincidence with inelastically scattered a-particles were detected in seven avalanche counters situated a6 indicated in the enlarged view of the scattering chamber. It is important to note that the focal plane detector system, which consists of two 2-dimensional position sensitive detectors (PSD) and a scintillator, allows for ray tracing. This is achieved by determining the incidence angle on the focal plane which has a one-to-one correspndence with the scattering angle. Determination of the vertical and horizontal angles of incidence are computed from the vertical and C4-178 JOURNAL DE PHYSIQUE

horizontal positions determined by the two PSD's. A scatter plot of the vertical angle versus the horizontal angle is shown in Fig. 19. Projection Spectra for the angles are also shown. By setting circular l-dimensional gates one can determine the contribution to the a-fission coincident spectra from different angular ranges.

horizontal angle

Fig. 19 - Scatter plot of the vertical angle of incidence to the horizontal angle of incidence for a-particles inelastically scattered at angles near 00. Projected one-dimensional spectra for both angles are also shown.

The experimental results for the 238~(cr,arf) measurement /8/ are shown in Fig. 20. In Fig. 20a the spectrum of inelastically scattered a-particles in coincidence with fission fragments detected by all seven fission detectors is shown. This was obtained for the full opening angle of the spectrograph which corresponds at 00 to scattering angles from -30 to +3O. Fig. 20b is the same as Fig. 20a but for the condition that a-particles correspond to the core scattering angles between -1.5O to +1.5O. The spectrum in Fig. 20c is obtained by subtracting the spectrum in Fig. 20b from that in Fig. 20a and so it corresponds to the lateral scattering angles i .e. 1.5OG lea, 1 rJO. There is a striking difference between the spectra in Figs. 20b and 20c, which appears as a rise in the fission yield in the spectrum of Fig. 20b for energies 3 9 MeV in comparison with a flat behaviour for the spectrum of Fig. 20c. This can be attributed to L - 0 strength.

To determine the shape and the total cross section of the L=a strength, which is due to the excitation and decay of the GMR in this excitation energy region, one can Fig. 20 - Fission-coincident inelastic alpha spectra for the opening angles of -30 to +3O (a) and -1.350 to 1.350 (b) . (c) refers to spectrum (a) minus spectrum (b) and thus to the lateral scattering angles ranging from 1.35O to 30. (d) and (el are obtained by subtracting different fractions of spectrum (c) from (b). The dashed lines represent eyeball fits to the data. (f) giant monopole strength distribution derived from spectrum (d) by assuming a fission probability for the GMR similar to that of the GDR (solid curve) in comparisor, with the expected monopole strength from the data of Morsch et al. 1491 (dotted curve). (see text for more details.)

channel II I I I 16 14 12 10 8 6 excitotion energy (MeV) subtract the apectrum of Fig. 20c from the spectrum of Fig. 20b after weighting the spectra by the corresponding solid angles. Another approach would be to assume that for energies below 0 MeV very little direct excitation of EO strength is C4-180 JOURNAL DE PHYSIQUE present. Direct excitation of all other multiples leads to nearly flat angular distributions between O0 and 3O, except for the 3fiw isoscalar dipole resonance which rises steeply in this angular range but is not expected to contribute much to the cross section between 6 and 16 MeV. The differential cross section for multistep inelastic excitation is assumed to be flat near O0 but nevertheless is not expected to produce giant-resonance-like bumps in the spectrum.

The spectrum of Fig. 20d was obtained by subtracting one fourth of the spectrum of Fig. 20c from that of Fig. 20b resulting in almost complete cancellation of counts for E < 8 MeV. This factor of 0.25 corresponds to an angular range for the core X part of -1.35O to +1.35O which is very close to the intended analysis region of -1.50 to +1.5O. The dashed curve drawn in Fig. 206 represents an eyeball fit to the data and it indicates the shape of the EO strength distribution. This shape is not strongly dependent on the normalization factor used for the spectrum of Fig. 20c before the subtraction procedure. This is illustrated in Fig. 20e where a normalization factor of one fifth was used instead of one fourth. The shape of the EO strength distribution is hardly affected although now it appears to be riding over a flat continuum.

The solid line in Fig. 20f is obtained from the dashed line in Fig. 2- by assuming that the fission probability of the GKR is similar in shape and magnitude to that of the WR. In this case, the total EO strength exhausted by the GuR is 80*20% of the EO EWSR. This corresponds to 17.6*4.4% of the EWsR in the fission channel only. The uncertainty in the EO strength is estimated from the uncertainty in the normalization factor and the DWBA analysis. The EO strength centroid is located at z 12.5 MeV with a total width -6 MeV. This broad EO strength distribution suggests a splitting of the monopole strength in the deformed 238~nucleus in agreement with theoretical predictions /45-47/. Such a broadening of monopole strength has been observed /48/ previously in lighter deformed nuclei. Also the splitting of monopole strength in 238~was investigated by Morsch et al. /49/ from a comparison of inelastic a-scattering data taken at Ea = 100 and 172 MeV. Their results are shown as the dotted curve in Fig. 20f and indicate an actual splitting of the GKR strength in close agreement with the theoretical calculation of Abgrall et al. /%/. This calculation predicts /46/ a splitting of AE - = 4.7 MeV in 238~for P-4.3. However, the actual value of the deformation parameter p in 238~is 0.23, which would result in a smaller splitting of the GMR strength. Using the analytic fonnula obtained by Jang /47/ for the Splitting of the GHR, a splitting of AE = 26A-'l3 - 4.2 is obtained using m.23 and the GMR centroid energy of 77~-"~ 12.6 MeV. Our data still suggest a smaller splitting of 3.5 MeV which manifests itself as a broadening of the EO strength distribution.

The various experiments on the fission decay of the isoscalar giant resonances in the actinide nuclei have been reviewed. The picture that emerges is one in which the fission probability for the GR bump at about 11 MeV (which may consist of G- 0, 2 and higher multipolarities) is lower than that of the GDR. Assuming that the fission probability of the GDR can be explained as due to statistical decay as discussed in the introduction, then the lower fission probability of the GR bump could be due to preequilibrium emission of neutrons. Of course, this also assumes that the lower fission probability is not due to a subtle effect where e.g. the fission probability of the nuclear continuum and the angular correlation of its fission decay conspire to give a decreasing fission probability as a function of excitation energy and also to hinder the observation of the GR bump in ejectile-fission coincident spectra.

w (MeV)

Fig. 21 - Comparison of E2/EO strength obtained from the (e,e'f) experiment 171 with the EO strength obtained from the 00 measurement 181.

This low fission probability of the GR bump is clearly not due to a low fission probability of the GMR. The amount of EO strength of = 18% between 8 and 16 MeY found in the fission channel from the Oo measurement indicates that the GMR has a fission probability very similar to that of the GDR. This raises the question of whether the GQR is the resonance responsible for the low fission probability of the GR bump. One can investigate this question by a comparison between the EZ/EO strength distribution obtained from the (e,elf)measurement /7/ and the EO strength distribution deduced from the O0 measurement. This comparison is shown in Fig. 21. The hatched area represents the latter measurement while the data points represent the former measurement. If no EO strength is present in the e--fission coincident spectra then 10% of the E2 EWSR would be present between 7 and 11.7 MeV. If, however, the EO contribution obtained from the 00 measurement is subtracted this would leave only about 7% of the E2 EWSR. One expects from theoretical considerations that most of the E2 strength (probably as much as 70% of the EWSR) is located between 7 and 12 MeV. This would lead to a fission probability of the GQR of about 0.10; much lower than that of the GDR. Thus I think it is very important to perform an (e,eSf) experiment which would extend the results of Dowell et al. /7/ to higher excitation energies. This would indeed tell us, by comparison with the results of the O0 measurement, whether there is substantive E2 strength above 12 MeV.

To conclude, there are still a number of questions left unanswered concerning the C4-182 JOURNAL DE PHYSIQUE

fission decay of giant resonances contributing to the GR bump at about 11 MeV. Before these are answered it is premature to speculate about the fission decay of more weakly excited giant resonances at higher excitation energy. Here the analysis would be further complicated by lst, 2nd, 3rd and probably higher chance fission contributions which are in addition strong functions of energy.

I would like to acknowledge my colleagues who have participated in the Oo measurement? S. Brandenburg, R. De Leo, A.G. Drentje, H. Janszen and A. van der Woude. I would also like to thank E.F. Garman for a careful reading of the manuscript. The author would like to acknowledge partial support by the 0,s. Department of Energy. This work was perfonned as part of the research program of the Stichting voor Fundamenteel Onderzoek der Materie (mM) with financial support from the Nederlandse Organisatie voor Zuiver Wetenschappelijk Onderzoek (ZWD).

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