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

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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 HAL Id: jpa-00224078 https://hal.archives-ouvertes.fr/jpa-00224078 Submitted on 1 Jan 1984 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. JOURNAL DE PHYSIQUE Colloque C4, suppl6ment au n03, Tome 45, mars 1984 page C4-155 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 fis- sion 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.
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