Analysis of the cross section for <Superscript>232</Superscript>Th photofission in the subbarrier energy region

A theoretical analysis of the most detailed experimental data on the subbarrier fission of ²³²Th is performed. The energy dependences of the components of the cross sections for photofission through specific channels characterized by the quantum numbers J ^π K are obtained on the basis of previously measured total photofission cross sections and angular distributions of fission fragments. A theoretical analysis of the subbarrier photofission of ²³²Th is performed on the basis of the results obtained for these components. This analysis relies on the one-dimensional fission model within which one assumes a three-humped structure of the fission barrier and introduces damping in the second and third wells by including an imaginary part in the potential used. Values are selected for the parameters of the fission barriers and are compared with similar data from other studies. It is shown that the energy dependences of the photofission cross sections in question can be described adequately.     

Investigation of the <Superscript>232</Superscript>Th and <Superscript>238</Superscript>U photofission

The average angular momenta of the ⁸⁴Br, ¹³²Sb, ¹³³Te, and ¹³⁵Xe fission fragments, produced as a result of ²³²Th and ²³⁸U photofission by bremmsstrahlung γ photons with an endpoint energy of 8.5 MeV, have been determined by measuring the isomeric yield ratios. The experimental isomeric yield ratios have been calculated taking into account the contribution from β decay of isobaric nuclei to the yields of isotopes studied.     

Isomeric yield ratios of <Superscript>238</Superscript>U and <Superscript>232</Superscript>Th photofission products

The isomeric ratios of fragments of photofission of ²³⁸U and ²³²Th have been determined experimentally at the bremsstrahlung end-point of 50 and 3500 MeV. Results of measurements enabled us to estimate the mean angular momenta of primary fragments and to compare them with the published data.     

Proton induced fission of <Superscript>232</Superscript>Th at intermediate energies

The mass-energy distributions and cross sections of proton-induced fission of ²³²Th have been measured at the proton energies of 7, 10, 13, 20, 40, and 55 MeV. Experiments were carried out at the proton beam of the K-130 cyclotron of the JYFL Accelerator Laboratory of the University of Jyväskylä and U-150m cyclotron of the Institute of Nuclear Physics, Ministry of Energy of the Republic of Kazakhstan. The yields of fission fragments in the mass range A = 60–170 a.m.u. have been measured up to the level of 10?4%. The three humped shape of the mass distribution up has been observed at higher proton energies. The contribution of the symmetric component grows up with increasing proton incident energy; although even at 55 MeV of proton energy the shoulders in the mass energy distribution clearly indicate the asymmetric fission peaks. Evolution of shell structure was observed in the fission fragment mass distributions even at high excitation energy.     

Indication for hyperdeformed cluster states in <Superscript>233</Superscript>Th

An activation technique was used to investigate relative yields of fission products from the reaction ²³²Th(n, f ) for neutron energies between 1.3 and 1.8MeV covering the region around the first hyperdeformed resonances. Intensities of characteristic γ-ray transitions were analyzed to search for changes in the mass distribution for neutron energies corresponding to the resonances and below the resonances. Relative increases in yield between 7 and 23% are observed for A ≈ 100 and 132 in the resonance region around 1.6MeV. It is proposed that the yield enhancement of daughter nuclei of the preformed fragments ¹³²Sn and ¹⁰¹Zr arises from cold fission of a di-cluster configuration. The experimental results support theoretical predictions for the existence of hyperdeformed octupole shapes based on the di-nuclear configuration ¹³²Sn + ¹⁰¹Zr.     

Nuclear-charge polarization at scission in the 12 MeV proton-induced fission of <Superscript>232</Superscript>Th

The most probable charges of secondary fragments, produced after neutron evaporation from primary fragments, have been evaluated using fractional cumulative and mass yields in the 12MeV proton-induced fission of ²³²Th . The nuclear-charge polarization of primary fragments at scission has been obtained by correcting the most probable charge of secondary fragments for neutron evaporation. The fragment mass dependence of the nuclear-charge polarization at scission shows good agreement with that for thermal neutron-induced fission of ²³⁵U , indicating that the nuclear-charge polarization is nearly insensitive to mass and excitation energy of the fissioning nucleus for asymmetric fission in the actinide region.     

Symmetric and asymmetric yields in the photofission of232Th,235U,238U and239Pu

Measurements are presented for the mass and energy distributions of fragments from the photofission of²³²Th,²³⁵U,²³⁸U and²³⁹Pu. The experiments were performed for bremsstrahlung endpoint energies between 15 and 55 MeV, using silicon surface barrier detectors. The results are discussed with respect to the competition between the symmetric and asymmetric fission modes.     

Independent yields of Kr and Xe fragments in the photofission of <Superscript>237</Superscript>Np and <Superscript>243</Superscript>Am odd nuclei

Results are presented that were obtained by measuring the independent yields of Kr (A = 89–93) and Xe (A = 135–142) appearing as fragments in the photofission of ²³⁷Np and ²⁴³Am odd nuclei. The respective experiments were performed in a beam of bremsstrahlung photons from electrons accelerated to an energy of 25 MeV at the microtron of the Laboratory of Nuclear Reactions at the Joint Institute for Nuclear Research (JINR, Dubna). Use was made of the procedure involving the transportation of fragments emitted from the target by a gas flow along a capillary and the condensation of inert gases in a cryostat at liquid-nitrogen temperature. The identification of Kr and Xe appearing as fragments was performed by the gamma spectra of their daughter products. The mass-number distributions of the independent yields of Kr and Xe isotopes were obtained, along with those for the complementary fragments (Y and La in the fission of ²³⁷Np and Nb and Pr in the fission of ²⁴³Am).     

Investigation of <Superscript>235</Superscript>U and <Superscript>239</Superscript>Pu photofission

Average angular momenta of fission fragments ⁹⁰Rb, ^129,130,132Sb, ^131,133Te, ^134,136I, and ¹³⁵Xe produced in photofission of ²³⁵U and ²³⁹Pu by bremsstrahlung γ rays with a threshold energy of 9.6 and 9.8 MeV, respectively, were determined by measuring the isomeric yield ratios. The experimental values of the isomeric ratios were calculated with account of the contribution from β decay of isobaric nuclei in the yields of the studied isotopes.     

Photofission cross sections of 232Th and 236U from threshold to 8 MeV

Measurements of the photofission cross sections of ²³²Th and ²³⁶U for an energy range of 5.0 to 8.0 MeV are reported. The Compton scattering facility at the Ames Laboratory Research Reactor was used for these measurements. Resonance structure at γ-ray energies of 6.3 and 7.6 MeV is observed in ²³²Th and at 5.3, 6.0, 6.5, and 7.5 MeV in ²³⁶U. The present results are compared with other photofission experiments and particle-induced fission studies. Suggestions for the interpretation of the resonances are given on the basis of Bohr's fission-channel theory and the existence of a double-humped fission barrier.     

Inverse-beta-decay reaction in the antineutrino flux from the fragments of <Superscript>232</Superscript>Th and <Superscript>233</Superscript>U fission

The energy spectra of antineutrinos produced in the beta decay of fragments originating from ²³³U and ²³²Th fission induced by neutrons are calculated. The relevant cross sections and the spectra of positrons produced in inverse beta decay are found. This study was motivated by the hypothesis (discussed over the past decade)t hat a self-sustained chain reaction proceeds at the center of the Earth (“georeactor”). According to the author of this hypothesis, the georeactor provides energy necessary for maintaining the Earth’s magnetic field. It is ²³⁵U and, probably, ²³²Th and ²³³U that serve as a nuclear fuel in this reactor. Data obtained in the present study can be guidelines in future experiments aimed at testing the hypothesis of the georeactor and at estimating the composition of its nuclear fuel within the development of geophysical and astrophysical investigations based on the observation of antineutrino fluxes in nature.     

Angular distribution of photofission fragment of even-even and odd mass nuclei studies using SSNTD technique

Fission fragment angular distribution measurements of even-even (²³²Th) and odd mass (²³⁷Np) nuclei were carried out using bremsstrahlung radiation in the energy range 7.4 to 9.2 MeV from variable energy microtron at Mangalore University. In the present work SSNTD lexan polycarbonate films were used to detect the fission tracks from both even-even (²³²Th) and odd mass (²³⁷Np) nuclei. From the knowledge of track density formed from photofission reaction of ²³²Th and ²³⁷Np, angular distribution of fission fragments was measured. The results obtained are discussed on the basis of Age Bohr theory in this paper.     

Influence of nuclear dissipation on fission dynamics of the excited nucleus <Superscript>248</Superscript>Cf within a stochastic approach

A stochastic approach to fission dynamics based on two-dimensional Langevin equations was applied to calculate the anisotropy of the fission fragments angular distribution and average pre-scission neutron multiplicities for the compound nucleus ²⁴⁸Cf formed in the ¹⁶O+²³²Th reactions. Postsaddle nuclear dissipation strength of (12–14) × 10²¹ s^?1 was extracted for Cf nucleus by fitting the results of calculations with the experimental data. Furthermore, it was found that the results of calculations for the anisotropy of the fission fragments angular distribution and pre-scission neutron multiplicities are very sensitive to the magnitude of post-saddle nuclear dissipation.     

Mean energies of prompt fission neutrons in the neutron-induced fission of <Superscript>235</Superscript>U at primary energies in the range <Emphasis Type="Italic">E</Emphasis><Subscript>n</Subscript> < 20 MeV

The spectra of neutrons accompanying the induced fission of ²³⁵U target nuclei are described theoretically. It is confirmed that a third neutron source must be introduced in order to reproduce the shape of experimental distributions at high energies of primary neutrons (previously, a third source was used in describing the spectra of neutrons emitted in ²³²Th and ²³⁸U fission). On the basis of experimental results and their analysis, the mean energy of fission neutrons is estimated as a function of the bombarding-neutron energy up to E _n = 20 MeV.     

Subthreshold photofission of even-even nuclei

Within quantum-mechanical fission theory, the angular distributions of fragments originating from the subthreshold photofission of the even-even nuclei ²³²Th, ²³⁴U, ²³⁶U, ²³⁸U, ²³⁸Pu, ²⁴⁰Pu, and ²⁴²Pu are analyzed for photon energies below 7 MeV. Special features of various fission channels are assessed under the assumption that the fission barrier has a two-humped shape. It is shown that the maximum value of the relative orbital angular momentum L _m of fission fragments can be found upon taking into account deviations from the predictions of A. Bohr’s formula for the angular distributions of fission fragments. The result is L _m ≈ 30. The existence of an “isomeric shelf” for the angular distributions of fragments from ²³⁶U and ²³⁸U photofission in the low-energy region is confirmed.     

Dynamical decay process of <Superscript>219, 220</Superscript>Ra<Superscript>*</Superscript> formed in <Superscript>10, 11</Superscript>B + <Superscript>209</Superscript>Bi reactions

The excitation functions for both the evaporation residue and fission have been calculated for ¹⁰B + ²⁰⁹Bi and ¹¹B + ²⁰⁹Bi reactions forming compound systems ^{219, 220}Ra^* , using the dynamical cluster-decay model (DCM) with effects of deformations and orientations of the nuclei included in it. In addition to this, the excitation functions for complete fusion (CF) are obtained by summing the fission cross-sections, neutron evaporation and charged particle evaporation residue cross-sections produced through the axn\ensuremath \alpha xn and pxn\ensuremath pxn (x = 2, 3, 4) emission channels for the ²¹⁹Ra system at various incident centre-of-mass energies. Experimentally the CF cross-sections are suppressed and the observed suppression is attributed to the low binding energy of ^{10, 11}B which breaks up into charged fragments. The reported complete fusion (CF) and incomplete fusion (ICF) excitation functions for the ²¹⁹Ra system are found to be nicely fitted by the calculations performed in the framework of DCM, without invoking a significant contribution from quasi-fission. Although DCM has been applied for a number of compound nucleus decay studies in the recent past, the same is being used here in reference to ICF and subsequent decay processes along with the CF process. Interestingly the main contribution to complete fusion cross-section comes from the fission cross-section at higher incident energies, which in DCM is found to consist of an asymmetric fission window, shown to arise due to the deformation and orientation effects of formation and decay fragments.     

Observation of deep subbarrier resonances in <Superscript>232</Superscript>Th fission

Pronounced subbarrier ²³²Th fission induced by resonance neutrons was observed.     

The study of the (α, α′ f) reaction at 120 MeV on 232Th and 238U: (II). Fission barrier properties deduced from fission probabilities and angular distributions

The fission probabilities and angular distributions of the fission fragments for the (α, α'f) reaction on ²³²Th and ²³⁸U at a bombarding energy of 120 MeV have been measured from about 4 to 14 MeV excitation energy. Evidence for sub-barrier resonances has been found, the negativeparity ones occurring at the same excitation energy where photofission resonances have been observed. The data are analyzed with the two-humped barrier model. For ²³⁸U the data are reasonably well fitted with barriers similar to those known from the literature. For ²³²Th though, the outer barrier parameters are quite different: the height E_B = 6.6 MeV and the width (khω)_B = 1.2 MeV. Also for ²³²Th, introducing an additional mass symmetric and axially asymmetric outer barrier, as was previously found necessary for ²³⁸U, does not result in a good fit to the data at higher excitation energies.     

Investigation of the reaction <Superscript>208</Superscript>Pb(<Superscript>18</Superscript>O, <Emphasis Type="Italic">f</Emphasis>): Folding angular distributions of fission fragments and gamma-ray multiplicity

Correlations between folding angular distributions of fission fragments and the gamma-ray multiplicity are studied for ¹⁸O + ²⁰⁸Pb interactions at energies of the beam of ¹⁸O ions in the range E _lab = 78–198.5 MeV. The probabilities are determined for complete-and incomplete-fusion processes inevitably followed by the fission of nuclei formed in these processes. It is found that the probability of incomplete fusion followed by fission increases with increasing energy of bombarding ions. It is shown that, for the incomplete-fusion process, folding angular distributions of fission fragments have a two-component structure. The width of folding angular distributions (FWHM) for complete fusion grows linearly with increasing energy of ¹⁸O ions. The multiplicity of gamma rays from fission fragments as a function of the linear-momentum transfer behaves differently for different energies of projectile ions. This circumstance is explained here by the distinction between the average angular momenta of participant nuclei in the fusion and fission channels, which is due to the difference in the probabilities of fission in the cases where different numbers of nucleons are captured by the target nucleus.     

Excitation energy dependence of fragment characteristics for the photofission of 232Th

Independent and cumulative product yields were measured for the photofission of ²³²Th with bremsstrahlung with endpoint energies 6.5, 7.0, 8.0, 11.0, 12.0, and 14.0 MeV, applying γ spectrometric techniques on catcherfoils and pneumatically transported ²³²Th-samples. The independent heavy fragment yields for the fission of the ²³²Th compound nucleus at excitation energies in the vicinity of the fission barrier were deduced. Postneutron mass, isobaric charge, isotopic mass distributions, isotonic and elemental yield distributions and proton odd-even effects were obtained from these independent yields. In the mass distributions a maximum yield is observed for mass splits with heavy fragments in the region of A = 142, corresponding with a high production of Ba(Z = 56) - isotopes. A slightly increased yield is also observed for mass splits with heavy mass in the vicinity of A = 134. The latter effect increases with increasing compound nucleus excitation energy. The similarity between the mass distributions of the N = 142 fissioning systems ²³²Th, ²³⁴U and ²³⁶Pu is striking. For low excitation energy the proton odd-even effect in the element distributions amounts to 30%, while on the other hand no sizeable neutron odd-even effect could be deduced from the isotonic distributions. The proton odd-even effects remain constant up to compound nucleus excitation energies of about 7.85 MeV. For higher compound nucleus excitation energies the proton odd-even effect drops rapidly. A possible explanation of these observations in terms of pair breaking at the outer barrier is proposed.