Proton-induced Fission Of 232Th Research Articles

Experimental cross sections and mass distribution of fission products of thorium-232 irradiated with protons in energy range 20–140 MeV

Proton-induced fission of 232Th was studied in a wide proton energy range 20–140 MeV. A large amount of experimental cross sections for individual and cumulative formation of 232Th fission products were determined by means of stacked-foil irradiation followed by high-resolution γ-ray spectrometry. The obtained results were compared with available literature data and theoretical calculations performed with cascade–evaporation–fission model. The fission-product mass distribution in the given proton energy range was described with a three-peak curve representing asymmetric and symmetric fission modes. The fraction of symmetric fission mode increased along with the incident proton energy up to 60–70 MeV reaching a plateau at the value around 0.65.

Proton-Induced Fission of 232Th at Low and Intermediate Energies

Mass–energy distributions of fission fragments are measured in the proton energy region of 7 to 55 MeV in order to study shell effects in the fissioning of 232Th nuclei induced by protons at low and intermediate energies. Experiments at proton energies of 13 to 55 MeV are performed using proton beams from the K-130 cyclotron at the University of Jyvaskyla (Finland) using the CORSET two-armed time-of-flight spectrometer. Experiments at proton energies of 7, 10, and 13 MeV are performed using the beam from the U‒150M cyclotron at the Institute of Nuclear Physics in Almaty (Kazakhstan) using a 2Е spectrometer for detecting fission fragments. It is found that the yield of symmetric fragments falls along with the proton energy, while the yield of asymmetric fragments rises. Deep below the Coulomb barrier at the lowest proton energy of 7 MeV, an increased yield of fission fragments is observed in the region of 60−70 a.m.u., which corresponds to the superasymmetric mode of fission.

Proton induced fission of 232Th 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 Jyvaskyla 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.

Mass-yield distributions of fission products from 20, 32, and 45 MeV proton-induced fission of 232Th

The yields of various fission products in the 19.6, 32.2, and 44.8 MeV proton-induced fission of 232Th have been determined by recoil catcher and an off-line γ-ray spectrometric technique using the BARC-TIFR Pelletron in India and MC-50 cyclotron in Korea. The mass-yield distributions were obtained from the fission product yield using the charge distribution corrections. The peak-to-valley (P/V) ratio of the present work and that of literature data for 232Th(p,f) and 238U(p,f) were obtained from the mass yield distribution. The present and the existing literature data for 232Th(p,f), 232Th(n,f), and 232Th(γ,f) at various energies were compared with those for 238U(p,f), 238U(n,f), and 238U(γ,f) to examine the probable nuclear structure effect. The role of Th-anomaly on the peak-to-valley ratio in proton-, neutron-, and photon-induced fission of 232Th was discussed with the similar data in 238U. On the other hand, the fine structure in the mass yield distributions of the fissioning systems at various excitation energies has been explained from the point of standard I and II asymmetric mode of fission besides the probable role of even-odd effect, A/Z ratio, and fissility parameter.

Feasibility of 99Mo production by proton-induced fission of 232Th

The current global crisis in supply of the medical isotope generator 99Mo/99mTc has triggered much research into alternative non-reactor based production methods for 99Mo including innovative radionuclide production techniques using ion accelerators. A novel method is presented here that has thus far not been considered: 232Th is used as target material to produce carrier-free 99Mo for 99Mo/99mTc generators by proton-induced fission (232Th (p,f) 99Mo). The thick target yields of 99Mo are estimated as 3.6MBq/μA·h and 21MBq/μA·h for proton energies of 22MeV and 40MeV, respectively, energies that are available from many cyclotrons. With respect to 99Mo reactor based methods using uranium targets, the presented concept using 232Th does not pose proliferation concerns, transport of highly radioactive target materials can be reduced and unused cyclotron capacities could be exploited. Radiochemical target processing could be based on existing technologies of extraction of 99Mo from reactor irradiated 235U. The presented method could be used for co-production of other radioisotopes of medical interest such as 131I.

Theoretical and experimental studies of the neutron rich fission product yields at intermediate energies

A new method to measure the fission product independent yields employing the ion guide technique and a Penning trap as a precision mass filter, which allows an unambiguous identification of the nuclides is presented. The method was used to determine the independent yields in the proton-induced fission of 232Th and 238U at 25 MeV. The data were analyzed with the consistent model for description of the fission product formation cross section at the projectile energies up to 100 MeV. Pre-compound nucleon emission is described with the two-component exciton model using Monte Carlo method. Decay of excited compound nuclei is treated within time-dependent statistical model with inclusion of the nuclear friction effect. The charge distribution of the primary fragment isobaric chain was considered as a result of frozen quantal fluctuations of the isovector nuclear density. The theoretical predictions of the independent fission product cross sections are used for normalization of the measured fission product isotopic distributions.

Scission shapes of pair fragments in asymmetric and symmetric fission modes

Scission shapes composed of two touching spheroids in asymmetric and symmetric fission modes have been deduced from static potential calculations with the experimental fragment total kinetic energy and excitation energies for a typical fragment pair of 103Nb and 130Sn in the proton-induced fission of 232Th. It was found that the fragment deformation of the heavy fragment 130Sn in the symmetric fission mode was extremely large compared with that in the asymmetric fission and those of the complementary fragment 103Nb. Such scission shapes also provide internal excitation energies of pair fragments in the two fission modes. The deduced total internal excitation energies of complementary fragments for the two fission modes are nearly the same as the excitation energy of the fissioning nucleus. The results suggest that the two fission modes are strongly characterized by the degrees of fragment deformation of the heavy fragments, not by the total internal excitation energies at scission.

Characteristics of the asymmetric mass distribution in proton-induced fission of actinides

To study the correlation between fragment mass distributions and shell structures of fission fragments, fragment mass and energy distributions in proton-induced fission of 232Th and uranium isotopes, 233,235,238U, were precisely measured using a double-TOF method. It was found that the lighter side of the heavier wing of asymmetric mass distributions shifts to larger fragment mass number corresponding to the N/Z value of the fissioning nucleus. The results are explained qualitatively by the change of the most probable mass number of fission fragments of Z = 50 proton-shell.

Isomeric yield ratios of fission products in proton-induced fission of232Th

Isomeric yield ratios of 11 fission products were measured in the system of 13 MeV proton-induced fission of232Th by an on-line ion-guide isotope separator. It was found that the closed shell structures of primary fragments and their complementary fragments affect the isomeric yield ratios. Isomeric yield ratios of121Cd (11/2−, 3/2+) and135Xe (11/2−, 3/2+) were measured precisely in the proton energy range of 13 to 26 MeV to investigate their energy dependence. It was found that the isomeric yield ratios increased slightly with proton energy. The results were discussed in connection with the deformation of fission fragments and fission modes.

Highly Asymmetric Mass Division in Low-Energy Proton-Induced Fission of 232Th and 244Pu

Highly Asymmetric Mass Division in Low-Energy Proton-Induced Fission of 232Th and 244Pu

Two deformation paths in proton-induced fission of 232Th

Velocities of complementary fission fragments have been accurately measured by a double velocity time-of-flight method in the proton-induced fission of 232Th with incident energies from 12.0 to 14.7 MeV. The results experimentally demonstrate for the first time the correlation among the threshold energy, the total fragment kinetic energy, and the mass division mode in fission: one type of the fission process has a lower threshold energy and leads to a compact scission configuration with reflection asymmetry, and the other type has a higher threshold energy and leads to a symmetric elongated scission configuration.

Bimodal nature of actinide fission

Mass and kinetic energy distributions have been measured in the proton-induced fission of 232Th and 238U with two time-of-flight counter telescopes in coincidence. A binary structure was observed in the velocity and total kinetic energy distributions of fission fragments, with A = 128–131 for 232 Th + p fission and A = 126–130 for 238U + p fission. From the present results, it is concluded that there are at least two kinds of scission configuration: elongated shapes probably associated with the symmetric path, and more compact shapes associated with the asymmetric path.

Studies on fission phenomena induced by charged particles using the JAERI tandem accelerator

Charged-particle-induced fission has been studied using the JAERI tandem accelerator. Characteristics of the symmetric mass division process at high angular momentum are investigated in the region of relatively light mass systems: 37Cl + 68Zn and 16O + 89Y. Observed widths of the mass and total kinetic energy distributions are broader than those expected from the liquid drop model. Pre-scission 4He particles in coincidence with fission products are measured in the 19F+ 197Au reaction. The multiplicity of pre-scission 4He is explained by a statistical model calculation which takes into account the reduced emission barrier for 4He. The correlation between fragment mass and total kinetic energy distributions is studied in proton-induced fission of 232Th using a double velocity TOF spectrometer. The result suggests the presence of two kinds of scission point configurations.

Luminescence response of thin plastic scintillator detectors to fission fragments from proton-induced fission of 232Th, 235U, and 238U

The luminescence response of a thin-film detector to fragments from the fission of 232Th, 235U, and 238U induced by 10–18 MeV protons was determined. Measurements were made in coincidence with signals from a surface-barrier solid-state detector, and the detector responses were displayed for each energy as two-parameter plots. Detector responses to 252Cf spontaneous-fission fragments were determined for comparison. Results are consistent with previous work at comparable energies. The possible significance of a previously observed change in symmetric-to-asymmetric fission ratio near 15 MeV bombarding energy, and the applicability of thin-film detectors to identification of unknown fissioning nuclides are discussed.