Induced Fission Fragment Research Articles

Stopping Force Analysis of 235U Elemental Fission Product Yields for En = 0.11–92.4 MeV

Most evaluated elemental fission product yield distributions are not experimentally measured. Instead, the majority of evaluated distributions are based on analytic expressions of the Zp-model for relevant cumulative yields. Here we report independent elemental fission product yield distributions of a 235U target for incident neutron energies ranging from 0.11 MeV through 92.4 MeV. Atomic numbers are calculated by an approach that combines a 2E analysis with a stopping force analysis method, developed within this paper. These analyses are applied to more than 6.1 × 106 fission fragment ionization tracks captured within the NIFFTE (Neutron Induced Fission Fragment Tracking Experiment) collaboration fission time projection chamber (fissionTPC). A 3-Z resolution was obtained with the fissionTPC spatial and energy resolutions. Tabulated results are presented for the atomic yield and experimentally derived Zp values as a function of pre-neutron-emission fragment masses for the complete range of incident neutron energies. The stopping-force-derived Zp values tend to support the unchanged charge distribution theory within uncertainty.

Time-of-flight fission fragment spectrometer of the MAVR facility

A time-of-flight system was created to register the reaction products and perform correlation measurements on a high-resolution magnetic analyzer (MAVR). The facility is located on the channel of the extracted beam of the U-400 cyclotron at the Laboratory of Nuclear Reactions JINR. The results of measuring the parameters of the time-of-flight spectrometer obtained on the basis of measuring the spontaneous fission fragments velocities of 252Cf are described. In addition, the results of the measurement of fission fragments in the 48Ca+238U reaction are presented. In this case, two time-of-flight arms were used, with the help of it is planned to carry out correlation measurements of induced fission fragments of a complex system. The created system also makes it possible to carry out correlation (three particles) measurements of fission fragments with particles registered in the focal plane of the MAVR magnetic spectrometer. Another purpose of the described system is to register elastically scattered ions, which is used as a monitoring system for the quality and composition of the beam hitting on the target of the MAVR facility.

Neutron-induced fission fragment angular distributions, anisotropy, and linear momentum transfer measured with the NIFFTE fission time projection chamber

The Neutron Induced Fission Fragment Tracking Experiment (NIFFTE) collaboration has performed measurements with a fission time projection chamber (fissionTPC) to study the fission process by reconstructing full three-dimensional tracks of fission fragments and other ionizing radiation. The amount of linear momentum imparted to the fissioning nucleus by the incident neutron can be inferred by measuring the opening angle between the fission fragments. Using this measured linear momentum, fission fragment angular distributions can be converted to the center-of-mass frame for anisotropy measurements. Angular anisotropy is an important experimental observable for understanding the quantum mechanical state of the fissioning nucleus and vital to determining detection efficiency for cross section measurements. Neutron linear momentum transfer to fissioning $^{235}$U, $^{238}$U, and $^{239}$Pu and fission fragment angular anisotropy of $^{235}$U and $^{238}$U as a function of neutron energies in the range 130 keV--250 MeV are presented.

Raman and FT-IR investigation of neutron and fission-fragment irradiated DAP polymer

The polymer Diallyl Phthalate (DAP), commonly used as solid-state nuclear track detector (SSNTD), has been characterized via optical microscopy and Raman and FT-IR spectroscopies in three distinct processes: i) without irradiation; ii) irradiated by neutrons and induced fission fragments and iii) as a function of the chemical etching time. The detectors were exposed to fission fragments from the induced fission of 235U and to the neutrons themselves. The optical microscopy provides information about the morphological changes, while the spectroscopic techniques provide physical and chemical information over the detector molecular structure. The irradiation and chemical etching induces molecular degradation and cross-linking of the detector chemical structure as observed in the Raman and FT-IR analyses. According to the spectral profile analyses, the strength of the functional groups OCOO, CH, CHCH, CCH, COC, CCH3, C(CH3)2, CCH3, CO, CH2 decrease and additional CO2 gases and OH groups are produced. Therefore, the characterization process performed in this work is essential to better comprehend the behavior of the DAP molecular structure under irradiation and chemical etching and also to better understand the track formation process which is related to the etching kinetics.

Fission fragment angular anisotropy in neutron-induced fission of U235 measured with a time projection chamber

Fission fragment angular distributions can provide an important constraint on fission theory, improving predictive fission codes, and are a prerequisite for a precise ratio cross section measurement. Available anisotropy data is sparse, especially at neutron energies above 5 MeV. For the first time, a three-dimensional tracking detector is employed to study fragment emission angles and provide a direct measurement of angular anisotropy. The Neutron Induced Fission Fragment Tracking Experiment (NIFFTE) collaboration has deployed the fission time projection chamber (fissionTPC) to measure nuclear data with unprecedented precision. The fission fragment anisotropy of $^{235}$U has been measured over a wide range of incident neutron energies from 180 keV to 200 MeV; a careful study of the systematic uncertainties complement the data.

Determination of the number of 238U target nuclei and simulation of the neutron induced fission fragment energy spectrum

A Monte Carlo simulation of the energy spectrum of the α particles emitted from a 238U3O8 sample is performed. Through comparing the simulated and measured energy spectra of the α particles, the non-uniformity of the 238U3O8 sample is obtained, and then the number of 238U target nuclei is determined. Using the obtained non-uniformity, a Monte Carlo simulation is developed to simulate the energy spectrum of neutron induced fission fragments. The simulated and measured energy spectra of neutron induced fission fragments agree well with each other.

Fine structure effect among heavy-ion induced fission fragments at near and above barrier energies

The fission decay analysis of 201Bi*, 206Po*, 212Rn*, 216Ra*, 227Pa* and 228U* nuclei produced in 19F-induced reactions at near and above barrier energies is carried out within the framework of dynamical cluster-decay model (DCM) based on quantum mechanical fragmentation theory (QMFT). The interplay between two modes of fission, i.e. symmetric (SF) and asymmetric (aSF), in the fission fragment mass distributions is investigated. For the present set of calculations the SF to aSF peak ratio is less than unity ($ \frac{P_{SF}}{P_{aSF}} < 1$), which in turn suggests the dominance of asymmetric fission for above mentioned compound nuclei. The role of shell corrections and deformations of fragments in the fission dynamics is duly addressed. The calculated fission cross-sections show nice agreement with the experimental data, except for few energies above the Coulomb barrier. This disagreement is associated with the possible presence of non-compound nucleus (nCN) fission as higher energies are more prone to the nCN process. The study of fission fragment mass distributions is further extended to the isotopic analysis of above mentioned pre-actinide and actinide nuclei at common centre-of-mass energy $ E_{c.m.} \approx 80$ MeV near the Coulomb barrier. It is observed that the lighter isotopes exhibit symmetric fission distribution, whereas heavier ones prefer to decay via asymmetric path. A transition between SF and aSF occurs around fissioning nuclei with $ N/Z \approx 1.4$, showing the triple humped mass distribution suggesting comparable contribution of symmetric and asymmetric fission. Finally, the most energetic asymmetric light ($ A_L$) and heavy ($ A_H$) fission fragments are identified for a long isotopic chain of pre-actinide and actinide nuclei. Interestingly, the heavier asymmetric fission fragments are located near a proton number around $ Z=50$ for the pre-actinide region and $ Z=54$ for actinide nuclei.

Fission cross-section measurement for 238U(n,f)134 ZX reactions induced by D-T neutrons

The D-T neutron induced fission fragments from 238U, i.e., 134Te, 134 mI and 134 gI, were measured by low-background HPGe-γ detector. The fission cross-sections of the 238U(n, f) 134Te, 238U(n, f))134 mI and 238U(n, f)134 gI reactions induced by D-T neutrons, i.e., 14.7 ± 0.3, 14.5 ± 0.3 and 14.1 ± 0.3 MeV, were measured precisely with the neutron activation technique, where the neutron flux was monitored by accompanying α-particle and the neutron energy was determined by the cross-section ratio of 90Zr(n, 2 n)89Zr to 93Nb(n, 2 n)92 mNb reaction. The fission cross-sections of the 238U(n, f)134Te reaction are 54.3 ± 3.5 mb, 52.6 ± 3.4 mb and 50.0 ± 3.2 mb, corresponding to the above energy points. However, the cross-sections of 238U(n, f)134 gI reactions were estimated to be below a value about 8.2 × 10−3 mb, since almost all the 134 gI nuclide, of which characteristic γ-rays were detected in the spectrum, originated from the 134Te decay. Meanwhile, the characteristic γ-rays of 134 mI were not found on any spectrum, and beyond the lower limit of measurement.

Dynamic picture of the formation of spins of induced fission fragments

A way of calculating the average spins of induced fission fragments is developed, based on the dynamic model of their angular distributions. The range of relaxation times for the degree of freedom associated with the orientation of the axis of symmetry of a fissioning nucleus relative to its total angular momentum is determined by analyzing experimental data on the energy dependences of average spins and the anisotropy of the angular distributions of fission fragments for the 12C, 16O + 232Th reactions at E сm = 55–150 MeV.

Commissioning the NIFFTE Time Projection Chamber: Towards the 238U / 235U (n, f) Cross-section Ratio

The Neutron Induced Fission Fragment Tracking Experiment (NIFFTE) collaboration is developing a Time Projection Chamber (TPC) to measure neutron-induced fission cross sections with unprecedented accuracy. Modifying TPC technology for nuclear physics applications is a challenging endeavor, and frequent testing and evaluation is required to ensure that all components are behaving as expected. In-beam tests of the NIFFTE TPC at the Los Alamos Neutron Science Center (LAN-SCE) commenced in 2010. An overview of the NIFFTE TPC experiments performed at LANSCE will be presented, along with preliminary performance results.

Comparison of Geant4 and MCNP6 for use in delayed fission radiation simulation

Neutron induced fission fragment distributions and delayed fission radiation are extremely important with reactor applications in fission cross sections and heating. Data on the fragment distributions are sparse so simulations use models or interpolations between known neutron energies. Different simulations perform different treatments of the distributions, and have different capabilities and flexibility in use. MCNP is a typical workhorse for fission simulations and coupled with burn-up codes such as CINDER can provide delayed radiation from fission. Geant4 is an extremely flexible physics based Monte Carlo simulation framework, but is not typically used for fission research. In this work the applicability of Geant4 for delayed fission radiation simulations is examined, with comparison to MCNP6 coupled with the CINDER2008 burn-up code. The Fisher and Engle fission experiment with the Godiva II subcritical assembly as a fission neutron source is used as a test case. Both simulations are adapted from that experiment and simulation results are compared with that experiment. Following Fisher and Engle, photons/fission/sec, MeV/fission/sec, and MeV/photon are examined. For the first two quantities results from both simulation codes are similar and are lower than experimental values, with Geant4 giving a higher value for earlier time bins and MCNP6/CINDER giving a higher value for the later time bins. For the last quantity both simulations are usually within uncertainty of the experimental values, with MCNP6/CINDER values consistently higher than both experimental and Geant4 values.

The NIFFTE project

The Neutron Induced Fission Fragment Tracking Experiment (NIFFTE) is a double-sided Time Projection Chamber (TPC) with micromegas readout designed to measure the energy-dependent neutron-inducedfission cross sections of the major and minor actinides withunprecedented accuracy. The NIFFTE project addresses the challenge ofminimizing major sources of systematic uncertainties from previousfission chamber measurements such as: target and beamnon-uniformities, misidentification of alpha and light chargedparticles as fission fragments, and uncertainties inherent to thereference standards used. In-beam tests of the NIFFTE TPC at the LosAlamos Neutron Science Center (LANSCE) started in 2010 and havecontinued in 2011, 2012 and 2013. An overview of the NIFFTE TPC statusand performance at LANSCE will be presented.

Facility at CIRUS reactor for thermal neutron induced prompt [formula omitted] spectroscopic studies

A facility for prompt γ-ray spectroscopic studies using thermal neutrons from a radial beam line of Canada India Research Utility Services (CIRUS) reactor, Bhabha Atomic Research Centre (BARC), has been developed. To carry out on-line spectroscopy experiments, two clover germanium detectors were used for the measurement of prompt γ rays. For the first time, the prompt γ–γ coincidence technique has been used to study the thermal neutron induced fission fragment spectroscopy (FFS) in 235U(nth, f). Using this facility, experiments have also been carried out for on-line γ-ray spectroscopic studies in 113Cd(nth, γ) reaction.

Systematics on fission fragment mass distribution of neutron induced 235U fission

Based on the neutron induced fission fragment mass distribution data up to neutron energy 20 MeV measured with the double kinetic energy method (KEM) and the radio active method (RAM), the systematics of fission fragment mass distribution was investigated by using 5 Gaussian model and the systematics parameters were obtained by fitting the experimental data. With the systematics, the yields of any mass A and at any energy in the region from 0 to 20 MeV of neutron energy can be calculated. The calculated results could well reproduce the experimental data measured with KEM, but show some systematical deviation from the data measured by RAM, which reflects some systematical deviations between the two kinds of measured data.The error of systematics yield was calculated in an exact error transformation way, including from the error of the experimental yield data to the error of the discrete parameters, then to the systematics parameters, and at last to the yield calculated with systematics.

Future Perspectives in Nuclear Structure: From high intensity stable to radioactive nuclear beams

Future perspectives in nuclear structure rely on radioactive nuclear beams as well as on high intensity beams of stable ions. Especially for neutron rich nuclei, deep-inelastic and multi-nucleon transfer reactions can be used to populate yrast and non yrast states. Particularly powerful is here the combination of large acceptance spectrometers with highly segmented γ-detector arrays. Such devices, eventually complemented by large coverage particle detectors, can provide the necessary channel selectivity to identify very rare signals. An example is the CLARA γ-ray detector array coupled with the PRISMA spectrometer at the Legnaro National Laboratories (LNL). The physics aims achievable with this setup will complement studies performed with current radioactive beam (RIB) facilities. With such set-up we have recently investigated the stability of the N=50 shell closure when moving towards more exotic systems. Here the comparison of the experimental data with shell model calculations seems to indicate a persistence of the N=50 shell gap down to Z=32. Future perspectives at LNL are based on an increase in intensity as well as on the availability of heavy ion species. Beams like 136 Xe or 208 Pb, which will be provided by the new PIAVE injector, can be used to drive the multinucleon flux toward the more exotic regions. Moreover a new ISOL facility (SPES) dedicated to the production and acceleration of radioactive neutron rich species is now under development at LNL. It will be based on an high intensity proton and deuteron LINAC. Induced fission fragments will be ionized and then accelerated using the presently existing superconductive LINAC (ALPI). Among the new instrumentation the concept of γ-ray tracking has been recently introduced in nuclear spectroscopy. Detectors based on γ-ray tracking have position resolution capabilities with excellent performances both in efficiency and in achievable Doppler correction. A new γ-ray detector array (AGATA) based on such technique is now under study in a wide european collaboration. 1. Probing Nuclear Structure with Radioactive and Stable ion beams

Surface activation by induced fission fragments implantation

A technique for surface activation by induced fission fragments is described. The 235U layer irradiated with thermal neutrons was used as a fragment source. Neutrons were generated by means of the 7Li(p,n)7Be reaction. The proton beam from the 2.5MeV Van de Graaf accelerator was employed. The depth distribution of the implanted fragments was determined and appeared to be ideally suited for a surface degradation study. The fragments suitable to serve as tracers were determined and their γ-spectra were measured. Advantages of the method are demonstrated through the application to abrasion testing of an electrostatic accelerator charge conveyer belt.

Role of entrance channel properties in heavy ion induced fission fragment angular distribution studies

Fission fragment angular distributions have been measured for14N+232Th,11B+235U and16O+181Ta systems at projectile energies, below and above the coulomb barrier of the respective systems. Detailed statistical model analysis of the entire data set have been reported.

Role of entrance channel properties in heavy ion induced fission fragment angular distribution studies

Fission fragment angular distributions have been measured for 14N+ 232Th, 11B+ 235U and 16O+ 181Ta systems at projectile energies, below and above the coulomb barrier of the respective systems. Detailed statistical model analysis of the entire data set have been reported.

Fullerene genesis by ion beams III. On the absence of latent tracks in GeV ion irradiated graphite

Irradiation of natural graphite with ∼ 1 GeV lead and uranium ions initiates a phase change to fullerene (C60) along the particle trajectories. High resolution transmission electron microscopy (Dunlop et al. [1]) reveals no tracks in the bulk in either kinematic or diffraction contrast. Small amoeboid-like surface patches are formed however, random in distribution and proportional in number, but not equivalent to the total ion fluence. The evidence suggests that molecules of C60 and other clusters are ejected from the entry and exit surfaces by electronic sputtering and are then deposited. A mechanism referred to as particle activated prompt anneal (or PAPA) is proposed to explain the absence of latent tracks in the bulk. A reverse cooling and annealing wave permits recovery by epitaxy of the disorder and phase changes induced by the primary heat wave. Radiation enhanced recovery processes may also play a role. PAPA can additionally explain the absence of induced fission fragment tracks in graphite, diamond, silicon and germanium.

Heavy-ion induced fission fragment angular distributions at near-barrier energies

Heavy-ion induced fission fragment angular distribution measurements performed in recent years have provided interesting results. From a systematic study involving several projectiles and actinide targets, the entrance channel mass asymmetry dependence of fission anisotropies has been brought out at energies above the fusion barrier. It has been observed that the fission fragment anisotropies, essentially for all target - projectile combinations involving actinide targets increase as bombarding energy is lowered below the barrier and are anomalously large when compared to the saddle-point model expectations. However, similar measurements carried out for spherical targets are consistent with the calculations based on the saddle-point model.