Correlations Of Fission Fragments Research Articles

Detection of correlated fission fragments with a twin Frisch-grid ionization chamber

Twin Frisch-grid ionization chambers (TFGICs) have been widely used in the measurement of total kinetic energy (TKE), angular distribution, mass, and charge of the correlated fission fragments. To study these correlated fission parameters, we have developed a TFGIC with a digital acquisition system for use at both the Chinese Mianyang research reactor (CMRR) and neutron science facility in the Institute of Nuclear Physics and Chemistry (INPC), as well as the China Spallation Neutron Source (CSNS). The distribution of the electric field was investigated with Garfield toolkit, the results showed that the ratio of the electric field of the collect region to the drift region (Ec /Ed ) must be greater than a critical value Z (1.78). An individual experiment, carried out with a compound alpha source (241Am and 243Am), was used to validate the preliminary performance of this detector. The effects of the electric field on the anode pulse height agreed well with the simulation results. Furthermore, the results also indicated that the energy resolution of (68.40 ± 1.07) keV at 5.39 MeV (243Am) and (69.67 ± 1.09) keV at 5.49 MeV (241Am) could be achieved. An initial in-beam fission experiment of 235U reaction with 14 MeV neutrons was also performed to test the setup, and the results showed that the reconstructed emission angle could achieve a resolution of 0.221 ± 0.002 on the difference of the cosines determined from both halves of TFGIC. The present work primarily reported the development of a TFGIC with a digital acquisition system in INPC.

Measurement of relative isotopic yield distribution of even-even fission fragments from U235 ( nth,f ) following γ -ray spectroscopy

A detailed investigation on the relative isotopic distributions has been carried out for the first time in case of even-even correlated fission fragments for the $^{235}\mathrm{U}({n}_{\text{th}},f$) fission reaction. High-statistics data were obtained in a prompt $\ensuremath{\gamma}$-ray spectroscopy measurement during the EXILL campaign at ILL, Grenoble, France. The extensive off-line analysis of the coincidence data have been carried out using four different coincidence methods. Combining the results from two-dimensional $\ensuremath{\gamma}\text{\ensuremath{-}}\ensuremath{\gamma}$ and three-dimensional $\ensuremath{\gamma}\text{\ensuremath{-}}\ensuremath{\gamma}\text{\ensuremath{-}}\ensuremath{\gamma}$ coincidence analysis, a comprehensive picture of the relative isotopic yield distributions of the even-even neutron-rich fission fragments has emerged. The experimentally observed results have been substantiated by the theoretical calculations based on a novel approach of isospin conservation, and a reasonable agreement has been obtained. The calculations following the semiempirical GEF model have also been carried out. The results from the GEF model calculations are found to be in fair agreement with the experimental results.

Pre-scission neutron multiplicity in the S32+W184 reaction

The neutron energy spectra in coincidence with both the correlated fission fragments in $^{32}\mathrm{S}+^{184}\mathrm{W}$ reaction forming the compound nucleus $^{216}\mathrm{Th}$ at an excitation energy of 71.31 MeV have been measured at various angles with respect to one of the fission fragments. Pre- and post-scission neutron multiplicities of $2.68\ifmmode\pm\else\textpm\fi{}0.48$ and $3.3\ifmmode\pm\else\textpm\fi{}0.20,$ respectively, have been extracted by analyzing angular correlation of the neutron energy spectra using moving source fitting procedure. The excess pre-scission neutron multiplicity in comparison to statistical model joanne2 code predictions have been converted into total fission delay. A comparative study of fission delay has been carried out for the $^{16}\mathrm{O}+^{208}\mathrm{Pb}$ reaction forming the compound nucleus $^{224}\mathrm{Th}$ at similar excitation energy as in $^{32}\mathrm{S}+^{184}\mathrm{W}$ reaction. Both the reactions have been analyzed within the frame work of joanne2 code including fission delay, deformation dependent level densities, particle binding energies, and transmission coefficients. The observed fission delay for both the reactions can be quantitatively understood if different formation time predicted by dynamical model hicol code are considered.

Development of Parallel Plate Avalanche Counter for heavy ion collision in radioactive ion beam

Abstract We have developed a position-sensitive Parallel Plate Avalanche Counter (PPAC) to detect the fission fragments and reconstruct the fission reaction plane in the experiment of studying nuclear equation of state (nEOS) by means of heavy ion collision (HIC). This experiment put forward high requirements for the performances of PPAC, such as the time resolution, efficiency and position resolution. According to these requirements we designed the PPAC with an active area of 240 mm × 280 mm working at low gas pressure. The results show that time resolution could be less than 300 ps. Position resolution is consistent with the theoretical calculation about 1.35 mm. Detection efficiency could be approaching 100% gradually with the voltage increasing in different gas pressures. The performances of PPAC have also been verified in beam experiment. Each set of anode wires can be accurately separated in the position spectrum. In the beam experiment, we also got the back-to-back correlation of fission fragments which is one of the direct signals characterizing binary decay.

Investigations of fission characteristics and correlation effects

We review the experimental results on the P-even and P-odd angular correlations of fission fragments in the fission of the 235U and 239Pu nuclei induced by unpolarized and polarized resonance neutrons, and on the TRI and ROT effects in the ternary and binary fission of actinides induced by polarized thermal neutrons. Also reported are the measured yields of prompt and delayed neutrons per fission event. The experimental data are analyzed within a novel theoretical framework developed by the JINR—RNC KI Collaboration, whereby the reduction of the multidimensional phase space of fission fragments to the JπK-channel space is consistently validated and the role of resonance interference in the observed correlation effects is revealed.

Fission fragment mass distributions via prompt γ-ray spectroscopy

The distribution of fragment masses formed in nuclear fission is one of the most striking features of the process. Such measurements are very important to understand the shape evolution of the nucleus from ground state to scission through intermediate saddle points. The fission fragment mass distributions, generally obtained via conventional methods (i.e., by measuring the energy and/or the velocity of the correlated fission fragments) are limited to a mass resolution of 4–5 units. On the other hand, by employing the γ-ray spectroscopy, it is possible to estimate the yield of individual fission fragments. In this work, determination of the fission fragment mass distribution by employing prompt γ-ray spectroscopy is described along with the recent results on 238U(18O, f) and 238U(32S, f) systems.

Some aspects of fission and quasifission processes

The discovery of nuclear fission in 1938–1939 had a profound influence on the field of nuclear physics and it brought this branch of physics into the forefront as it was recognized for having the potential for its seminal influence on modern society. Although many of the basic features of actinide fission were described in a ground-breaking paper by Bohr and Wheeler only six months after the discovery, the fission process is very complex and it has been a challenge for both experimentalists and theorists to achieve a complete and satisfactory understanding of this phenomenon. Many aspects of nuclear physics are involved in fission and it continues to be a subject of intense study even three quarters of a century after its discovery. In this talk, I will review an incomplete subset of the major milestones in fission research, and briefly discuss some of the topics that I have been involved in during my career. These include studies of vibrational resonances and fission isomers that are caused by the second minimum in the fission barrier in actinide nuclei, studies of heavy-ion-induced fission in terms of the angular distributions and the mass–angle correlations of fission fragments. Some of these studies provided evidence for the importance of the quasifission process and the attendant suppression of the complete fusion process. Finally, some of the circumstances around the establishment of large-scale nuclear research in India will be discussed.

Fission via compound states andJπKA. Bohr's channels: what we can learn from recent studies with slow neutrons

Last data on angular correlations of fission fragments from slow (s-wave) neutron induced binary fission of spin-aligned nuclei 235 U are discussed in the context of Jπ K A. Bohrs channels. Special attention is paid to K = 0 channel. Reasons for its suppression are specified for compound nucleus states of negative parity. A brief overview of recent data on T-odd angular correlations in ternary and binary (with emission of a third particle, a neutron or γ -quantum) fission induced by slow polarized neutrons is presented. On the basis of the developed theoretical approach it is shown that a valuable information on Jπ K fission channels at scission point can be inferred from these T-odd angular correlations.

Spatial- and Time-Correlated Detection of Fission Fragments

With the goal to measure angular correlations of fission fragments in rare fission decay (e.g. ternary and quaternary fission), a multi-detector coincidence system based on two and up to four position sensitive pixel detectors Timepix has been built. In addition to the high granularity, wide dynamic range and per pixel signal threshold, these devices are equipped with per pixel energy and time sensitivity providing more information (position, energy, time), enhances particle-type identification and selectivity of event-by-event detection. Operation of the device with the integrated USB 2.0 based readout interface FITPix and the control and data acquisition software tool Pixelman enables online visualization and flexible/adjustable operation for a different type of experiments. Spatially correlated fission fragments can be thus registered in coincidence. Similarly triggered measurements are performed using an integrated spectrometric module with analogue signal chain electronics. The current status of development together with demonstration of the technique with a 252 Cf source is presented. 1 Motivation

Study of deep inelastic collisions in the heavy-ion interaction of [formula omitted][formula omitted

Using mica as a passive detector and employing the 2 π -geometry technique (14.0 MeV/ u) 208 Pb + 238 U heavy-ion nuclear interaction has been studied. The particle tracks were converted into their actual lengths and polar angles through simple geometry. The elastic binary events so bifurcated were used to deduce the coefficients to analyze the three prong events. The relative velocities of the correlated fission fragments were calculated at the second stage of the reaction in order to construct the intermediate stage of the reaction, and hence, determine the pre-fission masses integrated over all angles and energies. The angular distribution of fission fragments within the reaction plane provided the information about the speed of the reaction and proximity effect.

Near-scission emission of intermediate mass fragments in12C+232ThatE/A=16and22 MeV

Intermediate mass fragments (IMFS) (IMF: 3<~ZIMF<~20) observed in coincidence with two correlated fission fragments following incomplete fusion in $^{12}$C+$^{232}$Th at E/A=16 and 22 MeV are investigated. IMFs emitted prior to significant deformation of the fissioning system, as well as IMFs emitted near scission, are distinguished based upon their characteristic kinetic energy and angular distributions. The yield distributions of IMFs emitted near scission in these12C induced reactions are compared with near-scission IMF yields in spontaneous and low-energy ternary fission. Comparisons are made to both experimental fusion-evaporation data and theoretical predictions of a statistical model. The excitation energy dependence of relative IMF yields for both isotropic and near-scission emission is also presented. Our results for near-scission emission suggest that the production of IMFs near scission is inconsistent with a statistical emission mechanism in which emission barriers follow a standard Z dependence. Dynamical model calculations are used to investigate the role of dissipation, angular momentum, N/Z, and kinetic energy on the fragment formation near scission.

Sequential fission decay induced by transfer reactions at the Coulomb barrier

In-plane and out-of-plane angular correlations of fission fragments detected in coincidence with projectile residues produced in nuclear collisions ${}^{19}{\mathrm{F}+}^{238}\mathrm{U}$ at 115, 120, and 125 MeV have been investigated. The data are described assuming sequential fission decay of targetlike nuclei produced in peripheral transfer reactions.

Recent studies in heavy ion induced fission reactions

Nuclear fission process involves large scale shape changes of the nucleus, while it evolves from a nearly spherical configuration to two separated fission fragments. The dynamics of these shape changes in the nuclear many body system is governed by a strong interplay of the collective and single particle degrees of freedom. With the availability of heavy ion accelerators, there has been an impetus to study the nuclear dynamics through the investigations of nucleus-nucleus collisions involving fusion and fission process. From the various investigations carried out in the past years, it is now well recognized that there is large scale damping of collective modes in heavy ion induced fission reactions, which in other words implies that nuclear motion is highly viscous. In recent years, there have been many experimental observations in heavy ion induced fission reactions at medium bombarding energies, which suggest possible occurrence of various non-equilibrium modes of fission such as quasi-fission, fast fission and pre-equilibrium fission, where some of the internal degrees of freedom of the nucleus is not fully equilibrated. We have carried out extensive investigations on the fission fragment angular distributions at near barrier bombarding energies using heavy fissile targets. The measured fragment anisotropies when compared with the standard saddle point model (SSPM) calculations show that for projectile-target systems having zero or low ground state spins, the angular anisotropy exhibits a peak-like behaviour at the sub barrier energies, which cannot be explained by the SSPM calculations. For projectiles or targets with large ground state spins, the anomalous peaking gets washed out due to smearing of the K-distribution by the intrinsic entrance channel spins. Recently studies have been carried out on the spin distributions of fission fragments through the gamma ray multiplicity measurements. The fission fragments acquire spin mainly from two sources: (i) due to rigid rotation of the nascent fragments at scission and (ii) due to statistical excitation of the spin bearing collective modes in the fissioning nucleus. One of the collective modes — the tilting mode depends on the K quantum number and is responsible for the emission angle dependence of fragment spin. In our studies, we have shown conclusively that the collective statistical spin modes get strongly suppressed for high K values corresponding to large rotational frequencies along the fission axis. These results bring out the importance of the dynamical effects in the heavy ion induced fusion-fission reactions. The present article will review the work carried out on the above aspects in heavy ion fission reactions as well as on the fission time scales, and some of the recent studies on the mass-energy correlations of fission fragments at near-barrier bombarding energies.

Evidence for Different Fission Behavior of Hot Nuclei Formed in Central and Peripheral Collisions of 40Ar+209Bi Reaction at 25 MeV/u

Correlated fission fragments from the reaction of 25 MeV/u 40Ar+209Bi and their further correlation with α particles have been studied for peripheral and central collisions simultaneously. The excitation energy at scission deduced from postscission multiplicity is about 172.5 MeV. The fission timescale deduced from prescission multiplicity is about 4×10-21 s. Systematic analysis of the mass and energy distributions of fission fragments as a function of the initial temperature of hot fissioning nuclei reveals the existence of different fission behavior of hot nuclei formed in central and peripheral collisions. Experimental data demonstrate the change of fission behavior at T similar 4 MeV.

Cross measurements of linear momentum transfer and energy dissipation in collisions between 290 MeV20Ne and238U

The20Ne + U reactions are investigated at 290 MeV bombarding energy. The linear momentum transfer and excitation energy are deduced eventwise from the respective measurements of the folding angle between correlated fission fragments and the neutron multiplicity. A simple incomplete fusion picture is shown to essentially account for the data. The sensitivity of the two measurements in order to infer the violence of a collision is discussed in details.

Fission induced in natU, natPb, 197Au, and 165Ho by 80 and 100 MeV pi + and pi -

Solid state nuclear track detectors having different detection thresholds have been employed to study the fission fragments and the light charged particles emitted in the interaction of 80 and 100 MeV ${\ensuremath{\pi}}^{\ifmmode\pm\else\textpm\fi{}}$ with targets of $^{\mathrm{nat}}\mathrm{U}$, $^{\mathrm{nat}}\mathrm{Pb}$, $^{197}\mathrm{Au}$, and $^{164}\mathrm{Ho}$. Cross sections for the production of correlated fission fragments (and other heavy charged particles) and the unpaired single charged particles were obtained in different Z and Z/\ensuremath{\beta} domains. Energies and masses of the reaction products are estimated. Angular correlations of the reaction products have also been obtained. A large fraction of protons and (to a lesser extent) alpha particles are found to be emitted in these interactions. It has been found that distributions of the fission fragments are only slightly anisotropic and that their mass distributions are wider than those obtained for nucleon-induced fission having similar excitation energies. The results have been compared with the scanty data available on the subject and with systems having comparable excitation energies.

Long range alpha particle associated neutron induced fission of U-235

Abstract The ternary fission component in 235U(n, f) was studied in detail using the detection system ‘DIOGENES’, measuring ternary alpha particles in coincidence with both correlated fission fragments. Present results on equatorial and polar ternary fission are discussed.

Reaching the Capacity of a Composite Nucleus for Energy and Spin Containment

The reaction 1080-MeV $^{40}\mathrm{Ar}$+$^{238}\mathrm{U}$ has been studied via correlated fission fragments and He particles. Between $\frac{1}{3}$ and \textonehalf{} of the reaction cross section goes into fusionlike events to form composite nuclei excited to 500-800 MeV. The probability for such high-energy containment drops dramatically for $^{40}\mathrm{Ar}$ projectiles between 27 and 44 MeV/u. Evaporative He particles for fusionlike events (multiplicity \ensuremath{\approx} 1.5) have been found to arise predominantly from the massively excited composite nuclei.

Study of solid state nuclear track detectors in the institute of Atomic Energy of the Academy of Sciences of China

ABSTRACT The main fields of research and achievements in the study of solid state nuclear track detectors (SSNTD) in the Institute of Atomic Energy of the Academy of Sciences of China are presented. The contents include (1) Study of parameters of SSNTDT's. A method to calibrate the detection efficiencies of SSNTD's for fission fragments by counting correlated fission fragments has been proposed and used. (2) Fast neutron spectrum and dose measurements. The threshold energy (0.3 Mev) and the detection efficiency curve of polycarbonate (PC)foil for 0.085-18 Mev neutrons by recoil tracks have been calibrated and other methods are used. (3) Studies of nuclear fission. The angular distributions of fission fragments have been studied with SSNTD's. Non-anomalous angular distributions and obvious structures have been observed near threshold energy of 238U(n, 2n'f) and 238U(n, f) reactions respectively. (4) Applications in geology, archaeology and analytic chemistry. Muscovite mica, tektites from Hainan Island in China and Peking Man have been dated by fission track method. The contents and microscopic distributions of uranium in minerals and water have been studied. (5) Meteorite study. The track densities and fission track age (3.9 ± 0.3 byr of Jilin Meteorite were measured and discussed.

Ionization chamber technique for studies of excited states and angular correlation of fission fragments

The angular correlations between the directions of emission of gamma rays of energies 0.7 MeV and 1.3 MeV and associated fission fragments were studied by a special electronic technique in conjunction with a multigrid ionization chamber and were found to be isotropic. In addition, it was observed that more than one fragment of a fission can emit the same gamma ray.