Detection Of Fission Fragments Research Articles

Instrumentation for correlated prompt [formula omitted] emission studies in coincidence with fission fragments

Recent theoretical and experimental results have brought renewed interest and focus on the topic of fission fragment angular momentum. Measurements of neutrons and γ rays in coincidence with fission fragments remain the most valuable tool in the exploration of fission physics. To achieve these scientific goals, we have developed a system that combines a state-of-the-art fission fragment detector and n−γ radiation detectors. A new twin Frisch-gridded ionization chamber has been designed and constructed for use with a spontaneous fission source and an array of forty trans-stilbene organic scintillators (FS-3) at Argonne National Laboratory. The new ionization chamber design we present in this work aims at minimizing particle attenuation in the chamber walls, and provides a compact apparatus that can be fit inside existing experimental systems. The ionization chamber is capable of measuring fission fragment masses and kinetic energies, whereas the FS-3 provides neutron and γ-ray multiplicities and spectra. The details of both detector assemblies are presented along with the first experimental results of this setup. Planned event-by-event analysis and future experiments are briefly discussed.

Development of THGEM-based Detectors for Nuclear Fission Studies

To optimize the exploration of Super Heavy Elements (SHE), the key challenge is to understand the dynamics of the fusion-fission through the measurements of mass and angular distribution of the fission fragments. For the detection of fission fragments, position sensitive Multi-Wire Proportional Counters are generally used due to their high gain, good temporal and position resolution. However, these detectors use fragile anode wires having a diameter of only 10 μm and therefore they are vulnerable. In the present work, a detector based on robust Thick Gaseous Electron Multiplier (THGEM) has been proposed. A numerical demonstration of THGEM-Multi Wire hybrid detector technology as a possible candidate for the new generation low-energy fission studies and their evaluation as a function of different possible geometric and electric configuration in low-pressure gas is discussed here.

Development of Micro-Pattern Gaseous Detectors for Nuclear Reaction Studies

One of the frontiers of today’s nuclear physics research is the synthesis of Super Heavy Elements (SHE). Fusion-fission dynamics, namely the competition between quasi fission and fusion is one of the key challenges to optimize the SHE. To have an insight into the dynamics, one requires the study of fission fragment mass and angular distribution near barrier energies for heavy-ion induced fission reactions. Recent successful installation of linear accelerators in India offers a unique opportunity to study the dynamics of nuclear reactions and formation process of SHE. For the effective utilization of these current, as well as upcoming facilities, development of novel detectors to study reaction dynamics, formation process of SHE with heavier projectiles and higher beam energies is needed. Gaseous detectors have undergone a rapid improvement in terms of spatial, temporal and energy resolution, rate capability, radiation hardness, ion feedback etc., ushering in a new genre of micro-structured devices based on semi-conductor technology, commonly known as Micro-Pattern Gaseous Detectors (MPGDs). Although many of the MPGD structures were primarily developed for high-rate tracking of charged particles in high energy physics experiments, stability of operation, simplicity of construction and relatively low cost make these detectors suitable for other applications, such as low-energy nuclear physics experiments. The present activities encompass a detailed evaluation of the operational conditions of Micromesh-Multi Wire and THGEM-Multi Wire hybrid detector operated in low-pressure isobutane gas with a view to optimizing their use in the detection of charged particles and fission fragments.

Characterization of Diamond and Silicon Carbide Detectors With Fission Fragments

Experimental fission studies for reaction physics or nuclear spectroscopy can profit from fast, efficient, and radiation-resistant fission fragment (FF) detectors. When such experiments are performed in-beam in intense thermal neutron beams, additional constraints arise in terms of target-detector interface, beam-induced background, etc. Therefore, wide gap semi-conductor detectors were tested with the aim of developing innovative instrumentation for such applications. The detector characterization was performed with mass- and energy-separated fission fragment beams at the ILL (Institut Laue Langevin) LOHENGRIN spectrometer. Two single crystal diamonds, three polycrystalline and one diamond-on-iridium as well as a silicon carbide detector were characterized as solid state ionization chamber for FF detection. Timing measurements were performed with a 500-µm thick single crystal diamond detector read out by a broadband amplifier. A timing resolution of ∼10.2 ps RMS was obtained for FF with mass A = 98 at 90 MeV kinetic energy. Using a spectroscopic preamplifier developed at INFN-Milano, the energy resolution measured for the same FF was found to be slightly better for a ∼50-µm thin single crystal diamond detector (∼1.4% RMS) than for the 500-µm thick one (∼1.6% RMS), while a value of 3.4% RMS was obtained with the 400-µm silicon carbide detector. The Pulse Height Defect (PHD), which is significant in silicon detectors, was also investigated with the two single crystal diamond detectors. The comparison with results from α and triton measurements enabled us to conclude that PHD leads to ∼50% loss of the initial generated charge carriers for FF. In view of these results, a possible detector configuration and integration for in-beam experiments has been discussed.

New developments of a fission chamber for very high radioactivity samples

A new fission chamber was developed and built for prompt-fission-neutron spectra (PFNS) measurements in neutron-induced fission of actinides, and is described in Taieb et al. (2016). It allowed us to minimize incident- and outgoing-neutron scattering, and optimize both time resolution and alpha to fission fragment discrimination. The fission chamber was validated and used for 238U and 252Cf PFNS measurements. In order to measure PFNS of samples with a natural alpha activity of the order of 10 MBq, further improvements to the detector were done and are presented in this work. Tests of the improved fission chamber lead to a fission fragment detection efficiency of 91% for a 14.41 MBq 240Pu deposit, and 99% for a 247 kBq 242Pu deposit. The fission chamber was successfully used for 239Pu PFNS measurements at the Weapons Neutron Research facility of the Los Alamos Neutron Science Center (WNR@LANSCE).

First investigation of the response of solar cells to heavy ions above 1 AMeV

Solar cells have been used since several decades for the detection of fission fragments at about 1 AMeV. The advantages of solar cells regarding their cost (few euros) and radiation damage resistance make them an interesting candidate for heavy ion detection and an appealing alternative to silicon detectors. A first exploratory measurement of the response of solar cells to heavy ions at energies above 1 AMeV has been performed at the GANIL facility, Caen, France. Such measurements were performed with 84Kr and 129Xe beams ranging from 7 to 13 AMeV. The energy and time response of several types of solar cells were studied. The best performance was observed for cells of 10x10 mm2, with an energy and time resolution of σ(E)/E=1.4% and 3.6 ns (FWHM), respectively. Irradiations at rates from a few hundred to 106 particles per second were also performed to investigate the behavior of the cells with increasing intensity.

An instrument for measuring T-odd asymmetries in the fission of heavy nuclei

We describe an experimental setup used to measure the T-odd asymmetry effects for prompt fission neutrons and γ-rays in binary fission of some heavy nuclei induced by polarized neutrons with resonance energy. The overall setup as well as the details of the design and performance of the key components like fission fragment detectors, neutron and γ detectors, and the electronics used in the data acquisition and processing system are presented. The experimental setup on beam line POLI of the FRM II reactor (Garching, Germany), as well as the neutron beam characteristics, polarization measurements are described, too.

Nuclear fission investigation with twin ionization chamber

In this paper, we report recent results obtained in the development of digital pulse processing mathematics for prompt fission neutron (PFN) investigations using a twin ionization chamber (TIC) along with a fast neutron time-of-flight detector (ND). Due to some ambiguities in the literature concerning a pulse induction on TIC electrodes by fission fragment (FF) ionization, we first presented a detailed mathematical analysis of FF signal formation on the TIC anode. The analysis was done using the Shockley–Ramo theorem, which gives the relation between charged particle motion between TIC electrodes and the so-called weighting potential. The weighting potential was calculated by direct numerical solution of the Laplace equation (neglecting space charge) for the TIC geometry and ionization caused by FFs. Formulae for GI correction and digital pulse processing algorithms for PFN time-of-flight measurements and pulse shape analysis are presented and used in experiments for PFN investigations of two reactions, [Formula: see text]U(n[Formula: see text],f) and [Formula: see text]Cf(sf). Results of the measurements were compared to literature data to demonstrate the feasibility of the new developed techniques. These results were necessary for the development of a new PFN investigation facility consisting of a position sensitive fission fragment detector combined with 32 liquid scintillation neutron detectors.

A Change in the Parameters of Si(Li) Detectors under Exposure to α Particles

A spectrometer based on a Si(Li) detector has been tested by exposure to α particles with the aim of determining changes in the detector characteristics versus the fluence. As a result of 60-day measurements with a total fluence of 6.2 × 109 α particles, it has been established that the dependence of the deterioration in the energy resolution of α-particle peaks on the fluence is described by a linear function with a slope of Δσ/ΔФ = (8.4 ± 0.4) × 10–10 keV/α. The measured amplitude of the α-particle momentum decreases linearly with increasing fluence with a slope of (–4.8 ± 0.6) × 10–9 keV/α. These effects do not prevent reliable discrimination between the signals from α particles and fission fragments until the fluence reaches a value of 1010 α particles when the Si(Li) detector is a part of a neutron source for detecting fission fragments.

Measurement of the234U(n,f) cross section in the energy range between 14.8 and 17.8 MeV using Micromegas detectors

The234U(n,f) reaction cross section was measured for three neutron energies 14.8, 16.5 and 17.8 MeV, relative to the238U(n,f) reference reaction. The in-beam measurements were carried out by using a set-up based on Micromegas detectors, while the quasi-monoenergetic neutron beams were produced by means of the3H(d,n)4He reaction at the 5.5MV Tandem T11/25 Accelerator Laboratory of the National Center for Scientific Research "Demokritos" in Athens (Greece). Additionaly,α-spectroscopy measurements were performed in order to determine the active mass of the samples and the corresponding impurities. In order to estimate the fission-fragment detection efficiency, Monte Carlo simulations were carried out using the GEF and FLUKA codes. Furthermore, simulations were also performed by coupling the NeuSDesc and MCNP5 codes for the determination of the neutron energy distribution in all the irradiated samples and the results were used in order to correct for the contribution of low energy parasitic neutrons in the fission yield. The final cross section data are presented, along with the methodology adopted for the treatment of the parasitic neutrons.

Surface decomposition and registration efficiency of CR-39 detector irradiated by electron beam

Poly allyl diglycol carbonate (CR-39) solid-state nuclear track detector is irradiated by a low-energy electron beam, and the decomposition of the irradiated samples is inspected with various techniques. The CR-39 viscosity is tested and showed that the primary effect of irradiation is surface degradation. The surface etching at different irradiation fluence is studied and correlated to the modified surface morphology. Moreover, the changed surface chemical composition is explored by Raman spectroscopy, which shows changes in the polymer structure after treatment. The surface wettability is investigated by the contact angle method. Surface wettability, free energy and polarity increase after irradiation. The performance of irradiated CR-39 is tested for the detection of fission fragments from 252Cf radioactive source. It is found that etching sensitivity and detection efficiency are enhanced after irradiation. The induced changes due to electron beam irradiation are pronounced and improve the etching characteristics, which control the registration efficiency of the CR-39 detector.

Study and detection of relativistic Solar Neutrons at ground level with the Spherical Proportional Counter

A novel method of high energy solar neutron detection is proposed with the Spherical Proportional Counter (SPC), taking advantage of the 209Bi(n,f) reaction. This reaction, is considered as a standard for high energy neutron detection, due to large cross section values in the 100 MeV – 1 GeV energy interval, obtained in the n_TOF facility at CERN. A thin spherical shell of Bismuth will be situated in the large volume of the SPC, serving as target for high energy neutrons bombarding the detector, thus resulting in fission fragment emission. Detailed simulation of the 209Bi(n,f) reaction with the INCL++ model, coupled with the ABLA07 de–excitation code is performed (cross section, mass & atomic number distribution, kinetic energy per fragment) in the 100 MeV – 10 GeV energy interval, together with SRIM for the fragments’ projected range in 209Bi. Experimental data from a 252Cf source are obtained, in order to validate the SPC’s efficiency in fission fragment detection. Calculations for the expected reactions in the 209Bi shell have been performed in different atmospheric depths (700 & 1000 g/cm2), and various spherical detector radii.

Fast-neutron-induced fission cross section of Pu242 measured at the neutron time-of-flight facility nELBE

The fast-neutron-induced fission cross section of $^{242}\mathrm{Pu}$ was measured at the neutron time-of-flight facility $n$ELBE. A parallel-plate fission ionization chamber with novel, homogeneous, large-area $^{242}\mathrm{Pu}$ deposits on Si-wafer backings was used to determine this quantity relative to the IAEA neutron cross-section standard $^{235}\mathrm{U}(n,f)$ in the energy range of 0.5 to 10 MeV. The number of target nuclei was determined from the measured spontaneous fission rate of $^{242}\mathrm{Pu}$. This helps to reduce the influence of the fission fragment detection efficiency on the cross section. Neutron transport simulations performed with geant4, mcnp6, and fluka2011 are used to correct the cross-section data for neutron scattering. In the reported energy range the systematic uncertainty is below 2.7% and on average the statistical uncertainty is 4.9%. The determined results show an agreement within 0.67(16)% to recently published data and a good accordance to current evaluated data sets.

Prompt Fission Neutron Investigation in 235U(nth,f) and 252Cf(sf) Reactions

The prompt neutron emission in thermal neutron induced fission of 235U and spontaneous fission of 252Cf was investigated by using digital signal electronics. The goal was to check a new revised data analysis software with fission fragment (FF) kinetic energy corrections after prompt fission neutron (PFN) emission. The revised software was used to reanalyze old data measured in EC-JRC-IRMM, where 252Cf(sf) reaction was investigated. Both measurements were done using similar twin Frisch grid ionization chamber for fission fragment detection with equivalent NE213 fast neutron detector. About 0.5*106 FF with PFN coincidences have been analyzed in both measurements. The fission fragment kinetic energy, mass and angular distribution were investigated along with prompt neutron time of flight and pulse shape analysis using a six channel synchronous waveform digitizer (WFD) with sampling frequency of 250 MHz and 12 bit resolution in the 235U(nth,f) reaction. Similar WFD with sampling frequency of 100 MHz was used for PFN investigation in 252Cf(sf) reaction. These two experiments were considered as a reference for further investigations with a new setup composed of position sensitive ionization chamber to detect FF and an array of 32 liquid scintillators recently constructed in Dubna to detect neutrons.

Pulse-height defect of Ar + CF[formula omitted] mixtures as a counting gas for fission-fragment detectors

For the detection of energetic charged particles, gaseous ionization detectors use a variety of counting gases. We studied the properties of non-flammable, high stopping-power Ar+CF4 as counting gas. We assess the usability of different mixtures for compact, high-luminosity fast ionization chambers for future applications in nuclear photonics. The pulse-height defect (PHD) of Ar+CF4 mixtures at different pressures has been extracted in comparison to P-10 gas (90%Ar+10%CH4) for fragments from 252Cf spontaneous fission in a twin Frisch-grid ionization chamber. For the different mixtures of Ar+CF4 (P-10), a PHD of 4.5–11MeV (4MeV) and 6–12MeV (5.5MeV) for light and heavy fragments, respectively, have been measured. Based on these findings, reconstructed 252Cf mass and total kinetic energy distributions are consistent with literature, which demonstrates success in the PHD correction procedure.

Dependence of the prompt fission γ-ray spectrum on the entrance channel of compound nucleus: Spontaneous vs. neutron-induced fission

Prompt γ-ray spectra were measured for the spontaneous fission of 240,242Pu and the neutron-induced fission of 239,241Pu with incident neutron energies ranging from thermal to about 100 keV. Measurements were made using the Detector for Advanced Neutron Capture Experiments (DANCE) array in coincidence with the detection of fission fragments using a parallel-plate avalanche counter. The unfolded prompt fission γ-ray energy spectra can be reproduced reasonably well by Monte Carlo Hauser–Feshbach statistical model for the neutron-induced fission channel but not for the spontaneous fission channel. However, this entrance-channel dependence of the prompt fission γ-ray emission can be described qualitatively by the model due to the very different fission-fragment mass distributions and a lower average fragment spin for spontaneous fission. The description of measurements and the discussion of results under the framework of a Monte Carlo Hauser–Feshbach statistical approach are presented.

Angular correlations in the prompt neutron emission of spontaneous fission of 252Cf

Neutron angular distributions in spontaneous fission of 252Cf are investigated. The CORA experiment, performed at IPHC Strasbourg, aims at elucidating neutron emission mechanisms in the fission process. The experimental setup is composed of the angle-sensitive twin ionisation chamber CODIS for the detection of fission fragments and the DEMON neutron detector assembly. The development of a simulation toolkit based on GEANT4 and MENATE_R is described, adapted as a strategy to investigate the influence of experimental conditions on the observed properties of neutrons emitted. Besides the kinematic neutron anisotropy in the laboratory system due to neutron evaporation from moving fragments, two additional effects are discussed which may have an influence on the angular distributions of neutrons: scission neutrons and dynamic neutron emission anisotropy in the CM system of fragments due to the spin carried by fragments. A new analysis method is presented to disentangle the dynamic anisotropy from the other effects in an independent way. For the dynamic anisotropy only an upper limit could be found. Results for the angular correlation $ (\mathrm{n},\mathrm{n})$ between two evaporated neutrons and the correlation $ (\mathrm{n},\mathrm{LF})$ between an evaporated neutron and the Light Fragment direction of flight are reported.

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.

Fission fragment velocity distribution measurement using time of flight technique

Two large area position sensitive Multi Wire Proportional Counters (MWPCs) have been developed for the detection of Fission Fragments (FFs). The detectors were characterized using FFs from 252Cf source to obtain position information by employing delay-line read out method and the resolution is about 1 mm in both X and Y directions. Velocity distribution of the FFs produced from spontaneous fission of 252Cf has been measured employing a time-of-flight (TOF) technique, using a barium fluoride (BaF2) detector and a MWPC. The most probable velocities obtained for the heavy and light fragment groups are 1.035 ± 0.003 and 1.378 ± 0.004 cm/ns, respectively. The energy dependence of fragment velocity distribution was also studied after degrading the energy of the fragments by using Mylar foils. It is observed that the width of the distribution for the heavy fission fragments decreases with the reduction in the energies, but for light fragments it does not show any significant dependence on the energy.

Measurement of the 234U(n, f ) cross-section with quasi-monoenergetic beams in the keV and MeV range using a Micromegas detector assembly

The 234U neutron-induced fission cross-section has been measured at incident neutron energies of 452, 550, 651 keV and 7.5, 8.7, 10 MeV using the 7Li (p, n) and the 2H(d, n) reactions, respectively, relative to the 235U(n, f ) and 238U(n, f ) reference reactions. The measurement was performed at the neutron beam facility of the National Center for Scientific Research “Demokritos”, using a set-up based on Micromegas detectors. The active mass of the actinide samples and the corresponding impurities were determined via $ \alpha$ -spectroscopy using a surface barrier silicon detector. The neutron spectra intercepted by the actinide samples have been thoroughly studied by coupling the NeuSDesc and MCNP5 codes, taking into account the energy and angular straggling of the primary ion beams in the neutron source targets in addition to contributions from competing reactions (e.g. deuteron break-up) and neutron scattering in the surrounding materials. Auxiliary Monte Carlo simulations were performed making combined use of the FLUKA and GEF codes, focusing particularly on the determination of the fission fragment detection efficiency. The developed methodology and the final results are presented.