Spectroscopy Of Fission Fragments Research Articles

Excitation energy dependency of the low-energy fission dynamics: Probing through prompt gamma-ray spectroscopy

Fission Fragment Spectroscopy (FFS) technique has been utilized to make simultaneous measurements of both the relative charge and mass yield distributions of even-even fission fragments produced from the fission of 236U at two different excitation energies, Eex. The necessary analysis has been carried out on the measured yield distributions by employing the Multimodal Random Neck Rupture Model (MM-RNRM). The relative contribution of two different types of asymmetric mode of fission – Standard I and Standard II has been extracted at Eex = 6.5 and 21.5 MeV. An attempt has been made to study the evolution of these two asymmetric fission modes with respect to the values of Eex of the concerned fissioning system at low-excitation regime. The probable evidence, although at the primitive stage, for an oscillatory behavior of the relative contributions of different low-energy fission modes with increasing values of Eex has been presented.

Advancements of $$\gamma $$-ray spectroscopy of isotopically identified fission fragments with AGATA and VAMOS++

$$\gamma $$ -ray spectroscopy of fission fragments is a powerful method for studies of nuclear structure properties. Recent results on the spectroscopy of fission fragments, using the combination of the AGATA $$\gamma $$ -ray tracking array and the VAMOS++ large acceptance magnetic spectrometer at GANIL, are reported. A comparison of the performance of the large germanium detector arrays EXOGAM and AGATA illustrates the advances in $$\gamma $$ -ray spectroscopy of fission fragments. Selected results are highlighted for prompt $$\gamma $$ -ray spectroscopy studies, measurements of short lifetimes of excited states with the Recoil Distance Doppler-Shift method, using both AGATA and VAMOS++ and prompt-delayed $$\gamma $$ -ray spectroscopy studies using AGATA, VAMOS++ and EXOGAM.

Development of a liquid scintillator based active fission target for FIPPS

An active fission target has been developed for the FIPPS instrument at ILL, enabling for the first time an efficient suppression of upbeta -delayed upgamma rays in high-resolution and high-efficiency upgamma -ray spectroscopy of fission fragments at a neutron beam. The target is based on a liquid scintillator in which the actinide is dissolved, resulting in a 4pi fragment detection. Measurements have been performed with ^{233, 235}U, with a fission tagging efficiency of 97.8(25)%. The high efficiency, together with the good time resolution of the scintillator target, provide high-selectivity data for upgamma -ray spectroscopy studies of fission fragments.

Excitations of the magic N=50 neutron-core revealed in Ga81

The high-spin states of the neutron-rich Ga81, with three valence protons outside a Ni78 core, were measured. The measurement involved prompt γ-ray spectroscopy of fission fragments isotopically identified using the combination of the variable mode spectrometer (VAMOS++) and the advanced gamma tracking array (AGATA). The new γ-ray transitions, observed in coincidence with Ga81 ions, and the corresponding level scheme do not confirm the high-spin levels reported earlier. The newly observed high-spin states in Ga81 are interpreted using the results of state-of-the-art large-scale shell model (LSSM) calculations. The lower excitation energy levels are understood as resulting from the recoupling of three valence protons to the closed doubly magic core, while the highest excitation energy levels correspond to excitations of the magic N=50 neutron core. These results support the doubly magic character of Ni78 and the persistence of the N=50 shell closure but also highlight the presence of strong proton-neutron correlations associated with the promotion of neutrons across the magic N=50 shell gap, only few nucleons away from Ni78.Received 11 January 2019Revised 30 April 2019DOI:https://doi.org/10.1103/PhysRevC.100.011301©2019 American Physical SocietyPhysics Subject Headings (PhySH)Research AreasEnergy levels & level densitiesNuclear structure & decaysRare & new isotopesProperties59 ≤ A ≤ 89TechniquesGamma ray spectroscopyShell modelSpectrometers & spectroscopic techniquesNuclear Physics

Effects of one valence proton on seniority and angular momentum of neutrons in neutron-rich Sb51122–131 isotopes

The neutron-rich $^{122-131}$Sb isotopes were produced as fission fragments in the reaction $^{9}$Be($^{238}$U,~f) with 6.2 MeV/u beam energy. An unique setup, consisting of AGATA, VAMOS++ and EXOGAM detectors, was used which enabled the prompt-delayed gamma-ray ($\gamma$) spectroscopy of fission fragments in the time range of 100 ns - 200 $\mu$s. New isomers, prompt and delayed transitions were established in the even-A $^{122-130}$Sb isotopes. In the odd-A $^{123-131}$Sb isotopes, new prompt and delayed $\gamma$-ray transitions were identified, in addition to the confirmation of the previously known isomers. The half-lives of the isomeric states and the $B(E2)$ transition probabilities of the observed transitions depopulating these isomers were extracted. The experimental data was compared with the theoretical results obtained in the framework of Large-Scale Shell-Model (LSSM) calculations in a restricted model space. Modifications of several components of the shell model interaction were introduced to obtain a consistent agreement with the excitation energies and the $B(E2)$ transition probabilities in neutron-rich Sn and Sb isotopes. The isomeric configurations in Sn and Sb were found to be relatively pure. Further, the calculations revealed that the presence of a single valence proton, mainly in the $g_{7/2}$ orbital in Sb isotopes, leads to significant mixing (due to the $\nu\pi$ interaction) of: (i) the neutron seniorities ($\upsilon_{\nu}$) and (ii) the neutron angular momentum ($I_{\nu}$). The above features have a weak impact on the excitation energies, but have an important impact on the $B(E2)$ transition probabilities. In addition, a constancy of the relative excitation energies irrespective of neutron seniority and neutron number in Sn and Sb was observed.

A gated LaBr[formula omitted](Ce) detector for border protection applications

We report on the dedicated implementation of the blocking technique for a LaBr3(Ce) detector as well as associated electronics and data acquisition system for border protection applications. The detector is meant to perform delayed γ-ray spectroscopy of fission fragments produced via photofission induced by a high intensity pulsed photon beam. The gating technique avoids saturation of the detection chain during irradiation. The resulting setup allows us to successfully perform delayed γ-ray spectroscopy starting only 30 ns after the gating operation. The measured energy resolution ranges from 5% to 6.5% at 662 keV depending on the γ-ray detection time after the gating operation.

A twin Frisch-grid ionization chamber as a selective detector for the delayed gamma-spectroscopy of fission fragments

We present a twin Frisch-grid ionization chamber. The detector is meant to provide high selective power for the study of delayed gamma-ray spectroscopy of fission fragments produced via 252Cf spontaneous fission. A mean energy resolution on the kinetic energy of fission fragments of 675keV (FWHM) is achieved and allows us to resolve masses of fragments for fission events where neutron emission is not energetically possible. The mean mass resolution measured for these particular events amounts to 0.54 mass units (FWHM). For fission events with neutron emission a resolution of 4 mass units (FWHM) is reported. Information on fragment emission angle is measured with a resolution of 0.1 on the difference of the cosines determined for both halves of the detector. A charge resolution of 4.5 charge units (FWHM) is also demonstrated.

γ-ray spectroscopy of fission fragments produced inPb208(O18,f)

Prompt gamma-ray spectroscopy of fission fragments produced in the heavy-ion induced fusion-fission reaction $^{208}\mathrm{Pb}(^{18}\mathrm{O},f)$ at $E=90$ MeV has been performed. The relative isotopic yields of the fission fragments and the fragment mass distribution have been studied. Structures in the mass distribution have been discussed in the light of earlier results. Relative yields of several odd-$A$ isotopes of Mo, Ru, Pd, and Cd and the odd-$A$ isotones with $N=62$ and 64 have been studied along with the yields of the neighboring even-$Z$, even-$N$ fragments and correlated to nuclear structural effects. The average total neutron multiplicity during fission has been measured to be $5.48\ifmmode\pm\else\textpm\fi{}0.59$. The level schemes of the two neutron-rich nuclei $^{110}\mathrm{Pd}$ and $^{116}\mathrm{Cd}$ have been studied from $\ensuremath{\gamma}$-ray triple coincidence data. A large number of transitions, previously reported only from $\ensuremath{\beta}$-decay studies, have been observed in $^{110}\mathrm{Pd}$ for the first time. The yrast band in $^{116}\mathrm{Cd}$ has been extended up to spin $({16}^{+})$. In addition, a rotational sequence built upon an excited ${5}^{\ensuremath{-}}$ state in $^{116}\mathrm{Cd}$ has been observed up to $({13}^{\ensuremath{-}})$. The level schemes have been discussed in the context of existing results, both experimental and theoretical, in the literature.

γ-ray spectroscopy of fission fragments from the cold-neutron235U induced fission with EXILL

A cold neutron induced fission experiment recently took place at the Institute Laue-Langevin (ILL) in Grenoble. The neutron beam was provided by the nuclear reactor facility at ILL and the detector setup that was used for the γ-spectroscopy of the fission products consisted mainly of the detectors of the EXOGAM array [1], thereby the name of the campaign is EXILL. The main purpose of our measurement was to investigate the nuclei in the region with N = 50 close to 78Ni as well as the nuclei close to the N = 82 shell closure. In this paper, the motivation of the experiment is described as well as the experimental setup and the status of the ongoing data analysis.

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.

Application of digital signal processing in nuclear spectroscopy

Digital signal processing algorithms for nuclear particle spectroscopy are described. The algorithms are presented as recursive procedures, which can easily be programmed using modern computer programming languages. The input data flow was formed by particle detector pulses, preprocessed by a charge-sensitive preamplifier and transformed into a sequence of equidistantly sampled signals by a sampling analog-to-digital converter. The pile-up elimination procedure, the influence of the number of bits of the sampling analog-to-digital converter on the energy resolution of the spectrometer, and ballistic deficit correction are also considered. The pile-up elimination method was developed for fission fragment spectroscopy in the case of high α radioactivity of fissile targets (239Pu, 241Am) using a Frisch-grid double ionization chamber. Since the intrinsic energy resolution of a spectrometer based on a double ionization chamber is much worse than that determined by the noise level of the charge-sensitive amplifier, an ultrapure germanium detector was used to provide the highest sensitivity to the errors introduced by measuring procedures. The effect of pile-up elimination procedure on the energy resolution was experimentally investigated at different source intensities. The main characteristics of the developed algorithms are compared with those of other processing procedures published in the literature.

Preparation of radioactive targets for tandem accelerator experiments by high vacuum evaporation–condensation

Radioactive thin film targets have been prepared by high vacuum evaporation–condensation in the hot-lab facility of the University of Munich. Targets made by this technique are of high density and excellent smoothness, being well suited for experiments aimed at the production and spectroscopy of fission fragments. Cold crucible electron beam heating is the preferred method for evaporation of actinide isotopic material of sufficient abundance, for instance 230Th, 232Th, 233U, 235U, 238U and 239Pu. Exotic isotopes, which are available only in submilligram quantities, are processed in a micro-evaporator module consisting of a chimney-type crucible heated by low-tension electron bombardment. This set-up is characterised by a high collection efficiency and was used for the production of 244Pu and 10Be targets.

Fission fragment spectroscopy with large Ge detector arrays

Fission fragment spectroscopy with large Ge detector arrays

5623207 Method for avoiding image occlusions in nuclear magnetic resonance tomography apparatus that are operated with multi-echo sequences

Large area (A = 8.5 cm2) surface barrier detectors have been tested for consistent time-of-flight and pulse height spectroscopy of fission fragments. Timing was measured with fast and narrow IR laser diode pulses; it yielded a resolution (97±14) ps and an integral width of (228±58) ps fwhm with detectors cooled −20o°C. Mass dependent plasma delay differences were obtained from correlated 252Cf fragment determination and linear momenta from two time-of-flight measurements for different distances. Pulse height spectroscopy performed with the Schmitt calibration constants yielded linear momenta that were 1.2% higher; this discrepancy is removed if the revised calibration cconstants yielded linear momenta that were 1.2% higher; this discrepancy is removed if the revised calibration constants derived with fragments of known energies of Weissenberger et al. are used.

Preparation of thin counting samples by volatilization-condensation

We describe a technique for the preparation of thin counting samples of metallic elements for a α-particle and fission-fragment spectroscopy. The elements are volatilized in a carrier gas stream through a capillary and condensed on a cooled thin collector foil.

Escape suppressed spectrometer arrays: A revolution in γ-ray spectroscopy

The design and performance of Escape Suppressed γ-ray Spectrometer Arrays are described. These arrays have revolutionised the field of γ-ray spectroscopy with their significant improvement in signal to background ratios enabling very weak photon lines to be detected in the presence of many stronger γ-rays. This enables the properties of nuclei to be studied at the highest spins and for very weak channels to be detected. The highlights of the new physics achieved with these arrays are described. These include: the observation of discrete superdeformed states up to spin 60 kh: the systematic studies of aligned many quasi-particle states in rotating nuclei; band terminations and phase changes at high spins; classical rotations and the disappearance of static pairing; correlations in continuum radiation; the spectroscopy of proton rich nuclei at and near the N=Z line; fission-fragment spectroscopy; the measurement of sub-Coulomb reaction cross-sections and the associated input angular momentum.

Bragg curve spectroscopy of fission fragments by using parallel plate avalanche counters

By using a charge-sensitive preamplifier and methods of energy spectroscopy we investigated the response of a parallel plate avalanche counter in order to scan the Bragg curve. The detector was operated in butabe, pentane and heptane which also served as stopping gases. In this paper, we report on results obtained with fission fragments from a 252Cf source. We achieved a separation of the light and heavy fragment groups in the interval of gas absorber thickness from about 500 μg cm −2 to 1.5 mg cm −2. The best energy loss resolution was achieved with pentane at E p ≃ 70 V cm −1 Torr −1 resulting in σΔE ΔE ≃ 13% . The corresponding shape of the ΔE spectrum reproduces correctly the nuclear charge distribution of the fragments.

Fission fragment spectroscopy with large surface barrier detectors

Large area ( A = 8.5 cm 2) surface barrier detectors have been tested for consistent time-of-flight and pulse height spectroscopy of fission fragments. Timing was measured with fast and narrow IR laser diode pulses; it yielded a resolution (97±14) ps and an integral width of (228±58) ps fwhm with detectors cooled −20o°C. Mass dependent plasma delay differences were obtained from correlated 252Cf fragment determination and linear momenta from two time-of-flight measurements for different distances. Pulse height spectroscopy performed with the Schmitt calibration constants yielded linear momenta that were 1.2% higher; this discrepancy is removed if the revised calibration cconstants yielded linear momenta that were 1.2% higher; this discrepancy is removed if the revised calibration constants derived with fragments of known energies of Weissenberger et al. are used.

Fission fragment spectroscopy by measurements in nuclear track detectors applied to the fission of silver induced by 80 MeV alpha particles

A method is presented for the analysis of track length distributions in mica track detectors and track diameter distributions in glass detectors, produced by the fission fragments of medium mass nuclei. Measured distributions are compared with distributions calculated from trial values for the total kinetic energy release, the center-of-mass motion, and the form of the fragment mass distributions. A FORTRAN computer program has been written that calculates the relevant quantities for the fission process under study and incorporates the characteristics of the stopping process of the fragments. The resulting calculated track length and track diameter distributions show only a moderate sensitivity towards the shape of the fragment mass distribution. A simple fitting procedure is shown to give estimates of the total kinetic energy release as well as of the center-of-mass motions for medium mass fissioning systems. The method is applied to data from the 80 MeV alpha-particle-induced fission of silver, and yields a value for the total kinetic energy release of this system which is higher than present theoretical estimates.