Fission cross section and fragment angular distribution in gold fission induced by 55 MeV alpha particles using solid state nuclear track detectors

The angular distribution of fission fragments in alpha induced fission has been studied at an incident energy of 55 MeV in¹⁹⁷Au target. The relative differential fission cross sections were measured at different angles between 10° and 170° and the resulting angular distributions fitted by least squares method with Legendre polynomials. In the present work, a correction for the self-scattering and self-absorption of fission fragments in the target itself was applied and a target of 3 mg/cm² was used to get good statistics. The anisotropy for 55 MeV alpha induced fission of gold was 2·83±0·43 and the fission cross section calculated by integrating the measured angular distributions over all the solid angles was 5·2±1.0 mb, confirming the value of 4·0±0·05 mb reported by Burnettet al but contrary to the high value of 10±3 mb reported by Ralarosyet al.     

Fission fragment angular distribution in alpha-particle-induced fission of actinide elements

Using Lexan polycarbonate plastic as the fission fragment track detectors, the fragment angular distributions have been measured in the cases of fission of²³²Th and²³⁸U induced by alpha particles of various energies ranging from 40 to 70MeV obtained from the 88″ variable energy cyclotron at Calcutta. The center-of-mass angular distributions have been calculated and fitted by a series of Legendre polynomials. TheW(10°)/W(90°) ratios (defined as anisotropy) were measured at several energies for both the targets. These data are utilized in calculation of the energy dependence ofK ₀ ² , the standard deviation of the distribution in the angular momentum projection on the nuclear symmetry axis at the saddle point. Values of Γ _f /Γ _η , i.e. the ratio of the fission width to neutron emission width have been determined for²³²Th and²³⁸U nuclei. The integral cross-section for alpha induced fission in each target was determined by numerical integration of the respective center-of-mass angular differential cross-sections. The results were compared with similar data available in the literature which served to resolve some of the discrepancies observed in earlier measurements. The results were also compared with theoretical cross-sections.     

Fragment angular distributions in electron-induced fission of 232Th

Fission fragment angular distributions have been observed in electron-induced fission of ²³²Th for electron energies 8.7 MeV ≦ E_e ≦ 30 MeV. For low energies (though above the fission threshold) the angular distributions contain both a dipole and a quadrupole component. The (90°/0°) anisotropy decreases rapidly for higher electron energies but reveals smaller peaks after the onset of second-, third- and fourth-chance fission suggesting that the effective fission barriers for ²³¹Th and ²²⁹Th in second- and fourth-chance fission, respectively, are both characterized by $\text{K =}\text{1}\text{2}$.     

In-beam investigation of ternary fission

Nuclear fission accompanied by long-range alpha particles has been investigated. Fission was induced by irradiating a natural uranium target with 13.5 MeV deuterons. The alpha energy and angular distributions are similar to those observed in spontaneous or thermal neutron induced fission. The correlation angle between alpha particles and light fission fragments has a most probable value of \(\bar \vartheta _{l f - \alpha } = 82.1^\circ \pm 0.9^\circ \) and a dispersion (FWHM) of Δ?=18.4°±1.2°. The mean value and dispersion of the energy distribution are \(\bar {\rm E}_\alpha = 14.8 \pm 1.0 MeV\) and ΔE=9.1±1.2 MeV (FWHM), respectively. The peak-to-valley ratio of the ternary fission fragment mass distribution is found to increase with increasing alpha energy. For near-symmetric mass division, a strong broadening of the angular distribution is observed. The results are discussed in the frame of a multichannel-fission model.     

The 6He emission accompanying the spontaneous fission of 252Cf

The results of a multiparameter investigation of the ⁶He emission accompanying the spontaneous fission of ²⁵²Cf are presented. The energy spectrum and the yield of the ⁶He particles are found to be in accord with previous measurements, but their angular distribution is observed to be narrower at 13° ± 3° (FWHM) than the value of ? 32° deduced in a previous investigation.Comparisons of the experimental with published calculated energy spectra and angular distributions for ³H, ⁴He and ⁶He particles are shown to provide evidence for a compact scission configuration in ternary fission.     

Search for E2 strength in electrofission of 238U and 232Th

We have measured the cross-section ratio σ^?/σ⁺ for the electron and positron induced fission of uranium and thorium in the energy range 10–40 MeV with detectors at 90° and in a 2π geometry. The observed periodic structure is discussed with respect to E2 admixtures and an anisotropy of the angular distribution, after the onset of each higher chance fission threshold, as well as with respect to the shape of the photofission cross section.     

Mechanisms for emission of4He in the reactions of 334 MeV40Ar with238U

Emission of⁴He in the reaction 334 MeV⁴⁰Ar+²³⁸U has been studied by triple coincidence measurements that allow the separate identification of fusion fission and sequential fission. For the⁴He evaporative spectra from fusion fission the composite system is shown to be the predominant contributor; whereas, for sequential fission the dominant emission is from the fragments. This result demonstrates a correlation between evaporative emission probability and lifetime expectancy of the composite system. To account for the observed⁴He spectra two other mechanisms are necessary in addition to nuclear evaporation. At forward angles, the⁴He spectra from both fusion fission and sequential fission exhibit higher intensities and larger energies than those expected from purely evaporative processes. This forward-peaked component must be related to a very rapid or pre-thermalization stage of the reaction. At backward angles yet another component is observed for fusion fission. As it is sensitive to the fragment masses but does not carry the kinematic shift characteristic of their full acceleration, this component must originate near to the time of scission. The average⁴He energy for this component is approximately 17 MeV (c.m.), and its intensity is correlated with a plane perpendicular to the fission fragment separation axis. These signatures are similar to those for long range alpha particle emission in low energy fission. Alpha particles evaporated from the composite nuclei in fusion-fission reactions are shown to be preferentially associated with fission events which result in the more symmetric masses. This result is consistent with the notion that mass asymmetric fission is a faster process than symmetric fission. Such a correlation between mass asymmetry and lifetime is an essential part of the “fast fission” or “quasifission” idea, which has attracted much current attention.     

Possible observation of light neutron nuclei in the alpha-particle-induced fission of 238U

Searches for nuclear-stable multineutrons among products originating from the fission of ²³⁸U nuclei that is induced by 62-MeV alpha particles were performed by the activation method. The reaction involving the transfer of four neutrons and occurring on the isotope ⁸⁸Sr, ⁸⁸Sr( ^x n, (x ? 4) n)⁹²Sr??⁹²Y, was used to identify nuclear-stable multineutrons. A line at the energy of E = 1384 keV was found in the measured gamma-ray spectra of irradiated samples. This line, together with the measured time dependence of the decrease in its activity, is indicative of the formation of the beta-active nucleus ⁹²Sr. This result was reproduced in repeated measurements. It suggests the possible existence of nuclear-stable multineutrons ( ^x n) for x ?? 6. The differential cross section for the ^x n yield at an angle of 30° in the alpha-particle-induced fission of ²³⁸U was about 6 × 10^?2 mb/sr.     

Quantum effects in the low temperature reorientation of N2− in KCl and KI

The electron paramagnetic resonance (EPR) spectra of KCl:N₂^? and KI:N₂^? were studied as a function of temperature and applied uniaxial stress in the temperature range 2–35°K. The molecular reorientation rates were determined for both 60 and 90° reorientations from the motional broadening and narrowing effects in the observed spectra. Although thermally activated in the range 15–35°K for KCl:N₂^?, the parameters deduced from the data are not consistent with a simple classical model of the motion. In both hosts the 90° rotation is markedly faster than the 60° reorientation, in conflict with simple models, but explicable in terms of the suppression of the tunneling matrix elements by polarization of the host crystal. In KI:N₂^?, the 90° reorientation is still apparent at 1·7°K and the spectra clearly show the influence of the symmetry of the nuclear spin states upon the reorientation kinetics, an effect predicted by Sussman.     

Measurement of the neutron spectra from the decay of the giant quadrupole resonance in 238U

We have performed the coincidence experiments (α, α'f) and (α, α'nf) on ²³⁸U for a beam energy Eα = 120 MeV and for α-scattering angles Θα' = 9°, 15°, and 17°. The 9° and 17° data correspond to maxima in the angular distribution for the isoscalar giant quadrupole resonance, whereas the 15° data lie close to a minimum. Using a set of fission counters which covered roughly 80% of 4π, we were able to reject neutrons associated with fission in the (α, α'n) spectrum. We present a first look at these spectra.     

Effects of radiations on the characteristics of alpha and fission tracks in CR-39 detectors

Abstract

The effects of neutron, gamma and alpha radiations on the alpha and fission fragment tracks registration and revelation properties of CR-39 detectors (CR-39 and CR-39(DOP) were studied. It was found that the ratio of the bulk etch rate of irradiated to unirradiated (V_G(irr.)/V_G(unirr.) detectors is linearly dependent on dose. An exponential decrease in fission track densities with increase in neutron fluence was observed. The ratio of V_G(irr.)/V_G(unirr.) was found to be high in CR-39 than that in CR-39(DOP) exposed to the same reactor neutron fluence. The decrease in fission track densities with increase in neutron fluence was observed to be faster in CR-39 than in CR-39(DOP). This indicates that doping with dioctyl phthalate improves the radiation resistance of CR-39 detectors. It was observed that in detectors exposed to an alpha flux of the order of 9.36 × 10⁶ / cm², the fission track density was reduced by 11% and thereafter it remained constant. The results also indicate that thermal neutron fluence up to 7.01 ×10¹¹ neutrons/cm² does not affect the alpha and fission track densities. I.R. spectra were also studied to find out the nature of chemical changes produced by these radiations on CR-39.     

Ternary fission involving4He emission in the16O (144 MeV)+232Th and12C (108 MeV)+197Au reactions

The results of coincidence measurements of⁴He emission with fission fragments in reactions of¹²C(108 MeV) ions with a¹⁹⁷Au target and of¹⁶O(144 MeV) ions with a²³²Th target are presented. On the basis of a Monte Carlo kinematic simulation of nuclear reactions the experimental energy spectra and velocity distributions of alpha particles have been analyzed. A conclusion has been drawn that the main source of⁴ He emission is evaporation from the fissioning compound nucleus. Substantial part of alpha particles was emitted from fully accelerated fission fragments. Some of⁴He nuclei with an average energy of about 16 MeV (in the CM system) emitted mainly perpendicular to the fission axis were identified as being similar long-range alpha particles produced in ternary fission of heavy nuclei at a low excitation energy. The emission multiplicities of these particles are considerably higher than those observed at a low excitation energy. The experimental results are compared with the statistical model predictions.     

Investigation of fragment yields from <Superscript>239</Superscript>U fission at anomalously high values of the total kinetic energy

The yields of fragments originating from ²³⁸U fission induced by 5-MeV neutrons are investigated. Accumulated statistics—2.5×10⁶ events of binary fission—make it possible to study fission-fragment yields at anomalously high values of the total kinetic energy. The spectra of the cold fragmentation of ²³⁹U are obtained. Events characterized by the total kinetic energy that is equal to the total reaction energy are found for some fragment masses. Methods of digital signal processing permit a highly reliable identification of these rare events. An interpretation of this phenomenon on the basis of the liquid-drop model of the fission process is proposed.     

Fission accompanied by alpha particle emission in the12C(85MeV)+232Th reaction

Inclusive and coincident spectra of alpha particles and fission fragments were measured for the²³²Th+¹²C (85 MeV) reaction to study the influence of the excitation energy and the angular momentum on the fission of the compound nucleus and to separate different alpha particle emission mechanisms. At backward angles α emission can be accounted for by the evaporation processes. At forward angles the most important contribution is given by the break-up fusion process. Mass distributions for compound nuclei²⁴⁴Cm (E^*=58 MeV,ff coincidences), and²⁴⁰Pu (E^*=37 MeV,ff α coincidences) were obtained. In the case of²⁴⁰Pu mass distribution has a shape different from those obtained in light ion reactions at the same excitation energy, indicating the strong influence of the entrance channel. The dependence of the mass distribution shape on the α particle energy is also examined.     

Angular distributions of fission and deep inelastic products of the system Ar+Au at 285 MeV

The angular distribution of the heavy products from the interaction of 285 MeV ions⁴⁰Ar on an Au target has been analysed through a large angular range (4°–165° lab angles). The results exhibit two components: the fission component corresponding to relatively symmetric masses, and a deep inelastic one which peaks in the angular range 100°–140° (c.m angles). The fission component exhibit the usual 1/sinθ pattern, proving that, in spite of a zero fission barrier, the life time of the system exceeds the rotation period. The relative cross sections for the fission and deep inelastic processes are discussed.     

Luminescence and EPR spectroscopy of neutron-irradiated single crystals of magnesium aluminium spinel

Neutron irradiated single crystals of stoichiometric MgAl₂O₄ and MgAl₂O₄:Mn²⁺ (0.15 wt. %) were studied using the methods of luminescence spectroscopy and versions of electron paramagnetic resonance. In stoichiometric MgAl₂O₄:Mn²⁺ single crystals, the transition of a part of manganese impurity ions from tetrahedral to octahedral coordination, caused by irradiation with fast fission neutrons, is detected using photoluminescence spectra. This fact confirms the partial inversion of a normal spinel due to neutron-irradiation. Using pulse EPR technique it is shown that an electron in the field of oxygen vacancy (i.e. from neutron-irradiation induced F⁺ centres, which are spatially separated from each other) is in hyperfine interaction only with neighbouring ²⁷Al nuclei with nuclear spin 5/2.     

Light particle emission in fusion-like 14N + 197Au collisions at E/A = 35 MeV

Single- and two-particle inclusive cross sections for light nuclei (p, d, t, ^3,4He, ^6–9Li, ^7,8,10Be) were measured for ¹⁴N induced reactions on ¹⁹⁷Au at E/A = 35 MeV. At forward angles, θ ≈ 20°, quasi-elastic peripheral reactions and more violent fusion-like collisions were discriminated by measuring the folding angle between two coincident fission fragments resulting from the decay of the heavy reaction residue. More pronounced correlation functions, but very similar emission temperatures are observed for fusion-like collisions as compared to peripheral collisions.     

Muon capture rates in233U,234U,235U,236U,238U,and237Np

The time spectra for muon induced fission of²³³U,²³⁴U,²³⁵U,²³⁶U,²³⁸U, and²³⁷Np were measured simultaneously, detecting both fragments in coincidence in a fast multi parallel plate avalanche detector. The observed mean lifetimes τ are 68.9 ± 0.3 ns, 70.6 ± 0.2 ns, 72.2 ±0.2 ns, 74.3 ± 0.3 ns, 77.0 ±0.4 ns, and 69.8 ±0.2 ns for the above isotopes, respectively. No second time component, tentatively ascribed to a possible fission isomer, was observed. The deduced total capture rates are compared with theoretical predictions.     

Temperature dependence of yields from multi-foil SPES target

The temperature dependence of neutron-rich isotope yields was studied within the framework of the HRIBF-SPES Radioactive Ion Beams (RIB) project. On-line release measurements of fission fragments from a uranium carbide target at $\ensuremath 1600 {}^{\circ}\mathrm{C}$\ensuremath 1600 {}^{\circ}\mathrm{C} , 1800 ^°C\ensuremath 1800 {}^{\circ}\mathrm{C} and 2000 ^°C\ensuremath 2000 {}^{\circ}\mathrm{C} were performed at ORNL (USA). The fission reactions were induced by a 40MeV proton beam accelerated into a uranium carbide target coupled to a plasma ion source. The experiments allowed for tests of performance of the SPES multi-foil target prototype loaded with seven UC₂/graphite discs (ratio C/U = 4 with density about 4g/cm³.     

Structural phase transformations in annealed cubic ZnS nanocrystals

The structural changes of cubic ZnS (cZnS) nanocrystals (NCs) doped with 0.2 at.% Mn²⁺ pulse annealed in vacuum and in air, up to 500 °C, were investigated by multifrequency electron paramagnetic resonance (EPR), X-ray diffraction (XRD), and transmission electron microscopy (TEM). The samples, prepared by a surfactant (Tween20)-assisted liquid–liquid reaction at pH = 6, consist of NCs with a tight size distribution around 3 nm and high crystallinity self-assembled into a stable mesoporous structure. The EPR spectra of the as prepared samples contain only the characteristic lines of the substitutional Mn²⁺(I) centers. No spectra from Mn²⁺ ions localized in (hydro)oxidized regions of the NCs surface were observed. The absence of such a surface layer could explain the stability of the cubic (sphalerite) structure observed by XRD and TEM in the investigated cZnS:Mn NCs annealed in vacuum up to 500 °C. The observation of the cubic-hexagonal transformation for the same NCs annealed in air supports the role of such layer in promoting this structural transformation. The narrowing of the EPR spectral lines above 200 °C with the increase in the average size of the cZnS:Mn crystallites was observed. The effect was more pronounced for the sample annealed in air. EPR also revealed the formation of minute amounts of substitutional Mn²⁺-type centers in a hexagonal ZnO structure at T ~ 300 °C, corresponding to the early stages of the thermally induced oxidation of the cZnS:Mn NCs.