Neutron Decay Widths Research Articles

Neutron and fission decay width of superheavy compound nuclei

We have systematically studied the level density parameter, nuclear level density and decay width of neutron and fission of superheavy nuclei. This paper is validated by comparing with available experiments for [Formula: see text]U and [Formula: see text]Pu. The angular dependence of level density parameter, level densities and ratio of [Formula: see text] and [Formula: see text] the thermal excitation energy (U) have been studied in the superheavy region [Formula: see text]. The role of entrance channel parameter on decay width of neutron and fission has been studied both in cold and hot fusion reactions in the superheavy region [Formula: see text]. Systematic variation of [Formula: see text] and [Formula: see text] is observed in both the cases. Finally, predicted [Formula: see text] for the synthesis of superheavy nuclei [Formula: see text] and 120.

Accurate estimation of the neutron and fission decay widths for hot fusion reactions

Yanez et al. [Phys. Rev. Lett. 112, 152702 (2014)] measured unusually high neutron decay width for the hot fusion reaction $^{25,26}\mathrm{Mg}+^{248}\mathrm{Cm}$ compared to the theoretical estimations available till that date. Various theoretical models have been developed since then to upgrade our understanding of the phenomenon but good agreement is not yet achieved. We have made an attempt for the same, which is based on a modified back-shifted Fermi gas model that also includes the shell and pairing energy correction. Though it works well for the neutron decay, it needs a dissipative effect also to evaluate the fission decay width correctly. It predicts the neutron to total decay width ratio 0.82--0.89 for dissipation coefficients 15--22 and thus closed to the measured value $0.89\ifmmode\pm\else\textpm\fi{}0.13$.

Neutron to dark matter decay in neutron stars

Recent proposals have suggested that a previously unknown decay mode of the neutron into a dark matter particle could solve the long lasting measurement problem of the neutron decay width. We show that, if the dark particle in neutron decay is the major component of the dark matter in the universe, this proposal is in disagreement with modern astrophysical data concerning neutron star masses.

Single-particle and collective motion in unbound deformed Mg39

Background: Deformed neutron-rich magnesium isotopes constitute a fascinating territory where the interplay between collective rotation and single-particle motion is strongly affected by the neutron continuum. The unbound $fp$-shell nucleus $ ^{39}\text{Mg} $ is an ideal candidate to study this interplay. Purpose: In this work, we predict the properties of low-lying resonant states of $ ^{39}\text{Mg} $, using a suite of realistic theoretical approaches rooted in the open quantum system framework. Method: To describe the spectrum and decay modes of $ ^{39}\text{Mg} $ we use the conventional Shell Model, Gamow Shell Model, Resonating Group Method, Density Matrix Renormalization Group method, and the non-adiabatic Particle-Plus-Rotor model formulated in the Berggren basis. Results: The unbound ground state of $ ^{39}\text{Mg} $ is predicted to be either a $ J^{\pi} = {7/2}^- $ state or a $ {3/2}^- $ state. A narrow $ J^{\pi} = {7/2}^- $ ground-state candidate exhibits a resonant structure reminiscent of that of its one-neutron halo neighbor $ ^{37}\text{Mg} $, which is dominated by the $ f_{7/2} $ partial wave at short distances and a $ p_{3/2} $ component at large distances. A $ J^{\pi}={3/2}^- $ ground-state candidate is favored by the large deformation of the system. It can be associated with the $ {1/2}^- [321] $ Nilsson orbital dominated by the $\ell=1$ wave; hence its predicted width is large. The excited $J^{\pi} = {1/2}^-$ and $5/2^-$ states are expected to be broad resonances, while the $ J^{\pi} = {9/2}^- $ and $ {11/2}^- $ members of the ground-state rotational band are predicted to have very small neutron decay widths. Conclusion: We demonstrate that the subtle interplay between deformation, shell structure, and continuum coupling can result in a variety of excitations in an unbound nucleus just outside the neutron drip line.

Measurement of α and neutron decay widths of excited states ofC14

The $^{12}\mathrm{C}$($^{16}\mathrm{O}$,$^{14}\mathrm{O}$)$^{14}\mathrm{C}$ reaction was studied at a beam energy of 234 MeV. The $^{14}\mathrm{O}$ ejectile was detected by a Q3D spectrometer at forward angles. The energies and angles of the excited $^{14}\mathrm{C}$ recoil break-up fragments were measured in coincidence with the $^{14}\mathrm{O}$ ejectile using a double sided silicon strip detector array at backward angles. A complete kinematic reconstruction of the reaction was performed to reconstruct the ${}^{14}{\mathrm{C}}^{*}\ensuremath{\rightarrow}{}^{10}\mathrm{Be}+\ensuremath{\alpha}$ and ${}^{14}{\mathrm{C}}^{*}\ensuremath{\rightarrow}{}^{13}\mathrm{C}+n$ decay channels and the branching ratios and widths of these decays were calculated. Theoretical decay branches were calculated using barrier penetrability factors and were compared to the measured ratios to provide information on the spins, parities, and configurations of the states. Neutron emission was found to be favored for the 11.73, 12.96, 14.87, 16.72, and 18.6 MeV states. The 14.87, 18.6, and 21.4 MeV states were found to have a considerable width for $\ensuremath{\alpha}$-decay and are candidates for the three bodied molecular cluster structure of $^{14}\mathrm{C}$.

Neutron decay width of one-particle resonant levels in deformed nuclei

The neutron partial decay width of one-particle resonant levels that are the bandhead states in odd-$N$ deformed nuclei is estimated in the formalism with a real-energy variable, using the width of one-particle resonance calculated in terms of eigenphase. It is shown that because of the presence of various ($\ensuremath{\ell}j$) components in a given one-particle resonant level of deformed nuclei, the neutron decay width can be quite different from the one obtained from assuming a spherically symmetric nuclear shape. As a numerical example, the formalism is applied to the case of the nucleus ${}_{4}^{13}$Be${}_{9}$, which lies outside the neutron drip line by 100--200 keV. Considering that the core of the nucleus consisting of four protons and eight neutrons is most likely prolately deformed, we point out the possibility that two lowest lying levels have ${I}^{\ensuremath{\pi}}=1/{2}^{+}$ for the $N=9$ nucleus, and the higher lying one may have a neutron decay width that is much narrower than the one expected for an ${s}_{1/2}$ level at the same resonance energy. The structure of some ${\ensuremath{\Omega}}^{\ensuremath{\pi}}=1/{2}^{+}$ one-particle resonant levels, which are smoothly connected to weakly bound one-particle levels as the potential strength becomes stronger, is also analyzed using the present formalism.

0+ analogue state in118Sb from117Sn(p,nγ) reaction

The analogue of the 0+ ground state in118Sn has been observed in the compound nucleus118Sb through117Sn(p,nγ)117Sb reaction. The neutron decays of this analogue resonance have been studied from the deexcitingγ-rays of the residual nucleus117Sb. From off resonance excitation functions, spin assignments have been made to states in117Sb, on the basis of Hauser-Feshbach formalism. The resonance parameters of the isobaric analogue resonance have been determined, including the total, proton and neutron decay widths.

Widths of Ξ Hypernuclear States

The $\Lambda$ and neutron decay widths of $\Xi$ hypernuclear states, based on calculated $\Xi N \to \lala$ mixing amplitudes, are estimated. The widths which result from using the Nijmegen Model D interaction are sufficiently small, of order 1.5 MeV, that experiments to observe $\Xi$ hypernuclear states using the $(K^-,K^+)$ reaction may be feasible.

0+ analogue state in 118Sb from 117Sn(p,nγ) reaction

The analogue of the 0+ ground state in 118Sn has been observed in the compound nucleus 118Sb through 117Sn(p,n γ) 117Sb reaction. The neutron decays of this analogue resonance have been studied from the deexciting γ-rays of the residual nucleus 117Sb. From off resonance excitation functions, spin assignments have been made to states in 117Sb, on the basis of Hauser-Feshbach formalism. The resonance parameters of the isobaric analogue resonance have been determined, including the total, proton and neutron decay widths.

Baryon- and lepton-number violation by electroweak instantons

We make a quantitative study of instanton-induced baryon- and lepton-number-violating processes in an SU(2) x U(1) electroweak gauge theory at zero and finite temperatures (in the ''dilute-instanton-gas'' approximation). As an example we consider a simplified model involving only the proton, neutron, electron, and electron neutrino. At zero temperature the total cross sections for p+n..-->.. e-bar+nu-bar and eleven other similar reactions are of order s x 10/sup -195/ cm/sup 2/, where s is the total center-of-momentum energy squared in GeV/sup 2/. The neutron decays via n..-->..p-bar+e-bar+nu-bar with a lifetime of the order 10/sup 146/ years. The cross sections and neutron decay width decrease with temperature because color-electric-charge screening reduces the self-dual-instanton density at finite temperature. At high temperature the cross sections (for a given s) and neutron decay width fall off as T/sup -47/3/ in this simplified model. It is suggested that correctly treating the instanton gas as very dense (as discussed by Berg, Luscher, and Stehr) and including finite-energy tunneling solutions could increase the predicted reaction rates.

Angular momentum misalignment in deep inelastic processes and angular distribution of sequentially emitted particles and gamma rays

The angular momentum misalignment for fragments produced in deep inelastic scattering is discussed in terms of the thermal excitation of angular-momentum-bearing modes in the intermediate complex. Analytical expressions for the in- and out-of-plane angular distributions are obtained for sequentially emitted particles and fission fragments. The angular momentum dependence of the ratio between particle and neutron decay widths is explicitly treated and found to be quite important. Similarly angular distributions are obtained both for dipole and quadrupole gamma decay. The theoretical results are compared with experimental angular distributions of sequential fission fragments, sequential alphas and gamma rays, and a good agreement is found.

Isotope Dependence of Widths of Hole Analog States

Recent experiments of the neutron pick up reaction with high energy resolution make it possible to extract the values of narrow widths of isobaric analog states in medium and heavy nuclei.!) Taketani et a1. 2 ) ob· served a systematic increase of the widths (20 keY ~40 keY) of the hole analog states (HAS) in the tin isotopes (A=lll~123). They estimated the spreading width due to the coupling of the analog state to the iso­ vector monopole state to be from 18 ke V to 16.5 keY and almost independent of the mass number A. They suggested therefore that the spreading width is dominant for the light isotopes and that the escape width causes the additional broadening for the heavy iso­ topes. In this short note, we examine their conjecture and elucidate the mechanism of these strong isotope dependence of the escape width. All the HAS's in the tin isotopes are un­ stable for proton, neutron and a-particle de­ cays. However, the a-decay width may be negligible as discussed in Ref. 2). We inves­ tigate the proton decay width in detail and consider the neutron decay width briefly. At first we make a rough estimate of the proton decay width, using the single particle escape width,3)

Competition between neutron emission and fission for234U,236U,240Pu and242Pu compound nuclei

In recent years the insufficiency of the s tandard level density expression has been demonstrated (1) and it was found necessary to derive the compound level densities directly from the spectrum of the single-particle levels (~). This approach automatically accounts for the effect of nuclear shells and in addition it allows to take into account the effects of pairing in a na tura l way (3). The problem of obtaining reliable leveI densities and thereby decay widths can therefore be solved if realistic single-particle spectra for the cases of interest are available. This approach seems part icular ly useful in the actinide nuclei, where the existence of a two-peaked barrier is a result of the variation with deformation in the spacings of single-particle levels near the Fermi surfaces (a). I t is now possible, if one knows the single-particle spectra at the relevant deformations (first minimum, fist maximum, second minimum, second maximum) to calculate by a self-consistent statistical model neutron, gamma and fission decay widths (s.s). One of the results of the previous calculations is the existence of a bumplike structure in the ratio Fn/F I of the neutron emission to fission widths at about 5 MeV above the neutron threshold. This structure is due to the different singlc-particle densities relevant for the neutron and fission channels and depends on the height of the fission barrier B relative to the neutron binding energy S; however the calculated structure persists as long as 0 < S B < 2 MeV. Moreover a similar s t ructure has been deduced for 241Cm from the excitation functions of the reactions 23spu(a, 2n) and 241Am(p, 2n) and should show up for a large number of nuclei (s).

Inelastic electron scattering and the electromagnetic properties of 9Be

The simple Nilsson model was calculated out to higher values of the deformation parameter η, and this was shown to produce the intrinsic state sequence, g.s. 3 2 −, 1 2 + at 1.67 MeV, and 1 2 − at about 3 MeV, for not unreasonably large values of η. Rotational bands based on the intrinsic states of the Nilsson model were used to predict the energy spectrum. Electromagnetic transition rates and neutron decay widths have been calculated, and compared with the measured (γ, n) cross section. The low- q extrapolation of electron scattering data provided evidence for an induced octupole deformation of the 9Be surface. This simple Nilsson model was extended to include coupling between the major shells N, N+2, N+4, and N+6, and the same low-lying intrinsic state sequence was obtained for a lower value of η. The implications of using the wave functions belonging to the larger basis set are briefly discussed. Elastic and inelastic electron scattering from 9Be were measured at q-values of 0.6, 0.8 and 0.9 fm −1 and angles 120° and 154°. Form factors for the states at 1.67,2.43 and 3 MeV were analyzed into longitudinal and transverse contributions, and a similar separation made for the continuum scattering up to 10 MeV. Wave functions from the extended Nilsson model calculations were used to compare with the experimental data. Good fits to the ground state charge and magnetic form factors were obtained, but the calculated C2 form factor for the 5 2 − 2.43 MeV state was low. The transverse form factor in the 3 MeV region was shown to be in good agreement with the model calculation provided Ml coupling to a broad 1 2 − state at 3 MeV was included. The data and calculations on the 1.67 MeV 1 2 + state were in disagreement. However, the transverse form factor in the region 4.5 to 6.5 MeV was reasonably well predicted by the Nilsson model.

A generalized r-matrix interpretation of the 89Y + n total cross section at low incident energies

The resonance structure observed in the 89Y(n, n) 89Y total cross-section measurements in the range of 0.9 to 1.2 MeV neutron energy is investigated using a comprehensive theory of nuclear reactions. A shell-model calculation which formed the initial stage of this study predicts satisfactorily the energies of the negative-parity states that contribute to the observed anomalies. The neutron decay widths for these resonances are evaluated using the model wave functions. The general trends in the energy dependence of the total cross section are satisfactorily reproduced by the theory. The factors that could contribute to the discrepancies between theory and experiment are discussed. The theoretical estimates of the damping widths for the two 1 − anomalies that occur in this region were within 20 to 25% of the experimental values and support the view that these are intermediate-type resonances. Their configurational structure as predicted by the model calculation suggests that they are the parent states of the T > components of the giant dipole resonance near 21.0 MeV in 90Zr. The distribution of E1 widths calculated for a proposed 1 − → 2 + (at 0.78 MeV) transition in 90Y indicates that an anomaly corresponding to these 1 − states can also be expected in the (n, γ) reaction.

Shell-model calculations of fission decay widths and probabilities in superheavy nuclei

The stability of excited superheavy nuclei with respect to fission is studied on the basis of realistic shell models and of the BCS Hamiltonian. A statistical approach is used to calculate the deformation probabilities of excited nuclei as well as the fission and neutron decay widths. The results of the calculations show a rapid washing out with energy of the shell effects responsible for the high fission barriers predicted in the superheavy region. The first chance fission probabilities are calculated for nuclei in the region between Z = 108 and Z = 126. Two single-particle models proposed by Nilsson and by Bolsterli et al. have been used in the calculations. The results are discussed in terms of the possible production of superheavy elements by means of standard nuclear reactions.

Fission of 210Po

The excitation functions of the reactions 206Pb(α, f) and 209Bi(p, f) and the anisotropy parameters characterizing, at various excitation energies, the angular distributions of fragments emitted in both reactions are analysed. In the analysis of the excitation functions the angular momentum of the fissioning nucleus is explicitly taken into account. In the calculation of the competing neutron decay width the influence of shell effects on the level density of 209Po has been considered. The effective fission threshold of 210Po (fission threshold plus pairing at the saddle point) is found to be ≈ 21.95 MeV, corresponding to a fissility parameter x ≈ 0.72, a value about 2 % higher than the one recently proposed by Myers and Swiatecki. The ratio I rig/ I eff at the saddle point seems to be about 15 % higher than. the one predicted by the liquid-drop-model calculations of Cohen and Swiatecki.