Nuclear structure and decay modes of Ra isotopes within an axially deformed relativistic mean field model

We examine the structural properties and half-life decay of Ra isotopes within the axially deformed Relativistic Mean-Field(RMF)theory with NL3 force parameters.We work out the binding energy(BE),RMS radii,two-neutron separation energies(S_(2n)),and some other observables.The results are in good agreement with the finite-range droplet model(FRDM)and experimental results.Considering the possibility of neutron magic number,theα-decay and cluster decay half-lives of Ra isotopes are calculated systematically using the Q-values obtained from the RMF formalism.These decay half-life calculations are carried out by taking three different empirical formulae.The calculated decay half-lives are found to be highly sensitive to the choice of Q-values.Possible shell or sub-shell closures are found at daughter nuclei with N=128 and N=126 when alpha and~8Be,~(12)C,~(18)O respectively are emitted from Ra isotopes.Though the cluster radioactivity is affected by the shell closure of parent and daughter,a long half-life indicates the stability of the parent,and a small parent half-life indicates that the shell stability of the daughter against decay.     

偶偶 Ra 同位素的八极形变研究

用反射不对称的相对论平均场理论(RAS-RMF)对偶偶Ra同位素的八极形变进行了系统研究,并与实验数据和有限力程小液滴模型(FRDM)结果进行了比较.结果表明：RAS-RMF理论很好地描述了Ra同位素的基态性质,计算的结合能、双中子分离能和形变与实验数据和FRDM一致.RAS-RMF理论获得的中子、质子密度分布清晰地展现出偶偶Ra同位素由球形逐渐转变为八极形变,到四极形变的过程,与实验观察的八极形变不稳定现象一致. 关键词： 反射不对称平均场(RAS-RMF) 八极形变 偶偶核     

邮票上的物理学史（49）——核裂变

发现核裂变的故事应当接着发现中子往下讲 .在发现中子之前 ,用得最多的轰击原子核的炮弹是α粒子 .例如 ,历史上的第一个人工核蜕变就是卢瑟福用钋源发出的α射线轰击氮核 ,把它嬗变成氧原子 .约里奥-居里夫妇用钋源α射线轰击各种原子核 ,轰击铍发现了中子 ,轰击铝发现了人工放射性 ,但对Ｚ >2 0的原子核 ,α粒子轰击却不能引起核反应 .这是因为 ,原子核的正电荷的推斥力使α粒子不能进入原子核 .发现中子后 ,1 93 4年 ,意大利物理学家费米 (邮票见下节 )和他的研究集体试着用中子而不是α粒子来轰击原子核以产生人工放射现象 .费米认为 ,…     

偶偶 Ra 同位素的八极形变研究

用反射不对称的相对论平均场理论(RAS-RMF)对偶偶Ra同位素的八极形变进行了系统研究,并与实验数据和有限力程小液滴模型(FRDM)结果进行了比较.结果表明：RAS-RMF理论很好地描述了Ra同位素的基态性质,计算的结合能、双中子分离能和形变与实验数据和FRDM一致.RAS-RMF理论获得的中子、质子密度分布清晰地展现出偶偶Ra同位素由球形逐渐转变为八极形变,到四极形变的过程,与实验观察的八极形变不稳定现象一致.     

The Th fission valleys

One of the interesting problems in the nuclear fission studies is the nature of the asymmetry of the fission fragment mass distribution (FFMD). In connection with recent experiments, the valleys on the potential energy surface of ²²⁶Th have been considered. The pre-scission nuclear shape calculated as a result of the minimization in multi-dimensional space of the deformation parameters with two constrains is shown to be of the type considered by Brosa et al.     

Symmetric and asymmetric fission of Ac isotopes near the fission threshold

Fission probabilities and fragment anisotropies were investigated for fission of ²²⁷Ac and ²²⁸Ac by means of direct reactions with a ³He beam on a ²²⁶Ra target. Triple-humped mass distributions are found also for excitation energies where second-chance fission is excluded. The fission barrier is higher for symmetric fission compared to that for asymmetric fission by 1.2 and 2.0 MeV, respectively, suggesting that the character of the mass split is already predetermined at the saddle point.     

Th microanalysis using fission tracks

External uniaxial stress effects on the radiation damage dynamics in a a-Fe b.c.c. structure are studied by means of the computer simulation method. Focused collision chains, defocused ones, and atom-atom collision cascades were simulated. The external uniaxial stresses affect the intervals between a vacancy and an interstitial atom produced due to a collision chain, i.e. changes a dynamical crowdion path. The external stresses change the number of stable point defects and their arrangement in the cascade development region if an atom-atom collision cascade develops in a stressed crystal. The studies of focused collision chains of energies close to the atom displacement threshold energies show that the latter can either increase or decrease due to the external stresses depending on the direction of the effect.     

Nuclear spectroscopy of molybdenum isotopes

The dependence of the properties of low-lying states of Mo isotopes on the mass number A = 83–117 is studied based on the collective and shell models of atomic nuclei. The variation of the nuclear shape with an increasing number of neutrons influences strongly the properties of excited states of Mo isotopes. Decay channels of isovector giant dipole resonance with emission of protons and neutrons are studied in the framework of the combined model. The basic mechanisms of the production of stable Mo isotopes in astrophysical nuclear reactions are described.     

Nuclear structure of 216 Ra at high spin

High-spin states of ²¹⁶Ra ( $\emph{Z}=88$ , $\emph{N}=128$ ) have been investigated through ²⁰⁹Bi(¹⁰B, 3n) reaction at an incident beam energy of 55?MeV and ²⁰⁹Bi(¹¹B, 4n) reaction at incident beam energies ranging from 65 to 78?MeV. Based on ??? coincidence data, the level scheme for ²¹⁶Ra has been considerably extended up to ~ 33 $\hbar$ spin and 7.2 MeV excitation energy in the present experiment with placement of 28 new ??-transitions over what has been reported earlier. Tentative spin-parity assignments are done for the newly proposed levels on the basis of the DCO ratios corresponding to strong gates. Empirical shell model calculations were carried out to provide an understanding of the underlying nuclear structure.     

Nuclear molecules in ternary fission

Experimental results on particle accompanied cold fission of ²⁵²Cf suggest the existence of a short-lived quasi-molecular state. Within our three-center phenomenological model, described in this paper, we found a possible explanation based on a new minimum in the deformation energy at a separation distance very close to the touching point. Half-lives of some quasimolecular states which could be formed in ¹⁰Be accompanied fission of ²³⁶U, ²³⁶Pu, ²⁴⁶Cm, ²⁵²Cf, ^252,256Fm, ^256,260No, and ²⁶²Rf are roughly estimated.     

Nuclear fission with inertial confinement

The possibility of inducing an explosive fission reaction in a small amount of fissionable material by a heavy-ion beam from a high-power accelerator driver developed for bringing about fusion in deuterium-tritium cylindrical targets with direct burning is analyzed. The consequences of the use of this method in the nuclear power industry are discussed.     

Nuclear dissipation,fission probability and neutron multiplicity prior to fission

We discuss the effects of nuclear dissipation on fission probabilities that are characteristic of a diffusion model of the fission process. Reproducing the experimental fission probabilities at low excitation energies fixes the ratioa _f/a_n of the level density parameters for a given strength of the reduced dissipation coefficientβ. These low energy constraints ona _f/a_n andβ balance the effects of transients on neutron multiplicities prior to fission at higher excitation energies. For the competitive decay of¹⁵⁸Er formed in the reaction¹⁶O+¹⁴²Nd at 207 MeV we show that dueto transients only the multiplicity of pre-fission neutrons is enhanced with respect to the prediction of the statistical model in a manner consistent with our earlier general analysis.     

Nuclear radii of Na and Mg isotopes

The effective root-mean-square (rms) matter radii of ^ANa (A = 20–23, 25–32) and ^AMg (A = 20, 22, 23, 27, 29–32) have been deduced from the measured interaction cross sections using a Glauber-type calculation. It was found that the increase of rms matter radii in Na isotopes is primarily a consequence of the increase of rms neutron radii. A correlation between the radii, corrected for quadrupole deformation, and the Fermi-energy difference was observed for both Na and Mg isotopes. This correlation can be explained by a model that assumes the valence nucleons are responsible for the changes in nuclear radii. The presence of a neutron skin is suggested for neutron-rich Na and Mg isotopes. An application to the nuclear equation of state (EOS) is discussed.     

Nuclear time-reversal violation and the Schiff moment of 225Ra

We present a comprehensive mean-field calculation of the Schiff moment of the nucleus 225Ra, the quantity that determines the static electric-dipole moment of the corresponding atom if time-reversal (T) invariance is violated in the nucleus. The calculation breaks all possible intrinsic symmetries of the nuclear mean field and includes, in particular, both exchange and direct terms from the full finite-range T-violating nucleon-nucleon interaction, and the effects of short-range correlations. The resulting Schiff moment, which depends on three unknown T-violating pion-nucleon coupling constants, is much larger than in 199Hg, the isotope with the best current experimental limit on its atomic electric-dipole moment.     

Nuclear reactions between oxygen isotopes

Elastic and inelastic cross sections have been measured for the system ¹⁶O + ¹⁷O at c.m. energies from 12.5 to 15.5 MeV, and for ¹⁶O + ¹⁸O at c.m. energies from 12 to 20 MeV, at angles between 60° and 125°. Position-sensitive detectors were employed, using the kinematic coincidence technique. The data have been analyzed with particular attention to the contributions of multiple-exchange processes.     

Nuclear spins of neutron-deficient thulium isotopes

Cyclotron-produced thulium isotopes have been investigated using the atomic-beam magnetic resonance method. The spin measurements have given the following results: ¹⁵⁹Tm (9 min) , ¹⁶⁰Tm (9 min) I = 1, ¹⁶¹Tm (37 min) , ¹⁶²Tm (21.5 min) I = 1, ¹⁶⁴Tm (2 min) I = 1, ^164m Tm (5 min) I = 6 and ¹⁶⁸Tm (87 d) I = 3. An interpretation based on the Nilsson diagram for odd protons clearly indicates a systematic variation in the nuclear deformation.     

Nuclear moments of neutron-rich aluminum isotopes

Ground-state magnetic-dipole moments (μ) of ^30-32Al and electric quadrupole moments (Q) of ^31,32Al have been measured with the β-NMR method using spin-polarized radioactive-isotope beams produced in projectile-fragmentation reactions. Beams of ^30-32Al were obtained by using RIKEN projectile-fragment separator RIPS after the fragmentation of ⁴⁰Ar projectiles at an energy of E = 95A MeV on a ⁹³Nb target. The obtained μ_exp[^30-32Al] and values agree well with shell-model calculations within the sd shell using the USD interaction. Also, Q_exp[³¹Al] was found to be small. Thus, we can conclude that these aluminum isotopes are located outside the island of inversion.     

Independent fission yields of short-lived Br and I isotopes in thermal neutron fission of235U

Independent fission yields of^88–91Br and^138–141I produced in thermal neutron fission of²³⁵U were determined at the SOLIS on-line isotope separator. The measurements were made for the first time using a negative surface ionization integrated target-ion source.     

The effect of the entrance channel on the fission of 216Ra

We have studied mass-energy distributions (MED) of fission fragments using two projectile-target combinations, ¹²C + ²⁰⁴Pb and ⁴⁸Ca + ¹⁶⁸Er, leading to the same compound nucleus ²¹⁶Ra at the excitation energy E*～40 MeV. It has been found that the contribution of the asymmetric mode in the case of the former reaction is 1.5%, and it is 30% in the case of the latter one. We connect such a sharp increase in the yield of asymmetric products in the ⁴⁸Ca + ¹⁶⁸Er reaction with the quasifission process, the MED of which have a clearly expressed shell structure. The characteristics of the fission fragment MED are of such a kind that they can be interpreted by analogy with the low-energy fission of heavy nuclei as a manifestation of an independent mode of nuclear decay which competes with the classical fusion-fission process.