Spin Cutoff Research Articles

Is (3599) Basov a large C-type super-fast rotator?

The rubble-pile model of asteroids deduced from the existence of a spin cutoff is an important paradigm in the interior structure of asteroids as a consequence of their collisional evolution. Many objects with size less than 200 ​m have been found to be rotating with a spin period less than 2.2 ​h but seldom so for bigger objects. Thus, asteroids with rotation periods significantly less than this spin cutoff limit are called super-fast rotators. Most often this picture of spin cutoff and hence the rubble-pile model has been applied to the S-type asteroids. In this study, we report the interesting case of (3599) Basov which is a member of the Themis family of C-class composition. The spin barrier of the Themis family asteroids has been shown to be P ​= ​3 ​h. What is unusual about (3599) Basov is that its rotation period has been confirmed to be P ​= ​2.157 ​± ​0.037 ​h which is significantly shorter than the critical value for centrifugal breakup. Furthermore, its size of 18 ​km is much larger than those of the S-class super-fast rotators. The physical property of (3599) Basov might therefore be quite unique. • C-type asteroids of rubble-pile structure should have a spin-cutoff at about 3 h • The rotation period of (3599) Basov is confirmed to be P=2.157±0.037 h and a C-type asteroid by photometry and spectroscopy observations. • If (3599) Basov has a rubble-pile structure, it should not have such a short rotation period unless it has a very large cohesive force.

Neutron and γ multiplicities as a function of incident neutron energy for the Np237(n,f) reaction

In the nuclear fission process, the excitation energy sharing mechanism between the fission fragments, the fragment angular momentum generation process and the interdependence between the two are still poorly known. The study of the neutron and $\ensuremath{\gamma}$ emission characteristics as a function of fragment and compound nucleus properties brings valuable information on these mechanisms since they decrease respectively the fragment excitation energy and angular momentum. In the 1980s, Naqvi and M\uller experimentally highlighted, with $^{237}\mathrm{Np}(n,f)$ direct fission reactions, that only the prompt neutron multiplicity for the heavy fragment increases with the incident neutron energy. This means that the additional excitation energy goes into the heavy fragment. This data set allows one to test the different energy sharing models. This paper investigates, using the fifrelin fission fragment deexcitation code, the impact of the models used to assign the fragment initial state on fission observables. It focuses on the impact of a constant or an energy dependent spin cutoff model, whose parameters define the initial total angular momentum distribution, coupled with an energy sharing model based on an empirical temperature ratio law ${R}_{T}(A)$ in which fission fragments are considered to behave as a Fermi gas. It shows that both spin cutoff models succeed in reproducing the experimental neutron multiplicities which are mainly driven by the ${R}_{T}(A)$ law. However, they predict different $\ensuremath{\gamma}$ observables and neutron-$\ensuremath{\gamma}$ multiplicity correlations. Therefore, to infer about the validity of the two models, it is necessary to measure neutron and $\ensuremath{\gamma}$ observables in correlation in order to have a deeper understanding of the fission mechanism.

Rotational behavior of thermally excited 56Fe, 58Fe and 60Fe isotopes

Thermal and rotational behaviors of neutron rich fp-shell isotopes such as [Formula: see text], [Formula: see text] and [Formula: see text] were analyzed microscopically within the framework of statistical theory of hot rotating nuclei (STHRN) for the angular momentum range (0–15)[Formula: see text] at excitation energy above 4[Formula: see text]MeV. Pair-breaking phenomenon and band-crossing phenomenon of Fe isotopes were discussed with and without the inclusion of BCS pairing. The STHRN method with BCS effect was extended to the Fe isotopes to determine the critical temperature [Formula: see text], where the pair-breaking phenomenon takes place and it is found to occur below [Formula: see text][Formula: see text]MeV. The observation of band-crossing phenomenon above the [Formula: see text] was explained without considering the effect of BCS pairing. The single particle energy levels were engendered from triaxially deformed Nilsson Hamiltonian. The outcomes of the present investigation on moment of inertia (MOI), back-bending phenomenon, spin cutoff parameter show a strong evidence of band-crossing phenomenon and it eventually led to a shape transition from spherical to noncollective oblate. Moreover, attention on separation energy of protons and neutrons reveals that the neutron pairs are responsible for band-crossing. STHRN method was able to reproduce results in good agreement with experiments and comparable with other theories such as projected shellmodel (PSM) and shellmodel Monte Carlo (SMMC) method.

Role of magic numbers in thermodynamic quantities of 206Pb and 138Ba using BCS and Lipkin-Nogami models

The level density is calculated using the Barden-Cooper-Schriefer (BCS) and Lipkin-Nogami (LN) models for 206Pb and 138Ba nuclei and compared with the experimental data. It has been shown that the calculated level densities using both models are highly matched with the experimental data. The spin cut off factor, moment of inertia, excitation energy, and entropy were calculated using the above mentioned models. The protons in the 206Pb nucleus occupy a closed shell and the 138Ba nucleus has a closed neutron shell. It was revealed that the contribution of the nucleons with magic number to the mentioned thermodynamic quantities is less than that of the nucleons from open shells at low temperature.

Energy and Temperature Dependences of the Spin Cutoff Parameter in 50 - 57Cr Isotopes

The spin cutoff parameters of 50 - 57Cr isotopes have been calculated using a superconducting Hamiltonian with the inclusion of the pairing effect. Their energy and temperature dependences have been studied through comparison with some well-known semi-empirical formulae. This study shows that the microscopic calculation results converge to the Fermi gas model prediction at higher energies. Also, an even-odd effect is evident in the spin cutoff parameters at low temperatures and disappears after the pairing phase transition.

Impact of nuclear inertia momenta on fission observables

Fission is probably the nuclear process the less accurately described with current models because it involves dynamics of nuclear matter with strongly coupled manybody interactions. It is thus diffcult to find models that are strongly rooted in good physics, accurate enough to reproduce target observables and that can describe many of the nuclear fission observables in a consistent way. One of the most comprehensive current modeling of the fission process relies on the fission sampling and Monte-Carlo de-excitation of the fission fragments. This model is implemented for instance in the FIFRELIN code. In this model fission fragments and their state are first sampled from pre-neutron fission yields, angular momentum distribution and excitation energy repartition law then the decay of both initial fragments is simulated. This modeling provides many observables: prompt neutron and gamma fission spectra, multiplicities and also fine decompositions: number of neutrons emitted as a function of the fragment mass, spectra per fragments, etc. This model relies on nuclear structure databases and on several basic nuclear models describing for instance gamma strength functions or level densities. Additionally some free parameters are still to be determined, namely two parameters describing the excitation energy repartition law, the spin cutoff of the heavy and light fragments and a rescaling parameter for the rotational inertia momentum of the fragments with respect of the rigid-body model. In the present work we investigate the impact of this latter parameter. For this we mainly substitute the corrected rigid-body value by a quantity obtained from a microscopic description of the fission fragment. The independent-particle model recently implemented in the CONRAD code is used to provide nucleonic wave functions that are required to compute inertia momenta with an Inglis-Belyaev cranking model. The impact of this substitution is analyzed on different fission observables provided by the FIFRELIN code.

Mass-number and excitation-energy dependence of the spin cutoff parameter

The spin cutoff parameter determining the nuclear level density spin distribution $\ensuremath{\rho}(J)$ is defined through the spin projection as ${\ensuremath{\langle}{J}_{z}^{2}\ensuremath{\rangle}}^{1/2}$ or equivalently for spherical nuclei, ${(\frac{\ensuremath{\langle}J(J+1)\ensuremath{\rangle}}{3})}^{1/2}$. It is needed to divide the total level density into levels as a function of $J$. To obtain the total level density at the neutron binding energy from the $s$-wave resonance count, the spin cutoff parameter is also needed. The spin cutoff parameter has been calculated as a function of excitation energy and mass with a super-conducting Hamiltonian. Calculations have been compared with two commonly used semiempirical formulas. A need for further measurements is also observed. Some complications for deformed nuclei are discussed. The quality of spin cut off parameter data derived from isomeric ratio measurement is examined.

Breaking of Cooper pairs in 108Pd

In this paper, breaking of Cooper pairs in [Formula: see text]Pd is investigated within the canonical ensemble framework and the BCS model. Our results show an evidence of two phase transitions, which are related to neutron and proton systems. Also, with consideration of pairing interaction, the role of neutron and proton systems in entropy, spin cutoff parameter and as a result in the moment of inertia are investigated. The results show minor role for the proton system at low temperatures and approximately equal roles for both neutron and proton systems after the critical temperature. Good agreement was observed between obtained results and the experimental data.

Role of neutron and proton system in spin cut off parameter and entropy of 89,90Y

The nuclear level densities, entropies and spin cut off parameters have been determined in 89,90Y nuclei using the BCS model with inclusion of pairing interaction. The results have a good agreement with the recent experimental data on the level densities measured by the Oslo group. In addition, the entropy excess of 90Y compared to 89Y as a function of temperature has been extracted. Also, the role of neutron and proton systems in the entropy excess as well as the spin cut off excess have been investigated using the entropy excess ratio and spin cut off excess ratio introduced in our previous publication. The role of the neutron system at low temperatures, the temperatures below critical temperature, in the semi-magic nucleus 89Y is similar compared to the closed shell proton system in the tin isotopes.

A NEW LARGE SUPER-FAST ROTATOR: (335433) 2005 UW163

Asteroids of size larger than 150 m generally do not have rotation periods smaller than 2.2 hours. This spin cutoff is believed to be due to the gravitationally bound rubble-pile structures of the asteroids. Rotation with periods exceeding this critical value will cause asteroid breakup. Up until now, only one object, 2001 OE84, has been found to be an exception to this spin cutoff. We report the discovery of a new super-fast rotator, (335433) 2005 UW163, spinning with a period of 1.290 hours and a lightcurve variation of $r'\sim0.8$ mag from the observations made at the P48 telescope and the P200 telescope of the Palomar Observatory. Its $H_{r'} = 17.69 \pm 0.27$ mag and multi-band colors (i.e., $g'-r' = 0.68\pm0.03$ mag, $r'-i' = 0.19\pm0.02$ mag and SDSS $i-z = -0.45$ mag) show it is a V-type asteroid with a diameter of $0.6 +0.3/-0.2$ km. This indicates (335433) 2005 UW163 is a super-fast rotator beyond the regime of the small monolithic asteroid.

Level Density Inputs in Nuclear Reaction Codes and the Role of the Spin Cutoff Parameter

The proton spectrum from the 57Fe(α, p) reaction has been measured and analyzed with the Hauser-Feshbach model of nuclear reactions. Different input level density models have been tested. It was found that the best description is achieved with either Fermi-gas or constant temperature model functions obtained by fitting them to neutron resonance spacings and to discrete levels and using the spin cutoff parameter with much a weaker excitation energy dependence than predicted by the Fermi-gas model.

Experimental differential cross sections, level densities, and spin cutoffs as a testing ground for nuclear reaction codes

Proton double-differential cross sections from 59Co(α,p)62Ni, 57Fe(α,p)60Co, 56Fe(7Li,p)62Ni, and 55Mn(6Li,p)60Co reactions have been measured with 21-MeV α and 15-MeV lithium beams. Cross sections have been compared against calculations with the empire reaction code. Different input level density models have been tested. It was found that the Gilbert and Cameron [A. Gilbert and A. G. W. Cameron, Can. J. Phys. 43, 1446 (1965)] level density model is best to reproduce experimental data. Level densities and spin cutoff parameters for 62Ni and 60Co above the excitation energy range of discrete levels (in continuum) have been obtained with a Monte Carlo technique. Furthermore, excitation energy dependencies were found to be inconsistent with the Fermi-gas model.

Role of neutrons and protons in entropy, spin cut off parameters, and moments of inertia

The nuclear level densities, spin cut off parameters, and entropies have been extracted in ${}^{116\ensuremath{-}119}$Sn and ${}^{162,163}$Dy nuclei using superconducting theory, which includes nuclear pairing interaction. The results agree well with the recent data obtained from experimental level densities by the Oslo group for these nuclei. Also, the entropy excess ratio proposed by Razavi et al. [R. Razavi, A.N. Behkami, S. Mohammadi, and M. Gholami, Phys. Rev. C 86, 047303 (2012)] for a proton and neutron as a function of nuclear temperature have been evaluated and are compared with the spin cut off excess ratio. The role of the neutron (proton) system is well determined by the entropy excess ratio as well as the spin cut off excess ratio. The moment of inertia for even-odd and even-even nuclei are also compared. The moment of inertia carried by a single hole is smaller than the single particle moment of inertia.

Feeding of the11/2−isomers in stable Ir and Au isotopes

Excited states in $^{191}\mathrm{Ir}$, $^{193}\mathrm{Ir}$, and $^{197}\mathrm{Au}$ were studied and absolute partial $\ensuremath{\gamma}$-ray cross sections were measured using the ($n,{n}^{'}\ensuremath{\gamma}$) reaction. A Compton-suppressed germanium-detector array (GEANIE) for $\ensuremath{\gamma}$-ray spectroscopy was used for the measurement and the broad-spectrum pulsed neutron source of the Los Alamos Neutron Science Center's WNR facility provided energetic neutrons. The energy of the incident neutrons was determined using the time-of-flight technique. Absolute partial $\ensuremath{\gamma}$-ray cross sections were measured up to incident neutron energy of 20 MeV for several transitions feeding directly the $11/{2}^{\ensuremath{-}}$ isomers and ground states in $^{191}\mathrm{Ir}$, $^{193}\mathrm{Ir}$, and $^{197}\mathrm{Au}$. The feeding of the $11/{2}^{\ensuremath{-}}$ isomers, which originate from the odd proton occupying the ${h}_{11/2}$ orbital, was found for the three targets to be very similar and increasing relative to the feeding of the corresponding ground state with increasing neutron energy up to ${E}_{n}~10$ MeV. Above this neutron energy the opening of the $(n,2n)$ reaction channel strongly affects the population of the isomers and leads to a decrease of their relative population compared to the population of the ground states. The experimental results are compared with theoretical predictions from the GNASH reaction model calculation implementing a version of the spin distribution for the pre-equilibrium reaction piece with either a compound nucleus spin distribution (CN-GNASH) or a Feshbach-Kerman-Koonin (FKK-GNASH) quantum mechanical spin distribution. The effects of the spin cutoff parameter values on the population of states are examined. Evidence is presented that FKK-GNASH provides a description of the experimental data that mitigates the need for adjustment of the level density parameter to fit the data.

Level densities for20<~A<~110

A recent study of nuclear level densities for $20<~A<~70$ found evidence that the level densities for nuclei off the stability line were lower than those for nearby nuclei on the stability line. This analysis has been extended to cover the mass range $20<~A<~110$ with results that support the original conclusions. As part of the study, the variations with energy and mass number of the parity ratio and spin cutoff parameter are examined.

High spin properties of 124Ba

The 124Ba nucleus is investigated on the basis of the method of statistical mechanics by assuming the nucleons to move in triaxially deformed Nilsson potential. The variation in the Fermi energies of protons and neutrons is studied as a function of spin and temperature. The Fermi energies determined as a function of angular momentum is used to study the dependence of shell correction on angular momentum using the Strutinsky smoothing procedure. The most important observation is that the shell correction is almost the same for all spins for 124Ba. The spin cutoff parameter and the single particle level density parameter are studied as a function of spin and temperature. Constant entropy lines drawn by plotting the excitation energy against angular momentum are found to be roughly at constant energy above the yrast line and are almost equally spaced. It is observed that no yrast traps are present for 124Ba.

Properties of112Cdfrom the(n,n′γ)reaction: Levels and level densities

Levels in ${}^{112}\mathrm{Cd}$ have been studied through the ${(n,n}^{\ensuremath{'}}\ensuremath{\gamma})$ reaction with monoenergetic neutrons. An extended set of experiments that included excitation functions, $\ensuremath{\gamma}$-ray angular distributions, and $\ensuremath{\gamma}\ensuremath{\gamma}$ coincidence measurements was performed. A total of 375 $\ensuremath{\gamma}$ rays were placed in a level scheme comprising 200 levels (of which 238 $\ensuremath{\gamma}$-ray assignments and 58 levels are newly established) up to 4 MeV in excitation. No evidence to support the existence of 47 levels as suggested in previous studies was found, and these have been removed from the level scheme. From the results, a comparison of the level density is made with the constant temperature and back-shifted Fermi gas models. The back-shifted Fermi gas model with the Gilbert-Cameron spin cutoff parameter provided the best overall fit. Without using the neutron resonance information and only fitting the cumulative number of low-lying levels, the level density parameters extracted are a sensitive function of the maximum energy used in the fit.

Influence of pairing correlations on the excitation energy, angular momentum, and parity dependence of nuclear level densities

We have studied the effects of pairing correlations on the excitation energy ${(E}^{*})$ dependence of the total nuclear level density, parity-projected level density, and spin cutoff factor for two heavy systems ${}^{152}\mathrm{Sm}$ and ${}^{160}\mathrm{Yb},$ within a microscopic approach based on static path approximation to a grand canonical partition function. For both systems considered, we find that pairing has significant effects on the ${E}^{*}$ dependence of total level densities. At moderate excitation energies ${(E}^{*}\ensuremath{\sim}10 \mathrm{MeV})$ it supresses the total level density nearly by two orders of magnitude. By fitting the total level density to a Bethe-type formula with energy backshift $({\ensuremath{\Delta}}_{\mathrm{BS}}),$ we find that ${\ensuremath{\Delta}}_{\mathrm{BS}}\ensuremath{\approx}2.5 \mathrm{MeV}$ for the excitation energy beyond $15 \mathrm{MeV}.$ At lower excitation energies, however, ${\ensuremath{\Delta}}_{\mathrm{BS}}$ is strongly ${E}^{*}$ dependent. We find that the effects of pairing on the spin cutoff factor and the parity-projected level densities (or parity asymmetry) is somewhat less important. We also compare our results for the spin cutoff factor with the ones conventionally employed in statistical model codes.

Structural properties of hot rotating40Ca

The yrast and yrare lines are obtained for {sup 40}Ca using a statistical method by assuming the nucleons to move in a triaxially deformed Nilsson harmonic oscillator potential. The backbending and yrast traps are observed in the case of {sup 40}Ca at certain spins. A shift in the first yrast minima towards higher angular momentum states with an increase in entropy is observed. A second backbending is found to occur at spin M=17{h_bar} for all entropies. The excitation energy, the nuclear level density, the single-particle level density parameter, and the spin cutoff parameter are determined as functions of temperature and angular momentum by minimizing the free energy. The Strutinsky method is used to study the variation of the shell correction with angular momentum for equilibrium shapes. The Fermi energies obtained as a function of angular momentum are used for calculating this shell correction. The nucleus is found to be spherical up to M=6{h_bar} and becomes oblate with a further increase in angular momentum. The shell correction is found to increase with the deformation having a minimum value for the spherical shape. {copyright} {ital 1997} {ital The American Physical Society}

Structural properties of hot deformed 122Xe at high spins.

The Xe122 nucleus is investigated using statistical theory for studying intrinsic properties as a function of temperature and spin assuming the nucleus to move in a triaxially deformed Nilsson potential. The single particle level density parameter is extracted as a function of temperature for various spins and is found to reach a constant value at higher temperature. The excitation energy is calculated as a function of angular momentum for different entropies and temperatures. The spin cutoff parameter as a function of angular momentum is determined for various temperatures from the rotational energy. The shell correction as a function of angular momentum is determined. The variation of proton and neutron Fermi energies with temperature and angular momentum is studied. The effect of the variation of nuclear level density with excitation energy on nuclear temperature is verified for different spins. © 1995 The American Physical Society.