Final Nucleus Research Articles

Band mixing in 29Si and 29P

The experimental data of the 28Si(α, t)29P and 28Si(α, 3He)29Si reactions at 45 MeV have been analysed in terms of full finite-range coupled-channels calculations. In the calculations, the reaction paths, direct and two step via the inelastic states of the initial and final nuclei have been treated in terms of the finite-range transfer theory. The spectroscopic amplitudes connecting the deformed initial and final nuclei in both the reactions are obtained from Nilsson's model. The data of both the reactions, populating the 1/2+, 3/2+ and 5/2+ states in the Kπ = 1/2+ band and the 3/2+ and 5/2+ states in the Kπ = 3/2+ band, are best described by about 20% mixing of between the two bands. The effects of normal Woods–Saxon (WS), squared WS and molecular type of α-nucleus potentials are also discussed.

Analysis of the 2 νββ decay and muon capture reactions for the mass A = 46 and A = 48 nuclei using the nuclear shell model

We discuss the two-neutrino double-beta decays of 46Ca and 48Ca by using the nuclear shell model with well-tested two-body interactions. We also discuss the ordinary muon-capture (OMC) reaction on the final nuclei, 46Ti and 48Ti, of these decays. The OMC leads to the virtual states of the intermediate nuclei, 46Sc and 48Sc, in these double-beta decays.

Neutrinoless double electron capture: A tool to search for Majorana neutrinos

The possibility of observing the neutrinoless double $\ensuremath{\beta}$ decay and thus proving the Majorana nature of neutrinos as well as providing a sensitive measure of its mass is a major challenge of today's neutrino physics. As an attractive alternative, we propose to study the inverse process---the radiative neutrinoless double-electron capture $(0\ensuremath{\nu}2\mathrm{EC})$. The associated monoenergetic photon provides a convenient experimental signature. Other advantages include the favorable ratio of the $0\ensuremath{\nu}2\mathrm{EC}$ to the competing $2\ensuremath{\nu}2\mathrm{EC}$ capture rates and, very importantly, the existence of a coincidence trigger to suppress the random background. These advantages partly offset the expected longer lifetimes. Rates for the $0\ensuremath{\gamma}2\mathrm{EC}$ process are calculated. High $Z$ atoms are strongly favored. A resonance enhancement of the capture rates is expected to occur at an energy release comparable to the $2P\text{\ensuremath{-}}1S$ atomic level difference. The resonance conditions are likely to be met for decays to excited states in final nuclei. Candidates for such studies are considered. The experimental feasibility is estimated and found encouraging.

Measurement of cross sections for gamma transitions induced in excited nuclei by the interaction of 1.1-GeV protons with silicon nuclei

The cross sections for the production of excited nuclei were measured in the reaction 28Si(p, xpyn)A* followed by a gamma transition to a state of lower excitation energy or to the ground state. The experiment was performed in an extracted proton beam from the accelerator of the Institute of Theoretical and Experimental Physics (ITEP, Moscow). The reaction in question was identified by a Ge(Li)-NaI(Tl) anticoincidence spectrometer that recorded prompt gamma radiation emitted by the excited final nucleus. The sensitivity of the experiment was 1.5 mb. The cross-section values were obtained for 24 gamma transitions in 17 nuclear products. The cross sections for spallation reactions were estimated. A comparison was performed with known experimental data and with the results of calculations by a semiempirical formula, as well as with the results obtained by simulating hadron interactions on the basis of the GEANT and INUCL codes.

Analysis of the 2νββ decay and muon-capture reactions for the mass A=46 and A=48 nuclei using the nuclear shell model

We discuss the two-neutrino double-beta decays of 46Ca and 48Ca by using the nuclear shell model with well-tested two-body interactions. We also discuss the ordinary muon-capture (OMC) reaction on the final nuclei, 46Ti and 48Ti, of these decays. The OMC leads to the virtual states of the intermediate nuclei, 46Sc and 48Sc, in these double-beta decays.

Structure of neutron-rich beryllium and carbon isotopes

Ogloblind, Ch. Schulzfl, S. Thummererfl aHahn-Meitner-Institut, Glienicker Strasse 100, 14109 Berlin, Germany bFlerov Laboratory of Nuclear Reactions, JINR, 141980 Dubna, Russia and Bulgarian Academy of Sciences, Sofia, Bulgaria ‚Dept. of Physics and Astronomy, Ohio University, Athens, Ohio 45701-2979, USA dInst. of General and Nuclear Physics, Kurchatov Institute, Moscow 123 182, Russia The structure of the neutron-rich isotopes 12Be, 16C and 17C has been studied using the (12C,gC) three-neutron transfer reaction on ™Be, 13C and 14C targets, respectively. Ex- cited states up to 17 MeV excitation energy were observed. In 16C and 17C the populated structures have a dominant (~d)~ neutron configuration, these are three-particles one-hole (3plh) states in 16C and three-particles zero-hole (3pOh) states in 17C. In 12Be a larger va- riety of configurations is expected. Besides the low-lying states with (2p2h) character, also odd parity (3p3h) configurations are populated. In addition, due to the pronounced a-n-a cluster structure of the ™Be ground state, molecular structures in I2Be with 6He+6He and 4He+8He configurations can be populated by the three-neutron transfer. 1. INTRODUCTION In neutron-rich Be- and C-isotopes the detailed structure of excited states is of strong interest, since pronounced a-cluster or even molecular structures as well as single-particle and particle-hole structures can be expected [l-31. To obtain more detailed experimental information, it is important to select appropriate reactions to populate in a direct way the distinct structures in the final nucleus:

Nuclear deformation and the double-beta decay

The effect of deformation on the two-neutrino double-beta decay (2νββ decay) for ground-state transition 76Ge → 76Se is studied in the framework of the deformed QRPA with separable Gamow-Teller residual interaction. A new suppression mechanism of the 2νββ-decay matrix element based on the difference in deformations of the initial and final nuclei is reported. An advantage of this suppression mechanism in comparison with that associated with ground-state correlations is that it allows a simultaneous description of the single-β and the 2νββ decay.

Angular momentum population in the projectile fragmentation ofU238at750MeV∕nucleon

A systematic study of the population probabilities of nanosecond and microsecond isomers produced following the projectile fragmentation of U-238 at 750 MeV/nucleon has been undertaken at the SIS/FRS facility at GSI. Approximately 15 isomeric states in neutron-deficient nuclei around A similar to 190 were identified and the corresponding. isomeric ratios determined. The results are compared with a model based on the statistical abrasion-ablation description of relativistic fragmentation and simple assumptions concerning gamma cascades in the final nucleus (sharp cutoff). This model represents an upper limit for the population of isomeric states in relativistic projectile fragmentation. When the decay properties of the states above the isomer are taken into account, as opposed to the sharp cutoff approximation, a good agreement between the experimental and calculated angular momentum population is obtained.

Properties of Some Bound States of the 56Fe Nucleus Excited by the 55Mn(p,γ)56Fe Reaction

Radiative decay of 21 resonances in the 55Mn(p,γ)56Fe reaction was studied in the proton beam energy region Ep = (1.3-1.8) MeV. Properties of bound states in the final 56Fe nucleus have been deduced from accurately measured very complex γ-ray spectra. About 150 levels have been firmly established for excitation energy up to ≈ 8 MeV and 93 of them have been observed for the first time or their established energy is substantially more accurate than in former publications or some other information is new. For each of the 57 levels, there has been established its decay and firmly observed at least one decay branch. Spin and parity have been newly determined or appreciated for 35 levels and for 35 levels the mean lifetime has been newly established by the DSAM method. Crude comparison of the observed level density with the statistical Fermi model prediction has been also performed.

Weak-interaction observables from nuclear-structure calculations of 0νββ decay transitions

The present status of the calculation of the nuclear matrix elements associated with neutrinoless double-beta decay is discussed. The currently available approaches, as well as the historical perspective, are presented in this review. The discussion is restricted to nuclear matrix elements relevant in the context of the light Majorana-neutrino-mediated decays to the ground state and excited states of the final nucleus. Special emphasis is laid on the quantitative description of the 0νββ decay transitions to the excited 0+ states, a subject of very recent interest in the double-beta community.

Investigation of α-nucleus interaction in the 27Al (α,α)27Al scattering and 27Al (α, d )29Si reaction

Full finite-range macroscopic calculations in the distorted-wave Born approximation have been performed using the molecular and Michel α-nucleus potentials to analyze the angular distributions of cross-sections of the 27Al(α, d)29Si reaction, at 26.5 and 27.2 MeV incident energies, leading to seven transitions up to the excitation energy E X = 4.08 MeV of the final nucleus. The parameters of the two types of the α-nucleus potentials are determined from the elastic-scattering data. Both the molecular and Michel potentials, without any adjustment to the parameters needed to fit the elastic-scattering data, are able in most cases to reproduce, simultaneously, the absolute cross-sections particularly at large angles, where the previous calculations failed to reproduce by orders of magnitude, and the gross pattern of angular distributions of the reaction. The deuteron-cluster spectroscopic factors for most of the seven transitions, deduced using the two α-27Al potentials, differ from those obtained in earlier works. The spectroscopic factor for the ground-state transition, deduced in the present work for the 25.8 MeV data, agrees well with the shell model prediction.

Low energy behavior of the astrophysical S-factor in radiative captures to loosely bound final states

The low-energy behavior of the astrophysical S-factor for E1 direct radiative captures a(p, γ)b leading to loosely bound final states (b=a+p) is investigated. We derive a first-order integral representation for S( E) and focus on the properties around zero energy. We show that it is the competition between various effects, namely the remnant Coulomb barrier, the initial and final centrifugal barriers and the binding energy, that defines the behavior of the S( E→0). Contrary to previous findings, we prove that S( E→0) is not determined by the pole corresponding to the bound state. The derivative S′(0) increases with the increase of the centrifugal barrier, while it decreases with the charge of the target. For l i = l f +1 the increase of the binding energy of the final nucleus increases the derivative S′(0) while for l i = l f −1 the opposite effect is found. We make use of our findings to explain the low energy behavior of the S-factors related to some notorious capture reactions: 7Be(p, γ) 8B, 14N(p, γ) 15O, 16O(p, γ) 17F, 20Ne(p, γ) 21Na and 22Mg(p, γ) 23Al.

Angular momentum population in the fragmentation of208Pbat 1 GeV/nucleon

A systematic study of the population probabilities of microsecond isomers produced following the fragmentation of 208Pb projectiles at 1 GeV/nucleon has been undertaken at the SIS/FRS facility at GSI Darmstadt. Gamma decays from approximately 20 isomeric states, mainly in the rare-earth and transitional nuclei with A∼180, were identified and the corresponding isomeric ratios deduced. The results are compared with a model based on the statistical abrasion-ablation description of relativistic fragmentation and simple assumptions concerning gamma cascades in the final nucleus (sharp cutoff). The model is found to represent an upper limit for the population of isomeric states in relativistic projectile fragmentation.

New data on the mechanism of decay of the giant dipole resonance in the 58Ni nucleus

New data on the mechanism of decay of the giant dipole resonance in the 58Ni nucleus are obtained from an analysis of the experimental cross sections for the photonucleon reactions 58Ni(γ, pi)57Co and 58Ni(γ, ni)57Ni. The method used in this analysis takes into account both the energy spread of the dipole strength concentrated in various isospin components of the giant dipole resonance and the spread of the spectroscopic strength of the populated nucleon-hole states over the levels of the final nuclei. The entire body of experimental spectroscopic information about the levels of the final nuclei 57Co and 57Ni is employed. It is found that the probability of the semidirect mechanism of decay of the giant dipole resonance in the 57Ni nucleus lies in the range 0.16–0.3. The probability of semidirect processes is much higher in the (γ, n) channel (0.28–0.62) than in the (γ, p) channel (0.07–0.17).

Distinguishing the Oνββ-decay mechanisms

Many new 0νββ-decay experiments are planned or in preparation. If the 0νββ-decay will be detected, the key issue will be what is the dominant mechanism of this process. By measuring only transitions to the ground state one can not distinguish among many of the 0νββ-decay mechanisms (the light and heavy Majorana neutrino exchange mechanisms, the trilinear R-parity breaking mechanisms etc.). We show that if the ratio of the 0νββ-decay half-lifes for transitions to the 0 + first excited and ground states is determined both theoretically and experimentally, it might be possible to determine, which 0νββ-decay mechanisms is dominant. For that purpose the corresponding nuclear matrix elements have to be evaluated with high reliability. The present work is giving strong motivations for experimental studies of the 0νββ-decay transitions to the first excited 0 + states of the final nuclei.

The single state dominance hypothesis and the two-neutrino double beta decay of 100Mo

The hypothesis of the single state dominance (SSD) in the calculation of the two-neutrino double beta decay (2νββ-decay) of 100Mo is tested by an exact consideration of the energy denominators of the perturbation theory. Both transitions to the ground state as well as to the 0+ and 2+ excited states of the final nucleus 100Ru are considered. We demonstrate that by experimental investigation of the single electron energy distribution and the angular correlation of the outgoing electrons, the SSD hypothesis can be confirmed or ruled out by a precise 2νββ-decay measurement (e.g. by NEMO III collaboration).

Elastic and inelastic scattering of π mesons on 7Li nuclei

Within the diffraction theory of multiple scattering, the differential cross sections for elastic and inelastic pion scattering on a 7Li nucleus are calculated for the case where the final nucleus is either in the ground or in the first excited state. The nuclear wave function is set to that in the αt cluster model. The sensitivity of the calculated observables to variations in the type of the wave functions for the alpha-particle and the triton cluster and for their relative motion is investigated. Various multiplicities of scattering and rescattering on the clusters constituting the 7Li nucleus are taken into account, and their contributions to the cross section are revealed. The results of the calculations are compared with experimental data at Eπ=143, 164, and 194 MeV.

On the conversion of charge-exchange cross sections to “GT” strength: Anomalies in odd- A nuclei

Gamow–Teller (GT) transitions are the most common weak interaction processes of spin–isospin (στ) type in atomic nuclei. They are of interest not only in nuclear physics but also in astrophysics; they play an important role in supernovae explosions and nucleosynthesis. The direct study of weak decay processes, however, gives relatively limited information about GT transitions and the states excited via GT transitions (GT states); β decay can only access states at excitation energies lower than the decay Q-value, and neutrino-induced reactions have very small cross-sections. However, one should note that β decay has a direct access to the absolute GT transition strengths B(GT) from a study of half-lives, Qβ-values and branching ratios. They also provide information on GT transitions in nuclei far-from-stability. Studies of M1γ transitions provide similar information. In contrast, the complementary charge-exchange (CE) reactions, such as the (p,n) or (3He, t) reactions at intermediate beam energies and 0°, can selectively excite GT states up to high excitation energies in the final nucleus. It has been found empirically that there is a close proportionality between the cross-sections at 0° and the transition strengths B(GT) in these CE reactions. Therefore, CE reactions are useful tools to study the relative values of B(GT) strengths up to high excitation energies. In recent (3He, t) measurements, one order-of-magnitude improvement in the energy resolution has been achieved. This has made it possible to make one-to-one comparisons of GT transitions studied in CE reactions and β decays. Thus GT strengths in (3He, t) reactions can be normalised by the β-decay values. In addition, comparisons with closely related M1 transitions studied in γ decay or electron inelastic scattering [(e,e′)], and furthermore with “spin” M1 transitions that can be studied by proton inelastic scattering [(p,p′)] have now been made possible. In these comparisons, the isospin quantum number T and associated symmetry structure in the same mass A nuclei (isobars) play a key role. Isospin symmetry can extend our scope even to the structures of unstable nuclei that are far from reach at present unstable beam factories.

Systematics of the low-energy pionic double charge exchange in nuclei

The experimental results for the $({\ensuremath{\pi}}^{+},{\ensuremath{\pi}}^{\ensuremath{-}})$ reaction on nuclei obtained in recent years reveal clear systematic features of this reaction. New data on ${}^{7}\mathrm{Li},$ ${}^{12}\mathrm{C},$ ${}^{16}\mathrm{O},$ and ${}^{56}\mathrm{Fe}$ supplementing the existing data base are presented. The data on ${}^{12}\mathrm{C}$ are partly at variance with previous results. The dependence of the cross sections on incident energy, scattering angle, and on the target mass is discussed for transitions leading to the ground state of the final nucleus or to the double isobaric analog state.

Properties of isovector 1+ states in A=28 nuclei and nuclear muon capture

A method is proposed for taking into account, in a calculation of partial rates of muon capture by nuclei, experimental information about strength functions for Gamow-Teller and isovector M1 transitions. The method, which amounts to choosing an orthogonal transformation that acts in the subspace of wave functions for excited states, requires neither modifying transition operators nor introducing effective charges. The matrix of the above transformation is constructed as a product of the matrices of reflection in a plane. All calculations are performed on the basis of the multiparticle shell model. Numerical results are obtained for isovector states in A=28 nuclei. Strength functions for Gamow-Teller and isovector M1 transitions in 28Si are considered, and the lifetimes of 1+ states in 28Al and the branching fractions for gamma decays of this state are calculated. Owing to taking into account experimental information about the properties of isovector states, the branching fractions for the γ decays of the 1+ state at 2.201 MeV in 28Al are successfully described for the first time. The above transformation of the wave functions changes substantially the distribution of partial rates of allowed muon capture by a 28Si nucleus among the 1+ states of the final nucleus 28Al in relation to the results of the calculations with the eigenfunctions of the Hamiltonian of the multiparticle shell model. The muon-capture rates calculated with the transformed functions agree well with experimental data.