States In Neutron-rich Nuclei Research Articles

Clustering states in neutron-rich nuclei

The study of clustering states in neutron-rich nuclei is an important subject of research in the field of nuclear physics, steadily growing in interest in the international scientific community. In this context, break-up reactions play an important role for the characterization of exotic states in radioactive light nuclei, like neutron halos around stable cores, α-clustering structures or exotic clusters. The CLIR (Clusters in Light Ion Reactions) experiment was performed at INFN - Laboratori Nazionali del Sud (LNS), aiming at the investigation of such states in light radioactive nuclei, by producing a radioactive beam at the FRIBs facility. Reaction products were detected by the CHIMERA multidetector, coupled with four telescopes of the FARCOS array. Calibrations of the tagging system and of the FARCOS telescopes have been performed, for which accurate procedures have been carried out. In this paper, results on the analysis will be presented. Moreover, a brief review on the new fragment separator FRAISE, currently under construction at LNS, will be given.

Clustering and molecular states in neutron rich nuclei

An investigation plan of different clustering and molecular states in neutron rich isotopes of Lithium, Beryllium, Boron and Carbon, in the context of an experimental campaign at INFN-Laboratori Nazionali del Sud, is presented. High statistics and new information on known states and study on new states will be possible thanks to the high intensities available for the exotic beams, delivered by the FRAISE facility and thanks to the opportunity of using very performing experimental apparatus, as well as the detectors CHIMERA, FARCOS, and NArCOS. Among these, the new hodoscope NArCOS, by detecting with high efficiency neutrons and providing high energy and angular resolutions, will allow also a precise study of reaction channels characterized by neutron emission.

Continuum random-phase approximation for γ transitions between excited states in neutron-rich nuclei

A characteristic feature of collective and particle-hole excitations in neutron-rich nuclei is that many of them couple to unbound neutron in continuum single-particle orbits. The continuum random phase approximation (cRPA) is a powerful many-body method that describes such excitations, and it provides a scheme to evaluate transition strengths from the ground state. In an attempt to apply cRPA to the radiative neutron capture reaction, we formulate in the present study an extended scheme of cRPA that describes gamma-transitions from the excited states under consideration, which decay to low-lying excited states as well as the ground state. This is achieved by introducing a non-local one-body operator which causes transitions to a low-lying excited state, and describing a density-matrix response against this operator. As a demonstration of this new scheme, we perform numerical calculation for dipole, quadrupole, and octupole excitations in $^{140}$Sn, and discuss E1 and E2 transitions decaying to low-lying $2^{+}_{1,2}$ and $3^{-}_{1}$ states. The results point to cases where the branching ratio to the low-lying states is larger than or comparable with that to the ground state. We discuss key roles of collectivity and continuum orbits in both initial and final states.

Cross-shell excitations in Ca46 studied with fusion reactions induced by a reaccelerated rare isotope beam

Discovering unexplored high-spin states in neutron-rich nuclei can open up a new direction to study band structure and the associated shell structure in isospin-asymmetric many-body systems. However, experimental reach has so far been limited to neutron-deficient or stable nuclei which are preferentially produced in fusion reactions used in such studies. Here, we report the first $\ensuremath{\gamma}$-ray spectroscopy with fusion reactions using a reaccelerated rare-isotope beam of $^{45}\mathrm{K}$ performed at the ReA3 facility of the National Superconducting Cyclotron Laboratory. Using particle and $\ensuremath{\gamma}$-ray coincidence techniques, three new higher-lying states around 6 MeV and five new $\ensuremath{\gamma}$-ray transitions were identified for $^{46}\mathrm{Ca}$, suggesting three independent band structures formed from different particle-hole configurations. The rotational-like band built on the ${0}_{2}^{+}$ state is established up to the tentatively assigned ${6}_{2}^{+}$ state. New results are compared to large-scale shell model calculations, confirming the validity of the effective interaction describing particle-hole excitations across the $Z=20$ and $N=28$ shell gaps in the vicinity of doubly magic $^{48}\mathrm{Ca}$.

Astromers in the Radioactive Decay of r-process Nuclei

Abstract Certain nuclear isomers are well known to affect nucleosynthesis with important observable consequences (e.g., 26Al and 180Ta). We study the impact of nuclear isomers in the context of rapid neutron capture process (r-process) nucleosynthesis. We demonstrate that nuclear isomers are dynamically populated in the r process and that some are populated far from thermal equilibrium; this makes them astrophysical isomers, or “astromers.” We compute thermally mediated transition rates between long-lived isomers and the corresponding ground states in neutron-rich nuclei. We calculate the temperature-dependent β-decay feeding factors, which represent the fraction of material going to each of the isomer and ground state daughter species from the β-decay parent species. We simulate nucleosynthesis following the decay of a solar-like r-process composition and include as separate species nuclear excited states with measured terrestrial half-lives greater than 100 μs. We introduce a new metric to identify those astromers most likely to be influential and summarize them in a table. Notable entries include many second peak nuclei (e.g., the Te isotopic chain) and previously overlooked isomers in stable nuclei (e.g., 119Sn, 131Xe, and 195Pt). Finally, we comment on the capacity of isomer production to alter radioactive heating in an r-process environment.

Double charge-exchange phonon states

We study double charge-exchange phonon states in neutron-rich nuclei, in particular the double isobaric analog states and the double Gamow-Teller excitations, induced by the double isospin operator $\sum_{i,j=1}^At_-(i) t_-(j)$ and spin-isospin operator $\sum_{i,j=1}^A{\sigma}(i) t_-(i){\sigma}(j) t_-(j)$, respectively. We employ quartic commutator relations to evaluate the average energies $E_{\rm DIAS} - 2E_{\rm IAS}$ and $E_{\rm DGTR}-E_{\rm DIAS} - 2(E_{\rm GTR}-E_{\rm IAS})$, and conventional double commutator relations to evaluate the average energies of $E_{\rm GTR}-E_{\rm IAS}$ and $E_{\rm IAS}$. We have found that the corrections due to quartic commutators follow the approximate laws: $E_{\rm DIAS} - 2E_{\rm IAS}\approx \frac{3}{2} A^{-1/3}$ MeV and $E_{\rm DGTR}-E_{\rm DIAS} - 2(E_{\rm GTR}-E_{\rm IAS})\approx 16 A^{-1}$ MeV. While the former is dominated by direct Coulomb effects, since Coulomb exchange cancels out to some extent with isospin symmetry breaking contributions originated form the nuclear strong force, the latter is sensitive to the difference in strength between the spin and spin-isospin chanels of the strong interaction.

The γ-γ fast-timing technique and the EXILL&FATIMA campaign

At the Institut Laue-Langevin in Grenoble, germanium-gated γ-γ fast-timing lifetime measurements of nuclear excited states in neutron-rich nuclei have been performed within a prompt γ-ray spectroscopy experimental campaign. We report on results obtained from the cold-neutron induced fission of 235U. The excited secondary fission products were stopped almost instantaneously within the thick target and the γ rays emitted were collected triggerlessly using the EXILL&FATIMA mixed array of HPGe and LaBr3(Ce) detectors. Precise lifetimes could be determined by analysing the γ-γ time difference spectra using the generalized centroid difference method. This picosecondsensitive method provides many advantages and is briefly explained. Still, the major source of systematic errors is related to the contribution of time-correlated Compton background. The EXILL&FATIMA results are discussed with respect to the typical energy-dependent timing behaviour of the background. According to the time response of the background, appropriate methods and a time correction for the sub-nanosecond regime are proposed.

Counting rate measurements for lifetime experiments using the RDDS method with the new generation γ-ray array AGATA

The differential Recoil Distance Doppler Shift (RDDS) method after multinucleon transfer (MNT) reactions to measure lifetimes of excited states in neutron-rich nuclei requires the use of a thick energy degrader for the recoiling ejectiles that are then detected in a spectrometer. This type of measurements greatly benefits from the use of the new generation segmented γ-ray detectors, such as the AGATA demonstrator which offers unprecedented energy and angular resolutions. In order to make an optimized choice of the material and the thickness of the degrader for lifetime measurements using the RDDS method after MNT, an experiment has been performed with the AGATA demonstrator. Counting rate measurements for different degraders are presented.

Cluster states in neutron-rich nuclei

The structure of light neutron-rich nuclei is investigated within the cluster model. Moments of inertia are systematized for low-lying bands of positive and negative parity. It is shown that the experimentally observed rotational bands can be described in the scope of a simple alpha particle model.

Exploring the evolution of the shell structure by means of deep inelastic reactions: recent results from LNL

Nuclear structure studies far from stability, which mainly rely on the availability of radioactive nuclear beams, can complementarily be addressed by means of high-intensity beams of stable ions. In such a context, deep-inelastic and multi-nucleon transfer reactions are a powerful tool to populate yrast and non-yrast states in neutron-rich nuclei. Particularly successful here is the combination of large acceptance spectrometers with highly segmented γ-detector arrays. Such devices can provide the necessary channel selectivity to identify very rare signals. Examples are the CLARA and AGATA γ-ray detector arrays coupled with the PRISMA spectrometer at the Legnaro National Laboratories (LNL) in Italy. Large data sets have been collected at LNL for nuclei close to the N=20, 28, 40, 50 and 82 shell closures as well as in new regions of deformation such as, for example, at A=60. Here I will shortly review some recent results.

Observation of a two-neutron cascade from a resonance inO24

A new resonance having a relative decay energy of $~0.6$ MeV has been observed above the two-neutron separation energy in $^{24}\mathrm{O}$ at an excitation energy of $~7.5$ MeV. The new level was found to directly feed the first excited state in $^{23}\mathrm{O}$, an $E=0.045(2)$-MeV resonance, through the measurement of neutron-neutron correlations. Energy level comparisons with $^{23}\mathrm{O}$ suggest that this new level in $^{24}\mathrm{O}$ is dominated by an unpaired neutron in the $\ensuremath{\nu}0{d}_{3/2}$ single-particle orbital coupled to a single hole in the $\ensuremath{\nu}0{d}_{5/2}$ single-particle orbital. Establishment of this two-neutron discrimination technique provides a means for investigating high-lying excited states in neutron-rich nuclei.

Exploring the evolution of shell structure by means of deep inelastic reactions

To understand the properties of a nucleus, apart from establishing the interaction between its components, it is necessary to determine the arrangement of the nucleon, i.e. the structure of a nucleus. Presently our knowledge about the structure of nuclei is mostly limited to nuclei close to the valley of stability or nuclei with a deficiency of neutrons. Only recently the availability of beams of unstable ions has given access to unexplored regions of the nucler chart, especially on the neutron rich side. An alternative and often complementary way to study the structure of neutron rich nuclear systems is offered by the use of high intensity beams of stable ions. Binary reactions with stable bems are a powerful tool to populate medium and high spin states in neutron rich nuclei. In this article I will discuss some selected examples which show the high potential of such reaction mechanism for the study of the spectroscopic properties of the neutron-rich nuclear many body systems.

Lifetime measurements of neutron-rich nuclei around 48Ca with the CLARA-PRISMA setup

The lifetimes of excited states in neutron-rich nuclei around the doubly-magic nucleus 48Ca have been measured using a novel technique that combines the Recoil Distance Doppler Shift method with the CLARA-PRISMA spectrometers. This is the first time such a method is applied to measure lifetimes of neutron-rich nuclei populated via a multinucleon transfer reaction. The experimental method will be discussed and the results for Ca nuclei will be compared with realistic simulations of the CLARA and PRISMA spectrometers.

Nuclear Structure far from Stability at LNL: From high intensity stable to radioactive nuclear beams

Future perspectives in nuclear structure rely on radioactive nuclear beams as well as on high intensity beams of stable ions. Deep-inelastic and multi-nucleon transfer reactions with stable beams of heavy elements can be used to populate yrast and non yrast states of neutron rich nuclei. Particularly powerful is here the combination of large acceptance spectrometers with highly segmented γ -detector arrays. Such devices, eventually complemented by large coverage particle detectors, can provide the necessary channel selectivity to identify very rare signals. An example is the CLARA γ -ray detector array coupled with the PRISMA spectrometer at the Legnaro National Laboratories (LNL). The physics aims achievable complement studies performed with current radioactive beam (RIB) facilities. With such set-up we have recently investigated the stability of the N=50 shell closure when moving towards more exotic systems. Here the comparison of the experimental data with shell model calculations seems to indicate a persistence of the N=50 shell gap down to Z=31. Future perspectives at LNL are based on an increase in intensity as well as on the availability of heavy ion species. Beams like 136 Xe or 208 Pb, provided by the new PIAVE injector, can be used to drive the multinucleon flux toward the most exotic regions. Moreover a new ISOL facility (SPES) dedicated to the production and acceleration of radioactive neutron rich species is now under development at LNL. Among the new instrumentation, the concept of γ -ray tracking has been recently introduced in nuclear spectroscopy. A new γ -ray detector array (AGATA) based on such technique is now under study in a wide european collaboration. The first sub-cluster of AGATA is foreseen to be installed at the PRISMA spectrometer.

Study of fission process and neutron-rich nuclei

Prompt triple γ coincidence studies of the nuclei produced in the spontaneous fission of 252Cf were carried out at Gammasphere. The relative yield matrix for the Ba-Mo and Ba-α-Zr charge splits was determined using an improved analysis. An upper limit for the second mode of 0.5 % is found for the binary case, while the ternary case shows evidence of a strong second mode. Half-lives of several states in neutron-rich nuclei were measured by using a new time-gated γ − γ − γ coincidence technique. A large deformation of β = 0.47 was observed in 104Zr using this method. New band structures were found in several odd-A Y, Nb, Tc, and Rh nuclei. These nuclei show a smooth evolution from axial symmetry in Y to maximum triaxiality in Rh. Particle plus triaxial rotor calculations have been made for these odd-A nuclei. The gamma band staggering in 108,110,112Ru will be discussed.

YRAST STATES IN 188,190Os NUCLEI

Yrast states in neutron rich nuclei 188 Os and 190 Os have been populated using the deep-inelastic reaction 82 Se + 192 Os at 460 MeV beam bombarding energy. A thick 192 Os target (>50 mg/cm 2) with 0.2 mm Ta backing was used to stop all of the recoils in the target, minimising the broadening of the lines due to Doppler shift. High fold γ-γ coincidences were aquired with the 4π spectrometer GASP detector array. Three and two dimensional gamma-ray matrices were used to construct level schemes of the nuclei of interest up to an yrast state of 22.

A New Antisymmetrized Molecular Dynamics Approach to Few-Nucleon Systems

A new method in the study of antisymmetrized molecular dynamics (AMD) is proposed. This method consists of a Jacobi-coordinate-basis AMD with the generator coordinate method (GCM). A simple example is presented for the case of in few-nucleon systems with the Volkov potential. Bound states for 2N, 3N, and 4N systems are compared with those obtained using the “primary AMD” and some other methods. We find this to be a promising method for investigating light nuclear systems. During the past decade, since the primary antisymmetrized molecular dynamics (AMD) was proposed, 1)–3) it has been successfully extended to systematic studies of neutron-rich nuclei, 4)–6) as well as excited states of nuclei. 7) Further developments of the AMD method have been realized by introducing several conventional ideas of conventional nuclear structure physics. 8)–19) The physical approaches mentioned above were formulated by making improvements of the AMD in Refs. 4)–7); for instance, a variational calculation was carried out after application of the spin parity projection (VAP) in order to investigate excited states 15) and the existence of exotic clusters in excited states of neutronrich nuclei. 16)–19) Another improvement realized through use of the superposition of selected Slater determinants was made in the AMD triple-S model, which can quantitatively describe properties of light nuclei. 10),11) These improvements were made in methods treating medium light nuclei in which the wave function is expressed in terms of a superposition of Slater determinants. However, this method has not yet been successfully applied to few-body problems. In order to allow for the application of the AMD method to few-body and very light nuclei, we present one modified AMD method which was developed using the Jacobi-coordinate-basis AMD plus the GCM technique. 20),21) One of the aims of this work is to introduce the N-body correlations in a “single Slater determinant”. The second is to remove the ambiguity of the zero-point oscillation energy. The third is to reduce the number of variational parameters in the AMD effectively with the GCM technique; using this technique, a very large number of the variational parameters can be fixed in the eigenvalue problem. Let us start from the trial wave function primary AMD. The wave function of N -nucleon system Ψ is given by

Investigations of short half-life states from SF of 252Cf

By using different time-gated triple γ coincidence data, the half-lives (T1/2) of several short lived states in neutron-rich nuclei have been studied. The first excited states in the ground state bands often decay by delayed γ emission. By creating triple γ coincidence spectra with time windows of 8, 16, 20, 28, and 48 ns, we have studied states with half-lives below 10 ns. The estimated half-lives of 102Zr, 137Xe, and 143Ba are in reasonable agreement with previously reported values. We extract the first estimates of the half lives of the 2+ states in 104Zr and 152Ce.

Half-life measurement of excited states in neutron-rich nuclei

Half-lives (T 1/2) of several states which decay by delayed γ transitions were determined from time-gated triple γ coincidence method. We determined, for the first time, the half-life of 330.6 + x state in 108Tc and the half-life of 19/2− state in 133Te based on the new level schemes. Five half-lives of 95,97Sr, 99Zr, 134Te and 137Xe are consistent with the previously reported ones. These results indicate that this new method is useful for measuring the half-lives.

RELATIVISTIC DESCRIPTION OF EXOTIC COLLECTIVE EXCITATION PHENOMENA IN ATOMIC NUCLEI

The low-lying dipole and quadrupole states in neutron rich nuclei are studied within the fully self-consistent relativistic quasiparticle random-phase approximation (RQRPA), formulated in the canonical basis of the Relativistic Hartree–Bogoliubov model (RHB), which is extended to include the density dependent interactions. In heavier nuclei, the low-lying E1 excited state is identified as a pygmy dipole resonance (PDR), i.e. as a collective mode of excess neutrons oscillating against a proton–neutron core. Isotopic dependence of the PDR is characterized by a crossing between the PDR and one-neutron separation energies. Already at moderate proton–neutron asymmetry the PDR peak is calculated above the neutron emission threshold, indicating important implications for the observation of the PDR in (γ, γ′) scattering, and on the theoretical predictions of the radiative neutron capture rates in neutron-rich nuclei. In addition, a novel method is suggested for determining the neutron skin of nuclei, based on measurement of excitation energies of the Gamow–Teller resonance relative to the isobaric analog state.