Β-decay Half-lives Research Articles

βdecay of nuclei around90Se: Search for signatures of aN=56subshell closure relevant to therprocess

Background: Nuclear structure plays a significant role on the rapid neutron capture process (r process) since shapes evolve with the emergence of shells and subshells. There was some indication in neighboring nuclei that we might find examples of a new N=56 subshell, which may give rise to a doubly magic 3490Se56 nucleus. Purpose: β-decay half-lives of nuclei around 90Se have been measured to determine if this nucleus has in fact a doubly magic character. Method: The fragmentation of a 136Xe beam at the National Superconducting Cyclotron Laboratory at Michigan State University was used to create a cocktail of nuclei in the A=90 region. Results: We have measured the half-lives of 22 nuclei near the r-process path in the A=90 region. The half-lives of 88As and 90Se have been measured for the first time. The values were compared with theoretical predictions in the search for nuclear-deformation signatures of a N=56 subshell, and its possible role in the emergence of a potential doubly magic 90Se. The impact of such hypothesis on the synthesis of heavy nuclei, particularly in the production of Sr, Y, and Zr elements was investigated with a weak r-process network. Conclusions: The new half-lives agree with results obtained from a standard global QRPA model used in r-process calculations, indicating that 90Se has a quadrupole shape incompatible with a closed N=56 subshell in this region. The impact of the measured 90Se half-life in comparison with a former theoretical predication associated with a spherical half-life on the weak r process is shown to be strong.Received 22 December 2011DOI:https://doi.org/10.1103/PhysRevC.85.035807©2012 American Physical Society

β-decay measurements of A ≃ 70 − 110 r-process nuclei at the National Superconducting Cyclotron Laboratory

The present paper reports on several r-process motivated β-decay experiments undertaken at the National Superconducting Cyclotron Laboratory. β-decay half-lives and β-delayed neutron-emission probabilities were measured for nuclei around the r-process A = 70–80 and A = 90 – 110 mass regions. The data are discussed on the basis of quasi-random phase approximation calculations. The emphasis is made on the impact of these data upon calculations of r-process abundances.

β-Decay Half-Lives of Very Neutron-Rich Kr to Tc Isotopes on the Boundary of ther-Process Path: An Indication of Fastr-Matter Flow

The β-decay half-lives of 38 neutron-rich isotopes from (36)Kr to (43)Tc have been measured; the half-lives of (100)Kr, (103-105)Sr, (106-108)Y, (108-110)Zr, (111,112)Nb, (112-115)Mo, and (116,117)Tc are reported here. The results when compared with previous standard models indicate an overestimation in the predicted half-lives by a factor of 2 or more in the A≈110 region. A revised model based on the second generation gross theory of β decay better predicts the measured half-lives and suggests a more rapid flow of the rapid neutron-capture process (r-matter flow) through this region than previously predicted.

β-DECAY HALF-LIVES AROUND 78Ni AND 132Sn IN A SELF-CONSISTENT APPROACH

β-decay half-lives around 78 Ni and 132 Sn are studied with proton-neutron Quasiparticle Random Phase Approximation on the basis of Skyrme-Hartree-Fock (SHF) method. We take into account tensor force correlation in calculating the ground and excited states and find it improves the half-life of 132 Sn . However, the Q value is still underestimated. We also discuss that half-lives do not strongly depend on type of particle-particle interaction in isospin T = 0 channel.

Network calculations for r-process nucleosynthesis

The r-process is known to be responsible for the synthesis of about half of the elements heavier than iron, nevertheless its astrophysical site has not yet been clearly ascertained, but observations indicate that at least two possible sites should contribute to the solar system abundance of r-process elements. The r-process being responsible for the production of elements heavier than Z = 56 operates rather robustly always resulting in a similar relative abundance pattern. From the nuclear-physics point of view the r-process requires the knowledge of a large number of reaction rates involving exotic nuclei that are not accessible by experiment and data have to be provided by theoretical predictions. We have developed for the first time a complete database of reaction rates that in addition to neutron-capture rates and β-decay half-lives includes the dominant reactions that can induce fission (neutron-capture, β-decay and spontaneous fission) and the corresponding fission yields. In addition, we have implemented these reaction rates in a fully implicit reaction network. The influence of the nuclear physics input constituted in the reaction rates based on the two mass models FRDM and ETFSI and on the astrophysical conditions simulating a cold or hot environment are examined.

β−-delayed spectroscopy of neutron-rich tantalum nuclei: Shape evolution in neutron-rich tungsten isotopes

The low-lying structure of W188,190,192 has been studied following β decays of the neutron-rich mother nuclei Ta188,190,192 produced following the projectile fragmentation of a 1-GeV-per-nucleon Pb208 primary beam on a natural beryllium target at the GSI Fragment Separator. The β-decay half-lives of Ta188, Ta190, and Ta192 have been measured, with γ-ray decays of low-lying states in their respective W daughter nuclei, using heavy-ion β−γ correlations and a position-sensitive silicon detector setup. The data provide information on the low-lying excited states in W188, W190, and W192, which highlight a change in nuclear shape at W190 compared with that of lighter W isotopes. This evolution of ground-state structure along the W isotopic chain is discussed as evidence for a possible proton subshell effect for the A~ 190 region and is consistent with maximization of the γ-softness of the nuclear potential around N~116.11 MoreReceived 18 September 2009DOI:https://doi.org/10.1103/PhysRevC.80.064308©2009 American Physical Society

Recent progress in measuring β half-lives of nuclei approaching the r-process waiting point A = 195

Heavy neutron-rich nuclei close to N = 126 were produced by fragmentation of a 1 A GeV 208 Pb beam at the FRS at GSI. The β-decay half-lives of 8 nuclides have been determined using a novel numerical analysis method. The comparison of the data with model calculations provides indications on the importance of first-forbidden transitions around A = 195 . The measured data indicate that the matter flow in the r-process to heavier fissioning nuclei may be faster than previously expected.

Benford's Law and β-Decay Half-Lives

The experimental values of 2059 β-decay half-lives are systematically analyzed and investigated. We have found that they are in satisfactory agreement with Benford's law, which states that the frequency of occurrence of each figure, 1–9, as the first significant digit in a surprisingly large number of different data sets follows a logarithmic distribution favoring the smaller ones. Benford's logarithmic distribution of β-decay half-lives can be explained in terms of Newcomb's justification of Benford's law and empirical exponential law of β-decay half-lives. Moreover, we test the calculated values of 6721 β-decay half-lives with the aid of Benford's law. This indicates that Benford's law is useful for theoretical physicists to test their methods for calculating β-decay half-lives.

On the weak axial-vector constant in N ≈ Z nuclei near the closed shells

Gamow-Teller transitions in nuclei of the regions close to the doubly closed shells at A = 16, 40, 56, and 100 are analyzed in the shell-model approach by considering the configuration mixing, core polarization, and by using the experimental values of the one-particle spectroscopic factors. The observed data on the β-decay half-lives are well reproduced by using the |GA/GV| value equal to 1.1(1).

Beta-decay of [formula omitted] nuclei near the [formula omitted] closed neutron shell

Systematic mean-field calculations of the β-decay rates for the nuclei with charge numbers Z = 42 – 49 approaching the possible r-process paths in vicinity of the spherical neutron shell at N = 82 are performed within the DF + CQRPA framework employing a self-consistent approach to the nuclear ground states based on the local energy-density functional (DF) theory. The calculated ground state properties are compared with the available experimental data. The beta-strength-functions of the Gamow–Teller and first-forbidden decays and the total β-decay half-lives are calculated within the continuum QRPA approach. If available, data and half-lives obtained on the basis of the Finite Range Droplet Model and the shell model are compared to our results. The effects of our calculated half-lives on the r-process abundances are explored in local r-process calculations.

A new analysis method to determine β-decay half-lives in experiments with complex background

This paper reports the first application of a new technique to measure the β-decay half-lives of exotic nuclei in complex background conditions. Since standard tools were not adapted to extract the relevant information, a new analysis method was developed. The time distribution of background events is established by recording time correlations in backward time. The β half-lives of the nuclides and the detection efficiency of the set-up are determined simultaneously from a least-squares fit of the ratio of the time-correlation spectra recorded in forward and in backward time, using numerical functions. The necessary numerical functions are calculated in a Monte-Carlo code using the known operation parameters of the experiment and different values for the two free parameters, half-life and detection efficiency, as input parameters.

Beta-decay rates

Major astrophysical applications involve a huge number of exotic nuclei. Their beta-decay properties play a crucial role in stellar explosive events. An important effort has been developed in last decades to measure the masses and β-decay properties of very neutron-rich nuclei at radioactive nuclear beam facilities. However, most of them cannot be synthesized in terrestrial laboratories and only theoretical predictions can fill the gap. We will concentrate mainly on the β-decay rates needed for stellar r-process modeling and for performing the RNB experiments. An overview of the microscopic approaches to the β-decay strength function is given. The continuum QRPA approach based on the self-consistent ground state description in the framework of the density functional theory is outlined. For the first time, a systematic study of the total β-decay half-lives and delayed neutron emission probabilities takes into account the Gamow–Teller and first-forbidden transitions. Due to the shell configuration effects, the first-forbidden decays have a strong impact on the β-decay characteristics of the r-process relevant nuclei at Z ≈ 28 , N > 50 ; Z ⩾ 50 , N > 82 and Z = 60 – 70 , N ≈ 126 . Suppression of the delayed neutron emission probability is found in nuclei with the neutron excess bigger than one major shell. The effect originates from the high-energy first-forbidden transitions to the states outside the ( Q β − B n ) -window in the daughter nuclei. The performance of existing global models for the nuclides near the r-process paths is critically analyzed and confronted with the recent RIB experiments in the regions of 78Ni, 132Sn and “east” of 208Pb.

Self-consistent relativistic QRPA studies of soft modes and spin-isospin resonances in unstable nuclei

The excitation phenomena in unstable nuclei are investigated in the framework of the relativistic quasiparticle random-phase approximation (RQRPA) in the relativistic Hartree-Bogoliubov model (RHB) which is extended to include effective interactions with explicit density-dependent meson-nucleon couplings. The properties of the pygmy dipole resonance (PDR) are examined in 132Sn and within isotopic chains, showing that already at moderate proton-neutron asymmetry the PDR peak energy is located above the neutron emission threshold. A method is suggested for determining the size of the neutron skin within an isotopic chain, based on the measurement of the excitation energies of the Gamow-Teller resonance relative to the isobaric analog state. In addition, for the first time the relativistic RHB + RQRPA model, with tensor ω meson-nucleon couplings, is employed in calculations of β-decay half-lives of nuclei of the relevance for the r-process.

Weak interaction rates for astrophysical applications

An important effort has been developed in recent decades to measure the masses and β-decay rates of very neutron-rich nuclei at radioactive ion beam (RIB) facilities. However, major astrophysical applications involve a huge number of exotic species. Most of them cannot be synthesized in terrestrial laboratories and only theoretical predictions can fill the gap. We concentrate on the self-consistent predictions of the β-decay rates needed for stellar r-process modeling and for performing the RIB experiments. The continuum QRPA approach based on the self-consistent ground-state description in the framework of the density functional theory is briefly described. The model for the large-scale calculations of total β-decay half-lives accounts for the Gamow-Teller and first-forbidden transitions. Due to the shell configuration effect, the first-forbidden decays have a strong impact on the total half-lives of the r-process relevant nuclei at N=126, Z=60–70. The performance of existing global models for the nuclides near the r-process paths at N=126 is critically analyzed and confronted with the recent RIB experiments in the region “east” of 208Pb.

Status of delayed-neutron precursor data: half-lives and neutron emission probabilities

We present in this paper a compilation of the present status of experimental delayed-neutron precursor data; i.e. β-decay half-lives ( T 1/2) and neutron emission probabilities ( P n) in the fission-product region (27 ≤ Z ≤ 57). These data are compared to two model predictions of substantially different sophistication: (i) an update of the empirical Kratz-Herrmann formula (KHF), and (ii) a unified macroscopic-microscopic model within the quasi-particle random-phase approximation (QRPA). Both models are also used to calculate so far unknown T 1/2 and P n values up to Z = 63. A number of possible refinements in the microscopic calculations are suggested to further improve the nuclear-physics foundation of these data for reactor and astrophysical applications.

Selective laser ionization of N≥82 indium isotopes: The new r-process nuclide 135In

Production yields and β-decay half-lives (T 1/2) of very neutron-rich indium isotopes were determined at CERN/ISOLDE using isobaric selectivity of a resonance-ionization laser ion-source. Beta-delayed neutron (βdn) multiscaling measurements have yielded improved T 1/2 for 206(6) ms 132In, 165(3) ms 133In and 141(5) ms 134In. With 92(10) ms 135In, a new r-process nuclide has been identified which acts as an important “waiting point” in the In isotopic chain for neutron densities in the range n n≃ 1024-1026 n/cm3, where the r-matter flow has already passed the A≃ 130 abundance peak region.

Isomeric state of 80Y and its role in the astrophysical rp-process

A careful investigation of the isomeric transition of the long-lived state at 228.5 keV excitation energy in 80Y has been done. The HIGISOL facility at the Jyvaskyla isochronous cyclotron has been used. We used the electron magnetic transporter to prepare an appropriate source and to measure the electron spectra in clean background conditions. The measured internal conversion coefficient αK = 0.50±0.07 allows unambiguous 1- identification for the 228.5 keV first excited isomeric state in 80Y. With a “bare" half-life of 6.8±0.5 s found in this work, this state is strongly populated in the rp-process during X-ray bursts and has therefore to be taken into account in X-ray burst model calculations. However, because of the similarity of the β-decay half-lives of isomeric and ground states, we find a maximum reduction in the effective β-decay lifetime of 80Y of only 17±2%. Our results pave the way for a future investigation of the impact of the isomeric state on the “effective" 80Y proton capture rate.

β-decay half-lives of new neutron-rich isotopes 167,168Tb and levels in 167,168Dy

$\ensuremath{\beta}$-decay half-lives of new neutron-rich isotopes ${}^{167}\mathrm{Tb}$ and ${}^{168}\mathrm{Tb}$ produced in the 20 MeV proton-induced fission of ${}^{238}\mathrm{U}$ have been determined to be 19.4(27) s and 8.2(13) s, respectively, using a gas-jet coupled on-line isotope separator. The present half-lives and those of the recently identified nuclei ${}^{159}\mathrm{Pm},$ ${}^{161}\mathrm{Sm},$ ${}^{165}\mathrm{Gd},$ ${}^{166}\mathrm{Tb}$ were compared with theoretical predictions. The recent calculation by the gross theory with the new one-particle strength functions shows quite good agreement with the experimental half-lives. Excited states of the daughter nuclides ${}^{167}\mathrm{Dy}$ and ${}^{168}\mathrm{Dy}$ have been established for the first time. Level energies of the first ${2}^{+}$ states in even-even Dy isotopes were found to show an irregular behavior at ${}^{164}\mathrm{Dy}$ and increasing deformation toward the neutron midshell around $N\ensuremath{\approx}104$.

β-decay half-lives of very neutron-rich isotopes of elements from Ti to Ni

The unknown β-decay half-lives of 22 isotopes far off stability (5 < T Z < 10) in the region from Ti to Ni were measured at GSI, Darmstadt. The nuclei were produced in a fragmentation reaction of 500 A.MeV 86Kr-projectile impinging on a thick Be target. The isotopes of interest were separated and identified with the fragment separator, FRS, by a combination of Bρ,Z, and ToF techniques. An additional range separation was performed by a selective implantation into granular detectors. The spatial and time correlations of the implant with the consecutively detected β-particles were used to determine the unknown half-lives. For nuclei far off stability, β-decay chains were measured and analyzed as well, leading to an even more reliable evaluation of the lifetimes. The large discrepancies found between the measured and the theoretical values emphasize that most recent theoretical work is not an improvement over calculations made almost a decade ago.

Semi-gross theory of nuclear β-decay

The gross theory of β-decay is refined to take into account shell effects of the parent nuclei, and the resulting theory is named semi-gross theory. In this theory, the one-particle energy distribution in the parent nucleus is taken to have structures, and the one-particle strength function is assumed to depend on the quantum numbers of the initial state of the decaying nucleon. β-decay (partial) half-lives are calculated for 1659 nuclides, and the results are compared with experimental data as well as with those calculated by the gross theory. The β-decay strength functions are shown for two selected nuclides, and briefly discussed. These numerical studies confirm that the semi-gross theory includes some part of the shell effects correctly, although there still remain deviations of the theoretical results from experimental data, which should, at least partly, be due to shell effects of the daughter nuclei. Comparison of the present results with microscopic theories shows that the overall accuracy of the semi-gross theory is comparable with those of the microscopic theories.