Microscopic calculations of weak interaction rates of nuclei in stellar environment for A = 18 to 100

We report here the microscopic calculation of weak interaction rates in stellar matter for 709 nuclei with A = 18 to 100 using a generalized form of proton-neutron quasiparticle RPA model with separable Gamow-Teller forces. This is the first ever extensive microscopic calculation of weak rates calculated over a wide temperature-density grid which includes 10⁷≤ T(K) ≤ 30 × 10⁹ and 10 ≤ρ Y_e (gcm⁻³) ≤ 10¹¹, and over a larger mass range. Particle emission processes from excited states, previously ignored, are taken into account, and are found to significantly affect some β decay rates. The calculated capture and decay rates take into consideration the latest experimental energy levels and ft value compilations. Our calculation of electron capture and β-decay rates, in the fp-shell, show considerable differences with a recently reported shell model diagonalization approach calculation. Received: 16 April 1999     

Search for tensor couplings in the weak interaction

The existence of exotic couplings in the weak interaction can be tested by a precise measurement of the β-ν angular correlation parameter a in nuclear beta decay. In the case of ⁶He, the ratio of tensor and axial-vector couplings can be determined. The goal of the LPC Trap experiment is to improve the limits on the existence of tensor currents. The LPCTrap setup is installed on the low energy beam line LIRAT of the SPIRAL/GANIL facility. The ⁶He⁺ ions are confined in a novel transparent Paul trap allowing the detection in coincidence of both the β-particle and the recoil ion. The detection setup enables to detect the position and energy of these two particles. The a parameter can then be extracted from the time of flight spectrum of the recoil ions. The first ⁶He⁺ decay events were observed in May 2005 during a commissioning run. Over 10⁵ coincidence events have been recorded during a second run in July 2006.     

Superallowed 0+ → 0+ beta-decay and CKM unitarity

Precise measurements of the ft-values for superallowed nuclear β-decay yield results that support conservation of the weak vector current but violate the unitarity of the Cabibbo-Kobayashi-Maskawa matrix by more than two standard deviations. This apparent violation of the Standard Model has led to considerable theoretical and experimental activity largely focussed on reducing uncertainties in the small correction terms that account for charge-dependent nuclear effects. This activity is outlined and the prospects for future sharpening of the unitarity test are assessed. Received: 21 March 2002 / Accepted: 16 May 2002 / Published online: 31 October 2002 RID="a" ID="a"e-mail: hardy@comp.tamu.edu     

New BCS and renormalized QRPA formalism with application to double beta decay

A new analysis of the renormalized proton–neutron quasiparticle random phase approximation based on simultaneous recalculation of the one-body density matrix and the pairing tensor has been used to study the double beta decay. We demonstrated that inclusion of the quasiparticle correlations at the BCS level reduces ground state correlations in the particle–particle channel of the proton–neutron interaction. We also simplified the RQRPA equations significantly obtaining a low-dimensioned set of linear equations for the quasiparticle densities. The formalism was applied to the double beta decay of ⁷⁶Ge. Received: 4 January 1999 / Revised version: 29 March 1999     

Effects of quantum space time foam in the neutrino sector

We discuss violations of CPT and quantum mechanics due to interactions of neutrinos with space-time quantum foam. Neutrinoless double beta decay and oscillations of neutrinos from astrophysical sources (supernovae, active galactic nuclei) are analysed. It is found that the propagation distance is the crucial quantity entering any bounds on EHNS parameters. Thus, while the bounds from neutrinoless double beta decay are not significant, the data of the supernova 1987a imply a bound being several orders of magnitude more stringent than the ones known from the literature. Even more stringent limits may be obtained from the investigation of neutrino oscillations from active galactic nuclei sources, which have an impressive potential for the search of quantum foam interactions in the neutrino sector. Received: 5 June 2000 / Accepted: 12 July 2000     

Ion traps --Precision measurements and more

Today ion traps are an important experimental tool. Applications range from high-precision measurements of masses and moments, realization of atomic clocks, to the study of ion chemical reactions. Ion traps have gained particular importance in the field of nuclear physics where they are used for the precise determination of nuclear binding energies, decay studies, and radioactive ion beam manipulation. Received: 21 March 2002 / Accepted: 16 May 2002 / Published online: 31 October 2002 RID="a" ID="a"e-mail: bollen@nscl.msu.edu     

A new measurement of 208Tl levels

A new determination for the levels of the nucleus ²⁰⁸Tl was performed by measuring the γ-rays following the β^--decay of ²⁰⁸Hg. Twenty-six γ-rays were assigned and three new levels of ²⁰⁸Tl at 1.728 MeV, 1.652 MeV and 1.362 MeV have been affirmed. A partial ²⁰⁸Hg decaying γ scheme was proposed. The experimental level structure was compared with a shell model calculation. Received: 12 August 2002 / Accepted: 19 December 2002 / Published online: 25 February 2003 RID="a" ID="a"e-mail: Li41zhang@yahoo.com Communicated by W. Henning     

(EC + β+) decay of 133Sm and 1.1 s isomer of 133Pr

The very neutron-deficient isotope ¹³³Sm was produced by irradiation of an enriched target of ⁹⁶Ru with ⁴⁰Ca and studied by using a helium-jet fast tape transport system in combination with X-γ and γ-γ coincidence measurements. A simple ( EC + β⁺) decay scheme of ¹³³Sm was proposed for the first time. As a by-product ¹³³Pr was produced simultaneously because of the ^98-102Ru content of the ruthenium target. The half-life of a 11/2^- low-lying isomeric state in ¹³³Pr was measured to be 1.1±0.2 s. Received: 22 March 2001 / Accepted: 18 July 2001     

Muon capture on neutron-rich nuclei

Future facilities will allow the exploration of extremely neutron-rich nuclei far from the valley of stability. It is discussed that the strong-neutron excess results in changes in the collective excitations of such nuclei compared to conventional stable nuclei. We propose muon capture as an experimental tool to explore such changes. We will quantify our discussion by the calculation of the total and differential muon capture rates on selected calcium isotopes between ⁴⁰Ca and ⁶⁰Ca. Our calculations are based on the random phase approximation and agree nicely with the measured rates for ⁴⁰Ca and ⁴⁴Ca. Received: 6 April 2001 / Accepted: 14 May 2001     

Contribution of the Δ(1232) to the (e, e'p) reaction along with Coulomb distortion effects

The Δ-resonance contribution has been included in the (e, e'p) reaction along with Coulomb distortion effects. We treat the resonance via a non-relativistic Δ current operator and use a Dirac Hartree single-particle model for the ground-state single-particle wave function and a relativistic optical model for the knocked-out proton wave function. It is assumed that the π-meson created by the virtual photon is absorbed in the target nucleus following the production of a Δ-resonance. Our DWBA calculation shows that the Δ-resonance contribution to the (e, e'p) reaction cross-section is 10-15% for an energy of 250 MeV transfered to the proton knocked out of the s-shell of ⁴⁰Ca, in the parallel and perpendicular kinematics. Received: 21 July 2001 / Accepted: 21 March 2002     

Low-energy weak-interaction studies

An overview is given of the present status of low-energy tests of the Standard Model in nuclear beta-decay and neutron decay, covering the unitarity problem, searches for right-handed currents, scalar- and tensor-type currents, tests of time-reversal violation, as well as experiments to set the neutrino mass scale. In view of the large amount of ongoing and planned experiments in this sector, many new results can be expected in the coming decade. Received: 21 March 2002 / Accepted: 16 May 2002 / Published online: 31 October 2002 RID="a" ID="a"e-mail: nathal.severijns@fys.kuleuven.ac.be     

Perturbative analysis of the 2νββ decays of <Superscript>100</Superscript>Mo and <Superscript>116</Superscript>Cd

We have performed a theoretical analysis of the ground-state-to-ground-state transitions in ¹⁰⁰Mo and ¹¹⁶Cd, based on the quasiparticle random-phase approximation and on a straightforward perturbative scheme. The results show that the single-state dominance found in the realistic calculations of the nuclear matrix elements, which is consistent with data, can be viewed as a result of the interference between few two-quasiparticle configurations. Received: 12 August 2002 / Accepted: 23 October 2002 / Published online: 18 February 2003 RID="a" ID="a"e-mail: civitarese@fisica.unlp.edu.ar Communicated by V.V. Anisovich     

The neutron and proton two-particle nucleus 134Sb: New low-spin states observed in the decay of 134Sn and an estimate of the energy of the 7- isomer

Excited states in the ¹³⁴Sb nucleus, populated in the β^--decay of ¹³⁴Sn, have been studied at the mass separator OSIRIS. The ¹³⁴Sn activity was produced via fast neutron-induced fission of ²³⁸U target. A main result was the discovery of a very low-lying first-excited state of ¹³⁴Sb, at 13keV, which has led to a strong revision of the level scheme. The new results are compared with different theoretical calculations and with the known data for the analogous neutron and proton two-particle nucleus in the ²⁰⁸Pb region. On the basis of this comparison, the energy of the ( πg _7/2νf _7/2)_7- isomer is estimated to be about 250keV, some 100keV lower than previously reported. Received: 21 March 2002 / Accepted: 16 May 2002 / Published online: 31 October 2002 RID="a" ID="a"e-mail: anovak@fuw.edu.pl     

Identification of the new isotope 241Bk

A new neutron-deficient berkelium isotope ²⁴¹Bk produced in the ²³⁹Pu(⁶Li, 4n) reaction has been identified using a gas-jet coupled on-line isotope separator. Cm K and L X-rays associated with the EC decay of ²⁴¹Bk were observed in the mass-241 fraction, and three γ transitions were attributed to the EC decay of ²⁴¹Bk through X-γ coincidences. The half-life of ²⁴¹Bk was determined to be 4.6±0.4 min which is 1/2-1/4 of that of theoretical predictions. The half-life value and the observed γ transitions can be consistently explained as a consequence of the allowed EC transition of π7/2⁺[633] → ν7/2⁺[624]. Received: 14 October 2002 / Accepted: 6 November 2002 / Published online: 17 January 2003 RID="a" ID="a"e-mail: asai@tandem.tokai.jaeri.go.jp Communicated by J. ?yst?     

High-precision measurement of the 14O half-life using a mass-separated radioactive beam

A high-precision measurement of the ¹⁴O half-life has been performed using a mass-separated radioactive beam in combination with a germanium detector set-up. This is the first ¹⁴O half-life measurement with a contamination-free source. The final result of 70.560 ± 0.049 seconds is in agreement with the generally adopted mean value. Received: 5 April 2001 / Accepted: 6 August 2001     

New region of deformation in the neutron-rich 60 24Cr36 and 62 24Cr38

The neutron-rich nuclei ₂₃ ^60-63V have been produced at GANIL via interactions of a 61.8A·MeV ⁷⁶Ge beam with a ⁵⁸Ni target. Beta-decay to ₂₄ ^60-63Cr has been investigated using combined β- and γ-ray spectroscopy. Half-lives of the ^60-63V nuclei have been determined, and the existence of a beta-decay isomer in the ⁶⁰V nucleus is strongly supported. The observation of low-energy 2⁺ states in ⁶⁰Cr (646keV) and ⁶²Cr (446keV) suggests that these isotopes are strongly deformed with β₂ ∼ 0.3. This is confirmed by shell model calculations which show the dominant influence of the intruder g and d orbitals to obtain low 2⁺ energies in the neutron-rich Cr isotopes. Received: 13 June 2002 / Accepted: 27 August 2002 / Published online: 17 January 2003 RID="a" ID="a"e-mail: sorlin@ipno.in2p3.fr Communicated by D. Guerreau     

Neutron interactions as seen by a segmented germanium detector

The GERmanium Detector Array (GERDA) is designed for the search for “neutrinoless double-beta decay” ( 0ν2β with germanium detectors enriched in ⁷⁶Ge . An 18-fold-segmented prototype detector for GERDA Phase II was exposed to an AmBe neutron source to improve the understanding of neutron-induced backgrounds. Neutron interactions with the germanium isotopes themselves and in the surrounding materials were studied. Segment information is used to identify neutron-induced peaks in the recorded energy spectra. The Geant4-based simulation package MaGe is used to simulate the experiment. Though many photon peaks from germanium isotopes excited by neutrons are correctly described by Geant4, some physics processes were identified as being incorrectly treated or even missing.     

Analysis of decay data from neutron-rich nuclei

The β-decays of the neutron-rich nuclei ¹²Be and ²⁹Ne have been studied. The statistical correlations between the almost identical half-lives of ¹²Be and its daughter ¹²B are analysed for a large sample of ¹²Be decay data. Stringent mutual bounds are obtained on the parameter set, leading to a precise determination of the ¹²Be half-life of 21.50±0.04 ms. From a simultaneous detection of β-particles and neutrons from the decay of ²⁹Ne the neutron emission probability, P_n, is determined to 17±5%. No indication of two-neutron emission is seen from this nucleus. An upper limit of 2.2% (90% confidence level) is established for P_2n. Received: 3 May 2001 / Accepted: 6 August 2001     

Weak-interaction-mediated rates on iron isotopes for presupernova evolution of massive stars

During the presupernova evolution of massive stars, the isotopes of iron, ^{54, 55, 56}Fe , are advocated to play a key role inside the cores primarily decreasing the electron-to-baryon ratio (Y_e) mainly via electron capture processes thereby reducing the pressure support. Electron decay and positron capture on ⁵⁵Fe , on the other hand, also have a consequential role in increasing the lepton ratio during the silicon burning phases of massive stars. The neutrinos and antineutrinos produced, as a result of these weak-interaction reactions, are transparent to the stellar matter and assist in cooling the core thereby reducing the entropy. The structure of the presupernova star is altered both by the changes in Y_e and the entropy of the core material. The aim of this paper is to report the improved microscopic calculation of Gamow-Teller (GT_±) strength distributions of these key isotopes of iron using the pn-QRPA theory. The main improvement comes from the incorporation of experimental deformation values for these nuclei. Additionally six different weak-interaction rates, namely electron and positron capture, electron and positron decay, and, neutrino and antineutrino cooling rates, were also calculated in presupernova matter. The calculated electron capture and neutrino cooling rates due to isotopes of iron are in good agreement with the large-scale shell model (LSSM) results. The calculated beta decay rates, however, are suppressed by three to five orders of magnitude.