Nuclear structure calculation ofβ + β +,β +/EC and EC/EC decay matrix elements

Nuclear matrix elements for double positron emisson ( ^{+ +}), positron emission/electron capture ( ⁺/ EC) and double electron capture (EC/EC) in the 2 decay mode and for ^{+ +} and ⁺/EC decay in the 0 mode are calculated for the experimentally most promising isotopes⁵⁸Ni,⁷⁸Kr,⁹⁶Ru,¹⁰⁶Cd,¹²⁴Xe,¹³⁰Ba and¹³⁶Ce within pn-QRPA. We point out that the matrix element for the 2 ⁺/EC decay differs from the 2 ^{+ +} matrix element, an effect not considered previously. For the neutrino accompanied decays our calculation predicts for the ⁺/EC and the EC/EC mode half lives which are shorter typically by 4–7 orders of magnitude than those for the double positron emission. However, even for the best candidates typical values for 2 ⁺/EC (2 EC/EC) are still in the range of 10²² ((some) 10²¹) years. For 0 decay we have calculated all matrix elements relevant for both, the mass mechanism and the right-handed currents for the first time complete. A detailed discussion of the differences between the 0 ⁺⁺, the 0 ⁺/EC and 0 ^{– –} decay is given.This work is supported in part by a Grant-in-Aid for Scientific Research (05243204) from the Ministry of Education, Science and Culture. One of us (M.H.) would like to thank the Japanese Ministry of Education, Science and Culture (Monbusho) for financial support. He also acknowledges valuable discussions with S.S.     

(EC+β +) decay of 130Nd and 140Tb

The available astrophysical S(E) factor data for the reaction ⁷Li (p,α)α at 10 < E < 1000keV exhibit an exponential increase at low energies due to the effects of electron screening. A parametrisation of the data using a non-resonant, direct process and two subthreshold resonances reproduces the data at energies E≥ 100keV, while at lower energies this calculated S _b(E) factor curve for bare nuclides drops below the data, which in turn represent the case of electron-shielded nuclides, i.e. the electron-shielded S _s(E) factor. The comparison between S _b(E) and S _s(E) leads to an electron-screening potential energy U _e = 350eV, which is much higher than the adiabatic limit of 175eV and not understood at present. The deduced value of S _b(0) is considerably smaller than the previously adopted value of 59keV b, significantly increasing the calculated abundance of ⁷Li in big-bang nucleosynthesis. The Trojan-horse method was applied to the reaction ⁷Li (p,α)α to determine the energy dependence of the S _b(E) factor for 10 < E < 370keV, free from the effects of the Coulomb barrier and electron screening. The THM results are close to the calculated S _b(E) curve and suggest that the THM may become a powerful way to obtain improved information on low-energy cross-sections and associated electron-screening effects in a model-independent way. Received: 9 September 2000 / Accepted: 5 December 2000     

The Β decay of the new isotope79Y

The neutron deficient isotope⁷⁹Y has been firstly identified and the decay properties were studied from-decay measurements in the reaction of⁵⁴Fe(²⁸Si,p2n at 92 MeV. Three rays of 152.5, 177.4, and 1106 keV were observed concerning the decay. The half life and Q_EC value were determined to be 14.9 ± 0.6 s and 7.12 ± 0.45 MeV, respectively. The decay scheme including spin-parity is discussed, and 5/2⁺ assignment for the ground state is proposed.We would like to thank to Dr. Y. Maeda for his valuable advice about the electroplating method of iron targets. We also thank to Mr. T. Maeda and Mr. S. Niiya for preparation of hardware for data-taking.     

EC/β+ decay of six medium-heavy nuclei

Previous experimental results of (EC+β+) decay for the medium-heavy nuclei reported by our group since 1996, including 153Er, 157Yb, 209Fr, 128Ce, 130Ce, and 128Pr have been briefly summarized. The observed low-lying states in their daughter nuclei have been reviewed in a systematic way and compared with different model calculations. Finally, some questions have been put forward for further study and discussion.     

Efficiency calibration of neutron detector array for β decay of exotic nuclei

The research of the in-beam efficiency calibration of Neutron Detector Array of Peking University using ¹⁷N and ¹⁶C beams was introduced in this paper. The efficiency of neutron wall and ball are comparable to the foreign similar devices and neutrons can be detected from low to high energies in high efficiency.     

Half-lives and fine structure for the α decay of deformed even-even nuclei

The α-decay properties of well-deformed even-even nuclei are systematically calculated within the multichannel cluster model (MCCM). Instead of working in the WKB framework, the quasibound solution to the coupled Schro¨dinger equation is presented with outgoing wave boundary conditions, and the coupling potential is taken into full account in terms of the general quantum theories. The calculated α-decay half-lives are found to agree well with the experimental data with a mean factor of less than 2. The fine...     

Investigation of very neutron deficient Ru and Rh nuclei with the help of (40Ca;yp+zαγ) reactions

⁴⁰Ca beams from the Munich tandem and from the Munich heavy ion postaccelerator have been used to produce very neutron deficient Ru and Rh nuclei with⁵⁸Ni (⁴⁰Ca,yp+zαγ) reactions. The nuclei⁹²Ru,⁹⁴Ru and⁹⁵Rh have been studied with in-beam γ ray spectroscopy consisting of the measurement of excitation functions, γ γ coincidences, γ ray angular distributions and life times with the recoil distance method. The following new level energies, spin parity values and half-lives have been deduced:⁹²Ru: 865.3 keV 2⁺; 1,855.8 (4⁺); 2,672.7 (6⁺); 2,834.6 (8⁺) 100±14 ns; 3,640.3 (10⁺), 4,206.1 (12⁺)⁹⁴Ru: 2,624.7 (5^?) 530 _?200 ⁺⁵⁰⁰ ps; 3,991.7 (10⁺); 4,197.9 (9^?); 4,489.3 (11^?) 780±120 ps; 4,717.1 (12⁺) 35±3ps; 5,568.4 (13^?); 5.826.8 (14^?); 6,360.3 (14^?); 6,614.7 (13⁺); 7,158.3 (14⁺); 7,774.1 (15); 8,412.1 (16); 9,042.2 (17)⁹⁵Rh: 1,350.7 (13/2⁺); 2,067.2 (17/2⁺); 2,236.2 (17/2^?) 15±5ns; 2,449.0 (21/2⁺) 2.1±0.3 ns; 3,241.2 (21/2^?); 3,724.1 (25/2⁺); 3.908.9 (25/2^?); 4,307.2 (27/2⁺); 4,457.9; 4,716.9; 5,063.2; 6,797.3; 7,627.4 The residual activities have been studied with multi spectra analysis'. The decay scheme of⁹⁴Rh(25±2 s) has been established.     

In-beamγ-ray spectroscopy of the neutron deficient100Ag

The neutron deficient nucleus¹⁰⁰Ag, three proton holes below and three neutron particles above the N=Z=50 shell closure at¹⁰⁰Sn, has been identified and studied by in-beam spectroscopy. The reactions⁴⁶Ti(⁵⁸Ni,3pn) at 231 MeV and⁶⁴Zn(⁴⁰Ca,3pn) at 167 MeV of the respective⁵⁸Ni and⁴⁰Ca beams were used, and states up to 8.7 MeV excitation energy and spin I?20 were found in two mainγ-ray cascades forming band-like structures of even and odd parity. Large scale shell model calculations suggest predominantπg ₉ ^2/?3 ν(d_5/2,g_7/2)³ andπg ₉ ^2/?3 ν(d_5/2,g_7/2)²h_11/2 structures with maximum spins I ^π = 19⁺ and I ^π =22^?, respectively, for the two level sequences. The influence ofπp_1/2 vs.νh_11/2 excitations is discussed for low lying odd-parity levels.     

EC/β+ decay of six medium-heavy nuclei

Previous experimental results of (EC+β⁺) decay for the medium-heavy nuclei reported by our group since 1996, including ¹⁵³Er, ¹⁵⁷Yb, ²⁰⁹Fr, ¹²⁸Ce, ¹³⁰Ce, and ¹²⁸Pr have been briefly summarized. The observed low-lying states in their daughter nuclei have been reviewed in a systematic way and compared with different model calculations. Finally, some questions have been put forward for further study and discussion.     

The decay of the neutron–rich nucleus 216Bi

The decay of the neutron–rich isotope ²¹⁶Bi, produced by proton–induced spallation at the PS Booster–ISOLDE facility, was investigated by β-γγ, αγ coincidence and spectrum-multiscaling measurements. A new method for reducing isobaric contamination enabled to cover the unknown region “east” of ²⁰⁸Pb for the isobaric chain A=216. The half-life of the β decay of ²¹⁶Bi was found as T_1/2= 135 ± 5 s. Its decay scheme was extended and the possible shell model configurations are proposed. Received: 13 July 1999 / Revised version: 22 September 1999     

Considering collective degrees of freedom in the β decay of odd nuclei

The β decay of around 30 odd nuclei was calculated within the dynamic collective model using expressions for the reduced matrix elements of the weak-interaction Hamiltonian for U-type and V-type β decay obtained earlier by the authors. All of the many-phonon states of the yrast band of an even-even core contributing to the formation of the excited states of nuclei were taken into account. For β transitions with an intensity of >1%, the calculated logft values differ from the experimental values by no more than 0.2.     

Laser ionization in β decay studies of Zn and Mn nuclei

Resonance ionization laser ion source (RILIS) technique has been used in the β-decay studies of ⁵⁹Mn and ⁵⁸Zn. The importance of the RILIS for production of these elements is discussed. The properties of the low-lying levels of the studied nuclei are discussed. This revised version was published online in August 2006 with corrections to the Cover Date.     

New search for 0νEC/EC and 2νEC/EC decay of 106Cd

The studies aimed at decreasing the background of the TGV-2 spectrometer have led to constructive changes in the cryostat and increased the sensitivity of the 32-detector spectrometer. New search for 0νEC/EC, 2νEC/EC) decays of ¹⁰⁶Cd was started with ～13.6 g of ¹⁰⁶Cd with enrichment of 75%. The evaluation of the experimental data accumulated for 4800 h was performed. Limits (at 90% CL) on the half-lives of 0νEC/EC resonant decay of ¹⁰⁶Cd to the 2741 keV excited state of ¹⁰⁶Pd (T _1/2 ≥ 6.5 × 10¹⁹ y) and on t 2νEC/EC decay to the ground state of ¹⁰⁶Pd (0⁺ → 0⁺, g.s.) (T _1/2 ≥ 1.7 × 10²⁰ y) were obtained. The limits on the other branches of ¹⁰⁶Cd decay—2νEC/EC decay to the 2⁺, 512 keV and 0 ₁ ⁺ , 1334 keV excited states of ¹⁰⁶Pd and 2νβ⁺β⁺ and 2νβ⁺ EC decays of ¹⁰⁶Cd to the ground and excited states of ¹⁰⁶Pd were improved.     

Spontaneous fission and alpha-decay properties of neutron deficient isotopes 257−253104 and 258106

Spontaneous fission and α-decay of^253?257104 and ²⁵⁸106 were investigated in irradiations of ^204,206,208Pb with ⁵⁰Ti and ²⁰⁹Bi with ⁵¹V, respectively. New spontaneous fission activities were identified and assigned to ²⁵³104, ²⁵⁴104, and ²⁵⁸106. The half-lives were measured as T_1/2 = (48 _?10 ⁺¹⁴ ) μs for ²⁵³104, T_1/2 = (23 ± 3) μs for ²⁵⁴104, and T_1/2 = (2.9 _?0.7 ^+1.3 ) ms for ²⁵⁸106. No indication for α-decay of any of these isotopes was found. For the α-decay branching ratios bα limits corresponding to bα ≤ 0.1 for ²⁵³104, bα ≤ 0.015 for ²⁵⁴104, and bα ≤0.2 for ²⁵⁸106 were obtained. These results prove a reduced fission probability for nuclei with neutron numbers N=152 up to Z=106 and a steep decrease of the fission half-lives for neutron numbers N < 152 up to element 104. α-decay data of ²⁵⁷104 and ²⁵⁵104 have been improved. An isomeric state decaying by α-emission was identified in ²⁵⁷104 and attributed to a low lying 11/2- [725] state. A small α-decay branch for the even — even nucleus ²⁵⁶104, indicated in an earlier experiment at SHIP, was confirmed, allowing a better founded extrapolation of experimental masses for even — even nuclei up to ²⁶⁴Hs (Z=108), the heaviest even — even nucleus identified so far.     

Study of the Β decay of6He with allowance for second-class currents and the antineutrino mass

The effects of the rest mass of the electron antineutrino are studied with allowance for the induced tensor form-factor F_c of the second-class current according to the Weinberg classification during the decay of the⁶He nucleus. We calculate the contribution of m and F_c to the electron-antineutron angular correlation coefficient A_e and the degree of longitudinal polarization P_e of the decay electrons. When m is nonzero, the coefficient A_e tends asymptotically to zero both at the beginning and endpoint of the spectrum, whether or not the second-class currents are taken into account. The series of qualitatively new experiments planned for the near future to measure the e- correlation for pure Gamow-Teller transitions (the⁶He nucleus could be a suitable nucleus here) with an accuracy of the order of 1% would allow the available estimate of the form-factor F_c to be improved substantially.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 9, pp. 62–67. September, 1989.     

Influence of strong magnetic field on β decay in the crusts of neutron stars

βdecay in the strong magnetic field of the crusts of neutron stars is analysed by an improved method. The reactions ⁶⁷ Ni(β^-)⁶⁷ Cu and ⁶² Mn\beta ^-62 Fe are investigated as examples. The results show that a weak magnetic field has little effect on βdecay but a strong magnetic field (B>10¹²G) increases β decay rates obviously. The conclusion derived may be crucial to the research of late evolution of neutron stars and nucleosynthesis in r-process.     

Measurement of the decay rate and the parameter αK* of the decay KL → μμγ

The rate of the decay of the neutral K meson into K _L ⁰ → μμγ has been measured with the NA48 detector at the CERN SPS. A total of 59 events has been observed with an estimated background of 14 events. The overall Kaon flux was determined to be 1.2 × 10⁹. This observation corresponds to a branching ratio of (3.4 ± 0.6_stat ± 0.4_sys × 10^-7. Using this branching ratio the parameter α_K* describing the relative strength of the two contributing amplitudes to this decay, through intermediate pseudoscalar or vectorlike mesons, was measured to be α_K* = -0.04+ _-0.21 ^+0.24 .     

Method of detecting rare superheavy nuclei decay events—the active correlation technique

Recently, a successful cycle of experiments on the synthesis of superheavy elements with Z = 112–118 has been accomplished with ⁴⁸Ca beam at the Flerov Laboratory of Nuclear Reactions (FLNR), JINR. From the viewpoint of detection of rare decay and background suppression this success was achieved owing to a new radical technique, the method of active correlations. It employs the search in a real-time mode for a pointer to probable correlation like recoil-alpha for switching the beam off. In the case of detection in the same detector strip an additional alpha-decay event of the “beam OFF” time interval is automatically prolonged. Reasonable scenarios of developing the method are considered.     

Candidate of the member of three-phonon multiplet observed in the β + +EC decay of 76Br

The decay scheme of ⁷⁶Br is established by the ⁷⁶Br(T _1/2?=?16.2 h) β ^?+?/EC study. The 1791.31 keV level, which was identified initially by Müller et al with ⁷⁶Se(p, p ^′) reaction more than three decades ago, is reconfirmed. However, the spin and parity have not been assigned by Müller et al. Now we consider it reasonable to assign the 1791.31 keV level as a new candidate of the three-phonon multiplet in ⁷⁶Se. In addition, in order to study the deformation of ⁷⁶Se which lies in transitional region between strongly deformed and spherical nuclei, we calculate the total Routhian surfaces (TRS) of positive-parity states by cranking shell model formalism.