Beta decay of the proton-rich nuclei 102Sn and 104Sn

The β decays of ¹⁰²Sn and ¹⁰⁴Sn were studied by using high-resolution germanium detectors as well as a Total Absorption Spectrometer (TAS). For ¹⁰⁴Sn, with three new β-delayed γ-rays identified, the total Gamow-Teller strength (B_GT) value of 2.7(3) was obtained. For ¹⁰²Sn, the γ-γ coincidence data were collected for the first time, allowing us to considerably extend the decay scheme. This scheme was used to unfold the TAS data and to deduce a B_GT value of 4.2(8) for this decay. This result is compared to shell model predictions, yielding a hindrance factor of 3.6(7) in agreement with those obtained previously for ⁹⁸Cd and ¹⁰⁰In. Together with the latter two, ¹⁰²Sn completes the triplet of Z ⩽ 50, N ≥ 50 nuclei with two proton holes, one proton hole and one neutron particle, and two neutron particles with respect to the doubly magic ¹⁰⁰Sn core.     

Nonmesonic decay of the Λ-hyperon in hypernuclei produced by p + Au collisions

The β-delayed proton precursor ¹⁴⁹Yb was synthesized in the reaction ¹¹²Sn(⁴⁰Ca, 3n) and identified by using a proton-gamma coincidence measurements in combination with a helium-jet fast tape transport system. Its β-delayed proton spectrum was observed. The half-life of ¹⁴⁹Yb was determined to be 0.7±0.2 s. The 253, 101, and 365 keV γ transitions in ¹⁴⁷Dy, following β-delayed proton emission of ¹⁴⁸Ho decay, were reported for the first time. Received: 14 March 2001 / Accepted: 26 September 2001     

Gamow-Teller beta decay of 105Sn

The β-decay of ¹⁰⁵Sn was re-investigated at the ISOL facility of GSI Darmstadt by using a total absorption spectrometer. The experimental results include the half-life and β-delayed proton branching ratio of 32.7(5)s and 1.1(4) ^. 10^-4, respectively, and the contribution of electron capture to the ¹⁰⁵Sn decay of 0.420(35). The Gamow-Teller strength distribution peaking at 3.6MeV was measured, yielding a summed Gamow-Teller strength of 3.0(4). The latter data are discussed in comparison with shell-model predictions based on an empirical interaction.     

(EC + β<Superscript>+</Superscript>) decays of the 11/2<Superscript>-</Superscript> isomer and 1/2<Superscript>+</Superscript> ground state of <Superscript>143</Superscript>Dy

The 1/2⁺ ground state and a 11/2^- isomer of very neutron-deficient isotope ¹⁴³Dy were produced by irradiation of an enriched target of ¹⁰⁶Cd with ⁴⁰Ca and studied by using a helium-jet fast tape-transport system in combination with proton-γ, X-γ and γ-γ coincidence measurements. A simple ( EC + β⁺) decay scheme of ^143mDy with a half-life of 3.0(3) s and a tentative ( EC + β⁺) decay scheme of ^143gDy with a half-life of 5.6(10) s are proposed. As a by-product, the 347- and 545-keV γ transitions in ¹³⁸Sm following the β-delayed proton emission of ¹³⁹Gd decay and the 323-keV γ transition in ¹³⁹Eu following the β-delayed proton emission of ¹⁴⁰Tb decay could be observed for the first time. Received: 20 August 2002 / Accepted: 28 October 2002 / Published online: 11 February 2003 RID="a" ID="a"e-mail: xsw@ns.lzb.ac.cn Communicated by D. Schwalm     

(EC +β+) decay study of 128Ce

The (EC+β⁺) decay of ¹²⁸Ce was reinvestigated in the ¹¹⁶Sn (¹⁶O, 4n) reaction by using a helium-jet tape transport system. The close half-lives of ¹²⁸Ce and ¹²⁹Ce made it difficult to separate the decay γ rays of both nuclei apart. Instead of the excitation-function measurements, both reactions of ¹⁶O+¹¹⁶Sn and ¹⁶O+¹¹⁷Sn were carried out, and from their comparisons, the decay γ rays of ¹²⁸Ce were clearly told from those of ¹²⁹Ce. Based on X-γ and γ-γ coincidence measurements, a detailed (EC +β⁺) decay scheme of ¹²⁸Ce has been proposed, which revises the previous one. Received: 29 September 1999 / Revised version: 5 November 1999     

Decay properties of N ≃ Z nuclei

This review focusses on recent results obtained by using fusion-evaporation reactions for the production of N ≃ Z nuclei, the on-line mass separator of GSI for the preparation of the radioactive samples, and charged-particle and γ-ray detectors for performing decay spectroscopy. The experimental results on prompt and β-delayed disintegration modes are discussed in comparison with theoretical model predictions. Received: 21 March 2002 / Accepted: 16 May 2002 / Published online: 31 October 2002 RID="a" ID="a"e-mail: e.roeckl@gsi.de     

Beta decay of 94Pd and of the 71 s isomer of 94Rh

The β decay of ⁹⁴Pd and of the 71s isomer of ⁹⁴Rh was investigated by using total γ-ray absorption techniques. Several levels in ⁹⁴Rh are established, including a new low-lying isomer characterized by a half-life of 0.48(3)μs and a de-exciting transition of 55keV. E2 multipolarity is determined for this transition by measuring the intensities of its γ-rays and the characteristic X-rays from its electron conversion. On the basis of the measured reduced β-decay transition rates to known ⁹⁴Ru levels and shell model considerations, the spin-parity of the 71s and the 0.48μs isomers of ⁹⁴Rh is assigned to be (4⁺) and (2⁺), respectively. The β-decay strength distributions measured for ⁹⁴Pd and the 71s isomer of ⁹⁴Rh yield Q _EC values of 6700(320) and 9750(320)keV for these decays and give evidence for the population of those states below and above the magic N = 50 gap that belong to both components of the 0g spin-orbit doublet.     

On the unusual properties of the 282 keV state in <Superscript>135</Superscript>Sb

Recently the first excited state in ¹³⁵Sb has been observed at the unexpectedly low excitation energy of only 282keV and interpreted as mainly d _5/2 proton coupled to the ¹³⁴Sn core. Based on theoretical considerations it was suggested that its low excitation energy is related to a relative shift of the proton d _5/2 and g _7/2 orbits induced by the neutron excess. We have measured the lifetime of the 282keV state by the advanced time-delayed βγγ(t) method. The measured half-life, T _1/2 = 6.1(4)ns, yields exceptionally low limits of B(M1;5/2₁ ⁺→7/2₁ ⁺)≤3.0×10^-4 μ ² _N and B(E2;5/2₁ ⁺→7/2₁ ⁺)≤54e ² fm ⁴. These strongly hindered M1 and slow E2 transition rates are similar to those for the transition de-populating the first excited state at 405keV in ²¹¹Bi. Results of shell model calculations with realistic interactions are presented. The M1 decay rate was found to be extremely sensistive both to the wave function and to the M1 effective operator.     

Mirror decay of 75Sr

The β-decay of ⁷⁵Sr to its mirror nucleus ⁷⁵Rb was studied at the ISOLDE PSB facility at CERN by means of β-delayed γ and proton spectroscopy. The decay Q-value and β-delayed γ intensity were measured for the first time. These results, 10.60±0.22 MeV and 4.5^+1.9 _-0.7%, together with accurate measurements of the β-decay half-life and β-delayed proton branching ratio yielded the Gamow-Teller strength 0.35±0.05 for the mirror transition. Implications of the results on studies of deformation effects and on the path of the rapid proton capture process are discussed. Received: 26 September 2002 / Accepted: 30 October 2002 / Published online: 18 February 2003 RID="a" ID="a"e-mail: jussi.huikari@phys.jyu.lfi Communicated by D. Schwalm     

Study of the N = 50 major shell effect toward 78Ni at PARRNe⋆

The γ-ray de-excitations following the β-decay of ⁸³Ga and the β-n decay of ⁸⁴Ga have been studied. The radioactive species were produced using the PARRNe on-line mass-separator installed at the IPN Orsay Tandem accelerator. Two γ-lines were attributed to ⁸³Ge with the aid of β-γ and γ-γ coincidences. The Z identification of the γ-lines was provided by time analysis of a buid-up/decay cycle. The excited levels of ⁸³Ge can be explained by the coupling of the single neutron state ν2d _5/2 to the first 2⁺ excitation of the ⁸²Ge core.     

β-delayed proton decays and spin assignments for 133Sm and 149Yb

The proton-rich isotope ¹³³Sm was produced via the fusion evaporation reaction ⁴⁰Ca + ⁹⁶Ru. Its β-delayed proton decay was studied by p-γ coincidence in combination with a He-jet tape transport system, and half-lives, proton energy spectra, γ-transitions following the proton emission, as well as β-delayed proton branching ratios to the low-lying states in the grand-daughter nucleus were determined. Comparing the observed β-delayed proton branching ratios with statistical model calculations, the best agreement is found assuming that only one level with the spin of 3/2 in ¹³³Sm decays or two levels with the spins of 1/2 and 5/2 decay with similar half-lives. The configuration-constrained nuclear potential energy surfaces of ¹³³Sm were calculated using the Woods-Saxon-Strutinsky method, which suggests a 1/2^- ground state and a 5/2⁺ isomer with an excitation energy of 120keV. Therefore, the simple (EC+β⁺) decay scheme of ¹³³Sm in Eur. Phys. J. A 11, 277 (2001) has been revised. In addition, our previous experimental data on the β-delayed proton decay of ¹⁴⁹Yb reported in Eur. Phys. J. A 12, 1 (2001) was also analyzed using the same method. The spin-parity of ¹⁴⁹Yb is suggested to be 1/2^-.     

Search for β-delayed fission of the heavy neutron-rich isotope 230Ac

The ²³⁰Ra has been produced via multinucleon transfer reaction and dissipative fragmentation by 60 MeV/u ¹⁸O ion irradiation of ²³²Th targets. The radium was radiochemically separated from the mixture of thorium and reaction products. Thin Ra sources were prepared and exposed to the mica fission track detectors, and measured by a HPGe gamma detector. It is likely that the β-delayed fission of ²³⁰Ac was observed for the first time and the β-delayed fission probability of ²³⁰Ac was tentatively found to be (1.19±0.40)×10^-8. Received: 2 November 2000 / Accepted: 30 January 2001     

Decay of 1.643 h <Superscript>95</Superscript>Ru and its daughter's level structure

The decay of ⁹⁵Ru has been investigated by means of γ-ray spectroscopy. The ⁹⁵Ru nuclei were produced by the reaction ⁹²Mo( α, n) ⁹⁵Ru at a beam energy of 17MeV. High-purity Ge detectors have been used singly and in coincidence to study γ-rays in the decay of ⁹⁵Ru to ⁹⁵Tc. 132 γ-rays are reported, among them, energies and intensities for 127 transitions have been determined. A decay scheme of ⁹⁵Ru with 31 levels is proposed which accommodates 127 of these transitions. Spins and parities for three new levels are proposed from calculated log ft values, measured γ-ray branching ratios, and in-beam experiment results of the daughter nucleus ⁹⁵Tc. Combining with the high-spin states observed by in-beam γ-ray spectroscopy of previous decay works, the structure of the excited states of ⁹⁵Tc is discussed in the framework of the projected shell model.     

Ion detection from beta decay and two-body decay experiments with laser-cooled atoms

Localized and cold samples of atoms produced with laser cooling and trapping techniques are a powerful tool for nuclear β-decay experiments. Recently we have concentrated on measurements of the momentum of the daughter ion produced, which leads to a variety of new observables. Angular distributions of the recoils with respect to the nuclear spin in β ⁺ decay are sensitive to non-standard model interactions. Measurements of the momentum of monoenergetic recoils from either electron capture or isomer γ decay would make it possible to search for particles with masses of 10s of keV.      

The β-decay of 22Al

In an experiment performed at the LISE3 facility of GANIL, we studied the decay of ²²Al produced by the fragmentation of a ³⁶Ar primary beam. A β-decay half-life of T_1/2 = 91.1±0.5ms was measured. The β-delayed one- and two-proton emission as well as β-α and β-delayed γ-decays were measured and allowed us to establish a partial decay scheme for this nucleus. New levels were determined in the daughter nucleus ²²Mg. The comparison with model calculations strongly favours a spin-parity of I^π = 4⁺ for the ground state of ²²Al.     

First measurement of β-decay properties of the proton drip-line nucleus 60Ga

By using the fusion-evaporation reaction ²⁸Si(³⁶Ar,p3n) and spectroscopy of β-delayed γ-rays and charged particles on mass-separated sources, β-decay properties of the neutron-deficient isotope ⁶⁰Ga were studied for the first time. The half-life of ⁶⁰Ga was determined to be 70(15) ms, and, based on βγγ coincidences, the isobaric-analogue state in ⁶⁰Zn was identified at 4851.9(7) keV. A semiempirical proton separation energy value of 40(70) keV was deduced for ⁶⁰Ga. The experimental results on half-life, mass excess, proton separation energy, and structure of the ⁶⁰Zn daughter states are discussed in comparison with various model predictions, including large-scale shell model calculations. Received: 4 September 2001 / Accepted: 12 November 2001     

β-delayed proton decays of 93Pd and 92Rh

β-delayed proton precursors ⁹³Pd and ⁹²Rh were produced by the irradiation of ⁵⁸Ni with a ⁴⁰Ca beam, and identified using proton-gamma coincidence measurements in combination with a helium-jet fast tape transport system. The half-lives of ⁹³Pd and ⁹²Rh were determined to be 1.3(2), 3.0(8) s, respectively. The measured energy spectrum of β-delayed protons and the estimated relative branching ratios to the final states in the daughter nuclei were fitted by a statistic model calculation, and then the ground-state spin and parity of ⁹³Pd were assigned as 9/2^±. Received: 13 April 2001 / Accepted: 22 August 2001     

β-delayed proton decay of the proton drip-line nucleus142Ho in the rare-earth region

New β-delayed proton precursor¹⁴²Ho was produced via heavy ion induced reaction¹⁰⁶Cd(⁴⁰Ca, p3n), and identified for the first time by using a He-jet fast tape transport system in combination with “p-γ” coincidence measurements. The β-delayed proton decay of¹⁴²Ho was observed and its half life was determined to be (0.4±0.1) s. By fitting the experimental relative branching ratios to final states in the proton daughter nucleus¹⁴¹Tb and the energy spectrum of β-delayed protons with a statistical model calculation, the ground-state spin of¹⁴²Ho was assigned as 5, 6 or 7. Nuclear energy-potential-surface (EPS) calculations were performed using the Woods-Saxon Strutinsky method. The calculated results favored the assignments of 7⁻ to¹⁴²Ho. As the by-products, some γ-transitions in the proton daughter nuclei following the β-delayed proton decays of precursors¹³⁹Gd,¹⁴⁰Tb,¹⁴²Tb, and¹⁴³Dy were reported here for the first time.     

Beta decay of 61Ga

The β decay of ⁶¹Ga to its mirror nucleus ⁶¹Zn has been measured for the first time by using on-line mass separation and β-delayed gamma-ray spectroscopy. The observed decay strength to the ground state implies superallowed character in accordance with the systematics of the mirror decays in the sd and fp shell. The β feedings observed to four excited states in ⁶¹Zn are consistent with earlier spin-parity assignments based on in-beam experiments. The ground-state spin and parity for ⁶¹Ga were determined to be 3/2⁻. Received: 28 April 1998 / Revised version: 6 October 1998     

Decay studies of N ≈ Z nuclei from 75Sr to 102Sn

Neutron deficient nuclei near ¹⁰⁰Sn have been produced by fragmentation of a 1 ^. AGeV ¹¹²Sn beam. The fragments were separated, identified and stopped in a highly segmented silicon strip detector stack. This detector measured the total energy of emitted β⁺-particles. γ-radiation was measured with surrounding detectors. The half-lives for many nuclides have been determined for the first time and give important information for the following topics: For the heaviest particle-stable odd-odd nuclei ⁹⁰Rh, ⁹⁴Ag and ⁹⁸In we observed for the first time fast β-decays, compatible with superallowed Fermi transitions and confirmed such decays for ⁷⁸Y, ⁸²Nb and ⁸⁶Tc. We have also observed long-lived T = 0 states in some of these nuclei. We measured the half-lives of all rp-process waiting-point nuclei from ⁸⁰Zr up to ^{92, 93}Pd. In addition we find the proton drip line nucleus ⁷⁷Y to decay dominantly via β-decay. To study the Gamov-Teller strength in the β-decay near the doubly magic ¹⁰⁰Sn we measured the half-life, β- and γ-spectrum of ¹⁰²Sn. We propose a level scheme for the daughter nuclide ¹⁰²In and deduce the Gamov-Teller strength (B _GT = 4.0±0.6). This is one of the largest values known. Received: 21 March 2002 / Accepted: 16 May 2002 / Published online: 31 October 2002 RID="a" ID="a"e-mail: thomas.faestermann@ph.tum.de