M. Karny

Radioactive decays at limits of nuclear stability

The last decades brought an impressive progress in synthesizing and studying properties of nuclides located very far from the beta stability line. Among the most fundamental properties of such exotic nuclides, usually established first, is the half-life, possible radioactive decay modes, and their relative probabilities. When approaching limits of nuclear stability, new decay modes set in. First, beta decays become accompanied by emission of nucleons from highly excited states of daughter nuclei. Second, when the nucleon separation energy becomes negative, nucleons start to be emitted from the ground state. Here, we present a review of the decay modes occurring close to the limits of stability. The experimental methods used to produce, identify and detect new species and their radiation are discussed. The current theoretical understanding of these decay processes is overviewed. The theoretical description of the most recently discovered and most complex radioactive process - the two-proton radioactivity - is discussed in more detail.

NEW ISOTOPES AND ISOMERS PRODUCED BY THE FRAGMENTATION OF 238U AT 1000 MEV/NUCLEON

Abstract Seven previously unobserved neutron-rich isotopes ( 209 Hg, 210 Hg, 211 Tl, 212 Tl, 218 Bi, 219 Po and 220 Po) have been identified among the fragmentation products of a 1000 MeV/nucleon 238 U beam incident on a beryllium target. The γ -ray decays of 9 known and 4 new μ s-isomers in nuclei around 208 Pb were also observed. Production cross sections and probabilities for populating isomeric states in the fragmentation reaction were determined and compared with a simple model based on the statistical abrasion-ablation approach.

Superallowed Gamow-Teller decay of the doubly magic nucleus 100Sn

The shell structure of atomic nuclei is associated with ‘magic numbers’ and originates in the nearly independent motion of neutrons and protons in a mean potential generated by all nucleons. During β+-decay, a proton transforms into a neutron in a previously not fully occupied orbital, emitting a positron–neutrino pair with either parallel or antiparallel spins, in a Gamow–Teller or Fermi transition, respectively. The transition probability, or strength, of a Gamow–Teller transition depends sensitively on the underlying shell structure and is usually distributed among many states in the neighbouring nucleus. Here we report measurements of the half-life and decay energy for the decay of 100Sn, the heaviest doubly magic nucleus with equal numbers of protons and neutrons. In the β-decay of 100Sn, a large fraction of the strength is observable because of the large decay energy. We determine the largest Gamow–Teller strength so far measured in allowed nuclear β-decay, establishing the ‘superallowed’ nature of this Gamow–Teller transition. The large strength and the low-energy states in the daughter nucleus, 100In, are well reproduced by modern, large-scale shell model calculations.

Coupling a total absorption spectrometer to the GSI on-line mass separator

Abstract A total absorption spectrometer, constructed and briefly operated at the on-line isotope separator OASIS at Lawrence Berkeley Laboratory, has been moved and installed at the on-line mass separator of GSI Darmstadt. At GSI, the spectrometer has been used for β-decay strength function measurements of nuclei in the vicinity of 100 Sn and 146 Gd.

Pulse pileup correction of large NaI(Tl) total absorption spectra using the true pulse shape

Abstract The usefulness of a simple numerical correction method of electronic pulse pileup distortion in measured energy spectra is investigated. The algorithm only requires the knowledge of the true pulse shape. The accuracy of the correction achieved for total absorption gamma-ray spectroscopy measurements with NaI(Tl) detectors is demonstrated.

Monte Carlo simulation of the response of a large NaI(Tl)total absorption spectrometer for β-decay studies

Abstract The response of a large NaI(Tl) total absorption spectrometer for β-decay studies is investigated.The accuracy of Monte Carlo simulations taking into account the light yield non-proportionality of NaI(Tl) and the detailed geometry is assessed through comparison with spectra measured for several radioactive sources.

Beta decay of 103In: Evidence for the Gamow-Teller resonance near 100Sn

Abstract The β decay of the neutron-deficient isotope 103 In was investigated by using total absorption γ-ray spectrometry on mass-separated sources. The measurement reveals a high-lying resonance of the β-decay strength in striking disagreement with high-resolution γ-ray data. The result is discussed in comparison with shell-model predictions.

A new pulsed release method for element selective production of neutron rich isotopes near 208Pb

Abstract A new method to reduce the isobaric contamination problem for the production of neutron rich Bi, Pb and Tl nuclei at on-line mass separators, based on the pulsed release of these radioactive species, is presented. The results of a feasibility study are reported.

Gamow–Teller strength distribution near 100Sn. The beta decay of 102In

One of the main motivations of today’s β-decay studies is to investigate whether the presence of many bound nucleons affects the value of the Gamow—Teller (GT) axialvector coupling constant. The quest for 100Sn is driven by the hope that its particulary simple decay scheme may help to perform improved tests of the models used to calculate the summed GT strength (B GT). Since 100Sn can not yet be produced in sufficient quantity, the experiments are restricted to nearby nuclei. In this paper we report on a study of the β decay of 102In, performed at the GSI Online Mass Separator. This odd—odd nucleus has a very complicated decay scheme and thus represents indeed a great challenge. The loss of numerous weak transitions in high resolution mesurements is suspected to be one of the main reasons for incomplete B GT measurements. Therefore, the experiment with a cube-like array of 6 Euroball Cluster high-resolution germanium detectors (Euroball Cube), with 7 crystals per Cluster, was complemented by the measurement with a total-absorption gamma spectrometer (TAS) [1].

Decay properties of very neutron-deficient isotopes of silver and cadmium

Abstract The β-decay properties of the lightest known silver and cadmium isotopes are investigated. Data for β-delayed γ-ray emission of 95–97Ag and for β-delayed proton emission of 96Ag and 97Cd are presented. The experimental findings are compared with predictions of shell-model calculations in which a variety of model spaces and effective interactions are considered.