H.V. Klapdor

Beta-decay half-lives of neutron-rich nuclei

Abstract Beta-decay half-lives have been calculated by a microscopic theory for nuclei with 6 ⩽ Z ⩽ 114 between the line of beta-stability and the neutron drip line. The calculated half-lives are independent of the present uncertainties of nuclear mass formulas. The accuracy of the predictions is considerably improved over that of an earlier prediction based on a “gross theory” of beta decay.

Beta-delayed fission and neutron emission: Consequences for the astrophysicalr-process and the age of the galaxy

Beta-delayed neutron emission and beta-delayed fission probabilities (Pn andPβ d j) were calculated for neutron-rich nuclei between the beta stability line and the neutron-drip line in the range 10≦Z≦100 and 75≦Z≦100, respectively. These results were obtained by applying recentβ-strength function calculations, fission barrier height predictions, and a neutron optical potential from infinite nuclear matter calculations.An area of ∼100% fission probability is predicted around Z=94,N=168 extending beyond the well-known island of spontaneous fission in that region.Astrophysicalr-process calculations were performed including the calculatedPn and Pβ d f values. This puts the method to determine the age of the Galaxy by the actinide chronometers on a reliable basis. An excellent overall agreement with the observedr-abundance distribution is obtained. The predictedr-process production ratios for the Chronometrie pairs232Th/238U,235U/238U and244Pu/238U result in an age of the Galaxy oftG=(20.8±42)×109a, which is by almost a factor two larger than earlier predictions by this method, but in accordance with recent astronomical observations from globular clusters.The predicted island of 100%β-delayed fission acts as a sink to ther-process with the consequence that no superheavy elements are produced in nature.

Calculation of double beta decay of 76Ge, 82Se, 128,130Te

Abstract Calculations of double beta decay rates of 76Ge, 82Se, 128,130Te, including Δh excitations and collective effects arising from spin-isospin and quadrupole-quadrupole forces are presented. The serious discrepancy between previous shell model calculations and experimental rates for 128,130Te is markedly reduced. The persisting discrepancy of a factor 10 for these nuclei and for 82Se could raise the question of a possible non-nuclear structure origin of the disagreement.

Calculation of double beta decay rates and the neutrino mass

Abstract Predictions for 2 v and 0 v double beta decay rates are given for all nuclei with A ⩾ 70, for which double beta decay is energetically allowed. These predictions are based on detailed nuclear structure studies of the beta strength distribution and replace earlier estimates basing mostly on phase space considerations. New and more stringent limits on the Majorana neutrino mass are deduced from existing double beta decay experiments. Since the collective effects arising from spin-isospin as well as quadrupole-quadrupole forces are found to lead to a strong reduction of the nuclear matrix elements for two-neutrino double beta decay, but to have only minor influence on the matrix elements M 0 v for the neutrinoless decay mode, the smaller limits for m v result mainly from the fact that the widely used scaling procedure underestimates the 0 v matrix elements. It is further discussed to what extent interference between different neutrinos affects the obtained mass limits.

Investigation of beta strength functions by neutron and gamma-ray spectroscopy

Abstract Neutron spectra from the decay of the β− delayed neutron precursors 55.6 sec 87Br, 24.5 sec 137I, 2.05 sec 85As and 1.71 sec 135Sb have been studied with high resolution 3He ionization chambers. By γ-ray measurements, the partial neutron emission probabilities to excited states in 84Se and 134Te have been determined independently. For the neutron-emitter nuclei 87Kr and 137Xe, from which neutron decay proceeds only to the ground states of the final nuclei, 86Kr and 136Xe, unique information on the shape of the β− strength function Sβ(E) is obtained. For the emitter nuclei 85Se and 135Te, correlations between neutron transition energies and differences in level energies in 84Se and 134Te, have defined some levels in 85Se and 135Te which are strongly fed by β− decay. It is shown that the experimental shape of the β− strength function cannot be explained by the widely used assumptions Sβ(E) = const or Sβ(E) ∼ ϱ(E) or by the “gross theory” of β-decay, but that it is the result of structures in the low-energy tail of the Gamow-Teller giant resonance (GTGR) expected from general nuclear structure considerations.

Nuclear Structure Effects on the Neutrinoless Double Beta Decay

The nuclear matrix elements of the 0ν ββ decay of76Ge,82Se,100Mo,128,130Te,136Xe and150Nd are calculated in the proton-neutron quasiparticle RPA with theG-matrix of the Paris potential. It is shown that the matrix elements are not sensitive to details of nuclear structure, in contrast to the 2ν ββ decay. We investigate effects of ground-state correlations and those of short-range correlations on the suppression of the nuclear matrix elements. We also discuss effective values of the neutrino mass which are deduced from experimental 0ν ββ decay half-lives.

Investigation of the level schemes of 73,75,77As via the (3He, d) reaction

Abstract The reactions 72, 74, 76 Ge( 3 He, d) were investigated at E lab = 23 MeV with a multigap and a Q3D magnetic spectrograph. Some 30 new levels up to E ∗ ≈ 4 MeV have been found. The level schemes of the odd As isotopes 73, 75, 77 As up to E ∗ ≈ 4 MeV seem to be rather independent of the neutron number. The good agreement of the low-lying level structure with the Coriolis-coupling model including a pairing force was verified and the vacancies of low-lying shell model states were extracted and compared with the simple pairing theory.

THE NEUTRINO MASS FROM DOUBLE BETA DECAY

Abstract New and more stringent limits for the mass of a Majorana neutrino are deduced from the existing experimental data on double beta decay by new calculations of the matrix elements for neutrinoless double beta decay.

Predictions of 2ν and 0ν double beta decay rates for nuclei with A ⩾ 70

Abstract Microscopic calculations of double beta ( ββ ) decay matrix elements are presented for the first time for all potential ββ decaying nuclei with A ⩾ 70. This replaces previous phase-space estimates for most of these nuclei. It is found that nuclear structure effects considerably influence the ββ decay rates. The best candidates for 2 ν as well as 0 ν ββ decay experiments are selected.

The beta strength function and the astrophysical site of ther-process

AbstractThe beta strength function has been calculated for ∼6000 nuclei between the line of beta stability and the neutron drip line. The calculations — performed by using a schematical Brown-Bolsterli model — yield more reliable beta decay half lives,Pn values andβ-delayed fission rates than the strongly oversimplified assumptions on Sβ used up to now in astrophysical calculations. The calculated beta rates are shown in this paper to be decisive in the discussion of the problem of the astrophysical site of ther-process, which is responsible for the production of the heavy elements in the universe. In particular we concentrate on the neutron capture processes during the explosive burning of He in massive stars, initiated by the outgoing shock wave from a supernova explosion. It is shown that as consequence of the revisedβ-decay rates explosive He-burning represents a very convincing alternative scenario to the classicalr-process, which avoids the presently existing problems of the latter:1.The computed relative abundances ofr-nuclei resemble on the average their solar system counterparts.2.The calculated abundance peaks essentially coincide in position with the observed ones.3.The new site of ther-process avoids the problems of overproduction of heavy elements and of the mass-cut. Our results are based on realistic stellar models and hydrodynamical explosion calculations which for the first time are applied here to the problem of heavy element nucleosynthesis. The results turn out to be rather insensitive to the details of those models. The shorterβ-decay half lives obtained are of importance also in the investigation of further astrophysical sites producing heavy elements such as then-processes in explosive C or Ne burning.