Gamow -Teller Beta Decay in Drip Line Nuclei

Abstract: Before radioactive ion beams became available, the Gamow-Teller (GT) giant resonance was studied using charge-exchange reactions such as (p,n) and (He,t). In beta-stable nuclei, which can be used as targets, the GT giant resonance lies above the ground state of the mother nucleus. In this thesis the GT strength distribution is studied for deformed proton drip line nuclei with 28 < Z < 72. For these nuclei a large part of the GT strength can be reached through beta decay and one does not have the problem of subtracting the background, which is always present in charge-exchange reactions. Proton-rich nuclei play an important role in the rapid-proton capture process (rp-process). To understand the rp-process one would like to know several decay and capture rates, e.g. beta decay rates and proton capture rates, of proton-rich nuclei and nuclei at the proton drip line. The distribution of the GT beta strength is estimated for the oblate and the prolate Hartree-Fock (HF) local minima of even-even nuclei, using the quasiparticle Tamm-Dancoff approximation (TDA) based on a deformed HF+BCS calculation with Skyrme interaction. The Skyrme interactions SIII and SG2 are used in the HF ground state calculation. The eigenstates are expanded in a harmonic oscillator basis. Pairing correlation is treated in the BCS approximation with a given pairing gap parameter (1 MeV). In the region of 28 < Z < 44 the distribution often depends sensitively on the nuclear shape (namely, oblate or prolate). In this region the possibility of observing beta-delayed proton emission depends sensitively on the excess of Z over Z=N. The prolate local minima and the oblate local minima lie energetically close for these nuclei. In the region of 54 < Z < 72 almost all the GT strength lies below the ground state of the mother nuclei. The GT strength distribution is not sensitive to deformation. For the isotones N=74 and N=76 close to the proton drip line, the total GT strength is always larger for the prolate shape compared to the oblate shape. Approaching the proton drip line the total GT strength increases. The observation of the total GT strength by beta-delayed charged-particle(s) emission will be of essential importance.

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