Shape study of the N=Z waiting-point nucleus 72Kr via beta decay

The Ph.D. thesis entitled “Shape study of the N=Z waiting-point nucleus 72Kr via beta decay” is devoted to the study of the shape of the ground state of the 72Kr nucleus. It is an N=Z nucleus in the mass region A~70-80 where shape transitions and the shape coexistence phenomena have been identified. Furthermore, this nucleus participates in the rp-process as a waiting point due to the slowdown of the process taking place at the arrival to this nucleus. The study of the properties of this nucleus is interesting from the Nuclear Structure point of view, for the phenomena occurring in its mass region and have been predicted for it, and from the Nuclear Astrophysics for the accurate performance of astrophysical calculations. The β+/EC decay of the 72Kr nucleus has been studied through two complementary experiments at the ISOLDE facility at CERN in Geneva (Switzerland). In one of them, the low-spin structure of the daughter nucleus, 72Br, has been revised via conversion electron spectroscopy where the conversion coefficients of low-energy transitions have been experimentally determined for first time. Those coefficients allowed us to determine the multipolarity of the transitions and evaluate the spin and parity of connected levels when compared to theoretical predictions for each multipolarity of the transition. In the second experiment, the Total Absorption Spectroscopy (TAS) technique was used to measure the beta feeding distribution of our interest. This study is motivated for the possibility of deducing information on the ground state deformation of the parent nucleus through the comparison of experimental distribution of B(GT) with theoretical predictions. Mean field calculations performed in QRPA (“Quasi-particle Random Phase”) approximation were already used for neighbouring nuclei such as 76Sr, 74Kr or 78Sr providing successful results. The main results which have been extracted in the present study are the following: - The value of 14 conversion coefficients of low-energy transitions in 72Br not previously known has been determined. Furthermore, an upper limit for the value of other two coefficients has been established. - The spin and parity of the 72Br ground state has been restricted to 3 possible values: 0+, 1+ y 2+. The most likely one, but not exclusively, according to our discussion is 1+. This work rejects the previously suggested value of 3+. - The multipolarity of pure character of 5 transitions and the mixed character of other 4 transitions in 72Br has been established. The multipolarity of other 6 transitions has not been firmly determined but it has been restricted. - The β+ feeding distribution in the 72Kr decay has been determined via the TAS technique and from it the total beta feeding distribution, including the β+ and EC components, has been deduced and the B(GT) distribution up to an excitation energy in 72Br of 2640 keV. The uncertainty on this distribution includes systematic and statistical components which have been estimated during the analysis and they are discussed in detail. - The comparison of the accumulated B(GT) distribution with theoretical predictions suggests a dominantly oblate deformation for the 72Kr ground state. This conclusion constitutes the first evidence on the sign of the 72Kr ground state deformation as well as the first determination of an oblate deformed nucleus inferred from its beta decay.