Lifetime measurements of excited states in the neutron-rich nucleus 47Ca

Chapter 1 Introduction In this thesis it will be discussed the lifetimes of some selected excited states of the neutron-rich nucleus 47 Ca, which is just one-neutron hole with respect to the doubly- magic nucleus 48 Ca. These states were populated via a multi-nucleon transfer reaction between an incident beam of 48 Ca, at a bombarding energy of 310 MeV, and a 208 Pb target. In order to measure the lifetimes of the excited states of interest from their γ-decay the Recoil Distance Doppler Shift method (RDDS) has been used. The RDDS methodconsists inplacing, afterathin target, anenergy degraderatavariable distance, and to measure with a γ-ray spectrometer, as function of such distance, the γ-rays emitted by the ions recoiling in vacuum, whose mass and atomic number is measured in a magnetic spectrometer. Depending whether a γ-ray, de-exciting a particular state, is emitted before or after the degrader, it exhibits different Doppler shifts. Each γ-ray transition is therefore splitted in two peaks corresponding to the two recoiling velocities. The relative intensities of the two peaks as a function of the target-degrader distance determinesthelifetimeofthestateofinterest. Theexperimenthasbeenperformedatthe Legnaro National Laboratories using the γ-ray spectrometer CLARA and the magnetic spectrometer PRISMA. In particular, I will discuss in detail the lifetimes of the 3999- (13/2 + , 11/2 + ), 2578- (3/2 + ) and 2599-keV (1/2 + ) excited states. The knowledge of the lifetimes of these states will allow to assign or give stringent limits to their spins and therefore to understand their character in terms of a shell-model description around the double shell-closure Z = 20 and N = 28. 1.1 Population of neutron-rich nuclei Themethodthathasbeenusedtopopulateneutron-richnucleiisamulti-nucleontransfer reaction [1] between an incident beam of 48 Ca, at a bombarding energy of 310 MeV [2], and a 208 Pb target. It consists onaninelastic peripheral collision where the two colliding nuclei transfer a certain number of neutrons and protons, in a very short interaction time (∼ 10 −22 ). The resulting products are separated, according to their mass, into 1