HPGe segmented detectors in γ-ray spectroscopy experiments with exotic beams

Most of the present knowledge of the nuclear structure is based on the properties of nuclei that lie in the valley of stability; it has become clear that it is in general not possible to extrapolate such knowledge to the region far off stability. Consequently, in order to obtain an overall comprehension of the nuclear force, it is mandatory to probe the structure of exotic nuclei. In recent years the availability of Radioactive Ion Beams (RIB) enabled the experimental study of nuclear systems far off stability and consequently gave the possibility to attach fundamental open problems in this research field. Gamma ray spectroscopy experiments with radioactive beams have to be performed in critical conditions due to low beam intensity, the presence of large background radiation and relevant Doppler effects. As a consequence it has become clear the need for developing a gamma detector array with a sensitivity much higher compared with the present generation Compton-suppressed detectors: a 4π gamma spectrometer, composed of highly segmented HPGe detectors and based on the concept of γ-ray tracking. In this thesis will first be described a Coulomb excitation measurement with the exotic 68Ni nucleus that presents all the challenging features of the experiments in which new generation tracking arrays are planned to operate. Such experiment has been carried out in April 2005 at RISING [63] set up in GSI laboratory. It consists of coulomb excitation of 68Ni at 600 MeV/u performed in order to study the low lying dipole strength by direct measurement of the gamma decay. The question how the giant dipole resonance strength evolves when going from stable to exotic nuclei is presently under discussion. It is in general expected by the calculations to find, in neutron rich medium heavy and heavy nuclei, a stronger fragmentation of the dipole strength (compared to stable nuclei) with significant components located in an energy domain well below that of the giant dipole resonance. In the literature such a lowlying concentration of dipole strength is usually denoted as Pygmy Dipole Resonance (PDR). The impact of that research activity goes even beyond nuclear structure field. In fact, since the energy dependence of the dipole strength directly affects the (γ,n) cross section, the presence of an increase in the low lying dipole strength could relevantly change our understanding and the description of the r-process. The results of the data analysis show the first evidence of such pygmy states in 68Ni; in addition it will appear clearly the need for a gamma detector array capable to provide an improved quality in Doppler correction and background rejection, namely a gamma ray tracking array. Recently the concept of γ ray tracking detectors has been developed and it has been shown that even a factor of 1000 in sensitivity over previous generation arrays could be gained using an array of detectors that enables the gamma ray path to be reconstructed. Reconstructing the trace of a γ ray inside the detector will permit a very precise Doppler correction since the position of the first interaction is obtained with a resolution of some mm, furthermore it will be possible to deduce the γ ray incoming direction and therefore distinguish the radiation of interest from the one not coming from the target, obtaining consequently much cleaner spectra. A second topic discussed in this thesis is the Pulse Shape Analysis in segmented HPGe detector for the gamma ray tracking. Pulse Shape Analysis (PSA) for determination of interactions position is a fundamental step in the functioning scheme of a gamma ray tracking array: the spatial localization of the interactions (hits) and their corresponding energetic release is basic information needed to reconstruct the path of a γ-ray inside an HPGe detector and it constitutes indeed the input of any tracking algorithm. Such information is encoded in the shape of the current pulse given by the detector following the interaction of γ radiation; in order to extract the spatial coordinates and energy of the γ ray interaction points specific PSA methods to process the detector signals have to be developed. In this thesis is described the Pulse Shape Analysis (PSA) process in highly segmented HPGe detectors for γ-ray tracking and a PSA algorithm for the decomposition of net-charge signal (Recursive Subtraction). The result of its extensive tests on simulated and real events are presented. The experimental data, on which the algorithm has been tested, were acquired during the in beam test of the MARS detector, performed at INFN Legnaro laboratories in July 2001 and during the in beam test of the AGATA symmetric cluster, performed at IKP Köln in August/September 2005. Finally in this thesis will be presented three ideas to exploit PSA techniques for applications that go also beyond the interaction localization (i.e. detector scan, improvement of timing performances of the detector). The first results are very encouraging but still there is the need for further development as will be pointed out in more detail.