Data di Pubblicazione:
2008
Citazione:
HPGe segmented detectors in γ-ray spectroscopy
experiments with exotic beams / F.c.l. Crespi ; F. Camera, R. Bonetti, G. Bellini. DIPARTIMENTO DI FISICA, 2008. 20. ciclo, Anno Accademico 2006/2007.
Abstract:
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 ba
Tipologia IRIS:
13 - Tesi di dottorato discussa entro ottobre 2010
Keywords:
gamma spectroscopy ; nuclear structure ; nuclear physics; radiation detection ; pygmy dipole resonance ; HPGe segmented detectors ; gamma ray tracking ; pulse shape analysis ; PSA
Elenco autori:
F.C.L. Crespi
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