1,721,164 research outputs found
A pulse shape analysis algorithm for segmented HPGe detectors
No full textA simple Pulse Shape Analysis method for the determination of the
number of interactions and their radial localization inside one HPGe segmented
detector segment is presented. The algorithm processes only the
net charge signal using a fast comparison procedure with a basis of reference
signals. The efficiency of the algorithm calculated for 600 keV
-rays
ranges between 65 to 95% depending on the complexity of the analyzed
event. The algorithm has been applied to real signals acquired during an
in-beam test of the MARS 25-fold segmented HPGe detector
Analysis of Nuclear Lifetimes Using the Gamma-ray Induced Doppler Shift Attenuation Method
Full text linkLifetime measurements allow extraction of fundamental information on the nature of the excited states of a nuclear system. Since nuclear lifetimes cover many orders of magnitude, a number of experimental techniques and detection setups have been developed depending on the range of the lifetime of interest. The Gamma-ray Induced Doppler Shift Attenuation (GRIDSA) Method presented here is applied to the measurement of very short lifetimes, in the femtosecond range. It allows determining the nuclear lifetime by measuring the Doppler shift of a gamma ray emitted from the state of interest, in different directions with respect to a coincident preceding gamma ray, populating the same state and inducing a recoil of the nucleus in the target material with velocities of the order of 104-105 m/s. We realized an experiment in order to test the GRIDSA technique for the measurement of fs lifetimes after (n,γ) reactions. The measurement was performed at the Institut Laue-Langevin (ILL) with the 8 Ge-clover detectors of the FIPPS array. Preliminary results are discussed
HPGe segmented detectors in γ-ray spectroscopy experiments with exotic beams
No full textMost 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
Study of the pygmy dipole resonance in 124Sn with AGATA
Full text linkThe γ decay of the pygmy dipole resonance of 124Sn was measured with the AGATA demonstrator coupled to an array of 9 large volume LaBr3:Ce scintillators. This resonance was populated by the inelastic scattering reaction 17O+124Sn at 20 MeV/u. With AGATA, the γ decay up to the neutron separation energy was measured with high resolution. The angular distribution was measured both for the γ rays and the scattered 17O ions. The present results are presented in comparison with the previous findings for (γ, γ') and (α,α'γ)
Isospin character of low-lying pygmy dipole states probed via inelastic scattering of <sup>17</sup>O
No full textThe pygmy dipole states were populated in 208Pb and 124Sn by the inelastic scattering of a 17O beam at the energy of 20 MeV/u, and their subsequent gamma decay was measured with the AGATA demonstrator array. Differential cross sections as a function of the angle were measured. The results are compared with (γ, γ') data. For the dipole transitions, a form factor obtained by folding a microscopically calculated transition density was used. This has allowed us to extract the isoscalar component of the 1- excited states
Study of the γ decay of high-lying states in 208Pb via inelastic scattering of 17O ions
No full textHigh-lying states in 208Pb nucleus were populated via inelastic scattering of a 17O beam at bombarding energy of 20 MeV/u. Their subsequent gamma decay was measured with the detector system AGATA Demonstrator based on HPGe detectors, coupled to an array of large volume LaBr3:Ce scintillators. Preliminary results in comparison with (γ,γ′) data, for states in the 5–8 MeV energy interval, seem to indicate that in that region the states belong to two different groups one with a isoscalar character and the other with a isovector nature. This is similar to what was observed in other stable nuclei with (α,α′γ) experiments. The multipolarity of the observed gamma transitions is determined with remarkable sensitivity thanks to angular distribution measurements. Data aiming at studying the neutron decay of the Giant Quadrupole Resonance in the 208Pb by the high resolution measurement of the following gamma decay are also presented in their preliminary form
Gamma decay of pygmy states from inelastic scattering of ions
No full textAn overview of relevant results on the study of 1− states focusing on their excitation with nuclear probes is given. Results obtained for the 90Zr , 124Sn , and 208Pb nuclei using the (17O,17O′γ) reaction are compared with available data obtained with the (γ, γ′, and (p, p′) reactions. These comparisons allow to learn on the nature of the populated states, particularly the E1 states, whose isospin character is presently poorly known. The DWBA description of the data is discussed in terms of different form factors, standard collective form factor and form factors obtained by folding microscopically calculated transition densities. The relevant aspects related to the used theoretical approach are also presented. The main objective of the analyses is the extraction of the values of the fraction of the energy weighted sum rule strength for the isoscalar dipole excitation. For completeness, in all cases, the DWBA analysis was made also for the excitations of 2+ and 3− states
Low-lying 1− and 2+ states in 124Sn via inelastic scattering of 17O
No full textThe γ decay of low-lying 1−and 2+ states up to the neutron separation energy in 124Sn populate by the inelastic scattering of 17O was measured. The Angular distributions were measured both for the γ rays and the scattered 17O ions. The results are presented
Application of a Simple PSA Algorithm for HPGe Segmented Detectors to in-Beam Data
No full textA simple Pulse Shape Analysis (PSA) algorithm is applied to signals acquired during the in-beam tests of two different highly segmented HPGe detectors. The issue of the algorithm, named Recursive Subtraction (RS), consists in the determination of the number of interactions in a detector segment and their radial localization. Following a comparison of the results obtained applying RS to the acquired signals with those of specific GEANT simulations, excellent agreement is found between the experimental number of interactions distribution and the calculated one. Some deviations between the experimental radial distribution and the calculated one are instead present. Evidence is provided that this cannot be adduced to the algorithm but most probably to a non perfect reproduction of the detector position response in the calculated signals use
HPGe detectors timing using pulse shape analysis techniques
No full textIn this work the Pulse Shape Analysis has been used to improve the time resolution of High Purity Germanium (HPGe) detectors. A set of time aligned signals was acquired in a coincidence measurement using a coaxial HPGe and a cerium-doped lanthanum chloride (LaGl(3):Ce) scintillation detector. The analysis using a Constant Fraction Discriminator (CFD) time output versus the HPGe signal shape shows that time resolution ranges from 2 to 12 ns depending on the slope in the initial part of the signal. An optimization procedure of the CFD parameters gives the same final time resolution (8 ns) as the one achieved after a correction of the CFD output based on the current pulse maximum position. Finally, an algorithm based on Pulse Shape Analysis was applied to the experimental data and a time resolution between 3 and 4 ns was obtained, corresponding to a 50% improvement as compared with that given by standard CFDs
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