Showing papers by "A. Bracco published in 2008"

A novel technique for the characterization of a HPGe detector response based on pulse shape comparison

Abstract: A novel technique for measuring a HPGe detector pulse shape as a function of the γ-ray interaction position inside the detector volume is presented. Exploiting a specific pulse shape comparison procedure, this technique allows to characterize the 3D position response of a HPGe segmented detector in a much shorter time as compared with the standard coincidence techniques. The method was first validated using a GEANT simulation of a 36-fold HPGe AGATA detector realized taking into account the effects of the electronic chain response and electrical noise on the calculated signal shape. This procedure was then applied to extract experimentally the position response of a non-segmented coaxial HPGe detector along the radial direction, using a 438 MBq 137Cs collimated γ-source. The results of this measurement show a dependence of the pulse shape as a function of γ-ray interaction radial coordinate consistent with that obtained with calculations. The signal acquisition rate reached using this characterization technique allows to realize a full scan of a large volume highly segmented HPGe detector in less than a week.

New sub-us Isomers in 125Sn, 127Sn, 129Sn and Isomer Systematics of 124-130Sn

Abstract: New sub-mu s isomers have been observed in the neutron-rich Sn isotopes. Sn-125,Sn-127,Sn-129 nuclei have been produced in a relativistic fission reaction of U-238 on a Be-9 target at 750 A.MeV and by the fragmentation of Xe-136 at 600 A.MeV populating high-spin yrast states. In addition to the already known mu s isomers, three new ones with sub-mu s half-lives have been observed. These yrast isomers are the high-spin members of the nu(d(3/2)(-1)h(11/2)(-2)) and nu h(11/2)(-n), seniority v = 3 multiplets leading to isomeric (23/2(+)) and (27/2(-)) states, respectively. Added to the already known 19/2(+)mu s isomers in this region the current work completes the systematic information of neutron-hole excitations toward the filling of the last h(11/2) orbital at N = 82. The results are discussed in the framework of state-of-the-art shell-model calculations using realistic interactions.

Resolution, efficiency and stability of HPGe detector operating in a magnetic field at various gamma-ray energies

Abstract: The use of High Purity Germanium detectors (HPGe) has been planned in some future experiments of hadronic physics. The crystals will be located close to large spectrometers where the magnetic fringing field will not be negligible and their performances might change. Moreover high precision is required in these experiments. The contribution of magnetic field presence and long term measurements is unique. In this paper the results of systematic measurements of the resolution, stability and efficiency of a crystal operating inside a magnetic field of 0.8 T, using radioactive sources in the energy range from 0.08 to 1.33 MeV, are reported. The measurements have been repeated during several months in order to test if any permanent damage occurred. The resolution at 1.117 and 1.332 MeV gamma-rays from a 60Co source has been measured at different magnetic fields in the range of 0–0.8 T and the results are compared with the previous data.

Probing the order-to-chaos region in superdeformed 151Tb and 196Pb nuclei with continuum gamma transitions.

Abstract: The {gamma} decay associated with the warm rotation of the superdeformed nuclei {sup 151}Tb and {sup 196}Pb has been measured with the EUROBALL IV array. Several independent quantities provide a stringent test of the population and decay dynamics in the superdeformed well. A Monte Carlo simulation of the {gamma} decay based on microscopic calculations gives remarkable agreement with the data only assuming a large enhancement of the B(E1) strength for 1-2 MeV {gamma} rays, which may be related to the evidence for octupole vibrations in both mass regions.

Probing the order-to-chaos region in superdeformed Tb151 and Pb196 nuclei with continuum γ transitions

Abstract: The γ decay associated with the warm rotation of the superdeformed nuclei 151 Tb and 196 Pb has been measured with the EUROBALL IV array. Several independent quantities provide a stringent test of the population and decay dynamics in the superdeformed well. A Monte Carlo simulation of the y decay based on microscopic calculations gives remarkable agreement with the data only assuming a large enhancement of the B(El) strength for 1-2 MeV y rays, which may be related to the evidence for octupole vibrations in both mass regions.

Transition to non-collective states at high spin in 124Xe

Abstract: Excited states in 124Xe were populated in the reaction 82Se(48Ca, 6n)124Xe and γ -ray coincidence relationships were measured with the Gammasphere spectrometer. Two new bands are observed and several of the previously known bands are extended in the high- as well as in the low-spin region. Two irregular high-spin structures are also added. The irregularities are a fingerprint of a transition from collective to non-collective behaviour. Configuration assignments to the new structures are proposed on the basis of systematics and by comparing experimental properties with calculations within the framework of the cranking model.

Extended investigation of superdeformed bands in 151,152 Tb nuclei

Abstract: A detailed study of known and new SD bands in Tb isotopes has been performed with the use of the EUROBALL IV gamma-ray array. The high-statistics data set has allowed for the extension of known SD bands at low and high spins by new gamma-ray transitions. These transitions, as it turns out, correspond to the rotational frequencies where the principal superdeformed gaps (Z=66,N=86) close giving rise to up- or down-bending mechanisms. This enables to attribute the underlying theoretical configurations with much higher confidence as compared to the previous identifications. Five new SD bands have been discovered, three of them assigned to the Tb-152 and the two others to the Tb-151 nuclei. Nuclear mean-field calculations have been used to interpret the structure of known SD bands as well as of the new ones in terms of nucleonic configurations.

Discrete decay of the yrast superdeformed band in the Tb151 nucleus

Abstract: The Euroball array has been used to search for linking transitions between the superdeformed (SD) and the normal deformed (ND) wells in Tb-151. Many. rays in the energy range 2-4 MeV have been observed in coincidence with the yrast SD band. It is proposed that the highest energy transition of 3748 keV and the strongest line (2818 keV) of the previously observed transitions both decay from the same SD level as their links with the ND states have been identified. The current spectra have insufficient statistics to completely identify the decay point in the SD band. Theoretical calculations covering SD bands in the A approximate to 150 region enable the two possible spin assignments to be compared with experimental data on proposed links in Gd-149 and Dy-152. It is concluded that the energy of the lowest SD band member in Tb-151 has an excitation energy of 12861 keV and a spin of 65/2(+).

Nuclear Spectroscopy near the proton drip-line in the Lanthanide region:the 122La nucleus

Abstract: One of the most exciting subjects in contemporary nuclear physics is the study of nuclei at the limits of stability with respect to particle emission. Recently, there has been an intensive experimental activity in measuring proton decay and a large variety of proton emitters were observed in the region of nuclei with 50 < Z < 82. Very recently, the proton radioactivity from 117La and 121Pr has been identified and the decay rate deviates significantly from calculations assuming spherical configurations, thus indicating the onset of large deformations in the drip line nuclei below Z = 69. Predictions by the microscopicmacroscopicmass model and by relativistic mean field calculations support these experimental results and suggest the position of the proton drip line. The lifetimes of the proton-decaying isomers are extremely sensitive to the orbital angular momentum ` of the emitted proton and can vary over several orders of magnitude when changing the angular momentum of the occupied orbital. The position of certain orbitals at the Fermi surface depends strongly on the beta2 deformation. It is therefore very important to determine the quadrupole deformation of the nuclei at and beyond the proton drip line, in order to locate the orbitals close to the Fermi surface and to estimate the lifetime of the isomers, which is essential when looking for new proton emitters. The deformation of the nuclei at the proton drip line in the A 130 mass region can be experimentally tested by comparing the lifetimes of proton emitters with theoretical predictions and by investigating the level structure of particle bound nuclei close to the drip line. For Z = 57 the proton drip line is predicted to correspond to the isotope 118La. The lightest La nucleus for which spectroscopic information has been published is 124La . This thesis bescribes an experiment realised to identify for the first time excited states of the doubly-odd nucleus 122 57 La65. Another aspect that make particularly interesting a study of these nuclei is a well-established phenomenon in two-quasiparticle rotational bands in odd-odd nuclei: the signature inversion. Despite the observation of signature inversion in many different nuclei, and numerous theoretical interpretations, a satisfactory explanation of the phenomenon has yet to be found. We want verify if the experimental level scheme of 122La is in agreement with the systematics of the other nuclei in this mass region. The common feature relating the studies of these neutron deficient nuclei is that they are populated with extremely low cross section and require special techniques to be identified. The standard efficient way to achieve this is the use of arrays of high-purity germanium detectors coupled to specific ancillary devices for their selection. Our experiment has been performed at the Legnaro National Laboratory (LNL) where heavy-ion fusion-evaporation reaction 40Ca(@ 200 MeV)+92Mo was performed to establish the scheme of 122La by using the GASP -ray spectrometer in conjunction with the ancillary charged-particle ISIS detector and the Neutron Ring. This experimental set-up led to numerous achievements in the A 130 mass region. Chapter 1 introduces to the physics of the nuclei close and beyond to the proton drip-line. In Chapter 2 we will briey see the various theoretical models created to describe several types of nuclear excitations and discuss in more detail the cranking model. Chapter 3 describes the experimental techniques of -ray spectroscopy and the setup used for the present measurement. In Chapter 4 the steps followed in the sorting of the raw data to produce particle-gated matrices are described, together with the techniques used to select the different reaction channels and to clean the spectra. Chapter 5 shows the systematic features of the rotational bands of the A 130 proton rich odd-odd nuclei. Chapter 6 will be devoted to the construction of the decay scheme and to the spins assignments, performed through the analysis of gated matrices, transition intensities, DCO, and the comparison with the systematics of the La isotopes. Finally, in Chapter 7, the results are discussed in the light of the deduced experimental quantities like single-particle spin alignment, dynamic moments of inertia, signature splitting, and B(M1)=B(E2) branching ratios to assign the Nilsson orbitals to the found bands.

Study of multi‐nucleon transfer reactions with light nuclei

Abstract: Multi-nucleon transfer reactions are useful tools to populate exotic nuclei, particularly the neutron-rich ones. In this view, two different experiments have been performed employing a stable ( 22 Ne) and a radioactive ( 24 Ne) beam, both impinging on a 208 Pb target. The first reaction has been studied using the CLARA-PRISMA-DANTE set-up at Laboratori Nazionali di Legnaro (Legnaro-Italy), while the second reaction was performed at Ganil (Caen-France) employing a SPIRAL radioactive beam of 24 Ne. In this case recoils and coincident γ rays were detected with the VAMOS-EXOGAM set-up. The data show that MNT reactions can selectively populate states of different nature and, therefore, are a good tool to study nuclear structure further away from stability.

Application of a simple PSA algorithm for HPGe segmented detectors to in-beam data

Abstract: A 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 used.