Gamma ray tracking with the AGATA demonstrator - A novel approach for in-beam spectroscopy

The Advanced GAmma Tracking Array (AGATA) employs the novel method of gamma-ray tracking (GRT), where all locations of energy depositions within the Ge crystal detector volume are used by computer algorithms to reconstruct the various simultaneous interactions of the measured radiation. The interaction positions are determined by Pulse Shape Analysis (PSA) algorithms that compare the measured and digitized signals with the information of a signal database comprising position dependent calculated sets of detector signals. The result of a detailed comparison between measured and calculated signals yields the position of each interaction point. The GRT algorithms rely on this precise position of the deposited energy as an input to reconstruct the initial gamma-rays from the full sequence of the different interactions in the detector. Within this thesis a computer program library was developed, providing software routines to calculate the position dependent detector signals of the highly segmented HPGe detectors. The currently used signal databases of all AGATA detectors were generated by this software package and computer library. Part of the computing is based on individual detector properties which were deduced from detailed characterisation measurements. Details of the library, the used routines and the needed characteristics of the detector system are described, this includes a precise measurement of the crystal axis orientation of the AGATA HPGe crystals. The second part of this thesis is dealing with the analysis of one of the first in-beam experiments performed with the AGATA demonstrator setup at the LNL in Italy. The experiment aimed for a spectroscopic investigation of neutron rich actinides from Thorium to Plutonium produced after multi-nucleon transfer reactions. For this purpose a 136Xe beam with an energy of 1GeV bombarded onto a 238U target. The fast beam like particles after the transfer reactions were identified by the magnetic spectrometer PRISMA. The gamma-rays were detected with the AGATA demonstrator consisting of five AGATA triple cluster detectors. An additional micro channel plate detector for particle detection was mounted inside the scattering chamber in order to request kinematic coincidences. The analysis procedures for the two complex sub-detectors AGATA and PRISMA were extended and adapted to the specific requirements of this new approach for actinide spectroscopy. First the complex analysis of the magnetic spectrometer PRISMA and solutions for unexpected detector behaviour like time drifts and aberration corrections are described. As a result the individual isotopes of elements from Barium to Tellurium were identified confirming the very high quality of the PRISMA spectrometer and its design parameters. The analysis of the gamma-ray spectra comprised a detailed PSA and GRT analysis of the AGATA demonstrator. This analysis included also data analysis developments for the AGATA collaboration. The data of the AGATA demonstrator, the PRISMA spectrometer and the ancillary detectors were merged to obtain background free Doppler corrected spectra for the beam- and target-like reaction products. The simultaneous Doppler correction for beam and target-like ions included an elaborate optimization procedure for unobservable experimental parameters. The gamma-ray spectra for the individual isotopes is consistent with the isotope identification of the PRISMA analysis. For the beam like particles gamma-ray spectra of the isotopes 128-139Xe are presented and discussed. For the target like nuclei gamma-ray spectra of the isotopes 236-240U are deduced. By gating on the remaining excitation energy after the multi-nucleon transfer reaction the neutron evaporation and fission of the excited actinide nuclei were suppressed. Coincidences between AGATA and PRISMA were exploited for the first time together with the particle coincidence between beam- and target-like nuclei. These triple coincidences allowed further background reduction. The results for the individual Xenon and Uranium isotopes demonstrate the successful operation of the AGATA demonstrator coupled to the PRSIMA spectrometer. The quality of the gamma-ray spectra show clearly that the novel pulse shape analysis and gamma-ray tracking methods fulfil expectations also for demanding in-beam gamma-ray spectroscopy experiments

Preliminary results of lifetime measurements in neutron-rich 53Ti

To study the nuclear structure of neutron-rich titanium isotopes, a lifetime measurement was performed at the Grand Accélérateur National d'Ions Lourds (GANIL) facility in Caen, France. The nucleiwere produced in a multinucleon-transfer reaction by using a 6.76 MeV/u 238U beam. The Advanced Gamma Tracking Array (AGATA) was employed for the γ-ray detection and target-like recoils were identified event-by-event by the large-acceptance variable mode spectrometer (VAMOS++). Preliminary level lifetimes of the (5/2−) to 13/2− states of the yrast band in the neutron-rich nucleus 53Ti were measured for the first time employing the recoil distance Doppler-shift (RDDS) method and the compact plunger for deep inelastic reactions. The differential decay curve method (DDCM) was used to obtain the lifetimes from the RDDS data

Preliminary results of lifetime measurements in neutron-rich 53Ti

To study the nuclear structure of neutron-rich titanium isotopes, a lifetime measurement was performed at the Grand Accélérateur National d'Ions Lourds (GANIL) facility in Caen, France. The nucleiwere produced in a multinucleon-transfer reaction by using a 6.76 MeV/u 238U beam. The Advanced Gamma Tracking Array (AGATA) was employed for the γ-ray detection and target-like recoils were identified event-by-event by the large-acceptance variable mode spectrometer (VAMOS++). Preliminary level lifetimes of the (5/2−) to 13/2− states of the yrast band in the neutron-rich nucleus 53Ti were measured for the first time employing the recoil distance Doppler-shift (RDDS) method and the compact plunger for deep inelastic reactions. The differential decay curve method (DDCM) was used to obtain the lifetimes from the RDDS data