The Advanced Gamma Ray Tracking Array AGATA

On behalf of the AGATA collaboration New accelerator facilities for radioactive-ion beams will enter into operation in the next few years, providing the opportunity to explore unknown territories of the nuclear landscape. The foreseen harsh experimental conditions require the construction of a new generation of γ-ray detector arrays based on the emerging technique of γ-ray tracking. The "Advanced GAmma Tracking Array" (AGATA), proposed in Europe, will be built out of 120 or 180 highly segmented Ge crystals operated in position sensitive mode by means of digital data techniques and pulse shape analysis of the segment signals. AGATA will be capable of measuring γ radiation in a large energy range (from ∼ 10 keV to ∼ 10 MeV), with the largest possible photopeak efficiency (25% at M γ = 30) and with good spectral response. The very good Doppler correction and background rejection capability of this γ-ray tracking array will allow to perform γ-ray spectroscopy experiments using fragmentation beams with sources moving at velocities up to β ∼ 0.5

The mutable nature of particle-core excitations with spin in the one-valence-proton nucleus ¹³³Sb

The γ-ray decay of excited states of the one-valence-proton nucleus ¹³³Sb has been studied using cold-neutron induced fission of ²³⁵U and ²⁴¹Pu targets, during the EXILL campaign at the ILL reactor in Grenoble. By using a highly efficient HPGe array, coincidences between γ-rays prompt with the fission event and those delayed up to several tens of microseconds were investigated, allowing to observe, for the first time, high-spin excited states above the 16.6 μs isomer. Lifetimes analysis, performed by fast-timing techniques with LaBr₃(Ce) scintillators, revealed a difference of almost two orders of magnitude in B(M1) strength for transitions between positive-parity medium-spin yrast states. The data are interpreted by a newly developed microscopic model which takes into account couplings between core excitations (both collective and non-collective) of the doubly magic nucleus ¹³²Sn and the valence proton, using the Skyrme effective interaction in a consistent way. The results point to a fast change in the nature of particle-core excitations with increasing spin

Lifetime measurements and the high-spin structure of Cl-36

1st NUBA International Conference on Nuclear Physics and Astrophysics -- SEP 14-21, 2014 -- Akdeniz Univ, Antalya, TURKEYWOS: 000354773900036High-spin states in Cl-36 were populated through the Mg-24(N-14,2p)Cl-36 reaction at E(N-14)=31 MeV. Lifetimes have been determined for fifteen states by applying the Doppler shift attenuation method. The results indicated the onset of collectivity in the high spin negative parity states. Large basis shell model calculations have been performed to understand the underlying structure of these states

Crosstalk corrections for improved energy resolution with highly segmented HPGe-detectors

WOS: 000269865500014Crosstalk effects of 36-fold segmented, large volume AGATA HPGe detectors cause shifts in the gamma-ray energy measured by the inner core and outer segments as function of segment multiplicity. The positions of the segment sum energy peaks vary approximately linearly with increasing segment multiplicity. The resolution of these peaks deteriorates also linearly as a function of segment multiplicity. Based on single event treatment, two methods were developed in the AGATA Collaboration to correct for the crosstalk induced effects by employing a linear transformation. The matrix elements are deduced from coincidence measurements of gamma-rays of various energies as recorded with digital electronics. A very efficient way to determine the matrix elements is obtained by measuring the base line shifts of untriggered segments using gamma-ray detection events in which energy is deposited in a single segment. A second approach is based on measuring segment energy values for gamma-ray interaction events in which energy is deposited in only two segments. After performing crosstalk corrections, the investigated detector shows a good fit between the core energy and the segment sum energy at all multiplicities and an improved energy resolution of the segment sum energy peaks. The corrected core energy resolution equals the segment sum energy resolution which is superior at all folds compared to the individual uncorrected energy resolutions. This is achieved by combining the two independent energy measurements with the core contact on the one hand and the segment contacts on the other hand.German BMBF [06KY2051]; European funding bodies; EU [R113-CF-2004-506065]This research was supported by the German BMBF under Grant 06KY2051. AGATA was supported by the European funding bodies and the EU Contract R113-CF-2004-506065

Structure of the N=50 As, Ge, Ga nuclei

The level structures of the N=50 83 As, 82 Ge, and 81 Ga isotones have been investigated by means of multi-nucleon transfer reactions. A first experiment was performed with the CLARA-PRISMA setup to identify these nuclei. A second experiment was carried out with the GASP array in order to deduce the ?-ray coincidence information. The results obtained on the high-spin states of such nuclei are used to test the stability of the N=50 shell closure in the region of 78 Ni (Z=28). The comparison of the experimental level schemes with the shell-model calculations yields an N=50 energy gap value of 4.7(3) MeV at Z=28. This value, in a good agreement with the prediction of the finite-range liquid-drop model as well as with the recent large-scale shell model calculations, does not support a weakening of the N=50 shell gap down to Z=28. © 2012 Elsevier B.V.Science and Technology Facilities Counci

High-spin level structure of 35 S

The nucleus 35S has been studied by in-beam ?-ray spectroscopy using the 24Mg(14N,3p) fusion-evaporation reaction at Elab=40 MeV. A level scheme extended up to J?=17/2+ at 8023 keV and J?=13/2- at 6352 keV has been established. Lifetimes of six excited states have been determined by applying the Doppler shift attenuation method. The experimental data have been compared with the results of large-scale shell model calculations performed using different effective interactions and model spaces allowing particle-hole excitations across the N=Z=20 shell gap. © 2014 American Physical Society

Reduction in the uncertainty of the neutron-capture cross section of 210Bi: Impact of a precise multipolarity measurement of the 2− → 1− main ground-state transition

The mixing ratio of the main 320-keV, M1 + E2 ground-state γ transition in 210Bi has been more precisely quantified, allowing a significant reduction in the uncertainty of measurements of the neutron-capture cross section to the ground state of 210Bi from 25% to 0.9%. Accurate values for neutron-capture cross sections to both the ground and long-lived 9− isomeric state at 271 keV in 210Bi are of particular importance as Pb-Bi finds increased usage in Accelerator Driven Systems

The γ-ray spectroscopy studies of low-spin structures in 210Bi and 206Tl using cold neutron capture reactions

The γ-coincidence studies of low-spin structures of 210Bi and 206Tl are presented. The 210Bi nucleus, populated in thermal neutron capture reaction, was investigated using EXILL HPGe array at Institut Laue-Langevin in Grenoble. The experimental results were compared to the shell-model calculations allowing to draw the conclusions on the nature of the low-spin excitations populated below the neutron binding energy in 210Bi (4.6 MeV). It has been found that some levels cannot be described by the valence proton and neutron couplings, but may arise from couplings of valence particles to the 3- octupole phonon of the doubly magic 208Pb core. Moreover, preliminary results of a low-spin structure measurements of 206Tl by the γ-coincidence technique, making use of the 205Tl(n,γ)206Tl reaction at the FIPPS prompt γ-ray spectroscopy facility of ILL are shown. The population of a large number of excited states of 206Tl above the ground state up to the neutron binding energy (at 6.5 MeV), within a few units of spin is expected. The analysis involving double and triple γ-coincidences and γγ-angular correlations will allow to significantly extend the experimental information on the energy and spin-parity of the levels in 206Tl. This will help shedding light on the proton-hole and neutron-hole couplings near the doubly magic core 208Pb