Lifetime measurement of neutron-rich even-even molybdenum isotopes

Background: In the neutron-rich A approximate to 100 mass region, rapid shape changes as a function of nucleon number as well as coexistence of prolate, oblate, and triaxial shapes are predicted by various theoretical models. Lifetime measurements of excited levels in the molybdenum isotopes allow the determination of transitional quadrupole moments, which in turn provides structural information regarding the predicted shape change. Purpose: The present paper reports on the experimental setup, the method that allowed one to measure the lifetimes of excited states in even-even molybdenum isotopes from mass A = 100 up to mass A = 108, and the results that were obtained. Method: The isotopes of interest were populated by secondary knock-out reaction of neutron-rich nuclei separated and identified by the GSI fragment separator at relativistic beam energies and detected by the sensitive PreSPEC-AGATA experimental setup. The latter included the Lund-York-Cologne calorimeter for identification, tracking, and velocity measurement of ejectiles, and AGATA, an array of position sensitive segmented HPGe detectors, used to determine the interaction positions of the gamma ray enabling a precise Doppler correction. The lifetimes were determined with a relativistic version of the Doppler-shift-attenuation method using the systematic shift of the energy after Doppler correction of a gamma-ray transition with a known energy. This relativistic Doppler-shift-attenuation method allowed the determination of mean lifetimes from 2 to 250 ps. Results: Even-even molybdenum isotopes from mass A = 100 to A = 108 were studied. The decays of the low-lying states in the ground-state band were observed. In particular, two mean lifetimes were measured for the first time: tau = 29.7(-9.1)(+11.3) ps for the 4(+) state of Mo-108 and tau = 3.2(-0.7)(+ 0.7) ps for the 6(+) state of Mo-102. Conclusions: The reduced transition strengths B(E2), calculated from lifetimes measured in this experiment, compared to beyond-mean-field calculations, indicate a gradual shape transition in the chain of molybdenum isotopes when going from A = 100 to A = 108 with a maximum reached at N = 64. The transition probabilities decrease for Mo-108 which may be related to its well-pronounced triaxial shape indicated by the calculations

Shape coexistence revealed in the N= Z isotope 72 Kr through inelastic scattering

The N= Z= 36 nucleus 72Kr has been studied by inelastic scattering at intermediate energies. Two targets, 9Be and 197Au, were used to extract the nuclear deformation length, δN, and the reduced E2 transition probability, B(E2). The previously unknown non-yrast 2 + and 4 + states as well as a new candidate for the octupole 3 - state have been observed in the scattering on the Be target and placed in the level scheme based on γ- γ coincidences. The second 2 + state was also observed in the scattering on the Au target and the B(E2;22+→01+) value could be determined for the first time. Analyzing the results in terms of a two-band mixing model shows clear evidence for a oblate-prolate shape coexistence and can be explained by a shape change from an oblate ground state to prolate deformed yrast band from the first 2 + state. This interpretation is corroborated by beyond mean field calculations using the Gogny D1S interaction

Metal-core pad-plane development for ACTAR TPC

With the recent development of active targets and time projection chambers (ACTAR TPC) as detectors for fundamental nuclear physics experiments, the need arose for charge collection planes with a high density of readout channels. In order to fulfill the mechanical constraints for the ACTAR TPC device, we designed a pad-plane based on a metal-core circuit with an conceptually simple design and routing for signal readout, named FAKIR (in reference to a fakir bed of nails). A test circuit has been equipped with a micro mesh gaseous structure (micromegas ) for signal amplification and a dedicated readout electronics. Test measurements have been performed with an $^{55}$FeX-ray source giving an intrinsic energy resolution (FWHM) of $22 \pm 1\%$ at 5.9 keV, and with a 3-alpha source for which a resolution of about $130 \pm 20$ keV at 4.8 MeV has been estimated. The pad-plane has been mounted into a reduced size demonstrator version of the ACTAR TPC detector, in order to illustrate charged particle track reconstruction. The tests preformed with the X-ray and the 3-alpha sources shows that results obtained from pads signals are comparable to the intrinsic result from the micro-mesh signal. In addition, a simple alpha particle tracks analysis is performed to demonstrate that the pad plane allows a precise reconstruction of the direction and length of the trajectories

Shape coexistence revealed in the N= Z isotope 72Kr through inelastic scattering

The N= Z= 36 nucleus 72Kr has been studied by inelastic scattering at intermediate energies. Two targets, 9Be and 197Au, were used to extract the nuclear deformation length, δN, and the reduced E2 transition probability, B(E2). The previously unknown non-yrast 2 + and 4 + states as well as a new candidate for the octupole 3 - state have been observed in the scattering on the Be target and placed in the level scheme based on γ- γ coincidences. The second 2 + state was also observed in the scattering on the Au target and the B(E2;22+→01+) value could be determined for the first time. Analyzing the results in terms of a two-band mixing model shows clear evidence for a oblate-prolate shape coexistence and can be explained by a shape change from an oblate ground state to prolate deformed yrast band from the first 2 + state. This interpretation is corroborated by beyond mean field calculations using the Gogny D1S interaction