TWO-PROTON RADIOACTIVITY: THE INTERESTING CASE OF 67Kr AND FURTHER STUDIES

We report on the observation of 67Kr that has been produced in an experiment performed at the RIKEN/BigRIPS facility. The two-proton decay of 67Kr has been evidenced and this nucleus is thus the fourth observed long lived ground-state two-proton emitter, after 45Fe, 48Ni and 54Zn. In addition, the decay of several isotopes in the mass region has been investigated. While for previous cases of two-proton radioactivity, the theoretical models could reproduce the measured data, this is not the case anymore for 67Kr. Two interpretations have been proposed to explain this discrepancy: a transition between real two-proton and sequential decay or the influence of deformation. These hypotheses will be tested in future experiments by measuring the angular and energy correlations of the emitted proton

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

Two-Proton Radioactivity of Kr 67

In an experiment with the BigRIPS separator at the RIKEN Nishina Center, we observed two-proton (2p) emission from Kr67. At the same time, no evidence for 2p emission of Ge59 and Se63, two other potential candidates for this exotic radioactivity, could be observed. This observation is in line with Q value predictions which pointed to Kr67 as being the best new candidate among the three for two-proton radioactivity. Kr67 is only the fourth 2p ground-state emitter to be observed with a half-life of the order of a few milliseconds. The decay energy was determined to be 1690(17) keV, the 2p emission branching ratio is 37(14)%, and the half-life of Kr67 is 7.4(30) ms

New neutron-deficient isotopes from Kr-78 fragmentation

Background: Because of its half-life of about 35 million years, 92Nb is considered as a chronometer for nucle-osynthesis events prior to the birth of our sun. The abundance of 92Nb in the early solar system can be derivedfrom meteoritic data. It has to be compared to theoretical estimates for the production of 92Nb to determine thetime between the last nucleosynthesis event before the formation of the early solar system. Purpose: The influence of a low-lying short-lived isomer on the nucleosynthesis of 92Nb is analyzed. The thermalcoupling between the ground state and the isomer via so-called intermediate states affects the production andsurvival of 92Nb. Method: The properties of the lowest intermediate state in 92Nb are known from experiment. From the lifetimeof the intermediate state and from its decay branchings, the transition rate from the ground state to the isomerand the effective half-life of 92Nb are calculated as a function of the temperature. Results: The coupling between the ground state and the isomer is strong. This leads to thermalization of groundstate and isomer in the nucleosynthesis of 92Nb in any explosive production scenario and almost 100 % survivalof 92Nb in its ground state. However, the strong coupling leads to a temperature-dependent effective half-life of92Nb which makes the 92Nb survival very sensitive to temperatures as low as about 8 keV, thus turning 92Nb atleast partly into a thermometer. Conclusions: The low-lying isomer in 92 Nb does not affect the production of 92Nb in explosive scenarios. Inretrospect this validates all previous studies where the isomer was not taken into account. However, the dramaticreduction of the effective half-life at temperatures below 10 keV may affect the survival of 92Nb after its synthesisin supernovae which are the most likely astrophysical site for the nucleosynthesis of 92Nb

GET electronics samples data analysis

The General Electronics for TPCs (GET) has been developed to equip a generation of time projection chamber detectors for nuclear physics, and may also be used for a wider range of detector types. The goal of this paper is to propose first analysis procedures to be applied on raw data samples from the GET system, in order to correct for systematic effects observed on test measurements. We also present a method to estimate the response function of the GET system channels. The response function is required in analysis where the input signal needs to be reconstructed, in terms of time distribution, from the registered output samples

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

Gamma-ray branching ratios in the decay of 49^{49}Cr

The $\gamma$ -decay branching ratios of $^{49}$V have been studied in the $\beta$ /EC decay of $^{49}$Cr. The $\gamma$-ray spectrum has been measured with a 70% relative efficiency co-axial germanium detector calibrated in efficiency with a precision of about 0.5%. The primary result is the measurement of the relative branching ratios of the three main $\gamma$ rays in the decay of $^{49}$Cr. Large discrepancies were found with a previous experimental work. The $^{49}$Cr sample was produced by proton impact on a ZrO$_{2}$ target and collected at the ISOLDE facility of CERN and enabled us to re-evaluate the decay scheme of $^{49}$Cr. The new data allow the use of this nucleus for future high-precision calibration work

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