Abstract: A new digital pulse-timing algorithm, to be used with the future neutron detector array NEDA, has been developed and tested. The time resolution of four 5 in diameter photomultiplier tubes (XP4512, R4144, R11833-100, and ET9390-kb), coupled to a cylindrical 5 in by 5 in BC501A liquict scintillator detector was measured by employing digital sampling electronics and a constant fraction discriminator (CFD) algorithm. The zero crossing of the CM algorithm was obtained with a cubic spline interpolation, which was continuous up to the second derivative. The performance of the algorithm was studied at sampling rates of 500 MS/s and 200 MS/s. The time resolution obtained with the digital electronics was compared to the values acquired with a standard analog CFD. The result of this comparison shows that the time resolution from the analog and the digital measurements at 500 MS/s and at 200 MS/s are within 15% for all the tested photomultiplier tubes.

Abstract: 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.

Abstract: Energy differences between analogue states in the T = 1/2 Mg-23-Na-23 mirror nuclei have been measured along the rotational yrast bands. This allows us to search for effects arising from isospin-symmetrybreaking interactions (ISB) and/or shape changes. Data are interpreted in the shell model framework following the method successfully applied to nuclei in the f(7/2) shell. It is shown that the introduction of a schematic ISB interaction of the same type of that used in the f(7/2) shell is needed to reproduce the data. An alternative novel description, applied here for the first time, relies on the use of an effective interaction deduced from a realistic charge-dependent chiral nucleon-nucleon potential. This analysis provides two important results: (i) The mirror energy differences give direct insight into the nuclear skin; (ii) the skin changes along the rotational bands are strongly correlated with the difference between the neutron and proton occupations of the s(1/2) “halo” orbit.

Abstract: Excited states in the Tz = 0, -1 nuclei Ga-62 and Ge-62 were populated in direct reactions of relativistic radioactive ion beams at the RIBF. Coincident gamma rays were measured with the DALI2(+) array and uniquely assigned to the A = 62 isobars. In addition, Ge-62 was also studied independently at JYFL-ACCLAB using the Mg-24(Ca-40,2n)Ge-62 fusion-evaporation reaction. The first excited T = 1, J(pi) = 2(+) states in Ga-62 and Ge-62 were identified at 979(1) and 965(1) keV, respectively, resolving discrepant interpretations in the literature. States beyond the first 2+ state in Ge-62 were also identified for the first time in the present work. The results are compared with shell-model calculations in the f p. model space. Mirror and triplet energy differences are analyzed in terms of individual charge-symmetry and charge-independence breaking contributions. The MED results confirm the shrinkage of the p-orbits' radii when they are occupied by at least one nucleon on average.

Abstract: In this work we present a comparison between the two liquid scintillators BC-501A and BC-537 in terms of their performance regarding the pulse-shape discrimination between neutrons and gamma rays. Special emphasis is put on the application of artificial neural networks. The results show a systematically higher gamma-ray rejection ratio for BC-501A compared to BC-537 applying the commonly used charge comparison method. Using the artificial neural network approach the discrimination quality was improved to more than 95% rejection efficiency of gamma rays over the energy range 150 to 1000 keV for both BC-501A and BC-537. However, due to the larger light output of BC-501A compared to BC-537, neutrons could be identified in BC-501A using artificial neural networks down to a recoil proton energy of 800 keV compared to a recoil deuteron energy of 1200 keV for BC-537. We conclude that using artificial neural networks it is possible to obtain the same gamma-ray rejection quality from both BC-501A and BC-537 for neutrons above a low-energy threshold. This threshold is, however, lower for BC-501A, which is important for nuclear structure spectroscopy experiments of rare reaction channels where low-energy interactions dominates.