L. M. Fraile

Beta-decay properties of the neutron-rich Kr-94-99 and Xe-142-147 isotopes

Beta-decay half-lives and delayed-neutron emission probabilities of the neutron-rich noble-gas isotopes Kr94-99 and Xe142-147 have been measured at the PSB-ISOLDE facility at CERN. The results are compared to QRPA shell-model predictions and are used in dynamic calculations of r-process abundances of Kr and Xe isotopes

Performance evaluation of SiPM detectors for PET imaging in the presence of magnetic fields

The multi-pixel photon counter (MPPC) or silicon photo-multiplier (SiPM), recently introduced as a solid-state photodetector, consists of an array of Geiger-mode photodiodes (microcells). It is a promising device for PET thanks to its potential for high photon detection efficiency (PDE) and immunity to high magnetic fields. It is also very easy to use, with simple electronic read-out, high gain and small size. In this work we evaluate the performance of three 1 × 1 mm2 and one 6 × 6 mm2 (2 × 2 array) SiPMs offered by Hamamatsu for their use in PET. We examine the dependence of the energy resolution and the gain of these devices on the thermal and reverse bias when coupled to LYSO scintillator crystals. We find that the 400 and 1600 microcells models and the 2 × 2 array are suitable for small size crystals, like those employed in high resolution small animal scanners. The good performance of these devices up to 7 Tesla has also been confirmed.

Sizeable beta-strength in 31 Ar (β3p) decay

We present for the first time precise spectroscopic information on the recently discovered decay mode beta-delayed 3p-emission. The detection of the 3p events gives an increased sensitivity to the high energy part of the Gamow-Teller strength distribution from the decay of Ar-31 revealing that as much as 30% of the strength resides in the beta 3p-decaymode. A simplified description of how the main decay modes evolve as the excitation energy increases in Cl-31 is provided.

Persistence of octupole correlations in Ra-231

Abstract The structure of 231 Ra, the heaviest odd Ra nucleus currently accessible for detailed spectroscopic investigation, has been studied in the β − decay of 231 Fr. The main purpose has been to verify whether fast B (E1) rates and significant octupole correlations recently established in 229 Ra persist in 231 Ra. The excited states in 231 Ra have been determined for the first time. Spins and parities have been deduced from conversion electron measurements, while level half-lives in the ps and ns ranges have been measured with the Advanced Time Delayed βγγ (t) method. The moderately fast B (E1) rates found for transitions connecting the lowest-lying K π =5/2 ± and K π =1/2 ± bands, reveal the persistence of octupole collective effects in 231 Ra, although the B (E1) rates are lower than in 229 Ra. These findings reinforce the differences in the B (E1) systematics between isotonic Ra and Th nuclei.

Be-11(beta p), a quasi-free neutron decay?

We have observed beta(-)-delayed proton emission from the neutron-rich nucleus Be-11 by analyzing a sample collected at the ISOLDE facility at CERN with accelerator mass spectrometry (AMS). With a branching ratio of (8.3 +/- 0.9). 10(-6) the strength of this decay mode, as measured by the B-GT-value, is unexpectedly high. The result is discussed within a simple single-particle model and could be interpreted as a quasi-free decay of the Be-11 halo neutron into a single-proton state.

Rare βp decays in light nuclei

Beta-delayed proton emission may occur at very low rates in the decays of the light nuclei Be-11 and B-8. This paper explores the potential physical significance of such decays, estimates their rates and reports on first attempts to detect them: an experiment at ISOLDE/CERN gives a branching ratio for Be-11 of ( 2.5 +/- 2.5) x 10(-6) and an experiment at JYFL a 95% confidence upper limit of 2.6 x 10(-5) for B-8.

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

The gamma-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 gamma-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 micros isomer. Lifetimes analysis, performed by fast-timing techniques with LaBr3(Ce) scintillators, reveals 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.

^{133}

The level scheme of 96Y was significantly extended and a new 201 ns isomer was located at 1655 keV excitation energy, with spin-parity assignment of 5± or 6−. The isomer decays to spherical low-spin structure by transitions with large hindrance and is fed by a short cascade which resembles the beginning of a rotational band. This is in analogy with the feeding and decay pattern of the 4− isomer in 98Y, here confirmed, by lifetime analysis, as a bandhead of a rotational structure with sizable deformation. It is suggested that the new isomer in 96Y arises from a shape change between deformed and spherical configurations, which indicates the appearance of deformation already at N = 57 in the yttrium chain. The experimental findings for 96Y are strengthened by theoretical calculations based on the complex Excited Vampir model.

Sb

Nuclear structure studies of exotic nuclei via the strength of E2 transitions; advanced time-delayed γγ spectroscopy at the extreme

New isomer in 96Y marking the onset of deformation at N = 57

Ultra fast inorganic scintillators, like LaBr3(Ce) are key detectors in Nuclear Physics and medical applications. On the other hand, fully digital acquisition systems, for which the pulses from the detectors are sampled at high rates, are being increasingly used. We present new digital pulse processing algorithms to extract time and energy signals from nuclear pulses produced by ultrafast detectors. We compare the results of the fully digital acquisition chain (DDAQ) to state-of-the-art results obtained with traditional electronics, based upon constant fraction discriminators (CFD), time to digital converters and multichannel analyzers. For this purpose, we have performed coincidence measurements using relatively large (1″×1.5″×1”) truncated cone and cylindrical LaBr3(Ce) and CeBr3 crystals coupled to ultra fast photomultipliers (PMTs). Coincidence measurements with Co-60 and Na-22 sources are acquired. The main advantage of fully digital processing is that many different algorithms may be applied to the same raw data set. Pulses from PMTs optimized for timing measurements were digitized to a switched capacitor array with a speed sampling of 5 Gs/s and a resolution of 16 bits, and to a fast digital oscilloscope able of 4 Gs/s and 8 bits resolution. With an in silico version of the analog CFD we obtain coincidence resolving times below 150 ps for Co-60, outperforming the standard acquisition system.