P. G. Bizzeti

On the alpha activity of natural tungsten isotopes

The indication for the alpha decay of 180-W with a half-life T1/2=1.1+0.8-0.4(stat)+-0.3(syst)x10^18 yr has been observed for the first time with the help of the super-low background 116-CdWO_4 crystal scintillators. In conservative approach the lower limit on half-life of 180-W has been established as T1/2>0.7x10^18 yr at 90% C.L. Besides, new T1/2 bounds were set for alpha decay of 182-W, 183-W, 184-W and 186-W at the level of 10^20 yr.

Linking transitions between the highly deformed states and the yrast states of normal deformation in 133Nd

Abstract Transitions linking the highly deformed band to low deformation states have been unambiguously established in 133 Nd in the very first experiment performed with the GASP array. the highly deformed band becomes yrast at 29 2 + , where it mixes with the positive parity band built upon the [404] 7 2 Nilsson state. The experimentally deduced interaction amounts to 13 keV. The measurement of the spins in the highly deformed intruder band sheds some light upon the reported backbend anomaly in this mass region.

STRUCTURE OF NEUTRON–RICH NUCLEI FROM DEEP–INELASTIC REACTIONS

Neutron rich nuclei have been populated using the 82Se+192Os deep-inelastic reaction. New experimental results on 188W, 188,190Os target-like nuclei, as well as 74,76,78Ge beam-like nuclei are presented.

Response of CdWO4 crystal scintillator for few MeV ions and low energy electrons

The response of a CdWO4 crystal scintillator to protons, α particles, Li, C, O and Ti ions with energies in the range 1 – 10 MeV was measured. The non-proportionality of CdWO4 for low energy electrons (4 – 110 keV) was studied with the Compton Coincidence Technique. The energy dependence of the quenching factors for ions and the relative light yield for low energy electrons was calculated using a semi-empirical approach. Pulse-shape discrimination ability between γ quanta, protons, α particles and ions was investigated.

Pulse height defect for Al27 and Mg24 ions in silicon surface barrier detectors

Abstract Silicon surface barrier detectors are shown to exhibit a “pulse height defect” for a low energy Al27 and Mg24 ions. The ions were produced inside the detector as the recoils of the Si28 (γ, p) and (γ, α) reactions by monochromatic γ-rays of 17.6 MeV. The pulse height defect ranges from 70 to 152 keV for Al27 ions of 91 keV and 225 keV respectively, while for Mg24 ions it is 138 keV and 145 keV for energies of 910 keV and 1105 keV. It is temperature independent (from −70°C to +20°C) and does not vary when the electric field is varied by a factor 2. The evidence suggests that the effect is due to a reduction of the efficiency in the electron-hole production when the ions are sufficiently slowed down.

Ersicson's fluctuations in the photodisintegration of Si28

Abstract The cross section of the reactions Si 28 (γ, p) Al 27 and Si 28 (γ, α) Mg 24 as been measured in an energy range ΔE = 117 keV around 17.62 MeV gamma ray energy. Six different transitions (γ, p 0 ), (γ, p 1 ), (γ, p 2 ), (γ, p 4 ), (γ, α 0 ) and (γ, α 1 ) have been separately analysed; they show strong, uncorrelated fluctuations, the amplitude of which agrees with Ericsons predictions (pure statistical reaction and purely E1 gamma absorption being assumed). No free parameter, as the effective number of channels, is required for the comparison. Some contribution of direct effect (up to, say, 50%) is not excluded. The relative yield of (γ, α) to (γ, p) transitions is, on the average, lower (by a factor about 10) than the statistical model would predict, on the basis of transmission coefficients. This is probably an example of the isobaric spin selection rules, as applied to photodisintegration by Gell-Mann and Telegdi.

Radioactive contamination of CaWO4, ZnWO4, CdWO4, and Gd2SiO5:Ce crystal scintillators

Radioactive contaminations of CaWO4, ZnWO4, CdWO4, and Gd2SiO5:Ce crystal scintillators were measured in the Solotvina Underground Laboratory. The Monte Carlo simulation of contributing radioactive sources, the time‐amplitude and pulse‐shape analyses of the experimental data were applied. CaWO4 and GSO scintillators are considerably polluted by uranium and thorium, while the total α activity in ZnWO4 and CdWO4 scintillators does not exceed the level of a few mBq/kg. Particular radioactivity was observed in CaWO4 (α active 180W), CdWO4 (β active 113Cd, 2v2β decay of 116Cd, 180W), and GSO (α active 152Gd) detectors. The radioactive contamination of the studied scintillation crystals is compared with that of commonly used detectors.

Gamma-decay study of 21Na and 21Ne, octupole bands in 21Ne

The reactions 16 O( 7 Li, 2n )a nd 16 O( 7 Li, np) populating 21 Na and 21 Ne have been studied at EL = 27 MeV using the GASP γ -detector array. The level scheme for 21 Na and 21 Ne has been extended to higher excitation energy. Spins and parities of the observed levels are assigned tentatively supporting th ei dentification of opposite parity structures connected by strong dipole transitions. The structure of these bands is interpreted as based on a reflection asymmetric cluster structure.

Lifetimes of lower excited states in27Si

SummaryThe mean lives of some levels in27Si have been measured with the Doppler-shift attenuation method, by means of the28Si(3He, α)27Si reaction. The γ-rays were detected in coincidence with alphas at the angle ϑ=60°. The results for the mean lives are the following:τm=(1.8±0.6) ps, (65−35+40) fs, > 5 ps and < 63 fs for the 0.957, 2.162, 0.781 and 2.647 MeV levels, respectively.RiassuntoSi sono misurate le vite medie di alcuni livelli eccitati del27Si col metodo dello spostamento Doppler attenuato. La reazione usata per ottenere il27Si eccitato era28Si(3He, α)27Si all’energiaEτ=6.6 MeV. I raggi γ venivano rivelati in coincidenza con le particelle α emesse a (60 ± 5)° rispetto alla direzione del fascio incidente. I risultati per le vite medie sono i seguenti:τm=(1.8±0.6) ps,τm=(65−35+40) fs,τm > 5 ps eτm < 63 fs, rispettivamente, per i livelli a 0.957, 2.162, 0.781 e 2.647 MeV.РеэюмеИспольэуя реакцию28Si(3He, α)27Si, были иэмерены средние времена жиэни некоторых уровней в27Si с помошью метода, основанного на уменьщении допплеровского сдвига. γ-лучи детектировались по совпадению с альфа-частицами при угле ϑ=60°. Реэультаты для средних времен жиэни следуюшие: τm=(1.8±0.6) рs, (65−35+40) fs, > 5 рs и > 63 fs соответственно для уровней 0.957, 2.162, 0.781 и 2.647 МзВ.

The CLARA-PRISMA setup installed at LNL: first results

CLARA is an array of 25 Clover (EUROBALL type) Ge detectors, placed at the target position of the large acceptance magnetic spectrometer PRISMA. Due to the granularity of the CLARA Ge array (100 crystals), the photopeak efficiency (≈3%) and the PRISMA large acceptance (≈80 msr, and 20% Δp/p), the setup is an excellent tool to investigate the structure of neutron-rich nuclei, populated in multinucleon transfer reactions and deep inelastic collisions with stable beams. Since March 2004 the setup is fully operational, and since then several experiments have been performed, with beams delivered by the LNL XTU-Tandem and the ALPI linac. In this contribution the preliminary outcome of the first experiments is shown.