D. De Pasquale

Mass and charge identification of fragments detected with the Chimera Silicon-CsI(Tl) telescopes

Abstract Mass and charge identification of charged products detected with Silicon–CsI(Tl) telescopes of the Chimera apparatus are presented. An identification function, based on the Bethe–Bloch formula, is used to fit empirical correlations between Δ E and E ADC readings, in order to determine, event by event, the atomic and mass numbers of the detected charged reaction products prior to energy calibration.

Radiation effects induced by MeV electron beams irradiating dense polyethylene (UHMWPE)

Ultra-high-molecular-weight-polyethylene (UHMWPE) is irradiated with 5 MeV electrons in vacuum at different doses. The radiation effects induced by the electron irradiation are investigated by using different physical analyses such as mechanical tensile stress measurement, hardness measurements, specific heat measurements, infrared molecular absorption, surface wetting, polymer darkness and morphological scanning electron microscopy. The results indicate that the effects induced by high electron dose absorption consist mainly of a dehydrogenation of the polymer with an increase of the C–C cross-linking bonds, of an increase of the macroscopic mechanical resistance of the polymer, and of a darkness increase of the polymer color. Obtained results are compared with those produced by ion irradiation of the polymer by using ion implants at different stopping powers. Some applications of electron-irradiated UHMWPE are presented and discussed.

Mechanical modifications in dense polyethylene induced by energetic electron beams

Effects of radiation damage in ultra high molecular weight polyethylene are investigated by using energetic electron beams. Special attention is devoted to the mechanical characterization of the polymer, before and after the modification induced by 5 MeV electrons, as a function of the absorbed dose. Elastic modules, ultimate tensile and compressive strengths and roughness and hardness, have been measured in pristine and electron irradiated polymers. Infrared spectroscopy, scanning electron microscopy and differential scanning calorimetry are employed in order to investigate the microscopic modifications induced by the electron energy release to the polymeric chains. Results indicate that the polymer loses hydrogen and becomes rich in carbon content. Cross-linking effects are responsible for the higher mechanical resistance, fragility and hardness of the polymer submitted to a high absorbed dose.

Measurements of gas desorption from polyethylene-UHMWPE irradiated by 5 MeV electrons

Gas desorption in vacuum from electron irradiated ultra high molecular weight polyethylene (PE) is measured with a high sensible mass quadrupole spectrometer. Measurements are performed in thick PE irradiated with 5 MeV electron beams at doses of the order of tens of kGy. The irradiation modifies the PE molecules producing dehydrogenation, emission of different C x –H y groups, C-enrichment and carbon cross-linking processes. Results indicate that the radiation damage depends on the dose and that a significant change of chemical and physical polymer properties is reached for a critical dose of 18 kGy.

Measurements of subthreshold photofission angular distributions of238U and fission channel analysis

SummaryMeasurements of photofission fragment angular distributions of238U between 5.1 and 6.0 MeV are presented. As γ-source the intense bremsstrahlung beam from the Catania University Microtron is used. The results are analysed within the framework of the double-humped barrier model with a potential constructed with three smoothly joined parabolae and a damping term in the second well. The values of the fission barrier parameters for the involved 2+0, 1−0 and 1−11 channels are extracted and a comparison with the theoretical predictions and experimental results in the literature is made. Delayed-fission contributions for the involved channels are taken into account in the 1−1 channel parameter determination.RiassuntoSono presentate misure di distribuzioni angolari dei frammenti della fotofissione del238U tra 5.1 e 6.0 MeV. Come sorgente γ è stato usato l’intenso fascio di bremsstrahlung ottenuto dal microtrone dell’Università di Catania. I risultati sono analizzati nell’ambito di un modello a doppia barriera con un potenziale costruito con tre parabole tangenti ed un termine di smorzamento in seconda buca. Si estraggono i valori dei parametri di barriera per i canali coinvolti 2+ 0, 1−0, 1−1 e si fa un confronto con previsioni teoriche e risultati sperimentali esistenti in letteratura. Nella determinazione dei parametri per il canale 1−1 si tiene conto dei contributi da fissione ritardata per i canali coinvolti.

Measurements of gas diffusion in polyethylene irradiated by 5 MeV electron beams

The diffusion coefficient of light gas species in polyethylene is measured through a special apparatus which uses a vacuum chamber and a mass quadrupole spectrometer in order to detect the diffused species. Measurements are performed in thin polyethylene films with different molecular weight, which have been irradiated in air with 5 MeV electron beams. The irradiation modifies the poly-ethylene-inducing cross-linking processes, which reduce the diffusion coefficient. Measurements of the diffusion coefficient as a function of the gas specie and of the absorbed dose are reported. The results indicate that the radiation damage depends on the dose, on the dose rate and on the electronic stopping power. A cross-section of the damage process can be deduced. The polymer diffusion is evaluated as a function of the electron irradiation dose. Special attention is given to black polyethylene, rich in additive carbon for industrial applications which can be improved by treatment with electron beams.

DETECTION OF BREAKUP FRAGMENTS IN INVERSE COULOMB DISSOCIATION EXPERIMENTS

In the framework of Coulomb dissociation experiments, performed to reconstruct the cross section of the inverse radiative capture process, the detection of breakup fragments is usually performed using separate detectors, which unavoidably are not sensitive to small relative angles. It is shown, by means of Monte Carlo simulations, that the phase-space constraints imposed by the detection geometry can bias, specially at very low relative energies, the extracted cross section, which turns out to be strongly dependent upon the assumed angular distribution of fragments in the intrinsic reference frame. A solution to the problem of the model-dependence of the response function is proposed, making use of a single three-stage detector for the identification and the relative energy measurement of the breakup fragments. A test measurement on the reaction 208Pb(16O, 12C-a)208Pb at 126 MeV incident energy has been performed, using a gas—Si—CsI tritelescope. The results indicate the feasibility of measuring very low relative energies in the Coulomb dissociation approach to the solution of important problems in nuclear astrophysics via the accurate determination of reaction rates at energies far below the Coulomb barrier. ( 1998 Elsevier Science B.V. All rights reserved.

Evidence for pair correlation effects in heavy ion reactions

The study of the 12C(14N,14N)12C reaction was performed at 28 and 35 MeV beam energies. The results were analyzed in the frame of the EFRDWBA (Exact-Finite-Range Distorted Wave Born Approximation) assuming the simultaneous and sequential transfer of a np pair. The angular distributions, fairly reproduced in the first case, confirm the validity of the generalized BCS (Bardeen-Cooper-Schrieffer) theory to explain this behaviour. Moreover, this process could be regarded as a possible Nuclear Josephson Effect.

Proton and α Sequential Emission in the 16 O + 58 Ni Deep Inelastic Reaction: A Semi-Classical Approach

The 132 MeV 16O + 58Ni reaction has been experimentally investigated by using coincident charged particle techniques. A closed-form theoretical approach, describing in a simple picture the non-equilibrium component and the evaporative one of the angular correlation between light particles and reaction residues emitted in a peripheral heavy-ion collision, is applied — in the hypotesis of a sequential process — to the (C,N,O)-α and (C,N,O)-p differential multiplicities for the 16O + 58Ni at 8.25 MeV/A deep inelastic collision. From this analysis some reaction mechanism information is deduced.

Electric-quadrupole inelastic transitions in27Al+32S collision atEinc=100 MeV

SummaryThe differential cross-sections of various single excitation inelastic transitions in the27Al+32S collision atEinc=100 MeV were measured down to σin < ≈ 10−2 mb/sr. They show backward oscillations which are interpreted as due to interference with the transfer of five nucleons. The analysis of the data is performed in the frame of EFR-DWBA. The nuclear deformations used for the inelastic excitations are those deduced, through the use of the Blair rule, from the already known Coulomb excitation of the involved levels in the colliding nuclei. This gives a very good agreement to the experimental data. By phase analysis of the oscillations, the transfer seems however to take place in more-than-one-step mechanism.RiassuntoLe sezioni d’urto di eccitazione inelastica sono state misurate nella collisione27Al+32S aEinc=100 MeV per transizioni di quadrupolo elettrico verso gli stati più bassi dei nuclei collidenti fino a valori di σin < ≈ 10−2 mb/sr. Si sono riscontrate ai grandi angoli oscillazioni attribuibili a interferenza con i contemporanei processi di transferimento di 5 nucleoni. L’analisi delle fasi di tali oscillazioni suggerisce di attribuire il meccanismo del trasferimento a un puro processo sequenziale a due stadi, confermando i risultati ottenuti nel caso del trasferimento elastico tra gli stessi nuclei alla stessa energia.РезюмеС точностью до σin < ≈ 10−2 мб/ср измеряются дифференциальные поперечные сечения различных неупругих переходов в соударениях27Al+32S при энергииEinc=100 МэВ. Измерения овнаруживают осцилляции, которые интерпретируются, как интерференция с передачей пяти нуклонов. Анализ данных проводится в рамках EFR-DWBA. Выводятся деформации ядер, используемые для неупругих возбуждений, используя правило Блеэра, из уже известного кулоновского возбуждения рассматриваемых уровней в соударяющихся ядрах. Получается хорошее согласие с экспериментальными данными. Фазовый анализ осцилляций свидетельствует, что перенос пяти нуклонов связан с механизмом, который имеет более чем один этап.