P. Mastinu

Pulse shape analysis of liquid scintillators for neutron studies

The acquisition of signals from liquid scintillators with Flash ADC of high sampling rate ð 1G S=sÞ has been investigated. The possibility to record the signal waveform is of great advantage in studies with g’s and neutrons in a high count-rate environment, as it allows to easily identify and separate pile-up events. The shapes of pulses produced by g-rays and neutrons have been studied for two different liquid scintillators, NE213 and C6D6: A 1-parameter fitting procedure is proposed, which allows to extract information on the particle type and energy. The performance of this method in terms of energy resolution and n=g discrimination is analyzed, together with the capability to identify and resolve pile-up events. r 2002 Elsevier Science B.V. All rights reserved.

A TELESCOPE WITH MICROSTRIP GAS CHAMBERS FOR THE DETECTION OF CHARGED PRODUCTS IN HEAVY-ION REACTIONS

Prototypes of a AE-E telescope, designed to detect and identify with low-energy threshold both light charged particles and heavy fragments, are described. They are based on a gas drift chamber which conveys primary ionization electrons on gas microstrip devices where multiplication occurs and the energy loss signals are generated. Silicon detectors or CsI(TI) crystals operate as residual energy detectors. The prototypes were tested both with a source and heavy ion beams. Performances, mainly related to energy resolution, charge identification and angle resolution, are reported.

Calibration of CsI(Tl) scintillators for heavy ions (3⩽Z⩽54) in a wide energy range (Eu⩽60MeV/u)

Abstract The light output response of CsI(Tl) scintillators, photodiodes readout, has been investigated for heavy ions with atomic number 3⩽ Z ⩽54 for energies up to 45 MeV/u. The detectors have been calibrated with particular attention to the dependence on the atomic number of the ions. The expression obtained with this procedure was applied to the calibration of the CsI(Tl) detectors of the MULTICS array, on the basis of only one renormalization coefficient. A complete compatibility has been found with recent data at energies up to 60 MeV/u.

A Method to Obtain a Maxwell–Boltzmann Neutron Spectrum at kT = 30 keV for Nuclear Astrophysics Studies

A method to shape the neutron energy spectrum at low-energy accelerators is proposed by modification of the initial proton energy distribution. A first application to the superconductive RFQ of the SPES project at Laboratori Nazionali di Legnaro is investigated for the production of a Maxwell–Boltzmann neutron spectrum at kT = 30 keV via the 7Li(p, n)7Be reaction. Concept, solutions and calculations for a setup consisting of a proton energy shaper and a lithium target are presented. It is found that a power dentisity of 3 kW cm–2 could be sustained by the lithium target and a forward-directed neutron flux higher than 1010 s–1 at the sample position could be obtained. In the framework of the SPES project the construction of a LEgnaro NeutrOn Source (LENOS) for Astrophysics and for validation of integral nuclear data is proposed, suited for activation studies on stable and unstable isotopes.

33S as a cooperative capturer for BNCT

(33)S is a stable isotope of sulfur for which the emission of an α-particle is the dominant exit channel for neutron-induced reactions. In this work the enhancement of both the absorbed and the equivalent biologically weighted dose in a BNCT treatment with 13.5keV neutrons, due to the presence of (33)S, has been tested by means of Monte Carlo simulations. The kerma-fluence factors for the ICRU-4 tissue have been calculated using standard weighting factors. The simulations depend crucially on the scarce (33)S(n,α)(30)Si cross-section data. The presence of a high resonance at 13.5keV was established by previous authors providing discrepant resonance parameters. No experimental data below 10keV are available. All of this has motivated a proposal of experiment at the n_TOF facility at CERN. A setup was designed and tested in 2011. Some results of the successful test will be shown. The experiment is scheduled for the period November to December 2012.

Isotopic composition of fragments in nuclear multifragmentation

In this Rapid Communication we present results from the analysis of the isotopic yields of fragments emitted in two selected reactions: the decay of the quasiprojectile in Au 1Au peripheral collisions at 35 MeV/nucleon, and the disassembly of the unique source formed in Xe1Cu central reactions at 30 MeV/nucleon. We find that the relative yields of neutron-rich isotopes increase with the excitation energy of the emitting sources. In the framework of a statistical multifragmentation model which reproduces fairly well the experimental observables, such behavior can be explained with the increase of the N/Z ratio of the hot primary fragments. This corresponds to the statistical evolution of the fragmentation mechanism as a function of the excitation energy, from the decay into few small fragments with a heavy residue to complete multifragmentation. PACS number~s!: 25.70.Pq, 24.60.2k Nuclear fragmentation and its connection to the behavior of nuclear matter at high excitation energy is the subject of intensive theoretical and experimental investigations @1#. Some general properties of this process are already established: at relatively small excitation energies ( E* < 2‐3 A MeV) there is a formation and decay of a longlived compoundlike nucleus system. This process can be described by evaporation/fissionlike models. At higher excitation energies ~close to the binding energy! there is a complete fast disintegration of the system into fragments, this process can take place in a finite breakup volume. In this case statistical models based on the hypothesis of a nuclear phase transition ~simultaneous decay! happen to be very successful @2,3#. In this Rapid Communication we present recent data on the isotope production in heavy-ion collisions at intermediate energies @4,5# with the aim of studying the isotopic content of fragments for different sources and excitation energies during the transition from the low energy decay to the multifragmentation. We will show that the behavior of the experimental isotopic yields, as a function of source size, isospin, and excitation energy, can be connected to the corresponding evolution of the N/Z ratio of the hot primary fragments in the breakup volume, which is an important ingredient in the disintegration process. In fact, there are both experimental and theoretical studies which show that the knowledge of the chemical composition of hot fragments helps in establishing the freeze-out conditions. ~1! Information about the density of the freeze-out volume can be obtained from the analysis of the velocity correlation functions and kinetic energies of the emitted fragments. Among the different methods @6,7# to extract the temperature of the system, the most popular is based on the statistical properties of the double isotope ratios. With this technique a nuclear caloric curve, as an experimental evidence of a nuclear liquid-gas phase transition @8#, was obtained. In any case, a correction for secondary decay of hot fragments produced in the freeze-out volume @9# is needed, and depending on their N/Z ratio, the secondary decay can proceed differently.

Isotopic composition of fragments in multifragmentation of very large nuclear systems: Effects of the chemical equilibrium

Studies on the isospin of fragments resulting from the disassembly of highly excited large thermal-like nuclear emitting sources, formed in the ^{197}Au + ^{197}Au reaction at 35 MeV/nucleon beam energy, are presented. Two different decay systems (the quasiprojectile formed in midperipheral reactions and the unique source coming from the incomplete fusion of projectile and target in the most central collisions) were considered; these emitting sources have the same initial N/Z ratio and excitation energy (E^* ~= 5--6 MeV/nucleon), but different size. Their charge yields and isotopic content of the fragments show different distributions. It is observed that the neutron content of intermediate mass fragments increases with the size of the source. These evidences are consistent with chemical equilibrium reached in the systems. This fact is confirmed by the analysis with the statistical multifragmentation model.

Calibrating the CsI(Tl) detectors of the GARFIELD apparatus

The energy and charge dependence of the light output of the CsI(Tl) detectors of the GARFIELD apparatus has been investigated for heavy ions with 5pZp16 in the energy range from 2.2 to 8:3 A MeV: The results have been compared to an analytical expression successfully used in previous calibration procedures at higher energies, and a rather good agreement was obtained between measured and calculated quantities. The resulting parameter set was successfully applied to another set of experimental data. The overall result demonstrates the validity of the above mentioned calibration procedure in a wide range of incident ion energies and masses. r 2002 Elsevier Science B.V. All rights reserved. PACS: 29.40.Mc

The Neutron Time-Of-Flight Facility n_TOF At CERN: Phase II

Neutron‐induced reactions are studied at the neutron time‐of‐flight facility n_TOF at CERN. The facility uses 6∼ns wide pulses of 20 GeV/c protons impinging on a lead spallation target. The large neutron energy range and the high instantaneous neutron flux combined with high resolution are among the key characteristics of the facility. After a first phase of data taking during the period 2001–2004, the facility has been refurbished with an upgraded spallation target and cooling system for a second phase of data taking which started in 2009. Since 2010, the experimental area at 185 m where the neutron beam arrives, has been modified into a worksector of type A, allowing the extension of the physics program to include neutron‐induced reactions on radioactive isotopes.

Time scale and freeze-out volume in the Xe + Cu reaction at 45 MeV/u

Correlations between Intermediate Mass Fragments were measured for the reaction Xe + Cu atE/A= 45 MeV/u. The velocity correlation function for central 3-fold events, depleted at small values of the relative coordinate, as typical for fast decay processes, reflects the mutual Coulomb repulsion between the emitted fragments. From the comparisons between a significant number of experimental observables and the predictions of the Berlin Multifragmentation Model, it appears that the data are compatible with a simultaneous multifragment emission.