R. Laforest

Signals for a transition from surface to bulk emission in thermal multifragmentation.

Excitation-energy-gated two-fragment correlation functions have been studied between E(*)/A = (2-9)A MeV for equilibriumlike sources formed in 8-10 GeV/c pi(-) and p+197Au reactions. Comparison with an N-body Coulomb-trajectory code shows an order of magnitude decrease in the fragment emission time in the interval E(*)/A = (2-5)A MeV, followed by a nearly constant breakup time at higher excitation energy. The decrease in emission time is strongly correlated with the onset of multifragmentation and thermally induced radial expansion, consistent with a transition from surface-dominated to bulk emission expected for spinodal decomposition.

Thermal excitation of heavy nuclei with 5–15 GeV/c antiproton, proton and pion beams

Abstract Excitation-energy distributions have been derived from measurements of 5.0–14.6 GeV/c antiproton, proton and pion reactions with 197 Au target nuclei, using the ISiS 4 π detector array. The maximum probability for producing high excitation-energy events is found for the 8 GeV/c antiproton beam relative to other hadrons, 3 He and p beams from LEAR. For protons and pions, the excitation-energy distributions are nearly independent of hadron type and beam momentum above about 8 GeV/c. The excitation energy enhancement for p beams and the saturation effect are qualitatively consistent with intranuclear cascade code predictions. For all systems studied, maximum cluster sizes are observed for residues with E ∗ /A∼6 MeV.

Effect of nucleon exchange on projectile multifragmentation in the reactions of {sup 28}Si+{sup 112}Sn and {sup 124}Sn at 30 and 50 MeV/nucleon

The multifragmentation of quasiprojectiles was studied in the reactions of a {sup 28}Si beam with {sup 112}Sn and {sup 124}Sn targets at projectile energies of 30 and 50 MeV/nucleon. The quasiprojectile observables were reconstructed using isotopically identified charged particles with Z{sub f}{<=}5 detected at forward angles. The nucleon exchange between projectile and target was investigated using the isospin and the excitation energy of the reconstructed quasiprojectile. For events with total reconstructed charge equal to the charge of the beam (Z{sub tot}=14), the influence of the beam energy and target isospin on the neutron transfer was studied in detail. Simulations were carried out employing a model of deep inelastic transfer, a statistical model of multifragmentation, and a software replica of the FAUST detector array. The concept of deep inelastic transfer provides a good description of the production of highly excited quasiprojectiles. The isospin and excitation energy of the quasiprojectile were described with good overall agreement. The fragment multiplicity, charge and isospin were reproduced satisfactorily. The range of contributing impact parameters was determined using a backtracing procedure.

FAUST : A NEW FORWARD ARRAY DETECTOR

A new forward array has been designed and constructed using silicon technology. The detectors telescopes are Si followed by CsI that are read out by photodiodes. This combination allows for isotopic resolution through Z = 6. The special edge-mounting of the silicon detector combined with the ring-like structure of the overall array has allowed for a design that minimizes dead area.

Isospin dependence of isobaric ratio and its relation to temperature

Abstract A dependence of the isobaric ratio Y( 3 H)/Y( 3 He ) on the N / Z ratio of the reconstructed quasiprojectile for the reaction of 28 Si beam with 112,124 Sn targets at two different projectile energies of 30 and 50 MeV/nucleon is presented. We demonstrate a linear dependence of the observable ln( Y( 3 H)/Y( 3 He ) ) on the N / Z ratio of the quasiprojectile and show the dependence of the slope on the reconstructed excitation energy of the quasiprojectile. We relate this slope dependence at a given excitation energy to the temperature of the fragmenting system. Using the model assumptions of the statistical multifragmentation model, a method of temperature determination is proposed. A caloric curve is constructed and compared to the result of the double isotope ratio method for the same set of the data and to the results of other studies.

Isospin dependence of isobaric ratio Y(H-3) / Y(He-3) and its possible statistical interpretation

Abstract A dependence of the isobaric ratio Y( 3 H)/Y( 3 He ) on the N / Z ratio of the reconstructed quasiprojectile for the reaction of 28 Si beam with 112,124 Sn targets at two different projectile energies of 30 and 50 MeV/nucleon is presented. We demonstrate a linear dependence of the observable ln( Y( 3 H)/Y( 3 He ) ) on the N / Z ratio of the quasiprojectile and show the dependence of the slope on the reconstructed excitation energy of the quasiprojectile. We relate this slope dependence at a given excitation energy to the temperature of the fragmenting system. Using the model assumptions of the statistical multifragmentation model, a method of temperature determination is proposed. A caloric curve is constructed and compared to the result of the double isotope ratio method for the same set of the data and to the results of other studies.

Inhomogeneous isospin distribution in the reactions of {sup 28}Si+{sup 112}Sn and {sup 124}Sn at 30 and 50 MeV/nucleon

We have created quasiprojectiles of varying isospin via peripheral reactions of {sup 28}Si+{sup 112}Sn and {sup 124}Sn at 30 and 50 MeV/nucleon. The quasiprojectiles have been reconstructed from completely isotopically identified fragments. The difference in N/Z of the reconstructed quasiprojectiles allows the investigation of the disassembly as a function of the isospin of the fragmenting system. The isobaric yield ratio {sup 3}H/{sup 3}He depends strongly on N/Z ratio of quasiprojectiles. The dependences of mean fragment multiplicity and mean N/Z ratio of the fragments on N/Z ratio of the quasiprojectile are different for light charged particles and intermediate mass fragments. Observation of a different N/Z ratio of light charged particles and intermediate mass fragments is consistent with an inhomogeneous distribution of isospin in the fragmenting system.

Breakup time scale studied in the 8 GeV/c pi- + 197Au reaction

Experimental data from the reaction of an 8.0 GeV/c pi- beam incident on a 197Au target have been analyzed in order to investigate the integrated breakup time scale for hot residues. Alpha-particle energy spectra and particle angular distributions supported by a momentum tensor analysis suggest that at large excitation energy, above 3-5 MeV/nucleon, light-charged particles are emitted prior to or at the same time as the emission of the heavy fragments. Comparison with the SMM and GEMINI models is presented. A binary fission-like mechanism fits the experimental data at low excitation energies, but seems unable to reproduce the data at excitation energies above 3-5 MeV/nucleon.

Caloric curve of 8 GeV/c negative pion and antiproton + Au reactions

The relationship between nuclear temperature and excitation energy of hot nuclei formed by 8 GeV/c negative pion and antiproton beams incident on 197Au has been investigated with the ISiS 4-pidetector array at the BNL AGS accelerator. The double-isotope-ratio technique was used to calculate the temperature of the hot system. The two thermometers used (p/d-3He/4He) and (d/t-3He/4He) are in agreement below E*/A ~ 7 MeV when corrected for secondary decay. Comparison of these caloric curves to those from other experiments shows some differences that may be attributable to instrumentation and analysis procedures. The caloric curves from this experiment are also compared with the predictions from the SMM multifragmentation model.

Caloric curve of8GeV/cπ−,p¯+197Aureactions

The relationship between nuclear temperature and excitation energy of hot nuclei formed by 8 GeV/c negative pion and antiproton beams incident on 197Au has been investigated with the ISiS 4-pidetector array at the BNL AGS accelerator. The double-isotope-ratio technique was used to calculate the temperature of the hot system. The two thermometers used (p/d-3He/4He) and (d/t-3He/4He) are in agreement below E*/A ~ 7 MeV when corrected for secondary decay. Comparison of these caloric curves to those from other experiments shows some differences that may be attributable to instrumentation and analysis procedures. The caloric curves from this experiment are also compared with the predictions from the SMM multifragmentation model.