D.S. Bracken

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.

The Indiana silicon sphere 4π charged-particle detector array

A low threshold charged particle detector array for the study of fragmentation processes in light-ion-induced reactions has been constructed and successfully implemented at the IUCF and Saturne II accelerators. The array consists of 162-triple-element detector telescopes mounted in a spherical geometry and covering 74% of 4π in solid angle. Telescope elements are composed of (1) an axial-field gas ionization chamber operated with C3F8 gas; (2) a 0.5 mm thick passivated silicon detector, and (3) a 2.8 cm thick CsI(Tl) scintillation crystal with photodiode readout. Discrete element identification is obtained for ejectiles up to Z ∼ 16 over the dynamic range 0.7 ≤ EA ≤ 95 MeV/nucleon. Isotopes are also distinguished for H, He, Li and Be ejectiles with 8 ≲ EA ≲ 95 MeV. Custom-designed electronics are employed for bias supplies and linear signal processing. Data are acquired via a CAMAC/VME/Ethernet system.

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.

Heating nuclear matter with GeV 3He beams

Abstract The heating curve for hot nuclei formed in the 4.8 GeV 3 He+ nat Ag, 197 Au reactions has been derived from reconstructed excitation-energy distributions and temperatures based on 2,3 H/ 3,4 He isotope ratios. Intranuclear-cascade predictions over-estimate the excitation-energy distributions for hot thermal-like residues, but are in general agreement with the data when contributions from preequilibrium emission are included. Both targets exhibit nearly an identical temperature vs. excitation energy dependence. The heating curve initially increases rapidly, undergoes a slope change near E ∗ /A = 2–3 MeV, and then follows a gradual monotonic increase up to E ∗ /A = 10 MeV. The results are in qualitative agreement with predictions of EES and SMM multifragmentation models.

Time dependence of multifragmentation in light-ion-induced reactions

Abstract The evolution of multifragmentation reactions has been investigated via large-angle correlations of IMFs emitted in 4.8 GeV 3 He bombardment of 197 Au nuclei. Relative IMF-IMF velocity data agree well with fission kinetic energy systematics for IMFs with Z ≳ 8, but increasingly deviate above the fission prediction as the IMF charge decreases. This result depends only weakly on IMF multiplicity. The results suggest that IMF emission occurs from an evolving system in which lighter fragments are emitted preferentially from a hotter, more dense source prior to breakup of a dilute residue that also produces heavier fragments. The data are compared with both time-dependent (EES) and simultaneous (SMM) multifragmentation models.

Evaporation residue as a dominant exit channel at high thermal energies in 3He + Ag reactions

Abstract In the reaction 3 He (1.8 GeV) + nat Ag , events are observed with a heavy fragment (HF), A≥10 , in coincidence with charged particles detected over 70% of 4π solid angle. Calorimetric measurements show high thermal excitation energies of the target primary fragment: 6–8 MeV per nucleon for HF mass ≳45 . For these excitation energies, the probability for having an evaporative residue is shown to be unexpectedly high when compared with current multifragmentation models. This result is interpreted as linked to the use of light ion projectiles at relatively low incident energy.

Saturation of deposition energy in relativisitic 3He-induced reactions

Abstract The 4π detector ISiS has been used to measure light-charged particles and intermediate-mass fragments emitted in the 1.8–4.8 GeV 3 He + nat Ag , 197Au reactions. Ejectile multiplicity and total event energy distributions scale systematically with projectile energy and target mass, except for the natAg target at 3.6 and 4.8 GeV. For this system, a saturation in deposition energy is indicated by the data, suggesting the upper projectile energy for stopping has been reached. Comparison of the experimental distributions with intranuclear cascade predictions shows qualitative agreement.

A 4 π charged-particle detector array for light-ion-induced nuclear fragmentation studies☆☆☆

Abstract Operating characteristics of the Indiana Silicon Sphere 4 π detector array are outlined. The detector geometry is spherical, with 90 telescopes in the forward hemisphere and 72 at backward angles, covering a total solid angle of 74% of 4π. Each telescope consists of a simple gas-ion chamber, operated with C3F8 gas, followed by a 0.5 mm thick ion-implanted silicon detector and a 28 mm CsI(Tl) crystal, readout by a photodiode. Custom-built bias supplies and NIM preamp/shaper modules were used in conjunction with commercial CFD, TDC and ADC CAMAC units.

Effects of in-medium cross sections and optical potential on thermal-source formation in p+{sup 197}Au reactions at 6.2-14.6 GeV/c

Effects of in-medium cross sections and of optical potential on preequilibrium emission and on formation of a thermal source are investigated by comparing the results of transport simulations with experimental results from the

Probing the nuclear EOS with GeV light-ion beams

p+^{197}\mathrm{Au}