D.A. Cebra

The light response of plastic scintillator and the calibration of large arrays

Abstract The light response function for both fast and slow plastic scintillator (BC-412 and BC-444) has been studied in a series of experiments with phoswich (fast/slow plastic scintillator) detectors. The light response of the slow plastic element was determined as a function of the charge (Z), the mass number (A) and the energy(E) deposited by incident particles that were stopped within the scintillator material. The light response of the fast scintillation element was determined as a function of Z, A and E for both stopping and for transmitted particles. The response function was found to be independent of particle species for fragments that were transmitted through the scintillator. The response functions are independent of experimental configuration and are not specific to individual detectors. A method for the analysis of phoswich spectra which employs these calibration functions is developed. This method is suitable for calibrating large numbers of detectors.

Bragg curve spectroscopy in a 4π geometry

Abstract Ionization counters employing Bragg curve spectroscopy have been constructed for use in a 4π geometry. These detectors compare very favorably in terms of both energy and charge resolution with small solid angle devices. These detectors have a large dynamic range because they are backed by scintillation detectors, and are thus capable of detecting and identifying particles with energies from 1 MeV/nucleon up to 200 MeV/nucleon.

Peripheral collisions of 40 MeV/u 14N with a 197Au target

Abstract A 32-phoswich detector array has been used in conjunction with two Si-telescopes to detect, in coincidence, light and projectile-like fragments produced in the reaction 14 N + 197 Au at 560 MeV. Inclusive and semi-exclusive cross sections are given. Angular correlations and invariant cross-section velocity distributions of light particles in coincidence with projectile-like fragments are used to deduce the dominant reaction mechanisms. The transfer of nucleons to the target and sequential breakup of the quasi-projectile with or without pickup of target nucleons are clearly seen. Some evidence is found for a prompt breakup of the projectile into 10 B + α followed by a dissipative interaction of the heavy fragment with the target.

Two-particle correlations from 500 MeV p+Ag and Be

Abstract Two-particle correlations between light particles at small relative momenta have been measured for proton-induced reactions on Ag and Be at 500 MeV. The results for p + Ag and p + Be are very similar to each other and to those observed in nucleus-nucleus collisions at similar total incident energy. This similarity suggests that the observed correlations are dominated by final state interactions rather than by details of the emitting system and therefore may cast doubt on the extraction of temperatures and radii from nuclear collisions.

Collective flow, multi-fragment emission and azimuthal asymmetries in intermediate energy nucleus-nucleus collisions

Abstract Using the MSU 4π Array with the NSCL K1200 Superconducting Cyclotron we have measured an excitation function for 40 Ar+ 51 V from 35 to 100 MeV/nucleon. We have observed a minimum in collective flow around 82 MeV/nucleon which marks the transition from attractive to repulsive scattering. Multi-fragment emission is observed to occur sequentially in reactions at 35 MeV/nucleon. As the beam energy is raised, the emission becomes more simultaneous. Using azimuthal asymmetries, we have extracted rotation-like phenomena. This signal for rotation is strong at 35 MeV/nucleon and nearly disappears as the beam energy is raised to 85 MeV/nucleon.

Mass dependence of pion production in heavy ion collisions near, but below threshold☆

Abstract We have measured the inclusive cross section for production of negative pions near mid-rapidity in 20 Ne + NaF , 139 La + 139 La and 197 Au + 197 Au collisions at E = 183 and 236 MeV/u. Au + Au is the heaviest system from which subthreshold pion production has been measured to date. The dependence of the pion cross section on pion energy, beam energy and associated charged particle multiplicity is consistent with previous results both above and below threshold. The dependence of the cross section on the mass of the colliding system varies only slightly as the beam energy is reduced below threshold, in contrast to some previous measurements. Comparison with theory suggests that at these energies the pion production process is still dominated by nucleon-nucleon collisions.

Impact-parameter independence of participant energy spectra measured in symmetric heavy-ion collisions

Abstract A reaction filter of nearly 4π sr geometry is presented. Its ability to tag heavy-ion collisions according to their centrality is established by a comparison of experimental data with computer simulations and BUU model calculations in which events of known impact parameter are filtered by the instrumental acceptance. On the one hand, energy spectra of light charged particles from reactions of 20 Ne at 35 and 45 MeV/nucleon with a NaF target show little dependence of their high-energy slopes upon the relative impact parameters determined by this technique. On the other hand, spectra from 20 Ne on 45 Sc and 197 Au targets display increasing slopes with increasing centrality. Such a difference between symmetric and asymmetric systems is consistent with the formation of a “participant” zone.

Directed Transverse Momentum and Multi-Particle Emission in Intermediate Energy Nucleus-Nucleus Collisions

We have measured directed transverse momentum and multi-fragment emission for the systems of 35 MeV/nucleon 40Ar-51V, 50 MeV/nucleon 12C + 12C, and 50, 70, and 130 MeV/nucleon 139La + 139La. We observe that directed transverse momentum increases with decreasing mass of the system at energies around 50 MeV/nucleon. In addition the directed transverse momentum goes to 0 as the beam energy is decreased from 130 to 50 MeV/nucleon for La + La. We present results for multiparticle emission from the same systems in terms of global momentum space variables. We demonstrate that there are observable differences between sequential and simultaneous emission dynamics using global variables. Our results for 35 MeV/nucleon Ar + V fall closer to the sequential simulations than the simultaneous.

4π Fragment Measurements at MSU and the Nuclear Equation of State

In the collision of two nuclei at intermediate energies (20 – 200 MeV/nucleon) a variety of phenomena can be studied including multifragmentation, the liquid-gas phase transition in nuclear matter, energy and momentum flow, the production of entropy, and the role of thermodynamics in these collisions.1’2 These phenomena involve correlations of many particles. In such experiments it is clear that the more complete the event characterization is, the more accurate will be the understanding of the underlying phenomena. Presented here are the results from the first set of experiments using the MSU 4π Array to study nuclear collisions at intermediate energies using the NSCL K500 Superconducting Cyclotron.