N. T. B. Stone

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.

Tracking fission-like processes in central collisions of 40Ar+232Th; E = 15–115 A MeV

Abstract Fission-like fragments and coincident charged particles have been measured in a 4π geometry over a wide energy range (15–115 A MeV) for the reaction 40 Ar+ 232 Th. The exclusive folding angle distribution data provide direct evidence that fission-like processes following incomplete-fusion are still an appreciable exit channel for beam energies as high as 115 A MeV.

High-order azimuthal correlation functions: powerful probes for collective motion in heavy ion reactions

Abstract We have investigated the utility of high-order azimuthal correlation functions as probes of collective motion from both rotation and flow at intermediate energies. Reaction simulations indicate new and distinct signatures for rotational collective motion which are important for its characterization and its distinction from collective flow. For the system Ar + Sc(35−115A MeV), experimental high-order correlation functions are used for a clear demonstration of the disappearance of collective flow at 93±4 A MeV. The method is direct and circumvents reaction-plane assignment.

Peripheral Reaction Mechanisms in Intermediate Energy Heavy-Ion Reactions

At beam energies up to E/A = 20 MeV deep-inelastic reactions are the dominant reaction mechanism for heavy-ion peripheral collisions. These reactions are characterized by broadening of the mass and charge distributions with increasing energy loss or excitation energy and by orbiting in the deflection functions. Excitation energy is produced through the relative momentum of exchanged nucleons. The deep-inelastic reaction mechanism is a very efficient way to produce hot nuclei at relatively low beam energies.

Projectile Like Fragments from 129Xe + natCu reactions at E/A = 30, 40, 50 MeV

There has been a great deal of experimental and theoretical interest in the modes of dissassembly of highly excited nuclear matter. However, the mechanisms by which these hot nuclei are formed is also important to the study of the energy dependence of the influence of the nuclear mean field. One method to form an excited system is via a damped reaction. First observed at energies just above the Coulomb barrier, damped reactions were thought to occur only at low energies. The persistance of the damped reaction mechanism into the intermediate energy regime, between 20 and 100 MeV per nucleon, has recently been seen experimentally[1, 2]. But how high in energy do damped reactions occur? In order to measure an excitation function for damped reactions, an experiment was performed at the National Superconducting Cyclotron Laboratory (NSCL) on the campus of Michigan State University (MSU). The experiment consisted of a 129Xe beam at energies of 30, 40, 50, and 60 MeV per nucleon incident on targets of nat Cu and nat Sc. Reaction products where detecting using the MSU 4πdetector system[3] augmented by the Maryland Forward Array[4] (MFA), a detector that covers between 1.4° and 2.9° from the beam in the laboratory.

The Disappearance of Fusion/Fission

The disappearance of fusion/fission has been studied for reactions of 40Ar + 232Th at incident energies ranging from 15 to 115 MeV/nucleon using the MSU 4π Array. We have studied these reactions using a variety of observables including fission fragment opening angles, fission fragment azimuthal correlations, intermediate mass fragment and light charged particle production, and event shape analysis. We observe a change in the decay characteristics of high momentum linear transfer collisions as a function of incident energy from fission-like to multifragment emission.

The Impact Parameter Dependence of the Disappearance of Flow

The Michigan State University 4π Array has been recently upgraded to include the High Rate Array (HRA), a close-packed 45 element phoswich array which covers laboratory polar angles from ≈ 3° to ≈ 18°. The HRA subtends all solid angle between the Maryland Forward Array and the detectors of the main Ball resulting in ≈ 90% geometric efficiency for the entire array. With this improved detector configuration, we studied the reaction 40Ar + 45Sc at beam energies from 35 to 115 MeV/nucleon. We present preliminary results on the impact parameter dependence of the disappearance of flow in nuclear collisions, extending the systematics of our previous work.

Search for the Decay of Non-Compact Geometries

Recent theoretical calculations have raised one of the most intriguing questions today in intermediate energy nuclear physics. This question concerns the formation of exotic shapes in nuclear matter. There have been many theorists, using a rich diversity of models to simulate nuclear dynamics, who have predicted the occurrence of such shapes.[1–5] The title “non-compact geometries” refers to the position-space distribution of the nucleons of a combined nuclear system shortly after a nucleus-nucleus collision, and implies short-lived configurations with novel shapes, e.g. toroids or bubbles. A solid sphere is the geometrical configuration with the minimum surface area for a given volume, i.e., the most compact shape. In this light, any shape other than a sphere can be called “non-compact.”

Radial and Directed Transverse Flow in Heavy-Ion Collisions

One of the fundamental problems remaining in the field of heavy-ion reaction dynamics is the description of nuclear matter in terms of an equation of state (EOS). Collective motion is ordered motion characterized by the correlation between particle positions and momenta of a dynamic origin. The study of collective flow in nucleus-nucleus collisions can provide information about the nuclear EOS.[1,2] Collective radial expansion of particle emission from central nuclear collisions, radial flow, is primarily attributed to the conversion of thermal and compressional energy into work through a pressure gradient in the hydrodynamic limit.[3] Consequently, the fragments acquire a net outward radial velocity in addition to their random thermal component, which is evident from the increased curvature in the single-particle energy spectrum. As impact parameter increases there is anisotropy in the pressure, resulting in a transverse flow of nuclear matter in the directions of lowest pressure. Collective transverse flow in the reaction plane disappears at an incident energy, termed the balance energy E bal ,[4] where the attractive scattering dominant at energies around 10 MeV/nucleon balances the repulsive interactions dominant at energies around 400 MeV/nucleon.[5,6] We present results from a systematic study for the incident energy and impact parameter dependence of collective flow from 40Ar+45Sc collisions at E = (35 – 115) MeV/nucleon. Comparison to predictions of dynamical transport models showing agreement with our measured values of flow observables are presented.

Absence of saturation in energy deposition in 40Ar + 232Th collisions at E = 15–115 AMeV

Abstract Fission fragment (FF), intermediate mass fragment (IMF), and light charged particle (LCP) production have been measured in 40 Ar + 232 Th collisions at E = 15–115 AMeV with the Michigan State University 4π Array. Trends in IMF and LCP production and in calculated excitation energy indicate there is no saturation in the deposited energy in central collisions of this system in the bombarding energy range studied.