Search

The normalized $^{238}$U(n,f)/$^{235}$U(n,f) cross section ratio has been measured using the NIFFTE fission Time Projection Chamber from the reaction threshold to $30$~MeV. The fissionTPC is a two-volume MICROMEGAS time projection chamber that allows for full three-dimensional reconstruction of fission-fragment ionization profiles from neutron-induced fission. The measurement was performed at the Los Alamos Neutron Science Center, where the neutron energy is determined from neutron time-of-flight. The $^{238}$U(n,f)/$^{235}$U(n,f) ratio reported here is the first cross section measurement made with the fissionTPC, and will provide new experimental data for evaluation of the $^{238}$U(n,f) cross section, an important standard used in neutron-flux measurements. Use of a development target in this work prevented the determination of an absolute normalization, to be addressed in future measurements. Instead, the measured cross section ratio has been normalized to ENDF/B-VIII.$\beta$5 at 14.5 MeV.

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.

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.

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.

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.

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.

We have added a new measurement at 100 MeV/nucleon to our previous excitation function for collective flow of light fragments from {sup 40}Ar+{sup 51}V collisions. In the earlier work, flow decreased as the beam energy was raised from 45 to 85 MeV/nucleon. This provided hints to the disappearance of flow, but the lack of measurements at higher beam energies precluded the observation of the reappearance of flow. At a beam energy of 100 MeV/nucleon the flow has reappeared and this allows an experimental determination of the region where attractive scattering balances with repulsive scattering in these collisions. A simultaneous least-squares fit of the flow data for light fragments yields (85{plus minus}10) MeV/nucleon for the balance energy.

Experiments at the Bevalac using the Plastic Ball and streamer chamber initiated the study of nucleus-nucleus collisions with close to 4π acceptance for charged particles. [l] Much was learned as a result of these experiments, and by the mid 1980s, there was a widespread belief that it would soon be possible to constrain unambiguously the equation of state of the compressed hadron gas that exists for a few fm/c during the early stage of the collision. During the years since then, we have come to a better understanding of the difficulties involved in inferring properties of the nuclear equation of state, and while these difficulties are surmountable, there are some indications that experimental observables are less strongly influenced by the state variables of interest than was initially assumed. [2,3] Such considerations motivate new measurements with good statistics and the smallest possible observational biases.

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.

The design of a Time Projection Chamber (TPC) detector with complete pad coverage is presented. The TPC will allow the measurements of high multiplicity (∼200 tracks), relativistic nucleus‐nucleus collisions initiated with the heaviest, most energetic projectiles available at the LBL BEVALAC accelerator facility. The front end electronics, composed of over 15,000 time sampling channels, will be located on the chamber.

We have measured the transverse momentum of protons as a function of rapidity in 50 MeV/nucleon C-induced peripheral collisions with C and Au targets. The reaction plane for each event was determined from the azimuthal angle of lightly damped coincident projectile fragments. High transverse momentum protons in the midrapidity region were found to be deflected towards the opposite side of the reaction plane with respect to the beam axis than the projectile fragments. This result is interpreted as projectile fragment scattering to positive angles due to the repulsive Coulomb interaction while midrapidity protons are simultaneously deflected to negative angles by the attractive component of the nuclear mean field.

By studying light charged particles emitted in coincidence with projectile-like fragments, we probe heavy ion reaction mechanisms that result in the production of light particles in peripheral collisions. Triple coincidence {alpha}-{alpha}-{sup 12}C correlations in the reactions of 35 MeV/nucleon {sup 16}O with Ni conclusively identify previously known sources of light particles. The fast, beam velocity component of {alpha} particles in coincidence with projectile-like fragments is associated with the breakup of the projectile. The correlations suggest that the origin of the intermediate velocity component of {alpha} particles is the target nucleus. Both of these components are characterized by {alpha} particles emitted during the early stages of the collision.

Azimuthal distributions of fragments with respect to the reaction plane are studied in the Ar+V system as a function of beam energy. Light charged particles are found to exhibit an enhanced emission in the reaction plane which increases with the mass of the detected particle. As the beam energy is increased, the asymmetry decreases. Possible mechanisms behind the asymmetry, such as rotational collective motion and directed transverse momentum, are discussed.

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.

We examine a variety of methods for determining both the impact parameter and the direction of the impact vector in symmetric nuclear collisions at intermediate energies. Two quantities, the particle multiplicity and the midrapidity charge, retain their dependence on the impact parameter after filtering through the acceptance of a typical 4\ensuremath{\pi} detector. By gating on these quantities we can select four ranges of impact parameters. There is some overlap of these ranges. It is noted that this is dependent on the model used to simulate the collisions. The midrapidity charge has the advantage that it integrates over the final fragments and should be less sensitive to how the collision zone disassembles. The angle of the impact vector is well reproduced with the method developed by Danielewicz and Odyniec. The difference between known and determined reaction plane has a half-width at half-maximum of less than 70\ifmmode^\circ\else\textdegree\fi{}. Some comparisons are made to experimental data.

Light charged fragments from the reactions $^{40}\mathrm{Ar}$${+}^{51}$V at 35 MeV/nucleon and $^{12}\mathrm{C}$${+}^{12}$C at 50 MeV/nucleon have been measured with a 4\ensuremath{\pi} array. Transverse collective motion is observed for all fragment types and increases in strength for the heavier fragments. The transverse momentum is qualitatively similar for the different impact parameters selected by the midrapidity charge. The global transverse momentum analysis includes corrections for correlations due to momentum conservation. The results are compared to those at higher energies to investigate whether the same or a new reaction phenomenon produces the collective motion. We outline some implications for the extraction of thermodynamic information from the kinetic energy of emitted fragments.

We report measurements for in-plane and out-of-plane correlations for p,d,t,/sup 3/He, and /sup 4/He from 40 MeV/nucleon C+C. Silicon-NaI light particle telescopes were placed at (theta,phi) = (45/sup 0/,90/sup 0/) and (45/sup 0/,180/sup 0/) and another telescope was moved from theta = 25/sup 0/ to 120/sup 0/ at phi = 0/sup 0/. No peak was observed in the ratio of in- to out-of-plane proton-proton correlations at energies corresponding to quasi-elastic scattering. The angular distributions of the ratio of in- to out-of-plane for p-p, d-d, t-t, /sup 3/He-/sup 3/He, and /sup 4/He-/sup 4/He peak around 35/sup 0/ and the maximum value increases monotonically with mass of the particles reaching a value near 4 for /sup 4/He-/sup 4/He. A calculation incorporating simple kinematic effects cannot reproduce the observed correlations for any of the paritcle pairs.

Angular correlations between light particles have been studied to probe the extent to which a thermally equilibrated system is formed in heavy ion collisions near the Fermi energy. Single-light-particle inclusive energy spectra and two-particle large-angle correlations were measured for 40 and 50 MeV/nucleon C+C, Ag, and Au. The single-particle inclusive energy spectra are well fit by a three moving source parametrization. Two-particle large-angle correlations are shown to be consistent with emission from a thermally equilibrated source when the effects of momentum conservation are considered. Single-particle inclusive spectra and light-particle correlations at small relative momentum were measured for 35 MeV/nucleon N+Ag. Source radii were extracted from the two-particle correlation functions and were found to be consistent with previous measurements using two-particle correlations and the coalescence model. The temperature of the emitting source was extracted from the relative populations of states using the quantum statistical model and was found to be 4.8/sub -2.4//sup +2.8/ MeV, compared to the 14 MeV temperature extracted from the slopes of the kinetic energy spectra.

Large angle correlations have been measured between two light particles (Z = 1,2) emitted from 40 MeV/nucleon C+C, Ag, Au, and 50 MeV/nucleon C+C reactions. The first particle is detected at theta = -45/sup 0/, and the second particle is detected in the same plane with -150/sup 0/less than or equal tothetaless than or equal to+150/sup 0/. The correlations are consistent with emission from a thermalized system that is constrained by momentum conservation effects. In addition, the decay of particle unstable states contributes heavily to the coincidence cross sections for p-p, p-t, p-/sup 4/He, d-/sup 4/He, t-/sup 4/He, and /sup 4/He-/sup 4/He correlations.

Alpha particles have been measured in coincidence with both projectile-like fragments (Z = 5--8) and light fragments (Z = 3--4) in the reactions of 35 MeV/nucleon /sup 16/O with /sup 58/Ni. These results provide an extension of previous systematics studying the evolution of nonequilibrium emission of ..cap alpha.. particles from reactions at 6 to 20 MeV/nucleon. Two nonequilibrium components can be identified: one consists of beam velocity ..cap alpha.. particles and the other consists of intermediate velocity ..cap alpha.. particles. Compared to lower bombarding energies, no drastic change in the reaction mechanism has been observed at the Fermi energy.

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.

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.