The Indiana Silicon Sphere


The ISiS project was initiated in 1990 in order to investigate nuclear many-body dynamics and equation-of-state physics with light-ion beams. Funds for the detector array were provided primarily by the US Department of Energy University Research Instrumentation (URI) program and Office of Nuclear and High Energy Physics, with additional support from Indiana University. Total cost of the project was $675,000, including subsequent upgrade funds. Local technical support was provided by the Indiana University Department of Chemistry and the IU Cyclotron Facility, the latter funded by the National Science Foundation.

The ISiS array was successfully assembled and implemented in 1993. Its inaugural experiment was performed at the Laboratoire National Saturne in Saclay, France (E228). Subsequently, it has been employed to study the mechanisms of energy deposition and multifragmentation in reactions that extend from the Fermi-energy/pion-threshold regime up to projectile energies of 15 GeV, where the excitation of baryonic resonances (Δ, N*, etc.) plays an important role in heating nuclear matter. The former experiments were performed at the Indiana University Cyclotron Facility in 1995 (E375) and the latter at the Brookhaven National Laboratory AGS accelerator in 1996 (E900) and 1998 (E900a). These were the first studies of hadron and helium beams incident on heavy target nuclei for which charged particles up to Z~15 have been measured with high geometric acceptance, low energy thresholds and discrete charge identification. Because the hot nuclei formed in these collisions disintegrate primarily as a consequence of thermal and Coulomb forces, they form an important complement to heavy-ion studies, where compression and rotation influence the dynamics strongly.

Below we describe the scientific goals of the ISiS program, the properties of the ISiS array, and briefly summarize some of the significant results obtained thus far. This is followed by a cast of characters and a list of publications from the project.

Scientific Goals

Determining the properties of hadronic matter as a function of temperature and density is one of the fundamental goals of nuclear physics. The description of these properties in terms of a nuclear equation of state (EOS) is essential not only for understanding the physics of complex nuclei, but also to account for astrophysical phenomena--for example, the aggregation of low-density hadronic matter in supernovae to form neutron stars and black holes. Of particular interest is determining the nuclear compressibility constant K for hot nuclear matter, for which considerable uncertainty still exists.

Until recently, our knowledge of the nuclear equation of state has been restricted to nuclear structure in or near the ground state. However, advances in accelerator and detector technology, as well as theory, during the past decade have made it possible to conduct much more rigorous tests of the predicted dependence of the EOS on density and temperature. Our research has focused on the use of light-ion beams for this purpose, using the Indiana Silicon Sphere (ISiS) 4π detector array to measure the multiparticle breakup states that signal the formation of hot nuclear matter at low density (ρ<ρ0) While significant recent progress has been made, a satisfactory description of the nuclear EOS remains a difficult challenge--largely due to the complex dynamics associated with heating finite nuclear matter to the expected phase-transition region and the fast time scale for disassembly. Further, the temperature-density space that must be explored demands data from many different projectile-target combinations and energies, as well as highly sophisticated detection systems. For example, the fast collective expansion that follows the initial compression in heavy-ion-induced reactions tends to mask purely thermal effects. These can be more cleanly isolated with hadron probes.

Central collisions between hadrons and heavy nuclei are unique in that they create highly excited nuclei with minimal compression and angular momentum. The heating mechanism proceeds via hard N-N scattering and the multiple excitations of baryonic resonances--temporarily creating a region of locally concentrated resonance matter in the core of the nucleus. The rapid evolution of such systems into regions of phase instability is indicated in Fig. 1 for a 4.8 GeV 3He beam. Here the collision trajectory predicted by a BUU (cascade plus mean field) calculation is superimposed on the temperature/density/entropy-per-nucleon phase diagram for infinite nuclear matter. Time steps are in units of 4 fm/c and the trajectory follows the maximum (not average) density, reflecting the projectile momentum front. The calculations indicate that the nucleus is rapidly heated to energies near the total binding energy of the target residue with little compression. This is followed by mass loss via emission of fast cascade ejectiles and expansion into the liquid-gas coexistence and spinodal instability regions on a time scale ( ≤ 40 fm/c) that is much shorter than that for collective nuclear response. At this point, the growth of density fluctuations leads to disassembly of the residue, or multifragmentation.

The Indiana Silicon Sphere 4π Detector Array (ISiS)

In order to investigate fragmentation processes in intermediate-to-high energy collisions, a versatile 4π electronic detector array is required. Such an array must possess the following characteristics:

The Indiana Silicon Sphere detector array ( Fig. 2) is based on a spherical geometry, designed primarily for the study of light-ion-induced reactions. It consists of 162 triple telescopes--90 in the forward hemisphere and 72 in the backward hemisphere--covering the angular ranges from 14° to 86.5° and 93.5° to 166°. The design consists of eight rings, each composed of 18 truncated-pyramid telescope housings. To increase granularity for the most forward angles, the ring nearest 0° is segmented into two components. A sketch of the detector configuration in the forward hemisphere is shown in Fig. 3. Each telescope is composed of:

  1. a gas-ionization chamber (GIC) operated at 25-40 torr of CF4 or 12-20 torr of C3F8;
  2. a 500 µm ion-implanted passivated silicon detector, Si(IP), and
  3. a 28 mm thick CsI(Tl) crystal with light guide and photodiode readout.

Detectors are operated in a common gas volume; vacuum isolation is provided by a 250 µg/cm2 polypropylene window supported by a cage-like structure. The telescope dynamic range permits measurement of Z ≈ 1 - 20 fragments with discrete charge resolution over the dynamic range 0.6 ≤ E ≤ 96 MeV. The Si(IP)/CsI(Tl) telescopes also provide particle identification (Z and A) for energetic H, He, Li and Be isotopes (E/A ≥ 8 MeV). The Si(IP) detectors constitute a critical component of the array in that they provide both excellent energy resolution and reliable energy calibration for the GIC and CsI(Tl) elements.

The combined solid angle and energy acceptance of the ISiS array is significantly improved compared to currently operating 4π arrays based on phoswich scintillator technology. The figure of merit here is the product of solid angle coverage and the fraction of the total fragment energy spectra that is above threshold. For ISiS, the total solid angle is 74% of 4π as determined by simulations with the GEANT code. The major acceptance advantage of the ISiS array is its very low detector thresholds (E/A ≈ 0.6 MeV compared to E/A ≈ 2.0 - 3.5 MeV for scintillator phoswich arrays). For these light-ion data, the detectors based on phoswich technology would miss about 90% of the cross section in the backward hemisphere.

Dynamic range is also important. Simulations indicate that for central collisions, our upper energy of 96 MeV/nucleon provides excellent multiplicity and total energy information for IMFs formed in central collisions. In addition. It is possible to obtain hit information on minimum-ionizing radiation up to 400MeV by reading out the CsI crystals in an event for which the silicon signal is too low to trigger any discriminator, but the CsI energy is large (>16MeV).

More information is available on the following topics:

For more information about the ISiS project or comments about this Web page, please contact:
     Vic Viola
or Luc Beaulieu.