The High Resolution Array Detector

When nuclei collide at high velocities, they can break up into smaller fragments. By looking at these fragments we try to reconstruct how exactly the nuclei break up. This can tell us a lot about the “states” these nuclei were in before breakup. The High Resolution Array (HiRA) is a detector that will allow us to detect these fragments and determine their most important properties: their electrical charges, their masses, their energies, and direction of motion.

Expanded Description

Nuclei colliding at high velocities can break up into smaller fragments. By studying these fragments information can be gathered on the properties of the nuclei that collided, and on precisely how they collide.

The High Resolution Array is capable of detecting these fragments. It not only measures the velocity and direction of the fragments with high precision, but also their electrical charges and masses. HiRA consists of 20 telescopes. Each telescope consists of a stack of two silicon strip detectors, followed by a Cesium Iodide (CsI) detector. These detectors will each produce an electronic signal when a fragment enters the detector. By examining the electronic signals produced by a fragment that goes through the two silicon strip detectors and is stopped in the CsI-crystal, its mass, electrical charge and velocity can be determined. The silicon detectors have small strips, 0.079 inches in width, running vertically on one side of a detector and horizontally on another. This divides the area of each telescope into 1,024 square 0.079''x 0.079'' pixels, allowing us to determine where the fragment hits the detector and therefore its direction of motion with high resolution.

The Importance of HiRA

There are many research programs planned for HiRA. One of the early ones concerns the nuclear burning processes that fuel the stars and make the elements that are found on the earth and elsewhere in the universe. Such burning processes combine lighter nuclei to form heavier nuclei and thereby release energy. The earth is, in fact, the ashes of these burning processes. One interesting question concerns how these burning processes are different in really hot environments like the surface of a neutron star. HiRA helps answer these questions by allowing study of the very short-lived nuclei that are the fuel on the surface of neutron stars.

The first measurements with HiRA were the proton separation energies of one of the proton-rich nuclei that fuels nuclear burning processes on the surface of a neutron star. This technique uses the relationship between mass m and energy E, (E=mc²) discovered by Einstein to turn the energy measurement into a mass measurement. Likewise, in the environment of a neutron star, knowledge of this seperation energy can tell how much energy is released by the nuclear reactions that either make this nucleus as the ashes of nuclear burning or burn it further to make even heavier nuclei. This tells whether this nucleus is important to the burning process and helps make the light that is observed in X-ray bursts, for example.

Technical Information

The high resolution array (HiRA) is an array of 20 telescopes each of which contain a 65 µm thick Si-strip detector, a 1.5 mm thick silicon-strip detector and four 4 cm thick CsI(Tl) crystals. The silicon-strip detectors have an active area of 6.2 x 6.2 cm² which is divided into vertical 32 strips on the front. The 1.5 mm thick silicon-strip detector is double sided and has 32 vertical strips on the front-side and 32 horizontal strips on the back, providing an angular resolution of 0.15° at the nominal distance of 35 cm from the target. At this distance the 20 telescopes cover 70% of the solid angle between scattering angles of 5° and 30°. The telescopes are designed such that they can be independently placed, which allows optimizing the geometry for a specific experiment. The high resolution (about 30 keV) of the silicon-detectors will allow excellent isotopic resolution up to Z=16.

Status: Operational

Location: S2 and S3 vaults

Contact person: Bill Lynch

Funding acknowledgement: The high resolution array (HiRA) was funded by the National Science Foundation under Major Research Instrumentation grant PHY-9977707, NSCL at Michigan State University, the Indiana University Cyclotron Facility, Washington University in St. Louis, and the INFN Milano.

Reference:

The High Resolution Array (HiRA) for Rare Isotope Beam Experiments, M.S. Wallace, M.A. Famiano, M.-J. van Goethem, A.M. Rogers, W.G. Lynch, J. Clifford, F. Delaunay, J. Lee, S. Labostov, M. Mocko, L. Morris, A. Moroni, B.E. Nett, D.J. Oostdyk, R. Krishnasamy, M.B. Tsang, R.T. de Souza, S. Hudan, L.G. Sobotka, R.J. Charity, J. Elson, and G.L. Engel, Nucl. Instrum. Meth. A 583 (2007) 302.
doi: 10.1016/j.nima.2007.08.248