R. Alfaro

LASSA: a large area silicon strip array for isotopic identification of charged particles

Abstract A new high resolution, charged particle detector array, LASSA, has been developed. LASSA consists of nine individual telescopes that each utilizes a Si–Si–CsI(Tl) stack to provide isotopic identification of fragments (1⩽Z⩽8) with good angular resolution over a wide dynamic range in energy. The energy range covered is E/A=2.4– 140 MeV for protons and E/A=4.8– 335 MeV for 16O ions. The front portion of each telescope is comprised of a 65 μm Si(IP) strip detector backed by a 500 μm Si(IP) strip detector. This second detector provides position information in two dimensions. Behind the second silicon detector is a cluster of four 6 cm thick CsI(Tl) crystals, each read-out by a photodiode. The design, construction, and performance characteristics of this detector telescope are described.

Fragment production in noncentral collisions of intermediate-energy heavy ions

The defining characteristics of fragment emission resulting from the non-central collision of 114Cd ions with 92Mo target nuclei at E/A = 50 MeV are presented. Charge correlations and average relative velocities for mid-velocity fragment emission exhibit significant differences when compared to standard statistical decay. These differences associated with similar velocity dissipation are indicative of the influence of the entrance channel dynamics on the fragment production process.

Excitation and decay of projectilelike fragments formed in dissipative peripheral collisions at intermediate energies

Projectile-like fragments (PLF:15<=Z<=46) formed in peripheral and mid-peripheral collisions of 114Cd projectiles with 92Mo nuclei at E/A=50 MeV have been detected at very forward angles, 2.1 deg.<=theta_lab<=4.2 deg. Calorimetric analysis of the charged particles observed in coincidence with the PLF reveals that the excitation of the primary PLF is strongly related to its velocity damping. Furthermore, for a given V_PLF*, its excitation is not related to its size, Z_PLF*. For the largest velocity damping, the excitation energy attained is large, approximately commensurate with a system at the limiting temperature

Fragment isospin as a probe of heavy-ion collisions

Isotope ratios of fragments produced at mid-rapidity in peripheral and central collisions of 114Cd ions with 92Mo and 98Mo target nuclei at E/A = 50 MeV are compared. Neutron-rich isotopes are preferentially produced in central collisions as compared to peripheral collisions. The influence of the size (A), density, N/Z, E*/A, and Eflow/A of the emitting source on the measured isotope ratios was explored by comparison with a statistical model (SMM). The mid-rapidity region associated with peripheral collisions does not appear to be neutron-enriched relative to central collisions.

Comparison of midvelocity fragment formation with projectilelike decay

The characteristics of intermediate mass fragments (IMFs: 3 Z 20) produced in midperipheral and central collisions are compared. We compare IMFs detected at midvelocity with those evaporated from the excited projectilelike fragment (PLF ∗ ). On average, the IMFs produced at midvelocity are larger in atomic number, exhibit broader transverse velocity distributions, and are more neutron rich as compared to IMFs evaporated from the PLF ∗ . These characteristics of midvelocity fragments are consistent with the low-density formation of the

Interplay of initial deformation and Coulomb proximity on nuclear decay

Alpha particles emitted from an excited projectilelike fragment sPLF * d formed in a peripheral collision of two intermediate-energy heavy ions exhibit a strong preference for emission towards the targetlike fragment. The interplay of the initial deformation of the PLF * caused by the reaction, Coulomb proximity, and the rotation of the PLF * results in the observed anisotropic angular distribution. Changes in the shape of the angular distribution with excitation energy are interpreted as being the result of forming more elongated initial geometries in the more peripheral collisions.