L. G. Sobotka

Systematics of complex fragment emission in niobium-induced reactions

Abstract Complex fragments of 3 Z ⪅35 have been detected in the reverse-kinematics reactions of 93 Nb plus 9 Be, 12 C and 27 Al at bombarding energies of E / A =11.4, 14.7 and 18.0 MeV. Velocity spectra and angular distributions show the presence of projectile and target-like components along with a component isotropic (in the reaction plane). This latter component aappears as a Coulomb ring in the invariant cross section plots indicating the presence of a binary decay which is confirmed by the coincidence data. Statistical model calculations indicate that for the Nb+Be and C reactions, the isotropic component is associated with the binary decay of compound nuclei formed in complete fusion reactions. The charge distributions for these two systems are consistent with the conditional barriers predicted with the rotating finite-range model. For the Nb+Al reactions, there is an additional isotropic component besides compound nucleus decay, which may arise from fast fission.

Universality of spectator fragmentation at relativistic bombarding energies

Abstract Multi-fragment decays of 129Xe, 197Au and 238U projectiles in collisions with Be, C, Al, Cu, In, Au and U targets at energies between E A = 400 and 1000 MeV have been studied with the ALADIN forward-spectrometer at SIS. By adding an array of 84 SiCsI(Tl) telescopes the solid-angle coverage of the setup was extended to θlab = 16°. This permitted the complete detection of fragments from the projectile-spectator source. The dominant feature of the systematic set of data is the Zbound universality that is obeyed by the fragment multiplicities and correlations. These observables are invariant with respect to the entrance channel if plotted as a function of Zbound, where Zbound is the sum of the atomic numbers Zi of all projectile fragments with Zi ⩾ 2. No significant dependence on the bombarding energy nor on the target mass is observed. The dependence of the fragment multiplicity on the projectile mass follows a linear scaling law. The reasons for and the limits of the observed universality of spectator fragmentation are explored within the realm of the available data and with model studies. It is found that the universal properties should persist up to much higher bombarding energies than explored in this work and that they are consistent with universal features exhibited by the intranuclear cascade and statistical multifragmentation models.

“The microball” Design, instrumentation and response characteristics of a 4π-multidetector exit channel-selection device for spectroscopic and reaction mechanism studies with Gammasphere

Abstract A 4π multidetector light-charged particle and light fragment detection spectrometer is described. It consists of 95 CsI(T1) scintillators closely packed to cover the angular range 4.0°–172°, arranged in 9 rings with increasing forward segmentation. The device is optimally designed to be used in conjunction with Gammasphere. The scintillator light is collected by silicon photodiodes that provide high quantum efficiency and minimal mass. The signals are processed through a charge sensitive preamplifier followed by a slow shaper. Particle identification for 1,2,3 H, 3,4 He ions and Li, Be and B ions is accomplished by pulse shape discrimination. The geometry, construction, energy calibration, gain stability, associated integrated electronics and the data acquisition system are discussed. The capabilities of this spectrometer as a channel selecting device in conjunction with Gammasphere are discussed. A second version of the device with thicker scintillators, that can stop more energetic charged particles and is useful for reaction mechanism studies, is also described.

Dwarf Ball and Dwarf Wall: Design, instrumentation, and response characteristics of a 4π CsI(Tl) plastic phoswich multidetector system for light charged particle and intermediate mass fragment spectrometry

A 4π multidetector intermediate mass fragment and charged particle spectrometer is described. It consists of the Dwarf Ball section with 65 CsI(Tl) plastic scintillator phoswiches closely packed to cover the angular range of 32°–168°, and the Dwarf Wall section, with 40 CsI(Tl) plastic phoswiches covering the angular range of 4°–32°. For each detector that fires, three regions of the photomultiplier anode current are separately integrated; one at early times for the fast plastic ΔE, another at intermediate times for the bulk of the signal from the CsI(Tl), and a third in the tail region of the CsI(Tl) signal. In addition, the times are recorded for each detector that fires. From this information, 1,2,3H, 3,4He and the elements from Li to Mn can be identified and their energies measured over a large dynamic range. The geometry, construction, energy calibration, gain stability, associated electronics, and an approximate light charged particle identification procedure are discussed. Examples of the performance of the spectrometer from heavy-ion induced reaction experiments are given. The capabilities of this device as a channel selecting device in conjunction with the spin spectrometer are also discussed.

Neutron and Proton Transverse Emission Ratio Measurements and the Density Dependence of the Asymmetry Term of the Nuclear Equation of State

Recent measurements of preequilibrium neutron and proton transverse emission from (112,124)Sn+(112,124)Sn reactions at 50 MeV/A have been completed at the National Superconducting Cyclotron Laboratory. Free nucleon transverse emission ratios are compared to those of A=3 mirror nuclei. Comparisons are made to Boltzmann-Uehling-Uhlenbeck (BUU) transport calculations and conclusions concerning the density dependence of the asymmetry term of the nuclear equation of state at subnuclear densities are made. Comparison to BUU model predictions indicate a density dependence of the asymmetry energy that is closer to a form in which the asymmetry energy increases as the square root of the density for the density region studied. A coalescent-invariant analysis is introduced as a means of reducing suggested difficulties with cluster emission in total nucleon emission.

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.

Neutron-proton asymmetry dependence of spectroscopic factors in Ar isotopes

Spectroscopic factors have been extracted for proton-rich 34Ar and neutron-rich 46Ar using the (p, d) neutron transfer reaction. The experimental results show little reduction of the ground state neutron spectroscopic factor of the proton-rich nucleus 34Ar compared to that of 46Ar. The results suggest that correlations, which generally reduce such spectroscopic factors, do not depend strongly on the neutron-proton asymmetry of the nucleus in this isotopic region as was reported in knockout reactions. The present results are consistent with results from systematic studies of transfer reactions but inconsistent with the trends observed in knockout reaction measurements.

Evolution of the giant dipole resonance in excited 120Sn and 208Pb nuclei populated by inelastic alpha scattering

Abstract The evolution of the giant dipole resonance (GDR) in 120 Sn and 208 Pb nuclei at excitation energies in the range of 30–130 MeV and 40–110 MeV, respectively, were studied by measuring high energy γ rays from the decay of the resonance. The excited states were populated by inelastic scattering of α particles at beam energies of 40 and 50 MeV/nucleon for 120 Sn and 40 MeV/nucleon for 208 Pb. A systematic increase of the resonance width with increasing excitation energy was observed for both nuclei. The observed width evolution was compared to calculations employing a model that adiabatically couples the collective excitation to the nuclear shape, and to a model based on the collisional damping of nucleons. The adiabatic coupling model described the width evolution in both nuclei well, whereas the collisional damping calculation could describe the width evolution only in 208 Pb. Light-particle inelastic scattering populates low angular momentum states in the target nucleus. The observed width increase is therefore interpreted to be predominantly due to fluctuations in the nuclear shape induced by temperature. This interpretation is consistent with the adiabatic model calculations and with recent angular momentum-gated measurements of the GDR in excited Sn isotopes.

A “dwarf ball”: Design, instrumentation, and response characteristics of a 4π light charged-particle multidetector system

Abstract A 4 π light charged-particle spectrometer is described. The spectrometer consist of 72 fast-low plastic scintillator phoswiches closely packed in a 4 π arrangement. The device is small enough to be enclosed in the spin spectrometer scattering chamber. For each detector that fires, the fast ΔE and slow E pulse heights and a time for each group of 16 detectors are recorded. From this information protons and α particles can be identified and their energies measured over a large dynamic range. The geometry, construction, electronics and data acquisition system are discussed. Examples are given of the performance of this spectrometer from an experiment in which Si ( ΔE , E ) heavy-ion telescopes were used as event triggers and the spin spectrometer detected γ rays and neutrons.

Energy resolution and energy}light response of CsI(Tl) scintillators for charged particle detection

This article describes the crystal selection and quality control utilized to develop and calibrate a high-resolution array of CsI(Tl) scintillator crystals for the detection of energetic charged particles. Alpha sources are used to test the light output variation due to thallium doping gradients. Selection of crystals with better than 1% non-uniformity in light output is accomplished using this method. Tests with a 240 MeV alpha beam reveal that local light output variations within each of the tested CsI(Tl) crystals limit the resolution to about 0.5%. Charge and mass dependences in the energy}light output relationship are determined by calibrating with energetic projectile fragmentation beams. ( 2001 Elsevier Science B.V. All rights reserved.