D.C. Hensley

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.

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.

Nuclear temperature measurements and feeding from particle unbound states

Abstract Inclusive fragment cross sections and cross sections of coincident γ -ray transitions from the decay of 8 Li, 7 Be, 10 B, 12 B and 13 C fragments were measured for 32 S induced reactions on Ag at 715 MeV. These measurements are compared to quantum statistical model calculations which include sequential feeding from particle unbounds states of heavier fragments. When the uncertainties in the sequential feeding calculations are taken into account, although the measured γ -ray fractions are consistent with temperature, T ⪖ 4 MeV; the calculations demonstrate that the investigated γ -ray transitions cannot be used to determine the temperature of highly excited systems.

Population of the entry states in heavy-ion fusion reactions☆

Abstract The excitation energy and angular momentum dependence of the population of the entry states following fusion of 136 MeV 20Ne with 146Nd has been measured with a new type of instrument. Statistical-model calculations reproduce the main features of the data. Structure effects for J⪆45 are evident in the entry lines. The entrance-channel orbital angular momentum distribution leading to fusion has been deduced.

Large deformation in A ~ 170 nuclei at high excitation energies

Abstract The γ -ray decay of the giant dipole resonance was measured in coincidence with fission fragments following the fusion reaction 16 O+ 159 Tb forming 175 Ta at 123.4 MeV excitation energy. A large splitting of the giant dipole resonance (GDR) energies in the compound system was observed corresponding to a large deformation of β =0.55.

Shape coexistence and disappearance of pairing correlations in 82Sr

Abstract Extensive high-spin band structures in 82 Sr have been established using proton- γ - γ coincidence techniques. On the basis of the Woods-Saxon cranking calculations with pairing, four of these bands are interpreted to have prolate, oblate, or triaxial shapes. Pairing correlations are predicted to be very weak at high spins in this nucleus, and calculations with no static pairing successfully reproduce the experimentally observed crossing frequencies and alignments despite the fact that none of the bands displays a rigid-rotor behavior. It is concluded that observation of rigid-rotor behavior is neither necessary nor sufficient for the disappearance of static pairing in nuclei.

The angular momentum dependence of complex fragment emission

Abstract Large fragment (A>4) production at high angular momentum is studied via the reaction, 200 MeV 45 Sc + 65 Cu. Comparisons of the fragment yields from this reaction (high angular momentum) to those from 93 Nb + Be (low angular momentum) are used to verify the strong angular momentum dependence of large fragment production predicted by equilibrium models. Details of the coincident γ-ray distributions not only confirm a rigidly rotating intermediate but also indicate that the widths of the primary L-wave distributions decrease with increasing symmetry in the decay channel. These data are used to test the asymmetry and L-wave dependence of emission barriers calculated from a rotating, finite range corrected, liquid drop model.

High Spin Phenomena in the Mass 100-200 Region Seen Through the Crystal Ball

The average properties of the gamma ray entry region and the decay from it are studied systematically, for 49 nuclear systems, in the spin spectrometer. Preliminary results are given for the mass the neutron number dependence of the gamma ray fold distribution and of unresolved γ spectra. The possibility of gating simultaneously on narrow regions of fold and excitation energy is exploited.

Alpha-particle emission as a probe of nuclear shapes and structure effects in proton evaporation spectra

Emission barriers and subbarrler anisotropies from a decay of Sn* and Yb* compound nuclei are examined in the light of calculations incorporating deformation effects in the decay process. For the Yb* systems deformation which increases with spin is necessary to explain the data. For the Sn* systems the spectral shapes and auisotropies can be explained without deformation. For systems lighter than Sn this probe is not sensitive to the deformation. Energy spectra and angular correla52 34 82 tions of evaporated protons from the Cr( S, 2n2p) Sr reaction were measured in coincidence with discrete transitions. Large shifts in proton spectra were observed when high spin states in different rotational bands are populated. They are interpreted as due to near-yrast stretched proton emission preferentially populating the yrast hand by subbarrier protons. Simulations show that channel selected proton spectra cannot be used as probes of deformation.