D.W. Stracener

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.

Dynamical production of intermediate-mass fragments in peripheral 209Bi+136Xe collisions at ElabA=28 MeV

Abstract Intermediate-mass fragments (IMF) from the reaction 209 Bi+ 136 Xe at E lab A =28 MeV have been measured in coincidence with other charged reaction products, as well as with neutrons. The IMF emission patterns are seen to exhibit signatures of two mechanisms — statistical emission from the fully accelerated massive reaction partners and a fast dynamical emission from a single effective source. The latter mechanism, possibly involving a multiple neck rupture, becomes dominating for least dissipative collisions characterized by low associated light particle multiplicities.

Reaction filters. Charged-particle multiplicity and linear momentum transfer

Abstract The relation between charged-particle multiplicity and linear momentum transfer to heavy reaction residues has been investigated with a 4π charged-particle detector for the reactions 36Ar+238U at E A =35 MeV and 14N+238U at E A =50 MeV . The multiplicity of charged particles at backward angles (θ > 35°) incrreases linear momentum transfer while the multiplicity of charged particles in the forward direction is almost independent of the linear momentum transfer.

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.

Time scale for the binary disintegration of the projectile in 48Ti+93Nb collisions at 19.1 MeV/u

Abstract The time scale for the binary disintegration of projectile-like fragments was investigated in 48 Ti+ 93 Nb collisions at 19.1 MeV/ nucleon. The influence of the target proximity to the decaying projectile-like fragment was utilized to determine the time interval between the primary re-separation of the projectile and target and the secondary projectile disintegration. This time interval for near symmetric decay of the projectile, is less than ≈ 200 fm/ c . Taking into account the time interval required to separate the two fragments of the projectile to their scission configuration, the projectile decay process is one which quite likely, or quite often, is initiated before the projectile and target-like fragments are fully separated.