J. M. Elson

“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.

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.

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.

Fast pulsed UV light source and calibration of non-linear photomultiplier response

A novel ultraviolet pulsed-light source with a digitally controlled amplitude has been developed and used to determine the response of the BURLE 8850 photomultiplier tube for light intensities close to that corresponding to single photoelectrons to those well beyond the linear response region.

Democratic Decay of 6Be Exposed by Correlations

The interaction of an E/A=70-MeV (7)Be beam with a Be target was used to populate levels in (6)Be following neutron knockout reactions. The three-body decay of the ground and first excited states into the α+p+p exit channel were detected in the High Resolution Array. Precise three-body correlations extracted from the experimental data allowed us to obtain insight into the mechanism of the three-body democratic decay. The correlation data are in good agreement with a three-cluster-model calculation and thus validate this theoretical approach over a broad energy range.

Democratic Decay ofBe6Exposed by Correlations

The interaction of an E/A=70-MeV (7)Be beam with a Be target was used to populate levels in (6)Be following neutron knockout reactions. The three-body decay of the ground and first excited states into the α+p+p exit channel were detected in the High Resolution Array. Precise three-body correlations extracted from the experimental data allowed us to obtain insight into the mechanism of the three-body democratic decay. The correlation data are in good agreement with a three-cluster-model calculation and thus validate this theoretical approach over a broad energy range.

Three-body decay of Be-6

Three-body correlations for the ground-state decay of the lightest two-proton emitter Be-6 are studied both theoretically and experimentally. Theoretical studies are performed in a three-body hyperspherical-harmonics cluster model. In the experimental studies, the ground state of Be-6 was formed following the alpha decay of a C-10 beam inelastically excited through interactions with Be and C targets. Excellent agreement between theory and experiment is obtained demonstrating the existence of complicated correlation patterns that can elucidate the structure of Be-6 and, possibly, of the A = 6 isobars.

Superdeformation in 91Tc

A high-spin rotational band with 11 gamma -ray transitions has barn observed in Tc-91. The dynamical moment of inertia as well as the transition quadrupole moment of 8.1(-1.4)(+1.9) eb measured for this band show the characteristics of a superdeformed band. However, the shape is more elongated than in the neighbouring A = 80-90 superdeformed nuclei. Theoretical interpretations of the band within the cranked Strutinsky approach based on two different Woods-Saxon potential parameterisations are presented. Even though an unambiguous configuration assignment proved difficult, both calculations indicate a larger deformation and at least three additional high-N intruder orbitals occupied compared to the lighter SD nuclei

Ground-state proton decay of 69Br and implications for the rp-process 68Se waiting-point

The first direct measurement of the proton separation energy, Sp, for the proton-unbound nucleus 69Br is reported. Of interest is the exponential dependence of the 2p-capture rate on Sp which can bypass the 68Se waiting-point in the astrophysical rp process. An analysis of the observed proton decay spectrum is given in terms of the 69Se mirror nucleus and the influence of Sp is explored within the context of a single-zone X-ray burst model.