D. P. Balamuth

Alignment Delays in the N = Z Nuclei 72Kr, 76Sr, and 80Zr

The ground state rotational bands of the N = Z nuclei (72)Kr, (76)Sr, and (80)Zr have been extended into the angular momentum region where rotation alignment of particles is normally expected. By measuring the moments of inertia of these bands we have observed a consistent increase in the rotational frequency required to start pair breaking, when compared to neighboring nuclei. (72)Kr shows the most marked effect. It has been widely suggested that these delayed alignments arise from np-pairing correlations. However, alignment frequencies are very sensitive to shape degrees of freedom and normal pairing, so the new experimental observations are still open to interpretation.

Channel selection for in-beam γ-ray spectroscopy using a 4π phoswich array

Abstract A modular 4π array of up to 26 charged particle detectors has been designed and constructed to provide channel selection for in-beam γ-ray spectroscopy. The system is useful for the identification and study of nuclei far from stability, populated in very weak fusion-evaporation channels. The individual detectors are phoswich scintillator telescopes of plastic and CaF2(Eu), optimized for identification of evaporation protons and alpha particles. The detector array is very compact, to allow large Ge γ-ray detectors to be placed close to the target. The system has been used for the identification of nuclei in the very neutron-deficient Ge region, produced with submillibarn cross sections.

Angular correlation study of levels in 16C

Abstract Angular correlation measurements using the 14 C(t, pγ) 16 C reaction result in the following spin assignments to levels in 16 C: E x = 1.766 MeV, J = 2; E x = 3.980 MeV, J = 2; E x = 4.083 MeV, J = 3; E x = 4.138 MeV, J = 4.

Instrumentation of double-sided silicon strip detectors for multi-particle detection

A compact, modular, low-cost, high-density readout system for double-sided silicon strip detectors (DSSD) has been developed. The system, composed of a customized high-density, multi-channel preamplifier unit combined with commercial CAMAC modules, provides for the individual readout of each of the 32 segments in a 16 × 16 crossed-strip DSSD. This system permits X–Y position sensitivity to be achieved for multiple-particle events, making double-sided silicon strip detectors powerful tools for the study of nuclear reactions leading to many-charged-particle final states.

Electromagnetic transitions in the N = Z + 1 nucleus 71Br

Abstract Electromagnetic transitions in the previously unknown neutron deficient nucleus 71 Br have been identified. Fifteen γ rays have been placed in a decay scheme using n- γ - γ coincidences. The level scheme deduced from the data is remarkably similar to that of 73, 75 Br; in particular, no evidence has been found for a theoretically predicted transition to stable oblate deformation with decreasing N .

Applications of complete reconstruction of multi-particle final states in fusion-evaporation reactions using a 4π detector

Abstract Measurement of the energies and directions of one or more charged particles emitted in a fusion-evaporation reaction permits the excitation energy of the remaining system of nucleons to be calculated. The measured excitation energy is strongly linked to the probability that additional particles will be evaporated. Observation of γ-rays in coincidence with the charged particles and forming a two-dimensional matrix of excitation energy vs E γ is thus useful for identifying the reaction channel associated with unknown γ-rays. Here we apply this method to the identification of in-beam γ-rays in 85 Zr and 84 Y, resulting from the pn and 2pn reactions respectively in the 27 Al+ 60 Ni system. Peak-to-background enhancements approaching an order of magnitude can be obtained by selection of events using kinematical criteria derived from the analysis. In addition, we demonstrate how the charged particle angular distribution obtained with a 4π detector can be used to identify reactions on light target contaminants because of differences in kinematic focussing.

Origin of correlations between evaporation protons and discrete gamma rays in heavy-ion fusion-evaporation reactions.

We have investigated correlations between evaporated protons and discrete {gamma} rays in the heavy-ion fusion-evaporation reactions {sup 58}Ni({sup 27}Al,3{ital p}){sup 82}Sr and {sup 58}Ni({sup 28}Si,4{ital p}){sup 82}Sr. 3{ital p}-{gamma} and 4{ital p}-{gamma} coincidences were measured using a 4{pi} charged-particle detector array. The events studied were completely reconstructed kinematically in order to determine the excitation energy of formation of {sup 82}Sr. Correlations arising from well-understood kinematic effects were measured and found to be well reproduced by predictions made by the statistical-model evaporation code PACE2. Dynamical correlations due to nuclear structure effects were searched for by measuring proton spectra in coincidence with discrete {gamma} rays from four rotational bands in {sup 82}Sr. Contrary to published results, no significant shifts of the first moment of the spectra were observed.

Operating experience with a multiple-processor system for data acquisition and reduction in nuclear physics

A multiprocessor system based on commercially available VMEbus components has been developed for the acquisition and reduction of event‐mode data in nuclear structure physics experiments. The system contains seven 68000 CPU’s and 14MB of memory. A simple operating system handles data transfer and task allocation, and a compiler for a specially designed event analysis language produces code for the processors. Design considerations are discussed for the hardware and software, with emphasis on the differences in requirements between nuclear and high‐energy physics applications. The system has been in operation for four years at the University of Pennsylvania Tandem Accelerator Laboratory. Computation rates several times that of a MicroVAX II have been achieved at a fraction of the cost. A recent improvement to the system has been the addition of WORM optical disks which allow the processing of gigabyte data sets without operator intervention.