Vertex Reconstruction Using a Single Layer Silicon Detector

Typical vertex finding algorithms use reconstructed tracks, registered in a multi-layer detector, which directly point to the common point of origin. A detector with a single layer of silicon sensors registers the passage of primary particles only in one place. Nevertheless, the information available from these hits can also be used to estimate the vertex position, when the geometrical properties of silicon sensors and the measured ionization energy losses of the particles are fully exploited. In this paper the algorithm used for this purpose in the PHOBOS experiment is described. The vertex reconstruction performance is studied using simulations and compared with results obtained from real data. The very large acceptance of a single-layered multiplicity detector permits vertex reconstruction for low multiplicity events where other methods, using small acceptance subdetectors, fail because of insufficient number of registered primary tracks.Comment: accepted for publication in Nucl. Instr. Meth.

A high efficiency, low background detector for measuring pair-decay branches in nuclear decay

We describe a high efficiency detector for measuring electron-positron pair transitions in nuclei. The device was built to be insensitive to gamma rays and to accommodate high overall event rates. The design was optimized for total pair kinetic energies up to about 7 MeV.Comment: Accepted for publication by Nucl. Inst. & Meth. in Phys. Res. A (NIM A

Vanishing spin alignment : experimental indication of triaxial 28Si+28Si\bf ^{28}Si + {^{28}Si} nuclear molecule

Fragment-fragment-$\gamma$ coincidences have been measured for $\rm ^{28}Si + {^{28}Si}$ at an energy corresponding to the population of a conjectured resonance in $^{56}$Ni. Fragment angular distributions as well as $\gamma$-ray angular correlations indicate that the spin orientations of the outgoing fragments are perpendicular to the orbital angular momentum. This differs from the $\rm ^{24}Mg+{^{24}Mg}$ and the $\rm ^{12}C+{^{12}C}$ resonances, and suggests two oblate $\rm ^{28}Si$ nuclei interacting in an equator-to-equator molecular configuration.Comment: 14 pages standard REVTeX file, 3 ps Figures -- Accepted for publication in Physical Review C (Rapid Communication

Silicon Pad Detectors for the PHOBOS Experiment at RHIC

The PHOBOS experiment is well positioned to obtain crucial information about relativistic heavy ion collisions at RHIC, combining a multiplicity counter with a multi-particle spectrometer. The multiplicity arrays will measure the charged particle multiplicity over the full solid angle. The spectrometer will be able to identify particles at mid-rapidity. The experiment is constructed almost exclusively of silicon pad detectors. Detectors of nine different types are configured in the multiplicity and vertex detector (22,000 channels) and two multi-particle spectrometers (120,000 channels). The overall layout of the experiment, testing of the silicon sensors and the performance of the detectors during the engineering run at RHIC in 1999 are discussed.Comment: 7 pages, 7 figures, 1 table, Late

Heavy-ion resonance and statistical fission competition in the Mg24+24Mg system at Ec.m.=44.4 MeV

The fully energy-damped cross sections of the Mg24+24Mg reaction at Ec.m.=44.4 MeV have been measured for all of the major fission channels. High-resolution Q-value spectra have been obtained for the large-angle yields in the Mg24+24Mg and Ne20+28Si channels. Calculations based on the transition-state model are found to reproduce the fully damped cross sections in all of the observed mass channels. The pronounced structure that is observed in the excitation-energy spectra for the more symmetric mass channels, even for the strongly damped yields, is shown to be qualitatively reproduced by assuming a spin-weighted population of the fragment states. There is no evidence, however, that the structure of the nascent fission fragments at scission may influence the population of states in the fragments. These results, taken together with earlier measurements of the resonance behavior of this system, suggest the coexistence of fission from the normal, compact compound nucleus with that from the deformed configurations believed to be responsible for the resonance behavior

Angular correlation, spin alignment, and systematics of mis-matched 12C + 12C inelastic scattering resonances

Particle gamma-ray angular correlation measurements have been used to study the spin alignment and magnetic-substate population parameters for the 21+ (4.443 MeV) state in 12C, populated in the 12C(12C,12C[02+]) 12C(21+) inelastic scattering reaction in the vicinity of a prominent, narrow peak in the scattering excitation function. The data show a strong alignment of the spin with the orbital angular momentum, and suggest that the cross section peak corresponds to a spin 14+ resonance at Ec.m. = 28.0 MeV. This energy is close to that where a strong peak is also observed in the 01+ + 0 2+ excitation function. A comparison between the data for these two channels lends some support to recent theoretical calculations of resonance behavior for angular-momentum-mismatched channels in 12C + 12C inelastic scattering

γ-ray spectroscopy of the A=23, T=1/2 nuclei 23Na and 23Mg: High-spin states, mirror symmetry, and applications to nuclear astrophysical reaction rates

Background: Obtaining reaction rates for nuclear astrophysics applications is often limited by the availability of radioactive beams. Indirect techniques to establish reaction rates often rely heavily on the properties of excited states inferred from mirror symmetry arguments. Mirror energy differences can depend sensitively on nuclear structure effects. Purpose: The present work sets out to establish a detailed comparison of mirror symmetry in the A=23, T=1/2 mirror nuclei 23Na and 23Mg both to high spin, and high excitation energy, including beyond the proton threshold. These data can be used to benchmark state-of-the-art shell-model calculations of these nuclei. Methods: Excited states in 23Na and 23Mg were populated using the 12C(12C,p) and 12C(12C,n) reactions at beam energies of 16 and 22 MeV, and their resulting γ decay was measured with Gammasphere. Results: Level schemes for 23Na and 23Mg have been considerably extended; highly excited structures have been found in 23Na, as well as their counterparts in 23Mg for previously known rotational structures in 23Na. Mirror symmetry has been investigated up to an excitation energy of 8 MeV and spin-parity of 13/2+. Excited states in the region above the proton threshold have been studied in both nuclei. Conclusions: A detailed exploration of mirror symmetry has been performed which heavily constrains expectations as to how mirror energy differences should evolve for different structures. Agreement with shell-model calculations provides confidence in using such estimations where real data are absent

Search for the 1/2+ intruder state in P 35

The excitation energy of deformed intruder states (specifically the 2p2h bandhead) as a function of proton number Z along N=20 is of interest both in terms of better understanding the evolution of nuclear structure between spherical Ca40 and the Island of Inversion nuclei, and for benchmarking theoretical descriptions in this region. At the center of the N=20 Island of Inversion, the npnh (where n=2,4,6) neutron excitations across a diminished N=20 gap result in deformed and collective ground states, as observed in Mg32. In heavier isotones, npnh excitations do not dominate in the ground states but are present in the relatively low-lying level schemes. With the aim of identifying the expected 2p2h - s1/2+ state in P35, the only N=20 isotone for which the neutron 2p2h excitation bandhead has not yet been identified, the S36(d,He3)P35 reaction has been revisited in inverse kinematics with the HELical Orbit Spectrometer (HELIOS) at the Argonne Tandem Linac Accelerator System (ATLAS). While a candidate state has not been located, an upper limit for the transfer reaction cross section to populate such a configuration within a 2.5 to 3.6 MeV energy range provides a stringent constraint on the wave function compositions in both S36 and P35

Selective population of states in fission fragments from the S32+24Mg reaction

The symmetric and near-symmetric mass fission yields from the S32+24Mg reaction have been studied in a particle-particle- coincidence measurement. Evidence is presented for a selective population of states in Si28 fragments arising from the symmetric fission of the Ni56 compound nucleus. A statistical-model calculation of the expected strength to specific mutual excitations of the fission fragments is presented and compared to the experimental results. This calculation is found to describe the structures observed at high excitation energy in the fission Q-value spectra quite well. Analysis of the -ray spectra indicates, however, that a specific set of states in Si28, corresponding to a highly deformed prolate band, is populated more strongly than expected based on a purely spin-weighted, statistical decay of the compound nucleus. It is suggested that the population pattern of states in the fission fragments may reflect nuclear structure effects at the point of scission

High-spin spectrum of 24Mg studied through multiparticle angular correlations

We describe the investigation of high-spin states in 24Mg populated with the 12C(16O,α)24Mg reaction at 62- and 68-MeV beam energy. The excited states were established through the coincident detection of up to four α and γ particles in complete decay cascades toward a final state of angular momentum zero, the ground state of either 20Ne or 16O. We describe a new analysis method for the angular correlations in such events and apply it with the goal to assign spin and parity to α-unbound states in 24Mg. We establish a number of high-spin, natural-parity states between spins 6 and 12, including the lowest known 10+ and 12 + levels. The energy systematics of positive- and negative-parity high-spin states are compared to the predictions of modern shell-model calculations