The high resolution array (HiRA) for rare isotope beam experiments

Abstract The High Resolution Array (HiRA) is a large solid-angle array of silicon strip-detectors that has been developed for use in a variety of nuclear structure, nuclear astrophysics and nuclear reaction experiments with short lived beta-unstable beams. It consists of 20 identical telescopes each composed of a thin ( 65 μ m ) single-sided silicon strip-detector, a thick (1.5 mm) double-sided silicon strip-detector, and four CsI(Tl) crystals read out by photodiodes. The array can be easily configured to meet the detection requirements of specific experiments. To process the signals from the 1920 strips in the array, an Application Specific Integrated Circuit (ASIC) was developed. The design and performance characteristics of HiRA are described

Development of a compact E×B microchannel plate detector for beam imaging

A beam imaging detector was developed by coupling a multi-strip anode with delay line readout to an E×B microchannel plate (MCP) detector. This detector is capable of measuring the incident position of the beam particles in one-dimension. To assess the spatial resolution, the detector was illuminated by an α-source with an intervening mask that consists of a series of precisely-machined slits. The measured spatial resolution was 520μm FWHM, which was improved to 413μm FWHM by performing an FFT of the signals, rejecting spurious signals on the delay line, and requiring a minimum signal amplitude. This measured spatial resolution of 413μm FWHM corresponds to an intrinsic resolution of 334μm FWHM when the effect of the finite slit width is de-convoluted. To understand the measured resolution, the performance of the detector is simulated with the ion-trajectory code SIMION

Searching for states analogous to the 12^{12}C Hoyle state in heavier nuclei using the thick target inverse kinematics technique

Identification of alpha cluster states analogous to the $^{12}$C Hoyle state in heavier alpha-conjugate nuclei can provide tests of the existence of alpha condensates in nuclei. Such states are predicted for $^{16}$O, $^{20}$Ne, $^{24}$Mg, $^{28}$Si etc. at excitation energies slightly above the multi-alpha particle decay threshold, but have not yet been experimentally identified. The Thick Target Inverse Kinematics (TTIK) technique can be used to study the breakup of excited self-conjugate nuclei into many alpha particles. The reaction 20Ne+${\alpha}$ was studied using a $^{20}$Ne beam at 12MeV/nucleon from the K150 cyclotron at Texas A&M University. The TTIK method was used to study both single ${\alpha}$-particle emission and multiple ${\alpha}$-particle decays. Events with alpha multiplicity up to four were analyzed. The analysis of the three ${\alpha}$- particle emission data allowed the identification of the Hoyle state and other $^{12}$C excited states decaying into three alpha particles. The results are shown and compared with other data available in the literature. Although the statistics for events with alpha multiplicity four is low, the data show a structure at about 15.2 MeV that could indicate the existence in $^{16}$O of a state analogous to the $^{12}$C Hoyle state. This structure is confirmed by the re-analysis of alpha multiplicity four events from a previous experiment performed at 9.7 MeV/nucleon with a similar setup but lower granularity. Moreover, the reconstructed excitation energy of $^{24}$Mg for these events peaks at around 34 MeV, very close to the predicted excitation energy for an excited state analogous to the $^{12}$C Hoyle state in $^{24}$Mg

Fusion enhancement at near and sub-barrier energies in 19^{19}O + 12^{12}C

Measuring the fusion excitation function for an isotopic chain of projectile nuclei provides a stringent test of a microscopic description of fusion. We report the first measurement of the fusion excitation function at near-barrier energies for the $^{19}$O + $^{12}$C system. The measured excitation function is compared with the fusion excitation function of $^{18}$O + $^{12}$C. A significant enhancement in the fusion probability of $^{19}$O ions with a $^{12}$C target as compared to $^{18}$O ions is observed. The experimental cross-sections observed at near-barrier energies are compared with a state-of-the-art microscopic model

Neutron-hole states in 45Ar from 1H(46Ar, d) 45Ar reactions

To improve the effective interactions in the pf shell, it is important to measure the single-particle and single-hole states near the N = 28 shell gap. In this paper, the neutron spectroscopic factors of hole states from the unstable neutron-rich 45Ar (Z = 18,N = 27) nucleus have been studied using the 1H(46Ar,d) 45Ar transfer reaction in inverse kinematics. Comparison of our results with the particle states of 45Ar produced in 2H(44Ar, p) 45Ar reaction shows that the two reactions populate states with different angular momenta. Using the angular distributions, we are able to confirm the spin assignments of four low-lying states of 45Ar. These are the ground state (f7/2), the first-excited state (p3/2), and the s1/2 and d3/2 states. While large basis shell-model predictions describe spectroscopic properties of the ground and p3/2 states very well, they fail to describe the s1/2 and d3/2 hole states. © 2013 American Physical Society.link_to_subscribed_fulltex

Mechanisms in knockout reactions

We report the first detailed study of the relative importance of the stripping and diffraction mechanisms involved in nucleon knockout reactions, by the use of a coincidence measurement of the residue and fast proton following one-proton knockout reactions. The measurements used the S800 spectrograph in combination with the HiRA detector array at the NSCL. Results for the reactions Be9(C9,B8+X)Y and Be9(B8,Be7+X)Y are presented and compared with theoretical predictions for the two reaction mechanisms calculated using the eikonal model. The data show a clear distinction between the stripping and diffraction mechanisms and the measured relative proportions are very well reproduced by the reaction theory. This agreement adds support to the results of knockout reaction analyses and their applications to the spectroscopy of rare isotopes. © 2009 The American Physical Society.link_to_subscribed_fulltex

Particle decay of Be12 excited states

The breakup of E/A=50 MeV Be12 fragments following inelastic scattering off of hydrogen and carbon target nuclei has been studied. The breakup channels α+He8, He6+He6, t+Li9, and p+Li11 were observed. Two doublets at excitation energies of 12.8 and 15.5 MeV were found for the α+He8 channel. A low-energy shoulder in the excitation-energy spectra at 10.2 MeV indicates one or more additional states. This work could not confirm the presence of He6-He6 rotational structure reported by Freer [Phys. Rev. C 63, 034301 (2001)], although possible peaks at excitation energies of 13.5 and 14.5 MeV were found for He6+He6 decay. Significant structure is observed in the excitation-energy spectrum for p+Li11 at 25-30 MeV which maybe associated with T=3 analog states. © 2007 The American Physical Society.link_to_subscribed_fulltex

Investigation of particle-unbound excited states in light nuclei with resonance-decay spectroscopy using a Be12 beam

Resonance-decay spectroscopy is used to study particle-unbound excited states produced in interactions of E/A=50 MeV Be12 on polyethylene and carbon targets. The particle-unbound states are produced in a variety of reaction mechanisms, ranging from projectile fragmentation to proton pickup. New proton-decaying excited states are observed in Li9(E*=14.1±0.1 MeV, Γ=207±49 keV) and Be10(E*=20.4±0.1 MeV, Γ=182±74 keV). In addition a new α-decaying state is observed in B13(E*=13.6±0.1 MeV, Γ≤320 keV). Also found was a Be8 state with E*=23 MeV, Γ=616±30 keV, which decays to the p+3H+α channel. Correlation between the fragments indicates that the decay is initiated by a proton emission to the 4.63-MeV state of Li7 and the spin of the state is J>2. A second T=2 state was confirmed in B12 at 14.82 MeV, which decays to the p+11Be, 3H+9Be, and α+8Li channels. Its width was found to be Γ≤100 keV and its spin is consistent with Jπ=2+. © 2008 The American Physical Society.link_to_subscribed_fulltex

Neutron spectroscopic factors of Ar34 and Ar46 from (p,d) transfer reactions

Single-neutron-transfer measurements using (p,d) reactions have been performed at 33 MeV per nucleon with proton-rich Ar34 and neutron-rich Ar46 beams in inverse kinematics. The extracted spectroscopic factors are compared to the large-basis shell-model calculations. Relatively weak quenching of the spectroscopic factors is observed between Ar34 and Ar46. The experimental results suggest that neutron correlations have a weak dependence on the asymmetry of the nucleus over this isotopic region. The present results are consistent with the systematics established from extensive studies of spectroscopic factors and dispersive optical-model analyses of Ca40-49 isotopes. They are, however, inconsistent with the trends obtained in knockout-reaction measurements. © 2011 The American Physical Society.link_to_subscribed_fulltex

The high resolution array (HiRA) for rare isotope beam experiments

The High Resolution Array (HiRA) is a large solid-angle array of silicon strip-detectors that has been developed for use in a variety of nuclear structure, nuclear astrophysics and nuclear reaction experiments with short lived beta-unstable beams. It consists of 20 identical telescopes each composed of a thin (65 μ m) single-sided silicon strip-detector, a thick (1.5 mm) double-sided silicon strip-detector, and four CsI(Tl) crystals read out by photodiodes. The array can be easily configured to meet the detection requirements of specific experiments. To process the signals from the 1920 strips in the array, an Application Specific Integrated Circuit (ASIC) was developed. The design and performance characteristics of HiRA are described. © 2007 Elsevier B.V. All rights reserved.link_to_subscribed_fulltex