A non-contact laser-based alignment system (LBAS) for nuclear-physics experiments

High-resolution reconstruction of reaction dynamics in nuclear-physics experiments with radioactive beams requires accurate knowledge of the beam-particle trajectories and the precise alignment of detectors with respect to the reaction target. In many cases, sub-millimeter position measurements of fragile beam-tracking and particle-detector systems are essential. We have constructed a laser-based alignment system (LBAS) which is a non-contact, high-precision alignment tool designed for applications where excellent spatial positioning must be achieved. The working principles and performance of the laser-based alignment system are presented.link_to_subscribed_fulltex

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

Isospin effects in 40,48Ca+40,48Ca collisions

The isospin dependence of two proton correlations is studied in 40Ca+40Ca and 48Ca+48Ca collisions at E / A = 80 MeV. Measurements were performed with the HiRA detector array complemented by the 4π Ball at NSCL. We observe a strong isospin dependence of the pp-correlation functions; however the emitting source radius extracted using the imaging technique yields no sensitivity to the isospin of the reaction system. We interpret this result as a consequence of smaller fraction of fast proton emission in the neutron rich 48Ca system. © 2010 Elsevier B.V. All rights reserved.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

Ground-state proton decay of Br69 and implications for the se68 astrophysical rapid proton-capture process waiting point

We report on the first direct measurement of the proton separation energy for the proton-unbound nucleus Br69. Bypassing the Se68 waiting point in the rp process is directly related to the 2p-capture rate through Br69, which depends exponentially on the proton separation energy. We find a proton separation energy for Br69 of Sp(Br69)=-785-40+34keV; this is less bound compared to previous predictions which have relied on uncertain theoretical calculations. The influence of the extracted proton separation energy on the rp process occurring in type I x-ray bursts is examined within the context of a one-zone burst model. © 2011 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

Isobaric multiplet mass equation for A=7 and 8

Deviations from the isobaric multiplet mass equation are presented and discussed for the A=7, T=3/2 quartet and the A=8, T=2 quintet. © 2011 American Physical Society.link_to_subscribed_fulltex

2p-2p decay of 8C and isospin-allowed 2 p decay of the isobaric-analog state in 8B

8C is found to decay to four protons and an α particle in two 2p emission steps. The correlations between the protons in the first step (8C to 6Be) exhibit a significant enhancement in the region of the decay phase space where the two protons have small relative energy, a region sometimes called the diproton region. The decay of the isobaric analog of 8C in 8B is also found to decay by 2p emission. This is the first case of isospin-allowed 2p decay between isobaric analog states. © 2010 The American Physical Society.link_to_subscribed_fulltex