S. P. Fox

Development of large area polycrystalline diamond detectors for fast timing application of high-energy heavy-ion beams

We have studied the effects of electrode fabrication and detector capacitance on the time resolution of large area electronic grade polycrystalline chemical vapour deposited diamond sensors that are suitable for time of flight measurements of heavy ions at relativistic velocities. Sensors were prepared both in house, with Al or Au metal contacts, and commercially fabricated with Au/diamond-like carbon contacts. He, Ar and a mixture of Ne and O beams at 16.3, 33.5 and 21-23 MeV/u, respectively were used on these devices whilst arranged in transmission geometry. Signal processing only began over one meter away from the sensors. The present approach, where we have large-area/large capacitance multi-strip detectors with processing electronics at some distance from the target, is compatible with anticipated space limitations in particle- identification and tracking setups at existing and planned nuclear fragmentation facilities. In a systematic study under these conditions, we demonstrate that the time resolution is limited by detector capacitance and energy deposition in the sensors. An intrinsic time resolution σ = (44±5) ps was achieved for a diamond detector of ∼ 14 pF capacitance. We conclude that, once further refinements are made, a large area time of flight detection system using polycrystalline diamond detectors would be able to provide time resolutions better than 40 ps, approaching the requirement for particle-identification in relativistic fragmentation experiments, such as those at the facility for antiproton and ion research, FAIR.

The construction and operation of a hybrid gas-silicon detector for studies of cluster breakup reactions

A hybrid detector system has been constructed consisting of a gas filled ionization chamber and a large area silicon semiconductor strip detector within the same gas volume. The construction and operation of these detectors is described, together with a brief description of the electronics and data acquisition system used in cluster breakup studies. The techniques for software correction and gain matching of the detector energy signals are described and results are presented for the resolutions in energy, position and time. In a typical application, elements up to Ca are easily resolved and the positions of individual ions are measured with a resolution of <0.2 mm in the reaction plane, whilst energy resolutions approaching 200 keV can be obtained from the Si strip detector over a small area. Some spectra are presented for measurements of the breakup of Mg-24 into C-12 + C-12.

Is γ -Ray Emission from Novae Affected by Interference Effects in the F 18 ( p , α ) O 15 Reaction?

The (18)F(p,α)(15)O reaction rate is crucial for constraining model predictions of the γ-ray observable radioisotope (18)F produced in novae. The determination of this rate is challenging due to particular features of the level scheme of the compound nucleus, (19)Ne, which result in interference effects potentially playing a significant role. The dominant uncertainty in this rate arises from interference between J(π)=3/2(+) states near the proton threshold (S(p)=6.411 MeV) and a broad J(π)=3/2(+) state at 665 keV above threshold. This unknown interference term results in up to a factor of 40 uncertainty in the astrophysical S-factor at nova temperatures. Here we report a new measurement of states in this energy region using the (19)F((3)He,t)(19)Ne reaction. In stark contrast to previous assumptions we find at least 3 resonances between the proton threshold and E(cm)=50 keV, all with different angular distributions. None of these are consistent with J(π)=3/2(+) angular distributions. We find that the main uncertainty now arises from the unknown proton width of the 48 keV resonance, not from possible interference effects. Hydrodynamic nova model calculations performed indicate that this unknown width affects (18)F production by at least a factor of two in the model considered.

SHARC: Silicon Highly-segmented Array for Reactions and Coulex used in conjunction with the TIGRESS γ-ray spectrometer

The combination of γ-ray spectroscopy and charged-particle spectroscopy is a powerful tool for the study of nuclear reactions with beams of nuclei far from stability. This paper presents a new silicon detector array, SHARC, the Silicon Highly-segmented Array for Reactions and Coulex. The array is used at the radioactive-ion-beam facility at TRIUMF (Canada), in conjunction with the TIGRESS γ-ray spectrometer, and is built from custom Si-strip detectors utilising a fully digital readout. SHARC has more than 50% efficiency, approximately 1000-strip segmentation, angular resolutions of Δθ ≈ 1.3 deg and Δ ≈ 3.5 deg, 25–30 keV energy resolution, and thresholds of 200 keV for up to 25 MeV particles. SHARC is now complete, and the experimental program in nuclear astrophysics and nuclear structure has commenced.

Two-neutron transfer reaction mechanisms in 12 C(6 He, 4 He) 14 C using a realistic three-body 6 He model

The reaction mechanisms of the two-neutron transfer reaction 12C(6He,4He) have been studied at Elab=30 MeV at the TRIUMF ISAC-II facility using the Silicon Highly-segmented Array for Reactions and Coulex (SHARC) charged-particle detector array. Optical potential parameters have been extracted from the analysis of the elastic scattering angular distribution. The new potential has been applied to the study of the transfer angular distribution to the 2+2 8.32 MeV state in 14C, using a realistic three-body 6He model and advanced shell-model calculations for the carbon structure, allowing to calculate the relative contributions of the simultaneous and sequential two-neutron transfer. The reaction model provides a good description of the 30-MeV data set and shows that the simultaneous process is the dominant transfer mechanism. Sensitivity tests of optical potential parameters show that the final results can be considerably affected by the choice of optical potentials. A reanalysis of data measured previously at Elab=18 MeV, however, is not as well described by the same reaction model, suggesting that one needs to include higher-order effects in the reaction mechanism.

Towards 26Na via (d,p) with SHARC and TIGRESS and a novel zero-degree detector

Nucleon transfer experiments have in recent years begun to be exploited in the study of nuclei far from stability, using radioactive beams in inverse kinematics. New techniques are still being developed in order to perform these experiments. The present experiment is designed to study the odd-odd nucleus 26Na which has a high density of states and therefore requires gamma-ray detection to distinguish between them. The experiment employed an intense beam of up to 3×107 pps of 25Na at 5.0 MeV/nucleon from the ISAC-II facility at triumf. The new silicon array SHARC was used for the first time and was coupled to the segmented clover gamma-ray array TIGRESS. A novel thin plastic scintillator detector was employed at zero degrees to identify and reject reactions occurring on the carbon component of the (CD)2 target. The efficiency of the background rejection using this detector is described with respect to the proton and gamma-ray spectra from the (d,p) reaction.

Search for the 2+ excitation of the Hoyle state in 12C using the 12C(12C,3α)12C reaction

A search for the 2+ excitation of the Hoyle state in 12C has been performed using the 12C(12C,3α)12C reaction at a beam energy of 101.5 MeV. An angular correlation analysis was used to suppress known contributions to the excitation energy spectrum, enhancing the experimental sensitivity. No strong evidence was found for new states in 12C between 9 and 11 MeV; rather upper limits for their excitation in the 12C+12C inelastic scattering reaction are determined.

TACTIC: A new detector for Nuclear Astrophysics Experiments

Directly measuring nuclear astrophysics reactions presents unique challenges. Low energy reaction products and small reaction cross sections are just two of the issues that the TACTIC detector addresses. TACTIC is the TRIUMF Annular Chamber for Tracking and Identification of Charged-particles detector being developed by TRIUMF and the University of York, UK. TACTIC is a cylindrical, active-target TPC providing high detection efficiency; a shielding cathode traps the ionization created by the beam and allows for higher intensities than typical TPCs. The 480 anode signals are collected through custom preamplifiers, digital electronics and acquisition systems. Acquisition and analysis software is also undergoing extensive development. Amplification of the small signals is accomplished using a Gas Electron Multiplier (GEM). The fill gas, He-CO2, provides both particle detection and a homogeneous, variable-thickness target for studying reactions on αs, such as 8Li(α,n)11B. A preliminary study of this flagship reaction was carried out in June 2009 and the results are providing feedback into the development of the final detector and infrastructure.

Influences on the triple alpha process beyond the Hoyle state

Christian Aa. Diget∗a†, Maria J. G. Borgeb, Rafik Boutamib, Peter Dendoovenc, Tommi Eronend, Simon P. Foxe, Brian R. Fultone, Hans O. U. Fynboa, Henrik B. Jeppesen f , Ari Jokinend, Björn Jonsong, Anu Kankainend, Iain Moored, Arto Nieminend, Solveig G. Pedersena, Heikki Penttiläd, Victor F. E. Pucknellh, Karsten Riisager f , Sami Rinta-Antilad, Olof Tengbladb, Youbao Wangd, Katarina Wilhelmseng, Juha Äystöd a Institut for Fysik og Astronomi, Aarhus Universitet, DK-8000 Århus, Denmark b Instituto Estructura de la Materia, CSIC, E-28006 Madrid, Spain c KVI, Zernikelaan 25, 9747 AA Groningen, Netherlands d Department of Physics, University of Jyväskylä, FIN-40014 Jyväskylä, Finland e Department of Physics, University of York, Heslington, YO10 5DD, UK f PH Division, CERN, CH-1211 Geneva 23, Switzerland g Fundamental Physics, Chalmers Univ. of Technology, S-41296 Göteborg, Sweden h CCLRC Daresbury Laboratory, Daresbury Warrington, Cheshire WA4 4AD, UK

Cross section measurements of the 3He(α, γ)7Be reaction using DRAGON at TRIUMF

We present our initial efforts with the DRAGON separator at TRIUMF facility towards obtaining the energy dependence of the astrophysical S-factor for 3He(?, ?)7Be reaction in the energy range of Ecm = 2 to 3 MeV that was recommended by the recent evaluations. A comparison between the existing data and our new complementary Madrid data, together with the recent theoretical calculations, is also given in the context of our ongoing work.