A. M. Laird

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.

Constraining nova observables: direct measurements of resonance strengths in 33S(p,\gamma)34Cl

The 33S(p,gamma)34Cl reaction is important for constraining predictions of certain isotopic abundances in oxygen-neon novae. Models currently predict as much as 150 times the solar abundance of 33S in oxygen-neon nova ejecta. This overproduction factor may, however, vary by orders of magnitude due to uncertainties in the 33S(p,gamma)34Cl reaction rate at nova peak temperatures. Depending on this rate, 33S could potentially be used as a diagnostic tool for classifying certain types of presolar grains. Better knowledge of the 33S(p,gamma)34Cl rate would also aid in interpreting nova observations over the S-Ca mass region and contribute to the firm establishment of the maximum endpoint of nova nucleosynthesis. Additionally, the total S elemental abundance which is affected by this reaction has been proposed as a thermometer to study the peak temperatures of novae. Previously, the 33S(p,gamma)34Cl reaction rate had only been studied directly down to resonance energies of 432 keV. However, for nova peak temperatures of 0.2-0.4 GK there are 7 known states in 34Cl both below the 432 keV resonance and within the Gamow window that could play a dominant role. Direct measurements of the resonance strengths of these states were performed using the DRAGON recoil separator at TRIUMF. Additionally two new states within this energy region are reported. Several hydrodynamic simulations have been performed, using all available experimental information for the 33S(p,gamma)34Cl rate, to explore the impact of the remaining uncertainty in this rate on nucleosynthesis in nova explosions. These calculations give a range of ~ 20-150 for the expected 33S overproduction factor, and a range of ~ 100-450 for the 32S/33S ratio expected in ONe novae.

The β-decay of 22Al

Abstract.In an experiment performed at the LISE3 facility of GANIL, we studied the decay of 22Al produced by the fragmentation of a 36Ar primary beam. A β-decay half-life of T1/2 = 91.1±0.5ms was measured. The β-delayed one- and two-proton emission as well as β-α and β-delayed γ-decays were measured and allowed us to establish a partial decay scheme for this nucleus. New levels were determined in the daughter nucleus 22Mg. The comparison with model calculations strongly favours a spin-parity of Iπ = 4+ for the ground state of 22Al.

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.

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.

Measurement of 19Ne spectroscopic properties via a new method of inelastic scattering to study novae

The accuracy of the predictions of the γ flux produced by a classical nova during the first hours after the outburst is limited by the uncertainties on several reaction rates, including the 18F(p,α)15O one. Better constraints on this reaction rate can be obtained by determining the spectroscopic properties of the compound nucleus 19Ne. This was achieved in a new inelastic scattering method using a 19Ne radioactive beam (produced by the GANIL-SPIRAL 1 facility) impinging onto a proton target. The experiment was performed at the VAMOS spectrometer. In this article the performances (excitation energy range covered and excitation energy resolution) and limitations of the new technique are discussed. Excitation energy resolution of σ = 33 keV and low background were obtained with this inverse kinematics method, which will allow extracting the spectroscopic properties of 19Ne.

TACTIC: the TRIUMF Annular Chamber for Tracking and Identification of Charged particles

An in-depth characterization of the TACTIC detector was performed using data from a 148Gd alpha source and some test runs with a stable ion beam. The detector is an active target time-projection chamber with a blind central region for maximizing beam tolerance and GEM-based electron amplification, equipped with a modern digitizing data acquisition system allowing the recording of full signals. The system was developed to study the reaction 8Li(α,n)11B, which is important for bridging the mass 8 gap in scenarios of low 4He density like Inhomogeneous Big Bang Nucleosynthesis and the production of r-process seeds in supernovae. Both energy resolution and tracking accuracy were found to agree with theoretical predictions and Geant4 simulations. The 8Li beam rate capability of the system is predicted to be of the order of 105s-1, several orders of magnitude higher than most previous measurements of the same reaction, while still maintaining a high detection efficiency of 70% to 80 %.

Experimental investigation of exotic clustering in 13B and 14C using the resonance scattering method

6 pags., 5 figs. -- 12th International Spring Seminar on Nuclear Physics, IOP Conf. Series. -- Open Access funded by Creative Commons Atribution Licence 3.0