D. Ottewell

The DRAGON facility for nuclear astrophysics at TRIUMF-ISAC: design, construction and operation

A facility for measuring cross-sections (resonance strengths) for reactions of astrophysical importance involving short-lived, radioactive reactants has been designed, built and installed at the new TRIUMF-ISAC Radioactive Beams Laboratory in Canada. Named DRAGON (Detector of Recoils And Gamma-rays of Nuclear reactions), it has been successfully commissioned with stable and radioactive heavy ion beams from ISAC. This report presents the main components of the facility, namely, the windowless gas target, the surrounding g detector array, the subsequent electromagnetic recoil mass separator, the focal plane detectors for recoils, the detection system for elastics, and the modular electronics and computer software used for the data acquisition. Examples of the operation of the facility for both stable beam reactions and the first radioactive beam reaction study, 21 Naðp;gÞ 22 Mg are also presented, along with future plans for the program. r 2003 Elsevier Science B.V. All rights reserved. PACS: 29.0

21Na(p,gamma)22Mg reaction and oxygen-neon novae.

The 21Na(p,gamma)22Mg reaction is expected to play an important role in the nucleosynthesis of 22Na in oxygen-neon novae. The decay of 22Na leads to the emission of a characteristic 1.275 MeV gamma-ray line. This report provides the first direct measurement of the rate of this reaction using a radioactive 21Na beam, and discusses its astrophysical implications. The energy of the important state was measured to be E(c.m.)=205.7+/-0.5 keV with a resonance strength omegagamma=1.03+/-0.16(stat)+/-0.14(sys) meV.

π±p differential cross sections at low energies

Abstract Differential cross sections for π − p and π + p elastic scattering were measured at five energies between 19.9 and 43.3 MeV. The use of the CHAOS magnetic spectrometer at TRIUMF, supplemented by a range telescope for muon background suppression, provided simultaneous coverage of a large part of the full angular range, thus allowing very precise relative cross section measurements. The absolute normalisation was determined with a typical accuracy of 5%. This was verified in a simultaneous measurement of muon proton elastic scattering. The measured cross sections show some deviations from phase shift analysis predictions, in particular at large angles and low energies. From the new data we determine the real part of the isospin forward scattering amplitude.

Inclusive measurements of pionic double charge exchange on 3He at low energies

Using the CHAOS spectrometer at TRIUMF pion momentum spectra and angular distributions were measured for the 3 yq . He p ,p reaction at incident energies from 65 to 120 MeV and total cross sections were deduced. The data are compared to Monte Carlo simulations for the conventional reaction process as well as to predictions for the production of the hypothetical p NN resonance d X with and without collision damping. q 1999 Published by Elsevier Science B.V. All rights

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 DRAGON facility for nuclear astrophysics at TRIUMF-ISAC

A facility for measuring cross-sections (resonance strengths) for reactions of astrophysical importance involving short-lived, radioactive reactants has been designed, built and installed at the new TRIUMF-ISAC Radioactive Beams Laboratory in Canada. Named DRAGON (Detector of Recoils And Gamma-rays of Nuclear reactions), it has been successfully commissioned with stable and radioactive heavy ion beams from ISAC. This report presents the main components of the facility, namely, the windowless gas target, the surrounding g detector array, the subsequent electromagnetic recoil mass separator, the focal plane detectors for recoils, the detection system for elastics, and the modular electronics and computer software used for the data acquisition. Examples of the operation of the facility for both stable beam reactions and the first radioactive beam reaction study, Naðp; gÞMg are also presented, along with future plans for the program. r 2003 Elsevier Science B.V. All rights reserved.

The 4He(π+,π−) reaction at low energies

Abstract Using the CHAOS spectrometer at TRIUMF the total cross sections and outgoing pion momentum distributions for the 4 He( π + , π − ) reaction were measured for π + kinetic energies from 70 to 130 MeV. At energies around 100 MeV the total cross sections show an excess by a factor 3 over conventional model calculations which could be ascribed to a contribution from a hypothetical d ′ dibaryon.

Energy response and reaction losses in plastic scintillators

Abstract The energy dependence of the scintillation response (light output) of plastic scintillator BC400 has been investigated for protons in the energy region of 60 to 220 MeV. In this region the scintillation exhibits a linear response, as well as a noticeable difference in the light output between stopping and passing-through (transmission) protons. A comparison between our results and theoretical calculations is presented. Losses due to edge effects have been separated from losses due to the bona-fide reaction of protons in the scintillator with the aid of multi wire proportional chamber (MWPC) trajectory information. The number of events associated with reaction losses was found to range from 10% to 25% of the total number of events, depending on the incident proton kinetic energy.

The differential cross section for proton-proton elastic scattering at 90° c.m. between 300 and 500 MeV

Abstract The absolute differential cross section for proton-proton elastic scattering has been measured at 90° c.m. for 300, 350, 400, 450 and 500 MeV. The statistical uncertainty of the measurements is 0.5% with an additional systematic normalization uncertainty of 1.8%. The results are compared to phase-shift analyses.

The 12C(π−,pp)X reaction at 165 MeV

Abstract Angular correlations of the 12 C(π - , pp)X reaction cross sections were measured over a wide angular range at an incident pion kinetic energy of 165 MeV and the results were compared to published data. A broad peak was observed in the angular distribution of the differential cross section that deviates from the angular distribution expected from a three-body phase space calculation for the two detected protons. The observed angular correlation disagrees with correlations previously reported in the literature.