Astrophysics with DRAGON: The 26g Al (p,γ) 27 Si Reaction Heather Crawford a,1 for the DRAGON Collaboration b a Simon Fraser University, Burnaby, B.C., Canada b TRIUMF, Vancouver, B.C., Canada Abstract The 26g Al(p,γ) 27 Si reaction is important for nuclear astrophysics, as 26 Al is directly observable in supernovae explosions due to its decay with a characteristic gamma. This allows comparison of observational data with models, the accuracy of which depends on how well known the reaction rates for the processes involved are. As the only direct destruction pathway for 26 Al aside from its beta decay, the 26g Al(p,γ) 27 Si reaction is an integral part of the 26 Al system, and an accurate measure of its rate, determined mainly by the strength of available resonance reactions, is critical. The strength of the 188 keV resonance is currently being directly studied for the first time in inverse kinematics, using the DRAGON facility at TRIUMF. A 26 Al radioactive beam incident on a windowless H 2 gas target gives rise to 27 Si recoils, which are detected through the coincidence of a prompt gamma, and a heavy ion signal at the end detectors. Data is being analyzed to separate true events from background and determine the thick target yield. Also important is an analysis of beam intensity and composition, using data from DRAGON detectors and faraday cups. Results from these latter aspects of the study will be reported on. Beam Contamination Summary The 26g Al(p,γ) 27 Si reaction is still ongoing at DRAGON, with a continuation of the experiment scheduled for this October to achieve improved statistics. However, the work completed this past summer has contributed to establishing a more standard method of beam normalization, which can be used for the upcoming and ideally other future experiments. In fact, this work produced two normalization methods which validated one another, producing values in agreement within 8% for over 150 experimental runs. Methods for monitoring contamination levels have also been considered and incorporated into normalization to determine the desired value for the calculation of resonance strength – the number of 26g Al particles on target. Acknowledgements I would like to thank the entire DRAGON group, particularly Dr. Chris Ruiz, Anuj Parikh, and Dr. Dave Hutcheon for their help and guidance in this work. I would also like to thank Dr. John D’Auria for allowing me the opportunity to work with the DRAGON group this past summer and gain invaluable experience. Measurement of Resonance Strength – Thick Target Yield Stellar nuclear reaction rates are usually dominated by a few narrow resonance reactions 4. These narrow resonances are characterized by resonance strengths (ωγ). Resonance strength is experimentally determined by measuring the thick target yield, given by the following equation: Experimental determination of the thick target yield requires accurate knowledge of the number of recoils detected, the number of beam particles incident on the target, the efficiency of the BGO array used for detection of gamma rays and the fraction of the recoils in the charge state used (CSF), shown by the following equation: Recoils are detected at the DRAGON end detectors using γ-heavy ion coincidence detection. Preliminary data analysis suggests that 11 recoils, or true events, were detected during the experiment. This work focused on determining the number of 26g Al beam particles on target over the course of the experiment. m and M = masses of the target and projectile nuclei respectively ωγ = resonance strength (ω is the statistical spin factor and γ is a level width dependent term) λ = de Broglie wavelength of projectile (1) ε = stopping power of the target material (2) The DRAGON Facility at ISAC-TRIUMF The DRAGON (Detector of Recoils And Gammas Of Nuclear reactions) facility is located at TRIUMF, Canada’s National Laboratory for Particle and Nuclear Physics. DRAGON 2 is a mass-separator used in the study of astrophysical radiative capture (p,γ or α,γ) reactions in inverse kinematics with a hydrogen or helium gas target as required. A schematic diagram of this system is shown in figure 1. An intense radioactive beam, produced at the ISAC facility, impinges on the DRAGON windowless gas target, which is surrounded by an array of 30 BGO (bismuth germanate) gamma detectors that detects prompt gammas emitted during any reactions that occur. Upon exiting the target, due to conservation of momentum, both recoils and beam particles continue together downstream and into the electromagnetic mass separator. DRAGON uses two stages of electromagnetic separation. Each separation stage is composed of a magnetic dipole that separates particles based on charge, focusing magnetic quadrupoles, and an electric dipole that separates particles based on mass. Once through the separator, recoils and any ‘leaky beam’ particles that make it through the separation arrive at the end detectors for detection. The 26g Al(p,γ) 27 Si reaction utilized a combination of a MCP (micro-channel plate) detector with a DSSSD (double-sided silicon strip detector). The DSSSD provides information on the number, energy and position of particles, as well as local timing information when used in tandem with the MCP detector. Figure 1 Motivation 26 Al 28 Si 27 Si 26 Si 27 Al 25 Al 24 Mg 25 Mg 26 Mg 26g Al undergoes positron decay to the first excited state of 26 Mg which then rapidly decays with a characteristic MeV gamma ray, meaning 26g Al can be directly observed by orbiting gamma telescopes. Direct observation of abundances allows comparison with calculated values from network calculations and models attempting to describe novae and supernovae explosions. The relevant reaction rates are important parameters within these models, and these rates, including that of the 26g Al(p,γ) 27 Si reaction, must be determined experimentally. Figure 2 The 26g Al(p,γ) 27 Si reaction, believed to occur in novae and supernovae exposions, involves radiative capture on a radioactive species. This reaction has a significant and direct impact 3 on the abundance of 26g Al, a relatively long-lived radionuclide which is produced as a part of the Mg-Al cycle, shown in figure 2. The number of beam particles includes both the number of 26g Al nuclei as well as the number of contaminant particles, including 26 Na and 26m Al. For calculation of the resonance strength, the number of 26g Al particles incident on the target is required, so the number of contaminant particles was determined and subtracted from the number of beam particles. 26m Al was detected using a pair of NaI detectors located on either side of a horn situated above the mass slits where this species was expected to be deposited. Since 26m Al decays through positron emission, some positrons make it into the horn and annihilate, emitting a pair of 511 keV gamma rays, which are detected in coincidence, by the pair of NaI detectors. After contamination levels of 26m Al were determined for each run; these are plotted in figure Na was similarly quantified using a HPGe detector pointed at the left mass slit where this species was expected to be deposited. Since 26 Na decays with a characteristic MeV gamma ray, the integral of the relevant gamma peak in the spectrum was used to determine the contamination levels. Again, after compensating for the charge state fraction and detection efficiency, the % contamination levels of 26 Na were determined run-by-run; these values are plotted in figure 10. Normalization of Beam Determination of the number of beam particles on target over the course of a given run involves a number of steps. A measure of absolute beam intensity was determined with the use of a Faraday cup ~ 2 metres upstream of the gas target Since beam intensity varies over the course of a two-hour run, a reliable monitor of relative beam intensity was chosen A relationship between the absolute beam intensity and the relative value was established (normalization factor) Monitor response was integrated over run, and normalization factor was used to determine the total beam on target Two different relative beam monitors within the DRAGON system were used for purposes of normalization for different groups of the 250 runs taken. (I) Current deposited on the left mass slit after the first mass separation is proportional to the number of beam particles and produces a good relative beam intensity profile (figure 4). The normalization constant was a simple ratio, defined as below. This ratio was calculated for each run and then averaged. The charge on the mass slit directly integrated, and used to determine the number of beam particles on target by multiplication by the normalization factor. The average number of beam particles on target over a 2-hour run determined using this method was on the order of particles. (3) Figure 4 (II) A surface barrier detector within the gas target, located at 30° to the beam axis, detects Rutherford elastically scattered protons, the number of which is proportional to the number of beam particles. Figure 5 shows a relative beam intensity profile. The number of protons was determined from the pulse height spectrum, which had virtually no background in the region where the protons appeared (figure 6). Given the well-known dependence of Rutherford scattering on the target pressure and beam energy, a normalization factor independent of these quantities was established (shown below). (4) Normalization values were calculated for runs in which the first 300s of the trigger rate spectrum showed a relatively constant beam intensity, as in figure 7. These values are shown in figure 8. Average normalization factors were then determined, and used with the total number of scattered protons determine the beam particles on target. On average, for a two hour run, the number of beam particles on target determined from this method was on the order of Figure 5 Figure 8 Figure 7 Figure 6 compensating for the charge state fraction of the contaminant species, and detection efficiency, the % Figure 10 Figure 9 The Experiment Intense radioactive 26 Al beam is incident on 6 Torr H 2 target with ~202 keV/u of energy Particles pass through target reaching resonance energy (188 keV in center of mass frame) near middle of target -- some react to produce 27 Si recoils, most pass straight through Particles that react emit a number of gamma rays (gamma ray cascade) which are detected by the BGO array surrounding the target (gamma signal) Recoils emerge with ~ same momentum as beam, with a small angular spread (  ~ 15 mrad), and move through the separator to isolate 4 + charge state 27 Si recoils, which are detected at the end detector (heavy-ion signal) 26 Al Gas Target H2H2  27 Si γ Figure 3 1 Author’s 2 D. Hutcheon et al., Nuclear Instruments and Methods in Physics Research A 498, 190 (2003). 3 Ruiz, C, E989: Astrophysical studies using 26Al ground-state and isomer beams, TRIUMF research proposal (internal), 4 C. Rolfs and W. Rodney, Cauldrons in the Cosmos (University of Chicago Press, 1988).