1. Abstract The 13 N(p,γ) 14 O reaction is very important for our understanding of explosive astrophysical sites, such as novae and supernovae. This reaction determines the conditions under which the CNO cycle changes to the Hot CNO cycle. If temperatures are hot enough, 13 N will capture a proton before it has chance to beta decay, forming 14 O which initiates the HCNO cycle. The beta decay of 14 O (t 1/2 = 70.6secs) is much quicker than the beta decay of 13 N (t 1/2 = 9.97mins), which means that the HCNO cycle produces energy much faster. The DRAGON collaboration at TRIUMF plans to measure the cross-section of the 13 N(p,γ) 14 O reaction at energies around the Gamow window, relevant to novae temperatures. This region of energy is lower than the resonance peak energy, which has been measured previously. As 13 N is radioactive, and is very close in mass to 13 C (a difference of 0.002383 amu), a pure 13 N beam is difficult to produce, because 13 C will contaminate the beam. Initially we studied the 13 C(p,γ) 14 N reaction so that its contribution could be compensated for when studying the 13 N(p,γ) 14 O reaction. The 13 C(p,γ) 14 N reaction was used to probe the DRAGON not only because it has similar properties, but because 13 C(p,γ) 14 N measurements have been made before by King et al 2. DRAGON The DRAGON (Detector of Recoils And Gammas Of Nuclear reactions) is situated at TRIUMF, Canada’s National Laboratory for Particle and Nuclear Physics, which houses the world’s largest cyclotron. DRAGON was designed to measure radiative capture reactions in inverse kinematics using a hydrogen or helium gas target. The DRAGON system is basically a 21m recoil mass spectrometer which can create elements via proton or alpha capture reactions, and then separates them based on mass, in two stages. Beam enters a windowless gas target box, which is surrounded by a closely- packed array of 30 gamma detectors made of BGO (Bismuth Germanium Oxide) scintillation crystals. A series of pumps are found either side of the entrance and exit to the target, and are used to keep the entrance and exit in vacuum (~10 -7 Torr), allowing the beam to pass cleanly through the target. On leaving the target, the products (recoils) of the nuclear reaction (together with original beam, known as leaky beam) enter the first stage of the mass spectrometer. The mass spectrometer is made up of magnetic dipoles (M), magnetic quadrupoles (Q), magnetic sextupoles (S), and electrostatic dipoles (E), arranged in a two stage mass separation: (QQMSQQQSE)(QQSMQSEQQ). The magnetic dipoles are used in such a way as to separate out the charge state of interest, and the quadrupoles and sextupoles focus the beam through the spectrometer. The electrostatic dipoles are used in such a way as to separate the recoils from the leaky beam using their different momentums. At the end of DRAGON is a choice of two end detectors, a double-sided-silicon-strip detector (DSSSD) and an ionization chamber (IC). The DSSSD measures energy, position, and time-of-flight. The IC measures energy and change in energy (ΔE). The IC was used for the 13 C(p,γ) 14 N experiment, and will be used for the future 13 N(p,γ) 14 O experiment, because it can be used to separate out all the contaminate elements, using ΔE measurements. Data Analysis from the 13 C(p,γ) 14 N reaction Figure 1 shows a typical coincidence gamma energy spectrum from a DRAGON run of the 13 C(p,γ) 14 N reaction. A coincidence gamma is one that is associated with a recoil heavy ion of 14 N as detected in the end detector of DRAGON. The cγ0 means that the data put into this spectrum is from the most energetic coincidence gamma ray detected by a single BGO per event by the BGO gamma array. The main peak will correspond to the energy of the gamma rays cascading from the 8MeV excited state to the ground state. The various other peaks are from either: the cascade gammas to other excited states, or from the main 8MeV gammas that did not deposit all of their energy into a single BGO. The data analyzer used by DRAGON is a MIDAS program which looks at runs online and offline. Analyzing a run offline means that we can pass the run through the analyzer many times, and by having made changes to the online data base (ODB), we can eliminate more and more unwanted background events. These changes in the ODB mean that we can also look at spectrums not set up in the online ODB. For example, instead of looking at the most energetic coincidence gamma per event, we can look at the sum of all gammas that trigger a BGO per event (see figure 2). By summing the gammas we have eliminated the lower energy peaks which were triggered by 8MeV gammas depositing their energy over more than one BGO. On analysis of the 14 N recoils, we see what appears to be “clipping” of lower energy recoils, in the peak (figure 3). The 13 C(p,γ) 14 N reaction has a large cone angle of approximately 19mrad, which is beyond the design limits of DRAGON (approximately 16mrad). Therefore, some recoils will not make it through the beam tubes out of the gas target box, but will be “clipped”, staying in the gas target box. To find out what percentage of recoils weren’t making it to the end detector, we needed to create and run GEANT simulations of DRAGON and this reaction. GEANT GEANT is a Detector Description and Simulation Tool. It is a program that simulates the way in which elementary particles pass through matter. It was originally designed for High Energy Physics but is also today used in medical and biological sciences, and astronautics. The main applications of GEANT for High Energy Physics are the tracking of particles through an experimental setup for the simulation of detector response, and the graphical illustration of the setup and of the particle trajectories. We used GEANT to create a replica of the DRAGON separator, for simulations of different astrophysical reactions, such as the 13 C(p,γ) 14 N reaction. Figure 1 Figure 2Figure 3 BGO simulations Due to the many excited states of 14 N, and hence the large amount of gamma cascades, I would start the analysis of 13 C(p,γ) 14 N by concentrating solely on the 8MeV ground state gamma, which could be compare with King et al 2. But how could I separate out the ground state gammas from the cascades? The GEANT simulation of DRAGON’s BGO array 3 was used to calculate the percentage of 8MeV gammas that deposited all of their energy in a single BGO. However, the BGO gamma array only covers 92% of the solid angle of the gas target, and hence not all gammas are registered. Of the gammas that did register, 85.3% deposited their energy in a single BGO, and 13.9% deposited their energy in a BGO and its neighbour. From the diagram of the simulated BGO array to the right, you can see that it is very complex, so defining a neighbouring BGO is difficult. To simplify, the GEANT simulation was updated to use a cuboid technique, where by if a BGO fires and another fires a certain distance away which is within the cube, then it is said to be a neighbouring BGO. 13 C(p,γ) 14 N simulations By creating an input file for the 13 C(p,γ) 14 N reaction, I was able to simulate this reaction through DRAGON to compare with the actual data. Figure 3 shows a recoil spectrum with a peak energy value of around 5MeV for an actual DRAGON run. Simulating the same conditions with the DRAGON GEANT simulation gave a peak energy of 6.55MeV (figure 4). This 1.5MeV energy difference was believed to happen as the recoils pass through the entrance window (a mylar foil) of the ionization chamber. To test this theory, I started working on creating an ionization chamber within GEANT for our DRAGON simulation. Other motivations for simulating the ionization chamber were to: a) get a proper estimate of energy straggling, b) find out what anode the recoil ion stops in, c) get a proper energy spectrum, d) compare with the real data and estimate the acceptance loss, e) simulate the correct geometry features of the energy loss, f) test recoils in different pressures within the ionization chamber. Creating an ionization chamber in the DRAGON simulation Using schematic diagrams of DRAGON’s actual ionization chamber, I was able to simulate a simple ionization chamber with ‘cuboids’ and ‘cylinders’. A lot of FORTRAN code was needed for the simulation and their were a lot of problems with the designing. After months of work, the simulated ionization chamber was operational, and the DRAGON simulation was able to track recoil particles through it. Figure 4 Summary Initially, the 13 C(p,γ) 14 N reaction experiment was used as an acceptance test for DRAGON. We were pushing the limits of angular acceptance of the DRAGON, due to the large 19.4mrad cone angle for this reaction. Also, the very small cross-section meant a simulation was necessary to investigate the acceptance loss specific to this reaction. My simulations of the 13 C(p,γ) 14 N reaction in DRAGON, with the new ionization chamber, are still continuing, with histogram updates, and running with different mistunes of distance, angle and percentage, of the beam in the gas target. Once complete, my analysis of the 13 C(p,γ) 14 N data compared with analysis from my 13 C(p,γ) 14 N simulations, will provide DRAGON with sufficient enough results to be able to compensate for this reaction occurring with the 13 N(p,γ) 14 O reaction. My creation of the ionization chamber in the DRAGON simulation will not only aid the DRAGONeers in distinguishing the different elements in their future 13 N(p,γ) 14 O data, but also help in the analysis of future reaction studies when the ionization chamber is used. Acknowledgements I would like to thank Dr Chris Ruiz, Dr Alison Laird, Dr Sabine Engel, Dario Gigliotti, and Mike Lamey, for their close help and support, throughout this project, and their friendship during my year at TRIUMF. Also, I like to thank Professor John D’Auria for giving me this excellent opportunity to come to this facility, and experience nuclear astrophysics outside of the classroom. 1 Author’s Email Address: ph91ab@surrey.ac.uk, 2 J. King et al., Nuclear Physics A 567 (1994) 354-376, 3 D.Gigliotti, Master’s thesis, University of Northern British Columbia (in preparation) 2003 Enter The DRAGON Investigating the 13 C(p,γ) 14 N reaction Aaron M. Bebington a, 1 (and the DRAGON Collaboration b ) a University Of Surrey, Guildford, Surrey, England b TRI-University Meson Facility (TRIUMF), Vancouver, BC, Canada