Efficiency calibration measurement and GEANT simulation of the DRAGON BGO gamma ray array at TRIUMF


A gamma ray array to detect the characteristic gammas emitted from astrophysically significant, radiative proton and aplpha capture reactions, was built as part of the Detector of Recoils And Gammas Of Nuclear reactions (DRAGON) spectrometer at ISAC/TRIUMF. The DRAGON array consists of a collection of 30 hexagonal BGO detectors measuring 7.62 cm long by 5.58 cm across the face. Experiments at DRAGON are affected by background due to "leaky beam" which reaches the end detector along with the reaction products of interest. In many cases the cross sections of these reactions are so small that it is impossible to distinguish the reaction recoils from leaky beam by using only the electromagnetic separator (EMS) of DRAGON. Further suppression of leaky beam is achieved by demanding a time of coincidence between reaction recoils and the associated gamma emitted from the reaction. To determine the rate of gamma/recoil ion coincidence events it is necessary to have an accutate estimate of the gamma array efficiency. Since it is impossible to measure this rate for all experimental conditions it is necessary to have a simulation which can estimate the efficiency of the array for a given set of experimental parameters (e.g gamma energy). A simulation was built with the particle-tracking program GEANT v3.21. The efficiency of the array was measured using calibrated sources of various gamma energies and compared to simulated results. For the cases where the activity of the source was not well know the sources were calibrated using a standard NaI detector of known efficiency. The agreement between simulation and measured differences is more than adequate for proposed DRAGON experiments. The analysis and results of the comparison between measured and simulated efficiency will be discussed in this thesis.

Author: Gigliotti, D. G.