THE OPTIMISATION OF A COMPTON CAMERA FOR THE IMAGING OF HIGH ENERGY GAMMA RAYS

1. THE OPTIMISATION OF A COMPTON CAMERA FOR THE IMAGING OF HIGH ENERGY GAMMA RAYS M. JONES1,A.J. Boston1, H.C. Boston1, R.J. Cooper1, M.R. Dimmock1, L.J. Harkness1 D.S. Judson1 P.J. Nolan1, D.C. Oxley1, B. Pietras1 M.J. Joyce2, R.O. Mackin2, B.D’Mellow2, M. Aspinall2, A.J. Peyton3, R. Van Silfhout3 (1)Department of Physics, University of Liverpool, Liverpool, UK, L69 7ZE. (2)Department of Engineering, Lancaster University, UK, LA1 4YR. (3)School of electronic & electrical Engineering, University of Manchester, Manchester, UK, M60 1QD. ABSTRACT – Over the last decade there has been a larger emphasis on homeland security and new applications. The Distinguish collaboration is developing a detection and imaging system to identify and quantify such materials as narcotics and explosives. This paper will discuss the physics motivation behind the project, the detector design and the current status of the optimisation of the device. The Physics motivation. • The DISTINGUISH collaboration is developing a highly specific detection and imaging system for the identification and quantification of narcotics and more notably, explosives. • Current imaging systems such as CAT scanners have high-sensitivity density information but poor specificity. • There is a need for a specific, sensitive, quick and automated device to help prevent future atrocities caused by explosives. • Explosives/narcotics contain combinations of the lighter elements such as Oxygen, Carbon and Nitrogen which have characteristic gamma rays. Oxygen - 6.13MeV. Nitrogen - 5.11MeV, 2.31MeV,1.64MeV. • The Compton imaging technique will be utilised to localise the source of the gamma rays. The energy range to be investigated in this work will be 0.2 to 2.2MeV. The cone reconstruction method will allow localisation of the source. Electronic collimation allows high imaging efficiency. The Detectors. • The detector illustrated by figure (a) is a High Purity Germanium detector (HPGe) which has a 12AC and 12DC orthogonal strip configuration and a 60mm x 60mm x 20mm active area. HPGe is used for its unprecedented energy resolution. The first detector (a) requires good energy resolution and position dependence which is provided by the HPGe detector. • The detector illustrated by figure(c) is a Caesium Iodide (CsI) detector which has an 8x8 pixel configuration and has an active area of 51.6mm x 51.6mm x 50mm. CsI is a high Z scintillation detector (a very dense material) and has very good stopping power which is ideal for an absorber detector. This detector is going to be used as the absorber in the Compton camera configuration. (Figures (b) and (d) are simulated illustrations of the single detectors). (a) (b) (c) (d) The Geant4 simulation toolkit. (e) (f) • The Geant4 simulation toolkit has been validated alongside experimental data which is illustrated in figure (e). L.J Harkness, et al. • The absolute efficiency curves shown in figure (e) show good agreement with one another above 200keV. • This validation was completed using a HPGe SmartPET detector. As discussed earlier this detector will be used as the scatter material. (g) L.J. Harkness, et al., Nucl. Instr. And Meth. A (2009). • CURRENT WORK. • Full Compton camera optimisation is underway using the Geant4 simulation toolkit. • Germanium Germanium and Germanium Caesium Iodide detector configurations are currently being investigated ((f) and (g) respectively). • Once simulated Compton camera is optimised the favoured setup will be looked at experimentally.