Gamma/hadron discrimination using composite cherenkov telescopes

Abstract

The Durham University Mk6 Ground Based Gamma Ray Telescope is a sensitive detector in the Very High Energy (VHE) band (~100GeV-10TeV). It is a 'composite' Cherenkov telescope consisting of three 42m(^2) mirrors, each of which is viewed by a Cherenkov photon detector. Together the three detectors provide the basis for a sophisticated coincidence trigger that gives the telescope its low energy threshold of ~300GeV. Analysis of high resolution Cherenkov images from the central detector allows gamma rays to be identified from the hadronic background. The Left and Right detectors also record two independent, medium resolution, Cherenkov images. This thesis has investigated the use of the images from the Left and Right detectors to provide extra gamma/hadron discrimination power. Two measurements, represented by the parameters dDist and LRconc, have been identified that are capable of improving current VHE source detection significance by 20-30%.In Chapter 1 some of the motivations behind the field of VHE gamma ray astronomy are discussed, along with brief explanations of VHE gamma ray production mechanisms. Potential astronomical sources of VHE photons are outlined in Chapter 2. In Chapter 3 the phenomenon of Cherenkov radiation emitted in the atmosphere from Extensive Air Showers (EAS) is introduced. The construction of Atmospheric Cherenkov Telescopes is considered in Chapter 4 and the Durham Mk6 telescope is detailed in Chapter 5. The moments method of analysing Cherenkov images is given in Chapter 6. Chapter 7 investigates the measurements that that could be made using data from the Left and Right detectors to identify gamma ray EAS from the hadron background. The effects on these measurements of various aspects of the Mk6 telescope's performance are also researched here and in further detail in Chapter 8. Finally a summary and some suggestions for future work are given in Chapter 9.