Development of a THGEM Imaging Detector with Delay Line Readout

Abstract

<p>Position sensitive detectors represent a class of particle detectors widely used in high-energy physics, astrophysics, biophysics and medicine for imaging the spatial distribution of various radioactive sources. In recent years, a new class of gas based detectors, so-called micropattern gas detectors (MPGDs), has emerged. While modern MPGDs rival solid state detectors in terms of spatial and temporal resolution, their cost of production is significantly lower. A Thick Gaseous Electron Multiplier (THGEM) imaging detector, with a two-dimensional delay line readout, has been constructed as a concept for a large area imaging detector with reasonable spatial resolution. The delay line based THGEM imaging detector is robust, easy to manufacture and cost effective alternative to direct readout techniques which frequently employ a large number of channels. Featuring an active area of 40 x 40 mm<strong>2</strong>, the prototype has been constructed using two 0.4 mm THGEMs and successfully operated in a low pressure, propane based, gas mixture. Two sets of orthogonal electrodes, connected to individual delay lines, serve as a two-dimensional anode readout. Adjacent electrodes are separated by approximately 3.4 ns of time delay and allow the interaction position to be calculated by measuring the time difference between delay line output signals corresponding to a common axis. Using modern field programmable gate arrays (FPGAs), a time-to-digital (TDC) data acquisition (DAQ) system has been developed. The TDC DAQ performs the position reconstruction algorithm and is capable of continuous event rates up to 1.8 MHz. The imaging capabilities of the detector have been assessed using a collimated alpha source and a wide X-ray beam. Under these aforementioned conditions, the detector was able to successfully resolve 1 mm diameter holes spaced 3 mm apart. With higher operating pressures, and using Xenon based gas mixture, it is expected the imaging detector should achieve sub-mm spatial resolution. The investigations presented in this thesis serve as a framework for the development of future THGEM imaging detectors.</p>