<p> In experimental microdosimetry one of the goals is to measure the absorbed dose in microscopic volumes of tissue. The traditional spherical tissue-equivalent proportional counter (TEPC) is the most common detector currently used for microdosimetry. A new microdosimetric detector based on a thick gas electron multiplier (THGEM) was developed. To investigate the feasibility of the THGEM type detector for microdosimetry, a prototype detector was designed and manufactured. The THGEM detector is robust, easy to manufacture and is cost effective. The THGEM foil is composed of a thin FR4-epoxy insulator coated with copper on both sides. The THGEM contains 32 holes each with a diameter of 0.35 mm and pitch of 0.64 mm. The sensitive volume of the detector is a right cylinder with a diameter of ~5 mm and height of ~5 mm and is located in the center of the detector. Systematic tests were conducted at the McMaster Accelerator Laboratory to investigate its overall performance. A neutron-gamma ray radiation field was generated using the 7Li(p,n) reaction. The detector was operated at low bias voltages initially to test the stability and then the relative multiplication gain was measured as a function of the operating high voltage. The detector performance was observed with different THGEM insulator thicknesses ranging from 0.12 mm to 1.48 mm. The multiplication gain was assessed and both neutron and gamma-ray radiation was detected by the THGEM detector. The spectra obtained with the THGEM detector were analyzed and compared to the data collected with the standard spherical TEPC. The investigations provided information about the THGEM detector operation for microdosimetry and the THGEM microdosimetric spectra observed are comparable to the standard TEPC data.</p> / Thesis / Doctor of Philosophy (PhD)

A new beta-ray detector using the Thick Gas Electron Multiplier (THGEM) technology is presented. Traditional proportional counters have been considered the standard for many decades for radiation contamination monitoring. However, it has always been challenging to detect low energy beta-emitters such as 3H and 14C. In order to extend the low energy cut-off of these beta particles, it is important to keep the electron multiplication gain as high as possible. To accomplish this goal, we have developed a new gaseous beta-ray detector using THGEMs. Founded on previous THGEM avalanche simulations [1] and predecessor detectors, a novel prototype THGEM beta-ray detector was designed and fabricated. Its signal performance, effective gain and gain stability were comprehensively studied for single and double-THGEM configurations using an alpha source. The first time THGEM detector response to beta-rays was observed for various operating conditions and compared with Monte Carlo N-Particle Transport 6 (MCNP6) Monte Carlo simulations. / Thesis / Master of Science (MSc)

Radiations can be classified as either ionizing or non-ionizing according to whether it ionizes or does not ionize the medium through which they propagate. X-rays photons and gamma rays are the typical examples of ionizing radiations whereas radiowave, heat or visible light are examples of non ionizing radiations. UV photons have some features of both ionizing and non-ionizing radiation. Both ionizing and non-ionizing radiation can be harmful to living organisms and to the natural environment. Hence the detection and measurement of radiation is very important for the well being of living organisms as well as the natural environment. Not only for safety reasons, have radiation detectors found their applications in various fields including medical physics, nuclear and particle physics, astronomy and homeland security. Industrial sectors that use radiation detection include medical imaging, security and baggage scanning, the nuclear power industry and defense. Gas electron multiplier (GEM) is one of the most successful representatives of gaseous detectors used for UV photon and X-ray photon detection. Recently there is a growing demand for large area photon detectors with sensitivity reaching to the level of single photon. They are used in spectroscopy and imaging in astronomy high energy physics experiments etc. Thick GEM (THGEM) is a mechanical expansion of standard GEM. It has all the necessary requirements needed for large area detector and offers a multiplication factor that permits efficient detection of light. Hence, the development and performance study of THGEM based radiation detector is chosen as the topic of study in the present thesis. The initial part of the thesis contains simulation studies carried out for the understanding the working of the detector and the effect of various design parameters of THGEM for the above said applications. Different steps for the fabrication of THGEM and the technical challenges faced during the process are discussed. In the view of application of the fabricated THGEM for UV photon detection, cesium iodide photocathode is prepared using thin film technology and characterized. The performance of the photocathode under various operating conditions is studied in terms of its photoemission property. The effect of vacuum treatment on the photoemission property of the photocathode exposed to moist air is studied in detail. A major portion of this thesis focuses on maximizing the detection efficiency of the UV photon detector realized using the fabricated THGEM coupled with the cesium iodide photocathode. Simulations are used at different stages to interpret the experimental observations. The electron spectrum obtained from the detector under study was analyzed. The dependence of secondary effect like photon feedback on the operating parameters is also discussed. The last portion of the thesis deals with the application of THGEM as an X-ray detector. The performance is evaluated in terms of the gain and energy resolution achieved. The thesis is organized as follows: Chapter 1 is divided into two sections. Section A gives a general introduction to different types of radiation detectors found in the present day and their working principles. This is followed by discussion about gas ionization based detector and its working principle in detail. A brief literature survey of the different types of micropattern gas detectors is also given in this section. In Section B of this chapter GEM and THGEM are introduced with discussion about their working principle and areas of application. Chapter 2 deals with the simulation study of THGEM undertaken to have a clear understanding of the detector’s working. Section A of this chapter gives an overview of the simulation tools used for the present thesis in particular ANSYS and GARFIELD. Section B presents the results of the simulation study highlighting the effects of different geometrical and operating parameters on the electric field distribution in and around the THGEM aperture. The relevance of the study to the detectors performance is discussed vividly for all the cases. In Chapter 3, the details of the different steps involved in THGEM fabrication are given. Design aspects involved, fabrication of the THGEM using standard PCB technology coupled with photolithography technique are discussed in this chapter. This is followed by an elaborate description of the test setup used for all the performance study. Preface In the view of application of THGEM as a UV photon detector, cesium iodide photocathode was prepared and characterized. Chapter 4 discusses about the CsI photocathode preparation and its characterization for the above said application. Photoemission property of the photocathode was analyzed under various operating parameters. The effect of vacuum treatment on the photocathode performance is a new aspect of this thesis. Its correlation with the microstructure of the film is reported for the first time. Chapter 5 deals with the application of THGEM as a UV photon detector. The study mainly focuses on the improvement of the detection efficiency of the detector. The effect of drift parameters on the electron transfer efficiency and hence on the detection efficiency of the detector is a major contribution of this thesis. There are no literature available which discusses this aspect of a UV photon detector. The experimental study has been supported with simulation results. In addition to the study on detection efficiency, electron spectrum has also been acquired from the UV photon detector. The spectrum has been analyzed under various operating conditions. Discussions about secondary effects like photon feedback prevailing in the detector output are also present in this chapter. Chapter 6 presents the results of THGEM as an X-ray detector. The performance of the detector has been evaluated in terms of the effective gain and energy resolution achieved under different operating conditions. The gain instability with time and its uniformity across the THGEM area are also studied. The effect of drift field on the energy resolution and its correlation with ETE is a new aspect of this work. Chapter 7 summarizes the salient features of the work presented in this thesis. Also the scope of future work based on this thesis is discussed at the end of the chapter.