Speakers
Description
Radiation dosimetry and radiation therapy are closely related because precise therapy requires precise measurement of the interaction of radiation with biological tissues. Early attempts to understand cellular radiation effects recognized that knowledge of the energy distribution at a scale comparable to the structures affected by irradiation was essential. It turned out that the microscopic distribution of energy deposition in volumes of lineal cellular and sub-cellular dimensions is of critical significance in biological damage. This realization led to the development of micro- and nano-dosimetry.
Among other measuring instruments, Tissue-Equivalent Proportional Counters (TEPC) play an important role in investigations related to these fields. Traditionally, TEPC designs have relied on single wire counter and parallel plate chamber configurations. However, recent studies [1] have indicated that it is important to fabricate TEPCs with very small cavity and avoid pile-up in high intensity beam, that is difficult using single-wire configuration. Use of Micro-Pattern Gaseous Detectors (MPGD), such as Gaseous Electron Multiplier (GEM), Thick GEM (THGEM) simplifies the construction of (i) a miniature counter with a small sensitive volume that reduce pile-up effects and (ii) multi-element counter configurations to increase the sensitivity of the TEPC to measure very low radiation field intensities. Despite such improvements, TEPCs based on MPGDs also suffer from problems related to efficiency, non-uniformity, space and surface charge accumulation. This is especially true for MPGDs such as GEM / THGEM, since they have a significant amount of insulating surface, made of materials such as Kapton, FR4, exposed to the active gas volume. In this presentation, we report results of recent numerical investigations on accumulation of space and surface charge in generic gaseous ionization detectors, which are based on THGEMs, and their effects on device response. Accumulation of surface and space-charge in gaseous ionization detectors are known to influence avalanche to streamer transition [2], onset of inefficiency and response non-uniformity [3]. While the former is delayed by the introduction of a THGEM rim, the latter can remain in control only if the charging up, and space charge accumulation, are entirely uniform, or below a reasonable threshold.
Numerical modelling of these phenomena using particle models faces insurmountable problems due to the fact that charge accumulation occurs over a large number of events and incorporation of space / surface charge effects through all these events becomes computationally intensive [4,5]. Fluid modelling such as those pursued in [6,7], on the other hand, misses important aspects related to statistical fluctuations. In this presentation, we will make an attempt to assess the pros and cons of using both the approaches. Standard HEP software such as Geant4 and Garfield++ will be used for particle modelling, while commercial FEM packages, such as COMSOL, will be used for modelling charge transport as a process described by drift-diffusion. Devices based on THGEMs, with and without rims, will be studied in reasonable detail using both models to understand what effect rim has on the detector response and avalanche to streamer transition from the perspective of charge accumulation on surface, and in the device volume.
| Name of the speaker | Dr. Purba Bhattacharya |
|---|---|
| Eligible for the Georges Charpak Young Scientist Award. | no |