Doctorate in Applied and Engineering Physics  

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Thesis Proposal for the

Doctorate Program in Applied and Engineering Physics (DAEPHYS)

Starting in the Academic Year 2016/2017


Proposal 001


Title:Double-Peak Response of Large-Area Avalanche Photodiodes to X-Rays
 
Objectives (recommended length: 2000-3000 char):
The main objective of this PhD program is to perform detailed studies of the pulses resulting from the x-ray interaction in large area avalanche photodiodes, in particular the assessment and comparison of the pulse rise-time and amplitude to fully understand the physics behind the visible effect of a double-peak structure in the pulse-height response of large-area avalanche photodiodes to low-energy x-rays, namely in the energy range of 2 keV to 8 keV, where the avalanche photodiode (APD) is most efficient .
A systematic study of the above pulse parameters as a function of the avalanche photodiode operation temperature and applied bias voltage, as well as the dependence of these parameters on the x-ray energy, will be carried out. The possible contribution of interactions taking place near the photodiode edges are also to be assessed. For this purpose, different parts of the APD surface will be irradiated by the x-ray source and the corresponding pulses produced inside the APD volume will be compared in terms of rise-time and amplitude.
Pulse processing techniques will be applied for correction and/or minimization of this effect in the performance of large area avalanche photodiodes. This is particularly important in the application of the APDs to x-ray spectrometry, where different x-ray lines need to be resolved in the same spectrum.
The APD double-peak response depends on the APD structure and electric field profile across the active volume. Studies will be performed with different prototypes of large area avalanche photodiodes of different designs and from different manufacturers: planar design APDs from Radation Monitoring Devices, Inc. (RMD) and bevelled-edge design APDs from Advanced Photonix, Inc. (API). These studies may be extended to the newly developed large area avalanche photodiodes from Hamamatsu Photonics.
The application of large area avalanche photodiodes to x-ray spectrometry will benefit from the rise-time correction of the double-peak response. The APD response to multiple x-ray lines obtained from fluorescence produced by x-ray sources on specific multi-element samples will be investigated and the application of the method to different areas of Physics will be discussed, in particular material analysis, x-ray astronomy and spectroscopy of muonic atoms.
 
Framework (recommended length: 500-2000 char):
Large-area avalanche photodiodes (LAAPDs) are silicon-based solid state detectors that convert photons into a charge current. They provide a compact, robust, magnetic field insensitive solution for light and x-ray detection with gains ~ hundreds and fast time response [1] Due to this, LAAPDs are extensively used in a large variety of physics [2-4], medical [2,4,5], and aerospace applications [2,6]
Recently, for RMD LAAPDs, we have observed two distinct responses to mono-energetic x-rays, which we assume to be due to x-ray absorption in different depths of LAAPDs. By constructing APD specific standard traces and using a pulse-by-pulse fitting technique we improved the energy resolution at 8.2 keV up to a factor of 2 and time resolution by 30% [7]
A full knowledge of this effect and the means to compensate for it are mandatory for the adequate application of these devices to x-ray spectrometry.
1. LMP Fernandes et al“Characterisation of large area avalanche photodiodes in X-ray and VUV-light detection”J Instrum 2,P08005,2007
2. D Renker“Properties of avalanche photodiodes for applications in high energy physics, astrophysics and medical imaging”Nucl Instrum Meth A486,164,2002
3. S Tanaka et al“Development of wideband X-rays and γ-rays spectrometer using transmission-type, large-area APD”Nucl Instrum Meth A582,562,2007
4. R Lecomte et al“Initial results from the Sherbrooke avalanche photodiode positron tomograph” IEEE Trans Nucl Sci 43(3),1952,1996
5. C Marriott et al“High-resolution PET imaging and quantitation of pharmaceutical biodistributions in a small animal using avalanche photodiode detectors”J Nucl Med 3(8),1390,1994
6. T Toitumi et al“In-orbit performance of avalanche photodiode as radiation detector onboard a pico-satellite Cute-1.7+APD II”J Geophys Res:Space Phys115,A05204,2010
7. M Diepold et al“Improved x-ray detection and particle identification with avalanche photodiodes”Rev Sci Instrum 86,053102,2015
 
Tasks (recommended length: 1000-3000 char):
For each type of LAAPD investigated in this project, the following tasks will be performed:

1. Study of the APD double-peak response to mono-energetic x-rays at room temperature, for different energies, from 2 keV to 8 keV or higher. The APD pulse rise-time and amplitude will be determined as a function of bias voltage and compared. The study at room temperature may not be possible for lower energy x-rays due to high dark current and subsequent high noise levels. However, for this study the chamber to incorporate the APD may work under air but light tight.

2. Study of the APD double-peak response as a function of temperature. This is important for lower energy x-rays due to the decreasing dark current and noise levels as temperature goes down. This way, the energy resolution improves and the double-peak distinction is possible. For this purpose the APD needs to be cooled down to temperatures below 0ºC, so a chamber operated in vacuum or nitrogen atmosphere is needed to incorporate the APD, while cooling may be performed by a liquid nitrogen based system or a Peltier element.

3. Study of the uniformity of the APD response over the active area to assess the contribution of interactions taking place near the photodiode edges, which may experience different avalanches due to distorted electric fields in the edges. Different parts of the APD surface will be irradiated by x-rays and the corresponding pulses produced inside the APD volume will be compared in terms of rise-time and amplitude.

4. Assuming the double-peak structure in the pulse-height distribution is correlated with the pulse rise-times, already observed in planar APDs from RMD, pulse processing techniques based on rise-time discrimination will be applied for correction of this effect in order to improve the final energy resolution obtained, which is very important for x-ray spectrometry applications, where different x-ray lines need to be resolved in the same spectrum. At the same time, the time resolution may be improved.

5. As the application of large area avalanche photodiodes to x-ray spectrometry will benefit from the rise-time correction of the double-peak response, the APD response to multiple x-ray lines obtained from fluorescence produced by x-ray sources on specific multi-element samples will be investigated. The application of the method to different Physics areas will be discussed, in particular material analysis, x-ray astronomy and spectroscopy of muonic atoms.
 
Research centre/lab or R&D unit hosting the thesis project:
LIBPhys-UC
 
University to which the thesis project will be presented:
UC - Universidade de Coimbra
 
DAEPHYS Scientific Domain in which the project fits:
Instrumentation
 
Relation of the project to the Scientific Domains of DAEPHYS:
The current proposal for a PhD work program can be fully integrated in the DAEPHYS Scientific Domain of Instrumentation. The various tasks to be performed in the present work program are related to both instrumentation development and analysis procedures applied to the obtained results. We expect the above relation to be reflected in the final results and instrumentation solutions at the accomplishment of the project.
 
Candidate profile:
Candidates should have a Master in Science degree in Physics or Engineering (physics, computer science or electronics) as well as background in data acquisition and/or data analysis, along with a strong commitment with the work program, both in data analysis and in experimental laboratory work. Candidates should also be willing to learn and/or apply methods/procedures used in the laboratory, and should have a reasonable knowledge of the English language, spoken as well as written.
 
Does this proposal involve more than one University?:
no
 
Synergies between the two Universities participating in the proposal:
DAEPHYS strongly encourages the presentation of thesis projects in co-supervision by researchers from two of the universities participating in the Program. In this field, explain the benefits resulting from the proposed co-supervision and the involvement of elements from the two universities, e.g. building critical mass teams, profiting from existing infrastructures or advanced equipments, profiting from expert technical know-how, etc. If the proposal involves only one University, write n/a.
(recommended length: 500-1000 char)
:
 
Does this proposal involve a company?:
no
 
Proposals involving a company:
DAEPHYS strongly encourages the presentation of thesis projects involving a company, preferably a high-tech company. These proposals have to: 1) be centered on a technological problem in which the partner company has been (or plan / would like to be) involved; 2) have a co-supervisor on the enterprise; 3) include part of the project to be carried out in the company.
(recommended length: 500-1000 char)
:

 

Supervisor

Name:Cristina Maria Bernardes Monteiro
Institution:LIBPhys-UC
email:cristina@gian.fis.uc.pt
 
link to CV or indication of ORCID ID:
0000-0002-1912-2804

 

Co-Supervisor

Name:Luis Manuel Panchorrinha Fernandes
Institution:LIBPhys-UC
email:pancho@gian.fis.uc.pt
 
link to CV or indication of ORCID ID:
0000-0002-7061-8768

 

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