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 011


Title:Boron nano-particles in Liquid Ionization Chambers for neutron detection
 
Objectives (recommended length: 2000-3000 char):
Until recently 3He Proportional Counters (PC) were the golden standard for thermal neutron detection. 3He has an excellent thermal neutron detection efficiency, good gamma-ray discrimination and, until recently, was an abundant material. The large demand of last decade, mostly motivated by homeland security applications, and the limited reserves of this gas caused what is now know to the scientific community as the 3He shortage crisis[1]. Besides homeland security (by far the largest consumer) other applications where 3He is used, such as large area detectors in neutron research facilities, nuclear reactor control, cryogenic systems, well logging and medical imaging have also suffered with the reduced availability of this material[1].
Most alternatives to 3He use 10B in the form of a solid layer. Neutron detection is in fact a 2 step process; a neutron is converted, via nuclear capture reaction, into 2 reaction products which are emitted in opposite directions, each carrying a defined fraction of the energy released in the reaction. The energy of these products is then measured, using a proportional gas, leading to neutron identification. In 3He PC, 3He acts both as conversion material for an incoming neutron and as proportional gas, absorbing the n-capture reaction products. In the case of solid layers (attached to the detector walls or other structures) one of the products is all ways emitted into the direction of the solid layer, where it is absorbed and its energy is lost. Such detectors present a very characteristic response [2], with 2 flat continuous distributions (one for each reaction product) super-imposed. This is a drawback for neutron identification, particularly in applications were a low energy gamma background is present, as it reduces the discrimination capability of the detector. Due to the limited range of the reaction product in a solid layer, the detection efficiency of such detectors is also limited to ~10% [2].
The student will evaluate the response of a detector in which the neutron conversion is done in a micro/nanoparticle made of 10B. Since the range of the n-capture reaction products is of few microns in most materials, both reaction product will escape the particle depositing a large fraction of their energy in the material that surround the micro/nano particle. A Monte Carlo simulation, already implemented, shows that full energy deposition is possible for particle diameters < 1micron. The absorbing material will be a liquid, such as neopentane and tetramethylsilane, used in the past for high energy calorimetry [3][4]. The liquid will support the micro/nano-particles and sample the energy of the n-capture reaction products. If successful, such technique has the potential to replace the 3He in many applications, particularly domestic security.
 
Framework (recommended length: 500-2000 char):
The n-capture reaction in 10B releases a total energy of 2.3 MeV, shared between an alpha particle (E=1.47 MeV) and a 7Li Ion (E=0.84 MeV). Assuming an average of 3 electrons per 100 eV of energy deposited [3] the conversion of the 2.3 MeV to produces ~69.000 primary electrons, a signal which is measurable without amplification in the liquid, with low-noise electronics.
An important factor for signal collection is the electron lifetime, related with the purity of the liquid. In [4] activated silica was used to assure good purification levels in large chambers with tetramethylsilane. The prototype to develop will be a multiwire proportional chamber, with several anode wires closely spaced to assure a fast collection of the converted primary electrons. This prototype will be assembled at LibPhys - UC laboratory and its as a function of particle concentration while irradiated by gamma rays and alpha particles will be evaluated. After the preliminary tests the prototype will be taken to the Paul Scherrer Institute, in Villigen, Switzerland, for exposure to a thermal neutron beamline.
LibPhys-Universidade de Coimbra and the I3N-Universidade de Aveiro researchers have a an extensive background in the development of detector systems for applications in fields as diverse as astrophysics, exotic atoms spectroscopy, UV and visible sensitive photomultipliers, cryogenic dual phase detectors, high rate particle beam and medical imaging systems, amongst others. The successful applicant will have the support from the researchers of these groups as well as the unique opportunity to develop is/her skills in the above mentioned areas.

[1] US Government Accountability Office “Neutron detectors Alternatives to using helium-3” Report to Congressional Requesters, 2011.
[2] K. S. McKinny, et al., IEEE TNS 2013.
[3] J.Engler, J. Phys. G: Nucl. Part. Phys. 22 (1996).
[4] A. Gonidec, et. al , CERN-EP/88-36, 1988.
 
Tasks (recommended length: 1000-3000 char):
1) Design and construction of a prototype chamber
A small, portable chamber will be projected. It will contain several parallel thin tungsten wires, stretched inside the liquid, in order to collect the primary charges. Micro-nano/particle dispersion and uniformity of the solution will be achieved, most likely by means of a liquid-circulation system. Other solutions, such as magnetic agitators, will be considered. The chamber design will take into account the requirement for an efficient liquid purification. Solutions such as the ones implemented in [4] (and others) will be considered. Care as to be taken in order to prevent the loss of micro-nano/particles during the liquid purification.
2) Micro/nano particle characterization
Detector response is greatly influenced by the micro/nano-particle size distribution. If these are too large, the alpha particle and 7Li ion will lose there a large fraction of their energy, which will not be sampled in the liquid. On the other hand, smaller particles are more difficult to manipulate, have a tendency to aggregate and can, potentially, react with the liquid. A Monte Carlo Simulation, already implemented with Geant4 for the case of a single wire, will be extended by the student to cover the chamber full geometry. The student will then run the simulation, determining the energy deposited in the liquid as a function of the particle size.
The student will also determine the reaction rate of micro/nano-particles made with several materials containg 10B. In the case that the micro/nano-particles are found to react with the liquid, a coating of the particles with an inert material will be used. The student will determine the optimal coating, which allows full energy deposition, while preventing particle reaction with the liquid.
3) Prototype characterization
The detector response will be characterized using alpha particles emitted by an 241Am source. Detection efficiency and SNR will be evaluated as a function of the micro/nano-particle concentration.
4) Prototype exposure to a thermal neutron beamline
The student will perform the irradiation of the prototype by a thermal neutron beamline, available at the Paul Scherrer Institute (PSI) in Villigen, Switzerland.


 
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:
This project requires a comprehensive knowledge of instrumentation for radiation detection, nuclear electronics as well as pulse processing and data analysis. It aims to develop a novel instrument for thermal neutron detection.
 
Candidate profile:
The candidate, preferably with a background in engineering physics or physics, should be very motivated and meticulous in his/her work. It should be a resilient person with the ability to find innovative solutions. Knowledge of nuclear instrumentation, pulse processing, programing and electronics are useful skills.
 
Does this proposal involve more than one University?:
yes
 
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)
: The researchers of LibPhys-Universidade de Coimbra have an extensive background in the development of radiation detectors for very diverse applications. Sealed prototypes with re-circulating gas and purification systems are routinely assembled at its labs. Research in neutron detection, with exposure of several prototypes to a thermal neutron beamline at PSI, is an ongoing line of research of the LibPhys-Universidade de Coimbra. I3N-Universidade de Aveiro is a dynamic group with very experient researchers. The members of this group are experts in the development of electronic systems for data acquisition and processing.
 
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:Fernando Domingues Amaro
Institution:Coimbra University
email:famaro@gian.fis.uc.pt
 
link to CV or indication of ORCID ID:
http://orcid.org/0000-0001-7315-0550

 

Co-Supervisor

Name:Carlos Davide da Rocha Azevedo
Institution:Aveiro University
email:cdazevedo@ua.pt
 
link to CV or indication of ORCID ID:
orcid.org/0000-0002-0012-9918

 

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