Doctorate in Applied and Engineering Physics  

Home
School
login
About the Program
Program Structure
Research Centers
Admission
Application
Scholarships
Contacts

Thesis Proposal for the

Doctorate Program in Applied and Engineering Physics (DAEPHYS)

Starting in the Academic Year 2016/2017


Proposal 009


Title:Implementation of a crystal-polarization dispersive XRF spectrometer
 
Objectives (recommended length: 2000-3000 char):
In this project the candidate will implement a polarized wavelength dispersive XRF spectrometer (PWXRF) for high-precision analysis of light and heavy trace elements of biological, environmental and geological samples.

Still based on the principle of X-ray fluorescence, this spectrometer will be an improvement relative to traditional direct X-ray fluorescence (XRF) techniques and tri-axial geometries [1]. The key element of this improvement is a conjugation of a Bragg crystal that serves as both energy selector of the primary X-ray beam and polarization cutoff of the scattered background. Such polarized X-ray optics significantly reduces the background, which enhances the performance compared to the direct excitation case for trace element analysis. It can thus be employed on low ppm applications.

The conjugation of a Bragg crystal provides further control of the excitation energy of the sample, allowing a standardless analysis. For that purpose, a complete characterization of the spectrometer will provide realistic instrumental response functions, which are required for the Fundamental Parameter algorithm [2, 3]. This validates the use of this spectrometer for a variety of different matrices (e.g. water, oil, soil or glass).

Spectrometers based in polarization optics of energy dispersive XRF (PEDXRF) has been being employed in several studies of analysis down to ppm level, achieved even in complex sample composition, e.g soils [4-7], plants [8,9] and liquids [10],

The analysis of biological and environmental samples with this new dispersive spectrometer will lead to precise quantification of elements with accuracies unattainable by the traditional direct energy dispersive XRF and PEDXRF method.

Based on this, determination of correlations among the various trace elements can thus give new insight of important biological processes of toxic elements elimination. Anthropogenic activities, such as mine exploitations, contaminate usually a large amount of water, soil and farmlands [11]. Toxic elements from them eventually will enter plants and food chains and cause damages to human health.

The research center LIBPhys-UNL has a 90 KV Philips generator and a water-cooled X-ray tube that provides a strong X-ray intensity of 100 W. According to preliminary simulations, such X-ray source can provide enough intensity for running a Bragg crystal simultaneously with fluorescence excitation of the sample, and giving a spectral data in a reasonable amount of time (150 to 600 s). A relative error bellow 10% was used as threshold for all elements between Na and U.

The Monte-Carlo ray-trace simulations were based in a variation of the program specifically design for a high-precision double-crystal spectrometer. Measurements of this spectrometer lead to a world-record absolute determination of X-ray transitions [12].
 
Framework (recommended length: 500-2000 char):
XRF spectrometry is an analytical technique which irradiates a sample with excitation X-rays and measures the element-specific fluorescence X-ray energies emitted from the sample. It is a non-destructive technique and is possible to reuse the sample after measurement. Since it is a non-contact analysis, the instrument will not be easily contaminated. Additionally, any sample from solid, powder and liquid can be analyzed.

Obtaining accurate and reliable quantification of light trace elements by using standard (XRF) is often a challenging task. This is because characteristic fluorescence lines have low fluorescence yields, which makes them barely standing above the background. This background occurs primarily by elastic (Rayleigh and Thomson) and inelastic (Compton) X-ray scattering at the sample.
A common way to improve light elements determination is based on the reduction of this background by employing polarization-cutoff geometries, such as energy polarization energy dispersive XRF (EDXRF) and the tri-axial XRF techniques.

As in other polarized optic techniques the background resulted from elastic and inelastic scattering is significantly reduced, at least one or two orders of magnitude, by a special geometrical combination that vanishes the polarization components that reaches the detector: While the scattered radiation is polarized parallel to the surface of the secondary target, if the detector is located in the same direction of this polarization plane, i.e., in a direction perpendicular to the scattering plane, no scattered radiation will arrive there, in agreement with dipole emission [13].

By combining a Bragg crystal in this modified PEDXRF geometry leads to further progress in the quantification process relative to EDXRF. X-ray diffraction selects a narrow region of allowed energies, or wavelengths, that will be reflected from the crystal. An almost monochromatic X-ray component with a scattered polarized component serves as excitation source.
 
Tasks (recommended length: 1000-3000 char):
The candidate will start by continuing the Monte-Carlo simulations of this spectrometer to get familiar with the geometry and its detection limits.

The choice of the crystal is crucial, therefore, a study the right crystal profile that meets both detection limits and financial constrains, is performed after. For this purpose, the program XOP (X-ray Oriented Programs) provides the necessary tools for investigate the properties of a variety of crystals [14].

Based on previous simulations and technical constrains, a design of the spectrometer will be projected in SolidWorks. After, the candidate will acquire and assemble the necessary components for a functional prototype of the spectrometer. Eventual technical constrains might lead to modifications of this prototype.

Once the prototype meets the required objectives of accurate determination of characteristic atomic lines and background reduction, a full characterization of various experimental parameters such as resolution and limits of detection for each element and background reduction will be performed. Simultaneously, this task will be supported with simulations. This study will be published in a peer-review journal.

This spectrometer can then be directly applied to relative quantifications of trace elements in studies of biological and environmental relevance.

Simultaneously, the potential of this spectrometer for obtaining standardless quantifications will be investigated by combining the fundamental parameter method with an instrumental response of the spectrometer obtained by simulations.


[1] Handbook of X-Ray Spectrometry, Rene Van Grieken
[2] D.K.G. de Boer et al Adv. X-ray Analysis (1990) 33 237.
[3] W.A. Abuhani, Powder Diffraction (2014) Volume 29 159-169
[4] L. Luo, et al. Enviro. Science and Pollution Research, (2014), 21, 8242-8260
[5] J. Heckel. X-Ray Spectrometre (1991) 20, 287–292
[6] T. D. Hettipathirana, Spectrochimica Acta Part B: Atomic Spectroscopy (2004) 59, 223–229
[7] X. Zhan, X-Ray Spectrometry (2005), 34, 207–212
[8] W.E. Stephens et al, Analytica Chimica Acta, (2004) 527, 89–96
[9] Z. Üstündağ. Spectroscopy Letters, (2011) 42 7-11
[10] Eva Marguıí et al.Anal. Chem. 2008, 80, 2357-2364
[11] F. E Huggins International Journal of Coal Geology (2002) 50, 169–214
[12] P. Amaro et al. Phys. Rev. Lett. 109, 043005
[13] L. Safari et al (2012) Phys. Rev. A 86, 043405
[14] Sanchez del Rio, et al (1998). Xop: SPIE proceedings 3448
 
Research centre/lab or R&D unit hosting the thesis project:
Department of Physics, UNL
 
University to which the thesis project will be presented:
UNL - Universidade Nova de Lisboa
 
DAEPHYS Scientific Domain in which the project fits:
Radiation, nuclear and atomic techniques
 
Relation of the project to the Scientific Domains of DAEPHYS:
The project fits into the theme of radiation and atomic techniques since it aims the development of an
X-ray spectrometer.
 
Candidate profile:
Physics, Physics Engineering and Biomedical Engineering
 
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:José Paulo Santos
Institution:UNL
email:jps@fct.unl.pt
 
link to CV or indication of ORCID ID:
http://orcid.org/0000-0002-5890-0971

 

Co-Supervisor

Name:Pedro Amaro
Institution:UNL
email:pdamaro@fct.unl.pt
 
link to CV or indication of ORCID ID:
http://orcid.org/0000-0002-5257-6728

 

Uploaded PDF document: (none)