Joint research project

Hard x-ray focussing and x-ray detectors for applications in medicine and plasma diagnostics

Project leaders
Claudio Ferrari, Frantisek Dubecky
Agreement
REPUBBLICA SLOVACCA - SAS - Slovak Academy of Sciences
Call
CNR-SAS 2016-2017
Department
Engineering, ICT and technologies for energy and transportation
Thematic area
Engineering, ICT and technologies for energy and transportation
Status of the project
Extended
Report for renewal
joint-report2015-imem-ieetris.pdf

Research proposal

1) Hard x-ray focussing

- Due to the negligible refraction index in materials to focus high energy x-rays the diffraction from crystals has been proposed. A lens is made of an array of single crystals properly oriented to focus parallel x-rays in a single point. Nevertheless the diffraction efficiency decreases very rapidly with the energy, due to the very limited diffraction range (Darwin width) of perfect crystals at high energy. This limitation can be overcome in imperfect (mosaic) crystals or in curved crystals, in which the diffraction planes are bent. In this case the diffraction width is given by the total lattice plane bending and the reflectivity value can be close to 100%.
As an original result of the previous bilateral project ("X-ray metrology and crystals surfaces for x-ray imaging") the proposers were able to demonstrate [1-2] that also crystals with moderate density (Si, Ge, GaAs) can diffract efficiently high energy x-rays if the diffracting planes are curved over a threshold given by the crystal density and structure factor and if the crystal thickness equals t=1/mu where mu=absorption factor.
In particular in medical imaging techniques such as SPECT (Single Photon Emission Tomography) and PET (positron Electron Tomography) the resolution and the dose to the patient are connected with the total dose to of ionizing radiation given to the patient. A focussing device able to improve the resolution and the sensitivity of the technique will permit a substantial reduction of the dose given to the patient and permit to detect thinner details.

- In this part of the project we intend to verify experimentally the relationship obtained in the previous work by measuring the diffracted intensity of bent crystals of Ge, GaAs and Si at high energy x-ray sources. Focusing properties of Laue lenses depend on the quality of crystals used to prepare lenses and on the homogeneity of bending of crystal lattice planes. Our experience in X-ray topography, diffractometry, including rocking curves imaging (RCI) and reciprocal space mapping (RSM), using sources with photon energy around 10 keV, will give us a possibility to characterize carefully and in detail the technology of crystal preparation, bending and orienting. Special double crystal camera with bendable first crystal is planned to be used for testing bent crystals for Laue lenses. High energy sources are rare in the scientific community but they are necessary to study focusing properties for real photon energies foreseen for human medical imaging.

- The second activity will be to design a gamma ray lens (size, curvature, type of crystal and of diffraction plane) to be used for nuclear medicine by design and ray tracing simulations. The basic physical design will come out of the energy range to be covered and of the geometry given by the object and laboratory dimensions. It is a great advantage to be able to "realize and test" the imaging optical elements in several alternatives in a computer, to compare several materials and geometries and to choose optimized solution before physical realization. It saves time and material. After such simulations the final design can be selected for technological realization and testing.

- Phase imaging is a new approach utilizing small but non-zero effect of refraction for X-ray imaging. We intend to perform a feasibility study for applying phase imaging in medical imaging to improve the resolution and reduce the dose required to obtain a contrast in medical imaging techniques based on x-ray absorption such as radiography and x-ray tomography.

[1] High diffraction efficiency in crystals curved by surface damage
Claudio Ferrari, Elisa Buffagni, Elisa Bonninia, and Dusan Korytar, J. Appl. Cryst. (2013). 46
[2] C. Ferrari et al., Optical Engineering 2014 53(4) Art. N.: 047104;


2) X-ray and gamma ray detectors
Basing on our previous long-term experience in the field [3, 4], novel x- and ³-ray semicon-ductor detectors with improved resistance against harsh environment applicable in medicine and hot plasma diagnostics will be developed and investigated. The best candidate with improved operation stability presents detectors based on 4H-SiC material. Such detectors are considered for hot plasma diagnostics and control. Development of peculiar ultrathin electrode will improve detection efficiency toward UV, EUV and soft X-rays up to 20 keV.
Semi-insulating GaAs-based detectors with novel metal electrode using low work function metals (Gd, Mg, Nd, Sr,...) will be fabricated also in 2D arrays and completed with the readout electronics allowing applications in digital X-ray imaging including medicine up to about 60 keV of photon energy range. In particular, completion with the MEDIPIX readout chip is in consideration with individual pixel array pitch 55x55 µm2. Preliminary results show promising perspective. Successful replacement of Si-based detector arrays by 2D GaAs detector chip could led to decrease of the X-ray dose to patient during screening by 80%.
[3] Dubecký, F., Ková
, J., Ková
, J., Zaeko, B., Osvald, J., Hubík, P., Kindl, D., Vanko, G., Gombia, E., Ferrari, C., et al.: 4H-SiC and novel SI GaAs-based M-S-M radiation hard photodetectors applicable in UV, EUV and soft X-ray detection: design, technology and performance testing, Proc. SPIE 8777B (2013) 8777.
[4] Dubecký, F., Dubecký, M., Hubík, P., Kindl, D., Gombia, E., Baldini, M., Ne
as, V.: Unexpected current lowering by a low work-function metal contact: Mg/SI-GaAs,. Solid-State Electr. 82 (2013) 72.

Research goals

We intend to study different bent crystals of Si, Ge and GaAs and to compare the experimental diffraction efficiency with that expected form theory.
The crystal perfection of the materials considered (Si, Ge, GaAs, Cu), their resistance to bending (dependent on thickness), and bending technology will be studied. X-ray laboratory characterization will be performed to choose suitable technology to obtain regularly bent crystals with minimized effect of anticlastic bending deteriorating the beam focusing.
A feasibility study will be performed to study, design and test phase contrast mode of x-ray imaging in medicine. Both flat and curved crystal optics are foreseen to be applied.


We intend to fabricate a set of detectors based on 4H-SiC with ultrathin electrode, with thick-ness less than 12 nm and chips (~4 pcs) of 2D array of SI GaAs detectors down to 55 µm pitch geometry. Fabricated detectors will be characterized by I-V, C-V, Hall, XPS and PC measurements and tested by ±-particles, X- and ³-rays. The CCE, energy resolution, and detection efficiency will be evaluated.

Last update: 27/11/2021