Simulations, testing and results for the pixelation of LYSO

Simulations, testing and results for the pixelation of LYSO crystals for
gamma detectors using sub-surface laser engraving techniques
Konstantinou
1
G. ,
Chil
1
R. ,
3
J.M. ,
Udias
Desco
1,2
M.
and Vaquero
1,2
J.J.
1 Departamento
de Bioingeniería e Ingeniería Aeroespacial, Universidad Carlos III de Madrid. Spain
2 Instituto de Investigación Sanitaria Gregorio Marañón, Madrid. Spain
3 Universidad Complutense de Madrid, CEI Moncloa, Madrid. Spain
Email: gkonstantinou@hggm.es
Web: http://image.hggm.es
Introduction
• 
One of the most common topologies for the scintillator crystals used in gamma radiation detectors for PET scanners is pixelation. The size of these pixels, the crystal
surface treatment and the reflector thickness inserted between crystals directly affects the energy and the spatial resolution as well as the sensitivity of the scanner.
•  Since the fabrication of pixels is laborious, complex and expensive. we focused our effort in researching the feasibility of creating pixels within monolithic LYSO scintillator
crystals using the proven technique of sub-surface laser engraving (SSLE) with a Nd:YAG laser.
•  To evaluate possible designs considering the limitations of the engraving procedure, we used the GEANT4 based GAMOS macro environment to analyze the light
propagation inside the engraved crystals, using information about the relative reflective quality and optical behavior of the engraved surfaces.
Materials-Methods
Focusing
unit xyz
Reflectance measurement results
Figure 5 depicts the optical behavior of
different grids of microcracks, tuning pulse
power, distance between cracks and
number of layers. Figure 6 shows
microscopy images of the different surfaces
illuminated with a blue incoherent light.
laser
Motor z
Motor xy
Figure 2 Surface measurement setting
Different crystals
Using the experience acquired through the
engraving of standard K9 glass blocks, we
proceeded with testing the engraving process for
different scintillators to evaluate the similarity of the
effects and expand the knowledge over the
process. In particular, blocks of BGO, GSO and
LYSO were engraved with a number of patterns.
A
C
D
Increasing
energy of points (A)
Figure 5 Top: Reflectance as a result
of density
and pulse power for two different configurations (B); Bottom:
Absorption length as a result of density of points (C) and
pulse power (D)
Energy
Figure 6: Engraved surfaces for different
combinations of energy and density.
Simulated surfaces results
In the figure 7, we exhibit the results of an extensive number of simulations of a single
detector element, namely the average FWHM of the image peaks produced by each
individual crystal, the peak to value ratio from a pixels row or column profile. The surface
reflective quality was found predominantly lambertian.
B
C
D
Figure 3: A) GSO engraved crystal; B) BGO
engraved crystal; C) Standard pixelated array; and
D) engraved crystal
GAMOS Simulation
The possibilities of surface configuration are
endless, while the outcome of each configuration
can’t be easily calculated with analytical methods.
To avoid extensive engraving efforts that would
yield no results, a simulation platform has been
developed using the GEANT4 based GAMOS
macro interface. In this way, the results from the
simulations can be used to select the best
candidates for engraving.
Density
Figure 1: SSLE engraving procedure using
a 532 nm Nd:YAG laser
Light measurements
A setting with incoherent illumination of the scintillation
expected wavelength range was set: a commercial LED of
known power and wavelength was positioned at a given
angle and illuminated the sample surfaces uniformly, while
microscopic images were. This simple setting also serves
for different goniometric experiments, supporting the
evaluation of the surface reflective quality.
B
A
Increasing density
SSLE system
We used a commercial SSLE Nd:YAG laser with
pulses of 6.8 to 7.2 ns and 1.2 to 2 W. Since the
configuration of the engraved grid designed plays a
crucial role in the overall reflectivity of the resulting
surfaces, we need to characterize the accuracy and
repeatability of the engraving system
Results-Analysis
Figure 7 Left to right: FWHM as a result of overall reflection; Peak/valley ratio as a result of simulated absorption length; and FWHM as a
function of the diffuse/specular percentage .
Comparison of engraved pixels with simulation
After having calibrated the simulation routine, using results from both engraved and
assembled pixelated arrays, we simulate the surface before engraving and compare the
simulated and engraved result (Table 1).
FWHM
Peak to
valley ratio
Simulated
0.43
0.46
Engraved
0.4
0.44
Discrepancy
7.5%
4.6%
#
A
Figure 4: Model used for the GAMOS simulation
Table 1 Comparison between simulated and
engraved pixel characterisation
Discussion
•  The SSLE reflective patterns collimate light in a similar manner as the reference
reflector, although up to now the best reflectivity renders up to a 60% of the total
reflection of the reference
•  The pixel resolvability as measured through the FWHM and peak to valley ratio is
approaching the figures of standard pixelated arrays.
•  Since the transparency of the reflective surfaces can be controlled, we can
compensate for the light sharing needed for Anger logic readouts integrating lightguides in the scintillation crystal block.
•  The size of pixels can be chosen to a micrometer precision to match the
characteristics of the read-out, such as SiPM pixel size.
Figure 8 Comparison between simulated (blue) and engraved (red) pixel profile
of the same calculated characteristics.
Engraved crystal results
Using the simulation results as a guideline, we
have managed to engrave a series of LYSO
crystals, at a depth of 13 mm and pixel size of
1.45x1.45 mm2.
Figure 9 Field flood diagram of a number of
engraved pixels at 13 mm depth
Conclusions, Future plans
• 
• 
• 
• 
The SSLE technique can be used to create reflective patterns inside monolithic scintillator crystals, to a depth higher than the existing limit of 10mm.
.
The simulations results show decreasing deviation from the measurement results, proving as an important guideline routine for the testing of more complex pixel topologies.
DOI patterns can be created either by manipulating the transparency of the surfaces layering different topologies of microcracks or by introducing distinct geometries.
Such modifications of the engraving pattern take place at practically no cost and constitute a highly promising solution for industrial fabrication, reducing substantially the
cost of the detector.
This work was partially funded by the TOPUS S2013/MIT-3024 project from the regional government of Madrid, the TEC2014-56600-R and RETOS RTC-2015-3772-1 from the Spanish Ministerio
de Economía y Competitividad, the INFIERI Network (Marie Curie ITN EU FP7, grant agreement no. 317446), and the Human Frontier Science Program grant RGP0004/2013.The authors
acknowledge the support for the simulations of Pedro Arce from CIEMAT, Madrid.