PROCEEDINGS OF THE 7 INTERNATIONAL ... - Fizika

I. Cibulskaite et al. / Medical Physics in the Baltic States 7 (2009) 72 - 75
DOSnrc (EGSnrc) [2] in each region of interest.
Estimated relative statistical uncertainty of calculated
doses was 0.8% in the case when 10 8 photon histories
were taken for the simulation, and 0.2% - for 10 9 photon
histories.
The spectra of the X-ray beam, entered to the code
system for interaction modelling, were generated for
different X-ray tube potentials from the range of (25 -
32) kV when the tilting angle of the Mo anode was 16 0 .
Generated X-rays were attenuated by 1 mm Be window
of the X-ray tube, 0.03 mm thick Mo filter and 3 mm
thick polymethylmethacrilic compression paddle of
mammography unit.
Additional filtration of the photons with the energies
below 10 keV and above 20 keV was undertaken due to
the fact, that Mo is characterized by K lines at the
energies: 17.38 keV (Kα2), 17.49 keV (Kα1), 19.61 keV
(Kβ1) and 19.96 keV (Kβ2) [3]. X-ray photon spectra
were calculated using IPEM Spectrum Processor [4] and
average photon energies for the applied X-ray tubes
potentials were estimated taking into account the special
geometry of the irradiation.
3. Results
Adapted for specific geometry EGSnrc code was used
for X-ray photon interaction with physical object
modelling. The main characteristic that was chosen was
radiation dose, that reflects information about energy
transfer, absorption and scatter in every segment of the
complex object irradiated by X-ray photons. The result
of modelling are total and scatter doses in selected
objects. It was considered that the total dose represents
the energy, transferred to the target and the scatter dose
is defined as a dose, which can be traced back to the
photons that have been scattered in Compton and
Rayleigh effects and to the photons that were created
trough relaxation processes after Compton and
photoelectric effect.
Modelling of low energy (25-32) keV X-ray induced
processes was performed for the constructions
consisting of Si wafer (active volume) protected by
different coating materials which are commonly used in
radiation detector constructions. Some characteristics of
the investigated coatings are provided in Table 1.
X-ray interaction effects in the detector construction are
dependent on X-ray attenuation and scattering processes
in the coating material, which are influenced at least by
the coating structure (polycrystalline, amorphous,
polymer-like), coating density and thickness, and the
construction of the layered structure (with or without
chemical bonding between detector and coating
material).
A thin air gap existing between the coating and detector
material was inserted into modelling geometry in the
case of free standing polymeric foils. Nanothick
interlayer structure between bulk material and coating
was considered for the modelling of the structures
73
having direct deposited or grown coating layers on the
top of the active volume.
The thickness of Si target in investigated constructions
varied within the range of 0.1 mm – 3.0 mm, the
thickness of protective coatings was from the interval of
100 nm - 1.0 mm.
Table 1. Coating materials
Coating
type
Coating material Zeff Density,
g/cm 3
Free Polyethylene
5.68 0.93
standing H – 14 %, C – 85 %
polymeric
foils
Mylar,
Polyethylenetereftalate
H – 4 %, C – 62 %, O
– 33 %
6.71 1.38
Rubber
H – 11%, C – 88%
5.75 0.92
Carbon fibre 6.10 1.75
Direct
deposited
/ grown
layers
SiO2
DLC film synthesized
from C2H2: H – (24 %
11.5
diverse
2.32
1.60
2.12
-
to 38%)
SiOx-containing DLC
film, synthesized from
HMDSO:
main components, O
and
Si (17% - 24%)
diverse 1.87 -
2.20
Comparing the modelling results, radiation induced
scatter dose of X-ray photons was higher in all coated Si
specimens of different thickness, as compared to
uncoated ones, while the total dose was less, due to the
photon energy absorption in coating material. Increasing
for scatter dose and decreasing for the total dose
tendency was observed with the increasing density of
coating material within the whole energy range of
mammography examinations, when the thickness of
coating was kept constant.
Some variations of calculated scatter doses in Si targets
of different thickness coated by different coating
materials of the same thickness, representing results of
the investigation are shown in Fig. 1. Lines in this figure
indicate the highest and lowest scatter doses
corresponding to the coating material, which were
evaluated within the group of coated samples, having
the same Si target thickness. A broadening of the range
between the highest and lowest scatter dose within the
group indicates the influence of coating material
(increased density) on scatter dose in coated Si when the
thickness of the active Si volume is decreased. The
highest scatter dose was estimated for Si-DLC film
containing SiOx combination, which represented the
highest density (2.2 kg/m 3 ) of all investigated coatings.
Scatter doses calculated for Si coated by nanothick DLC
film were included in Fig 1 for the comparison of the
results.