AbstractID: 9713 Title: New Generation Portal Sensor Based on Thin-Film Cadmium Telluride
for Clinical High Energy X-Ray Imaging
Supporting document for:
NEW GENERATION PORTAL SENSOR BASED ON THIN-FILM
CADMIUM TELLURIDE FOR CLINICAL HIGH ENERGY X-RAY IMAGING
Introduction: In radiation therapy the size of a portal sensor is dictated by the largest
available treatment field size, typically 40x40 cm2, not easily implemental with crystalline
semiconductors. Currently most popular EPIDs are manufactured from hydrogenated
amorphous silicon (a-Si:H). As a material with low atomic number and electron density,
it is not the best choice for detection of high energy x-rays and charged particles,
generally exhibits low quality images, and exhibits poor radiation hardness. Only in the
recent years semiconductor deposition techniques has been developed to a stage
where materials other than silicon can be manufactured with quality adequate for use in
large-area devices such as in solar cells. We propose new design for a portal imager
based on thin-film Cadmium Telluride (CdTe), which is expected to improve the device
in both imaging and dosimetric applications in radiation therapy. While crystalline CdTe
material is widely used in small-scale radiation detectors and has superior radiation
hardness, thin film devices have not been considered from this prospective. We present
the result of our investigation of thin film CdTe as a possible large area detector in
comparison with a-Si system of the same design.
Method and Materials: The first step in our feasibility study was assessment of
radiation hardness of CdTe compare to a-Si. It was accomplished by evaluating
performance of thin-film CdTe and a-Si:H solar cells after 6 MV photon beam irradiation.
The dose delivered to the devices was 10 times larger than typical EPID lifetime dose.
+
-
14
Normalized efficiency
1.4
γ -ray s
e-
2
Fluence (10 no./cm )
0
1
2
3
4
5
1.2
1.0
0.8
0.6
CdTe type1
CdTe type2
aSi:H type1
aSi:H type2
0.4
0.2
0.0
0.0
M etal
C dT e
Fig. 1 Sketch of the proposed detector system, with
metal plate as a conversion layer, enhancing the dose
deposited to CdTe.
0.5
1.0
1.5
2.0
2.5
3.0
6
Radiation dose (10 rads)
Fig. 2 Radiation hardness comparison between CdTe
and a-Si solar cells of two types, irradiated under
6MV photon beam of Elekta SL15 linac.
After material evaluation we considered several possible configurations for the thin-film
CdTe detector. Due to very small thickness (in a range of 30-100 microns) the sensor
has to be combined with a metal plate facilitating conversion of high-energy photons to
AbstractID: 9713 Title: New Generation Portal Sensor Based on Thin-Film Cadmium Telluride
for Clinical High Energy X-Ray Imaging
charge carriers directly, maximizing the dose deposited in the sensor layer (Fig.1). The
optimum thickness of metal plate for different converter materials was simulated with
MCNP5 package. We modeled an image detection procedure under 6MV photon beam
of the linear accelerator Eleckta-SL25 currently commissioned in our laboratory. We
compared performance of a-Si and CdTe detector systems in combination with Cu plate
commonly used in current EPID. To predict the imaging performance, we referred to the
parameter DQE (Detective quantum efficiency), which shows how efficiently an imaging
system transfers the information content of a radiation beam from the input to the output
of the system. It was calculated as
2
∞
m1
DQE (0) =
, m j = AEDs ( E )E j dE ,
m2
0
where AEDs(E) is the absorbed energy distribution, obtained from MCNP5 simulations
using energy distribution of the pulse height tally.
Results: Fig.2 shows efficiency values normalized to the initial efficiencies for CdTe
and a-Si thin film devices, corrected for the radiation-induced transmission loss in glass
substrate. We found CdTe cells to have superior radiation stability, much better than
that of a-Si devices under the same experimental conditions. This finding justified our
next step of detector design modeling.
0.009
0.007
0.008
0.006
0.007
0.006
0.004
DQE(0)
DQE(0)
0.005
0.003
CdTe
a-Si
0.002
0.005
Metal, density
3
Al, 2.7 g/cm
3
Cu, 8.93 g/cm
3
Pb, 11.34 g/cm
3
W, 19.25 g/cm
0.004
0.003
0.002
0.001
0.001
0.000
0.0
0.2
0.4
0.6
0.8
Cu thickness, cm
Fig.3 30 µm thick sensor with Cu plate, primary
6MV spectrum input.
1.0
0
1
2
3
4
Mass Thickness, g/cm
5
6
2
Fig.4 DQE(0) vs. metal plate mass thickness for Al, Cu,
Pb and W .
As expected, the higher absorption of CdTe resulted in a higher DQE(0) values as
shown in Fig. 3. We also simulated DQE(0) for 30 µm thick CdTe in combination with
different metals of varying thickness (Fig. 4) to find detector design with the highest
DQE. Even though Al shows the highest DQE(0) values, its corresponding large
physical thickness leads to excessive signal spreading, therefore denser metal would be
preferable for the converter layer. In the future, we plan to extend our study to DQE(f) to
further optimize material type and its thickness.
Conclusion: We found the thin film CdTe is better suited for electronic portal imaging
device than a-Si in terms of radiation hardness and x-ray absorption at energies
applicable in clinical radiation therapy.