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N. Kazuchits et al. / Medical Physics in the Baltic States 7 (2009) 76 - 78
3. Results and discussion
Temporal dependence of current for several γ-rays
137 Cs dose rates for the synthetic diamond detector is
shown in figure 1a. For comparison, the similar
characteristic for IIa-type natural diamond detector is
shown in figure 1b. For both detectors maximum
irradiation dose rate is equal 6,02 сGy/s and bias
voltage is 30 V.
As shown in figure 1, both detectors require preirradiation
dose about 10 Gy for current stabilization.
Requirement of pre-irradiation is character feature of
all types of diamond detectors and related to trapping
of charge carriers on the local states in the band gap of
material. It is necessary to note that sensitivity of
synthetic diamond is about 20 times higher than that of
natural diamond. Nevertheless, the synthetic diamond
detector current fluctuations are higher and reach the
value about 3%.
Fig. 1. Temporal dependence of current for several γ-rays
137 Cs dose rates for synthetic (a) and natural (b)
diamond detectors
Knowledge of the detector dose rate response is of
importance for accurate dosimetry. Figure 2 shows
such dependences for both types of detectors under the
γ-rays irradiation of 137 Cs source.
For natural and synthetic diamond detectors this
dependence is close to linear and can be approximated
by the equation [5]:
i = i0 + RD ∆ , (1),
where i - detector current, i0 - detector current without
irradiation (dark current), D – the dose rate, R –
sensitivity, and ∆ - linearity index.
Fit of the dependences in figure 2 with expression (1)
with fitting parameters R and ∆ gives ∆ = 1,011,
R = 0,1 μC/Gy, and ∆ = 0,978, R = 1,7 μC/Gy for
natural and synthetic diamond respectively.
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Fig. 2. Dose rate dependence of current for natural and
synthetic diamond detectors under γ-rays 137 Cs source
exposure
The suitability of the synthetic diamond detector for γrays
registration was investigated by measuring depthdose
curves and profiles in 6 and 18 MeV scanning
photon beam of “Varian” linear accelerator. Relative
dose rate distribution measurements by both PTW
natural diamond and synthetic diamond detectors were
carried out in water phantom. Detectors were placed
perpendicular to the beam axis.
Depth-dose curve of a 6 MeV photon beam measured
by the PTW natural diamond detector is presented in
figure 3.
Fig. 3. Depth-dose curve of a 6 MeV photon beam measured
by PTW natural diamond detector
It was not possible to obtain such dependence for
synthetic diamond detector because of its current
exceeds maximum threshold of used PTW registration
equipment.
For this reason further dose rate distribution
measurements were carried out while detectors were
submerged in water phantom on depth of 150 mm.
Radiation field distribution is shown in figures 4 and 5
for natural and synthetic diamond detectors
respectively. Parameters of registered radiation field
are presented in table 1. As shown in figure 5, synthetic
diamond detector measurements with integration time
of 0,5 s are characterized by higher signal fluctuations
in comparison with natural diamond detector. Such