We are investigating the dependence of efficiency of
diamond detector samples on accumulated radiation dose.
We have Sr90 -source of known activity and detector.
The dose rate and dose obviously depend on the distance
between detector and source. The dose also depends on
time of exposure.
So we could use time scale to measure relative dose and
compare effects, provided we don’t change anything else.
But it is much better to calibrate dose rate and dose in SI
units, as normal people would do.
In SI the absorbed dose is measured in Gray (Gy) = Joule / kg.
And dose rate is measured in Gy per second.
The energy in dose definition is the energy deposited in matter
by ionizing particles. This energy produces charge carriers and
if we apply bias voltage, we could observe and measure
resulting current.
We have measured current with Si detector and obtained the
following results
Time 5 min.
Approx. 500
points
Dark current
~30nA.
Temperature
stable within 1°C
Bias voltage
100 V = full
depletion.
Error in distance measurement is about 2 mm
Error in current measurement is about 0.03 nA
Now we have to covert current to dose. Or, more exactly, to dose rate
I / e will give us number of charge carriers Ni generated per second
Then we multiply Ni by ionization energy (3.6 eV for Silicon) and
get deposited energy Ei in eV units.
The next step is to convert energy in eV to Joules. 1 eV = 1.6x10-19 J
The last step is to divide this
energy by detector mass and we
get the following nice picture.
The silicon detector is
0.5x0.5x0.28 mm and
density of Si is 2.33 g/cm3
Weight is 1.63x10-5 kg
To cross-check this we could use the count rate measurement.
Katerina have calculated charge produced in Si detector per one
count with our setup.
The difference
between dose rate
calculated by both
methods is on this
figure.
Now it’s time to calculate dose for diamond detector. Diamond
have different density (3.52 g/cm-3) and dE/dx.
To convert dose we need to know density ratio (which is si/ dm
= 0.662) and energy loss ratio.
The energy loss could be calculated from Bethe-Bloch formula
and as a first approximation it gives us the same density ratio.
(Z/A for Silicon and Diamond both equals 2).
So, from Bethe-Bloch we get the same dose for Diamond, as for Si.
Another good estimation will be the ratio of energy loss for MIP,
obtained from literature. This gives us 50 keV/40 keV = 1.25
And for dose the conversion coefficient will be 1.25*0.662 = 0.828
There are also some Geant simulations for deposited energy in
Silicon and Diamond made by Katerina.
From this simulations we obtain ratio of 0.213/0.138 = 1.54
Multiplying this by density ratio we get 1.02 which is very close
to result from Bethe-Bloch formula.
So, with different methods we get dose estimations which are
different by 1.2 times. Which is not bad.