We are investigating the dependence of efficiency of diamond detector samples on accumulated radiation dose. We have Sr 90  -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. Dark current ~30nA. Approx. 500 points Temperature stable within 1°C Error in distance measurement is about 2 mm Error in current measurement is about 0.03 nA Bias voltage 100 V = full depletion.

Now we have to covert current to dose. Or, more exactly, to dose rate I / e will give us number of charge carriers N i generated per second Then we multiply N i by ionization energy (3.6 eV for Silicon) and get deposited energy E i in eV units. The next step is to convert energy in eV to Joules. 1 eV = 1.6x 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/cm 3 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.