My supervisor asked to calculate my radioactive exposure, after working with P32, for our annual radioactivity report. My first thought was that don’t remember very well how to do that calculation, so I went back to the radioactivity manual I got during my training. I looked for a formula to calculate exposure and I see that the manual says:
The theoretical dose to an individual in the vicinity of a point source of radioactivity is defined as:
Perfect! Now I only need to The specific Gama Ray Constant. Wait a minute!! Does P32 emits gamma rays? The manual gives me a list of isotopes and their gamma ray constants but P32 is not in the list!
How am I supposed to calculate my exposure then?
Ok so after hours of reading the manual carefully I decided to look for answers at the place where most people would start looking… Wikipedia
Well, it turns out P32 does not emit gamma rays. Perfect, that is a start, but HOW I CALCULATE MY EXPOSURE THEN?
Wait! the manual uses the terms exposure and dose, but I do not understand the difference (At least by reading the manual). So, I decided to look for other manuals.
After several more hours, I found the best manual explaining the concept between exposure and dose. So, “Dose is a measure of energy deposited by radiation in a material, or of the relative biological damage produced by that amount of energy given the nature of the radiation”, while “Exposure is a measure of the ionizations produced in air by x-ray or gamma radiation. The term exposure (with its ‘normal’ definition) is sometimes used to mean dose. (e.g. ‘He received a radiation exposure to his hand.’)”
Ahhh! that is why I was confused, people use “exposure” to mean “dose”. So, I do not have to calculate my exposure but my dose to P32.
Now, how can I calculate my dose?
Most of the radiation dose calculated wil be for the skin (hands) since my body was protected.
Looking at the Radionuclide Safety Data Sheet of P32 I realize that the Dose Rate by P32 is 348 rad/hour per 1 mCi/cm
Rad is a unit of absorbed dose
Rem is a unit of dose equivalent (since dose of alpha particles is not the same as beta or gamma)
For beta emitter 1 Rad = 1 Rem
Ok now I can use the formula above. I do not have a gamma ray constant but I have the P32 Dose Rate which will replace the T in the formula. The average activity (A) used per experiment was 0.74 MBq. The exposure time (T) lets say is 10 hour (probably an overestimation). The distance (d) from the source was 30 cm2 approximately.
((348 rem/hour per 1 mCi/cm) * 0.02 mCi * 10 hours)/ (20cm) = 3.48 rem
3.48 rem to SV
1Sv/100rem * 3.48 rem/year = 0.0348 Sv/year = 34.8 mSv/year
Ok that dose is very low compared with the maximum dose per year of UBC nuclear energy workers and even general public.
Lets consider the time I worked with the whole source with has 9.25MBq activity
Then time (t) would be marginal 0.25 hours and the activity (A) will increase to 0.25 mCi
((348 rem/hour per 1 mCi/cm) * 0.25 mCi * 0.25 hours)/ (20cm) = 1 rem
1Sv/100rem * 1 rem/year = 0.01 Sv/year = 10 mSv/year
So my total dose is 34.8 mSv/year + 10 mSv/year = 44.8 mSv/year
Note: This calculations are wrong. The radioactive safety commitee told me they will publish a guideline of how to do this calculation soon.