# Medical Conversion of Gy/s to Becquerels

1. Aug 20, 2011

### Physicsboi

Is it possible to convert Gy/s to Becquerels. I understand that they are a measurement of different things, dose etc. but if it is possible, what further information would I need?

I am trying to find the activity of Caesium- 137, from the Tammiku, Estonia accident. Some of the infromation that has been provided is that it gives
0.7 Gy/s dose.

2. Aug 21, 2011

### EricVT

You would need to know the exposure rate constant for your particular isotope (the "gamma"), which also depends on how the isotope was contained (if at all).

You would probably also assume a point-source distribution so you can get at the exposure rate in air at some distance "d" by calculating:

[Exposure Rate] = [Activity] x [Gamma] / d^2

If you multiply this by a factor that converts from exposure in air to absorbed dose in tissue (sometimes called a roentgen-to-rad conversion factor, or f-factor) you can get an approximate dose rate in rad/s. From there converting to Gy/s is a matter of dividing by 100.

So working backwards if you know that the dose rate is 0.7 Gy/s = 70 rad/s, then:

[70 rad/sec] = [Activity] x [f in rad/R] x [Gamma in R-m^2/Ci-hr]/[d in m^2]

Activity = [70 rad/sec] x [d in m^2] / {[f in rad/R] x [Gamma in R-m^2/Ci-hr]}

"f" is energy and material dependent. For Cs-137 radiation (662 keV) and for dose in soft tissue, it's value is around 0.96-0.97 rad/R.

The number I'm familiar with for the gamma value of unfiltered Cs-137 is 0.326 R-m^2/Ci-hr.

So you would need to take those values (or other published ones you find that you think would be more appropriate) and then do some unit conversions to get everything to cancel, then make a guess on how far the irradiated person was from this point-source approximation of the Cs-137 activity to get "d" and you would have an estimate in Ci. That can then be converted to Bq.

Obviously this is full of approximations and simplifications, but with nothing to go by except for "0.7 Gy/s" it's about the best you can do.