# Do you have to know which radiation type are you measuring with a geiger counter to correctly estimate the equivalent dose rate?

for a while now, I have been working on my own DIY geiger counter. I was able to make some more or less functional devices, which all were working around old soviet SBM-20 GM tube. I want to do a better one with a LND712 tube, which would then be also capable of measuring alpha particles.

I wanted to achive something similar to for example the GAMMA SCOUT, and their way of choosing the required radiation type (the slider, which bloks the open tube window by a specific material, blocking the unwanted radiation types).

The problem, or more likely my cofusion, is that I have found a video of the geiger counter teardown, where could be seen, that the circuit board is not connected to the slider, so the device circuit board does not know what type of radiation the user wants to be blocked. And since the geiger counter does not only display the measured values in Bq or CPM, but mainly in uSv (equivalent or effective dose), how do they come across the fact, that for example alpha particles do have higher weight factor (20), so 1 Gy of gamma particles will do smaller damage than 1 Gy of alpha particles?

How can the device show accurate readings in uSv/h when it doesn't know which radiation type is being blocked by the covering material?

*I am new here, so if my question is somehow incorrectly asked, I can try to explain the point better.

Yes, you need to the know radiation type to calculate the biological harm $$H$$ from the measured dose $$D$$.
From Equivalent dose - Calculation: $$H=\sum_R W_R D_R$$ where
$$H$$ is the equivalent dose in sievert,
$$D_R$$ is the absorbed dose in grays of radiation type $$R$$,
$$W_R$$ is the radiation weighting factor of radiation type $$R$$

The factors $$W_R$$ are different for different types of radiation.
Here are some examples from Relative biological effectiveness - Relation to radiation weighting factors ($$W_R$$):

$$\begin{array}{c|c} \text{Radiation} & W_R \\ \hline \text{x-rays, gamma rays, beta particles} & 1 \\ \hline \text{protons} & 2 \\ \hline \text{alpha particles} & 20 \end{array}$$

• Thank you Thomas, that is definitely helpfull. My second point would be, how is it possible, that a certified reliable geiger counter does not take that into account Jun 4 at 19:25
• @Steporkak I guess the manufacturer wanted to keep it simple for the layman by avoiding this complication. Jun 4 at 19:33
• It's worse than you say. A geiger counter just counts, it doesn't measure $H$, so you have to guess the energy and absorption depth. Jun 4 at 20:24