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If the gamma rays are at 662 keV [1], you'd need about 8cm of rock [2] to block 50%. For comparison, sunglasses typically block 70%-80% of visible light.

[1] https://en.wikipedia.org/wiki/Caesium-137

[2] https://pdfs.semanticscholar.org/7248/4e8995c811d31837199d7c...

Most rock is semi-transparent to gamma rays, which is how the lost source is intended to be used, to determine density of rock.

When you think of things like Gamma or x-rays you cant relate them back to "light" because while there are similarities there are also a number of differences.

Gamma penetrates rocks, these sensors are used to help determine the density of rock by measuring how much was stopped by the material.

This is also why Gamma sources are so dangerous, because things like a coat, a car door are effectively "transparent" to them.

If there was just "one small stone" in front of the source it would almost be like it wasn't present at all.

If something is easy to block, then it's probably not very dangerous.

So probably not the case.