Conversions from MeV to Rad or Gy

[comasic]

I am seeking some light hearted assistance with a conversion from MeV to Rad or Gy.
Im a little new to these units and can't find any helpfull pointers on how to convert them.

This was the real world question that was raised after a conversation about global problems
that could be encountered in a doomsday (http://www.mondovista.com/endtime2x.html)type event.


Question:
How deep would you have to dig a bunker, to reduce the radiation from a super solar flare if
the flare emitted gamma rays that had a median energy of 3.5 trillion electron-volts, or 3500
times the mass-energy of a proton.

Given that proton mass energy equivelent is 938.272 013 MeV
and that every layer of 3.6 inches or 9 cm of packed soil will reduce gamma radiation by half.

The final safe radiation level in the underground bunker should be zero.
Also note thet there is no need to take into consideration air or cloud cover above the bunker.

humm... sounds simple?

if i knew what 1 Rad or 1 Gy was in Mev i would be laughing...
but i could not google anything about the methods of conversion.
Any assistance would be appreciated.

P.S
I can dig the hole myself but calculating how deep it should be is the problem ;-)

[gendou2]

There is no MeV equivalent for Rad or Gy.

MeV is a measure of energy, while Rad and Gy are both measures of energy per unit mass.

1 Gy = 100 Rad = 1 Joule per Kg = 6.2415 × 1012 MeV per Kg

http://en.wikipedia.org/wiki/Rad_(unit)
http://en.wikipedia.org/wiki/Gray_(unit)
http://en.wikipedia.org/wiki/Joule_(unit)

[comasic]

Thank you for pointing out the basic units and pointing me in the right direction.
Im sure a conversion to a common unit is possible at some point but after days
of google and searching i have
found that some people have spent years researching and calculating radiation
from killer solar flares and still find it hard to explain how its calculated.

So I guess i was dreaming of finding a quick solution.

I did however find another topic that explains why the question was asked.
Could a Solar Flare destroy life on our planet? (http://www.physicsforums.com/showthread.php?t=194860)

Thank you for your input.

[daveb]

One thing to also note is that since gamma rays are not charged particles, they are attenuated, not stopped completely. This means that theoretically you need an infinitely thick barrier to reduce exposure to zero. You can, however, reduce to near zero so as to be practically harmless. Also, the mass-energy of the proton is irrelevant for this problem, since it's asking about gammas, not charged particles.

As for a conversion from MeV to Rad, again, there is no standard formula, but you can use the formula

Exposure (in Roetgen) = 1.829E-8 x Number of Gammas x Average Gamma Energy (in MeV) x the mass energy absorption coefficient (in square cm per gram, and this is a function of the gamma energy and the temperature/pressure of the air).

This shows that you also need to know how many gammas are emitted, even if you do have air at STP (the coefficient at this energy is somewhere around 1E-2). A single photon is practically harmless, but a trillion of them would be fatal (around 600 R acute exposure). A non-stochastic safe level would be around 100 R. Of course, your risk of cancer would still go up at 100 R, but you wouldn't die from it.