Cosmic Radiation

On Thursday 25 September 2003 12:36, Al Globus wrote:
(...)
> Are you sure this is true?

I am never sure about anything, except mathematical truths. :-)

> I don't think so. Why do you think the
> atmosphere blocks essentially all incoming radiation?
(...)

I thought that the atmosphere as an efficient blocking was also your opinion,
from this quote in an old (some days ago) message of yours:

"The effect of the blocking from atmosphere 'down under' is trivial
compared to the blocking by 6,000 miles of rock. The reason you need
double the atmosphere is that I assume *no* cosmic radiation comes from
under your feet on Earth. Thus, on a colony, you get cosmic radiation
from twice as many directions, requiring twice the shielding provided
by the atmosphere."

That is, you were assuming that if you have some atmosphere-equivalent shield
in both sides (like in my membrane example) then you get radiation at
Earth-surface levels.

Also, the atmosphere is equivalent to, I think, a wall of concrete four meters
thick. Such a wall would be a considered an over-safe radiation protection
against fission reactor cores and rays from particle accelerators, wouldn't
it? And those artificial sources of radiation are much deadlier than the
radiation existent in space, aren't they?

But there are too many "aren't they" and "wouldn't it"s in my argumentation so
far. So lets try to answer the question once and for all with some
approximate calculation:

In http://www.clavius.org/envsun.html , it is mentioned that in the Apollo 14
mission the astronauts got the highest radiation dosage ever detected in a
mission, 2.85 REM. I assume that the mission took a week (like all manned
missions to the Moon) and so that represents a worst-case of 150 REM/year .
Sorry for the indirect calculation, but I found the data for yearly radiation
exposition in extra-magnetospheric space nowhere...

Now it comes the difficult part: I couldn't find half-length values for cosmic
radiation shielding materials, but then I found in
http://www.atse.org.au/publications/occasional/occ-samarin.htm that the
half-length for biological tissue when it comes to gamma rays, neutrons and
beta decays is 35 mm. Then, for that kind of radiation, you would need
approximately 315 mm of biological tissue to lower 150 REM to the 0.3 REM
average dosage on Earth surface. Since bio tissue is basically water, a
shield of 30cm of water (or 10 cm of concrete) would be sufficient, in this
calculation... But 30cm of water is just 1/30 of the mass-per-surface ratio
of the atmosphere, so either the half-length for cosmic rays is much higher
or the current shielding thickness estimations are one order of magnitude
exaggerated. (I believe more in the first option. That or I made a mistake in
my calculation. :-)

So, does anyone have half-length data for cosmic rays? Then we will have the
ultimate answer to this question that has being debated for so many
messages...

Lucio