[Leica] Radioactivity

[Adam Bridge <abridge@idea-processing.com>]

On 1/28/00 at 12:28 PM, owner-leica-users@mejac.palo-alto.ca.us (Leica Users
digest) wrote:

I'm not sure quite why we are discussing this......BUT....half-life isn't a
measure of radioactivity. The Curie is the measure of radioactivity - it's unit
of measure is "number of disintigrations/second".

That said, a long half-life implies, on the whole, a substance with is not
extremely radioactive.

Consider: suppose you have 1 mole of two substances: one with a half-life of 1
hour and one with a half-life of 1 year. And suppose for the sake of argument
that they both decay giving off gamma rays of the same intensity.  Which is the
most radioactive? 

Well the Curie count of the short half-life substance is definately much higher,
since in one hour half the original material will have decayed into something
else. The second substance will have emitted far fewer gammas and so the
biological damage it could inflict is less. The number of disintigrations in the
first substance will be great, hence it's Curie content will have been higher:
more radioactive.

It's also true, in general, that nucleides which have very short half-lives
general decay into substances which are themselves unstable and they decay by
some process or the other -- usually with a longer half-life until they end up
as something stable.

There's a rule of thumb: a Curie of Co-60 will deliver about 1 REM in an hour of
exposure at a distance of 1 meter.  A REM is a measure of biological damage due
to radiation. The last time I looked the limits for civilian exposure to
radiation where .5 REM/year over and above what you get from medical x-rays etc.
The limits for the hands, etc are larger by an order or magnitude or so. My
information may be out of date since those limits were set not on the effect on
any one person but by the effect on the gene-pool of cumulative radiation
exposure.

An acute dose of 450 REM will kill about 50% of those who are subjected to it.

Now.......back to range finders!


> "Naturally occuring thorium 232 has a half
> life of about 13 billion years, which implies that it is not very 
> radioactive."
> 
> Of course this statement is false, its the other way around, the shorter a 
> radioactive element takes to decay the less radioactive it is.  A 13 billion 
> year half life is a very long time for something.  Also your information is 
> incorrect concerning the half life of thorium which should be 14 billion 
> years. Here are the details.