From: shaw@s119.es.llnl.gov (Henry Shaw 510 423-4645)
Newsgroups: sci.environment
Subject: Re: Are Your Light Bulbs Radioactive? (Gatorade results)
Date: 15 Sep 92 01:24:48 GMT
Distribution: na
Organization: Earth_Sciences_Dept.,_LLNL


  <steinman@is.morgan.com> (Jan Steinman) writes
   In article <10503@sun13.scri.fsu.edu> jac@ds8.scri.fsu.edu (Jim Carr) writes:

     JC> Good idea.  But do not stop there.  Suggest that they put a
     JC> bottle of GatorAde (sorry you folks at UF, any of the
     JC> "replacement" sport drinks) next to it.  <quite a nice effect,
     JC> BTW> Compare to CF bulb.  Look for any warning label on the
     JC> respective packages.  A learning experience!  Also might compare
     JC> to a banana and your own body.

 JS>  I didn't think Mr. Carr knew what he was talking about, but was
 JS>  unwilling to reply without some data...

He didn't know what he was talking about.  For a sci.* group, there are 
pitifully few messages here containing anything but wild assertions.

 JS> I counted Gatorade this weekend. Since I don't know offhand the decay
 JS> products of potassium (my college texts are 3,000 miles away)....

K has three naturally occurring isotopes:

 K-39 93.2581%
 K-40  0.0117%
 K-41  6.7302%

K-40 is the radioactive isotope.  It essentially has two different 
decay modes.  About 89% of the time, it decays via a 1.31 MeV beta- 
emission.  This mode has a halflife of 1.43x10^9 years and the resulting daughter is a stable Ca-40 nucleus.  About 11% of the time, K-40 decays 
via electron-capture accompanied by the emission of a 1.46MeV gamma 
photon.  This branch has a halflife of 1.2x10^10 years and results in a 
stable Ar-40 nucleus.  (This decay forms the basis of K-Ar geological 
dating methods.)   Very rarely (< 0.001% of the time, a K-40 nucleus 
will decay via positron, i.e. beta+, emission).  The "combined" halflife 
of K-40 resulting from the two important branches is thus:  
 
1/T(total) = 1/T(beta-) + 1/T(EC) 
           = 6.99x10^-10 + 8.33x10^-11 
           = 7.83x10^-10/yr

T(total) = 1.28x10^9 years, where T is the relevant halflife.


 JS> I opened the lid and positioned the alpha window about 4 cm from the
 JS> liquid surface, in the hope of capturing as many ionizing events as
 JS> possible. The label claims it contains a total of 100mg of potassium.

You don't say how large a volume this 100mg is contained in.  Is it one 
serving?  I would have thought that a large bottle of Gatorade has much 
more than 100mg of K in it.  But never mind.

Let's calculate what is called the "specific activity" of natural K.
The specific activity is simply the expected number of decays per 
unit time per unit mass of K.

One g of natural K contains 6.02x10^23/39.1 = 1.54x10^22 atoms.  0.0117% of these are K-40, so there are N = 1.8x10^18 atoms of K-40/(g K).

The number of decays per unit time (the "activity") in a sample is 
given by A = N*ln(2)/T, where T is the halflife and N is the number of 
radioactive nuclei in the sample.   Plugging in, one finds that 
A = 9.76x10^8 decays/year/(g K) = 30.9 decays/s/(g K).  One curie is 
defined as 3.7x10^10 decays/sec, so a natural K has a specific activity 
of ~840pCi/(g K)

100mg of K should thus result in about 3.9 decays/s, which you are 
unlikely to be able to detect with a standard G-M detector.

The human body contains about 140g K (for a 140kg person), so a typical
body burden of K-40 is about 4326 decays/s, or about about 0.12uCi.

Someone should check my math, but as I remember, this is about the right 
number.
 
 JS> Here are the results:

 JS> 		Gator	20mg K	banana	normal	nuke	flight
 JS> day	14.74	15.23	15.71	15.69	56.11	404.1
 JS>night	14.35	15.09	15.39	15.36


 JS> Once again, no statistical significance, let alone "quite a nice
 JS> effect". My guess is that solar activity was lower while counting the
 JS> Gatorade than with the other samples -- over years of background
 JS> monitoring, I've found solar activity to be the greatest corelant to
 JS> increased bacground count.


I don't know what units these are, but it's not surprising that you 
didn't see an effect given the above calculations.


It's instructive to do a similar calculation for C-14, which is the 
other major naturally occurring radioisotope in the human body. 
C-14 has a halflife of ~5730 years (it is constantly produced 
in the atmosphere by nuclear reactions between cosmic ray 
secondaries and nuclei of atoms in the air.)  Carbon had a 
natural specific activity of about 6.1pCi/(g C) prior to 
the atmospheric testing of nuclear weapons in the 50's and 
60's.  Your "standard person" contains about 16,000 g of C, 
so a typical body burden of C-14 is about 0.1uCi, approximately the 
same as that for K-40. The body burden of "post-bomb people" (i.e., us)
should be 1 to 2 times that of the pre-bomb case.  

I'm assuming your detector is some variant of a geiger counter.  It 
sounds like it has a thin-window detector that allows it to detect 
alphas.  Right?  If so, you will not be able to detect C-14 activity, 
even if it is high, because of the low energy of the decay.

If you are looking for "hot" samples, try counting some "natural phosphate
rock" fertilizer.  Make sure it is not a processed as it is possible that 
the processing method could have removed some of the uranium present in the 
feed material, which was probably a phosphate-rich rock known, naturally 
enough, as phosphorite.  Phosphorites can contain several hundreds of ppm
of uranium.  Bone and blood meal (other phosphate-rich materials used as fertilizers) should not be particularly active.

-- 
Henry Shaw, Earth Sciences Dept., LLNL |    All opinions are my own.
Internet: shaw4@llnl.gov               |  
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