[ RadSafe ] observations on iodized salt

[Thompson, Dewey L]

John:

That "lite" salt contains the compound Potassium Chloride, chemical symbols of (KCl).  The idea is to replace some of the Sodium in the diet for those of us who pour on table salt, which is Sodium Chloride (NaCl).  For those salts that "add iodine", some Potassium Iodide is added (KI).  The amount of KI added is very small, and for the purposes of your question (and my following discussion) we will ignore it.  

There are three "forms" of Potassium found on earth, K-39, K-40, and K-41.  Approximately 93% of the Potassium has 39 neutrons (K-39), and approximately 7% has 41 neutrons (K-41).  Neither K-39 or K-40 are radioactive.  

Approximately 0.0118% of all Potassium on earth has 40 neutrons, (K-40), and is radioactive.  It has a "half life" of 1.27 billion years, meaning that it was a "primordial" isotope.  Now, 89% of the time, K-40 decays with a beta particle, and 11% of the time it decays with a strong gamma, and associated "positive" beta particle.  These particles are what you have been seeing with the friskers.  

Potassium Chloride is quite stable, the molecule is called a "metal halide", which basically means that the molecular bonding of very tight, which means the stuff does not degrade much while sitting on the shelf.  

Now.  You can calculate the activity of a given quantity of material if you know what isotope is in it, and how much.  (Activity = KN, where N is the number of atoms and K is the probability of decay per given unit of time).  I'm not going to bore you with the math example here, but I do wish to comment on your remark on the "old" containers.  (If you do perform the math, you should get about 0.2 microcuries/lb, or 16 MBq/gm).

The half life of K-40 is very long.  This means the activity of a given sample won't change during our life times.  So the difference you were seeing was in the percentage of Potassium Chloride in the mixture.  I suspect the "old" container had a larger fraction of NaCl than the "new" container.

Oh, yes............  No.  NaCl is not radioactive.  You should NOT see any counts frisking table salt.

Dewey 

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<Snip>

We use a Ludlum model 14C and a pancake probe with the 0.1 scale calibrated to cpm.  We would take a couple of tablespoons of Morton Lite Salt, a mixture of iodized salt and potassium chloride, and count it during radiation safety training classes.  Background was less than 1000 cpm, a newly opened package of this salt would peg the meter on the 0.1 scale.  Older, opened containers (2 years) still near max reading (6000 cpm).  I used it none the less.  I have never tried to count plain iodized salt.  Would I get different results?

John

John G. Center, Jr.
Radiation Safety Officer
3922 Wood Hall
Western Michigan University
1903 W. Michigan Ave.
Kalamazoo, MI  49008-5410

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