RE: Physical properties of isotopes?

["Jim Otton" <jkotton@usgs.gov>]

Radsafers,

In addition to generally slight chemical diferences due to mass for many

isotopes, there are some chemical differences caused by alpha-recoil

phenomena.  The one that I am familiar with is uranium.  U-238 decays to

U-234 by alpha emission.  In water-rock systems in nature where uranium is

often sorbed to mineral grain  surfaces in contact with water in pore

spaces, alpha-recoil tends to kick the U-234 atom into pore spaces where it

may remain in solution or be resorbed.  If physico-chemical conditions are

just right, the U-234 may remain in solution and the U-234/U-238 activity

ratio in the water will increase.  Most often these increases are modest

(activity ratios of 1.0 to 1.3), in rare cases the increases may be severe.

The effect is an apparent increased solubility for U-234 compared to U-238

under some conditions.  There is a fair amount of literature on this.  A

good reference on this and other uranium-series disequilibrium phenomeon and

their uses in geosciences is Ivanovich, M. and Harmon, R.S., eds., 1992,

Uranium-series disequilibrium: Oxford, Clarendon Press, 910 p.

In contrast, U-238 and U-235 behave identically in natural systems such that

the U-235/U-238 activity ratio is constant at about 0.0467.  Deviations from

this activity ratio in U isotopic measurements of natural systems are

usually attributable to errors in measuring the activity of U-235.  The only

exception to this in nature is the Oklo (Gabon) natural reactor where U-235

was concentrated sufficiently 1.7-1.8  billion years ago to create a natural

nuclear reaction.



Jim Otton

U.S. Geological Survey



-----Original Message-----

From: owner-radsafe@list.vanderbilt.edu

[mailto:owner-radsafe@list.vanderbilt.edu]On Behalf Of Andrew Mattox

Sent: Monday, September 24, 2001 6:49 AM

To: radsafe@list.vanderbilt.edu

Subject: Re: Physical properties of isotopes?





Compounds containing radioisotopes will behave slightly differently than

their stable counterparts.



There is a great deal of environmental research that is based on the

fact that compounds containing the stable isotopes, C-12 vs.C-13 and

N-14 vs. N-15 do not behave exactly the same in the environment because

of the small difference in mass. I don't have a general reference at

hand but contact me off list at amattox@mbl.edu if you need a text.



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