[ RadSafe ] Advection / Diffusion of Radon through Media

[Geo>K0FF]

Dan, as a geologist you know that we use a gamma signature of the lower 
daughter of Uranium to identify and quantify eU in natural rocks. This tag 
is 1.76 MeV from Bi-214 and all geologist's gamma spectrum analyzers are set 
up to quantify this isotope. Of course if the gaseous radon has escaped the 
rock, the lower daughters will no longer be there in the same vicinity of 
the decay chain parents.

Obviously a certain amount of radon DOES escape the matrix, as it winds up 
in the atmosphere.

It seems to me that in any rock sample, one could accurately measure the 
upper chain components, say Pa-234m, then measure the lower daughter Bi-214 
and work out a ratio. This is just an idea as I have not looked into any 
established methodology for doing this using another pair of upper/lower 
constituents.

Regardless of the pair chosen, the measured deficit in the lower progeny 
would indicate the migration of the intermediate progeny, either by 
emanation or by geochemical leaching. A typical instrument for U analysis 
would be a Ludlum Dual Channel Stabilized Analyzer Model 2218 with a 2"dia. 
by 1/2" thick stabilized and shielded sodium iodide probe. At least that's 
what I use.

Rebound energy of the radon-222 ion does indeed impart significant momentum 
to it, as well as the alpha decay particle from the radium-226. This 
momentum is shared between the two ions according to their mass. Being much 
more massive, the speed of radon is much slower than the alpha particle. 
Still it is measured in km/s. It becomes an ion because the atom is sped 
away faster than the electrons can follow, so they simple shed off the outer 
shell!

Where that radon atom winds up is a matter of too many electrical and 
physical interactions to predict, but it should be somewhat easy to measure 
the final results using the simple gamma spec methodology mentions.Most ores 
and rocks that I have had a chance to examine do indeed have all the lower 
daughters, but the exact Secular equilibrium is never the same between 
samples from different regions. Each mine, even different parts of the same 
mine, are slightly different.

As you have mentions, the mere act of processing the ore/rock, i.e. cutting 
a slab, will change the geochemical and physical conditions enough to 
completely change the emanation and leach rates!

Certainly a quick GAMMA SURVEY with simplest equipment can indicate the 
total (relative to a known sample)  amount of inclusions of NORM, many of 
which would have the POSSIBILITY of creating radon. (Note: GM tubes 
universally reject gammas from K-40 as the energy level is above the usable 
range of GM tubes. The efficiency of a particular tube can be measured at 
K-40 by using a 50 pound bag of Potassium Chloride Salt Substitute, as sold 
at Wal Mart for water softeners. Be sure to close any beta windows first. ). 
Also use of a thin window pancake detector will give some idea as to how 
much of the NORM is close to the surface. Alphas close to the surface can 
easily be removed using a 3X5 card shield. If there are no alphas near the 
surface, there will be little likelihood of radon emission ( in my opinion)
If an unusual quantity of NORM is found, then more exacting GAMMA 
SPECTROSCOPY is needed to qualify the mix, and a RADON EMANATION test to 
check on the release. No inexpensive or easy shortcuts to the latter tests 
that I can think of.

These are just thoughts rolling off the top of my head and would certainly 
be a staring place in my own investigations.


George Dowell
NLNL
New London Nucleonics Lab
----- Original Message ----- 
From: "Dan W McCarn" <hotgreenchile at gmail.com>
To: "'Radsafe'" <radsafe at radlab.nl>
Sent: Wednesday, August 20, 2008 7:18 PM
Subject: [ RadSafe ] Advection / Diffusion of Radon through Media


> Hello:
>
> Perhaps this has been done somewhere before:  How does one go about
> estimating that component of radon that emanates from a granite or other
> material? There are several factors that I can think of:
>
> 1 - Nature of uranium mineralization: a) contained within minerals e.g.
> zircon, monazite; biotite and b) epigenetic mineralization via solutions
> precipitating U minerals in pore spaces and fractures.  Most granites show
> secular equilibrium in the uranium series.  Is this a macroscopic or
> microscopic property?  Does the 5 MeV or so recoil dislodge the radon that
> far away from the origin in a mineral grain? Since granites are massive 
> rock
> bodies, emanation of radon and subsequent decay would / could occur within
> the same granite, except near the margins.
>
> Does the accumulation of alpha decays e.g. U-238, Th-234, U-234, Th-230,
> Ra-226 ⇒ Rn-222 make it more accessible to mobilization? Because U-234 is
> more easily leachable than U-238 (Wyoming Basins, Kazakhstan), this 
> suggests
> to me that the Ra-226 has been fairly well dislocated prior to decay to
> Rn-222.
>
> 2 - Dual porosity matrix - the nature of the permeability associated with
> fractures or porous fractions of a material vs. that portion that is
> contained within a mineral grain.  I can imagine that if the rock was 
> porous
> / permeable enough to be an aquifer, that the radon would advect at the 
> same
> rate as the water.  This is borne-out by borehole measurements in and near
> sandstone U deposits.
>
> 3 - Distance to a surface (e.g. fracture or actual surface of material)
>
> Empirically, does a 1 or 2 cm slab of uniform composition granite emanate 
> at
> the same rate per unit surface area as a 10 cm slab?
>
> Dan ii
>
> Dan W. McCarn, Geologist; 3118 Pebble Lake Drive; Sugar Land, TX 77479; 
> USA
> HotGreenChile at gmail.com   UConcentrate at gmail.com
>
>
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