RE: Ra-226 Field Screening

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

Dan,

For many years researchers at the USGS used Scintrex GAD-6

gamma-spectrometers (3-inch NaI crystal detectors) to make field

measurements of K, U, and Th.  These field spectrometers were purchased in

the mid-1970s for uranium and thorium resource investigations, but have been

used since for a variety of investigations including soil and bedrock radon

source studies, gamma "logging" of outcrop for correlation with subsurface

gamma logs, calibration of airborne gamma surveys, and measurements of

radium-226 and Ra-228 at NORM-contaminated, oil- and gas-production sites.



These spectrometers were calibrated at concrete pads set up by the

Department of Energy at the airport in Grand Junction.  These pads had

precisely known K, U, and Th contents.  U and Th were in full equilibrium

with all their decay products. See

http://www.eml.doe.gov/about/news/99Inter/1999Inter.htm  for some recent

info on these pads.



Use of these field instruments to measure U and Th assumes that U was in

equilbrium with Bi-214 and that Th was in equilibrium with Tl-208 in the

bedrock or soil material being measured.  Since that is often not the case,

the results of the gamma measurements were usually expressed as eU or eTh

where e means "equivalent".



The instruments were not shielded for cosmic-sourced gamma, thus an

additonal set of meaurements was made to determine cosmic background by

taking the instrument out on a lake in a small aluminum boat several hundred

yards from shore.



Raw gamma counts for the K, U, and Th channels on the instrument were

converted to eU, eTh and K measurements (in ppm, ppm and percent,

respectively, for a uranium exploration geologist) using formulas preloaded

on a small, handheld Sharp computer.  The readings can also be given in

activity concentrations (for health physicists, environmental cleanup

specialists).  Almost all of these USGS instruments have fallen into

disrepair (damaged crystals, worn-out electronics) and there are few USGS

projects that need such instruments presently. For example, my work has

moved away from oilfield NORM to other aspects of oilfield environmental

issues.



In oilfield NORM work, we used the GAD-6 instrument to measure eRa-226 and

eRa-228 in soils, again assuming equilibrium between the two radium isotopes

and the measured Bi-214 and Tl-208 isotopes.



Use of the instrument and the associated calculations assumes that the

measurements are made with the detector resting on a flat surface

(hemi-spherical geometry for the sensed gammas).  About 90% of the

gamma-rays reaching the instrument from the soil or bedrock are emitted from

a hemispherical volume of about 40 cm in radius.  The radius varies

dependent upon soil density, but 40 cm is a good rule of thumb.  The

calculated apparent value is an average for the entire volume.  Note,

however, that any given gamma signal detected in the crystal can be derived

from a broad array of distributions of gamma-sources in the subjacent soil

or bedrock.  For example, a thin layer of radium-enriched oil tank sludge at

the surface over soil of background activity concentrations could yield the

same gamma signal as a hemispherical volume of soil of uniform but lower

activity concentrations.



Field-rugged, 4-channel, gamma spectrometers from Scintrex cost about

$20,000, the last time I checked 3 or 4 years ago.  There are several other

companies marketing various devices to measure gamma activity in the field.

The sophistication of these instruments is widely ranging from Ge-Li

detectors requiring liquid nitrogen in the field to simpler, single-channel

instruments that can be used if the limitations are understood. Note that

some of the limitations and assumptions are severe enough to make various

instruments unsuitable for some purposes.



We have some data for oilfield-NORM contamination where field spectrometer

measurements were followed up by radiochemical analysis of core at the point

of measurement.



Jim Otton

U.S. Geological Survey













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

From: owner-radsafe@list.vanderbilt.edu

[mailto:owner-radsafe@list.vanderbilt.edu]On Behalf Of Dan Hoffman

Sent: Thursday, July 18, 2002 10:52 AM

To: radsafe@list.vanderbilt.edu

Subject: Ra-226 Field Screening





I am interested in talking with someone with experience in using a field

screening method to evaluate Ra-226 in soils and how such methods relate to

conventional laboratory analysis.  If anyone has information on this

subject, please contact me directly.  Thanks for the assistance.



Daniel E. Hoffman, CHP,CIH,CSP

Vice President EH&S Services

Pangea Group

743 Spirit 40 Park Dr., Suite 232

Chesterfield, MO 63005

Office 636-519-4877

Mobile 314-707-5606



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