3
Estimation of Combined Uranium and Thorium
Content with a Scintillation Counter
Some samples exceeded the laboratory’s upper detection
limit for thorium (1000 ppm Th). A simple method was
developed to estimate total U and Th content in highly min-
eralized sandstones. Archive samples of 500g or more were
weighed using a scale accurate to 0.5g. The Exploranium
GR-130 handheld scintillation counter was placed on a
stable surface away from other radioactive sources in order
to have a low background radiation reading. The unit was
run in the “survey” mode with 30 seconds between read-
ings. The scintillation counter was allowed to equilibrate
for at least one minute, and the background reading was
recorded (counts per second, cps). Each sample was placed
directly in front of the scintillation counter and was again
allowed to equilibrate for at least one minute. Each sample’s
radioactivity was recorded. The background radiation read-
ing was then subtracted from each sample’s radioactivity.
As the sample’s radioactivity depends not only on its mass,
but also its composition (U and Th content, K is not pres-
ent in significant amounts), the sample’s background sub-
tracted radioactivity was divided by the sample’s mass to
create a “specific activity” for each sample. “Specific activ-
ity” is used here in quotes, as specific activity is defined as
the activity per unit mass of a radionuclide in the field of
nuclear physics. Here, “specific activity” refers to the activ-
ity per sample mass. By correlating these “specific activities”
to known U+Th values, the resulting line of best fit can
be used to estimate U+Th values from the easy to measure
“specific activity” of unknown samples, provided they con-
tain enough U and Th to measure above background radia-
tion levels.
RESULTS
Four sites containing heavy mineral sandstones have been
recently studied for this project (Figure 1). The Hogback
Deposit is located in San Juan County between Shiprock
and Farmington. The Sanostee Deposit is located on the
Navajo Nation in San Juan County south of Shiprock. The
Flat Top Hill Deposit (also known as Standing Rock) is
located in McKinley County on the Navajo Nation north-
west of Crownpoint. The Farr Ranch Deposit (also known
as Star Lake) is also located in McKinley County on the
Navajo Nation and is southwest of Cuba.
Ground Radiometric Maps
Each area was surveyed with a handheld scintillation coun-
ter to help define the extent of the mineralized sandstones.
Figures 2 through 5 show the results from these radiometric
surveys.
The survey of the Sanostee Deposit (Figure 2) consisted
of 234 stations focused around the ledges of the mesa where
the most mineralized sandstones were exposed. The survey
shows a 1200 meter long, northwest trending zone of min-
eralization that is likely shallowly buried under the thicker
portion of the mesa and eroded away entirely between the
central and northwestern portion of the deposit. The sur-
vey also shows a nearly 200 meter long, narrower mineral-
ized zone southwest of the main zone of mineralization,
also trending northwest. The most radioactive stations were
above 2000 cps, with the highest value of 4500 cps located
on the ledge at the very northwest extent of the survey. It
is possible that these mineralized zones extend along strike
to the northwest, but may be buried deeper. Extension to
the southeast is also possible, but the mesa cliffs out around
600 meters from the southern end of the surveyed area.
The survey of the Flat Top Hill Deposit (Figure 3) con-
sists of two separate mineralized zones that lie on strike with
each other. Flat Top Hill, host to what is referred to here as
Flat Top Hill East Deposit, is an 800 meter long, north-
west oriented mesa with distinct incisions on its northeast
face. The extension of this deposit, referred to as Flat Top
Hill West, lies roughly 1.6 kilometers to the northwest on
a less prominent, narrow ridge roughly 1.3 kilometers long.
The radiometric survey of Flat Top Hill East consisted of
Figure 2. Ground radiometric survey of the Sanostee
Deposit showing a 1200 meter long zone of mineralized
heavy mineral sandstone and a less extensive zone to the
southwest of the northern end
Estimation of Combined Uranium and Thorium
Content with a Scintillation Counter
Some samples exceeded the laboratory’s upper detection
limit for thorium (1000 ppm Th). A simple method was
developed to estimate total U and Th content in highly min-
eralized sandstones. Archive samples of 500g or more were
weighed using a scale accurate to 0.5g. The Exploranium
GR-130 handheld scintillation counter was placed on a
stable surface away from other radioactive sources in order
to have a low background radiation reading. The unit was
run in the “survey” mode with 30 seconds between read-
ings. The scintillation counter was allowed to equilibrate
for at least one minute, and the background reading was
recorded (counts per second, cps). Each sample was placed
directly in front of the scintillation counter and was again
allowed to equilibrate for at least one minute. Each sample’s
radioactivity was recorded. The background radiation read-
ing was then subtracted from each sample’s radioactivity.
As the sample’s radioactivity depends not only on its mass,
but also its composition (U and Th content, K is not pres-
ent in significant amounts), the sample’s background sub-
tracted radioactivity was divided by the sample’s mass to
create a “specific activity” for each sample. “Specific activ-
ity” is used here in quotes, as specific activity is defined as
the activity per unit mass of a radionuclide in the field of
nuclear physics. Here, “specific activity” refers to the activ-
ity per sample mass. By correlating these “specific activities”
to known U+Th values, the resulting line of best fit can
be used to estimate U+Th values from the easy to measure
“specific activity” of unknown samples, provided they con-
tain enough U and Th to measure above background radia-
tion levels.
RESULTS
Four sites containing heavy mineral sandstones have been
recently studied for this project (Figure 1). The Hogback
Deposit is located in San Juan County between Shiprock
and Farmington. The Sanostee Deposit is located on the
Navajo Nation in San Juan County south of Shiprock. The
Flat Top Hill Deposit (also known as Standing Rock) is
located in McKinley County on the Navajo Nation north-
west of Crownpoint. The Farr Ranch Deposit (also known
as Star Lake) is also located in McKinley County on the
Navajo Nation and is southwest of Cuba.
Ground Radiometric Maps
Each area was surveyed with a handheld scintillation coun-
ter to help define the extent of the mineralized sandstones.
Figures 2 through 5 show the results from these radiometric
surveys.
The survey of the Sanostee Deposit (Figure 2) consisted
of 234 stations focused around the ledges of the mesa where
the most mineralized sandstones were exposed. The survey
shows a 1200 meter long, northwest trending zone of min-
eralization that is likely shallowly buried under the thicker
portion of the mesa and eroded away entirely between the
central and northwestern portion of the deposit. The sur-
vey also shows a nearly 200 meter long, narrower mineral-
ized zone southwest of the main zone of mineralization,
also trending northwest. The most radioactive stations were
above 2000 cps, with the highest value of 4500 cps located
on the ledge at the very northwest extent of the survey. It
is possible that these mineralized zones extend along strike
to the northwest, but may be buried deeper. Extension to
the southeast is also possible, but the mesa cliffs out around
600 meters from the southern end of the surveyed area.
The survey of the Flat Top Hill Deposit (Figure 3) con-
sists of two separate mineralized zones that lie on strike with
each other. Flat Top Hill, host to what is referred to here as
Flat Top Hill East Deposit, is an 800 meter long, north-
west oriented mesa with distinct incisions on its northeast
face. The extension of this deposit, referred to as Flat Top
Hill West, lies roughly 1.6 kilometers to the northwest on
a less prominent, narrow ridge roughly 1.3 kilometers long.
The radiometric survey of Flat Top Hill East consisted of
Figure 2. Ground radiometric survey of the Sanostee
Deposit showing a 1200 meter long zone of mineralized
heavy mineral sandstone and a less extensive zone to the
southwest of the northern end