Prompt Fission Neutron Logging Tool
Typical gamma-ray logging tools measure radioactive decay products
which occur in the uranium decay chain rather than the 235U of interest.
Over a long period of geologic time the decay products measured
by gamma ray logging tools will be directly proportional to the
uranium in the formation provided that geologic processes have not
caused the uranium to be separated from the gamma emitters being
measured.
If the uranium has migrated to another location, the gamma ray
log can not be relied upon to indicate the correct grade. The grade
calculation made from the gamma can be either optimistic or pessimistic.
Uranium may have moved into an area of low gamma thus increasing
the grade or out of an area of high gamma thus decreasing the grade.
When this occurs, the ore body is said to be in disequilibrium.
The Prompt Fission Neutron (PFN) logging tool overcomes this problem
by measuring the 235U in the formation.
In the PFN tool, a pulsed neutron source electronically generates
108 14 MeV neutrons per second
which ultimately cause fission of 235U
in the formation. The thermal and epi-thermal neutrons returning
to the tool from the formation are counted in separate detector
channels to provide a measure of 235U free from variations in neutron
output and borehole factors common to both channels. The tool also
contains a standard scintillation gross gamma ray counter. The tool
has no electric logs (resistivity and self-potential). The lowest
practical grade measurement is about .02%.
The tool must be calibrated by taking measurements in environments
of known grade and porosity. We refer to these calibration standards
as “test pits”. An acceptable test pit is a 1 meter
diameter and 1 meter deep polyethylene tank, usually installed in
an excavation so only the top of the tank is exposed, filled with
a specific grain size sand into which is poured a solution of uranium
dissolved in nitric acid. From this, one can derive weight of uranium/volume.
Multiple pits are required to establish a calibration curve. The
minimum is three grade pits (high, medium, and low) and one barren
pit. Other useful pits are pits with varying bore diameters to establish
hole size factors. Tools have been very stable for long periods
of time but users should establish a regular program to verify the
tool’s calibration.
Assaying the mineralized zones in a borehole can be done either
in a parked mode where the tool is stopped and an assay taken or
in a continuous mode. The mode used by all of our customers is the
continuous mode, logging at 1 meter per minute. Customers use their
regular gross gamma logging tool, which usually contain electric
logs as well, to identify mineralized zones of interest. Then, go
back in the hole with the PFN tool to assay those zones.
The tool is provided with Windows logging software and the data
is written to a standard file type easily imported into several
plotting programs. We also sell plotting software and computerized
well logging systems and logging tools.
The PFN tool requires two rack mount power supplies and one microprocessor
interface box that supplies depth and communicates with the PFN
tool. The size of the microprocessor interface box is about 100
mm X 100mm X 25mm.
The tool is approximately 70 millimeters in diameter, 3 meters
in length and weighs 25 kilograms.
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