Gamma Ray Scintillation Detector Calibration


The internal discriminator setting in gross count scintillation detector gamma ray logging tools is not particularly critical, but it is a mistake to simply guess or to determine the setting by "feel".  The following procedure was originally written as a technician's bench guide; it is adaptable to all gross count gamma ray tools (Geiger-MŘeller detector tools require no discriminator calibration procedure because unlike scintillation detectors, the detector output is not proportional to the energy of the impinging gamma ray).  This approach guarantees that the tool will have the greatest possible sensitivity to all gamma rays capable of penetrating the tool housing and at the same time have the discriminator setting as far above the "noise floor" as is consistent with maximum sensitivity to those gamma rays of energy levels that can reach the detector.  Gross errors in setting the discriminator can result in a log with altered "character".

Check Sources

Two small check sources are required for meaningful calibration of gamma ray scintillation detectors in gross count downhole logging tools.  An Americium-241 (Am-241) source of a few microCuries (ÁCi) is used to set the detector preamp gain and to establish the lower discrimination threshold.  Am-241 emits both alpha and gamma radiation (alpha radiation is utilized in AmBe "chemical" neutron sources), but here we are interested in low energy or "soft" gamma rays.  Am-241 produces 59.5 keV gammas with a 36% probability of decay, and 14 keV gammas with a 43% probability of decay, though the latter are so weak that few, if any, can penetrate the detector housing.  Theoretically, a .062 inch standard ruggedized stainless steel crystal housing will allow only about 22% of 60 keV gammas, and no 14 keV gammas, to reach the crystal itself.  Conversely, the same crystal housing is essentially transparent (>95% transmission) to KUT (K, U, Th) energies encountered in the wellbore.  A Cesium-137 (Cs-137) source of a few microCuries is used to verify proper detector operation at higher gamma energies.  Cs-137 (actually the decay product Ba-137m) emits 662 keV (.662 MeV) gammas with a 90% probability of decay.  A small Radium source or even Thorium treated lantern mantles (no longer manufactured) are satisfactory substitutes for a Cs-137 source.

Using the Am-241 Source

The Am-241 check source is sealed in such a way that no alpha particles can escape.  The Am-241 source must be placed in very close proximity to the scintillation crystal housing to produce a decent count rate.  It is also critical that the source be oriented with the proper side toward the detector or the metal backing of the source will greatly attenuate the radiation.  Proper orientation is marked on the Am-241 check source envelope.

Preamp Gain

With the Am-241 check source in place as described above, adjust preamp gain so that the Am-241 gamma ray pulse height is 10-20% or so of maximum preamp pulse height output.  In other words, if the preamp maximum pulse height output is 5 volts, then the Am-241 pulses should be around 1 volt or a little less.  Remove the Am-241 and place the Cs-137 check source near the detector.  Half or more of the Cs-137 pulses should be in saturation, reaching the maximum possible pulse height for the particualr preamp design.

GO/MLS COSMOS Discrimination

For GO/MLS COSMOS tools, increase the discrimination sensitivity setting by turning the trim pot clockwise (decreasing dc bias voltage) until the tool begins to oscillate (counting the high voltage power supply ripple).  Reduce the discrimination sensitivity until the oscillation just stops.  Note the background count rate.  Place the AM-241 check source on the scintillation crystal housing as described above, in the position that yields the maximum count rate.  It may be desirable to tape the Am-241 check source on the detector housing to prevent accidental movement.  A count rate of a few hundred counts per second (cps) should be attainable (a 5 ÁCi Am-241 check source produced 600 and 700 counts on two COSMOS tools equipped with 1" x 4" ruggedized crystals, with the crystals inside their inner tool housings).  Reduce the discrimination sensitivity by turning the trim pot counter-clockwise (increasing the dc bias voltage) until the count rate is approximately 5% of the maximum Am-241 count rate plus the background count rate.  For example, if the background count rate is 50 and the maximum Am-241 count rate is 600, the adjusted count rate should be about 80.  The exact percentage is by no means critical, it is only important that a significant response to the "soft gammas" be observed.  Remove the Am-241 check source and verify a reasonable background count rate.  Bring the Cs-137 (or lantern mantles, etc.) near the tool to verify proper counting of higher energy gamma rays.

Happy calibrating!

API Test Pit Traceable Gamma Ray Calibrator

Lantern Mantles and Cs-137 Button Check Sources

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Last 10-20-10