Downhole energy calibration of gamma ray detectors

An apparatus for estimating a parameter of interest of a formation intersecting a borehole, comprising:

• a radiation sensor;
• a carrier configured to convey the radiation sensor into the borehole;
• and
• a source generator configured to intermittently generate a calibration radiation source in the carrier, the calibration radiation source comprising a radioisotope producing radiation to which the radiation sensor is exposed, wherein the radioisotope is a positron emitter producing radiation by annihilation of the positrons emitted from the radioisotope.

A method for estimating a parameter of interest of a formation intersected by a borehole using a radiation sensor disposed on a carrier in the borehole, comprising:

• intermittently generating a calibration radiation source in the carrier, the calibration radiation source comprising a radioisotope producing radiation to which the radiation sensor is exposed, by:
• applying a current to a capacitor bank over a first time interval while making a measurement using a response from the radiation sensor;
• discharging the capacitor bank to a plasma focus device to generate the calibration radiation source;
• generating a calibration response using measurements from the radiation sensor while the radioisotope is within a window of calibration emissions sufficient for calibration;
• and
• allowing the radio isotope to decay to a reduced radiation level such that a radiation measurement is substantially unaffected by the radioisotope by ceasing activation of the plasma focus device and waiting for a second time interval before repeating the measurement cycle.