A spectrometer for downhole applications can be calibrated without using radioactive sources. A spectrum of measured gamma ray counts can be received from a spectrometer in a calibration mode. A spectrum-to-window ratio can be used to determine a relationship between voltage level applied to a photoreceptor of the spectrometer and gain factors. A voltage level associated with a gain factor of one can be identified for use by the spectrometer in a non-calibration mode. The measured gamma ray counts and reference shapes for a plurality of radioactive elements can be used in a least squares fit process to determine an offset to apply to the spectrometer in the non-calibration mode.

Aspects of the subject technology relate to a system configured to apply a voltage setting to a sensor of a downhole tool. The system is configured to identify an active gain control model for a temperature-sensitive sensor for a downhole tool, receiving temperature data for the downhole tool, and apply a high voltage setting for the first sensor based on the temperature data and the active gain control model. The system is further configured to collect sensor data from the sensor operating in a wellbore using the high voltage setting and update the active gain control model based on the sensor data.

Embodiments are directed generally to an ionizing-radiation beamline monitoring system that includes a vacuum chamber structure with vacuum compatible flanges through which an incident ionizing-radiation beam enters the monitoring system. Embodiments further include at least one scintillator within the vacuum chamber structure that can be at least partially translated in the ionizing-radiation beam while oriented at an angle greater than 10 degrees to a normal of the incident ionizing-radiation beam, a machine vision camera coupled to a light-tight structure at atmospheric/ambient pressure that is attached to the vacuum chamber structure by a flange attached to a vacuum-tight viewport window with the camera and lens optical axis oriented at an angle of less than 80 degrees with respect to a normal of the scintillator, and at least one ultraviolet (“UV”) illumination source facing the scintillator in the ionizing-radiation beam for monitoring a scintillator stability comprising scintillator radiation damage.

Embodiments are directed generally to an ionizing-radiation beamline monitoring system that includes a vacuum chamber structure with vacuum compatible flanges through which an incident ionizing-radiation beam enters the monitoring system. Embodiments further include at least one scintillator within the vacuum chamber structure that can be at least partially translated in the ionizing-radiation beam while oriented at an angle greater than 10 degrees to a normal of the incident ionizing-radiation beam, a machine vision camera coupled to a light-tight structure at atmospheric/ambient pressure that is attached to the vacuum chamber structure by a flange attached to a vacuum-tight viewport window with the camera and lens optical axis oriented at an angle of less than 80 degrees with respect to a normal of the scintillator, and at least one ultraviolet (“UV”) illumination source facing the scintillator in the ionizing-radiation beam for monitoring a scintillator stability comprising scintillator radiation damage.

A spectroscopic gamma and neutron detecting device includes a scintillation detector that detects gamma and thermal neutron radiation, the scintillation detector including signal detection and amplification electronics, and a stabilization module configured to measure a pulse height spectrum of neutron radiation, determine a thermal neutron peak position in the neutron pulse height spectrum originating from cosmic ray background radiation, monitor the thermal neutron peak position in the neutron pulse height spectrum during operation of the spectroscopic gamma and neutron detecting device, and adjust the signal detection and amplification electronics based on the thermal neutron peak position in the neutron pulse height spectrum, thereby stabilizing the spectroscopic gamma and neutron detecting device.

A spectroscopic gamma and neutron detecting device includes a scintillation detector that detects gamma and thermal neutron radiation, the scintillation detector including signal detection and amplification electronics, and a stabilization module configured to measure a pulse height spectrum of neutron radiation, determine a thermal neutron peak position in the neutron pulse height spectrum originating from cosmic ray background radiation, monitor the thermal neutron peak position in the neutron pulse height spectrum during operation of the spectroscopic gamma and neutron detecting device, and adjust the signal detection and amplification electronics based on the thermal neutron peak position in the neutron pulse height spectrum, thereby stabilizing the spectroscopic gamma and neutron detecting device.

Methods and apparatus for calibrating radioactive sources are described. An array of scintillation detectors form a receptacle within which a sample or sample container can be retained by a holder. The scintillation detectors are coupled via light transducers such as photomultiplier tubes (PMTs) to independent electronic counters. Coincidence processing of time-tagged events yields a correlated cent rate. One or more corrections can be applied as needed, for background counts, deadtime, or random coincidences. Voltage tuning of PMTs yields improved reproducibility. Variations are disclosed. 1% accuracy has been demonstrated over a range N of 10 kBq-3 MBq, covering a gap in the capabilities of conventional technology.

A PET scanner includes a ring of detector modules encircling an imaging region. Each of the detector modules includes one or more sensor avalanche photodiodes (APDs) that are biased in a breakdown region in a Geiger mode. The sensor APDs output pulses in response to light from a scintillator corresponding to incident photons. A reference APD also biased in a breakdown region in a Geiger mode is optically shielded from light and outputs a voltage that is measured by an analog to digital converter. Based on the measurement, a bias control feedback loop directs a variable voltage generator to adjust a bias voltage applied to the APDs such that a difference between a voltage of a breakdown pulse and a preselected logic voltage level is minimized.

A PET scanner includes a ring of detector modules encircling an imaging region. Each of the detector modules includes one or more sensor avalanche photodiodes (APDs) that are biased in a breakdown region in a Geiger mode. The sensor APDs output pulses in response to light from a scintillator corresponding to incident photons. A reference APD also biased in a breakdown region in a Geiger mode is optically shielded from light and outputs a voltage that is measured by an analog to digital converter. Based on the measurement, a bias control feedback loop directs a variable voltage generator to adjust a bias voltage applied to the APDs such that a difference between a voltage of a breakdown pulse and a preselected logic voltage level is minimized.

A system for obtaining downhole azimuthal imaging information includes a pressure housing. The system also includes a source arranged within the pressure housing, the source including a directable electron beam. The system further includes an anode positioned proximate the source, within the pressure housing, the anode having a tapered face adapted to interact with the directable electron beam and direct an x-ray beam away from the anode. The system also includes a detector arranged proximate the anode, the anode being between the source and the detector, wherein the detector receives scattered x-rays, from the x-ray beam, the received scattered x-rays corresponding to imaging information to determine one or more properties of a wellbore.