Temperature compensation of radiation sensitive detectors for downhole use

Abstract

Radiation sensitive detection systems for subsurface exploration include a radiation sensitive detector and an electrical circuit electrically connected to the radiation sensitive detector and configured to transform a signal received from the radiation sensitive detector and to perform a temperature compensation of the transformed signal with a temperature compensation circuit. The temperature compensation circuit includes at least one of a thermistor, a field-effect transistor (FET), and / or a bipolar transistor.

Description

65NUL-510648-WO-2_INT 1042PCTTEMPERATURE COMPENSATION OF RADIATION SENSITIVE DETECTORS FOR DOWNHOLE USECROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of an earlier filing date from U. S. Provisional Application Serial No. 63 / 728,370 filed December 5, 2024, the entire disclosure of which is incorporated herein by reference.BACKGROUND

[0002] The subject matter disclosed herein generally relates to measurement systems utilized in subsurface exploration and evaluation, and more particularly is directed to temperature compensation of radiation sensitive detectors.

[0003] It is well known that logging-tools and measurement-while-drilling (MWD) tools, which are utilized for making measurements while traversing boreholes through subsurface formations, encounter large variations in borehole temperatures. In general, temperatures increase with depth, and very high temperatures are frequently encountered. Many types of logging tools and several types of MWD tools contain scintillation detectors for measuring radiation (e.g., gamma ray, neutron, charged particles, etc.). Scintillation detectors incorporate a scintillator (crystal) for converting radiation or charged particles to light, photomultiplier tubes for converting the light to electronic signals, and electronic circuits for processing the electronic signals. These components are subject to variation with temperature.

[0004] In some radiation sensitive detector configurations, analog data acquisition chains are utilized, where signals obtained at a photomultiplier tube are amplified and fed to a discriminator. A logical signal is output which may be counted. With a gain of an amplifier chain fixed, the total amplification gain is determined by the electron amplification gain of the photomultiplier. The amplification gain of the photomultiplier depends upon an applied high voltage. In counting applications, where a discriminator analyzes the signal from the photomultiplier tubes, a plateau measurement is performed to find an optimal operating voltage. To perform the plateau measurement, typically two count rate curves are measured at different temperatures, such as a relatively lower temperature and a relatively higher temperature (e.g., 25°C and 175°C). The two curves form a large gap between them because the total analog gain of the detector and the analog front-end decreases with increasing temperature and a low-level discriminator is typically kept constant with temperature. It is desirable to compensate the gain loss caused by temperature changes in general. However, the65NUL-510648-WO-2_INT 1042PCTcompensation of gain loss by increasing the high voltage has the drawback that the voltage-gain curve is an exponential curve for a constant count rate, which limit the effective working area. Accordingly, improved methods and systems for compensation for gain loss may be desirable.SUMMARY

[0005] According to some embodiments, radiation sensitive detection systems for subsurface exploration are provided. The radiation sensitive detection systems for subsurface exploration include a radiation sensitive detector and an electrical circuit electrically connected to the radiation sensitive detector and configured to transform a signal received from the radiation sensitive detector and to perform a temperature compensation of the transformed signal with a temperature compensation circuit. The temperature compensation circuit includes at least one of a thermistor, a field-effect transistor (FET), and / or a bipolar transistor.

[0006] According to some embodiments, subsurface exploration systems are provided. The subsurface exploration systems include a string configured to be deployed into a borehole formed in an earth formation and a downhole tool assembly arranged on the string and configured to make measurements of the earth formation. The downhole tool assembly includes a radiation sensitive detector and an electrical circuit electrically connected to the radiation sensitive detector. The electrical circuit is configured to transform a signal received from the radiation sensitive detector and to perform a temperature compensation of the transformed signal with a temperature compensation circuit. The temperature compensation circuit includes at least one of a thermistor, a field-effect transistor (FET), and / or a bipolar transistor.BRIEF DESCRIPTION OF THE DRAWINGS

[0007] The following descriptions should not be considered limiting in any way. With reference to the accompanying drawings, like elements are numbered alike:

[0008] FIG. 1 is a view of a drilling system that may incorporate sensor systems and assemblies in accordance with embodiments of the present disclosure;

[0009] FIG. 2 is a simplified schematic illustration of a radiation sensitive detector assembly that may incorporate embodiments of the present disclosure;

[0010] FIG. 3 A is a schematic plot of counts obtained by a conventional radiation sensitive detector assembly;

[0011] FIG. 3B is an illustration of the counts as a function of energy level of the system of FIG. 3 A at an operating voltage of 1,700V;65NUL-510648-WO-2_INT 1042PCT

[0012] FIG. 4A is a schematic plot of counts obtained by a radiation sensitive detector assembly in accordance with an embodiment of the present disclosure;

[0013] FIG. 4B is an illustration of counts as a function of energy level of the system of FIG. 4A at an operating voltage of 1,550V;

[0014] FIG. 5 is a circuit diagram of an example radiation sensitive detection system having a temperature compensation circuit including a negative temperature coefficient (NTC) thermistor in accordance with an embodiment;

[0015] FIG. 6 is a circuit diagram of an example radiation sensitive detection system having a temperature compensation circuit including a field-effect transistor (FET) in accordance with an embodiment;

[0016] FIG. 7 is a circuit diagram of an example radiation sensitive detection system having a temperature compensation circuit including both a negative temperature coefficient (NTC) thermistor and a field-effect transistor in accordance with an embodiment;

[0017] FIG. 8 is a circuit diagram of an example radiation sensitive detection system having a temperature compensation circuit including a positive temperature coefficient (PTC) thermistor in accordance with an embodiment;

[0018] FIG. 9 is a circuit diagram of an example radiation sensitive detection system having a temperature compensation circuit including both a positive temperature coefficient (PTC) thermistor and a field-effect transistor in accordance with an embodiment;

[0019] FIG. 10 is an example illustrative plot of relative pre-amp gain of a system having a combination of a thermistor and an FET compensation in accordance with an embodiment of the present disclosure; and

[0020] FIG. 11 is a circuit diagram of an example radiation sensitive detection system having a temperature compensation circuit having passive transformation elements in accordance with an embodiment.DETAILED DESCRIPTION

[0021] A detailed description of one or more embodiments of the disclosed apparatus and method are presented herein by way of exemplification and not limitation with reference to the Figures.

[0022] FIG. 1 shows a schematic diagram of a system for performing subsurface operations (e.g., downhole, within the earth or below the earth’s surface and into a formation). As shown, the system is a drilling system 10 that includes a drill string 20 having a drilling assembly 90, also referred to as a bottomhole assembly (BHA), conveyed in a wellbore or65NUL-510648-WO-2_INT 1042PCTborehole 26 penetrating an earth formation 60. The drilling system 10 includes a derrick 11, for example a conventional derrick, erected on a floor 12 that supports a rotary table 14 that is rotated by a prime mover, such as an electric motor (not shown), at a desired rotational speed. The drill string 20 includes a drilling tubular 22, such as a drill pipe, extending downward from the rotary table 14 into the borehole 26. A disintegrating tool 50, such as a drill bit attached to the end of the drilling assembly 90 or a reaming tool (not shown), also known as a reamer, disintegrates the geological formations when it is rotated to drill the borehole 26. The drill string 20 is coupled to a drawworks 30 via a kelly joint 21, swivel 28, traveling block 25, and line 29 through a pulley 23. During the drilling operations, the drawworks 30 is operated to control the weight-on-bit (WOB), which affects the rate of penetration. The operation of the drawworks 30 is well known in the art and is thus not described in detail herein.

[0023] During drilling operations a suitable drilling fluid 31 (also referred to as the “mud”) from a source or mud pit 32 is circulated under pressure through the drill string 20 by a mud pump 34. The drilling fluid 31 passes into an inner bore of the drill string 20 via a desurger 36, fluid line 38 and the kelly joint 21. Fluid line 38 may also be referred to as a mud supply line. The drilling fluid 31 is discharged at the borehole bottom 51 through an opening (not shown) in the disintegrating tool 50. The drilling fluid 31 circulates uphole through the annular space 27 between the drill string 20 and the borehole 26 and returns to the mud pit 32 via a return line 35. A sensor SI in the fluid line 38 provides information about the fluid flow rate. A surface torque sensor S2 and a sensor S3 associated with the drill string 20 respectively provide information about the torque and the rotational speed of the drill string. Additionally, one or more sensors (not shown) associated with line 29 are used to provide the hook load of the drill string 20 and other desired parameters relating to the drilling of the borehole 26. The system may further include one or more downhole sensors 70 located on the drill string 20 and / or the drilling assembly 90.

[0024] In some applications the disintegrating tool 50 is rotated by rotating the drilling tubular 22 about their longitudinal axis (not shown). However, in other applications, a drilling motor 55 (such as a mud motor) disposed in the drilling assembly 90 is used to rotate the disintegrating tool 50 and a portion of drilling assembly 90 about their longitudinal axis and / or to superimpose or supplement the rotation of the drill string 20 (rotary mode). In either case, the rate of penetration (ROP) of the disintegrating tool 50 into the earth formation 60 for a given formation and a drilling assembly largely depends upon the weight-on-bit and the rotational speed of the disintegrating tool 50. In one aspect of the embodiment of FIG. 1, the drilling motor 55 is coupled to the disintegrating tool 50 via a drive shaft (not shown) disposed65NUL-510648-WO-2_INT 1042PCTin a bearing assembly 57. If a mud motor is employed as the drilling motor 55, the mud motor rotates the disintegrating tool 50 when the drilling fluid 31 passes through the drilling motor 55 under pressure. The bearing assembly 57 supports the radial and axial forces of the disintegrating tool 50, the downthrust of the drilling motor and the reactive upward loading from the applied weight-on-bit. Stabilizers 58 that may be coupled to the bearing assembly 57 and / or at other suitable locations on the drill string 20 act as centralizers, for example for the lowermost portion of the drilling motor assembly and other such suitable locations. The drilling motor 55 may include an Adjustable Kick Off sub (AKO). The deployment of an AKO provides the build of inclination of the borehole when drilling in a sliding mode (i.e., no drill string rotation and the disintegrating tool is only driven by the rotating rotor of the drilling motor). Alternatively, a deviated borehole may be drilled by using a deflection device, such as a steering unit or device (not shown), that enables an operator to steer the disintegrating tool (e.g., drill bit) in a desired direction. A steering unit comprises one or more force application devices that may be actuated and controlled hydraulically, electrically, or both.

[0025] A surface control unit 40 receives signals from the downhole sensors 70 and devices via a sensor 43 placed in the fluid line 38 as well as from sensors SI, S2, S3, hook load sensors, sensors to determine the height of the traveling block (block height sensors), and any other sensors used in the system and processes such signals according to programmed instructions provided to the surface control unit 40. For example, a surface depth tracking system may be used that utilizes the block height measurement to determine a length of the borehole (also referred to as measured depth of the borehole) or the distance along the borehole from a reference point at the surface to a predefined location on the drill string 20, such as the disintegrating tool 50 or any other suitable location on the drill string 20 (also referred to as measured depth of that location, e.g. measured depth of the disintegrating tool 50). Determination of measured depth at a specific time may be accomplished by adding the measured block height to the sum of the lengths of all equipment that is already within the wellbore at the time of the block height measurement, such as, but not limited to drilling tubulars 22, drilling assembly 90, and disintegrating tool 50. Depth correction algorithms may be applied to the measured depth to achieve more accurate depth information. Depth correction algorithms, for example, may account for length variations due to pipe stretch or compression due to temperature, weight-on-bit, wellbore curvature and direction. By monitoring or repeatedly measuring block height, as well as lengths of equipment that is added to the drill string 20 while drilling deeper into the formation over time, pairs of time and depth information are created that allow estimation of the depth of the borehole 26 or any location on the drill65NUL-510648-WO-2_INT 1042PCTstring 20 at any given time during a monitoring period. Interpolation schemes may be used when depth information is required at a time between actual measurements. Such devices and techniques for monitoring depth information by a surface depth tracking system are known in the art and therefore are not described in detail herein.

[0026] The surface control unit 40 displays desired drilling parameters and other information on a di splay / monitor 42 for use by an operator at the rig site to control the drilling operations. The surface control unit 40 contains a computer that may comprise memory for storing data, computer programs, models and algorithms accessible to a processor in the computer, a recorder, such as tape unit, memory unit, etc. for recording data and other peripherals. The surface control unit 40 also may include simulation models for use by the computer to process data according to programmed instructions. The control unit responds to user commands entered through a suitable device, such as a keyboard. The surface control unit 40 can output certain information through an output device, such as s display, a printer, an acoustic output, etc., as will be appreciated by those skilled in the art. The surface control unit 40 may be adapted to activate alarms 44 when certain unsafe or undesirable operating conditions occur.

[0027] The drilling assembly 90 may also contain other sensors and devices or tools for providing a variety of measurements relating to the earth formation 60 surrounding the borehole 26 and for drilling the borehole 26 along a desired path. Such devices may include a device for measuring formation properties, such as the formation resistivity or the formation gamma ray intensity around the borehole 26, near and / or in front of the disintegrating tool 50 and devices for determining the inclination, azimuth and / or position of the drill string. A logging-while-drilling (LWD) device for measuring formation properties, such as a formation resistivity tool 64 or a gamma ray device 76 for measuring the formation gamma ray intensity, made according an embodiment described herein may be coupled to the drill string 20 including the drilling assembly 90 at any suitable location. For example, coupling can be done above a lower kick-off subassembly 62 for estimating or determining the resistivity of the earth formation 60 around the drill string 20 including the drilling assembly 90. Another location may be near or in front of the disintegrating tool 50, or at other suitable locations. A directional survey tool 74 that may comprise means to determine the direction of the drilling assembly 90 with respect to a reference direction (e.g., magnetic north, vertical up or down direction, etc.), such as by a magnetometer, gravimeter / accelerometer, gyroscope, etc. may be suitably placed for determining the direction of the drilling assembly, such as the inclination, the azimuth, and / or the toolface of the drilling assembly. Any suitable direction survey tool may be utilized.65NUL-510648-WO-2_INT 1042PCTFor example, the directional survey tool 74 may utilize a gravimeter (accelerometer), a magnetometer, or a gyroscopic device to determine the drill string direction (e.g., inclination, azimuth, and / or toolface). Such devices are known in the art and therefore are not described in detail herein.

[0028] Still referring to FIG. 1, other LWD devices (generally denoted herein by numeral 77), such as devices for measuring rock properties or fluid properties, such as, but not limited to, porosity, permeability, density, salt saturation, viscosity, permittivity, sound speed, etc. may be placed at suitable locations in the drilling assembly 90 for providing information useful for evaluating the earth formation 60 (i.e., subsurface formation ) or fluids along borehole 26. Such devices may include, but are not limited to, acoustic tools, nuclear tools, nuclear magnetic resonance tools, permittivity tools, and formation testing and sampling tools.

[0029] The above-noted devices may store data to a memory downhole and / or transmit data to a downhole telemetry system 72, which in turn transmits the received data uphole to the surface control unit 40. The downhole telemetry system 72 may also receive signals and data from the surface control unit 40 and may transmit such received signals and data to the appropriate downhole devices. In one aspect, a mud pulse telemetry system (including a mud pulser) may be used to communicate data between the downhole sensors 70 and devices and the surface equipment during drilling operations. A sensor 43 placed in the fluid line 38 may detect the mud pressure variations, such as mud pulses responsive to the data transmitted by the downhole telemetry system 72. Sensor 43 may generate signals (e.g., electrical signals) in response to the mud pressure variations and may transmit such signals via a conductor 45 or wirelessly to the surface control unit 40. In other aspects, any other suitable telemetry system may be used for one-way or two-way data communication between the surface and the drilling assembly 90. including but not limited to, a wireless telemetry system (such as an acoustic telemetry system, an electro-magnetic telemetry system), a wired pipe system, or any combination thereof. The data communication system may utilize repeaters in the drill string or the wellbore. In a wired pipe system, one or more wired pipes may be made up by joining drill pipe sections, wherein each pipe section includes a data communication link for electrical signals, such as a wire or any other type of conduit for electrical signal, that runs along the pipe. The data connection between the pipe sections may be made by any suitable method, including but not limited to, electrical or optical line connections, including optical, induction, capacitive or resonant coupling methods. A data communication link may also be run along a side of the drill string 20, for example, if coiled tubing is employed.65NUL-510648-WO-2_INT 1042PCT

[0030] The drilling system described thus far relates to those drilling systems that utilize a drill pipe to convey the drilling assembly 90 into the borehole 26, wherein the weighton-bit is controlled from the surface, typically by controlling the operation of the drawworks. However, a large number of the current drilling systems, especially for drilling highly deviated and horizontal wellbores, utilize coiled-tubing for conveying the drilling assembly subsurface. In such an application a thruster is sometimes deployed in the drill string to provide the desired force on the disintegrating tool 50. Also, when coiled-tubing is utilized, the tubing is not rotated by a rotary table but instead it is injected into the wellbore by a suitable injector while a downhole motor, such as drilling motor 55, rotates the disintegrating tool 50. For offshore drilling, an offshore rig or a vessel is used to support the drilling equipment, including the drill string.

[0031] Still referring to FIG. 1, a resistivity tool 64 may be provided that includes, for example, a plurality of antennas including, for example, transmitters 66a or 66b and receivers 68a or 68b. Resistivity can be one formation property that is of interest in making drilling decisions. Those of skill in the art will appreciate that other formation property tools can be employed with or in place of the resistivity tool 64.

[0032] Analog data acquisition chains are utilized during LWD natural radiation monitoring and / or measurement (e.g., gamma-ray, neutron, charged particle). For example, with reference to FIG. 2, a schematic diagram of a radiation sensitive detector 200 is illustratively shown. The radiation sensitive detector 200 includes a photomultiplier tubes (PMT) device 202 that is arranged to collect and direct radiation 204 (e.g., gamma rays, neutrons, charged particles, etc.) received from a surrounding formation. The signals received from the PMT device 202 are amplified within an amplifier 206 and then fed to a discriminator 208 (e.g., having upper-level discriminator(s) ULD) and / or lower-level discriminator(s) LLD). The output from the discriminator 208 may be a logical signal that is counted at a counter 210.

[0033] With the gain of the amplifier chain fixed (amplifier 206). the total amplification gain is determined by the electron amplification gain of the photomultiplier 202, which depends on an applied high voltage. The applied high voltage may be provided or supplied from a high voltage power supply 212. In counting applications, where the discriminator 208 analyzes the processed PMT signal, a simple procedure for finding the working voltage is to perform a so- called plateau measurement.

[0034] Translating the pulse-height spectrum into an integral representation in dependence of the external photomultiplier gain, i.e., the photomultiplier voltage, yields a plateau response curve. The curve is developed by using a source such asl37Cs and counting65NUL-510648-WO-2_INT 1042PCTall pulses higher than a fixed discriminator level. The total counts are then plotted as a function of the photomultiplier voltage. The rising portion of the integral response curve then corresponds to the discriminator moving through the photopeak, through the Compton scattering region of the distribution, and through the low-energy and X-ray peak regions. Formally, the integral counting curve I(Lo) with lower threshold Lois calculated from the differential of the spectrum S(E), I(Lo) = ∫S(E)dE. For a given electronic threshold, the gain required to bring a point Loon the spectrum to the threshold level T is K · G · Lo= T, with K a conversion factor and G the gain. This yields G = const · Lo-1. Consequently, the plateau curve is generated by transforming the integral curve into units of gain.

[0035] The plateau region still exhibits a slope, which corresponds to the dark background of the photomultiplier that would appear at lowest energies of the spectrum. The length of the plateau is determined by the stability of the tube at high gain and by the characteristic variation of gain with voltage. As the voltage on the photomultiplier is increased, the dark current of the tube increases as various so-called regenerative effects increase. At some high voltage and gain, almost all photomultipliers exhibit breakdown phenomena such as glowing of the dynodes under electron bombardment leading to light scattered back to the photo cathode, glass charging effects, after-pulses, and thermionic emission of electrons from the cathode or dynodes materials with low work functions. Thus, the plateau terminates at the upper end via an avalanche region of increased count rates due to the aforementioned noise and dark current contributions.

[0036] Referring now to FIG. 3A, a typical result of a plateau test of a radiation sensitive detector is illustratively shown (Plot 300). Along the horizontal axis is voltage (volt) and along the vertical axis is count rate (counts per second). There are two count rate curves 302, 304 which are measured at two different temperatures: 25°C (curve 302) and 175°C (curve 304). The two curves 302, 304 define a large gap 306 between them (e.g., about 150V-200V range). The gap 306 is present because the total analog gain of the detector and the analog front-end decreases with increasing temperature and the Low-Level Discriminator (LLD) is relatively constant with changing temperature.

[0037] If there were no noise effect from the detector, there would be no intersection between these curves 302, 304 at the right-hand side of plot 300. This is because the converted lower energy threshold is greater at higher temperature. For example, for a high voltage value of 1700V, this level is approximately 50keV at 175°C and 15keV at 25°C. FIG. 3B illustrates an example spectrum (plot 308) acquired for 1700V at 25°C and 175°C, respectively. As noise65NUL-510648-WO-2_INT 1042PCTincreases with higher voltage at higher temperature, the count rate curve at 175°C has an exponential form at higher voltage, as illustrated by plots 300, 308.

[0038] The selection of 1700V for plot 308 is based on a region of overlap 310 of the two curves 302, 304 in plot 300. The region of overlap 310 is defined by and between a high voltage upper limit (HV-UL) and a high voltage lower limit (HV-LL). The region of overlap 310 has a voltage range due to acceptable variations and deviation. From the region of overlap 310, an operating voltage (OV) may be selected for configuring a detector assembly and operation thereof. In this case, a voltage of 1700V was selected for the operating voltage (OV), which falls within the region of overlap 310.

[0039] The region of overlap 310 defines the length of the plateau. In this example embodiment, the plateau (or region of overlap 310) has a length of about 70 V. ranging between about 1660V to about 1730V, with an operating voltage (OV) selected at about 1700V. It is noted that the intersection point in this configuration is close to or at the high voltage upper limit (HV-UL) (e.g., about 1730V). As used herein, the term operating voltage (OV) refers to the voltage applied to a photomultiplier tube (PMT) during sensing, detecting, measuring, or the like (i.e., operation of a downhole detector assembly or device).

[0040] To minimize the gap, either the gain of the analog front-end must increase with temperature or the LLD must decrease with temperature. One disadvantage of having the LLD decrease with temperature is that any preamplifier of the detector system will be much more prone to noise at higher temperature. Accordingly, implementing solutions with an inverse LLD relative to temperature may result in unacceptable levels of noise at higher temperatures. In view of this, embodiments of the present disclosure are directed to downhole detection / sensor systems that compensate for temperature through inclusions of a system with the gain of an analog front-end that increases with temperature.

[0041] In accordance with embodiments of the present disclosure, at least one of a thermistor (e.g., negative temperature coefficient (“NTC”) or positive temperature coefficient (“PTC”)), a bipolar transistor, and / or a field-effect transistor (FET) in the ohmic region may be used to compensate the analog gain with respect to temperature. In embodiments that employ an FET or a bipolar transistor, a control voltage is required, whereas embodiments that employ a thermistor may not require such a control voltage.

[0042] Referring now to FIGS. 4A-4B, a plateau test of a radiation sensitive detector in accordance with an embodiment of the present disclosure is shown in FIG. 4A (plot 400), and FIG. 4B illustrates a spectrum acquired for 1550 V at 25°C and 175°C (plot 402), which is based on an operating voltage obtained from plot 400. As shown in FIG. 4A, there are two65NUL-510648-WO-2_INT 1042PCTcount rate curves 404, 406, which are measured at 25°C (curve 404) and 175°C (curve 406). The two curves 404, 406 define a gap 408 between them. As illustrated, the gap 408 is significantly smaller than the gap 306 shown in FIG. 3A. For example, in this configuration, and using a downhole detector system in accordance with an embodiment of the present disclosure, the gap 408 is about 20V-50V. As a result of the smaller gap 408, the length of a plateau (e.g., region of overlap 410) is increased. For example, the length of the plateau (region of overlap 410) is about 90 V in this example, and extends from about 1500V at the HV-LL to about 1590V at the HV-UL, with the operating voltage (OV) selected as 1550V. Accordingly, the operating voltage (OV) is decreased as compared to the system used for FIGS. 3A-3B. FIG.4B illustrates the spectrum acquired for 1550V at 25°C and 175°C, illustrating that both effects are positive for reliability at downhole use and for life-time of the detector.

[0043] To achieve the lower voltage plateau and detector reliability, the electronics of the detector system / assembly are provided to compensate for temperature, as noted above. For example, embodiments of the present disclosure are directed to incorporating at least one of a negative temperature coefficient (NTC) thermistor, a positive temperature coefficient (PTC) thermistor, a field-effect transistor (FET) in the ohmic region, and / or a bipolar transistor within the electronic circuitry of the radiation sensitive detector.

[0044] Referring now to FIG. 5, a schematic diagram of a radiation sensitive detection system 500 in accordance with an embodiment of the present disclosure is shown. The radiation sensitive detection system 500 may be arranged as part of a downhole assembly, bottomhole assembly, or the like, as will be appreciated by those of skill in the art. The radiation sensitive detection system 500 is configured with a temperature compensation circuit 502 which enables temperature compensation to reduce the operating voltage of the radiation sensitive detection system 500, such as shown in FIG. 4A.

[0045] In operation, radiation 504 (e.g., gamma rays, neutrons, charged particles, etc.) may be received at a photomultiplier tube (PMT) 506 from a downhole formation 508. The photons received at the PMT 506 will induce a current (input current) which is directed into the electrical circuitry of the radiation sensitive detection system 500 for transformation or processing of the input current. In this configuration, active processing or transformation is performed, with the current directed to a protection resistor 510 (Rprotect) that is arranged at an input to the analog circuitry components of the radiation sensitive detection system 500. The current is then directed to a parallel set of components, arranged after the protection resistor 510. As shown, a first operational amplifier 512 (with supply voltage Vcc / -Vcc) and a combination of passive components 514 are arranged in parallel to form a charge amplifier.65NUL-510648-WO-2_INT 1042PCTThe combination of passive components 514 includes a charge amplifier gain resistor 516 (Rdiarge) and a charge amplifier capacitor 518 (Ccharge). The amplified voltage will then be directed through a high-pass filter 520 having a high-pass filter capacitor 522 and a high-pass filter resistor 524. The filtered voltage is then directed to a second operational amplifier 526 (with supply voltage Vcc / -Vcc). The second operational amplifier 526 is electrically connected to the temperature compensation circuit 502.L0046J The temperature compensation circuit 502 of FIG. 5 is configured as an NTC thermistor assembly, which includes an initial gain resistor 528 (Rgain0), an additional resistor 530 (Rgaini), and an NTC thermistor 532 (Rtherm). Inclusion of the temperature compensation circuit 502 allows for analog gain compensation over a large range of temperature. For example, the analog gain can be adjusted by changing the resistance of the initial gain resistor 528 (Rgaino), the additional resistor 530 (Rgaini), and / or the NTC thermistor 532 (Rtherm). The NTC thermistor 532 has a value that decreases with increasing temperature, thus providing for temperature compensation, as described herein. In this illustrative configuration, the NTC thermistor 532 is arranged in series with the additional resistor 530.

[0047] With the temperature compensation circuit 502, the total gain of the analog front-end increases with increasing temperature, and this increased gain allows for compensation of signal loss of the PMT 506. Arranged in parallel with the second operational amplifier 526 is a low-pass filter 534 having a low-pass filter resistor 536 and a low-pass filter capacitor 538. The radiation sensitive detection system 500 may output an output voltage 540 that is temperature compensated and measurable by downstream components and systems, as will be appreciated by those of skill in the art. For example, the radiation sensitive detection system 500 illustrated in FIG. 5 may be configured as a pre-amplification stage that processes the signals of the collected photons at the PMT 506 into a suitable format for downstream data processing. This system allows for mitigation of gain losses and thus improves the fidelity and accuracy of the measurement as compared to systems without such temperature compensation circuit 502. Additionally, by minimizing the impact of temperature, the operating voltage of the system is reduced, and component lifetime and quality will be therefore improved.

[0048] The temperature compensation circuit 502 is arranged in electrical connection with an electrical circuit that is configured to amplify a signal received at the PMT 506. The electrical circuit includes, for example and without limitation, the protection resistor 510, the first operational amplifier 512, the charge amplifier 514, the high-pass filter 520, the second operational amplifier 526. and the low-pass filter 534. The temperature compensation circuit 502 may thus be integrated within the electrical circuit. The output of the electrical circuits of65NUL-510648-WO-2_INT 1042PCTthe radiation sensitive detection system 500 may be directed to signal processing components, which may be part of a downhole tool, bottomhole assembly, or the like, as will be appreciated by those of skill in the art. For example, the output voltage 540 may be a signal which is processed to measure radiation data of a downhole formation (e.g.. gamma rays, neutrons, charged particles, etc.). The radiation sensitive detection system 500 may be used, for example, during while-drilling operations to obtain measurement-while-drilling and / or logging-while-drilling data and information regarding a formation surrounding a borehole in which the radiation sensitive detection system 500 is located.

[0049] Referring now to FIG. 6, a schematic diagram of a radiation sensitive detection system 600 in accordance with an embodiment of the present disclosure is shown. The radiation sensitive detection system 600 may be arranged as part of a downhole assembly, bottomhole assembly, or the like, as will be appreciated by those of skill in the art. The radiation sensitive detection system 600 is configured with a temperature compensation circuit 602 which enables temperature compensation to reduce to the operating voltage of the radiation sensitive detection system 600, as described above.

[0050] The radiation sensitive detection system 600 is similar to that shown and described with respect to FIG. 5. For example, in operation, radiation 604 (e.g., gamma rays, neutrons, charged particles, etc.) may be received at a photomultiplier tube (PMT) 606 from a downhole formation 608. The photons received at the PMT 606 will induce a current (input current) which is directed into the electrical circuitry of the radiation sensitive detection system 600 for transformation thereof. First the current is directed to a protection resistor 610 (Rprotect) that is arranged at an input to the analog circuitry components of the radiation sensitive detection system 600. The current is then directed to a parallel set of components, arranged after the protection resistor 610. As shown, a first operational amplifier 612 (with supply voltage Vcc / -Vcc) and a combination of passive components 614 are arranged in parallel to form a charge amplifier. The combination of passive components 614 includes a charge amplifier gain resistor 616 (Rcharge) and a charge amplifier capacitor 618 (Ccharge). The amplified voltage will then be directed through a high-pass filter 620 having a high-pass filter capacitor 622 and a high-pass filter resistor 624. The filtered voltage is then directed to a second operational amplifier 626 (with supply voltage Vcc / -Vcc). The second operational amplifier 626 is electrically connected to the temperature compensation circuit 602.

[0051] The temperature compensation circuit 602 of FIG. 6 is configured as a fieldeffect transistor (FET) assembly, which includes an initial gain resistor 628 (Rgaino). an additional resistor 630 (Rgaini), and an FET 632 (QI). The FET 632 is provided with a control65NUL-510648-WO-2_INT 1042PCTvoltage 633. Inclusion of the temperature compensation circuit 602 allows for analog gain compensation over a large range of temperature. For example, the analog gain can be adjusted by changing the resistance of the initial gain resistor 628 (Rgaino), the additional resistor 630 (Rgaini), and / or the control voltage 633 of the FET 632 (QI). In some embodiments, it will be appreciated that the FET 632 may be replaced by a bipolar transistor, or a bipolar transistor may be arranged in combination with the FET 632, without departing from the scope of the present disclosure. In this illustrative configuration, the FET 632 is arranged in series with the additional resistor 630.

[0052] With the temperature compensation circuit 602, the total gain of the analog front-end increases with increasing temperature, and this increased gain allows for compensation of signal loss of the PMT 606. Arranged in parallel with the second operational amplifier 626 is a low-pass filter 634 having a low-pass filter resistor 636 and a low-pass filter capacitor 638. The radiation sensitive detection system 600 may output an output voltage 640 that is temperature compensated and measurable by downstream components and systems, as will be appreciated by those of skill in the art. For example, the radiation sensitive detection system 600 illustrated in FIG. 6 may be configured as a pre-amplification stage that processes the signals of the collected photons at the PMT 606 into a suitable format for downstream data processing. This system allows for mitigation of gain losses and thus improves the fidelity and accuracy of the measurement as compared to systems without such temperature compensation circuit 602. Additionally, by minimizing the impact of temperature, the operating voltage of the system is reduced, and component lifetime and quality will be therefore improved.

[0053] Referring now to FIG. 7, a schematic diagram of a radiation sensitive detection system 700 in accordance with an embodiment of the present disclosure is shown. The radiation sensitive detection system 700 may be arranged as part of a downhole assembly, bottomhole assembly, or the like, as will be appreciated by those of skill in the art. The radiation sensitive detection system 700 is configured with a temperature compensation circuit 702 which enables temperature compensation to reduce to the operating voltage of the radiation sensitive detection system 700, as described above. In this embodiment, the circuitry of the radiation sensitive detection system 700 and the transformation of the input current is substantially similar to that shown and described above, and thus like features will not be described again.

[0054] In this configuration, the temperature compensation circuit 702 is arranged with both an NTC thermistor 704 (Rtherm) and an FET 706 (and / or bipolar transistor), with associated control voltage 708. and an initial gain resistor 710 (Rgain0) (e.g., first resistor). Associated with the NTC thermistor 704 is a first additional resistor 712 (Rgaini) (e.g., second resistor) and65NUL-510648-WO-2_INT 1042PCTassociated with the FET 706 (QI) is a second additional resistor 714 (Rgaina) (e.g., third resistor). The temperature compensation circuit 702 enables the adjustment or customization as described above, such as by adjusting the resistances of the FET 706 (QI), the initial gain resistor 710 (Rgaino), the first additional resistor 712 (Rgain1), the second additional resistor 714 (Rgain2), and / or the control voltage 708 of the FET 706 (QI). In this embodiment, the NTC thermistor 704 is arranged in series with the first additional resistor 712 and the FET 706 is arranged in series with the second additional resistor 714.

[0055] Referring now to FIG. 8, a schematic diagram of a radiation sensitive detection system 800 in accordance with an embodiment of the present disclosure is shown. The radiation sensitive detection system 800 may be arranged as part of a downhole assembly, bottomhole assembly, or the like, as will be appreciated by those of skill in the art. The radiation sensitive detection system 800 is configured with a temperature compensation circuit 802 which enables temperature compensation to reduce the operating voltage of the radiation sensitive detection system 800, such as shown in FIG. 4A.

[0056] In operation, radiation 804 (e.g., gamma rays, neutrons, charged particles, etc.) may be received at a photomultiplier tube (PMT) 806 from a downhole formation 808. The radiation 804 received at the PMT 806 will induce a current (e.g.. input current) which is directed into the electrical circuitry of the radiation sensitive detection system 800. An active transformation is performed in which the current is first directed to a protection resistor 810 (Rprotect) that is arranged at an input to the analog circuitry components of the radiation sensitive detection system 800. The current is then directed to a parallel set of components, arranged after the protection resistor 810. As shown, a first operational amplifier 812 (with supply voltage Vcc / -Vcc) and a combination of passive components 814 are arranged in parallel to form a charge amplifier. The combination of passive components 814 includes a charge amplifier gain resistor 816 (Rcharge) and a charge amplifier capacitor 818 (Ccharge). The ampli tied voltage will then be directed through a high-pass filter 820 having a high-pass filter capacitor 822 and a high-pass filter resistor 824. The filtered voltage is then directed to a second operational amplifier 826 (with supply voltage Vcc / -Vcc). The second operational amplifier 826 is electrically connected to the temperature compensation circuit 802. It will be appreciated that this description of the electrical connections and current within the radiation sensitive detection system 800 may be substantially similar to the above-described embodiments.

[0057] The temperature compensation circuit 802 of FIG. 8 is configured as a positive temperature coefficient (“PTC") thermistor assembly, which includes an initial gain resistor 828 (Rgain), an additional capacitor 838 (Ciowpass), and a PTC thermistor 836 (Rtherm). Inclusion65NUL-510648-WO-2_INT 1042PCTof the temperature compensation circuit 802 allows for analog gain compensation over a large range of temperatures. For example, the analog gain can be adjusted by changing the resistance of the initial gain resistor 828 (Rgaino) and / or the resistance of the PTC thermistor 836 (Rtherm). The PTC thermistor 836 has a value that increases with increasing temperature, thus providing for temperature compensation, as described herein.

[0058] With the temperature compensation circuit 802 illustrated in FIG. 8, the total gain of the analog front-end increases with increasing temperature, and this increased gain allows for compensation of signal loss of the PMT 806. The radiation sensitive detection system 800 may output an output voltage 840 that is temperature compensated and measurable by downstream components and systems, as will be appreciated by those of skill in the art. For example, the radiation sensitive detection system 800 illustrated in FIG. 8, and / or the other configurations described herein, may be configured as a pre-amplification stage that processes the signals of the collected photons at the PMT 806 into a suitable format for downstream data processing. This system allows for mitigation of gain losses and thus improves the fidelity and accuracy of the measurement as compared to systems without such temperature compensation circuit 802. Additionally, by minimizing the impact of temperature, the operating voltage of the system is reduced, and component lifetime and quality will be therefore improved.

[0059] The temperature compensation circuit 802 is arranged in electrical connection with an electrical circuit that is configured to amplify a signal received at the PMT 806. The electrical circuit includes, for example and without limitation, the protection resistor 810, the first operational amplifier 812, the charge amplifier 814, the high-pass filter 820, and the second operational amplifier 826. The temperature compensation circuit 802 may thus be integrated within the electrical circuit of the radiation sensitive detection system 800. The output of the electrical circuit(s) of the radiation sensitive detection system 800 may be directed to signal processing components, which may be part of a downhole tool, bottomhole assembly, or the like, as will be appreciated by those of skill in the art. For example, the output voltage 840 may be a signal which is processed to measure radiation data of the downhole formation 808 (e.g., gamma rays, neutrons, charged particles, etc.). The radiation sensitive detection system 800 may be used, for example, during while-drilling operations to obtain measurementwhile-drilling and / or logging-while-drilling data and information regarding a formation surrounding a borehole in which the radiation sensitive detection system 800 is located.

[0060] Referring now to FIG. 9, a schematic diagram of a radiation sensitive detection system 900 in accordance with an embodiment of the present disclosure is shown. The radiation sensitive detection system 900 may be arranged as part of a downhole assembly, bottomhole65NUL-510648-WO-2_INT 1042PCTassembly, or the like, as will be appreciated by those of skill in the art. The radiation sensitive detection system 900 is configured with a temperature compensation circuit 902 which enables temperature compensation to reduce to the operating voltage of the radiation sensitive detection system 900, as described above. In this embodiment, the circuitry of the radiation sensitive detection system 900 and active transformation of the input current is substantially similar to that shown and described above, and thus like features will not be described again.L0061 J For example, the radiation sensitive detection system 900 is configured to receive radiation 904 at a photomultiplier tube (PMT) 906 from a downhole formation 908. Similar to that described above, a current is directed to a protection resistor 910 (Rprotect) that is arranged at an input to analog circuitry components of the radiation sensitive detection system 900. A first operational amplifier 912 and a combination of passive components 914 are arranged in parallel to form a charge amplifier. The passive components 914 includes a charge amplifier gain resistor 916 (Rcharge) and a charge amplifier capacitor 918 (Ccharge). An amplified voltage is then directed through a high-pass filter 920 having a high-pass filter capacitor 922 and a high-pass filter resistor 924. The filtered voltage is then directed to a second operational amplifier 926 which is electrically connected to the temperature compensation circuit 902.

[0062] In this configuration, the temperature compensation circuit 902 is arranged with both a PTC thermistor 936 (Rthenn) and an FET 932 (and / or bipolar transistor), with associated control voltage 933, and an initial gain resistor 928 (Rgain) (e.g., first resistor). Associated with the FET 932 (QI) is a second additional resistor 930 (Rgainz) (e.g., third resistor). The temperature compensation circuit 902 enables the adjustment or customization as described above, such as by adjusting the resistances of the FET 932 (QI), the initial gain resistor 928 (Rgain), the second additional resistor 930 (Rgain0), and / or the control voltage 933 of the FET 932 (QI). In this embodiment, the FET 932 is arranged in series with the second additional resistor 930.

[0063] It will be appreciated that other configurations of the temperature compensation circuits described herein are possible without departing from the scope of the present disclosure. For example, duplication of one or more of the types of temperature compensation circuits may be employed, with multiple FETs, multiple thermistors, and / or combinations thereof. A combination of the two types of temperature compensation can enable adjustment of the gain to fit degradation of the PMT. In embodiments with both FET and thermistors (NTC or PTC), for example, the relative pre-amp gain that is output by the radiation sensitive detection systems described herein (i.e., before other processing) can be increased by applying a control voltage to the FET. In one such example of operation, and as shown in the Plot 100065NUL-510648-WO-2_INT 1042PCTof FIG. 10, in a combined configuration (e.g., similar to that of FIG. 7 or FIG. 9), a normalized pre-amp gain may be increased from about 3 at 175°C (with OV control voltage) to about 3.5 at 175°C (with 2V control voltage). It will be appreciated that the values illustratively plotted in FIG. 10 may be representative of an illustrative embodiment, and it will be appreciated that variations in the values may be present and not depart from the scope of the present disclosure. In some embodiments, the temperature and / or gain values may be varied by 10% or more, depending on the specific application, components, and operation thereof, as will be appreciated by those of skill in the art. As illustrated, higher control voltages result in an increase in the gain curve with higher temperatures.

[0064] Additionally, in some embodiments of the present disclosure, a passive transformation of the signals from the photomultiplier tube (PMT) may be performed, rather than the active transformation of the above-described embodiments. For example, with reference to FIG. 11, a schematic diagram of a radiation sensitive detection system 1100 in accordance with an embodiment of the present disclosure is shown. The radiation sensitive detection system 1100 may be arranged as part of a downhole assembly, bottomhole assembly, or the like, as will be appreciated by those of skill in the art. The radiation sensitive detection system 1100 is configured with a temperature compensation circuit 1102 which enables temperature compensation to reduce to the operating voltage of the radiation sensitive detection system 1100, as described above.

[0065] In this passive temperature compensation configuration, photons are received at a PMT 1104 and will induce a current (input current) which is directed into the electrical circuitry of the radiation sensitive detection system 1100. As an initial processing, the input current is directed to passive transformation circuitry 1106, which in this configuration includes a capacitor 1108 (Ciransform) and a resistor 1110 (Rtransform). The capacitor 1108 and the resistor 1110 of the passive transformation circuitry 1106 are arranged in parallel, with an output of the passive transformation circuitry 1106 being directed through a high-pass filter 1112 having a high-pass filter capacitor 1114 and a high-pass filter resistor 1116. The filtered voltage is then directed to a second operational amplifier 1118 which is electrically connected to the temperature compensation circuit 1102. The temperature compensation circuit 1102 of this illustrative configuration is similar to that of the embodiment of FIG. 5, including, without limitation an NTC thermistor 1103 (Rtherm), and thus the configuration thereof will not be described again. Although the temperature compensation circuit 1102 is similar to that of FIG.5, it will be appreciated that the temperature compensation circuits described with respect to65NUL-510648-WO-2_INT 1042PCTthe other embodiments may be employed with the passive transformation circuitry 1106 of FIG. 11.

[0066] The passive transformation circuitry 1106 is configured to transform the signal received from the PMT 1104. In contrast to the active transformations performed in the above¬ described embodiments, the passive transformation circuitry 1106 does not include amplification of the signal with active amplifiers or the like. Even without such amplification of the signal, the temperature compensation of the present disclosure may be performed on the transformed (but not amplified) signal, to minimize temperature dependency of radiation sensitive detection systems. Accordingly, embodiments of the present disclosure include both active and passive initial processing of PMT signals.

[0067] Embodiments of the present disclosure are directed to an electrical (e.g., analog) solution for minimizing the temperature dependency of radiation sensitive detection systems and / or logging devices that are used in downhole or subsurface exploration. Such temperature compensation circuits can reduce subsurface data processing steps and / or may decrease operational voltages or actively control the gain (e.g., decreasing voltage to improve component lifetime and quality). For example, in accordance with some embodiments of the present disclosure, a conventional electrical circuit associated with a photomultiplier tube (PMT) for detecting radiation (e.g., gamma rays, neutrons, charged particles, etc.) may include a temperature compensation circuit to adjust the analog gain through inclusion of one or more thermistors (NTC or PTC), field-effect transistors (FET) in the ohmic region, and / or combinations thereof.

[0068] By adjusting gain relative to temperature dependence, the radiation plateau region in the integral count rate response representation among ambient (e.g., 25°C) and elevated (e.g., 175°C) temperature may be increased in length. Furthermore, count rate responses at low and elevated temperatures equilibrate in the plateau region, leading to smaller count rate deviation when increasing the temperature. Moreover, advantageously, the detector can be operated at relatively lower high voltages compared to conventional systems (e.g., 1550V versus 1700V).

[0069] As noted above, thermistors and / or field-effect transistors may be incorporated into the temperature compensation circuits. The inclusion of an NTC or P TC thermistor allows analog gain compensation over a large range of temperatures. With the thermistor, the total gain of the analog front-end increases with increasing temperature, this gain compensates the signal loss of PMT. The inclusion of FET (field-effect or bipolar transistor) allows for adjusting analog gain compensation over a large range of temperatures by using a set-point (control)65NUL-510648-WO-2_INT 1042PCTvoltage or current at the input of the transistor. The total gain of the analog front-end increases with increasing temperature, and this gain compensates the signal loss of PMT. Furthermore, combinations of thermistor(s) and field-effect transistor(s) enable the possibility adapting the analog gain to fit the PMT degradation, such as by adjusting the control voltage at the transistor input. By having an improved analog solution at the radiation sensitive detector assembly or system, subsurface processes and components may be simplified.L0070 J Set forth below are some embodiments of the foregoing disclosure:

[0071] Embodiment 1: A radiation sensitive detection system for subsurface exploration comprising a radiation sensitive detector and an electrical circuit electrically connected to the radiation sensitive detector and configured to transform a signal received from the radiation sensitive detector and to perform a temperature compensation of the signal with a temperature compensation circuit, wherein the temperature compensation circuit comprises at least one of: a thermistor; a field-effect transistor (FET); and / or a bipolar transistor.

[0072] Embodiment 2: The radiation sensitive detection system of any preceding embodiment, wherein the temperature compensation circuit comprises at least one thermistor and at least one FET.

[0073] Embodiment 3: The radiation sensitive detection system of any preceding embodiment, wherein the at least one thermistor and the at least one FET are arranged in parallel.

[0074] Embodiment 4: The radiation sensitive detection system of any preceding embodiment, wherein the at least one thermistor and the at least one FET are electrically connected at one end.

[0075] Embodiment 5: The radiation sensitive detection system of any preceding embodiment, wherein the transformation of the signal comprises an amplification of the signal, the system further comprising: signal processing components arranged to receive an amplified and temperature compensated signal from the electrical circuit.

[0076] Embodiment 6: The radiation sensitive detection system of any preceding embodiment, wherein the electrical circuit comprises a first resistor arranged at a position between the amplifier components and the signal processing components, and a second resistor electrically connected at a position between the first resistor and the amplifier components.

[0077] Embodiment 7: The radiation sensitive detection system of any preceding embodiment, wherein the second resistor is arranged in series with at least one thermistor, the system further comprising a third resistor arranged in series with at least one FET.65NUL-510648-WO-2_INT 1042PCT

[0078] Embodiment 8: The radiation sensitive detection system of any preceding embodiment, wherein the at least one of the thermistors, the FET, and / or the bipolar transistor is arranged in series with the second resistor.

[0079] Embodiment 9: The radiation sensitive detection system of any preceding embodiment, wherein the first resistor is a PTC thermistor.

[0080] Embodiment 10: The radiation sensitive detection system of any preceding embodiment, wherein at least one of the thermistors, the FET, and / or the bipolar transistor is arranged in series with the second resistor.

[0081] Embodiment 11: The radiation sensitive detection system of any preceding embodiment, wherein the radiation sensitive detector comprises a photomultiplier tube.

[0082] Embodiment 12: The radiation sensitive detection system of any preceding embodiment, wherein the electrical circuit comprises amplifier components arranged between the radiation sensitive detector and the temperature compensation circuit.

[0083] Embodiment 13: The radiation sensitive detection system of any preceding embodiment, wherein the amplifier components comprise a first operational amplifier, a high-pass filter, and a second operational amplifier.

[0084] Embodiment 14: The radiation sensitive detection system of any preceding embodiment, wherein an operating voltage of the radiation sensitive detector is based on a plateau measurement using the temperature compensation circuit.

[0085] Embodiment 15: The radiation sensitive detection system of any preceding embodiment, wherein the radiation sensitive detector is configured to detect gamma rays, neutrons, or charged particles.

[0086] Embodiment 16: A subsurface exploration system comprising: a string configured to be deployed into a borehole formed in an earth formation; and a downhole tool assembly arranged on the string and configured to make measurements of the earth formation, wherein the downhole tool assembly comprises: a radiation sensitive detector; and an electrical circuit electrically connected to the radiation sensitive detector and configured to transform a signal received from the radiation sensitive detector and to perform a temperature compensation of the transformed signal with a temperature compensation circuit, wherein the temperature compensation circuit comprises at least one of: a thermistor; a field-effect transistor (FET); and / or a bipolar transistor.

[0087] Embodiment 17: The subsurface exploration system of any preceding embodiment, wherein the downhole tool assembly further comprises a drill bit, wherein the string is a drill string.65NUL-510648-WO-2_INT 1042PCT

[0088] Embodiment 18: The subsurface exploration system of any preceding embodiment, wherein the radiation sensitive detector is configured to be operated during a drilling operation to obtain while-drilling radiation data.

[0089] Embodiment 19: The subsurface exploration system of any preceding embodiment, wherein the temperature compensation circuit comprises at least one thermistor and at least one FET.

[0090] Embodiment 20: The subsurface exploration system of any preceding embodiment, wherein the at least one thermistor and the at least one FET are arranged in parallel.

[0091] Embodiment 21: The subsurface exploration system of any preceding embodiment, wherein the at least one thermistor and the at least one FET are electrically connected at one end.

[0092] The use of the terms “a” and “an” and “the” and similar referents in the context of describing the invention (especially in the context of the following claims) are to be construed to cover both the singular and the plural, unless otherwise indicated herein or clearly contradicted by context. Further, it should be noted that the terms “first,” “second,” and the like herein do not denote any order, quantity, or importance, but rather are used to distinguish one element from another. The terms “about”, “substantially” and “generally” are intended to include the degree of error associated with measurement of the particular quantity based upon the equipment available at the time of filing the application. For example, “about” and / or “substantially” and / or “generally” can include a range of ± 8% of a given value.

[0093] The teachings of the present disclosure may be used in a variety of well operations. These operations may involve using one or more treatment agents to treat a formation, the fluids resident in a formation, a borehole, and / or equipment in the borehole, such as production tubing. The treatment agents may be in the form of liquids, gases, solids, semisolids, and mixtures thereof. Illustrative treatment agents include, but are not limited to, fracturing fluids, acids, steam, water, brine, anti-corrosion agents, cement, permeability modifiers, drilling muds, emulsifiers, demulsifiers, tracers, flow improvers etc. Illustrative well operations include, but are not limited to, hydraulic fracturing, stimulation, tracer injection, cleaning, acidizing, steam injection, water flooding, cementing, etc.

[0094] While the invention has been described with reference to an exemplary embodiment or embodiments, it will be understood by those skilled in the art that various changes may be made, and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to65NUL-510648-WO-2_INT 1042PCTadapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the claims. Also, in the drawings and the description, there have been disclosed exemplary embodiments of the invention and, although specific terms may have been employed, they are unless otherwise stated used in a generic and descriptive sense only and not for purposes of limitation, the scope of the invention therefore not being so limited.

Claims

65NUL-510648-WO-2_INT 1042PCTWhat is claimed:

1. A radiation sensitive detection system (500, 600, 700, 800, 900, 1100) for subsurface exploration characterized by:a radiation sensitive detector (506, 606, 806, 906, 1104); andan electrical circuit electrically connected to the radiation sensitive detector and configured to transform a signal received from the radiation sensitive detector and to perform a temperature compensation of the transformed signal with a temperature compensation circuit (502, 602, 702, 802, 902, 1102), wherein the temperature compensation circuit comprises at least one of:a) a thermistor (532, 704, 836, 936, 1103);b) a field-effect transistor (FET) (632, 706, 932); and / orc) a bipolar transistor.

2. The radiation sensitive detection system of claim 1, wherein the temperature compensation circuit comprises at least one thermistor (532, 704, 836, 936, 1103) and at least one FET (632, 706, 932).

3. The radiation sensitive detection system of claim 2, wherein the at least one thermistor and the at least one FET are at least one of arranged in parallel or are electrically connected at one end.

4. The radiation sensitive detection system of any preceding claim, wherein the transformation of the signal comprises an amplification of the signal, the system further comprises:signal processing components arranged to receive an amplified and temperature compensated signal (540, 640, 840, 940) from the electrical circuit.

5. The radiation sensitive detection system of claim 4, wherein the electrical circuit comprises a first resistor (528, 628, 710, 828, 928) arranged at a position between the amplifier components and the signal processing components and a second resistor (530, 630, 712, 930) electrically connected at a position between the first resistor and the temperature compensation circuit.

6. The radiation sensitive detection system of claim 5, wherein the second resistor (712) is arranged in series with at least one thermistor (704), the system further comprising a third resistor (714) arranged in series with at least one FET (706).

7. The radiation sensitive detection system of claim 5, wherein the at least one of the thermistor, the FET, and / or the bipolar transistor is arranged in series with the second resistor.65NUL-510648-WO-2_INT 1042PCT8. The radiation sensitive detection system of claim 5, wherein the first resistor is a PTC thermistor (836, 936), preferably, wherein the at least one of the thermistor, the FET, and / or the bipolar transistor is arranged in series with the second resistor.

9. The radiation sensitive detection system of any preceding claim, wherein the radiation sensitive detector comprises a photomultiplier tube.

10. The radiation sensitive detection system of any preceding claim, wherein the electrical circuit comprises amplifier components arranged between the radiation sensitive detector and the temperature compensation circuit.

11. The radiation sensitive detection system of claim 10, wherein the amplifier components comprise a first operational amplifier (512, 612, 812, 912), a high-pass filter (520, 620, 820, 920), and a second operational amplifier (526, 626, 826, 926).

12. The radiation sensitive detection system of any preceding claim, wherein an operating voltage of the radiation sensitive detector is based on a plateau measurement using the temperature compensation circuit.

13. The radiation sensitive detection system of any preceding claim, wherein the radiation sensitive detector is configured to detect gamma rays, neutrons, or charged particles.

14. A subsurface exploration system characterized by:a string (20) configured to be deployed into a borehole (26) formed in an earth formation (60); anda downhole tool assembly (90) arranged on the string and configured to make measurements of the earth formation, wherein the downhole tool assembly comprises the radiation sensitive detection system (500, 600, 700, 800, 900, 1100) in accordance with any of claims 1-13.

15. The subsurface exploration system of claim 14, wherein the radiation sensitive detector is configured to be operated during a drilling operation to obtain while-drilling radiation data.

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