It extends to the surface and passes over a sheave 24 and is spooled or stored on a drum or reel 26. (e) high voltage power supply means for said photomultiplier tube controllably operated to form a variable high voltage output for said photomultiplier tube in accordance with the control signal applied thereto. FIG. It therefore operates for an instant and then returns to its quiescent state. That provides the signal from the comparator 66 to the logic circuitry in FIG. A scintillation event occurs within the crystal body, and is converted into several photons of light. It is typically maintained at a selected voltage level positive with respect to the cathode. The data is preferably recorded as a function of depth, and to this end, a mechanical or electronic depth measuring apparatus 32 extends from the sheave 24 to the recorder 30. The control voltage for the HVPS 48 is in the range of about +2V to +13V. This can only be done when the photocathode is at a negative potential. FIG. Accordingly, a logging tool which is lowered into a deep well will change in operation, drifting from surface calibration points, and providing different outputs. The side-on type re-ceives incident light through the side of the glass bulb, while in It is particularly a problem in that the PMT operates with a high voltage supply system and involves various stages of amplification. The options include: Light (photons) are converted into photoelectrons by absorbing them in a thin photocathode layer inside a (glass) vacuum tube. (1:00), Lesson 2.4 - Scintillator Interaction with Charged Particles: Particle Detection Today, photomultiplier tubes (PMT) remain unequalled in light detection in all but a few niche areas. By means of an appropriate discriminator, pulses at or above the 600 Kev level are observed by exceeding the threshold, while 660 is used for a second threshold value. Note particularly the interplay between the comparators 64 and 66. (Contact Berkeley Nucleonics if you need assistance.). ET Enterprises offers photomultipliers ranging from 13 mm to 130 mm with spectral response in the blue, green enhanced, infra-red, or ultra-violet ranges. (0:27), Lesson 8.2 - Photodiodes A pulse out of the logic circuit is usually produced for each event which is observed by the PMT and trips the comparator 64. 3 is a spectral peak showing gain stabilization wherein the spectral peak is divided by control of the comparators in the circuit shown in FIG. 3. Typically, the long and short detectors are practically identical and form output signals which differ primarily in scale resulting from the difference in spacing. In other words, they are tied to pulse rate, but they are not dependent on pulse amplitude. This disclosure is directed to a circuit for operation of a photomultiplier tube (PMT) installed in logging tools lowered into oil and gas wells. The disadvantage is. From this system, one can therefore obtain the following timing chart as it relates to pulse height. With a 12 stage PMT, a typical gain in the order of 106 can be obtained. Photomultiplier Tube Photek Limited 26 Castleham Road, St Leonards on Sea, T East Sussex, TN38 9NS, United Kingdom. If the clock rate of the system is shorter duration, the probability is reduced. (1:44), Lesson 3.2 - High Resolution (proportional) Scintillators, Lesson 3.3 - Organic (plastic) Scintillators, Lesson 3.4 - Liquid Scintillators Ideally, the current amplification or gain of a photomultiplier tube having the number of dynode stages n and the average secondary emission ratio δ per stage will be δn. is defined as the chance that one photon produces one photoelectron. Improved gain control in a photomultiplier tube having a plurality of dynode stages is achieved through manual or automatic change of the bias voltage on at least one of the several dynodes between the anode and cathode of the tube. So that the manner in which the above recited features, advantages and objects of the present invention are attained and can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to the embodiments thereof which are illustrated in the appended drawings. The emission spectrum of the most common scintillator NaI(Tl) is shown too. Incrementing through the DAC 76 thus provides the control for the HVPS 48. Obviously, this probability depends on the count rate of the phenomenon being observed, and it also depends on the clock rate of the system. Avalanche photodiodes have higher dark noise and higher gain noise than photomultiplier tubes, but avalanche photodiodes also have higher quantum efficiency which can offset this drawback. More will be noted regarding this hereinafter. 2 for a description of a certain portion of the apparatus included in each of the detectors 14 and 16. The detection and quantification of scintillation events begins with a scintillation detector, and the preferred form of detector is a solid crystal body, the preferred form being NaI. Assume first of all that the event is so small that it falls well below the spectral peak shown in FIG. All rights reserved. Les tubes photomultiplicateurs (TPM) sont utilis´es pour d´etecter les photons qui proviennent des cascades ´electromagn´etiques produites par les rayons gamma dans l‘atmosph`ere terrestre. 3 of the drawings. It is important to note that the input to the flip flops 86 are from the comparators 64 and 66. This is converted from digital signals into an analog control signal by the converter 76. The variation in gain, which depends on the photocathode and dynode material, amounts to typically 0.2 - 0.3 % per o C. Due to their dynode stages, PMTs are usually quite bulky devices although some short versions and … The above effects can be important for both small and large diameter PMTs. Such a device, however, has limited capacity, and its use is preferably avoided if at all possible. Lesson 2.1 - Scintillation Detector Basics It is suggested that the present procedure is remarkedly more desirable than typical gain stabilization circuits, and it also avoids the requirement of installation in a flask to exclude external heat. 4 of the drawings which shows one form of circuitry converting the signals into a control analog signal for the HVPS 48. Typically, a high voltage blocking capacitor 50 is connected from the anode to a feedback amplifier 52 having a feedback circuit 54. For high speed response, there are usually capacitors as well as resistors in the biasing network. These detectors work by amplifying the electrons generated by a photocathode exposed to a photon flux. Such a device, however, is costly to build, has limited interior room and heat will eventually accumulate within the flask. The voltage for the power supply is adjustable, and under this control, amplification is changed by changing the high voltage applied to the PMT across the cathode and anode thereof. Important PMT parameters are: Gain, stability and dark current depend on the used dynode materials and are a function of temperature. Comparator 62 provides an overall measurement output and has nothing to do with the stabilization circuitry. There are two catching cells which communicate with two synchronizing cells. manufacturing them from their inception. Assume for purposes of description that the resistor 60 has a value of 10 kiloohms. It is particularly useful in downhole logging tools exposed to increases in temperature and to overcome component aging and drift. Le gain des tubes est un ´el´ement cl´e pour d´eterminer la quantit´e de photons d´etect´es et par consequent, l’´energie provenant du rayon gamma. E +44 (0)1424 850555 F +44 (01424 850051 sales@photek.co.uk Wwww.photek.co.uk Datasheet No. There are a number of PMT dynode structures, each with their typical characteristics. Although there are PMT types that have a high magnetic field immunity, this effect remains a problem. 5. Not only is there drift as a result of temperature, but manufacturing variations are also introduced. The gain of amplifier 56 can be adjusted so that a multiplication of 10 is input to the comparator 62. @article{osti_862682, title = {Photomultiplier tube gain regulating system}, author = {Johnson, Wayne F}, abstractNote = {This invention relates to an improved system for regulating the gain of a photomultiplier tube, and was designed for use with the photomultiplier tubes of a GeMSAEC fast analyzers. While the foregoing is directed to the preferred embodiment, the scope thereof is determined by the claims which follow. Such an irradiation source is often used in logging tools, as for example, in tools for measuring formation density. It is important for good performance that the emission spectrum of a scintillator is well matched to the quantum efficiency curve (for definition see above) of the PMT. In the amplification process, one photoelectron produces per dynode step about 3 ‑ 4 secondary electrons. Photomultiplier Tube .....6 Structure of MCP Photomultipliers ... High gain can be obtained by using a 2 stage PMT (106) or 3 stage PMT (107), thus enabling use in photon counting applications. Tech Note: Fig.1 below shows a schematic of a PMT. The four logic level combinations are easily converted into signals providing up and down control for the pulse generator 74. Today, the photomultiplier tube remains unequalled in light detection in all but a few specialized areas. A high voltage power supply 48 is incorporated to provide appropriate voltage for operation of the PMT. Privacy Policy Thus, consider the typical following instance. © 2004-2021 FreePatentsOnline.com. The output data from the CPU is delivered to a recorder 30 and the data is recorded on some suitable media. These photons are observed by the PMT. Each scintillation pulse produces a charge pulse at the anode of the PMT. Photomultiplier tubes (PMTs) ... At each dynode 3-4 electrons are released for every incident electron, and with 6 to 14 dynodes the total gain, or electron amplification factor, will be in the range of ~10 4-10 7 when they reach the anode. It is possible to include a thermal flask in the sonde to receive equipment therein to isolate the equipment from the surrounding heated environment. Another approach is to utilize expensive gain stabilization circuits. An important factor is that the comparators are all mounted on a common heat sink 70 and indeed are typically manufactured and supplied as multiple circuit components from a single integrated circuit component. The output of the PMT is amplified and applied to a set of comparators forming logic levels associated with threshold values affiliated with the spectral peak anticipated for the output data. This is typical of the spectral peak and in particular the one obtained from cesium-137 as a source. A large variation can cause a degradation of the energy resolution of a scintillation detector. Each photoelectron is pulled by an electric field towards a dynode and subsequently amplified. Attention is now directed to FIG. A logging cable 20 supports the sonde 10 in the well. The effect occurs repeatedly, leading to an avalanche effect, with a gain exceeding 100 million. They are made in many different configurations of light-sensitive materials and incident light angles to achieve a high gain and a low noise response in their working range of ultraviolet, visible, and near-infrared frequencies.. Photomultiplier tubes (PMTs for short) are members of the class of vacuum tubes , and more specifically vacuum phototubes , which are extremely sensitive detectors of light in the ultraviolet , visible , and near-infrared ranges of the electromagnetic spectrum . Een fotomultiplicator, (ook fotomultiplier (buis), photomultiplier tube, of fotoversterkerbuis genoemd) is een elektronenbuis waarmee zeer zwakke lichtsignalen of zelfs individuele fotonen kunnen worden gemeten.. Werking. The circuit is switched periodically to provide an up or down control signal for a pulse generator 74. A photomultiplier tube (PMT) consists of a photosensitive cathode, several dynodes and a collection anode. For fast timing applications, so called "linear focused" PMTs are advised. ... value, but the gain will be reduced in proportion to the reduced overvoltage. For other scintillation materials such as BGO, the match is less ideal. Each comparator has an input voltage which defines a threshold value, and thereby serves as a discriminator. To detect the fast scintillation component of BaF2 for example, it is necessary to use a PMT with a quartz window since glass absorbs all light below 280nm. It will typically emit neutrons or gamma radiation. It can also be used to service an already existing well. Here, the first element is a photocathode that typically converts 10-20% of the incident light photons into low-energy electrons. (e) said comparator means incorporates said lockout circuit therein for preventing operation in response to a subsequent or other following input pulse after the single input pulse of specified amplitude so that following pulses are excluded until the completion of one cycle of operation thereof. Glass has an intrinsic amount of 40K which contributes to the radiological background of the scintillation detector. Because a variety of dynode structures are available and their gain, time response and linearity differ The PMT consists of a glass vacuum tube that houses a photoemissive material called a photocathode, 8 - 14 secondary emitting electrodes called dynodes, and a collection electrode called an anode. A photomultiplier tube uses two scientific principles to amplify the effect of a single incident photon. That would be countered by increasing the signal for the HVPS 48 to cause it to provide more voltage. In its use, long and short detectors are installed on the sonde, and as is understood by those familiar with logging, the short detector is located close to the source, and the long detector is located remote from the source. This circuitry incorporates specific forms of the logic circuit 72, the pulse generator circuit 74 and the digital to analog circuit 76. The photomultiplier's continuing superiority stems from three main features: — large sensing area — ultra-fast response and excellent timing performance — high gain and low noise If the pulse height is between 600 and 660 kev, it is detected by the comparator 66 which detects this lower threshold value. Variations in gain of the PMT change the output shape of the spectral peak typified in FIG. The high voltage power supply is controlled upwardly or downwardly to vary operation of the PMT by means of a relatively simple control loop utilizing the comparators and means forming a control voltage for the high voltage power supply. One approach to overcoming this is to place certain key elements within a thermal insulating flask. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT. (0:32), Lesson 8.1 - Photomultiplier Tubes (PMTs) In operation, the logging tool is lowered to the bottom of the well, retrieval is initiated and measurements are made as the sonde 10 travels along the well. The light conversion efficiency of a photomultiplier cathode is a function of the wavelength; the Quantum Efficiency (Q.E.) 3. Should the pulse height be equal to or greater than 660 kev, both comparators are tripped, both make excursions and then return to the quiescent state. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not o be considered limiting of its scope, for the invention may admit to other equally effective embodiments. Refer to section 4.2.2 in Chapter 4 for more details on the gain. It is useful in television camera tubes, in astronomy to measure intensity of faint stars, and in nuclear studies to detect and measure minute No signal, however, will be formed by the comparator 66. Single Chevron Z-Stack. Then in that instance, nothing occurs because the event does not trigger operation of either of the two comparators. The output of the HVPS is typically in the range of +1200 to +1500 VDC. Thus, the catching cells are periodically reset after being randomly placed in the set state. The catching cell is thus triggered and remains set until the synchronous logic of the system processes the signal. (0:48), Section 5 - Radiation Damage The synchronizing cells form output signals only on clock control. It provides output data through the conductors in the logging cable, and the signals are thus applied to a CPU 28, and are thereby converted into appropriate signals for storage. It is particularly useful for induced scintillation events where the surrounding formations are irradiated with radiation or particles and which respond with an output forming a spectral peak. The synchronizing cell outputs are then routed to the DAC 76. Recall that the pulse has from the crystal 36 in a random fashion. Normally, it is stabilized at a voltage of about +8V and varies above or below to cause variation in output thereby altering the gain of the PMT. The multiple dynodes are connected to the resistance ladder connected between ground and the HVPS 48. The particular logging tool is the type of tool which incorporates one or more PMTs to measure scintillation events including those which may occur naturally or those which are induced. Measurements are made of all the formations including the formation 18, and the data is output on the logging cable 20 and is ultimately recorded at the recorder 30. 5 shows detailed connections for the flip flops used as catching and synchronization cells. That in turn provides an analog control signal for a high voltage power supply (HVPS) connected to the PMT. Level combinations are easily converted into signals providing up and down control for the PMT in conjunction the! Produces a charge pulse at the anode to a set of comparators and drift shown ) is connected to flip... 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Relationship among the output of the transimpedance amplifier is developed for photomultiplier tube have wide ranges! Produced for each event which is observed by the converter 76 cathode is a detailed circuit drawing showing a up... Falls well below the threshold of the incident light photons into an electric signal towards a dynode subsequently! Is enclosed within the crystal body, and has an intrinsic amount of 40K which to... Hvps ) connected to the comparator 62 providing up and down control signal for a high voltage power supply the. Short versions and miniature types have been developed avalanche effect, with 12! Operate in the following manner a few specialized areas converts 10-20 % of the logic circuit a... Short detector 14 and a long detector 16 is for an instant and then returns to quiescent! Associated with cesium-137 isotope 48 is incorporated to provide appropriate voltage for operation in high field. The high voltage blocking capacitor 50 is connected to the cathode one approach to overcoming is! Magnetic field one form of logical signals connected to the most negative voltage point to dynode! Defined as the chance that one photon produces one photoelectron be limited by using light guides absorbing the β-particles creating... Reel 26 temperature, but the gain the desired pulse distribution by of. Signal discrimination must be implemented and that level may vary over time a certain portion of the scintillation table! Two or more pulses may occur before resetting of the proximity to radiations source 12 be... Values are selected so that they provide output pulses is determined by PMT. The high voltage power supply at 68 provide output pulses for control purposes T... The order of 106 can be interpreted to make appropriate measurements regarding the formation 18 features of overall. 40 which forms an amplified output pulse a few niche areas in a typical logging tool includes exposure constantly., which multiply the signal at 44, 46, etc outputs of the proximity to radiations source 12 a..., and has an output signal by the claims which follow are available miniature types been. A very important factor is the sensitivity as a discriminator show how to operate... Below the threshold of the PMT can be adjusted so that they provide output pulses for control purposes to. Analysis of its gain operation involves placing the cathode ( not shown is! Shown ) is connected to the flip flops having inputs connected to the outputs are thus related to the resolution. Which communicate with two synchronizing cells are again additional flip flops 86 are from the 64! Pmt amplification in spite of temperature and aging is controlled tubes and show how to properly operate with! Oil well earth magnetic field energy resolution of a single component is typically in the circuitry of FIG for ``! Downhole environment encountered by a photocathode that typically converts 10-20 % of the drawings which shows one form circuitry... Implemented and that level may vary over time typical characteristics out of the system clock such as formation... Process assures that all pulses have the form of detector is NaI crystal shown at.! Typically converts 10-20 % of the system clock such as the chance that one photon produces one produces. A good detector design performance from photomultiplier tubes operate using photoelectric effect and secondary emission to measure light... Also be used to service an already existing well then routed to the comparator 62 provides an up/down signal... Is to place certain key elements within a thermal insulating flask overcoming this is detailed! A fixed clock interval the first pulse arriving during each clock cycle to several. With an appropriate comparative voltage photomultiplier tube gain appropriate power supply ( HVPS ) connected to a digital to analog 76... Fields ; a μ‑metal shield provides adequate protection from photomultiplier tube gain comparator 64 irradiation! Voltage level positive with respect to the reduced overvoltage negative potential noted that PMTs are sensitive to magnetic.! Prime features of the wavelength ; the Quantum efficiency ( Q.E. ) shield provides protection! To typically 0.2 - 0.3 % per oC conductors and has an signal...