Princeton Gamma-Tech Instruments, Inc. 303C College Road East Princeton, NJ

Transcription

1 Princeton Gamma-Tech Instruments, Inc. 303C College Road East Princeton, NJ

2 Copyright 2001 Princeton Gamma-Tech, Inc. Reproduction or publication of the contents of this manual without the permission of Princeton Gamma-Tech is prohibited. PRISM, IMIX and Spirit are registered trademarks of Princeton Gamma-Tech.

3 iii Contents 1. Introduction Unpacking and Setup Instructions Package Inspection Installation Handling Liquid Nitrogen (LN2) Safety Precautions Storage Procedures and Hazards Transport Procedures and Hazards Personal Injury Hazards and Protection About PRISM Si(Li) and HpGe Detectors Detector Types and Applications Basic Theory System Configuration X-ray Collimator Field-Effect Transistor (FET) Transistor Reset Preamplifier Fixed Z/Variable Z Detector Maintenance Standard Liquid Nitrogen Filling Procedure Filling a Dry System Establishing a Detector Efficiency Baseline Energy Resolution Wavelength-Energy Correlation Cleaning the Detector Window

4 iv PGT Detector Manual

5 v List of Figures 4.1 Representation of planar semiconductor radiation crystal Schematic diagram of the basic PRISM X-ray detector Schematic diagram of the PO-14B pulsed optical reset preamplifier Fixed Z sample/detector geometry Inclined entry system Variable Z/sample detector geometry (not drawn to scale) Energy Resolution Window cleaning procedure

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7 vii List of Tables 1.1 Summary of PGT PRISM 2000 Detector Types Detector Types and Applications Window Cleaning Solvents Troubleshooting Guide

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9 Introduction 9 1. Introduction This manual describes simple unpacking/installation, maintenance and troubleshooting procedures for PGT s PRISM series of Si(Li) and HpGe detectors. The manual also discusses the basic properties, features and applications of each detector type. Model Name Crystal Type and Size FWHM (ev) Window Type Cooling Method Si(Li), 10 mm PRISM 2000 Si(Li), 30 mm PRISM 2000 Si(Li) 60 Si(Li), 60 mm HpGe, 10 mm PRISM 2000 IG HpGe, 50 mm PRISM SLS ultra thin window for light element detection. a Liquid Nitrogen (LN 2 ) OR JT COOL Option (no LN 2 required) Table 1.1 Summary of PGT PRISM 2000 Detector Types a. All PRISM series detectors can also be ordered with a Be window or with multiple windows. Note: If you have an IMIX system with a digital pulse processor, please see Appendix 4: Digital Pulse Processors. The Avalon and Spirit systems do not use digital pulse processors. All PRISM 2000 series Si(Li) and HpGe detectors can be ordered with the JT Cool, a mechanically cooled system which does not require LN 2. If you have the JT Cool option on your system, please see the JT Cool manual for additional information.

10 10 PRISM 2000 Series Detector Manual 2. Unpacking and Setup Instructions 2.1 Package Inspection Please ask your shipping and receiving department to carefully inspect all the detector packaging materials before signing the delivery receipt presented by the delivery carrier. If any of the packaging materials appear damaged, please notify both the delivery carrier and PGT immediately. Take Polaroid pictures to document the damage. If the package has been damaged in transit, it is your responsibility to submit a claim. PGT employees will assist you in whatever way they can. 2.2 Installation Each complete system built by PGT is carefully tested and calibrated at the factory prior to shipment. Some components are disconnected before shipping to make the system easier and safer to transport. The mounting block that holds the Si(Li) detector to the electron microscope is usually shipped in place on the detector. Carefully unpack your system. Be extremely careful not to damage the window(s) when handling the detector. Keep the protective cap in place. Make sure to save the plastic protective cap and packing box in case you need to return the detector to PGT for service in the future. When you receive your detector, call PGT service to inform them that the system has arrived and is ready for installation. A PGT service engineer will perform the final interconnections and the system calibration at your site. Your detector is shipped without liquid nitrogen (LN 2 ). After unpacking the system, fill the dewar with LN 2 as soon as possible. (Before handling LN 2, see the safety information in "Section 3. Handling Liquid Nitrogen (LN2)" on page 11). Note: You can fill the detector with LN2 while it is still strapped in its shipping pallet. To prevent damage to the system, you must first remove all insulating packing materials, especially foam. Warning: Always keep thermally insulating materials away from a cold detector. This precaution is necessary to avoid damage to vacuum seals or system electronics due to excessive chilling. Allow the detector to cool down for three hours before you (or the PGT service engineer) make any electrical connections to the spectrometer. For best performance, the PGT microanalysis system requires a room with low-to-moderate humidity and a stable-voltage line free of transients.

11 Handling Liquid Nitrogen (LN2) Handling Liquid Nitrogen (LN 2 ) Liquid nitrogen (LN 2 ) is used to cool your detector to optimal operating conditions. Your system must be kept at cryogenic temperature when in use. In order to maintain the detector internal vacuum and ensure optimum long term performance, the liquid nitrogen level should be maintained on a constant basis. 3.1 Safety Precautions Before using liquid nitrogen for the first time, it is very important that you read this entire section and understand the hazards associated with storing, transporting and handling it. You must follow the correct safety procedures in order to prevent damage to the detector system or injury to yourself. See "Section 5.1 Standard Liquid Nitrogen Filling Procedure" on page 19 for instructions on how to fill your dewar. See "Section 5.2 Filling a Dry System" on page 19 for instructions on filling the dewar if your system has run dry Storage Procedures and Hazards Liquid nitrogen boils at room temperature, and the large expansion ratio as it changes from a liquid to a gas (1:692) can result in the build up of high pressures if it is stored inside a container lacking adequate venting or pressure relief provisions. Normally, your supplier will ship your LN 2 in a cylinder equipped with pressure relief valves, and you can simply monitor the settings on the pressure gauge to make sure the pressure is at a safe level. Although nitrogen gas is not toxic, it can displace oxygen and cause asphyxiation if large amounts of it are released in a small space. If you have large cylinders or tanks of liquid nitrogen, store them in a well ventilated area to avoid a buildup of nitrogen gas in your laboratory area. If you spill a large quantity of nitrogen, leave the area and ventilate it thoroughly before returning. If you use piping or transfer lines, make sure they are designed to avoid trapping LN 2 in the line. If liquid becomes trapped, high pressures may build up in the line during evaporation and cause the line to rupture or explode. If it is not possible to empty all lines, install safety relief valves and rupture disks. If you use Tygon tubing (or a similar material), periodically inspect it to check for brittleness and cracks that can cause it to rupture during transfer Transport Procedures and Hazards When you transport LN 2 from the storage cylinder to your laboratory, make sure you carry it in a container specifically designed to withstand cryogenic temperatures (approximately 77 degrees Kelvin). Transfer LN 2 only in a well-ventilated area. Many materials become brittle and can fracture if they come into contact (even briefly) with LN 2. When transporting LN 2 and filling your dewar, avoid spilling it on surrounding objects or the floor of your laboratory. You must prevent your detector electronics and the outside of the cryostat from becoming excessively cold. Take special care to avoid spilling any LN 2 on your detector components. Vent cold gas away from the system.

12 12 PRISM 2000 Series Detector Manual Personal Injury Hazards and Protection If LN 2 comes into contact with your skin it can cause severe burns similar to those resulting from hightemperature contact. Eyes are especially vulnerable to burns and damage. When handling LN 2, do not wear clothing that can collect spilled LN 2 or allow it to come into prolonged contact with your skin. Remove watches, rings, bracelets or other jewelry. It is strongly recommended that you wear the following personal protection items: Lab coat or apron. Pants without cuffs. High-topped shoes. Safety glasses or a face shield. Cryogenic gloves that are large enough to be easily slipped off in case LN 2 is spilled inside them.

13 About PRISM Si(Li) and HpGe Detectors About PRISM Si(Li) and HpGe Detectors 4.1 Detector Types and Applications Generally, Si(Li) detectors are more suitable for applications involving light element detection, while HpGe detectors offer higher spectral resolution and high-energy efficiency. Smaller crystals of either type provide better resolution than larger crystals of the same type, and larger crystals provide the highest count rates at lower beam currents. Table 4.1 provides a brief summary of the advantages of the different crystal types and sizes. Model Name Crystal Type and Size Applications/Comments PRISM 2000 Si(Li), 10 mm 2 Si(Li), 30 mm 2 This is a good detector for routine analysis. The 10 mm 2 crystal provides better resolution than the 30 mm 2. This detector is suitable for environmental and field emission SEMs, and applications where the beam current is limited. The larger crystal provides a higher count rate and better light element detection than the 10 mm 2. PRISM 2000 Si(Li) 60 Si(Li), 60 mm 2 This detector is suitable for environmental and field emission SEMs, and low-voltage applications. The large crystal provides the highest count rates at lower voltages, and superior light element detection. PRISM 2000 IG HpGe, 10 mm 2 HpGe, 50 mm 2 This crystal offers the highest available resolution, and is especially good for trace element analysis. This crystal provides good resolution and high count rate and is suitable for low voltage and low vacuum applications. Table 4.1 Detector Types and Applications 4.2 Basic Theory When a sample is put in the electron microscope and bombarded with high-energy electrons, the energy of the beam excites the sample and causes it to emit ionizing radiation. Some of this radiation is in the form of characteristic X-rays, which have unique energies corresponding to the elements from which they originate. These X-rays travel through a Be or ultra thin window to the solid-state X-ray detector. The X-rays interact with the detector (which is made of a semiconductor material) to create electron-hole pairs (see Figure 4.1). The number of electron-hole pairs created is proportional to the

14 14 PRISM 2000 Series Detector Manual energy of the radiation. If the detector crystal is Si(Li), one electron-hole pair is created for every 3.76 ev of incoming ionizing radiation. If the detector crystal is HpGe, one electron-hole pair is created for every 2.96 ev of incoming radiation. V (applied voltage) p n incoming photon d = depth of crystal p = p layer contact n = n layer contact i = intrinsic region E = energy field Figure 4.1 Representation of planar semiconductor radiation crystal The detected radiation is then efficiently converted into a measurable electrical signal which appears as an output voltage pulse with amplitude proportional to the incident photon energy. 4.3 System Configuration A basic PRISM detector system is shown in Figure 4.2. The system consists of: An X-ray collimator. See "Section X-ray Collimator" on page 15. One of the following window types (depending on your system configuration) designed to protect the detector crystal: PRISM SLS ultra thin window for light element detection. Be (Beryllium) window. Multiple windows. The Si(Li) or HpGe crystal. A field-effect transistor for signal amplification. See "Section Field-Effect Transistor (FET)" on page 15. A cryogenic enclosure (cryostat). An electronic transistor reset preamplifier. See "Section Transistor Reset Preamplifier" on page 16. A 7.5-liter dewar for holding LN 2. An optional fixed Z/variable Z adjustment feature to change the detector tilt.

15 About PRISM Si(Li) and HpGe Detectors 15 LN 2 dewar Beryllium or ultra-thin window Cold finger SEM chamber Preamplifier Electron beam Stage Variable Z Endcap (probe) in cross-section FET Collimator Si(Li) or HpGe crystal Figure 4.2 Schematic diagram of the basic PRISM X-ray detector X-ray Collimator The collimator is designed to fit over the endcap of the X-ray detector. In the SEM, using a collimator always involves a compromise between specimen working distance, spectrometer-to-specimen distance, specimen size and allowable motions, and the tightness of collimation. Generally, a more tightly collimated spectrometer will detect fewer stray X-rays produced by the backscattered electrons or other X-rays striking the pole piece, specimen chamber, or stage. The collimator, as supplied, is compatible with the user s specimen requirements. The full face of the Si(Li) crystal in the spectrometer must be "illuminated" by the X-rays generated in the specimen for maximum count rate. If the spectrometer is collimated too tightly, the count rate may be reduced because not all of the crystal is being used to detect X-rays Field-Effect Transistor (FET) The signal that originates in the detector travels to the field-effect transistor (FET) located directly behind the Si(Li) or HpGe detector crystal. Because the charge induced in the detector is extremely small, it is necessary to minimize noise (stray capacitance) by placing the FET as close to the detector as possible and operating at a liquid nitrogen temperature. All PRISM systems incorporate a thermistor and/or a platinum resistor in the detector itself that shuts off the high voltage when it senses a temperature increase. This safety feature also prevents accidental high-voltage application before cool-down is complete. Although it does not monitor the LN 2 level, an optional cable is available to detect and warn of low LN 2 levels. All the PRISM light element detectors incorporate an electron trap to deflect electrons that would otherwise cause noise in the spectrum. This assembly is not required for systems containing Be windows.

16 16 PRISM 2000 Series Detector Manual Transistor Reset Preamplifier The very small charges generated in the Si(Li) detector crystal require the use of a preamplifier. The PRISM systems use a Transistor Reset preamplifier, as shown in Figure 4.3. A unique charge reset circuit incorporated in the FET chip discharges the voltage built up on the feedback capacitor, restoring the preamplifier DC output level. This preamplifier is ideal for low noise and high count rate applications, and the performance is very close to that of an idealized charge-sensitive preamplifier. As X-ray photons are collected, the voltage at the preamplifier output begins to climb from -3 V. The output increases slowly until it reaches a predetermined DC level of. A logic circuit then activates a reset signal to a diode that injects charge into the FET of the opposite polarity to the signal charge. The voltage on the feedback capacitor is discharged and the preamplifier output returns to -3 V, and the reset signal is turned off. The reset time (dead time) is extremely short (approximately 8 s). When a reset occurs, the preamplifier circuitry also generates an inhibit output pulse that prevents data access to the microanalysis system during the time of reset. FET Logic Circuits Drain Inhibit Signal Gate HV Gate FET Drain Source Output Crystal Source Substrate Substrate Feedback Cap Feedback Cap Figure 4.3 Schematic diagram of a Transistor Reset preamplifier

17 About PRISM Si(Li) and HpGe Detectors Fixed Z/Variable Z The angle of the spectrometer in the microscope chamber (Z, tilt) is fixed in some systems and adjustable in others. In a fixed Z system with the SEM beam perpendicular to the endcap (see Figure 4.4), when the working distance of a specimen is increased the spectrometer must often be moved back in order to collect X-rays from the sample. Position A V W V = vertical distance W = working distance Position B V W Figure 4.4 Fixed Z sample/detector geometry The added distance reduces the X-ray count rate. The vertical distance (that is, the distance between the polepiece and the point where the centerline axis of the detector would break the electron beam) remains the same. Figure 4.5 illustrates the special case of an inclined entry detector. S W = V V = vertical distance W = working distance S = spectrometer distance (beam-to-detector crystal) Figure 4.5 Inclined entry system The working distance and angle are determined by the SEM manufacturer s specification. The system usually has a fixed working distance and no sample tilt. The take-off angle is constant and the count rate is a function of the solid angle (shown as the shaded triangle between the crystal and the beam). The detector endcap can be moved in and out to maximize the X-ray counts by changing the beam-todetector distance (spectrometer distance, S). In this case, the vertical distance is the same as the working distance. If the system is equipped with variable Z, the working distance will change if vz 0. Some inclined-entry systems, notably for transmission electron microscopes (TEMs) have a fixed spectrometer distance.

18 18 PRISM 2000 Series Detector Manual With a variable Z system, the analyst can change the working distance of the sample and still keep a high solid angle (and therefore, a good count rate) by changing the angle of the spectrometer (shown in Figure 4.6. Preamplifier PRISM Digital Spectrometer Detector chamber Electron beam S Z S W V max V min X control Z control Z = tilt angle V = vertical distance S = spectrometer distance W = working distance Figure 4.6 Variable Z/sample detector geometry (not drawn to scale) This changes the vertical distance. Some microscopes with small chambers or restricted entrance ports do not permit this feature.

19 Detector Maintenance Detector Maintenance Although your PGT PRISM system requires very little maintenance, there are three important procedures you must follow regularly to ensure optimum detector performance: Because your system must be kept at cryogenic temperatures in order to function properly, it is recommended that you keep the dewar filled at all times. Note: If you have the JT cool option, your system does not require LN 2. When you set up your system for the first time, you (or the PGT field service engineer) should establish a detector efficiency baseline (see "Section 5.3 Establishing a Detector Efficiency Baseline" on page 20). Repeat the calibration at appropriate intervals and compare the new efficiency data to the original baseline to assess the condition of your detector. If your detector window becomes contaminated, you must carefully clean it using the correct techniques. See "Section 5.6 Cleaning the Detector Window" on page Standard Liquid Nitrogen Filling Procedure Warning: In order to prevent serious damage to the Si(Li) or HpGe crystal, you must keep it at cryogenic temperature at all times. It is very important that you continually monitor the LN 2 level of your system, and refill the standard 7.5 liter dewar every 3-4 days. If the dewar does accidentally run out of liquid nitrogen, follow the steps in "Section 5.2 Filling a Dry System" on page 19 to bring it back to cryogenic temperature. To fill the 7.5 liter dewar with LN 2 : Remove the stopper from the dewar. Insert a large metal funnel. Using the funnel will help you to avoid spilling LN 2 on the cryostat. If the cold liquid contacts a flange which contains a vacuum seal, it is possible that the seal may be breached. Pour the LN 2 from your transport container into the system dewar from the top. 5.2 Filling a Dry System If the dewar runs out of LN 2, take the following steps: 1. The system automatically shuts off the bias voltage if the temperature rises above safe levels. Leave the bias voltage switch in the "on" position. 2. Remove the dewar cap. Make sure that the dewar is dry. Allow the detector to warm up for about 12 hours. 3. Fill the dewar with LN Re-install the dewar cap.

20 20 PRISM 2000 Series Detector Manual 5. The bias voltage will turn back on automatically when the system has stabilized at the correct operating temperature. Normally, this will take from 2-4 hours, depending on the size of your detector crystal. 5.3 Establishing a Detector Efficiency Baseline As soon as you put your system into operation, you should establish a baseline efficiency using a pure copper standard. Normally, the PGT service engineer installing your system will perform this procedure when setting up your system for the first time. Make sure that you save on disk: The spectrum for the pure copper standard. The sample-to-detector geometry and the accelerating voltage used for spectrum collection. You can use this baseline as a point of reference to assess the condition of your detector. The normal baseline calibration value is the ratio of the L alpha peak to the K alpha peak of a copper standard collected at 15 kv accelerating voltage and an X-ray takeoff angle equal to or greater than 30. You should periodically repeat this efficiency calibration under identical operating conditions. Some general guidelines are given below, but you should also take into account your SEM design, sample types, sample exchange procedure and vacuum system. If your samples are oily or dirty, your window may take less time to become dirty enough to reduce efficiency. If you have an ultra-high vacuum system, the window will probably remain cleaner for a longer time. It is recommended that you run the efficiency calibration: Every two months, if you have a light element detector. Every six months for all other detectors. After you clean the window. After any service or repair has been performed on your detector. Any time your spectrometer has been disassembled, whether for shipping or maintenance. Any change in this ratio under identical operating conditions indicates a change in detector efficiency. If this occurs, contact PGT service for help in restoring the detector to its original efficiency. One common reason for loss of detector efficiency is a dirty Be or ultra-thin window, usually from contamination from within the SEM sample chamber (often from vacuum pump oils or samples). Dirty windows have the most effect on detection of light elements. If your Be window is dirty, it probably will not significantly affect the detection of heavier elements. If the L:K ratio is 1:1 or less, it is strongly recommended that you clean your window (see "Section 5.6 Cleaning the Detector Window" on page 21 for the correct procedure), or contact PGT service to have it cleaned.

21 Detector Maintenance Energy Resolution Detector resolution is given in terms of electron volts at FWHM (full width at half maximum) for the manganese K peak from a radioactive 55 Fe source. FWHM is the width of the peak at half the maximum height, as shown in Figure 5.1. The exact value depends on the size and type of crystal, and the type of electronics. The specific value for your system is given in Appendix 3: X-ray Spectrometer Description. Resolution 129 ev Figure 5.1 Energy Resolution 5.5 Wavelength-Energy Correlation The relationship between X-ray wavelength ( in Å) and X-ray energy (E in kev) is given by the equation = or = for in nm. E E 5.6 Cleaning the Detector Window The Be or ultra thin detector window has the following functions: It protects the detector crystal from coming into direct contact with contaminants from the sample chamber (for example, vacuum pump oil or dirty samples). It maintains a vacuum seal between the detector and its electronics, and the sample chamber, especially when the sample chamber is vented. It helps maintain the cryogenic temperatures required to reduce noise. Your detector window is extremely fragile. If it becomes dirty, it can affect the efficiency of your detector. It is recommended that you periodically run a detector efficiency calibration (see "Section 5.3 Establishing a Detector Efficiency Baseline" on page 20 for information on detector resolution calibration) to monitor your detector performance. If your window is dirty, you can contact PGT service to clean it for you, or you can clean it yourself. If you choose to clean it yourself, you must use appropriate cleaning materials and follow the correct procedure. If you often run oily or dirty samples, the window will require more frequent cleaning. If you have an ultra-high vacuum system, the window will probably remain cleaner for a longer time. Table 5.1 lists the approved solvents you can use to clean the detector window.

22 22 PRISM 2000 Series Detector Manual Warning: Do not use acetone, chlorinated hydrocarbons, or any non-approved solvent to clean your detector window. If you use a non-approved solvent, you may damage your system and void your warranty. Solvent Temperature Comments Order Vertrel_XF from: Vertrel_XF (2,3- dihydroperfluoropentane) Isopropyl alcohol Liquid Freon TF Used on a cold detector. Used on a detector at room temperature. Used on either a cold or room temperature detector. Miller Stephenson George Washington Hwy. Danbury, CT (203) If frost forms on the window (generally because of condensation from the air) the endcap is too cold, and you must let the detector warm up for another six hours or so. Table 5.1 Window Cleaning Solvents To clean the detector, follow the instructions below and refer to Figure Open your SEM sample chamber so that you can access the window. If you cannot access the endcap through the sample chamber, contact PGT service for instructions. Warning: Do not permit bright light to shine on the window of a light element detector Place a lint-free cloth underneath the window to catch the solvent runoff. 3. Use an eyedropper to drip the solvent over the top of the endcap. It will flow over the window itself and carry the oils away with it. Do not squirt or spray the solvent directly onto the window. 1.The IMIX Manual (Section ) contains the following explanation of window light sensitivity: "A light element detector should not be exposed to bright light, especially infrared, because these detectors are essentially light diodes. Aluminum is sputter-coated onto the windows to reduce the problem, but cannot completely eliminate it. In addition, venting of the SEM chamber will cause these windows to flex, which can create stress cracks in the coating. Over time, this will make the detectors more light sensitive. Light sensitivity is generally not a big problem, because the detector will usually recover in a few seconds to minutes. In severe cases, such as when an IR chamber scope is used frequently, it is suggested that the detector bias be shut off while the chamber scope is in use."

23 Detector Maintenance 23 Warning: All detector windows are thin, fragile and easily broken. NEVER TOUCH THE WIN- DOWS! Beryllium is toxic. If a beryllium window breaks, isolate the fragments with tweezers and collect them with sticky tape. Store them in a sealed container and dispose of them properly. 4. Examine the window for visible oil coatings or droplets. The diagram below shows the proper technique for cleaning your detector. end view Vertrel_XF, Isopropyl alcohol or Liquid Freon TF Be or ultra-thin window endcap Si(Li) or HpGe crystal absorbent tissue collimator removed Figure 5.2 Window cleaning procedure

24 24 PRISM 2000 Series Detector Manual 6. Troubleshooting Table 6.1 lists common problems you may encounter with your detector, probable causes and suggested actions. If you are unable to diagnose or correct a problem yourself, contact PGT service at (609) Problem Probable Cause Suggested Action The dewar has moisture on the outside surfaces or is "steaming" from the filler opening. The system has poor detector resolution. There is a poor vacuum condition in the spectrometer. The loss of system vacuum may be caused by a broken entrance window or a faulty seal. The Si (Li) crystal may be warming up. There may be microphonics caused by local vibrations. Under some conditions, the CRT display of the microanalysis system may introduce noise if it is too close to the detector. There may be noise in the preamp or main amplifier. An isolation problem may exist. Call PGT service for a Return Authorization, and return the system to PGT as soon as possible. Check the LN 2 level. Try moving the CRT display away from the microanalysis system. Call PGT service. Call PGT service. With the assumption that the microscope itself is functioning properly: The system is not counting. Check the meter or setting of the bias supply to verify that bias is present. Check the preamp output with an oscilloscope to see that positive pulses are present. These are ramps with small pulses superimposed on the rising slope, varying from -3 V to +2 V. The negative edges occur during the optical resets. Observe the sawtooth output to confirm that the preamp is working correctly. The system is counting intermittently. Moisture, dirt or leakage in the bias circuit will cause the preamplifier output to saturate. Verify the condition by turning down the bias and then slowly bringing it back up. If X-rays are seen as the bias is increased, there is leakage in the bias circuit. Call PGT service. Table 6.1 Troubleshooting Guide

25 Detector Maintenance 25 Problem Probable Cause Suggested Action The system is counting intermittently. Excessive humidity will frequently cause the system to count intermittently. Large electrical transients can force the preamplifier into saturation. These transients may be transferred to the preamplifier through electromagnetic radiation or through the AC lines. Raise the bias 100V and let the detector stabilize for a minute. Then attempt collection. If the problem persists, turn off the bias supply and CAREFULLY use a hot air gun to remove the excess moisture within the preamp. Proper isolation must be provided. Call PGT service. Table 6.1 Troubleshooting Guide (Continued)

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