Tư vấn an toàn bức xạ » So sánh đầu dò GM và đầu dò dạng Ion chamber
Mô tả ngắn:Về bản chất hai loại đầu dò này đều là một bình không khí, GM được áp thế cao, Ion chamber được áp thế thấp.
ANALYTICAL X-RAY SAFETY MANUAL
5.0 RADIATION MONITORING
Radiation survey instruments are used to detect potential radiation hazards, measure radiation intensities, monitor the effectiveness of shielding arrangements, and estimate exposure to personnel. There are two main categories of radiation monitoring devices: gas filled detectors and scintillation detectors.
Gas detection instruments are based on the principle that ions are produced when radiation passes through a gas-filled chamber. Electrons liberated in the chamber are attracted to the center electrode (anode) by a positive voltage potential. Positive ions are attracted towards the walls (cathode) of the chamber. This produces an electrical pulse or current which can be detected and recorded on an instrument known as a scaler.
There are three types of gas filled radiation detectors: ionization chambers, proportional counters, and geiger-mueller detectors. The primary difference between these detectors is the voltage applied to the chamber. The kind of detector used is based on the intensity and the type of radiation field encountered. Scintillation detectors operate on the principle that certain materials scintillate or give off light when exposed to radiation. There are two major types of scintillation detectors, crystal and liquid.
At very low applied voltages, ion pairs created by radiation passing through the chamber may recombine before they are collected and counted. As the voltage of a gas filled detector is increased, virtually every ion pair produced by the incident radiation will be captured. The current flowing through the meter is therefore directly proportional to the activity of the source. This feature makes these detectors very useful as radiation monitoring devices. Survey instruments operating at this voltage are called ionization chambers or "cutie pies". Because almost all ion pairs are collected, this instrument is used when it is necessary to accurately determine exposures. Ionization chambers are also used to determine exposures in areas of high radiation intensities. Ionization chambers, however, lack sensitivity and are relatively slow to respond to changing fields and are not useful for detecting leaks in analytical x-ray systems.
As the voltage of the tube is increased further, electrons experience greater acceleration and achieve sufficient energy to create secondary ionization and electrons in the gas. This amplification is termed an avalanche and dramatically increases the size of the electrical pulse at the central anode. Gas multiplication can create millions of ion pairs per ionizing event, in contrast to the ionization chamber which creates one ion pair. Although an avalanche has occurred, gas amplification is proportional to the energy of the initiating event in this voltage region. Radiation monitoring devices operating in this region are called proportional counters. With sufficiently thin windows alpha particles, which produce a large number of ions in the gas, can be distinguished from beta particles. In addition, the counter can be used to measure the energies of incoming x-rays and is often used as a detector in analytical x-ray equipment.
Geiger-Mueller (GM) Counters
Primary ionizations produced by the incident x-ray photons are accelerated by a very high voltage potential in a Geiger-Mueller Counter. Secondary ionizations are created from collisions with the accelerated ions. The secondary ions, in turn, are also accelerated and achieve sufficient energy to create ions. This process continues with the resulting formation of an avalanche of billions of ion pairs produced from a single ionization event. Because of this avalanche of electrons, a very large electrical pulse is produced a the anode making the GM counter a very sensitive device for the detection of radiation. A small amount of a halogen gas is often added to the tube as a quenching agent to stop the avalanche. The size of the electrical pulse is independent of the type of initiating radiation depositing energy in the tube. Unlike the proportional counter, GM counters cannot distinguish between types of radiation or energies. Because gas amplification has now reached its maximum volume, the same current flow will result regardless of the number of primary ions produced by a single incident photon.
The GM counter is the most widely used area survey instrument for the detection of low-level radioactive contamination and leaks in shielding arrangements. It is very sensitive, relatively inexpensive, and rugged. With sufficiently thin windows, alpha, beta and low energy x-rays can be detected. Besides being unable to distinguish between different ionizing events, GM counters will respond differently when exposed to the same dose rate produced by photons of different energies. Therefore, GM counters should only be used as a detection instrument and not for quantitative measurements of radiation unless they have been specifically calibrated for those energies.
Another major disadvantage of GM counters is their limitation to low radiation fields, typically below 200 mR/hr. Once ionizations have been initiated in a GM tube it becomes insensitive for a short time, called the dead time, and will not respond to further ionizing events. As a result, the number of counts recorded will be less than the true count rate. This error is relatively small at low radiation intensities, however, in high radiation fields large errors can be introduced. At very high exposure rates, where events are interacting with the tube much faster than the dead time, the counter may actually saturate and read zero.
Scintillation detectors release light when exposed to x-rays or gamma rays. Scintillation detectors are of two types: solid and liquid. Most solid scintillation crystals are composed of sodium iodide with a small amount of thallium added as an "activator". The crystal is coupled to a photomultiplier tube that converts the light flashes to amplified electrical pulses. The number of pulses are directly proportional to the intensity, and the size of the pulse is directly proportional to the energy of the incident radiation. These pulses are then analyzed by a counter, spectrometer, oscilloscope, or computer.
Because scintillation crystals are solid, rather than gaseous, their higher density and atomic number makes them very efficient and sensitive instruments for the measurement of x-rays and gamma rays. Portable scintillation detectors are even more sensitive than GM counters because of their increased efficiency. Scintillation detectors, however, are not as rugged as geiger counters because the crystal is hygroscopic and can absorb water from the atmosphere.
Liquid scintillation detectors employ organic compounds that give off light when radioactive materials are added to the solution. The light is detected by photomultiplier tubes and analyzed and counted in a manner similar to solid scintillation detectors. These types of detectors are commonly used to detect weak alpha, beta, and gamma radiations.
SURVEYING ANALYTICAL X-RAY EQUIPMENT
Radiation surveys should be conducted with a thin window (1-2 mg/cm2) geiger counter and a low energy ionization chamber. With its fast response and sensitivity, the geiger counter is ideal for detecting leaks in the shielding and around couplings. Once the leak has been detected, the exposure rate can be accurately determined with a low energy ionization chamber. Care should be taken in the choice of instruments to ensure they are sensitive to the energy of the x-rays being generated, and have been calibrated for low energy x-rays.
The exposure rate from leakage and scatter may vary from a few mR/hr to several R/hr. Surveyors should be aware that geiger counters can saturate and read zero when placed in high radiation fields. Ideally, the geiger counter should be used with an audible indicator. This gives an immediate response to radiation and allows the surveyor to watch the placement of the probe.
Measurements of exposure rates with a detector that has a cross sectional area greater than the x-ray beam will result in readings less than the true value if the instrument was calibrated in a field with a large cross section. Correction factors of 6000 or more may be required for typical instruments. An approximate correction can be made by multiplying the measured value by the ratio of the detector area to the beam area. Accuracy, however, is not of prime importance when surveying analytical x-ray equipment. The geiger counter should be used to detect leaks and to trace the leaks back to their source. Appropriate actions, such as adding shielding, can then be taken to correct the problem.
Monitoring should be conducted periodically and whenever the arrangement or design of the equipment is changed. Analytical x-ray equipment should be surveyed to detect and correct unsafe conditions that could lead to excessive radiation exposures to workers and the public. The following procedures should be followed when surveying analytical x-ray equipment:
1. Note the location and function of the various components including switches, warning lights, x-ray tube, shutters, ports, collimators, and the beam trap.
2. Ensure that unused x-ray ports are effectively shielded and secured.
3. Check the shutter for smooth operation.
4. Equipment should have no missing parts; shielding and couplings should fit properly and be properly secured.
5. Any modifications to safety device should be noted.
6. Check for radiation damage or corrosion to parts exposed to the primary beam.
7. Ensure that warning labels and signs are properly posted.
8. Place a sample in position, turn the tube on to the normal maximum voltage and current.
9. With all shutters closed, check the tube housing and shutter couplings for leakage. Open the shutter and check all areas around the shutter, collimator, sample, and ports for excessive scattered radiation levels. Do not place the probe into the primary or diffracted beam path. Check the equipment throughout its range of motion.
10. Monitor the radiation levels at the boundary of the equipment to ensure that radiation levels to the public are within acceptable limits.
11. Track small leaks back to the source and apply lead foil to the area that is leaking radiation.
12. Diffraction units with more than one operational port should be checked individually.
Personnel monitoring is used to detect and measure radiation exposure to individuals. The purpose of personnel monitoring is to document the exposure a worker receives in order to determine if radiation exposure limits have been exceeded, and to aid in keeping exposures as low or reasonably achievable. Personnel monitors are relatively inexpensive, reasonably reliable, and portable. They are usually worn on the belt, shirt pocket, collar, or finger. Personnel monitoring devices designed to measure low energy radiation should not be worn in a shirt pocket.
Personnel monitoring is required if there is a possibility that a radiation worker will receive greater than one-fourth of the maximum permissible dose (MPD). However, monitoring devices are usually issued if there is a possibility that a worker will receive greater than 10% of the MPD.
Whole body badges are generally used to measure large radiation fields such as those that might be encountered from diagnostic x-ray units. These badges are of limited value when working around analytical x-ray equipment. Scattered radiation is of low energy and easily attenuated by the air. In addition, small diffracted beams may not be detected by the badge.
The use of finger badges can be of some value to detect scatter radiation and increases the probability of detecting exposure to the diffracted or primary beam. However, due to the small diameter of the beam, exposure to the fingers may go undetected.
Although finger and whole-body badges may be of limited value in describing the radiation dose to a worker, they can be helpful. Increased exposure, where no exposures were noted in the past, might point to leakage, an unsafe safety device, or unsafe operating procedures. Operators of open beam equipment and those who assemble and align systems, must wear finger badges and should wear whole-body badges. Operators of units that are completely closed, have no measurable scattered radiation, and cannot be operated with an open beam should wear finger badges.
The film badge consists of one or more photographic emulsions contained in light tight envelopes inside a plastic holder. These emulsions have varying degrees of sensitivities to x-rays, gamma rays, beta particles, and neutrons. Windows and filters are built into the badge to distinguish between different types of radiation. An estimation of radiation energies can also be made. The film is developed and the density of the exposed film is proportional to the exposure received by the film badge. The degree of darkening is then compared with film exposed to known quantities of radiations. Film can also discriminate between primary beam exposure and scattered radiation. Scatter radiation produces a fuzzy image while a distinct image is produced from primary beam exposure.
Film badges have several advantages that have made them the most popular form of personnel monitoring devices. They are inexpensive, reasonably accurate and sensitive, provide a permanent record of exposure, and supply information on the type and approximate energy of the radiation exposure.
Film badge monitors also possess several disadvantages that have led to a decline, recently, in their popularity. Film badges are not as accurate or reliable as other personnel monitoring devices, nor do they detect exposures from low energy photons very well. In addition, film badges are relatively insensitive, the lower limit of detection is typically between 20 to 30 mR. Artificially high readings can result from false darkening as a result of improper handling, heat, humidity and age. In order to limit false darkening, film badges should not be worn for periods in excess of a month.
Pocket dosimeters are small, pencil shaped ionization chambers that directly measure ionizations due to radiation. The chamber is charged before use and a scale is adjusted to read zero at this voltage potential. As radiation passes through the chamber, ions are created. These ions are collected at the electrodes of the chamber and neutralize or discharge the dosimeter. This discharge is directly proportional to the quantity of radiation entering the chamber and is read on a voltage meter that is calibrated in radiation units. Some pocket dosimeters can be read directly from an internal scale while others must be inserted into a dosimeter reader.
The major advantage of a pocket dosimeter is its ability to supply an immediate readout of radiation exposure. Because of this, pocket dosimeters are typically used only in high radiation area. It is important to ensure that the dosimeters can detect the energy of the x-ray field being emitted. Pocket dosimeters are also reasonably accurate and sensitive, however, they are expensive, easily damaged, and give false readings due to charge leakage.
The newest type of personnel monitoring device, which is rapidly becoming the most popular, is the thermoluminescent dosimeter (TLD). This device looks similar to a film badge but uses a calcium fluoride or lithium fluoride crystal to measure radiation exposure. When radiation strikes the crystal, electrons absorb the energy and are promoted to higher energy states in the crystalline lattice. Upon heating, these excited electrons fall back into their original energy states releasing the stored energy as ultra-violet light. The amount of this light is directly proportional to the radiation dose received by the crystal. The light is then quantified by a photomultiplier tube.
TLD monitors have several advantages and few disadvantages. They are more accurate, reliable, and sensitive than film badges and pocket dosimeters, and their response to low as well as high energy photons is more uniform. These properties make them ideal for use around analytical x-ray units. TLD monitors are also relatively sensitive; radiation doses as low as 5 mR can be detected. TLD monitors are not influenced by normal heat and moisture, which allows the monitors to be worn for as long as three months without loss of information. TLD monitors are also reusable, having lost the "memory" of the previous radiation exposure when heated. They can also be read quickly so that radiation doses can be obtained within a few minutes.The primary disadvantage of TLD monitors is cost. The monitoring program can be twice as expensive as film badge monitoring. However, this cost can be reduced because TLD monitors can be read every three months instead of monthly. Costs are expected to be lowered in the future and TLD monitors will probably replace film badges as the method of choice in personnel dosimetry programs.
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