Figure 1-3

Introduction
Radiation detection makes radiography work to give us pictures of the insides of specimens and allows us to do it safely. Either photographic emulsions or electronic imaging devices detect penetrating radiation (gammas, X-rays or neutrons) to form images related to the density thickness of a specimen. That is, the greater thickness or density portions of a specimen absorb more radiation than other parts, which then makes an exposure on the detection system that is a latent image or shadow picture of the specimen. The latent image on film converts to something we can see by developing the film.

Radiation safety for radiography depends greatly upon a combination of common sense and detectors that provide information about where radiation exists, the quantity of radiation at each location and the extent of the radiographer's exposure to the radiation. This short article contains information on the radiation detectors used in the area of radiation safety.

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Knowing how radiation detectors work can help you use them properly.

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Gas Filled Detectors
Gas filled detectors make up the most often used kinds of radiation detectors. They include Geiger-Muller (GM) tubes, proportional counters and ion chambers. Generally, they may all be represented as two electrodes with an applied voltage between them to collect ions formed by radiation interacting with the gas between them in the detector. The two electrodes may be flat plates, rings or a cylinder that forms the outside of the detector and surrounds a center wire, such as shown in Figure 1.

If only a small voltage is applied, then collection of only the ions created by the radiation interacting with the gas fill occurs (collection of primary ions). Collection of additional ions begins as the voltage between the electrodes increases. The additional ions (secondary ions) form when the primary ions accelerate toward an electrode and collide with the gas molecules that are in their path. Most ionization occurs when electrons are collected at the center wire (anode), when the applied voltage is very high and the positive ions are collected at the outside wall (the cathode) of the detector. When the volume of gas surrounding the anode is completely ionized, ion collection for an individual ionizing event or radiation interaction does not occur until the ions are collected or neutralized. The detector must collect essentially all of the ions before another can even be detected. This process limits the rate at which individual events can be detected.

Figure 2 illustrates the relationship of ion collection in a gas filled detector versus the applied voltage. In the ion chamber region, only primary ions are collected. In the proportional region, primary ions and a number of secondary ions proportional to the primary ions originally formed are collected. In the GM region, a maximum number of secondary ions are collected when the gas around the anode is completely ionized. The detector goes into continuous discharge if the voltage increases beyond the GM region for a detector and no individual radiation interaction or events can be detected. Note that discrimination between kinds of radiation is possible in the ion chamber and proportional regions but not in the GM region. Each kind of radiation forms different numbers of primary ions in the detector. However, in the GM region the number of secondary ions collected per event remains the same no matter how many primary ions initiate the event. The collection of the large number of secondary ions in the GM region is known as an avalanche and produces a large voltage pulse. The large pulse simplifies the electronics that are necessary for instruments such as survey meters.

 

Instruments
Collection of only primary ions in the ion chamber region requires expensive, sensitive electronics but provides information on true radiation exposure. Such expensive and delicate instruments are used in radiography only when necessary for accurate radiation exposure values, when multiple energies of radiation are present such as with X-ray machines and usually under laboratory conditions but not under field conditions. Proportional detectors discriminate between types of radiation but require additional and very stable electronics. Such is the case of detecting neutrons in radiation fields of both neutron and gamma radiation. Such equipment is very expensive but worth the cost when required. GM detectors offer low cost but suffer from a lack of ability to discriminate, slow detection rates and failure to detect individual events in high radiation fields (when they become saturated).

Proper design of the GM survey instrument solves most of the disadvantages and takes advantage of the large electrical pulses produced per radiation interaction event by keeping the equipment simple, inexpensive, rugged and dependable. To overcome the lack of discrimination, the instruments are calibrated for the energy of gamma radiation similar to that being used by the radiographer. Most often, gamma radiation from Cs-137 at 0.662 MeV provides the calibration. Only small errors occur when the radiographer uses Ir-192 (average energy about 0.34 MeV) or Co-60 (average energy about 1.25 MeV). Use of metal shields around the GM tubes may help compensate for the differences in gamma energy of the different radiation sources.

Proper design also helps the GM survey instrument register radiation fields as low as 2.6 x 10^-7 C/kg (1 mR) per hour and as high as 2.6 x 10^-4 C/kg (1 R) per hour as required in the Code of Federal Regulations used by the Nuclear Regulatory Commission or compliant regulations used in agreement states. The 2.6 x 10^-7 C/kg (1 mR) requirement allows the instruments to be able to meet regulatory survey requirements of 5.2 x 10^-7 C/kg (2 mR) in any 1 h at barricades. And the 2.6 x 10^-4 C/kg (1 R) requirement assures the user that the detector has not gone into saturation (continuous discharge because ionizing events are occurring faster than ion collection) in radiation fields that large. If the detector goes into saturation, the instrument reads 0, which could be dangerous for a radiographer in a high radiation field. Again, good design permits the GM detector survey meters to meet regulatory requirements and still provide ruggedness, simplicity of use and dependability at a reasonable cost for the radiographer. The radiographer must, however, turn the survey meter on and use the survey meter on the job to provide the safety available.

Saturation of GM detectors in high radiation fields, which prevents ordinary GM tube survey meters from working, points out another advantage of ion chambers: in high radiation fields the ion chamber instruments simply read higher or peg at the highest reading on the meter. Specially designed GM survey meters have a circuit that pegs the meter when the GM tube goes into saturation and is unable to produce additional pulses. Most meters simply use a small tube that does not saturate until the radiation field is greater than 2.6 x 10^-4 C/kg (1 R).

The alarming dosimeters most often utilize GM detectors for the same reasons that survey meters use them. However, the pocket dosimeters used by radiographers operate in the ion chamber region (see Figure 3). Since the pocket dosimeters operate in the ion chamber region, only primary ions are collected and errors in correctly reading different radiation energies may be minimized. The applied voltage makes a small, metalized fiber spring away from the post where it is attached. The shadow of the fiber is seen on a screen inside the dosimeter. As the primary ions (formed by the radiation) neutralize the applied voltage, the fiber moves toward the post and its shadow moves across a calibrated scale on the screen. A pocket dosimeter may read a wide range of energies, from X-ray to gamma, correctly while a GM survey meter cannot because the dosimeter operates in the ion chamber region and the survey meter operates in the ion avalanche GM region. Both should be calibrated to be certain they read correctly, and they should not be used outside of the energy range specified by the manufacturer.

 

Thermoluminescent Dosimeters
Film or thermoluminescent dosimeter badges give radiographers a way of determining the total radiation exposure for a period of time. Radiation exposed film turns black when it is developed. Plastic, aluminum, cadmium and/or lead filters in the holder for the film packet allow determination of types of radiation and its approximate energy. Lead filters out low energy radiation so that the film records only high energy radiation exposure under the lead filters. Plastic filters out most beta particles. Aluminum and cadmium filter out different energies of X-rays as well as beta particles. Cadmium converts neutrons into gamma radiation so that the neutron exposure may be measured. Unfortunately, film also detects heat and mechanical damage so the film dosimeters must be handled carefully.

The thermoluminescent dosimeter badges contain small pieces of material such as lithium fluoride or manganese sulfate which absorb the radiation in such a fashion that some of the electrons in the material remain in excited or high energy states for a long time. When the thermoluminescent dosimeter materials are heated, they release light in quantities related to their radiation exposure. The materials can be reused, are relatively rugged and give reliable information over long periods of time. Also, the badges are generally smaller than film badges.

 

Conclusion
Again, none of the radiation safety detectors protect a radiographer who does not use them or who uses them improperly. Knowing how radiation detectors work can help you use them properly. While regulations require their use, only your good sense will make you use them when you are doing radiography.

 

* Frank A. Iddings, 1635 Rob Roy Ln., San Antonio, TX 78251; (210) 647-7717; e-mail <profiddings@satx.rr.com>.

 

Copyright © 2001 by the American Society for Nondestructive Testing, Inc. All rights reserved.

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