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External Exposure Monitoring

How can a person be exposed to radiation?
Simply stated, an individual can be exposed to radiation from either an internal (inside the body) or external (outside the body) source of radiation. In a previous chapter of this basic introduction to concepts of radiation and radioactivity, we discussed the ways of evaluating internal radiation exposures. This section focuses only on evaluating exposures from external sources.

How do you evaluate the magnitude of an external exposure?
One way of assessing external exposures is to measure the exposure rate in the location of interest then simply multiplying that value by the amount of time spent in that location.

Are there other ways?
Yes. Another typical approach is to use devices called “personnel dosimeters”

Is the use of personnel dosimeters a common practice?
Yes. In fact, the use of personnel dosimeters is one of the most important aspects of a monitoring program. These devices not only detect but measure the amount of exposure. They permit radiation protection professionals to not only ensure people are not being exposed excessively, but to demonstrate that regulatory dose limits have not been exceeded.

Are there regulations that apply to external exposure monitoring?
Yes. Regulatory requirements for the assessment and recording of external exposures are set forth by different federal agencies such as the Nuclear Regulatory Commission (NRC) and the Department of Energy (DOE). The armed services (Army, Navy, Marine Corps, Air Force, and the Coast Guard) have their own set of regulations which closely parallel those issued by the NRC. In addition, many states have their own set of radiation protection regulations addressing this particular issue.

How did these requirements evolve?
NRC, DOE, and other radiation protection requirements were developed and are periodically revised to attain consistency with radiation-related guidance that has been submitted by the Environmental Protection Agency (EPA) to the President of the United States. This guidance is based on current recommendations provided by consensus standards organizations.

What particular NRC and DOE requirements are applicable?
The NRC’s major governing regulation is Title 10 of the Code of Federal Regulations, Part 20 (10 CFR 20), Standards for Protection Against Radiation. The counterpart within the DOE is 10 CFR Part 835, Occupational Radiation Protection.

Can the amount of external exposure I receive be measured directly?
Not really. External exposures usually involve a derived or inferred quantity since directly measuring the dose (or energy imparted) to every organ or tissue in the body with extreme accuracy is not realistic.

What are some of the key components of an external monitoring program?
There are several important elements of external monitoring programs, all of which are necessary for their successful operation. For example, the staff who operate the program must be well-trained. In addition, the monitoring devices must be suitable for the radiations present at the facility, and their use parameters (deployment duration, position on the body, etc.) must be designed to ensure the measured result is representative of the true exposure. Finally, the records associated with the measurements and their interpretation are almost as important as the measurements themselves. In the real world of exposure monitoring, “if you didn’t write it down, you didn’t measure it”.

Tell me more about the monitoring devices. What are they and how do they work?
Personnel monitoring devices, sometimes called “personnel dosimeters”, are essentially devices designed to be worn or carried by an individual for the purpose of measuring the exposure he or she receives.

Can you give me some examples of external radiation monitoring devices?
One of the most popular types of monitoring devices contains small chips of salt called “thermoluminescent dosimeters”, or TLDs. Another type that contains film much like your dentist uses for x-raying your teeth is called a “film badge”. Both of these types of monitoring devices require processing before the data are available.

Are there any devices that can tell me what my exposure is right away?
Yes. There are small instruments that work like gas-filled survey meters. These devices can be direct-reading, in that you can look at a needle on a scale to see how much accumulated exposure you have received, or they can “chirp” or make noise whenever the accumulated exposure reaches a pre-determined level.

Can TLD, film or direct reading devices be used interchangeably?
No. Primary personnel monitoring devices, that is those typically used for the official measurement of the exposure received for record-keeping purposes, include TLD and film badges. Audible-alarm dosimeters and direct-reading pocket ionization chambers are examples of supplemental dosimetry – devices often worn with or located near the primary dosimeter. The latter devices should not be used as the official record of the exposure received.

How does a Thermoluminescent Dosimeter work?
A thermoluminescent dosimeter, or TLD, can contain a variety of different materials. When these materials are exposed to radiation, the absorbed energy is “trapped” and held indefinitely. When the materials are heated at a later date in a device known as a “TLD reader” (the basis for the word “thermo”), the trapped energy is released in the form of light (lumunescence). The amount of light is then related to the radiation dose.

What are the advantages of using TLDs for radiation monitoring purposes?
The fact that the absorbed radiation energy is trapped indefinitely is a prime advantage because the user can decide when the TLD will be read and the results reported. It is not uncommon for a period of several months to pass between the radiation exposure and the read-out of the TLD.

Another advantage is that TLDs come in a variety of materials, sizes and shapes. These dosimeters are small, light, easy to handle, and can be worn comfortably by the individual. They are capable of covering a wide range of radiation exposures, from just a few millirem to thousands of rem. In addition, they have applicability for environmental measurements as well – a situation which is possible because certain types of TLDs are very sensitive to very low radiation exposures (the type of exposure levels found in the environment).

Is there a down side associated with the use of TLDs?
A TLD can only be read once, that is, once the trapped electrons have been heated, the amount of light produced, and the results recorded, the TLD cannot be re-read to confirm the results. It is also possible for a certain degree of “fading” to take place whereby some of the trapped electrons leave their excited energy states and return to the ground state prior to being read in the reader. This potentially results in a lower estimate of the individual’s exposure. (Many TLD materials, however, do not have this problem, and for those that do, correction factors can be applied to account for this situation.) Finally, some TLD materials exhibit energy dependence, which means its response is dependent upon the energy of the radiation that interacted with it.

Are TLDs only used in badges?
Absolutely not. In fact, the thermoluminescence principle dates back a number of years. Following the atomic bomb explosions in Japan in 1945, pieces of ceramic roof tiles were collected, along with ornamental tiles and brick, from various locations near the point of detonation. Because these materials also exhibit thermoluminescent properties, they were heated and the light emitted was used to estimate the radiation exposure at the location of the tile. Researchers then applied these values to exposure estimates for humans who were in the same vicinity.

Have thermoluminescence properties in other materials been used here in the U.S.?
Yes. A similar application was used to assess the external impact of radioactive fallout on individuals in St. George, Utah, who received radiation exposures from atmospheric weapons testing in the United States in the 1950’s and 1960’s. For this study, pieces of ceramic tile from toilets, sinks, and similar household “amenities” were collected and analyzed by thermoluminescent dosimetry to obtain the exposure estimates.

Any more?
Actually, the moon exhibits thermoluminescense. The moon is bombarded with cosmic radiation on a constant basis. The energy from this radiation is stored or trapped in the moon’s surface while it is dark and cold. When the sun moves across the moon’s face, the surface is heated and the energy is released in the form of light! We see a case of thermoluminescence whenever we see the moon!

What is a film badge?
Film badges have been used to monitor external radiation exposures for over 100 years. These devices consist of a piece of film similar to what you purchase for your camera that is placed in a small holder or “badge”. When the film is exposed to radiation, it darkens, just like it does when it is exposed to visible light. The amount of darkening is proportional to the amount of radiation exposure. In other words, the darker the film, the higher the radiation exposure.

How is the darkness of the film determined?
The degree of darkening is determined using a device known as a “densitometer”. This machine compares the amount of light that passes through the film in the badge to the amount of light that passes through film that has been carefully exposed to known amounts of radiation. Until fairly recently, processing and analyzing film badges was quite labor-intensive, and required trained workers to perform the task. Now days, electronics and computers are used to process and read the film quickly and efficiently.

Are there advantages to the use of film for external exposure monitoring?
Yes. Even though film is not as widely used as TLD-based devices, it still presents some important advantages. For example, unlike TLD, film can be read as many times as is desired to confirm or recheck prior results. In other words, the information contained on the film is not destroyed once it is read. This is an extremely useful feature when questions arise regarding the exposure received by an individual.

Are there other good things about the use of film?
Yes. A second and equally important advantage is that the type of radiation that is exposed to the film (i.e., low energy x-rays, high energy x-rays, gamma radiation, neutron radiation), and the radiation energy, can be determined. This is typically accomplished in two ways. First, the film is surrounded with a series of filters that cause various responses on the film depending upon the type and energy of the radiation. Second, the film in the badge is compared to film that has been darkened by various well-characterized radiation types at different (but known) exposure levels.

Anything else?
Film has the additional advantages of being small, light, easy to handle, and comfortable to wear; it is able to provide useful information over a wide range of radiation exposures; provide information as to whether an acute or chronic exposure to radiation occurred based on the appearance of the film; and able to determining the orientation of the exposure, that is, whether an exposure occurred from the front or the back of the badge.

But what about the down side?
Interpreting the results from a piece of exposed film is not as easy as it sounds. Even though it can be interpreted and re-interpreted many years after the exposure occurred, the calibration films (i.e., those exposed to known sources of radiation) prepared at the time of the original exposure must be preserved. This is because film can fade if it is stored improperly – just like your photographs at home will fade over time.

How does an alarming dosimeter work?
An alarming dosimeter contains a small Geiger-Mueller (G-M) detector and some electronics. Typically, the electronic circuit causes a series of “beeps” or “chirps” when the G-M detector responds to radiation.

Can an alarming dosimeter be used to estimate a person’s exposure to radiation?
If calibrated properly, the number of “chirps” can be made equivalent to a known amount of radiation exposure. For example, if the device is set to “chirp” every time the G-M counter measures one milliroentgen, a dosimeter that “chirps” five times over the next hour is telling you that it has measured an exposure of five milliroentgens.

Is that how alarming dosimeters are typically used?
No. When these devices are used in industry, their primary purpose is to make noise when a pre-set exposure or exposure rate has been exceeded. As such, it serves as a warning device rather than as a true dosimeter to measure the exposure received.

What should I do if I want to be sure to use an alarming dosimeter properly?
It is most important that the device be checked for functionality before work in a radiation field occurs. At that time, the device is typically pre-set to a specific alarm set point. However, it is equally important that the device be calibrated periodically to ensure the set points correspond to true radiation exposures.

Are there disadvantages in using alarming dosimeters?
Yes. These dosimeters can create a false sense of security for workers as they can misfunction. A dosimeter that has been dropped, or that has low batteries, or that is worn such that the G-M detector inside cannot “see” the radiation exposure, serves little purpose. An alarming dosimeter should never be used as a substitute for a radiation survey instrument, for job pre-planning, or for basic common sense.

What is a direct-reading dosimeter?
A direct-reading dosimeter, also called a “pocket meter” or “pocket ionization chamber”, is a small air-filled instrument, typically the size of a short, fat pen. It operates on the principle of radiation ionizing air, and it is capable of responding, primarily, to photon radiation (i.e., gamma rays, x-rays) and sometimes high-energy beta radiation. Specially-modified direct-reading dosimeters can also be used to measure neutron radiation.

How does a direct-reading dosimeter work?
A direct-reading dosimeter contains both a “fixed” and a “movable” quartz fiber. When first put into use, an electrical charge is placed on both fibers. Because of their similar charge, the two fibers “repel” each other. As radiation enters the chamber and ionizes the air that is inside, the charge on the fibers is neutralized, and they begin to move closer together. The degree of movement, which is proportional to the amount of exposure received, can be seen by observing one of the fibers through an eyepiece that is on the end of the device.

Is that why they call them “direct-reading” dosimeters?
Yes. And this is perhaps the greatest advantage of using a direct-reading dosimeter. A user can determine his/her exposure at any time by holding the pocket dosimeter up to a light source and directly reading the value off a numerical scale. This allows workers to keep track of the amount of radiation exposure received over each day’s work.

I assume there are disadvantages too, right?
Unfortunately, yes. The greatest disadvantage associated with these devices is that they are fragile. Simply dropping one of them can cause the two fibers to “discharge”. In addition, they can “leak” some of their charge, meaning the fibers inside the dosimeter move even though there is no radiation exposure occurring. Finally, if the user forgets to “charge” the dosimeter before use, the device serves no purpose. It is for these reasons that a film badge or TLD is almost always used in conjunction with a direct-reading dosimeter.

Why do nuclear facilities have personnel monitoring programs?
Personnel monitoring programs are designed and conducted at nuclear facilities for several reasons. Among these are: protecting the health of personnel; identifying poor work practices; detecting changes in radiological conditions; verifying the effectiveness of engineering and process controls; meeting ALARA (“as low as is reasonably achievable”) considerations; demonstrating compliance with regulatory requirements; and keeping adequate records.

When is personnel monitoring required from a regulatory standpoint?
From a regulatory standpoint, personnel monitoring is required under certain conditions – typically when an individual has the potential to receive 10% of the regulatory exposure limit from occupational exposure (i.e., as part of his or her work). For facilities licensed by the nuclear regulatory commission, adult occupational workers must be monitored if they have the potential to receive 5,000 millirem. For minors and declared pregnant women, however, monitoring is required when the individual is likely to receive 50 millirem.

Is there any particular category of occupational worker that requires additional consideration?
Yes, the NRC has grown increasingly concerned about a group of individuals known as industrial radiographers. These individuals typically use multi-curie sealed sources of radiation to “radiograph”, or take pictures of pipes, to examine the adequacy of welds and to perform related activities. For a variety of reasons, these intense radiation sources can and have caused exposures in excess of regulatory limits. In some cases, very serious overexposures that resulted in observable health effects occurred.

What has the regulatory agency done about this?
For one, the NRC has issued specific regulations that impose additional external monitoring requirements for radiographers. These regulations require radiographers and their assistants to wear a direct reading pocket dosimeter and either a film badge or thermoluminescent dosimeter. An alarming device is also required except for permanent radiography facilities where alarming/warning devices are in routine use.

I imagine some records must be kept when one is monitored, right?
Absolutely! Records documenting external exposures received by workers are required by federal and state agencies. These records must be maintained to document compliance, and they must be retained until their disposition is authorized by the overseeing agency.

What kinds of records are usually kept?
Examples of required records include those related to results of individual external exposure measurements; documentation of occupational exposures received during both current and prior years; data necessary to allow future verification or reassessment of recorded exposures; results of surveys, measurements, and calculations used to determine individual occupational exposures; results of maintenance and calibration performed on personnel monitoring devices; training records; results of internal audits; and declarations of pregnancy.

Should my employer tell me the results of my monitoring?
Yes, reports are required which include, but are not limited to, radiation exposure data for monitored individuals; and records of exposure for terminating employees. In addition, an annual radiation dose report must be provided to each individual that was monitored over the past year.

What are “hot particles” and what is their impact on personnel monitoring?
Typically, only the exposure to a small area on the anterior region of the body is evaluated as a measure of the whole body exposure. The possibility exists that other, more localized areas, could have been highly exposed. If an overexposure occurs, it may be necessary to reconstruct the exposure situation (never the preferred method). A prime example is the occurrence and detection of so-called hot particles, also known as fleas or specks, at nuclear facilities (primarily nuclear power plants) around the country. Originating primarily from defects in the cladding surrounding reactor fuel elements or activation of Co-59 in the core, microscopic particles of very high specific activity are formed and can attach to clothing or exposed areas of the body. These hot particles are electrically charged and can therefore “hop” from one location to another, potentially resulting in highly localized, non-uniform beta/beta-gamma exposures. In sum, the advent of more sensitive dosimetric equipment has created the ability to detect these particles, but at the same time, the appearance of hot particles has become a challenge from a personnel monitoring standpoint.

Where should TLD, film badges, direct-reading dosimeters and alarming dosimeters be placed on the body?
Interestingly, several of the major radiation protection regulations (e.g. 10 CFR Part 20 and 10 CFR Part 835) do not state where personnel monitoring devices should be placed on the worker. However, assistance in this regard can be found in other radiation-related guidance documents. To determine the whole body exposure, dosimeters should be placed on the trunk of the body (between the neck and the waist) and positioned so that the front of the badge holder is facing the source of the radiation. The dosimeter should be attached to the anterior portion of the torso. Although this seems to happen more often than not, dosimeters should not be attached to loose fitting clothing or worn on neck chains.

What if I am interested in the dose to specific parts of the body?
Exposure to the lens of the eye is one concern for radiation protection professionals, and there is a separate regulatory limit for the eyes. For uniform exposures, a measurement at the surface of the torso is usually assumed to be equivalent to the exposure in the location of the eye. Nonuniform exposures, however, which include localized beams of radiation, x-ray machines, or beta sources, would require the placement of a dosimeter somewhere near the eyes. Typically, a dosimeter is mounted on the side of the head or on the forehead, such that it hangs close to the eye.

Are there other localized monitoring methods?
Yes. Another circumstance is estimating the radiation exposure to the embryo/fetus of a pregnant worker. Again, wearing a conventional whole body personnel dosimeter (e.g., TLD or film badge) between the neck and the waist typically provides an adequate result. However, in the case of non- uniform fields or when exposures begin to approach the applicable exposure limit, an additional dosimeter is mounted near the mother’s waist or abdomen.

What about the hands?
The arms below the elbow and the legs below the knee are called extremities. Like the eyes, the extremities have a specific regulatory exposure limit. The limit is much higher than the limit for the whole body because there are few blood-forming organs in the extremities, thus they are less sensitive to radiation exposure. When monitoring the exposure of the extremities, dosimeters are typically placed at the most exposed location. Ring badges, wrist badges, toe badges, and ankle badges that are custom-designed to provide as little movement restriction as possible are readily available for this purpose.

What is the National Voluntary Laboratory Accreditation Program?
The National Voluntary Laboratory Accreditation Program, abbreviated “NVLAP”, provides accreditation for personnel dosimetry programs. The NVLAP accreditation process is designed to assess how precise and how accurate the devices are, and how competent the processor is in providing the service. Competence, as used here, requires not only that the dosimeters perform adequately, but also that the processor demonstrates it has the staff, facilities and equipment, procedures, records and reports, and a quality assurance program to ensure reliable results. Any facility that provides personnel monitoring services is afforded the opportunity to attain accreditation. Once received, NVLAP accreditation is valid for a period of one year. After this period, re-accreditation is required.

Why was NVLAP developed and what is its goal?
Concerns about personnel dosimetry performance date back to the 1950’s. Efforts to implement dosimetry performance standards have been attempted several times, but were always unsuccessful, primarily because only the performance of the dosimeter and not the dosimetry processor was addressed. The goal of the National Voluntary Laboratory Accreditation Program remains the satisfactory performance of personnel dosimeters, and of equal import, that of the processor.

What is DOELAP?
The U. S. Department of Energy, abbreviated “DOE”, was of the opinion that NVLAP did not completely meet its needs or the needs of its contractors. As a result, a separate and distinct dosimetry testing program – the Department of Energy Laboratory Accreditation Program (DOELAP) – was developed. This program differs from NVLAP accreditation primarily in the required level of dosimeter precision and accuracy, and in the categories for which the dosimeter must be accredited.

Do you have any final words about external exposure monitoring?
The key point in this tutorial is that personnel monitoring programs exist to protect individuals, and to satisfy ALARA, regulatory, and recordkeeping/report requirements. Monitoring is performed for occupational workers and, on occasion, for visitors to a facility that uses radiation, radioactivity or radiation-producing machines. Measurement results are primarily evaluated for whole body, skin, extremity, and eye exposures using typical monitoring devices such as TLD’s, film, and direct- reading dosimeters. In all cases, however, each device has advantages and disadvantages, which must be weighed before a deployment decision is made.

Where can I obtain more information about monitoring for external radiation exposures?
There are a number of excellent references that discuss monitoring and dose assessment methods in great detail. Quite a few of them are listed in the “Bibliography” that is located in this web page’s “Tool Box”. If you don’t find the information you need there, please don’t hesitate to “Ask a CHP”.