This course meets the requirements for 3 hours of continuing education credit in jurisdictions which recognize NYS Dept. of Education approval; however participants should be aware that some boards have limitations on the number of hours accepted in certain categories and/or restrictions on certain methods of delivery of continuing education. A certificate will be emailed to you upon successful completion. Show
Listed below are some of the probable prompt and delayed effects of certain doses of radiation when the doses are received by an individual within a twenty-four hour period. Dosages are in Roentgen Equivalent Man (Rem)
The delayed effects of radiation may be due either to a single large overexposure or continuing low-level overexposure. Example dosages and resulting symptoms when an individual receives an exposure to the whole body within a twenty-four hour period.
The Code of Federal Regulations (CFR) is the system used by the US Federal Government to organize the rules published in the Federal Register by the executive departments and agencies. The CFR is divided into 50 titles that represent broad areas subject to Federal regulation. Title 10 of the code applies to energy and parts 0 through 50 of Title 10 apply to NRC rules. Part 19, Notices, instructions and reports to workers: inspection and investigations; Part 20, Standards for protection against radiation; and Part 34, Licenses for industrial radiography and radiation safety requirements for industrial radiographic operations, are areas of the Code that are of primary interest when addressing radiation safety in industrial radiography.
The NRC regulations can be accessed on the Internet at: More than half of the states in the U.S. have entered into “agreement” with the NRC to assume regulatory control of radioactive material use within their borders. As part of the agreement process, the states must adopt and enforce regulations comparable to those found in Title 10 of the Code of Federal Regulations. Some of the requirement of the Code, such as exposure limits, responsibilities and procedures will be discussed in the following pages.
As discussed in the introduction, concern over the biological effect of ionizing radiation began shortly after the discovery of X-rays in 1895. Over the years, numerous recommendations regarding occupational exposure limits have been developed by the International Commission on Radiological Protection (ICRP) and other radiation protection groups. In general, the guidelines established for radiation exposure have had two principle objectives: 1) to prevent acute exposure; and 2) to limit chronic exposure to “acceptable” levels. Current guidelines are based on the conservative assumption that there is no safe level of exposure. In other words, even the smallest exposure has some probability of causing a stochastic effect, such as cancer. This assumption has led to the general philosophy of not only keeping exposures below recommended levels or regulation limits but also maintaining all exposure “as low as reasonable achievable” (ALARA). ALARA is a basic requirement of current radiation safety practices. It means that every reasonable effort must be made to keep the dose to workers and the public as far below the required limits as possible.
Regulatory Limits for Occupational Exposure
1) the more limiting of:
2) The annual limits to the lens of the eye, to the skin, and to the extremities, which are:
The shallow-dose equivalent is the external dose to the skin of the whole-body or extremities from an external source of ionizing radiation. This value is the dose equivalent at a tissue depth of 0.007 cm averaged over and area of 10 cm2. The lens dose equivalent is the dose equivalent to the lens of the eye from an external source of ionizing radiation. This value is the dose equivalent at a tissue depth of 0.3 cm. The deep-dose equivalent is the whole-body dose from an external source of ionizing radiation. This value is the dose equivalent at a tissue depth of 1 cm. The total effective dose equivalent is the dose equivalent to the whole-body. Declared Pregnant Workers and Minors
Non-radiation Workers and the Public Controlling Radiation ExposureWhen working with radiation, there is a concern for two types of exposure: acute and chronic. An acute exposure is a single accidental exposure to a high dose of radiation during a short period of time. An acute exposure has the potential for producing both nonstochastic and stochastic effects. Chronic exposure, which is also sometimes called “continuous exposure,” is long-term, low level overexposure. Chronic exposure may result in stochastic health effects and is likely to be the result of improper or inadequate protective measures. The three basic ways of controlling exposure to harmful radiation are: 1) limiting the time spent near a source of radiation, 2) increasing the distance away from the source, 3) and using shielding to stop or reduce the level of radiation.
Time
Dose = Dose Rate x Time When using a gamma camera, it is important to get the source from the shielded camera to the collimator as quickly as possible to limit the time of exposure to the unshielded source. Devices that shield radiation in some directions but allow it pass in one or more other directions are known as collimators. This is illustrated in the images at the bottom of this page.
Distance Inverse Square Law: I1 / I2 = D22 / D12
Shielding
As was discussed in the radiation theory section, the depth of penetration for a given photon energy is dependent upon the material density (atomic structure). The more subatomic particles in a material (higher Z number), the greater the likelihood that interactions will occur and the radiation will lose its energy. Therefore, the more dense a material is the smaller the depth of radiation penetration will be. Materials such as depleted uranium, tungsten and lead have high Z numbers, and are therefore very effective in shielding radiation. Concrete is not as effective in shielding radiation but it is a very common building material and so it is commonly used in the construction of radiation vaults. Since different materials attenuate radiation to different degrees, a convenient method of comparing the shielding performance of materials was needed. The half-value layer (HVL) is commonly used for this purpose and to determine what thickness of a given material is necessary to reduce the exposure rate from a source to some level. At some point in the material, there is a level at which the radiation intensity becomes one half that at the surface of the material. This depth is known as the half-value layer for that material. Another way of looking at this is that the HVL is the amount of material necessary to the reduce the exposure rate from a source to one-half its unshielded value. Sometimes shielding is specified as some number of HVL. For example, if a Gamma source is producing 369 R/h at one foot and a four HVL shield is placed around it, the intensity would be reduced to 23.0 R/h. Each material has its own specific HVL thickness. Not only is the HVL material dependent, but it is also radiation energy dependent. This means that for a given material, if the radiation energy changes, the point at which the intensity decreases to half its original value will also change. Below are some HVL values for various materials commonly used in industrial radiography. As can be seen from reviewing the values, as the energy of the radiation increases the HVL value also increases.
Approximate HVL for Various Materials when Radiation is from a Gamma Source
Approximate Half-Value Layer for Various Materials when Radiation is from an X-ray Source
Note: The values presented on this page are intended for educational purposes. Other sources of information should be consulted when designing shielding for radiation sources.
Since X-ray and gamma radiation are not detectable by the human senses and the resulting damage to the body is not immediately apparent, a variety of safety controls are used to limit exposure. The two basic types of radiation safety controls used to provide a safe working environment are engineered and administrative controls. Engineered controls include shielding, interlocks, alarms, warning signals, and material containment. Administrative controls include postings, procedures, dosimetry, and training.
Engineered Controls
Administrative Controls ResponsibilitiesWorking safely with radiation is the responsibility of everyone involved in the use and management of radiation producing equipment and materials. Depending on the size of the organization, specific responsibilities may be assigned to various individuals and/or committees.
Radiation Safety Officer
The minimum qualifications, training, and experience for RSOs for industrial radiography are as follows: (1) Completion of the training and testing requirements of Sec. 34.43(a) of Part 10 of the Federal Code of Regulations, (2) 2000 hours of hands-on experience as a qualified radiographer in industrial radiographic operations, and (3) Formal training in the establishment and maintenance of a radiation protection program. Radiation Safety Committee
System Users
Standard Operating Procedures
Written operating procedures must be developed and made available to anyone that will be working with radiation sources or X-ray producing equipment. These procedures must be specific to the equipment and its use in a particular application. Simply making the equipment manufacturers operating instructions available to workers does not satisfy this requirement. The operating procedure must be followed at all times unless written permission to deviate is received from the Radiation Safety Officer.
Emergency Procedures
Transporting the Exposure Device Preparing for an Exposure
The majority of over exposures in industrial radiography are the result of the radiographer not knowing the location of a gamma emitter and failing to conduct a proper radiation survey. Exposure vaults are equipped with warning lights and safety interlock switches which provide a margin of safety for workers. A survey must be performed occasionally to verify that vaults are not “leaking” radiation and that the safety devices are performing properly. However, when conducting radiography with gamma emitters in the field, the radiographer must rely heavily on measurements with a survey meter since other safety devices are uncommon. A series of surveys must be taken and some of the results from these surveys must be documented when transporting and working with gamma emitters in the field. Approaching the Exposure Device
Making an Exposure Retracting the Source This process is repeated for each exposure. The survey results must be documented when the exposure device is returned to the vehicle for transportation, and when it is placed back into its storage location.
Instruments used for radiation measurement fall into two broad categories: – rate measuring instruments and – personal dose measuring instruments. Dose measuring instruments are those that measure the total amount of exposure received during a measuring period. The dose measuring instruments, or dosimeters, that are commonly used in industrial radiography are small devices which are designed to be worn by an individual to measure the exposure received by the individual. An analogy can be made between these instruments and the odometer of a car because both are measuring accumulated units. The radiation measuring instruments commonly used in industrial radiography are described in more detail in the following pages.
There are many different models of survey meters available to measure radiation in the field. They all basically consist of a detector and a readout display. Analog and digital displays are available. Most of the survey meters used for industrial radiography use a gas filled detector. Gas filled detectors consists of a gas filled cylinder with two electrodes. Sometimes, the cylinder itself acts as one electrode, and a needle or thin taut wire along the axis of the cylinder acts as the other electrode. A voltage is applied to the device so that the central needle or wire become an anode (+ charge) and the other electrode or cylinder wall becomes the cathode (- charge). The gas becomes ionized whenever the counter is brought near radioactive substances. The electric field created by the potential difference between the anode and cathode causes the electrons of each ion pair to move to the anode while the positively charged gas atom is drawn to the cathode. This results in an electrical signal that is amplified, correlated to exposure and displayed as a value.
Depending on the voltage applied between the anode and the cathode, the detector may be considered an ion chamber, a proportional counter, or a Geiger-Müller (GM) detector. Each of these types of detectors have their advantages and disadvantages. A brief summary of each of these detectors follows.
Ion Chamber Counter Collection of only primary ions provides information on true radiation exposure (energy and intensity). However, the meters require sensitive electronics to amplify the signal, which makes them fairly expensive and delicate. The additional expense and required care is justified when it is necessary to make accurate radiation exposure measurements over a range of radiation energies. This might be necessary when measuring the Bremsstrahlung radiation produced by an X-ray generator. An ion chamber survey meter is sometimes used in the field when performing gamma radiography because it will provide accurate exposure measurements regardless of the radioactive isotope being used.
Proportional Counter Like ion chamber detectors, proportional detectors discriminate between types of radiation. However, they require very stable electronics which are expensive and fragile. Proportional detectors are usually only used in a laboratory setting.
Geiger-Müller (GM) Counter
The electronic circuit of a GM counters counts and records the number of pulses and the information is often displayed in counts per minute. If the instrument has a speaker, the pulses can also produce an audible click. When the volume of gas in the chamber is completely ionized, ion collection stops until the electrical pulse discharges. Again, this only takes a fraction of a second, but this process slightly limits the rate at which individual events can be detected. Because they can display individual ionizing events, GM counters are generally more sensitive to low levels of radiation than ion chamber instruments. By means of calibration, the count rate can be displayed as the exposure rate over a specified energy range. When used for gamma radiography, GM meters are typically calibrated for the energy of the gamma radiation being used. 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). Since the Geiger-Müller counter produces many more electrons than a ion chamber counter or a proportional counter, it does not require the same level of electronic sophistication as other survey meters. This results in a meter that is relatively low cost and rugged. The disadvantages of GM survey meters are the lack of ability to account for different amounts of ionization caused by different energy photons and noncontinuous measurement (need to discharge).
Comparison of Gas Filled Detectors The graph to the right shows the relationship of ion collection in a gas filled detector versus the applied voltage. In the ion chamber region, the voltage between the anode and cathode is relatively low and only primary ions are collected. In the proportional region ,the voltage is higher, and 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. Note that discrimination between kinds of radiation (E1 and E2) is possible in the ion chamber and proportional regions. Radiation at different energy levels 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 what the energy of the radiation that initiated the event. The GM counter gives up the ability to accurately measure the exposure due to different energies of radiation in exchange for a large signal pulse. This large signal pulse simplifies the electronics that are necessary for instruments such as survey meters.
Direct Read Pocket Dosimeter
By pointing the instrument at a light source, the position of the fiber may be observed through a system of built-in lenses. The fiber is viewed on a translucent scale which is graduated in units of exposure. Typical industrial radiography pocket dosimeters have a full scale reading of 200 milliroentgens but there are designs that will record higher amounts. During the shift, the dosimeter reading should be checked frequently. The measured exposure should be recorded at the end of each shift. The principal advantage of a pocket dosimeter is its ability to provide the wearer an immediate reading of his or her radiation exposure. It also has the advantage of being reusable. The limited range, inability to provide a permanent record, and the potential for discharging and reading loss due to dropping or bumping are a few of the main disadvantages of a pocket dosimeter. The dosimeters must be recharged and recorded at the start of each working shift. Charge leakage, or drift, can also affect the reading of a dosimeter. Leakage should be no greater than 2 percent of full scale in a 24 hour period.
Digital Electronic Dosimeter Some Digital Electronic Dosimeters include an audible alarm feature which emits an audible signal or chirp with each recorded increment of exposure. Some models can also be set to provide a continuous audible signal when a preset exposure has been reached. This format helps to minimize the reading errors associated with direct reading pocket ionization chamber dosimeters and allows the instrument to achieve a higher maximum readout before resetting is necessary. Audible Alarm Rate Meters and Digital Electronic DosimetersAudible alarms are devices that emit a short “beep” or “chirp” when a predetermined exposure has been received. It is required that these electronic devices be worn by an individual working with gamma emitters. These devices reduce the likelihood of accidental exposures in industrial radiography by alerting the radiographer to dosages of radiation above a preset amount. Typical alarm rate meters will begin sounding in areas of 450-500 mR/h. It is important to note that audible alarms are not intended to be and should not be used as replacements for survey meters.
Most audible alarms use a Geiger-Müller detector. The output of the detector is collected, and when a predetermined exposure has been reached, this collected charge is discharged through a speaker. Hence, an audible “chirp” is emitted. Consequently, the frequency or chirp rate of the alarm is proportional to the radiation intensity. The chirp rate varies among different alarms from one chirp per milliroentgen to more than 100 chirps per milliroentgen. Film BadgesPersonnel dosimetry film badges are commonly used to measure and record radiation exposure due to gamma rays, X-rays and beta particles. The detector is, as the name implies, a piece of radiation sensitive film. The film is packaged in a light proof, vapor proof envelope preventing light, moisture or chemical vapors from affecting the film. A special film is used which is coated with two different emulsions. One side is coated with a large grain, fast emulsion that is sensitive to low levels of exposure. The other side of the film is coated with a fine grain, slow emulsion that is less sensitive to exposure. If the radiation exposure causes the fast emulsion in the processed film to be darkened to a degree that it cannot be interpreted, the fast emulsion is removed and the dose is computed using the slow emulsion. The film is contained inside a film holder or badge. The badge incorporates a series of filters to determine the quality of the radiation. Radiation of a given energy is attenuated to a different extent by various types of absorbers. Therefore, the same quantity of radiation incident on the badge will produce a different degree of darkening under each filter. By comparing these results, the energy of the radiation can be determined and the dose can be calculated knowing the film response for that energy. The badge holder also contains an open window to determine radiation exposure due to beta particles. Beta particles are effectively shielded by a thin amount of material. The major advantages of a film badge as a personnel monitoring device are that it provides a permanent record, it is able to distinguish between different energies of photons, and it can measure doses due to different types of radiation. It is quite accurate for exposures greater than 100 millirem. The major disadvantages are that it must be developed and read by a processor (which is time consuming), prolonged heat exposure can affect the film, and exposures of less than 20 millirem of gamma radiation cannot be accurately measured. Film badges need to be worn correctly so that the dose they receive accurately represents the dose the wearer receives. Whole body badges are worn on the body between the neck and the waist, often on the belt or a shirt pocket. The clip-on badge is worn most often when performing X-ray or gamma radiography. The film badge may also be worn when working around a low curie source. Ring badges are worn on a finger of the hand most likely to be exposed to ionizing radiation. A LIXI system with its culminated and directional beam would be one example where monitoring the hands would be more important than the whole body.
How it works Heating the crystal causes the crystal lattice to vibrate, releasing the trapped electrons in the process. Released electrons return to the original ground state, releasing the captured energy from ionization as light, hence the name thermoluminescent. Released light is counted using photomultiplier tubes and the number of photons counted is proportional to the quantity of radiation striking the phosphor. Instead of reading the optical density (blackness) of a film, as is done with film badges, the amount of light released versus the heating of the individual pieces of thermoluminescent material is measured. The “glow curve” produced by this process is then related to the radiation exposure. The process can be repeated many times.
Andrews, H., Radiation Biophysics, Englewood Cliffs, New Jersey: Prentice Hall, Inc., 1974 Iddings, F., “Radiation Detection for Radiography”, Materials Evaluation, American Society for Nondestructive Testing, Columbus, OH, August 2001 National Research Council, “Health Exposure to Low Levels of Ionizating Radiation”, BEIR V, Washington D.C., 1990 Shapiro, J., Radiation Protection – A Guide for Scientists and Physicians, 4th Ed., Cambridge, MA: Harvard University Press, 2002 United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR), Ionizing Radiation Sources and Biological Effects, New York, NY, 2000 “Ionizing Radiation Exposure of the Population of the United States”, NCRP Report No. 93, 1987 “Radiation Safety Study Guide for Users of Analytical X-ray Systems”, Environment, Safety, Health and Assurance, Ames Laboratory, Ames, IA, April 1996 Code of Federal Regulations, Title 10, Energy, GPO Access web site @ NDT Education Resource Center Developed by the Collaboration for NDT Education
This course meets the requirements for 3 hours of continuing education credit in jurisdictions which recognize NYS Dept. of Education approval; however participants should be aware that some boards have limitations on the number of hours accepted in certain categories and/or restrictions on certain methods of delivery of continuing education. A certificate will be emailed to you upon successful completion. |