One rad is equal to erg a unit of energy and mechanical work of energy deposited into 1 g of absorber. The SI unit for absorbed dose is the gray Gy and is equal to 1 joule J of energy deposited into 1 kg of absorber. Another concept used to assess the long-term risk of biological damage is equivalent dose. This measure is derived from the product of absorbed dose in rad or gray and a radiation weighting factor W R for the radiation being observed.
The W R compares the biological damage and resulting risk of any type of radiation to a standard. The conventional unit for equivalent dose is the roentgen equivalent human rem. For neutrons, the W R ranges from 5 to 20 depending on the energy of the neutron.
Protection from external radiation exposure is related to the principle of ALARA , or attempting to keep exposures as low as reasonably achievable. Health care professionals should pay particular attention to clothing and surfaces when treating a contaminated patient. Proper, timely removal of a patient's clothing—taking care to avoid aerosolizing radioactive powder—can reduce substantial amounts of contamination.
Radioactive material can be spread from 1 surface to another, therefore efforts should be made to control contamination at the source. Instrumentation that detects, quantifies, and identifies ionizing radiation is then needed to provide a greater measure of safety for workers when working with radioactive materials or casualties.
Physicians must understand the basics of radiologic instrumentation—and of each device's readings—in order to manage the medical treatment setting. Office-based physicians, who may not have immediate access to nuclear medicine or radiation safety personnel, should nonetheless have a cursory understanding of the differences between instruments. Resources with appropriate instrumentation, even in these noninstitutional settings, should be sought before an incident occurs.
It is beyond the scope of the present article to give more than an overview of radiation instrumentation.
They measure the amount of the radionuclide activity present on a surface. Dose rate meters measure the exposure of a given area to a radiation field, including the interactions with the radiation, number of ionizations occurring, and relating this information to dose rate energy transfer. These measures are important for personnel protection in the hospital and in the field.
Dosimeters , which measure an accumulated radiation dose, are usually static devices that must be sent to special laboratories to determine the amount of radiation dose over time. Some dosimeters can measure accumulating dose in real time, and they use either analog or digital display screens. The GM refers to the original tube's designers, and the instrument is often called a Geiger counter. Most often, a GM detector consists of a tube contained within a pancake probe Figure 1. Although design and appearance vary according to manufacturer, all detectors do essentially the same thing: reveal ionizations by means of an inert gas within the instrument.
Depending on their configurations, GM detectors can be used for both contamination measurements and dose rate measurements. Readings from this instrument will be used to help determine the extent and magnitude of radioactive contamination. Radioactive contamination will have to be managed by the treating physician, especially if there is the potential for internal contamination, which may enter the body via a contaminated wound or a nasal passage. Figure 2. The pancake probe is used primarily to detect radioactive contamination.
Thus, they are easily absorbed by materials and can be easily shielded against. Note the reading is displayed in counts per minute cpm. Activity is measured in disintegrations per minute dpm , disintegrations per second dps, or Bq , or curies. A GM detector cannot measure every disintegration that occurs, but rather detects a percentage of them.
Because various radiation types with their respective energy levels have differing potentials for creating ionization within the GM tube, the interpretation of cpm into dpm is dependent on knowing what the radioactive material is. In short, the instrument's efficiency is limited to detection of disintegrations.
A physicist should be consulted for interpretation of the readouts. Operators should also take a background reading of the sensitive side of the instrument away from the area being surveyed. As a general rule, a person is considered contaminated when the reading is 2 to 3 times the normal background level typically 20 to 40 cpm.
A variety of sources contribute to background radiation levels, including natural sources eg, naturally occurring radioactive materials in the earth's crust , sources used in medicine eg, technetiumm, radioactive iodines , medical uses of ionizing radiation eg, x-rays , industrial uses of radioactive materials eg, industrial radiography, industrial irradiators , and radiation from outer space.
When patients initially present with symptoms, they should be quickly surveyed to determine if they are contaminated, and if so, to what magnitude. If contamination is detected, the patient's clothing should be removed, placed in a plastic bag, labeled, double-bagged, and then moved to a storage area for later disposition.
Keep in mind that the clothing will provide a good sample for isotope identification. Surveys of personnel, facilities, and equipment need to be thorough and methodical, gathering readings from all surfaces. If an increase in count rate is observed or heard, the detector should be held over the area with the elevated reading for approximately 10 seconds to ascertain the contamination level.
Then, the naked patient's body area should be surveyed in the following order: open wounds, facial orifices, intact skin. Decontamination priorities should follow the same order. When taking readings with a pancake probe, it is important to be aware of the common mistakes. One should make sure the unit is on; make sure that sound is switched to on if sound is being used to locate contamination; and also make sure the correct multiplier range selector is selected.
An analog readout requires changes in the multiplier to take accurate readings; therefore, care must be taken to use a multiplier that will give a correct measurement. Conversely, digital readouts already factor in the multiplier by means of autoranging, but one should always verify which units of measure are being displayed because on some detectors the units can change automatically. One should also pay attention to the distance from the surface and survey speed. Health care professionals need not know the intricacies of all detector types.
Despite a similar appearance, a dose rate meter has a very different function from a contamination survey meter. This instrument measures the rate at which irradiation or exposure is occurring. Therefore, its primary function is to protect personnel before contamination. Often, the detector is internal to the meter housing.
This is usually true for GM and sodium iodide detectors.
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The dose rate meter will be used in hospital or in the field by emergency medical workers and fire and hazardous materials personnel. Field personnel can use these instruments to help determine the risk from radiation to personnel entering an area. Once unstable medical and surgical conditions have been controlled, radiologic conditions can be addressed, though physics personnel may be allowed to proceed with surveys of patients as long as they do not interfere with emergency medical care.
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Ideally, one should be able to identify a toxicant and measure how much of it is present. Radioactive materials and ionizing radiation are no different. Of note, because of the presence of multiple types of radiation, one may not be able to isolate the exact emitter type. Nonetheless, these techniques may help an operator to form an approximation. If one can determine the type of radiation, risk evaluations can be made. Some assumptions can be made, however, on the basis of context and history. The Table lists locations where some commonly encountered radioactive materials are found.
Some of these devices are very sophisticated and accurate, but they are mainly used in laboratories and are not portable. Others are portable but may lack the high-energy resolution of the laboratory models. All of these instruments can be quite expensive. Whether in a laboratory setting or in the field, a spectrometer's operations, as well as interpretation of its readings, requires considerable training and experience; therefore, physicians would not be likely to use one.
These laboratory identification instruments involve incredibly time-consuming processes, sometimes several weeks, but will still likely be used for the analysis of excreta to help determine internal dose. Figure 3 lists facilities with these types of equipment. Figure 3.
Facilities that can assist health care professionals during a radiologic or nuclear incident. A dosimeter is designed to measure accumulated dose. The function and design are variable, and the instrument is often attached to the outer layer of clothing, just above the waist. Functionally, dosimetry can be divided into real time and delayed.
Delayed reading dosimeters cannot be read immediately. They must be sent to a special laboratory for analysis. Examples are optically stimulated luminescent dosimeters and thermoluminescent dosimeters. Noz and G. Maguire Jr. William D. Erwin University of Texas M.
Health Physics and Radiation Protection - PHYS5018
Anderson Cancer Center Houston, Texas. This Article doi: Classifications Book Review. Services Email this article to a colleague Alert me when this article is cited Alert me if a correction is posted Similar articles in this journal Download to citation manager. Google Scholar Articles by Noz, M.