TYPES OF RADIAC INSTRUMENTS

The detection of nuclear radiation is of vital importance to personnel. Serious injury or death can result from exposure to sufficient quantities of these invisible rays and particles. In considering the effects on personnel exposed to radiation, we need two kinds of information: (1) the intensity of the radiation field and (2) the total dose or quantity of radiation received per exposure or time interval. Intensity may be defined as the strength of the radiation. It is expressed as a quantity of radiation per unit of time. The quantity unit used is the roentgen or the rad, and the time unit is usually the hour. Therefore, intensity is expressed as roentgens per hour (R/hr) or as rads per hour (rad/hr). Dosage is expressed in two values; the exposure dose, which is measured in R, and the absorbed dose, which is measured in rad. An added factor in the use of rad is that it expresses the dose from any type of radiation, whereas the R relates only to gamma radiation or X rays. A large number of detection devices scaled in R are still in use. Therefore, R will be used in this book along with rad. The rad and R may be assumed to be equivalent for the purpose of the manual.

Information on intensity and dosage is essential in measuring the extent and degree of radiological contamination. It permits the calculation of safe entry time and stay time for personnel in contaminated areas. Also, it provides an objective means for withdrawing personnel who are nearing the critical point of radiation exposure. Finally, it is useful in anticipating the severity of radiation sickness. Data needed for these and other calculations can be gathered by various radiac instruments.

No hand-held radiac available for military use will measure both radiation intensity and dose. Therefore, the different kinds of measurements must be made by separate instruments. The device that measures radiation intensity is called a dose rate, or survey, meter. It provides the information needed to calculate the radiological hazards of occupying a contaminated area or handling contaminated equipment. It also provides the information necessary to calculate the approximate length of time personnel can safely remain in a radiologically contaminated area. The device that measures the total radiation received by an individual is called a dosimeter. Medical officers must have dose information to predict the severity of radiation sickness, to make the prognosis, and to provide appropriate medical treatment.

The two types of radiation detection and measuring instruments may be compared to automobile speedometers and odometers (mileage indicators). The dose rate, or survey, meter measures the intensity of radiation in R/hr or rad/hr and is like the speedometer that indicates the speed of an automobile in miles per hour. The dosimeter measures the total exposure in R or in rad, without regard to time. Therefore, it is like the odometer which records the total distance traveled in miles, without regard to time.

Fixed radiac systems are installed on most Navy combatants. The instruments provide information on gamma dose rates at the location of the detector and at the main readout installations, usually the bridge area and DCC. The information from these instruments may be used to estimate the dose rates at locations on the ship other than the bridge.

SURVEY METERS IN NAVY USE

Areas are monitored for radioactivity by means of survey meters (dose rate meters). Field monitoring will reveal not only the extent of the radiological contamination but also how "hot" various parts of the area really are. The term hot, when used in reference to radiological contamination, means that a certain area has a high contamination reading. Survey meters are also used to monitor food, water, and the skin and clothing of personnel for radiological contamination.

The Navy has radiacs that can be used to make alpha, beta, gamma, and neutron surveys. The following sections discuss the survey meters currently used by the Navy. No attempt has been made to discuss the specialized radiacs used around nuclear power plants or propulsion units.

Alpha Meters

The alpha particle is a relatively large and heavy nuclear particle that has a short range in

matter. The alpha particles that you will most 0.00025 inch thick) Mylar film, coated on both likely encounter are the 5.1 MeV alpha particles sides with evaporated aluminum, is placed over the emitted by plutonium-239. Alpha particles of this detector. The Mylar film provides the required energy can be stopped by a sheet of paper or light protection of the PM tube. Thus, a practical ordinary clothing. These alpha particles have a scintillation detector requires that the phosphor be range of only about 1 inch in air. Because of the protected from external sources of light and at the short range of alpha particles in air, alpha same time be sufficiently thin to permit the detectors must be placed within one-eighth to one-transmission of alpha particles. The fragile Mylar quarter of an inch from the surface being surveyed. film is presently the best material available for

The AN/PDR-56 (series A through E and G alpha detector "windows." through H) (fig. 8-8) is the Navy's standard alpha The AN/PDR-56 has a meter readout calisurvey meter. It is used to locate and measure brated in four ranges of counts per minute (c/min): alpha contamination of (1) the skin and clothing of 0 to 1,000, 0 to 10,000, 0 to 100,000, and 0 to personnel and (2) the surfaces of equipment, 1,000,000. These values are directly related to buildings, or land. surface plutonium-239 contamination levels of 0-

The detector of an AN/PDR-56 is of the 10, 0-100, 0-1,000, and 0-10,000 micrograms per scintillation type. Alpha scintillation detectors are square meter (ng/m 2 ). extremely fragile devices by necessity. Detection involves (1) the release of alpha particle energy in Beta-Gamma Radiacs a phosphor with the resultant production of a scintillation of light and (2) the conversion of the energy in the light scintillation to electrical energy in a photomultiplier (PM) tube. Since the PM tube is light sensitive, the phosphor must be protected ^{from ambient light. A very thin (approximately} 

_{The radiacs described in the following paragraphs can detect and measure both gamma and X rays. The detecting elements of some of these instruments are so constructed that by moving an} integral shield (the beta shield), an operator can

Figure 8-8.-AN/PDR-56.

expose the ports to the sensitive volumes of the detectors. This permits the detection of beta particles aIong with gamma rays. Judicious use of the beta window permits an operator to determine if a surface is contaminated with a beta or a lowenergy gamma-ray emitter.

AN/PDR-27.- The AN/PDR-27 (series J through S) (fig. 8-9) is a portable, watertight, battery-powered, low-range survey radiac. Two Geiger-Mueller (GM) tubes are mounted in an extendable probe. A spare Geiger-Mueller tube set is included in the carrying case. The probe is fitted with a beta shield. Six alkaline D-cell batteries (BA-3030/u) power the unit. If the alkaline batteries are unavailable, you may use carbon-zinc, D-cell batteries (BA-30). The AN/PDR-27 provides both visual and audible indications of gamma and beta radiation levels. The visual indication is shown on a meter. The audible indication is heard through a headset. The radiation measurement is in milliroentgens per hour (mR/hr). The unit is capable of detecting and measuring gamma radiation when the beta shield is in place. It is also capable of detecting beta and gamma radiations together when the beta shield is removed. There are four linear scales on the AN/PDR-27. The scales are 0 to 0.5 mR/hr, 0 to 5 mR/hr, 0 to 50 mR/hr, and 0 to 500 mR/hr. Beta radiation can be detected on the lower two scales only.

Figure 8-9.-AN/PDR-27. 8-15

AN/PDR-43.- The AN/PDR-43 (series A actuator is spring-loaded so that the detector movable beta shield. An internal radiation radiac. The detector consists of one GM tube through E) (fig. 8-10) is the standard Navy responds to it only when you so desire. Two check source is provided so you can conduct which is installed in the forward end of the high-intensity survey meter. It is a portable, an alkaline D-cell batteries (BA-3030/u) power the operational check of the unit. The check source watertight, battery-powered, high-range survey ^{radiac. The detector has a beta window and a radiac. You may use two carbon-zinc, D-cell}

Figure 8-10.-AN/PDR-43. 8-16

batteries (BA-30) if alkaline batteries are not available.

The AN/PDR-43 is capable of detecting and measuring gamma radiation alone. The unit can also detect gamma and beta radiations combined. The unit has three meter scales. The scales are 0 to 5 R/hr, 0 to 50 R/hr, and 0 to 500 R/hr. The AN/PDR-43 has a range selector switch which has five positions. These positions are OFF, BATTERY, 500 R, 50 R, and 5 R. On the bottom of the radiac is a function selector switch. This switch has three positions: BETA, GAMMA, and OPERATION CHECK. When the function selector switch is in the GAMMA position, only gamma radiation is detected by the GM tube. When it is in the BETA position, the end window of the GM tube is exposed to detect beta and gamma radiations combined.