Patty's Industrial Hygiene, Physical and Biological Agents. Группа авторов

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СКАЧАТЬ Electronic Dosimeters

      Electronic dosimeters are small, lightweight (∼100 g), and accurate (∼±10%) dosimeters that measure and display instantaneous dose rate and integrate over time to obtain, display, and store the cumulative dose. Personal electronic dosimeters are approximately energy independent from about 60 keV to about 1.5 MeV, can measure dose rates of 0.1 mrem (1 μSv) per hour to 1000 mrem (10 mSv) per hour, and store doses from 0.1 mrem (1 μSv) to 1000 mrem (10 Sv). They may also have an alarm feature set to go off at a preset dose rate or integrated dose, and information can be downloaded into a computer for dose‐tracking and record‐keeping purposes.

      11.2.5 Pocket Dosimeters

Pocket dosimeters are of the size and shape of a fountain pen. Two different types of pocket dosimeters are typically used. One of these, a condenser type, is simply a capacitor that is fully charged before exposure to radiation. Ionization produced by the radiation neutralizes the charge on the capacitor, and thus reduces the voltage across the capacitor. This reduction in voltage is directly proportional to the radiation dose. The instrument is charged before exposure, and the dose read after exposure, in a charger–reader. Commercially available condenser‐type dosimeters measure integrated X‐ray or gamma ray doses up to 200 mrem, with an accuracy of about ±15% for quantum energies of about 50 keV to 2 MeV.

      The second type of pocket dosimeter is direct reading (and therefore is often called an “SRD,” a self‐reading dosimeter). It operates on the principle of a gold‐leaf electroscope. A gold‐plated quartz fiber is displaced electrically by charging it to a potential of about 200 V at which time the instrument reads zero. An image of the fiber is focused on a scale by an optical system within the SRD and is viewed through a lens at the other end of the instrument. Exposure to radiation discharges the fiber, thus allowing it to move back toward its uncharged position. The amount of discharge, and therefore the position of the fiber, is directly proportional to the radiation dose. These dosimeters read up to 200 mrem of X‐ray or gamma radiation with an accuracy of ±15% across an energy span of about 50 keV to 2 MeV.

12 RADIATION SAFETY SURVEYS

      A survey is a systematic set of environmental measurements made in order to determine one or more of the following:

       An unknown radiation source

       Dose rate from known sources

       Surface contamination

       Atmospheric contamination

      The surveyor must choose the proper instruments and use them correctly in order to make these determinations.

      12.1 Choosing a Health Physics Instrument

      The choice of a health physics surveying instrument for a specific application depends on a number of factors. Some general requirements are the same as those for other industrial hygiene surveying instruments, such as portability, mechanical ruggedness, and reliability. They also should

       be calibrated for the radiation that they are designed to measure;

       be sufficiently sensitive and appropriate for the radiation levels to be detected and measured;

       be responsive to the radiation being measured. This important point can best be clarified with a practical example. A commonly used beta–gamma probe has a 30‐mg cm−2 wall. This probe would be worse than useless if one wished to survey for a low beta energy contaminant, such as 14C or 35S, or for an alpha‐emitting contaminant such as 210Po. The range of the radiation from each of these radioisotopes is less than the probe's wall thickness, and therefore would not be detected. If there were to be severe contamination of this type, then the probe would falsely indicate safe conditions;

       be used to make measurements in a manner appropriate to the instrument's response time. A short time constant signifies that the instrument has a short response time. This means that the instrument responds to rapid changes in radiation level, such as would be experienced when passing the probe over a small area of contamination or over a crack in a radiation shield. A fast response time, however, means a decrease in sensitivity. Furthermore, a fast response time may result in rapid fluctuations in the meter reading, thus making it difficult to obtain an average reading. Many instruments offer a range of response times, the appropriate one being selected by the surveyor.

      12.2 Surface Contamination

      Surface contamination can be located with a sensitive detector, such as a thin end‐window G–M counter. After finding a contaminated area, a dose measuring instrument may be employed to measure the dose rate from the contamination. The main concerns about contamination are transmission of the contaminant into the body via inhalation or ingestion following tactile transfer to the skin, or transmission of the contaminant to other clean areas. To estimate the transmission hazard, a smear test is performed to determine whether the surface contamination is fixed or whether it is loose, and therefore transmissible. Smear tests may also be conducted in areas where high background radiation levels exist. A smear test consists of wiping the suspected area with a piece of specifically purposed filter paper, and then measuring the activity on the paper using an appropriate detector. The area to be smeared is determined by the extent of the contamination and the physical conditions under which the survey is made; an area of 100 cm² is usually smeared. It must be emphasized that a smear test is a qualitative test whose chief purpose is to allow an estimate to be made regarding the degree of transmissibility of the contaminant. If significant transmissible contamination is found, then prompt decontamination procedures are recommended.

      12.3 Air Sampling

Air sampling in a radioactive environment is essentially the same as sampling in the ordinary industrial environment. The air sampling systems used in the two environments are exactly the same. First, an appropriate collecting device, chosen based on the nature of the contaminant, is used, such as a filter, a liquid absorber, or a charcoal‐adsorbing medium. Then, a metering device for measuring the volume of air sampled, and finally a pump for sucking the air through the collector is used. Although the overall sampling strategy is the same for both nonradioactive and radioactive environments, there are some differences in the details. The size of the sample is determined by the analytical capabilities of the laboratory. In the case of radioactivity, the half‐life of the contaminant may influence the sample size. If the half‐life is very short, the collected activity will decay while the sample is still being collected, and may reach a steady state where the quantity of activity that decays is equal to the quantity collected. Under these conditions, increasing the sampling time does not increase the sample size. If this steady‐state activity is less than the lower limit of detection by the analytical procedure, then the volumetric sampling rate (not the sampling time) must be increased.

      Another air sampling detail deals with the actual quantity of the contaminant. For example, OSHA's PEL for airborne lead is 0.05 mg m⁻³. For 1 μm mass median aerodynamic diameter (MMAD) particles, the PEL corresponds to 3.13 × 10⁸ particles m⁻³. NRC's DAC for lead‐210 is 1 × 10⁻⁴ μCi m⁻³, which corresponds to a 1‐μm‐sized particle concentration of 8 particles m⁻³. If the ²¹⁰Pb were uniformly dispersed throughout the air, and if one wished to determine 50% of the DAC, that is, 5 × 10⁻⁵ μCi m⁻³, and if the lower limit of detection of the counting system is 5 × 10⁻⁶ μCi, the required sample volume would be calculated as

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