Sec. 19.7] THE SAFE HANDLING OF RADIOACTIVE MATERIALS 465 



development the indicated accumulated dose is determined by densitometer 

 measurement of the film darkening. 



Neutron film meters are similar to gamma-ray meters except in the use of 

 proton-sensitive emulsions. The area behind the cadmium registers only 

 recoil proton tracks due to fast neutrons, while the uncovered area registers 

 both proton recoils and protons from the reaction N 14 (n, p)C 14 . Dose is 

 estimated from dark-field microscopic examination of the number of tracks 

 per unit area. The weekly tolerance dose of fast and slow neutrons is 

 approximately 4,500 tracks per square centimeter [3,5]. 



Film meters have been used in other forms such as rings worn on a finger. 

 These and other forms are particularly useful in working with beta emitters 

 when there is danger of excessive hand dose which would not be registered 

 on more remote film meters. 



b. Pocket Meters. Two forms of pocket meters have been developed for 

 registering personnel exposure to gamma radiation. The first of these is a 

 pen-sized air ionization chamber, or air condenser, charged to approximately 

 150 volts before it is worn. It is read, preferably at the end of each day, by 

 discharging it through a minometer or a string electrometer. Since it is 

 calibrated for only one gamma energy, a considerable error can result from 

 exposure to gamma radiation of very different energy. The useful range is 

 approximately to }/± r, and saturation can occur at high radiation levels. 

 These meters are subject to accidental discharge from rough treatment, high 

 humidity, and insulator leakage and should accordingly be treated with 

 reasonable care. 



The second meter is a pen-sized dosimeter consisting of a fiber electrometer 

 with a scale and magnifying lens at one end with which to view the fiber 

 position. Once charged, the dose received can be read at any time. These 

 instruments are useful only for high radiation intensities and are subject to 

 the same disadvantages as the air condenser with regard to accidental dis- 

 charge and false dosage indication. 



c. Air Contamination. Surveys of air contamination due to radioactive 

 dust are especially important in areas handling appreciable quantities of 

 alpha emitters or those particularly hazardous beta-gamma emitters which 

 when ingested or breathed remain fixed in the body. The maximum per- 

 missible level for each isotope is somewhat uncertain, but for plutonium the 

 Clinton Laboratories have set the limit at 3 X 10 _u microcurie per cc, and for 

 beta emitters, the value >~ 10~ 7 microcurie per cc. 



Both precipitators and air filters are used for air monitoring. The former 

 consists of a cylindrical aluminum foil in which dust is precipitated electro- 

 statically as air is drawn through at a known uniform rate. From Geiger- 

 counter measurements of the accumulated alpha, beta, and gamma activity 

 on the unrolled foil, the air contamination can be estimated. The second 



