466 ISOTOPIC TRACERS AND NUCLEAR RADIATIONS [Chap. 19 



instrument measures contamination by drawing air through filter paper 

 surrounding a thin-walled Geiger counter. The counter output can be 

 applied to a recording instrument which then gives a continuous record of 

 accumulated activity. The most useful correlation between accumulated 

 activity and dose received by the lungs is had when the rate of air flow in 

 either instrument above is made equal to the average lung minute volume 

 of approximately 8 liters per sec. 



d. Radiation Fields. Appreciable intensities of gamma radiation and fast 

 neutrons over large areas are sometimes encountered when, due to inadequate 

 shield thickness, direct radiation is not sufficiently attenuated or when the 

 shielding, improperly constructed, allows scattered radiation to leak out. 

 Radiation fields such as these are quickly and easily detected when radiation 

 surveys are diligently carried out. This is best accomplished through routine 

 monitoring while work with active materials is in progress and by careful 

 surveys of new shield assemblies before work is started. Only in this way 

 can it be made certain that shielding is sufficient and does not scatter or 

 transmit radiation into easily accessible areas. In no instance should shield- 

 ing, including the storage vault and hood, be regarded as safe for a large 

 quantity of active material until a survey has been taken. 



Portable ionization chambers and electroscopes, now available commer- 

 cially in many forms, are probably the most dependable devices for gamma- 

 ray detection in open areas. However, the greater sensitivity and faster 

 response of portable counter tubes can often be used to advantage in detecting 

 radiation leakage through cracks and joints in shielding. Fast neutrons are 

 detected most effectively with hydrogen-recoil ionization chambers and 

 counters, while slow neutrons are best detected with boron trifluoride-filled 

 or boron cathode counters. 



Beta radiation as such cannot be considered as a possible hazard at con- 

 siderable distances from active material, but in the immediate vicinity of 

 apparatus containing the active material the intensity can be great enough 

 to deliver the daily tolerance dose in a few seconds exposure. When it is 

 necessary for the hands to have access to apparatus, particularly above open 

 containers, it is extremely important to measure the dosage rate before that 

 part of the procedure is undertaken. The most convenient monitoring 

 instruments for this purpose are portable thin- window counters; either 

 cylindrical-tube or bell-type counters may be used effectively. These 

 instruments should be conveniently at hand in the laboratory at all times, and 

 especially is this important when new procedures are being worked out or 

 when particularly active quantities of material are handled. 



e. Surface Contamination. Contamination of working surfaces is inevit- 

 able at one time or another, and in hoods it is practically unavoidable. Even 

 when the surface activity delivers less than the daily tolerance dose, the 



