XVII. 



RADIOACTIVE TRACERS 643 



stantaneous clicking noise. This lack of delay is of advantage in 

 that survey measurements can be made much more rapidly if the 

 meter responds rapidly. In general, choice between types of instru- 

 ments depends on the particular application. The GM tube meters 

 should, for accurate work, be calibrated with sources of different kinds 

 and energies of radiation, while the ionization chamber type gives a 

 more accurate measurement of the ionization producible by the radia- 

 tion in air or in tissue. 



The GM tube type can be made more sensitive than the ion cham- 

 ber type. Currently available GM tube survey meters are provided 

 with three scales: 0.2, 2, and 20 mr. per hour full scale {82,84,87). 

 Since, in an eight hour day, tolerance is 12.5 mr. per hour these meters 

 cover the range between tolerance and background, which is about 

 0.01 to 0.02 mr. per hour in the absence of all radioactive contamina- 

 tion. 



The ion chamber type of survey meter is another wartime develop- 

 ment originally designed for monitoring contamination by a-particle 

 emitters. Later modifications were developed capable of measuring 

 any radiation and were provided with screens to permit discriminating 

 measurement of the different types of radiation. Several of these are 

 now available commercially (87,93,94), having sensitivity scales 

 ranging from 2 to 2000 mr. per hour full scale. Other types will prob- 

 ably be available in the near future. Of those made available re- 

 cently several models with fairly good sensitivity toward all radiation 

 (93) and for 7 raj^s only {87) have proved satisfactory. 



There are several a.c.-operated, nonportable instruments that 

 give a direct meter reading of the radiation incident upon the ion 

 chamber. These can be made somewhat more accurate than the 

 portable models but they are useful only for a qualitative or semi- 

 quantitative measure of the strength of sources since an accuracy of 

 3-5% is all that can be expected from a reading on the meter dial 

 {82,85). Experience shows that the instruments should be cali- 

 brated; this can be done by using the formula developed in Section 

 Gl if strong enough 7 sources (~ 1 mc.) are available. Operating 

 manuals that are helpful for a technician moderately experienced in 

 electronics are furnished by the manufacturers. 



H. PLANNING A RADIOCHEMICAL LABORATORY 



The potential hazards involved in handling radioactive materials 

 necessitate serious thought about the construction of the laboratory in 



