sparrow: cytogenetic effects of ionizing radiations 65 



3.7 x 10 10 disintegrations per second, without regard to the number 

 of particles released per disintegration, their energies, or their prop- 

 erties. It is possible to convert a known number of disintegrations 

 into a standard unit of dosage measurement if both the number of 

 particles emitted and their average energy is known. Dose rates 

 obtained from several gamma emitting isotopes are shown in Table 3. 

 Note that the dose rate per curie varies widely with different isotopes. 



There are several different units currently in use for the meas- 

 urement of amounts of radiation produced in air or the amount of 

 energy absorbed in tissue or similar material. The oldest unit still in 

 general use is the roentgen (abbreviation: r). It is defined as "that 

 quantity of X or gamma radiation such that the associated corpuscular 

 emission per 0.001293 gram of air produces, in air, ions carrying 1 

 e.s.u. of electricity of either sign". A roentgen is equivalent to the 

 absorption of approximately 98 ergs/gm in water or tissue. One 

 roentgen of X or gamma rays will produce 2.083 X 10° ion pairs per 

 cc of air at standard temperature and pressure, 1.6 X 10 12 ion pairs 

 per gram of tissue or about 1 .8 ion pairs per \i 3 . For specific values of 

 different radiations at various energies see Table 2 of Lea (93). 



A second unit, less commonly used, is known as the roentgen- 

 equivalent-physical (abbreviation: rep) and can be used for any ioniz- 

 ing radiation. A rep is defined as that quantity of corpuscular radi- 

 ation which produces in tissue, per gram of tissue, an amount of 

 ionization equivalent to that produced by 1 r of gamma radiation in 

 air. 5 Both of these units have the disadvantage that they do not 

 measure the energy absorbed but depend on the amount of ionization 

 produced in air. In order to avoid this difficulty, a newer unit, the 

 rad, has been adopted recently. It is a unit of absorbed dose and 1 rad 

 equals 100 ergs/gm. For X-rays, one rad equals the amount of energy 

 released by 1.08 r in water. 



More detailed information about the dosage units and measure- 

 ments of ionizing radiation are given in many books and articles 

 (43, 68, 72, 83, 98, 124, 193, for example). A summary titled "Recom- 

 mendations of the International Commission on Radiological Protec- 

 tion" has recently been published (74). This or a similar article 



B Unfortunately, the definition of the rep has undergone some revision and its value 

 has appeared in the literature both as energy absorbed per unit mass or per unit volume. 

 The rad unit avoids this difficulty. See glossary (111) for further comments. 



