170 ISOTOPIC TRACERS AND NUCLEAR RADIATIONS [Chap. 7 



products in equilibrium with radium was recommended b}^ the Commission 

 in 1930 [2]. A curie of any radium product, therefore, is that quantity of 

 the isotope which decays at the rate of 3.7 X 10 10 disintegrations per sec. 



In addition to the restricted use of the curie recommended by the Com- 

 mission, it has become widely adopted as a measure of the quantity of any 

 radioactive isotope. More generally, the fractional units millicurie (mc) and 

 microcurie (juc) are used; these are the quantities of the isotope that decay 

 at the rates of 3.7 X 10 7 and 3.7 X 10 4 disintegrations per sec, respectively. 

 The weight or the number of atoms of an isotope equivalent to 1 millicurie 

 is directly proportional to the half-life 



N = 3.7 X 10 7 



W = 3.7 X 10 7 



0.693 

 AT 



N 



where N = number of atoms per millicurie of isotope 

 W — weight in gm per millicurie of isotope 

 T = half-life of isotope, in sec 

 A = atomic number 

 N = Avogadro's number 



Failure to distinguish between total ionizing events and total disintegra- 

 tions has led to occasional confusion in the use of the curie for some isotopes 

 that do not have simple decay schemes. When beta particles and gamma 

 rays are emitted, the number of events detected (corrected for geometry, 

 absorption, and efficiency) is sometimes not equal to the number of dis- 

 integrations, and it is essential, therefore, to know the decay scheme for the 

 radioisotope. This must include the number of particles per disintegration, 

 the percentage and energy of each, and the percentage of internal conversion 

 of gamma rays if they are present. Only when this detailed information is 

 known can the disintegration rate of an isotope be ascertained from measure- 

 ments of its radiation and its quantity expressed in curies. 



Detection of the beta particles, for example, from a radioisotope that emits 

 a beta particle and gamma ray in succession may lead to a disintegration rate 

 that is too high if there are also internal conversion electrons. For the same 

 reason, detection of the gamma rays may well lead to a low value if the con- 

 version coefficient is unknown. Similarly, the decay rate of a radioisotope 

 that disintegrates by either positron emission or K capture cannot be deter- 

 mined unless the relative probabilities of the two processes are known, or 

 both the positrons and x-rays are detected. 



b. Rutherford. 1 rutherford = 1/(3.7 X 10 4 ) curie = }i 7 millicurie. 



