GENERAL PROCEDURES FOR RADIOASSAY 177 



thereafter be used for the actual calibration, as follows: At the time of 

 administration of radioactivity to the plant or animal, a known alicjuot is 

 diluted to enable the preparation of a standard sample of about 8()()0 to 

 4000 counts/min. In practice it is wise to prepare such standards in 

 replicate. These standards are counted in exactly the same way as the 

 biological samples. This means that the standard and biological samples 

 must be mounted in the same type of container and measured at the same 

 distance from the counter tube. If the biological samples are to be 

 counted in solid form and have appreciable mass, then self-absorption 

 corrections may also have to be made, as indicated in the next section. 

 If solution counting is used, the calibration and biological samples must 

 consist of the same volume in the same size vessel. 



If, for example, it is known that 0.01 per cent of the dose administered 

 to the animal has 4000 counts/min, then the biological-sample count X 

 0.01/4000 will give the percentage of dose in the measured sample. By 

 using a calibration sample of this counting rate, which in itself has a small 

 coincidence loss, it is usually not necessary to apply coincidence correc- 

 tions below counting rates of 10,000 to 12,000 counts/min. 



The calibration standard can be counted as often as necessary during 

 the sample measurements, and this compensates for decay or for changes 

 in sensitivity of the counter. With short-lived isotopes it is usually more 

 convenient to record the time of measurement and correct by calculation 

 to the reference time at which the calibration standard was counted. It 

 is clear that sample counting rates should not be used in excess of those 

 known to be acceptably linear. If a sample has too high a counting rate, 

 the measurement can be made with a suitable absorber placed between 

 the sample and the counter tube. This count has to be evaluated with 

 the calibration standard counted in the same way. Serious errors may 

 result if the absorber is not located in the identical geometric position 

 for both standard and sample. 



Self -absorption of Beta Particles. A large proportion of radioisotope 

 measurements are based on the counting of beta particles emitted. It 

 will be recalled from Chap. 3 that beta particles have a definite range and 

 are stopped by relatively small amounts of material. In the radioactivity 

 measurements of a sample, many of the beta particles originating from 

 within the sample will be absorbed by the mass of the sample itself and 

 therefore will not be counted. This behavior is called self -ah sorption and 

 is so important, especially with the lower-energy beta emitters, that it is 

 often the determining factor in the sample preparation. To indicate an 

 order of magnitude of this effect, it takes about 6 mg/cm- of sample to 

 cause a 50 per cent reduction in the counting rate of C'"* (0.155 Mev), 

 whereas for Ca^* (0.254 Mev) about 15 mg/cm- produces a 50 per cent 

 reduction. Actually the beta counting rate of a given sample will be 



