Sec. 13.4] STANDARDIZATION OF RADIOACTIVE SAMPLES 361 



A typical alpha-particle ionization chamber that may be used for obtain- 

 ing absolute measurements is shown in Fig. 100. The samples, usually 

 deposited in a thin layer or electropolated on platinum, are placed on electrode 

 A. The electric field between the electrodes is of the order of 1,000 volts 

 per cm. All alpha particles that cross the sensitive volume of the chamber, 

 and produce in the order of 3,000 ion pairs, give rise to a voltage pulse on the 

 collecting electrode which may be amplified with a linear amplifier and 

 counted by a conventional scaling circuit. Usually about 50 per cent of all 

 the alpha particles emitted by the sample travel through the sensitive volume, 

 and each of these will cause a count in the instrument. Most of the remain- 

 ing 50 per cent will go into the electrode and be absorbed there. However, 

 a small fraction of these particles (approximately 2 per cent for 5-mev alpha 

 particles) will be reflected by the electrode and counted in the chamber, as 

 indicated by the measurements of Ghiorso et al. [6] on reflection of alpha 

 particles by platinum. 



Alpha-particle ionization chambers calibrated by a parimary standard can 

 be used for direct determination of absolute disintegration rates of other 

 alpha emitters, which was the technique used by Kohman for the above- 

 mentioned determination of the rate of emission of alpha particles from Ra 226 . 

 The probable error of such absolute determinations of alpha disintegration 

 rates may be of the order of 0.5 per cent. In measurements of aged alpha- 

 particle samples it is necessary in some instances to take into account diffu- 

 sion of the radioactive material into the sample support [39]. 



13.4. Standards for Beta-particle and Gamma-ray Emitters. Techni- 

 cally, standardization of beta-particle and gamma-ray emitters presents a 

 much more difficult problem than that of alpha particles. The energy dis- 

 tribution of beta particles as well as the scattering of these particles is respon- 

 sible for much of the difficulty. In the case of the gamma rays, the problem 

 arises partly from the fact that these radiations have been detected mainly 

 through their secondary particles. Several independent methods of stand- 

 ardization are available. The most important of these are as follows: 



1. Standardization by coincidence measurements. 



2. Long-life beta-particle standards. 



3. Beta-particle standardization by direct measurement of the charge of 

 the particles. 



4. Indirect standardization by calorimetric measurement of the rate of 

 energy emission. 



5. Ionization measurement of beta particles and gamma rays. 



6. Standardization of beta-counter geometry. 



A discussion of each of these methods is given below. It should be borne 

 in mind that the response of all known instruments to beta and gamma rays 

 is a function of the energy of these radiations. The standardization of the 



