STANDARDS OF MEASUREMENT — BRTGGS 175 



RADIATION STANDARDS 

 THE INTERNATIONAL ROENTGEN 



The international roentgen is the quantity of X-rays that will 

 liberate one electrostatic unit of charge from 1 cm.' of air by 

 ionization under specified standard conditions. It is realized at the 

 National Bureau of Standards by means of a standard ionization 

 chamber developed by Taylor and Singer (7), which has now been 

 adopted as the ionization standard of six other countries besides 

 our own. The calibration of X-ray dosage meters in roentgens is 

 carried out by means of this standard chamber. 



RADIUM STANDARDS 



Radium is certified by the National Bureau of Standards in terms 

 of the weight of radium element, but the measurement which is 

 actually made is the determination of the ratio of the gamma-ray 

 intensity of the specimen to that of a radium standard. The Bureau 

 has three primary radium standards carrying certificates from the 

 International Radium Commission. The first of these (Interna- 

 tional value 15.44 mg.) was prepared by Madame Curie (1913) and 

 the other two (International values 38.10 mg. and 20.36) by Honig- 

 sclmiid (1936) from radium chloride of high purity. The Curie 

 standard when corrected for decay is in accord within 0.1 percent 

 with the values assigned by Honigschmid to the standards he pre- 

 pared. Radium standards must be systematically corrected for de- 

 cay. The initial rate of decay is about 0.043 percent per year. 



STANDARDS OF RADIANT ELUX 



The National Bureau of Standards maintains standards of radiant 

 flux, based upon absolute measurements by Coblentz (8). Lamps are 

 now supplied by the Bureau with certificates giving the total radi- 

 ation in microwatts per square centimeter at a specified distance 

 from the source. 



THE STANDARD OF BRIGHTNESS 



International agreement has now been reached regarding the adop- 

 tion of a primary standard of brightness, defined as the intensity 

 of the radiation from the interior of a blackbody at the temperature 

 of pure platinum at its freezing point. This standard was originally 

 proposed by Waidner and Burgess in 1910 but was first actually 

 realized at the National Bureau of Standards in 1931 by Wensel, 

 Roeser, Barbrow, and Caldwell (9), following the development of 

 the thoria crucible at the Bureau. 



