1937: NBS Hönigschmid standard
In 1934, because of continuing concerns about the stability of the 1913 standards, the International Radium Standards Commission invited Hönigschmid to prepare a new set of standards. The starting material was an anhydrous radium chloride salt that Hönigschmid was using for a measurement of the atomic mass of the element. It had been highly purified such that the barium contamination was less than 0.003 atom percent. Three of the twenty 1937 standards came to North America. The NBS received Standards No. XIV and XV, which contained 38.10 mg and 20.36 mg of radium, respectively. After the death of Marie Curie in July of 1934, her daughter Irène Joliot-Curie took her place on the International Radium Standards Commission. She signed the certificates for the 1937 standards along with Stefan Meyer and Ernest Rutherford. Note the closing line that, "These statements are considered correct to 0.3 %." This would correspond to an uncertainty of 60 µg for the mass of radium in Standard No. XV. This level of accuracy in mass measurements was readily achievable for someone like Hönigschmid.
Three of the Hönigschmid standards are shown here. From left to right they are U.S. Standards Nos. XIV (Hönigschmid No. 5437) and XV (No. 5440) and the primary radium standard of Germany (No. 5426). The RaCl2 crystals within the tubes are clearly visible. A proper gamma-ray intercomparison required that the salt be evenly distributed in the tube, and that the tube axis be parallel to the near side of the electroscope.