W.B. Mann

Backscattering of alpha particles from thick metal backings as a function of atomic weight

Abstract Alpha-particle backscattering from thick metal backings has been studied using two separate counters with geometries of 2 π and approximately 1 π steradians.

A RECALIBRATION OF THE NATIONAL BUREAU OF STANDARDS TRITIATED WATER STANDARDS BY GAS COUNTING

Abstract The National Bureau of Standards H 3 standards were prepared in the winter of 1954–1955 using microcalorimetric measurements. They have now been re-calibrated by means of the National Bureau of Standards length-compensated internal gas counters. The new value for their activity is 1.96 per cent lower than that previously measured, but this difference also involves uncertainties in the half-life and the average beta energy for H 3 . A value for the ratio of the combined evaporation and reaction rates with zinc, for HTO and H 2 O, equal to 0.92 has been obtained.

The half lives of two excited states in 239Np

The half-life of the 74 keV (52-) state of 239Np has been determined to be 1·380 nsec ± 0·032 nsec where the uncertainty represents the sum of estimated bounds to the systematic error, ±0·010 nsec, and twice the standard error, ±0·022 nsec. The half-life of the 118 keV (72-) state of 239Np has been found to be less than 0·04 nsec.

The calibration of the National Bureau of Standards' tritiated-toluene standard of radioactivity

Abstract The difficulties encountered, and their solution, when calibrating a radioactivity standard of tritiated toluene by comparison with the NBS tritiated-water standard by means of liquid-scintillation counting, are described. Leaks in the seals of commercial vials, giving rise to uncontrolled oxygen quenching, were eliminated by modifying the seals. White plastic caps gave higher count rates than black plastic caps. A new and highly precise method of dilution was used. The ratio of the activities of the two standards was determined with a standard error of less than 0.1 percent.

The half-life of carbon-14: Comments on the mass-spectrometric method

An assessment has been made of the mass-spectrometer results in the recent NBS determination of the half-life of carbon-14. Evidence is presented to show that the uncertainties due to effusive separation and the dependence of the sensitivity constant on the isotopic abundance of carbon-14 are small. It is proposed, however, to reduce the value of the half-life of carbon-14 determined by Mann, Marlow and Hughes(1) to 5745 ± 50 years.

The preparation and maintenance of standards of radioactivity

Abstract A historical survey is made of the preparation and measurement of international radium standards. The need for standards of other radioisotopes is explained. Standardization is subdivided into primary and secondary methods; 4πβ counting, β-γ and γ-γ coincidence counting, gas counting, and microcalorimetry are discussed as primary methods, and the maintenance of standards thus obtained is considered. The present state of international comparison of several radioisotopes is reported and discussed together with a policy for wider international standardization.