Michael P. Unterweger

Comprehensive Review and Critical Evaluation of the Half-Life of Tritium

As part of the preparation and calibration of three new National Institute of Standards and Technology (NIST) tritiated-water radioactivity Standard Reference Materials (SRMs), we have performed a comprehensive review and critical evaluation of the half-life of tritium (hydrogen-3). Twenty three experimentally-determined values of the half-life of tritium, reported between 1936 and 2000, were found. Six of these values were updated by later values. Two values were limits. Two values were deemed to be outliers. The 13 remaining values were evaluated in several ways. The results are compared with the results of other recent evaluations and all are found to be in good agreement. Our final recommended value for the half-life of tritium is the average of the adopted values from the four most recent evaluations, (4500 ± 8) d, where 8 d corresponds to one standard uncertainty.

Preparation and calibration of the 1978 National Bureau of Standards tritiated-water standards

Abstract The National Bureau of Standards activity standards for tritiated water, SRM 4926 and SRM 4927, have been recalibrated by the method of internal gas-proportional counting, and a new series of standards has been prepared. These standards are used in measurements made by the International Atomic Energy Agency and the World Meteorological Organization (IAEA/WMO) Tritium Monitoring Network. The agreement between the 1961 and 1978 gas counting measurements is 0.7%, assuming the adopted half-life of 12.35 yr. The results are, however, in complete agreement if, instead, a half-life of 12.44 yr is used. A half-life of 12.43 ± 0.05 yr, based on measurements of the NBS tritiated-water standards over a span of 18 yr, is suggested.

New and revised half-life measurements results

Abstract The half-lives of many radionuclides have been measured in the Radioactivity Group of NIST over the last three decades. The results of these measurements for many long-lived radionuclides such as 60Co, 137Cs, 85Kr, 22Na, 133Ba, 207Bi, 152Eu, 1 33Eu, 134Eu, 135Eu, and 125Sb have been recently revised. The results for the half-lives of the many radionuclides measured over the last three decades are tabulated. Comparisons with the recommended values from the International Atomic Energy Agency Coordinated Research Program (IAEA-CRP) are given for the long-lived radionuclides.

Half-life Measurements at the National Institute of Standards and Technology

For nearly half a century the half-lives of many radionuclides have been measured with increasing precision. The results of these measurements for many long-lived radionuclides, such as 60Co, 137Cs, 85Kr, 133Ba, 207Bi, 152Eu, 154Eu, and 155Eu, have been updated recently by the Radioactivity Group of NIST. These long-lived radionuclides are used extensively to calibrate various nuclear counting and monitoring systems. The long-term precision of these calibrations can be greatly affected by the uncertainties in the calibrant half-lives. Results for the half-lives of many radionuclides measured over the last four decades are tabulated. In addition, values of the half-lives of several short-lived radionuclides used in nuclear medicine are addressed, which are critical in determining the correct dosage given in patient treatment, are addressed. Comparisons with the recommended values from the International Atomic Energy Agency Coordinated Research Program and the Evaluated Nuclear Structure Data File from Brookhaven National Laboratory are presented and any apparent disagreements noted.

Comments on the NBS tritiated-water standards and their use

Abstract A panel of consultants of the International Atomic Energy Agency has recommended the adoption of the National Bureau of Standards tritiated-water standard and the half life of 12.43 yr for use in the WMO/IAEA precipitation survey. Factors are given to convert results obtained using a previous standard and half life to the newly recommended standard and half life. The preparation of a new low-level tritiated-water standard with an activity concentration of 1.312 s −1 g −1 is described.

The standardization and decay scheme of rhenium-186

Abstract Rhenium-186 has been standardized for activity by the CIEMAT/NIST method of 4πβ liquid-scintillation efficiency tracing with tritium, with an uncertainty (equivalent to 1 SD) of 1.61%. Half-life measurements with a pressurized ionization chamber give a T 1 2 = 89.25 ± 0.07 h . Photon emission rates were measured with semiconductor detectors, and the probability of emission of the principal γ-ray at 137 keV was found to be P7 = 0.0945 ± 0.0016.

Liquid-scintillation counting techniques for the standardization of radionuclides used in therapy

Abstract Radionuclides are increasingly used in therapeutic nuclear medicine. The CIEMAT/NIST method of standardizing high-energy beta-particle emitters is being applied to a list of candidate radionuclides developed by the US nuclear medicine community. Standards and standard methods are needed by the pharmaceutical manufacturers in North America before these nuclides can be widely distributed. Solutions standardized by liquid-scintillation counting are used to establish counting efficiencies for Cerenkov counting and NaI(T1) well crystals, and potentiometer settings for commercial radionuclide calibrators. Results are presented for a number of beta-particle-emitting radionuclides.

Intercomparison of internal proportional gas counting of 85Kr and 3H

Abstract An international intercomparison of internal proportional gas counting of 85 Kr and 3 H has been carried out under the auspices of EUROMET, a European Collaboration in Measurement Standards. A sample of 85 Kr mixed with inactive krypton gas was measured at three laboratories and a sample of 3 H mixed with nitrogen gas was measured at four laboratories. In each case agreement was achieved within the stated uncertainties. The experimental techniques employed at each of the participating laboratories are described and a statement of the results and conclusions is provided.

The standardization of samarium-153

Samarium-153 has been standardized by 4 pi beta liquid-scintillation counting, with an uncertainty of 0.4%. The probability per decay for the 103.2-keV gamma ray was measured, using two germanium detectors, to be 0.298 +/- 0.004. The half life, based on liquid-scintillation measurements over 6.4 half lives and pressurized-ionization-chamber measurements over four half lives, was found to be 46.27 +/- 0.02 h. The uncertainties given are one estimated standard deviation (of the mean when applicable) for random and non-random components.

Update of NIST half-life results corrected for ionization chamber source-holder instability.

As reported at the ICRM 2011, it was discovered that the source holder used for calibrations in the NIST 4πγ ionization chamber (IC) was not stable. This has affected a large number of half-life measurement results previously reported and used in compilations of nuclear data. Corrections have been made on all of the half-life data based on the assumption that the changes to the ionization chamber response were gradual. The corrections are energy dependent and therefore radionuclide specific. This presentation will review our results and present the recommended changes in half-life values and/or uncertainties.