Larry L. Lucas

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.

Study of the scattering correction for thick uranium-oxide and other α-particle sources—II: Experimental

Abstract Theoretical values of the 2π α-particle counting rate, C2π, including scattering, divided by the disintegration rate, N0, are calculated for α-particle sources mounted on flat backing materials. The theoretical values (Part I) and experimental values (Part II) are in good agreement, and show that 1. (a) C2π/N0 values as a function of α-particle energy and source thickness can be calculated on the basis of a simple physical model. 2. (b) The scattering correction, expressed as a fraction of the disintegration rate, decreases with increasing source thickness. 3. (c) The α-particle scattering in uranium-oxide is much less than the scattering in platinum, in disagreement with previous estimates. The calculations are readily extended to other source compositions and α-particle energies.

Calibration of the massic activity of a solution of 99Tc

Abstract A solution of 99Tc was calibrated in terms of massic activity using the NIST 4πβ(LS)–γ-anticoincidence counting system. 60Co was used as the efficiency-tracing radionuclide. Measurements were also made using conventional 4πβ liquid-scintillation (LS) counting. 3H was used as the efficiency-monitor radionuclide. The LS detection efficiency ratios, (e99/e3), were calculated theoretically. The calculated detection efficiency ratios depend strongly upon the assumed shape of the beta spectrum of 99Tc.

Calibration of the National Institute of Standards and Technology tritiated-water standards

Massic activity values obtained for NBS/NIST tritiated water SRMs 4927 and 4927E, using internal gas-proportional counting, are reported. The massic activity ratios of all of the available NBS/NIST tritiated water SRMs and samples of the Level 1 and Level 3 tritiated water standards produced by the National Physical Laboratory (NPL) in 1996 were measured using an extensive series of liquid-scintillation intercomparisons. New massic activity values obtained for the NPL standards, using the gas counting results for SRM 4927E and the results of the liquid-scintillation intercomparison, are also reported. A new determination of the half-life of tritium was made using massic activity data for SRM 4927 over a period of 38 years. The new value is (4504+/-9) (one standard deviation) days. This value differs significantly from the previous NBS/NIST value but is in good agreement with the value recommended in the Evaluated Nuclear Structure Data File (ENSDF).

The half life of plutonium-240

Abstract The alpha-particle-emission rates of solutions of plutonium-239 metal were determined from defined-solid-angle-counter and liquid-scintillation-counter measurements. These results were combined with composition data obtained from other laboratories and the half life of 239 Pu was calculated to be 24,112 yr. Associated with this value is a standard deviation of the mean of ±16yr and a systematic error of ±50yr.

Massic Activity Ratios of the NBS/NIST Tritiated-Water Standards Issued Between 1954 and 1999

As part of the preparation and calibration of three new National Institute of Standards and Technology (NIST) tritiated-water radioactivity Standard Reference Materials (SRMs), the massic-activity (activity of the sample divided by the mass of the sample) ratios of all of the available NBS/NIST tritiated-water SRMs issued between 1954 and 1999 were measured using liquid-scintillation (LS) counting. Four of the tritiated-water standards (SRMs 4361, 4926B, 4927C, and 4927D) were not available for measurement. All of the other tritiated-water standards (SRMs 4361B, 4361C, 4926, 4926C, 4926D, 4926E, 4927, 4927B, 4927E, and 4927F) were available, having been stored in flame-sealed glass ampoules. Where possible, massic-activity ratios measured by liquid-scintillation counting are compared with massic-activity ratios calculated from gravimetric dilution factors. The agreement is well within the stated uncertainties. For two of the tritiated-water standards that were not available for measurement (SRMs 4361 and 4926B), massic-activity ratios calculated from gravimetric dilution factors are available.

New Spherical Gamma-Ray and Neutron Emitting Sources for Testing of Radiation Detection Instruments

The National Institute of Standards and Technology (NIST) has developed new gamma-ray and neutron emitting sources for testing radiation detection systems. These radioactive sources were developed for testing of detection systems in maritime applications. This required special source characteristics.

Test Report-Exposure and Ambient dose Equivalent Rate Measurements in Support of the ITRAP+10 Testing

In this work we studied the response of two different Victoreen instruments as a function of the exposure rate, the instrument orientation and photon energy. The rate dependence for both instruments is of the order of 8 % over the range of exposure rates tested (0.5 mR/h to 1000 mR/h). Regarding the instrument orientation dependence investigation, a significant difference is observed between the two instruments. While the Victoreen 451P shows no significant dependence with instrument orientation, the Victoreen 451P-DE-SI shows a significant dependence of up to 20 % between the three different orientations. Finally, the energy dependence measurements of both instruments reveal that the instruments measure lower exposure rate values compared to the reference values for the low energy x-rays by about 20 %. While, for the Co gamma-ray beam measurements, both instruments are about 5 % higher than the reference value.

A pulse-recording system for radioactivity measurementsContribution of the National Institute of Standards and Technology. Not subject to copyright.

Abstract The NIST Radioactivity Group is developing a computer-based pulse-recording system to use for radioactivity measurements. The system consists of a dual ADC module and a plug-in board for an industry-standard-architecture personal computer. The host computer (or another computer) can be programmed to simulate many different logic systems (coincidence, anticoincidence, selective sampling, correlation counting, etc.), so that the pulse data from a given measurement can be analyzed using a wide range of timing parameters and types of logic.