S.A. Woods

Standardisation and measurement of the decay scheme data of 237Np

Within the framework of a EUROMET project, a solution of 237Np/233Pa at equilibrium has been standardised by 4pi(PC)-gamma counting. An intrinsic Ge spectrometer was employed to measure relevant gamma-ray emission probabilities. Good agreement was obtained for those pertaining to the decay of 233Pa, but not for the gamma-ray emissions following the decay of 237Np. The cause of such discrepancies is discussed.

Standardisation of 11C.

The increasing use of positron emission tomography for medical imaging and the availability of short-lived positron emitters has raised concerns about the accuracy of calibration of secondary standard measurement systems and the viability of using a single long-lived positron emitter as a reference calibration source for all positron emitters. Potential problems arise because the 511 keV quanta arising from positron annihilation are not generally produced at the same point as the original disintegration. In addition, the secondary standard may also be responsive to the associated bremsstrahlung radiation. The magnitude of both effects depends on the positron end-point energy. In order to resolve these problems, it is necessary to produce absolute standards of these positron-emitting radionuclides and the work presented here details the results of such work with 11C.

Metrology of plutonium for environmental measurements

Abstract Since its discovery in 1940, plutonium has become widely distributed in the worlds environment by a number of routes. Due to the perceived hazard of the element, considerable resources have been devoted to the measurement of plutonium in nuclear waste, nuclear effluent, human beings and the environment. Plutonium is, however, difficult to measure because all of the commonly encountered isotopes have very low abundance γ-ray emissions making conventional absolute 4πα-γ-coincidence counting techniques impractical for the nuclides 236Pu, 238Pu, 239Pu, 240Pu, 242Pu and 244Pu. Instead, the technique of using a tracer nuclide (in this case 241Am) has been employed at the National Physical Laboratory (NPL) to standardize 239Pu and 242Pu and is applicable to the other isotopes mentioned above. 241Pu has some different problems in that it is a s-emitter with a low (∼20 keV) end point energy and, again, no associated γ-ray. It decays to 241Am which can be measured quite easily, but to achieve calibration by ingrowth with an acceptable uncertainty could take many years. 241Pu is calibrated using liquid scintillation counting by comparison with aqueous 3H, with some corrections made for differing end point energies and β spectrum shapes. In recent years, the technique of Inductively Coupled Plasma Mass Spectrometry (ICP-MS) has been applied to the field of plutonium measurement and it appears to offer advantages in the routine, high-throughput analysis of plutonium. However, work commissioned by NPL suggests that there are significant problems in the calibration of ICP-MS equipment, with large variations in the response obtained from different plutonium nuclides. In the U.K. measurement community, the most pressing need is for suitable matrix reference materials and standardized chemical yield tracers, incorporating 236Pu, 242Pu or 244Pu. Each of these have some problems in that they are difficult to produce or procure and that isotopic purity can be a problem. The questions to be asked here are; “Is there a continuing global demand for these yield tracers?” if so “Who is prepared to fund production and purification?” and “Are National Standards Laboratories prepared to take on such work?”. These questions and some possible answers will be discussed in the paper.

The standardisation of 229Th for an environmental yield tracer

The naturally occurring long-lived nuclides of thorium (228Th, 230Th and 232Th) are all alpha-emitters that are of interest both to the nuclear and non-nuclear industries and levels of these nuclides need to be monitored in both the working and natural environment. In order to do this in a manner where the results of radiochemical analysis are traceable to national standards, a chemical yield tracer has to be employed. 229Th has been identified as a suitable tracer for such purposes and has been used in the past. This paper describes the absolute standardisation of 225Th by 47pi alpha-gamma coincidence counting. The results of this work are presented together with an analysis of the standardisation uncertainties that were achieved in practice.

Standardisation and decay data of 186Re

A solution of 186Re was standardised using the 4pi(PC)-gamma-coincidence technique. Two gamma channels were employed to provide separate detection efficiency data arising from the two decay modes (beta and electron capture): the resulting data were analysed using a two-dimensional extrapolation. Calibration factors for a high pressure re-entrant ionisation chamber as well as for the NPL secondary standard radionuclide calibrator were also determined. An efficiency-calibrated germanium spectrometer was used to measure the emission probabilities of the X- and gamma-rays.

A second intercomparison of 222Rn measurement systems in European laboratories

Abstract A recent intercomparison exercise (established under the auspices of EUROMET) involving seventeen laboratories was conducted by NPL, who generated and standardised a set of glass-encapsulated 222Rn samples and then dispatched a unique subset of the samples to each participant. The reported measured activities of the samples were normalised to the NPL values. The spread of results, for the laboratories who had participated in an earlier exercise, was of the order of ± 5%; the results of 12 of the participants agreed at the 1 σ level.

Compact radioactive aerosol monitoring device for early warning networks.

In the frame of the European metrological research project MetroERM, a compact portable aerosol sampling and measurement device was developed at Jožef Stefan Institute. The system incorporates a CeBr3 scintillation detector positioned centrally within a concertinaed filter assembly and an improved high flow rate air pump. It provides continuous on-line low level airborne radioactive particulate monitoring for field station use via 3G network communications. The calibration of the device was performed at National Physical Laboratory (NPL) with filters, spiked with a certified mixed nuclide solution. Additionally first tests were performed in an environment with an elevated radon concentration.