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Direct measurements and improved calculation techniques for nuclear decay

Since the early 20th century, metrologists have supported the nuclear industry by providing primary radioactivity standardisation. Today, radioactivity measurements can reliably be delivered to just a few parts per thousand for most types of decay using standard instruments. However, for other types — specifically low-energy beta and electron capture decay — uncertainties of several percent are typical, due to the low detection efficiency of low-energy decay and imprecise data analysis methods. For example, nuclides such as the α-decaying Americium-241 (²⁴¹Am), show better achievable uncertainty compared with others such as Iron-55 (⁵⁵Fe), which decays by low-energy electron capture. Americium-241 and Iron-55 standards are frequently used as reference sources for gamma-ray and X-ray spectrometers as well as for contamination monitors. In addition, they have various industrial applications. The list of radionuclides requiring improved decay data is extensive and also includes isotopes that are highly relevant in fields such as nuclear medicine and Earth sciences.

A type of low temperature calorimeter has been developed, called a magnetic microcalorimeter (MMC). These are capable of low energy detection with outstanding high energy resolution and are applicable to all types of radioactive decay. However, until the work of this project these devices were custom-made, so unsuitable for activity standardisation.

Completed EMPIR project Towards new primary activity standardisation methods based on low-temperature detectors (20FUN04, PrimA-LTD) developed new magnetic microcalorimeter detectors and associated source preparation techniques, and their capability for ultra-high-resolution spectrometry was experimentally demonstrated. In addition, theoretical calculation methods for beta decay and complex electron-capture processes were significantly improved.

Project outputs:

• Good practice guide on the preparation of 4π radionuclide sources containing α emitter (²⁴¹Am) for Low Temperature Detector measurements.

This good practice guide describes how to prepare radioactive samples with known activities (Bq) of ²⁴¹Am to be measured by low temperature detectors.

• High quality sources were prepared by ion implantation of ⁵⁵Fe which yielded unprecedented energy resolution when measured with newly developed, custom-designed MMCs.
• Energy spectra of ⁵⁵Fe and Iodine-129 (¹²⁹I) were measured and compared to new extended theoretical calculations which allow for nuclear structure effects and atomic effects. These experimental and theoretical improvements were combined to develop a specific analysis method of measured beta spectra. Applied to Samarium-151 (¹⁵¹Sm) spectra, fundamental data, such as the transition energy and branching ratio of the decay were determined with an unexpected, excellent precision, as described in a peer-reviewed publication. A first comparison of preliminary spectra of ¹²⁹I and ⁵⁵Fe confirms that the methodology developed within the project gives new insights for the understanding of decay spectra.
• New MMC detectors were designed and fabricated, which are adapted for the nuclides of interest within this project: this includes detectors for direct ion implantation and detectors that can be reused for further measurements. Dedicated MMC detectors optimised for direct ion implantation of ⁵⁵Fe were designed and manufactured. In addition, a microfabrication process to enclose the implanted material in a 4π geometry was developed and successfully applied. First results from MMC measurements confirm the superior quality of those sources in terms of energy resolution and thermalisation of the decay energy.
• New absorber preparation techniques were developed using microfabrication techniques that increase the energy resolution of measurements. This includes porous gold and silver nanofoams – applicable to Iodine radioisotopes – and electroplating of ²⁴¹Am on gold foils. It was shown that these sources can be standardised using MMCs as well as with established techniques. Significant progress was made for the analysis of MMC data of multichannel systems aiming at activity determination which requires a thorough understanding of pile-up and deadtime. First activity determinations showed good agreement for established techniques and MMCs.

Benefits

Many users of radioactive materials will benefit from the improved nuclear decay data and resulting improvement in activity determination provided by this project.

The nuclear power industry uses decay data to determine the residual heat and its evolution with time in nuclear reactors and in nuclear waste management. For example, the average energy per ¹²⁹I decay depends on the shape of the beta spectrum and the maximum beta energy. The work of this project means that this data will be available with significantly improved accuracy.

The Environmental monitoring of radioactivity will be improved as a result of using decay data with smaller uncertainties.

Nuclear medicine will also benefit because, for example, the improved beta spectrum calculation methods from this project can be used. This plays a role in some radiopharmaceuticals that can be used for diagnosis or therapy. More accurate decay data allow a more accurate calculation of the dose per administered activity, whose determination will also become more accurate with the results of this project.

The newly available nuclide data are likely to be taken up by NMIs and DIs resulting in a better service and better-quality radioactive sources for commercial customers.

Project coordinator Ole Nähle from PTB said

‘The enormous potential of MMCs for high-precision determinations of radionuclide data has once again been highlighted, and this technology will become an indispensable tool in radionuclide metrology in the future.’

This EMPIR project is co-funded by the European Union's Horizon 2020 research and innovation programme and the EMPIR Participating States.

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