National Physical Laboratory Research Articles

Preface

The NEWRAD Conferences bring together participants from National Metrology Institutes (NMIs) and key user communities in advanced radiometry. The 15th International Conference on New Developments and Applications in Optical Radiometry (NEWRAD 2023) was held at the National Physical Laboratory (NPL) in Teddington, UK, from 11th to 15th September 2023. The confernce was organised by NPL’s Optical Radiometric Metrology (ORM) group, who also handled abstract submissions and website administration. The NEWRAD 2023 conference saw 143 registered participants, with a total of 129 papers presented. Among these, 41 were oral presentations, complemented by 7 invited talks. Additionally, 88 posters were showcased during breaks and dedicated sessions, sparking lively discussions and idea exchange. The NEWRAD 2023 conference proceedings comprise contributions from the conference presentations. The submitted papers underwent peer review following the procedures of the Journal of Physics: Conference Series. I wish to thank the diligent reviewers and the dedicated editorial staff at the Journal of Physics: Conference Series for their meticulous work in facilitating the publication of these proceedings. I also wish to thank Nicole George, Dan Moore, Roger Hughes, Paris Aitken-Smith, and the entire organizing teams at NPL for hosting an exceptional conference. Finally, I want to express sincere appreciation to the NEWRAD scientific committee, who diligently reviewed and selected all the abstracts submitted to the conference, and provided unwavering support in organizing this event. Undoubtedly, this issue will serve as a valuable reference for radiometry. The venue of the next NEWRAD Conference is being organised and will be announced very shortly. List of NEWRAD scientific committee member is available in this Pdf.

Peer Review Statement

All papers published in this volume have been reviewed through processes administered by the Editors. Reviews were conducted by expert referees to the professional and scientific standards expected of a proceedings journal published by IOP Publishing. • Type of peer review: Single Anonymous • Conference submission management system: Morressier • Number of submissions received: 20 • Number of submissions sent for review: 19 • Number of submissions accepted: 19 • Acceptance Rate (Submissions Accepted / Submissions Received × 100): 95 • Average number of reviews per paper: 1.9 • Total number of reviewers involved: 23 • Contact person for queries: Name: Martin Dury Email: martin.dury@npl.co.uk Affiliation: National Physical Laboratory

Warfare and wind tunnel: engineers, physicists, and the evolution of combat aircraft 1914-1918

Abstract The British aircraft industry was transformed across the course of the First World War. Annual output grew exponentially, while by 1917-1918 the intensity of wartime research and development saw technologically advanced machines seeking air supremacy in all theatres of operation, most notably the Western Front. Capability had been inhibited by the absence of reliable, high performance power units, but ultimately production and procurement problems were resolved. This heroic narrative warrants qualification given the high ratio of poor to outstanding aircraft and the reality that in August 1914 Britain was by no means wholly unequipped to wage war in the air: the development programmes of the National Physical Laboratory and Royal Aircraft Factory, and the Royal Naval Air Service’s collaboration with science-based companies such as Short Brothers run counter to a popular belief that machines were still crudely built and militarily ineffective. There existed a working partnership between pioneer engineers and designers, schooled in applied science at metropolitan technical colleges, and graduate physicists and mathematicians encouraged by far-sighted dons to join a fledgling but fast-growing industry. That fusion of theory and practice, academic and industrial, was consolidated once British science’s human assets were mobilised to wage ‘industrial war’. Yes, aviation saw huge changes across the War, but the continuity is striking: before 1914 designers and air frame manufacturers were drawn from both the shop floor and the varsity, a wartime infusion of research scientists generated remarkable advances in aeronautics and in consequence a vital extension of Allied air power; and after 1918 companies survived retrenchment thanks to a durable combination of the entrepreneurial engineer and the experimental analyst.

Ensuring the Global Comparability of XPS Data Through Intensity Scale Calibration: Summary of ISO 5861:2024

ABSTRACTFor quantitative analyses of data acquired from x‐ray photoelectron spectroscopy (XPS) instruments to be globally comparable across industry and academia, a consistent and traceable intensity calibration method is required. ISO 5861 was prepared by Technical Committee ISO/TC 201 Surface Chemical Analysis, Subcommittee 7, Electron Spectroscopies, and describes an intensity calibration method for x‐ray photoelectron spectrometers that use quartz‐crystal monochromated Al Ka x‐rays. The method employs low‐density polyethylene reference spectra that are corrected for any instrument geometry and are traceable to the true reference spectra for gold, silver and copper held by the National Physical Laboratory (NPL), UK. This international standard generates a spectrometer relative response function for use in quantitative XPS analysis that is consistent with NPL's intensity calibration method to within 5% relative error.

National radiotherapy dosimetry audit in the UK – A vision and roadmap

Radiotherapy dosimetry audit is an important element in the overall quality assurance of radiotherapy delivery and has become established as best practice prior to implementation of new equipment, techniques or clinical trials.The Institute of Physics and Engineering in Medicine (IPEM) Interdepartmental Dosimetry Audit group (IDA) facilitates regional audits across the UK through a network of 8 regional chairs. This is a well-established and successful network utilising local staff resources and dosimetry equipment from each region to conduct audits for new radiotherapy equipment, techniques and annual reference audits. All dosimeters used are traceable to the National Physical Laboratory's (NPL) primary standard.NPL and the National Radiotherapy Trials Quality Assurance Group (RTTQA) perform audits for new techniques and clinical trials in the UK. The IPEM IDA, NPL and RTTQA have a joint vision of a national audit network so that comprehensive audits can be offered to all UK centres and bring together all UK dosimetry audit data within a centralised database to facilitate coordinated national audits. This will also allow centres to access and review their data for anonymous benchmarking against other UK centres to support quality improvement.Increasing the frequency and complexity of national audits to reflect current clinical practice is often inhibited by the cost of purchasing appropriate equipment. IPEM have now funded two phantoms that will enable national audits of gynae brachytherapy and head and neck external beam treatments through 2024–25.The aim of this Position Paper is to provide an update on the activities of the IPEM IDA and present the future vision and roadmap for the three UK dosimetry audit groups.

Traceable characterisation of fibre-coupled single-photon detectors

Abstract The detection of single photons plays an essential role in advancing single-photon science and technologies. Yet, within the visible/near-infrared spectral region, accurate fibre-based optical power measurements at the few-photon level are not yet well-established. In this study, we report on a fibre-based setup, enabling traceable optical power measurements at the few-photon level in this spectral region. The setup was used to calibrate the detection efficiency (DE) of four single-photon avalanche diode (SPAD) detectors. The relative standard uncertainties on the mean DE values obtained from repeat fibre-to-detector couplings ranged from 0.67% to 0.81% (k = 2). However, the relative standard deviation of DE values, which ranged from 1.38% to 3.20% (k = 2), poses a challenge for the metrology of these devices and applications that require high accuracy and repeatability. We investigated the source of these variations by spatially mapping the response of a detector’s fibre connector port, using a focused free-space beam, allowing us to estimate the detector’s spatial non-uniformity. In addition, we realise a novel calibration approach for fibre-coupled SPADs in a free-space configuration, enabling a direct comparison between the fibre-based setup and the National Physical Laboratory’s established free-space facility using a single SPAD. Finally, we investigated alternative coupling methods, testing the repeatability of different fibre-to-fibre connectors in addition to direct fibre-to-detector couplings: SPADs from three manufacturers were tested, with both single-mode and multi-mode fibre.

One-Port Coaxial Line Sample Holder Characterisation Method of Dielectric Spectra.

A technique for solving the one-port closed coaxial transmission line sample holder scattering equation for complex permittivity inversion for lossy materials is presented. A non-linear least-squares procedure is used for the determination of parameters for the specification of the spectral functional form of the complex permittivity. The method allows for accurate retrieval of many low- and high-permittivity dielectric materials in the frequency range of 1 GHz to 3 GHz inserted into the coaxial cell. Using this method, the complex permittivity of a number of liquids and a Maltese soil known as Bajjad soil have been extracted by measurements using a short terminated coaxial transmission line sample holder. The proposed novel inversion method is mainly based on the reflection coefficient of the test material. The measured results of the complex permittivity of liquid dielectrics such as ethanol, methanol, and TX100 are validated and compared with previously published data obtained from measurements made by the National Physical Laboratory (NPL) using a two-port measurement setup made with the same commercial coaxial transmission line sample holder used in the one-port setup. Since the technique allows broadband measurements, it has been used to characterise the soil dielectric spectrum in the frequency range of 1-3 GHz, which is also compared with results from a two-port setup of the same coaxial line. The experimental results are a validation of the proposed approach for different types of materials.

An assessment of the ageing of the absolute spectral responsivity of trap detectors at krypton ion laser emission wavelengths

We have assessed the absolute spectral responsivities of three-element trap detectors calibrated against the National Physical Laboratory primary standard cryogenic radiometer at eight krypton ion laser emission wavelengths over an approximate period of 20 years. As expected, we observed that the absolute spectral responsivities decrease with time. Additionally, we found that as the wavelength decreases, the ageing rate increases. At 356.4 nm, an average responsivity decrease of around −1800 ppm yr−1 is reported. While at longer wavelengths, the slowest rate of ageing, approximately −40 ppm yr−1, occurs at 799.3 nm. Interestingly, the year-on-year changes exhibit a relatively linear trend, allowing us to predict the future performance of the trap detectors. It is important to note, however, that predicted performance cannot be guaranteed and regular recalibration against a primary standard is essential, especially if low calibration uncertainties are required.

171Yb+ optical clock with 2.2×10−18 systematic uncertainty and absolute frequency measurements

A full evaluation of the uncertainty budget for the ytterbium ion optical clock at the National Physical Laboratory (NPL) was performed on the electric octupole (E3) 2S1/2→2F7/2 transition. The total systematic frequency shift was measured with a fractional standard systematic uncertainty of 2.2×10−18 . Furthermore, the absolute frequency of the E3 transition of the 171Yb+ ion was measured between 2019 and 2023 via a link to International Atomic Time (TAI) and against the local caesium fountain NPL-CsF2. The absolute frequencies were measured with fractional standard uncertainties between 3.7×10−16 and 1.1×10−15 , and all were in agreement with the 2021 BIPM recommended frequency.

RF and microwave metrology for quantum computing – recent developments at the UK’s National Physical Laboratory

Abstract Development of large-scale quantum computing systems will require radio frequency (RF) and microwave technologies operating reliably at cryogenic temperatures down to tens of milli-Kelvin (mK). The quantum bits in the most promising quantum computing technologies such as the superconducting quantum computing are designed using principles of microwave engineering and operated using microwave signals. The control, readout, and coupling of qubits are implemented using a network of microwave components operating at various temperature stages. To ensure reliable operation of quantum computing systems, it is critical to ensure optimal performance of these microwave components and qubits at their respective operating temperatures, which can be as low as mK temperatures. It is, therefore, critical to understand the microwave characteristics of waveforms, components, circuits, networks, and systems at cryogenic temperatures. The UK’s National Physical Laboratory (NPL) is focussed on developing new microwave measurement capabilities through the UK’s National Quantum Technologies Programme to address various microwave test and measurement challenges in quantum computing. This includes the development of various measurement capabilities to characterize the microwave performance of quantum and microwave devices and substrate materials at cryogenic temperatures. This paper summarizes the roadmap of activities at NPL to address these microwave metrology challenges in quantum computing.

LIME: Lunar Irradiance Model of ESA, a new tool for absolute radiometric calibration using the Moon

Abstract. Absolute calibration of Earth observation (EO) sensors is key to ensuring long-term stability and interoperability, and it is essential for long-term global climate records and forecasts. The Moon provides a photometrically stable calibration source within the range of the Earth's radiometric levels and is free from atmospheric interference. However, to use this ideal calibration source, one must model the variation in its disc-integrated irradiance resulting from changes in Sun–Earth–Moon geometries. The Lunar Irradiance Model of the European Space Agency (LIME) is a new lunar irradiance model developed from ground-based observations acquired using a lunar radiometer operating from the Izaña Atmospheric Observatory near Mount Teide, located in Tenerife, Spain. Nightly top-of-atmosphere (TOA) irradiance is determined using the Langley plot method, and each observation is traceable to the international system of units (SI) through the radiometer calibration performed at the National Physical Laboratory (NPL). Approximately 590 lunar observations acquired between March 2018 and December 2022 currently contribute to the model parameter derivation, which builds on the widely used ROLO (Robotic Lunar Observatory) model analytical formulation. This paper presents the strategy used to derive LIME parameters: the characterisation of the lunar radiometer, the derivation of nightly top-of-atmosphere lunar irradiance and a description of the model parameter derivation, along with the associated metrologically rigorous uncertainty. The model output has been compared to PROBA-V, Pléiades and Sentinel-3B, as well as to the VITO implementation of the ROLO model. Initial results indicate that LIME predicts 3 %–5 % higher lunar-disc-integrated irradiance than the ROLO model for the visible and near-infrared channels. The model output has an expanded (k=2) radiometric uncertainty of ∼ 2 % at the lunar radiometer wavelengths, and it is expected that planned observations until at least 2024 further constrain the model parameters in subsequent updates.

2022 CEOS International Thermal Infrared Radiometer Comparison. Part I: Laboratory Comparison of Radiometers and Blackbodies

Abstract An international comparison of field deployed radiometers for sea surface skin temperature (SSTskin) retrieval was conducted in June 2022. The campaign comprised a laboratory comparison and a field comparison. In the laboratory part, the radiometers were compared with reference standard blackbodies, while the same was done with the blackbodies used for the calibration of the radiometers against a transfer standard radiometer. Reference values were provided by the National Physical Laboratory (NPL), traceable to the primary standard on the International Temperature Scale of 1990. This was followed by the field comparison at a seaside pier on the south coast of England, where the radiometers were compared against each other while viewing the closely adjacent surface of the sea. This paper reports the results of the laboratory comparison of radiometers and blackbodies. For the blackbody comparison, the brightness temperature of the blackbody reported by the participants agreed with the reference value measured by the NPL transfer standard radiometer within the uncertainties for all temperatures and for all blackbodies. For the radiometer comparison, the temperature range of most interest from the SSTskin retrieval point of view is 10°–30°C, and in this temperature range, and up to the maximum comparison temperature of 50°C, all participants’ reported results were in agreement with the reference. On the other hand, below 0°C the reported values showed divergence from the reference and the differences exceeded the uncertainties. The divergence shows there is room for improvement in uncertainty estimation at lower temperatures, although it will have limited implication in the SSTskin retrieval.

Ray Freeman. 6 January 1932—1 May 2022

Ray Freeman pioneered many of the important advances in the technique of liquid-state nuclear magnetic resonance spectroscopy: he changed fundamentally the ways in which chemists and structural biologists use NMR. Born in Nottingham, he was an undergraduate chemistry student at Oxford after National Service working with radar, and completed his Part II and doctoral research with Rex Richards (FRS 1959). After brief spells with Anatole Abragam (ForMemRS 1983) in Saclay, France, and at the National Physical Laboratory in Teddington, he worked for 10 years in the research department of Varian Associates, Palo Alto, USA. He was appointed as lecturer in physical chemistry and tutorial fellow of Magdalen College, Oxford, in 1973, and subsequently promoted to Aldrichian praelector. In 1987 he was appointed to the John Humphrey Plummer chair at the University of Cambridge, where he remained active in research long after his retirement in 1999, publishing his last paper at the age of 84.

Interlaboratory comparison of two homemade pencil type ionization chambers

Computed tomography (CT) is a diagnostic exam responsible for the highest dose values received by the patients during the procedure. Over the years, this kind of equipment has been improved to ensure that the patients do not receive unnecessary doses. For this reason it is important to keep a quality control program for the CT equipment. To perform the dosimetry in CT beams, the instrument used is usually a pencil type ionization chamber with a sensitive volume length of 10 cm. However, this kind of detector is available with different sensitive volume lengths. The aim of this study was to compare the response of two homemade pencil type ionization chambers with sensitive volume lengths of 10 cm and 30 cm in the laboratories of Instituto de Pesquisas Energéticas e Nucleares (IPEN, Brazil) and National Physical Laboratory (NPL, UK). The characterization tests were performed, and the results obtained are within the international recommended limits. The only difference observed in the response of the two chambers in both laboratories is the fact that the ionization current obtained in all the tests at the IPEN is higher than at the NPL, because of the beam characteristics.

Final report, ongoing key comparison BIPM.QM-K1, ozone at ambient level, comparison with NPL, June 2022

Main textAs part of the ongoing key comparison BIPM.QM-K1, a comparison has been performed between the ozone national standard of United Kingdom maintained by the National Physical Laboratory (NPL) and the common reference standard of the key comparison, maintained by the Bureau International des Poids et Mesures (BIPM). The instruments have been compared over a nominal ozone amount fraction range of 0 nmol mol−1 to 500 nmol mol−1.To reach the main text of this paper, click on Final Report. Note that this text is that which appears in Appendix B of the BIPM key comparison database https://www.bipm.org/kcdb/.The final report has been peer-reviewed and approved for publication by the CCQM, according to the provisions of the CIPM Mutual Recognition Arrangement (CIPM MRA).

The National Physical Laboratory: Driving innovation with new quantum standards

The National Physical Laboratory: Driving innovation with new quantum standards

Recent metrological approaches relevant to the characterisation of hardmetals

Metrology provides the underpinning science that supports the practical use of testing standards; ensuring that different users, be it academia, government or industry, develop a common language regarding the capabilities of many materials sectors. For the hardmetal community, after a century of fundamental and effective applications, it isn't surprising that there are already a considerable number of formal International Standards. For example, ISO and ASTM are well known [1], and over time these have evolved, with international consensus, from metrological studies. Currently important properties of hardmetals such as Stiffness, Density, Hardness, Toughness, Abrasion Resistance, Macroscopic Strength and Phase and Magnetic properties are comprehensively supported by these international standards. However, there are other testing strategies where improved knowledge could contribute to enhanced carbide functionality. These might include high resolution microscopy, indentation strength measurements, micro-tribology, thermal fatigue testing and modelling in all its diverse approaches which are not yet addressed through standards, though they often feature strongly in published academic papers. The hardmetal research community is international, diverse and productive in addressing these many facets and frequently embed metrological studies that are relevant to the practical characterisation of hardmetals.In the UK the Materials Group at the National Physical Laboratory, NPL, has a prime remit to focus on metrology and our contribution to the WC-Co 100 year anniversary publication provides some specifics of typical metrological issues that need further attention before addressing the formal stage of standardisation. In this celebratory paper we have chosen examples of recent metrological activities that fall under this remit and which have only partly been contributed to the scientific published domain. These five examples are High Hardness measurements, Dislocation Analysis, Micro-Tribology, Nanoindentation and Structural Characterisation.

Proof-of-Principle of Absolute Dosimetry Using an Absorbed Dose Portable Calorimeter with Laser-Driven Proton Beams

Charged particle beams driven to ultra-high dose rates (UHDRs) have been shown to offer potential benefits for future clinical applications, particularly in the reduction of normal-tissue toxicity. Studies of the so-called FLASH effect have shown promise, generating huge interest in high dose rate radiation studies. With laser-driven proton beams, where the duration of the proton burst delivered to a sample can be as short as hundreds of picoseconds, the instantaneous dose rates are several orders of magnitude higher than those used for conventional radiotherapy. The dosimetry of these beam modalities is not trivial, with conventional active detectors, such as ionisation chambers, experiencing saturation effects making them unusable at the extremely high dose rates. Calorimeters, measuring the radiation-induced temperature rise in an absorber, offer an ideal candidate for the dosimetry of UHDR beams. However, their application in the measurement of laser-driven UHDR beams has so far not been trialled, and their effective suitability to work with the quasi-instantaneous and inhomogeneous dose deposition patterns and the harsh environment of a laser-plasma experiment has not been tested. The measurement of the absorbed dose of laser-driven proton beams was conducted in a first-of-its-kind investigation, employing the VULCAN-PW laser system of the Central Laser Facility (CLF) at the Rutherford Appleton Laboratory (RAL), using a small-body portable graphite calorimeter (SPGC) developed at the National Physical Laboratory (NPL) and radiochromic films. A small number of shots were recorded, with the corresponding absorbed dose measurements resulting from the induced temperature rise. The effect of the electromagnetic pulse (EMP) generated during laser–target interaction was assessed on the system, showing no significant effects on the derived signal-to-noise ratio. These proof-of-principle tests highlight the ability of calorimetry techniques to measure the absorbed dose for laser-driven proton beams.

A novel radiometric calibration of an infrared thermometer

A new temperature calibration scheme was proposed and implemented on the National Physical Laboratory infrared radiometer Absolute Measurements of Blackbody Emitted Radiance (AMBER) operating in the wavelength range from 9 µm to 11 µm and equipped with a cryogenically cooled mercury cadmium telluride detector. The scheme involves measurements of the relative spectral responsivity of the filter radiometer including the spectral transmittance of the window and lens. It requires calibration at two reference temperatures, but does not involve measurement of the zero-radiance signal. The scheme uses a Ga-point blackbody as one reference point, and a variable temperature blackbody source at close to −30 °C with its temperature measured with a standard platinum resistance thermometer as the other. The measured temperature was compared using the variable temperature blackbody as the source against the temperature measured by the standard platinum resistance thermometer and agreement within the uncertainties was confirmed at intermediate temperatures. The scheme has the advantage that it makes full use of the small uncertainty realised by the fixed-point blackbody. The scheme was successfully implemented in an international comparison of sea surface temperature measuring radiometers where AMBER served to provide the reference.

Quantitative validation of Monte Carlo SPECT simulation: application to a Mediso AnyScan GATE simulation

BackgroundMonte Carlo (MC) simulations are used in nuclear medicine imaging as they provide unparalleled insight into processes that are not directly experimentally measurable, such as scatter and attenuation in an acquisition. Whilst MC is often used to provide a ‘ground-truth’, this is only the case if the simulation is fully validated against experimental data. This work presents a quantitative validation for a MC simulation of a single-photon emission computed tomography (SPECT) system.MethodsAn MC simulation model of the Mediso AnyScan SCP SPECT system installed at the UK National Physical Laboratory was developed in the GATE (Geant4 Application for Tomographic Emission) toolkit. Components of the detector head and two collimator configurations were modelled according to technical specifications and physical measurements. Experimental detection efficiency measurements were collected for a range of energies, permitting an energy-dependent intrinsic camera efficiency correction function to be determined and applied to the simulation on an event-by-event basis. Experimental data were collected in a range of geometries with ^{99text {m}}Tc for comparison to simulation. The procedure was then repeated with ^{177}Lu to determine how the validation extended to another isotope and set of collimators.ResultsThe simulation’s spatial resolution, sensitivity, energy spectra and the projection images were compared with experimental measurements. The simulation and experimental uncertainties were determined and propagated to all calculations, permitting the quantitative agreement between simulated and experimental SPECT acquisitions to be determined. Statistical agreement was seen in sinograms and projection images of both ^{99text {m}}Tc and ^{177}Lu data. Average simulated and experimental sensitivity ratios of (0.991 pm 0.011) were seen for emission and scatter windows of ^{99text {m}}Tc, and (0.897 pm 0.014) and (0.839 pm 0.014) for the 113 and 208 keV emissions of ^{177}Lu, respectively.ConclusionsMC simulations will always be an approximation of a physical system and the level of agreement should be assessed. A validation method is presented to quantify the level of agreement between a simulation model and a physical SPECT system.