Table of contents

The International Committee for Weights and Measures (CIPM) has consultative committees covering various areas of metrology. The Consultative Committee for Ionizing Radiation (CCRI) differs from the others in having three sections: Section (I) deals with radiation dosimetry, Section (II) with radionuclide metrology and Section (III) with neutron metrology. In 2003 a proposal was made to publish special issues of Metrologia covering the work of the three Sections. Section (II) was the first to complete their task, and their special issue was published in 2007, volume 44(4). This was followed in 2009 by the special issue on radiation dosimetry, volume 46(2). The present issue, volume 48(6), completes the trilogy and attempts to explain neutron metrology, the youngest of the three disciplines, the neutron only having been discovered in 1932, to a wider audience and to highlight the relevance and importance of this field.

When originally approached with the idea of this special issue, Section (III) immediately saw the value of a publication specifically on neutron metrology. It is a topic area where papers tend to be scattered throughout the literature in journals covering, for example, nuclear instrumentation, radiation protection or radiation measurements in general. Review articles tend to be few. People new to the field often ask for an introduction to the various topics. There are some excellent older textbooks, but these are now becoming obsolete. More experienced workers in specific areas of neutron metrology can find it difficult to know the latest position in related areas. The papers in this issue attempt, without presenting a purely historical outline, to describe the field in a sufficiently logical way to provide the novice with a clear introduction, while being sufficiently up-to-date to provide the more experienced reader with the latest scientific developments in the different topic areas.

Neutron radiation fields obviously occur throughout the nuclear industry, from the initial fuel enrichment and fabrication processes right through to storage or reprocessing, and neutron metrology is clearly important in this area. Neutron fields do, however, occur in other areas, for example where neutron sources are used in oil well logging and moisture measurements. They also occur around high energy accelerators, including photon linear accelerators used for cancer therapy, and are expected to be a more serious problem around the new hadron radiation therapy facilities. Roughly 50% of the cosmic ray doses experienced by fliers at the flight altitudes of commercial aircraft are due to neutrons. Current research on fusion presents neutron metrology with a whole new range of challenges because of the very high fluences expected.

One of the most significant features of neutron fields is the very wide range of possible neutron energies. In the nuclear industry, for example, neutrons occur with energies from those of thermal neutrons at a few meV to the upper end of the fission spectrum at perhaps 10 MeV. For cosmic ray dosimetry the energy range extends into the GeV region. This enormous range sets a challenge for designing measuring devices and a parallel challenge of developing measurement standards for characterizing these devices.

One of the major considerations when deciding on topics for this special issue was agreeing on what not to include. Modelling, i.e. the use of radiation transport codes, is now a very important aspect of neutron measurements. These calculations are vital for shielding and for instrument design; nevertheless, the topic has only been included here where it has a direct bearing on metrology and the development of standards. Neutron spectrometry is an increasingly important technique for unravelling some of the problems of dose equivalent measurements and for plasma diagnostics in fusion research. However, this topic is at least one step removed from primary metrology and so it was felt that it should not be covered, particularly as a compendium of papers on spectrometry for radiation protection has been published relatively recently [1]. The CIPM Mutual Recognition Arrangement (CIPM MRA), whereby national measurement standards and certificates issued by different national metrology institutes (NMIs) can be recognized internationally, is covered only briefly, although the key comparisons which underpin the CIPM MRA are highlighted.

The papers included in this issue concentrate on the primary physical quantities—neutron source emission rate and neutron fluence, papers on the latter quantity covering the wide range of neutron energies for which standards are required. Neutron cross sections are fundamental to neutron physics and their importance in neutron metrology is also covered.

A large amount of work by acknowledged experts in neutron metrology has gone into the preparation of this special issue and we are indebted to them for their time and effort. The list of contributors begins with the authors of the papers but also includes the referees who provided invisible but invaluable input. We are grateful for the support and encouragement of Professor Georgio Moscati, president of the CCRI when the work was proposed, Dr Kim Carneiro the current president, and Dr Penny Allisy-Roberts the executive secretary of the CCRI.

When this work was first proposed a list of potential topics was drawn up by the then chairman of Section (III) Dr Horst Klein. It is a measure of his insight and knowledge of the field that the resulting document matches almost exactly the original plan he drew up. This special issue is thus a tribute to his very extensive contribution to the field. We sincerely hope its contents provide an accurate picture of the present state of neutron metrology in view of Dr Klein's conviction of the importance in metrology of getting things right.

Reference

[1] Thomas D J and Klein H (ed) 2003 Neutron and photon spectrometry techniques for radiation protection Radiat. Prot. Dosim.107 1–204

In some branches of metrology the quantity, or quantities, to be measured and the requirements for standards of this quantity, or these quantities, are clear-cut, although the accuracies required may not always be obvious. Good examples are time, length and mass. The quantities measured in neutron metrology and the requirements and uses of these quantities are less well known. This paper lays the foundation for subsequent more detailed discussions of the problems and issues faced by national metrology institutes when developing and providing standards.

Radionuclide neutron sources are of fundamental importance to neutron metrology. They are the reference standards which allow quantities such as neutron fluence and neutron dose equivalent to be realized in laboratories around the world. In order to do this it is essential that the sources themselves are well characterized in terms of neutron emission rate, emission anisotropy and spectral fluence Φ_E of emitted neutrons. A summary of the types of radionuclide neutron sources is given followed by detailed descriptions of the primary standardization techniques in use today, focusing on the manganese bath technique. Methods for determining the anisotropy of emission are discussed, followed by a brief description of the spectral fluence of the different source types. Finally, an overview of international comparison exercises in this area is included.

Over the 50-year history of Section III (Neutron Measurements) of the Consultative Committee for Ionizing Radiation of the International Committee for Weights and Measures, only a single interlaboratory comparison of thermal neutron measurements has been completed, although a new comparison of this kind is in progress at the time of this writing. The initial comparison was perhaps considered to have demonstrated a sufficient degree of agreement that its repetition was not of particularly urgent priority. Thermal neutron measurements are indeed an old and well-developed field, as is seen in the many references cited in this paper. This paper describes briefly the production of thermal neutron fields from fast neutron sources, the moderation of neutron spectra towards thermal equilibrium, various conventions for describing thermal neutron fields, corrections for self-absorption and scattering in detector materials and the selection of detectors based on established thermal neutron cross-section standards.

Monoenergetic neutron reference fields are required for the investigation of the energy-dependent response and the calibration of neutron detectors and dosemeters for various applications ranging from nuclear physics and nuclear data measurements to radiation protection. In a series of two separate publications the metrological aspects of the production and the measurement of fast neutrons are reviewed. In this first part, requirements for the nuclear reaction used to produce the neutron field as well as methods for target characterization and the general layout of reference facilities are discussed. The second part will focus on the most important techniques for field characterization. This includes the determination of the neutron fluence as well as the spectral neutron distribution and the determination of the fluence of contaminating photons.

Monoenergetic neutron reference fields are required for the calibration of neutron detectors and dosemeters for various applications ranging from nuclear physics and nuclear data measurements to radiation protection. In a series of two separate publications the metrological aspects of the production and measurement of fast neutrons are reviewed. In the first part, requirements for the nuclear reactions used to produce neutron fields as well as methods for target characterization and the general layout of reference facilities were discussed. This second part focuses on the most important techniques for field characterization and includes the determination of the neutron fluence as well as the spectral neutron distribution and the determination of the fluence of contaminating photons. The measurements are usually carried out relative to reference cross sections which are reviewed in a separate contribution, but for certain conditions 'absolute' methods for neutron measurements can be used which are directly traceable to the international system of units (SI).

This paper provides an overview of high-energy quasi-monoenergetic neutron sources and facilities above 20 MeV around the world. Various technical matters are discussed which are required in characterizing the neutron fields by spectrometry, fluence and beam profile measurements. Important topics regarding the calibration of neutron detectors are also introduced with emphasis on beam monitoring, tail correction, background subtraction and fluence-to-dose conversion. Efforts to standardize the high-energy neutron fluence in Japan and by the German national metrology institute in collaboration with Belgian and South African institutions are also presented.

The use of simulated workplace neutron fields, which aim at replicating radiation fields at practical workplaces, is an alternative solution for the calibration of neutron dosemeters. They offer more appropriate calibration coefficients when the mean fluence-to-dose equivalent conversion coefficients of the simulated and practical fields are comparable. Intensive Monte Carlo modelling work has become quite indispensable for the design and/or the characterization of the produced mixed neutron/photon fields, and the use of Bonner sphere systems and proton recoil spectrometers is also mandatory for a reliable experimental determination of the neutron fluence energy distribution over the whole energy range. The establishment of a calibration capability with a simulated workplace neutron field is not an easy task; to date only few facilities are available as standard calibration fields.

The calibration of a neutron-sensitive device can range from a simple calibration factor at a single energy or energy distribution to a full response characterization over the entire energy range to which the device is sensitive. As the responses of neutron-sensitive devices and the fluence-to-dose-equivalent conversion coefficients can vary with neutron energy and incident angle, both simulation and experiments in standard neutron fields are required. Although several ISO standards present calibration principles in general and detailed discussion on many specific areas, there are certain omissions and limitations that this paper intends to highlight, along with some new recommendations derived from the recent literature, mainly focused on the effective centre, corrections for geometry and neutron scattering, as well as the problem of calibrating in terms of personal dose equivalent.

The measurement of a neutron cross section can be simplified if it is measured relative to a neutron cross section standard. Then it is not necessary to measure the neutron fluence. The standards in effect provide determinations of the neutron fluence. These standards are not defined standards but instead working standards. As such it is important to make measurements to improve the quality of the standards. Also better techniques for evaluating the standards must be promoted. The history of the evaluation process for the standards is discussed from the first rather primitive one to the latest very sophisticated evaluation that was an international cooperative effort. An overview of the main detectors used for applications of the standards is given.

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