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Prototypes of a novel pocket-sized active neutron dosemeter, based on a sensor made from the scintillator material Cs2LiYCl6:Ce (CLYC) coupled to a silicon photomultiplier (SiPM), were exposed to several well-characterised neutron fields produced at the National Physical Laboratory (NPL), UK. 6Li-enriched CLYC is extremely interesting as a dosemeter sensor because it can detect not only gamma rays but also thermal and fast neutrons, gammas being distinguishable from neutrons by pulse shape discrimination. Thermal and low energy neutrons are detected by the 6Li(n, α)3H reaction, which gives rise to a well-defined peak in the pulse height spectrum. Fast neutrons typically interact via 35Cl(n, p) or (n, α) reactions, and the kinetic energy of the reaction product gives rise to a pulse height spectrum that relates fairly straightforwardly to the original neutron energy spectrum. Because of this spectrometry capability, such a dosemeter has the potential to retain its accuracy to a much greater extent, compared with conventional devices, when used in radiation fields that differ from the one used for calibration. SiPMs are low-power and compact, allowing the prototype dosemeters produced for testing to fit entirely within a standard existing personal dosemeter housing. Tests were carried out in the low-scatter neutron facility at NPL, with measurements made both on-phantom and free-in-air. The former were done to evaluate the device's performance as a personal dosemeter, and the latter to explore its potential as a very light neutron area survey meter. In this paper the experimental results are presented, performance issues encountered during the trials are discussed, and preferred application scenarios are proposed.

As part of its wide-ranging neutron metrology capabilities, the National Physical Laboratory in the UK has a thermal neutron facility in which accelerator-produced neutrons are moderated within a large assembly or pile of graphite bricks. The neutron field has previously been well characterised in terms of the fluence rate and energy spectrum at various irradiation positions. However, recent changes to the structure (enlarging the central irradiation cavity, for example) have prompted a renewal and extension of this work. We have also used Monte Carlo modelling to improve our understanding of the pile's performance.

The neutron sensor in an active neutron-sensitive personal dosemeter typically comprises a planar silicon diode topped with a converter layer. Neutrons interact in the converter to produce charged particles that are then detected in the diode. However, in the dosimetrically important energy region of a few keV to a few MeV, the efficiency of such devices is small because of the small cross section for producing the charged particles and their very limited range within the converter. The efficiency can in principle be improved by etching fine structures such as trenches or wells in the silicon, and filling these with the converter, so that a considerably larger amount of converter is within one particle range of the silicon. This study quantifies the potential improvements in efficiency for a selection of structure sizes and fast monoenergetic neutron energies. It also investigates how seriously these improvements are degraded if there is a dead layer at the surface of the silicon.

New standard (n,f) cross sections other than 235U are important to study the relevant cross sections for Generation-IV power plants. A specific need for such standards is for performing new experiments with quasimonoenergetic neutron beams, such as those produced by Van de Graaf accelerators. Neutrons down-scattered to low energies in the experimental environment, so called room-return, become relevant for this type of measurements. Hence, a standard (n,f) cross section with a fission threshold is of great interest, in order to suppress the contribution from room-return background. For this reason we have performed two experiments at the VDG of the National Physical Laboratory to measure absolutely the (n,f) cross sections of 235U, 238U and 237Np in the fast neutron energy region. Our preliminary results are in agreement with the most up-to-date evaluations.

The response of neutron personal dosemeters as a function of neutron energy and angle of incidence is typically measured by mounting the dosemeters on a slab phantom and exposing them to neutrons from an accelerator-based or radionuclide source. The phantom is placed close to the source (75 cm) so that the effect of scattered neutrons is negligible. It is usual to mount several dosemeters on the phantom together. Because the source is close, the source distance and the neutron incidence angle vary significantly over the phantom face, and each dosemeter may receive a different dose equivalent. This is particularly important when the phantom is angled away from normal incidence. With accelerator-produced neutrons, the neutron energy and fluence vary with emission angle relative to the charged particle beam that produces the neutrons, contributing further to differences in dose equivalent, particularly when the phantom is located at other than the straight-ahead position (0° to the beam). Corrections for these effects are quantified and discussed in this article.

A sample of the new plastic scintillator recently formulated by Zaitseva et al., and shown by them to have efficient pulse shape discrimination properties, was exposed to pure gamma and mixed neutron/gamma radiation from different sources, and the pulse shapes recorded by a digitising oscilloscope. Separate average neutron and gamma pulse shapes were derived, using pulse shape discrimination where necessary. A theoretical pulse shape formula, based on the conventional two-lifetime description of pulse shape formation in organic scintillators, was then fitted to these measured shapes to obtain numerical values for the lifetimes and proportions of the scintillator excitations. It is found that, although the conventional model explains much of the observed pattern of values, there are hints of behaviour that is not consistent with its assumptions.

Cross section measurements in the fast energy region are being demanded as one of the key ingredients for modelling Generation-IV nuclear power plants. However, in facilities where there are no time-of-flight possibilities or it is not convenient to use them, using the 235U(n,f) cross section as a benchmark would require a careful knowledge of the room scatter in the experimental area. In this paper we present measurements of two threshold reactions, 238U(n,f) and 237Np(n,f), that could become a standard between their fission threshold and 2.5 MeV, if the discrepancies shown in the evaluations and in some experimental data can be solved. The preliminary results are in agreement with the present ENDF/B-VII.1 evaluation.

A comparison of four algorithms for the digital discrimination of neutrons and γ‐rays in organic scintillation detectors is described. The mixed fields from four different isotopic neutron sources have been investigated with a fast digital processing system. This system has been used to digitise the raw sampled data for each individual pulse collected for each of the mixed radiation fields. The discrimination methods considered are the digital Charge Comparison Method, Pulse Gradient Analysis, Neutron–γ Model Analysis and Simplified Digital Charge Collection. The ratio of the number of γ‐ray counts to the number of neutron counts, often referred to as the R-factor, has been calculated for each mixed radiation field. These provide a basis for reference where future R-factor analysis of isotopic sources is to be carried out for the purposes of calibrating discrimination systems or when estimating the γ‐ray rejection effectiveness of neutron detector systems based on fast liquid scintillators. The efficacy of each of these methods has been assessed in terms of the figure-of-merit (based on the separation of the event distributions). The SDCC technique is observed to provide the best discrimination performance in this research.

The discrimination of neutron and γ-ray events in an organic scintillator with the pulse gradient analysis (PGA) method is verified against that achieved via the digital measurement of time of flight (TOF). Events arising from the reaction have been detected with an LS-301 organic liquid scintillator and recorded with a fast digital sampling oscilloscope. Both qualitative (via the cross-referencing of scatter diagrams) and quantitative (via the neutron/γ-ray ratio) comparisons are reported. Discrimination afforded by the PGA method is observed to be consistent with that achieved by digital TOF.

The authors have measured the emission anisotropy from a (252)Cf spontaneous fission neutron source in an X1 encapsulation. The measurements were made in a large low-scatter laboratory using a long counter, and data were taken at angles varying in 10 degrees steps from 0 degrees to 180 degrees relative to the cylindrical axis of the source. Corrections were made for room scatter, loss of neutrons due to air scatter and detector dead time. Calculations corresponding to these measurements were subsequently carried out using the two Monte Carlo codes MCNP and MCBEND, and the results are compared with the measurements and with each other.

A fast digital sampler has been employed in place of conventional analogue electronics to measure the rise times and amplitudes of pulses from a hydrogen-filled spherical proportional counter of the SP2 type. The rise-time information was used to distinguish neutron events from gammas, and separated neutron events were recorded at energies below those that would have been possible with amplitude measurements alone. The electronics were considerably simpler than those required for an equivalent analogue system.

A selection of the 14-MeV neutron spectra obtained at the JET Joint Undertaking tokamak during the deuterium–tritium operating campaign in 1997 are presented and analyzed. While several neutron spectrometers were operational during this campaign, the present paper is concerned solely with one: the tandem annular-radiator proton-recoil spectrometer (or proton recoil telescope, for brevity). During neutral beam heating with combined d- and t-beams, analysis of the spectra can define the core fuel composition (D:T) ratio. The spectra are sensitive to the population balance of the fast ions streaming in directions parallel and opposite to that of the injected beams. During ICRF heating of minority deuterium in bulk tritium plasmas, the spectra provide measurements of the effective temperature of the fast-deuteron energy tail and of its relative strength, which vary with the deuterium concentration. This information contributes to the overall understanding of the fusion performance of the various operating scenarios.

The UK national standards laboratory, the National Physical Laboratory (NPL), provides an extensive range of facilities for measurements on neutron sources and neutron-sensitive devices. This paper describes recent improvements and extensions to the neutron standards. Correction factors used in source strength measurements have been improved, as have the scatter corrections used in monoenergetic and source-based irradiations. Characterisation of the thermal field facility has been improved by activation measurements. Experimental facilities developed recently include a system for measuring personal dosemeter responses at several energies simultaneously, and a neutron source tailored to simulate typical workplace fields.

All major JET systems have been fully commissioned for D-T and the DTE1 series of experiments has started with the D-T fuel mixture and operating conditions foreseen for ITER. In the area of ITER physics, significant results have been produced in both D-D and D-T. In D-D, the LH threshold power database has been extended, the bounds on edge-electron temperature and density in ELMy H-modes have been defined and the advantages of Types I and III ELMy discharges have been compared. In D-T plasmas, the isotope effect on H-mode threshold power and transport has been determined so that a more accurate assessment can be made of the ignition margin and heating requirements for ITER. Trace tritium experiments have provided first particle transport measurements and an assessment of the ITER reference ion-cyclotron resonance-frequency heating scenarios has been started, In the area of fusion performance, record D-D neutron yields have been obtained by controlling the plasma and current profiles in hot ion ELM-free H-modes and optimized shear modes. In D-T, internal transport barriers have been readily established in optimized shear discharges and Alfven eigenmodes have been observed.

Based upon JET experience, the measurement of multichannel collimated neutron fluxes and of the neutron spectrum gives time-dependent information on spatial profiles of neutron emission and alpha–particle birth, the total neutron emission (fuel burn-up rate), plasma position, effects of plasma instabilities, triton burn-up, ion temperature, and fuel densities. The design for a horizontally viewing neutron camera for ITER is based upon the prototype and upgrade versions of the JET neutron emission profile monitor and the JET spectrometers. It is proposed that vertically stacked modules are installed in the ITER biological shield in a fan shaped viewing geometry, aimed at a focal point located at the slit opening of a preshield designed to reduce the streaming neutron flux. Each module contains a pair of sight lines with adjustable collimation, allowing for multiple detector neutron flux monitoring and neutron spectroscopy over a wide operating range. The modular system allows flexibility in detector choice...

Hydrogen-filled proportional counters perform well as neutron spectrometers in the energy region from a few tens of keV up to ∼1.5 MeV. Unfortunately, gamma rays also generate signals in these detectors. It is possible in principle to distinguish the two types of event via the rise time of their respective signal pulses, but the data acquisition system needed for this is complex to assemble and adjust if one uses conventional modular analogue electronics. In this work a digital sampling system, in conjunction with custom software, was used to measure and acquire amplitude and rise time data from type SP2 counters. The interpretation of the data was supported by a Monte Carlo calculation. The performance of the system is compared with that of a conventional 1-parameter analogue system, and the potential of the digital technique to supplant conventional methods is discussed.

The JET neutral beam injection system has proved to be both effective and reliable as a plasma heating device. The ion heating and plasma fuelling characteristics of the system are ideally suited to the production of high fusion performance plasmas while the flexibility in the choice of beam species (H, D, T, 3 He or 4 He) and the ability to inject into almost any JET plasma configuration allows a wide variety of related physics experiments to be carried out. The capability to inject (for the first time) tritium beams was essential to the successful execution of the first tritium experiments in which 1.7 MW of power from DT fusion reactions was generated.

An account is given of the high performance plasmas established by development of the H-mode regime in JET in the experimental campaigns up to 1992. High performance in this case is measured in terms of the confinement enhancement achieved over the L-mode scaling as measured using the plasma diamagnetism. Three JET H-mode regimes have achieved enhancement factors (H G DIA ) over Goldston L-mode scaling of 2.5 < H G DIA < 4.0. These are the Pellet Enhanced Performance (PEP) H-MODE, the high bootstrap fraction (high β POL ) H-mode and the Hot Ion (HI) H-mode. The phenomenology of these three regimes is reviewed and contrasts and common threads are elucidated

The neutron spectrometry system described in this report is in routine use at the Joint European Torus (JET). Designed for the D-D phase of operations it consists of a complementary set of compact neutron spectrometers housed in a shielding block in the torus hall. The collimation system, which has an adjustable aperture, defines a horizontal line of sight with a significant directional component tangential to the plasma axis. The shielding, the spectrometers, and the auxiliary sensors and systems are described. The operating limits of the spectrometers, and the methods of data analysis, are discussed. Examples of analysed data are given.

A neutron detector capable of measuring the time‐dependent yield of 14‐MeV neutrons from a D–D plasma producing predominantly 2.5‐MeV neutrons has been developed. The detector consists of a thick polythene recoil proton radiator backed by a graphite foil attached to a large area totally depleted ion‐implanted diode. Protons scattered in the forward direction by 14‐MeV neutrons pass through the graphite foil and are registered in the diode. Recoil protons from 2.5‐MeV neutrons, however, are prevented from reaching the diode by the foil. When operated with a 1.5‐MeV energy bias, the measured neutron detection efficiency for 15‐MeV neutrons is 3.2×10−3 per neutron. The corresponding figure for 3.1‐MeV neutrons is a factor of 540 lower. The neutron detector and its laboratory calibration are described, as is its deployment at the Joint European Torus where it serves as a triton burn‐up monitor.

The JET neutron profile monitor was used to study the transport of tritium into a magnetically confined deuterium plasma. Trace amounts of tritium were introduced through a gas valve beyond the plasma edge into a variety of plasma confinement regimes. The d(t,n)α fusion cross section is two orders of magnitude greater than the d(d,n)3He cross section and so a puff of tritium which has a negligible effect on the plasma nevertheless produces a large d–t neutron signal. The profile monitor consists of two cameras each made up of a fan-shaped array of collimated lines of sight. It was used to measure the d–d and d–t neutron profiles simultaneously. This article describes the detection system, its operation and assesses the difficulties due to scattered neutrons. The profiles can be used to determine tritium density and transport coefficients.

Bonner sphere (BS) sets which use activation foils as the central thermal neutron sensor have advantages over active BS systems in certain environments, for example, pulsed fields, or fields with high photon components. In such environments, they may be the only type of neutron spectrometer which can be used. This paper describes work, using both measurements and calculations, to validate the response functions for a BS set based on gold activation foils. As an illustration of the use of such a system, a measurement is described of the contaminant neutron spectrum in the treatment room of a 21 MV hospital linear accelerator providing photon beams for radiotherapy.

A special class of ITER diagnostics consists of those which extend the primary or tokamak vacuum outside the cryostat wall. This diagnostic set consists of the x-ray crystal spectrometers (XCS) [1], the vacuum ultra-violet spectrometers (VUV) [2], the neutral particle analysers (NPA) [3] and the low-field-side microwave reflectometers [4]. Two XCS systems are included, a high resolution five radial channel array system (XCS-A) designed to view specific spectral lines for temperature and rotation velocity profiles and a survey instrument package (XCS-S) to monitor a broad spectrum for identifying impurity concentrations. The vacuum vessel rough-out line is also located at this port.

This paper presents the reference design of the radially viewing horizontal neutron profile monitor for ITER. It incorporates integral neutron shielding, and with full machine integration, should provide control and plasma diagnostic information based on the deduced neutron emission profile. Information can be obtained on total neutron emission (fuel burn-up rate), alpha-particle birth profile, plasma position and the time dependent effects of plasma instabilities such as sawteeth.

JET was extensively modified in the 1992/93 shutdown. The new pumped divertor and many new systems were brought into operation early in 1994. Operations have progressed to 4MA plasma current and, with substantial additional heating, H-mode confinement results confirm the expected scaling. The high power handling capability of the pumped divertor with sweeping is estimated at 20MW for 20s. H-mode plasmas have large Type I ELMs. With lower hybrid heating alone, 2MA full current drive has been achieved with good efficiency, with ICRF power, effective heating and direct electron heating have been demonstrated.

A unique, digital time pick-off method, known as sample-interpolation timing (SIT) is described. This method demonstrates the possibility of improved timing resolution for the digital measurement of time of flight compared with digital replica-analogue time pick-off methods for signals sampled at relatively low rates. Three analogue timing methods have been replicated in the digital domain (leading-edge, crossover and constant-fraction timing) for pulse data sampled at 8 GSa s−1. Events arising from the 7Li(p, n)7Be reaction have been detected with an EJ-301 organic liquid scintillator and recorded with a fast digital sampling oscilloscope. Sample-interpolation timing was developed solely for the digital domain and thus performs more efficiently on digital signals compared with analogue time pick-off methods replicated digitally, especially for fast signals that are sampled at rates that current affordable and portable devices can achieve. Sample interpolation can be applied to any analogue timing method replicated digitally and thus also has the potential to exploit the generic capabilities of analogue techniques with the benefits of operating in the digital domain. A threshold in sampling rate with respect to the signal pulse width is observed beyond which further improvements in timing resolution are not attained. This advance is relevant to many applications in which time-of-flight measurement is essential.

Analysing powers for d(n,n)d elastic scattering were measured to high precision at 14.5 MeV and were found to be 0.085+or-0.01 at 150 degrees and 0.185+or-0.013 at 120 degrees (centre of mass). On comparing these results with those for d(p,p)d elastic scattering, no significant difference was found at 150 degrees . At 120 degrees , firm conclusion cannot be drawn until the energy trend of nuclear analysing powers between 14 and 20 MeV is settled.

Two 3He ionisation chambers were used at the Joint European Torus (JET) to measure neutron energy spectra for discharges in which deuterium neutral beams were injected into deuterium plasmas. By comparing the measured spectra with theoretical calculations, the fraction of the total neutron yield due to thermonuclear reactions was deduced. Calculated thermonuclear fractions agree well with the experimental results.