Stephen David Monk

The generation of Bessel beams at millimetre-wave frequencies by use of an axicon

An axicon is used to generate a Bessel beam at 90 GHz in the millimetre-wave region of the spectrum. The Bessel beam has a central intensity maximum of approximately 4 mm in diameter that is maintained over a propagation distance greater than 60 mm

A field-portable, laser-diode spectrometer for the ultra-sensitive detection of hydrocarbon gases

We have developed a field-portable optical gas sensor for the ultra-sensitive detection of ethane. The system is based on an adaptation of a commercially available system, which uses a cryogenically cooled, lead-salt laser diode at 3.34 μm and a multi-pass astigmatic Herriott sample cell. We have adapted this system to a second derivative wavelength modulation scheme giving a lower detection limit of less than 100 parts per trillion for a one second measurement time. Our custom-designed software controls every aspect of the instrument operation from spectral scanning of the laser diode, to automatic calibration, optical alignment, spectral analysis and complete data logging.

A portable energy-sensitive cosmic neutron detection instrument

The construction and testing of a portable energy-sensitive neutron instrument are described. This instrument has been designed and constructed for the primary purpose of characterizing cosmic-ray neutron fields in the upper atmosphere and in cosmic reference field facilities. The instrument comprises a helium-3 proportional counter surrounded by 15 mm of lead and 140 mm of polyethylene creating a spherical structure with a diameter of 34 cm. The instrument also incorporates 12 boron-coated diodes, six on the outside of the polyethylene layer with six placed within the structure. The dimensions, materials, and arrangement of these in the instrument have previously been optimized with the MCNPX Monte Carlo simulation software to provide a compromise between the requirements of portability and spectral response. Testing took place at several locations and experimental data from the instruments operation at the high-altitude Jungfraujoch laboratory in the Swiss alps are presented.

Dynamic modelling and parameter estimation of a hydraulic robot manipulator using a multi-objective genetic algorithm

ABSTRACT This paper concerns the problem of dynamic modelling and parameter estimation for a seven degree of freedom hydraulic manipulator. The laboratory example is a dual–manipulator mobile robotic platform used for research into nuclear decommissioning. In contrast to earlier control model-orientated research using the same machine, the paper develops a nonlinear, mechanistic simulation model that can subsequently be used to investigate physically meaningful disturbances. The second contribution is to optimise the parameters of the new model, i.e. to determine reliable estimates of the physical parameters of a complex robotic arm which are not known in advance. To address the nonlinear and non-convex nature of the problem, the research relies on the multi-objectivisation of an output error single-performance index. The developed algorithm utilises a multi-objective genetic algorithm (GA) in order to find a proper solution. The performance of the model and the GA is evaluated using both simulated (i.e. with a known set of ‘true’ parameters) and experimental data. Both simulation and experimental results show that multi-objectivisation has improved convergence of the estimated parameters compared to the single-objective output error problem formulation. This is achieved by integrating the validation phase inside the algorithm implicitly and exploiting the inherent structure of the multi-objective GA for this specific system identification problem.

An investigation towards real time dose rate monitoring, and fuel rod detection in a First Generation Magnox Storage Pond (FGMSP).

The First Generation Magnox Storage Pond (FGMSP) is located on the Sellafield Nuclear Site, housing legacy spent Magnox nuclear fuel. Some of which has since corroded, forming a layer of Corroded Magnox Sludge (CMS) creating one of the largest decommissioning challenges the UK has faced. In this work the composition, physical properties and potentially high hazard nature of CMS are discussed, as are the gamma emission spectra of spent Magnox fuel rods typical of the ilk stored. We assess the potential use of a RadLine gamma detector to dose rate map this area and provide fuel rod detection. RadLine consists of a small scintillator, fibre optic cable and photon counter. The probe has the unusual advantage of not being electrically active and therefore fully submersible underwater, with the option to deploy hundreds of metres in length. Our experimental method encompasses general purpose Monte Carlo radiation transport code, MCNP, where we describe the modelling of CMS and pond liquor in comprehensive detail, including their radiological spectrum, chemical composition data, and physical properties. This investigation concludes that the maximum energy deposited within the scintillator crystal due to ambient CMS corresponds to a dose rate of 5.65Gy h(-1), thus above this value positive detection of a fuel rod would be anticipated. It is additionally established that the detectable region is within a 20cm range.

Proposal to characterise legacy Sellafield ponds using SONAR and RadLine

Sellafield Nuclear Reprocessing Plant in Cumbria contains storage ponds built in the 1950s which was originally intended to hold spent nuclear fuel for reprocessing, and eventual production of weapons grade plutonium. Parts of the spent fuel have corroded; some are buried under a layer of sediment or intertwined with other debris and removal and destruction of this nuclear waste is not a trivial task due to elevated radiation levels. We propose a system in collaboration with the National Nuclear Laboratory (NNL) to characterise the ponds using a system containing three main parts; an ultrasonic SONAR system used to physically map the pond, scintillator based radiation detector (known as RadLine™) used to map the pond from a radiation point of view, and bespoke software intended to combine the physical and radiation plots of this environment to create an overall 3D source map.

Development of a portable isotropic neutron spectrometer.

A portable neutron spectrometry and dosimetry measurement device is in the process of development, which will be capable of accurate characterisation of neutron fields. The instrument is an amalgamation of two well-known techniques the Bonner-sphere and the surface barrier detector. Recent developments in semiconductor materials have allowed the growth of large-area thin-film based semiconductors upon high density polyethylene substrates, on which this detector is formed. Herein we discuss three plausible detection devices and test their results.

Design of a Detector for Characterizing Neutron Fields for Single-Event Effects Testing

We show how a moderating neutron detector can be designed for use as a proxy for devices subject to neutron-induced single-event effects (SEE). Such a detector can have a flat response to high energy neutrons, and can be used to characterize neutron fields for SEE testing and evaluation.

Characterisation of the TRIUMF neutron facility using a Monte Carlo simulation code

Here, the characterisation of the high-energy neutron field at TRIUMF (The Tri Universities Meson Facility, Vancouver, British Columbia) with Monte Carlo simulation software is described. The package used is MCNPX version 2.6.0, with the neutron fluence rate determined at three locations within the TRIUMF Thermal Neutron Facility (TNF), including the exit of the neutron channel where users of the facility can test devices that may be susceptible to the effects of this form of radiation. The facility is often used to roughly emulate the field likely to be encountered at high altitudes due to radiation of galactic origin and thus the simulated information is compared with the energy spectrum calculated to be due to neutron radiation of cosmic origin at typical aircraft altitudes. The calculated values were also compared with neutron flux measurements that were estimated using the activation of various foils by the staff of the facility, showing agreement within an order of magnitude.

Sub-aquatic response of a scintillator, fibre optic and silicon photomultiplier based radiation sensor

We describe here, the sub-aquatic response of the RadLine® detector (a small, novel, remotely operated radiation detection instrument) when irradiated with gamma doses between 6 and 400 Svhr-1. The National Nuclear Laboratorys (NNL, UK) RadLine® consists of an inorganic scintillating crystal coupled to a fibre optic cable which transports scintillation photons to a detector at the other end. A CCD camera is normally used for photon collection, however in this paper we trial a newer technology; the silicon photomultiplier (SiPM), namely SensLs MiniSL. SiPMs have performance characteristics similar to photomultiplier tubes (PMT), whilst benefiting from the practical advantages of solid-state technology which include; low operating voltage, robustness, compactness, insensitivity to magnetic fields and over-exposure to light. The MiniSL was chosen as its peak photon wavelength is well matched to the output from the scintillation crystal, as well as its fast recovery time (within the nano-second range). We use a clinical radiotherapy linear accelerator (linac) machine which produces x-rays by accelerating elections onto a target which then emits x-rays by Bremsstrahlung. The linac is rated at 6MeV in energy, with a peak of approximately 2MeV. The machine is capable of generating a highly precise dose at known distances between treatment head and scintillation crystal. Analysing the data gathered we were also able consider how the RadLine® might perform in larger aquatic environments for example First Generation Magnox Storage Ponds (FGMSP). Built in the 1950s they were originally intended to hold spent nuclear fuel for reprocessing, however now parts of the spent fuel have corroded; some of which are buried under a layer of sediment. Removal is not a trivial task due to elevated radiation levels, and the complexity of the environment. RadLine® has the potential to be of significant use for this and in other similar situations.