Tuesday 1st October 2002
Scientific Museum, Bushy House, NPL

Agenda and Contents

Welcome and Introduction	Martyn Sene
Dosimetry at the boron neutron capture facility at Studsvik, Sweden	Per Munck
Dosimetry for the BNCT programme	Stuart Green
The DRPS neutron dosimetry service	Roger Stokes
The HMS Sultan neutron generator	James Brushwood
The future of Consort	Simon Franklin
The Duo-Sphere neutron detector	Jack Roskell
The Siemens EPD-N2 neutron/photon dosimeter	Jonathan Coleman
Testing and calibration of Euratom Under Water Neutron Coincidence Counters at NPL	Adrian Tolchard
Testing and calibration of lithium gadolinium borate capture gated spectrometer in thermal and monoenergetic neutron fields at NPL	Andrew Williams
Novel neutron logging methods to extend the life of North Sea oil wells	Alex Pereira
Experimental validation of MCNP results for a novel design of neutron survey meter	Rick Tanner
Investigation of radiation doses at aircraft altitudes during a complete solar cycle: DOSMAX- A collaborative research programme	David Bartlett
Recent developments in the CAA/ MSSL/ NPL/ VAA collaboration measuring cosmic radiation doses in aircraft	Graeme Taylor
Visits to the facilities of the NPL Neutron Metrology Group
Members present and apologies

 

Report of the Eleventh Meeting of the Neutron Users' Club
held at the National Physical Laboratory
Tuesday 1st October 2002

 

Welcome and Introduction

Martyn Sené (Head of Centre) welcomed the participants to NPL and especially those attending for the first time. He described some of the history of the NPL during its hundred years of existence and how the laboratory site was being redeveloped and a new state-of–the-art laboratory complex being built.

Martyn stated that NPL is a key part of the National Measurement System (NMS) which provides technical infrastructure to ensure that measurement in the UK is traceable, consistent and internationally recognised and that excellence in measurement science enables regulatory compliance, improved standard of living and economical benefits

The DTI-funded NMS programme includes :

• Maintenance and development of standards and facilities.
• Participation in the international measurement system
• Research at the frontiers of measurement and materials science
• R and D related to standards of measurement.
• Knowledge/ Technology transfer.

The Ionising Radiation Programme at NPL has three technical themes, Radiation Dosimetry, Radioactivity metrology, Neutron Standards plus Knowledge/ Technology Transfer.

The Neutron Users Club was formed to promote information exchange amongst the relevant user communities and to report on, and inform, the present and future direction of the NMS Ionising Radiation Programme.

Talks

 

Dosimetry at the boron neutron capture facility at Studsvik, Sweden

Per Munck af Rosenschöld, Medical Radiation Physics, Lund University, Sweden

BNCT is a modal type of therapy, including 1) an intra-venous infusion of a tumour-seeking boron-compound and 2) an irradiation of the target volume with thermal neutrons. Boron has a neutron capture cross-section comparatively much higher than the materials in tissue (nitrogen, carbon, hydrogen, oxygen etc). Following neutron capture in boron an alfa-particle is release and the remaining lithium ion recoils in the opposite direction, the total path-length being in the order of a cell diameter. These facts facilitate, in principle, an irradiation of the tumour cells while the normal tissue is spared.

A facility designed to operate clinical trails has been built in Studsvik, Sweden. The current protocol is for treatment of malignant glioma. The treatment involves a 6h infusion of the boron compound BPA and (usually) a two-field irradiation approximately 2-3hrs after the end of the infusion. 25 patients have been treated so far.

The present abstract focus on some dosimetry aspects. The patient is subjected to mixed radiation fields of different biological effectiveness. Therefore, individual quantification of the components is imperative. The main absorbed dose components in healthy tissue are the boron and nitrogen capture absorbed dose, the fast neutron absorbed dose (hydrogen recoil) and the photon absorbed dose. The reference dosimetry has been carried out using ionization chambers and activation probes in a PMMA phantom. The ionization chambers are of two types; one magnesium-walled flushed with argon gas and one A-150-walled flushed with tissue-equivalent gas. The chambers are used for photon and fast neutron dosimetry. The achievable accuracy using the twin ionization chamber technique is adequate for photon dosimetry while the fast neutron dosimetry poor by conventional standards. However, the fast neutron component is rather small and does not compromise the safety of therapy. The activation wires and foils were used to determine the thermal neutron fluence. The accuracy of the determination of the thermal neutron fluence using gold foils is satisfactory.

The measured in-phantom parameters (fluences and absorbed doses) have been converted into patient dosimetry using the treatment planning system (TPS). However, individual boron concentrations vary between patients, therefore the boron concentration was monitored during treatment by taking blood samples continuously during the infusion and before and after irradiation. The absorbed dose components were subsequently added and their individual biological effectiveness was taken into account.

Future plans involve improving the in-vivo dosimetry system and the dosimetry of the fast neutron component.

 

Dosimetry for the BNCT programme

Stuart Green, University Hospital Birmingham and University of Birmingham

This talk reviewed two submissions to the recent 10^th International Congress on Neutron Capture Therapy in Essen, Gemany (September 2002). The first is a Birmingham / Helsinki collaborative effort and the second is a Birmingham / Bristol collaboration. Both are covered in detail in the Birmingham University PhD thesis of Mark Gainey.

A preliminary in phantom dosimetry intercomparison using TEPCs and ICs performed at the FiR 1 reactor in Finland.

M. B. Gainey^[1], S. Green^[2], J. Uusi-Simola^[3], M. Kortesniemi^[4], H. Koivunoro^[3], T. Seppälä^[3] and A. Kosunen^[5]

¹School of Physics and Astronomy, The University of Birmingham, Edgbaston, Birmingham, B15 2TT, UK., ²Department of Medical Physics, University Hospital Birmingham NHS Trust,  Birmingham, B15 2TH, UK, ³Department of Physical Sciences, University of Helsinki, Finland, ⁴Department of Radiology, Helsinki University Hospital, Helsinki, Finland, and  ⁵Radiation and Nuclear Safety Authority, STUK, Helsinki, Finland.

Summary

A preliminary dosimetry inter-comparison has been performed using tissue-equivalent proportional counters, TEPCs, and ionisation chambers, ICs, at the FiR 1 TRIGA reactor of VTT (Technical Research Centre of Finland), Finland. Measurements were made at different depths in a water-filled PMMA phantom (140 x 150 x 190 mm) along the clinical beam axis, viz. 32, 50, 75 and 125mm to determine the neutron and photon dose components in the mixed radiation field at these positions. The measurements were normalised per unit monitor unit of the beam monitoring system in accordance with normal procedure at the facility. An absolute comparison has been made between the two detector types and Monte Carlo calculations performed using MCNP4B.

An investigation into the distribution of boron additives within the walls of gas filled radiation detectors.

M. B. Gainey^[1], S. Green^[2], C. Oyedepo^[3], A. H. Beddoe^[2]

¹School of Physics and Astronomy, The University of Birmingham, Edgbaston, Birmingham, B15 2TT, UK., ²Department of Medical Physics, University Hospital Birmingham NHS Trust, Birmingham, B15 2TH, UK, and ³ Interface Analysis Centre, Bristol University, Bristol, UK

Summary

Radiation detectors are fundamental to the dosimetry of radiotherapy. Of particular interest to BNCT are ionisation chambers, ICs, and tissue-equivalent proportion counters, TEPCs. It has become common practice to add boron additives to the bulk material of the cavity walls of these radiation detectors. However, little work has been done to study the distribution of these ‘boron particles’ within the matrix of the cavity wall. This may have a profound effect on the evaluation of the boron dose due to the boron neutron capture reaction, BNCR, because of the short range of the ⁴He and ⁷Li ions. This paper reports experimentally determined distributions of the boron additive using secondary ion mass-spectrometry, SIMS, and its implication in the evaluation of the BNCR dose.

 

The DRPS neutron dosimetry service

Roger Stokes, Dstl Radiological Protection Services

Dstl Radiological Protection Services (DRPS) is part of the UK Ministry of Defence (MOD).  One of its roles is to provide the MOD with a personal neutron dosimetry service.  DRPS issues approximately 1,200 neutron dosemeters per month, the majority of these to personnel involved in the nuclear-powered submarine programme.  The DRPS neutron dosemeter consists of two CR-39 detection elements housed inside a polypropylene holder.  Fast neutrons interact in both the holder and the CR-39 to generate recoil protons, which may subsequently leave tracks of damage in the CR-39.  These damage tracks are enlarged with powerful chemical etchants, and counted using an NE Technology Limited Autoscan 60 reader.  The use of appropriate calibration factors allows the number of tracks to be converted to an effective dose.

One limitation of the DRPS neutron dosemeter is that it does not respond to neutrons with energies less than 100 keV.  DRPS aims to overcome this limitation by adding a lithium converter to its dosemeter.  The converter is placed in direct contact with the CR-39 detection elements.  Low-energy neutrons interact with lithium-6 nuclei in the converter to produce alpha particles and tritons.  These secondary charged particles subsequently leave tracks of damage in the CR-39.  The tracks can be enlarged and counted in the normal manner. 

DRPS has collaborated with the Centre For Industrial Polymers at Leeds University to produce two types of lithium converter; the first consists of lithium fluoride dispersed in polypropylene, while the second is a lithium-doped ionomer.  Preliminary trials to assess the neutron response and environmental stability of these converters have given promising results.  The next stage in this work is to optimise the lithium concentration so that the energy response of the dosemeter is as ‘flat’ as possible.

 

The HMS Sultan neutron generator

Presented by J. Brushwood

JB presented details of the finialised design and commissioning of the recently installed Neutron Generator Facility (NGEN) at HMS Sultan.  This facility is driven by a GENIE-16 14 MeV sealed tube D-T neutron source manufactured by Sodern.  The manufacturers stated output for normal operating conditions is 2 ´ 10⁸ n s^-1 into 4p with an expected tube lifetime of 4000 hours.  In addition to continuous output both pulse and burst modes of operation are possible.  A shielding design study has been conducted using the two well-known Monte Carlo codes MCNP and McBEND to realise an optimized shield design.  The key constrain of this study was the imposition of a dose limit of 1 mSv h^-1 on the external walls of the Irradiation Facility.

The NGEN facility is located within a previously existing Irradiation Facility housing both a gamma and X-ray exposure facility.  The D-T generator itself is located within a 5 cm lead cylinder surrounded by approximately 100 cm of polyethylene.  Two beam ports have been incorporated into both the lead and polyethylene to allow for a variety of experimental configurations.  All construction and development work was conducted in house by Nuclear Department staff.

The shielding study predicted that the highest external dose rate would be 0.8 mSv h^-1.  As part of the extensive commissioning trials neutron and gamma ray dose rate surveys were undertaken, the maximum observed dose rate was found to be 0.6 mSv h^-1.  Subsequent to these successful commissioning trials Approval to Operate was granted.  Standard practical experiments and a fast sample transfer system are now being developed for both educational and research purposes.

 

The future of Consort

Simon Franklin

The Reactor Centre has been through some difficult times.  The presentation shows what has been done to keep the facility available for users such as members of Neutron Users Club, and reinforces the statement that we are very alive and kicking.

Since the User’s Forum in June 2000, a great deal of effort has been made to secure the future of the reactor. A strategic plan has been produced, with a new mission statement.  We are planning for a major review of finances in 2003/4, which will ascertain how feasible refuelling might be. If the green light is given, then the lifetime could be 30 years or so. If not, the reactor will decommission between 2010-2015.

Much of the work can be labelled as responding to regulatory power.  The quinquennial review of decommissioning planning has been accepted by the Nuclear Installations Inspectorate, HSE, with a report on the website. The reactor has been undertaking Periodic Safety case Review for the Nuclear Installations Inspectorate, HSE, and the final report is due at the end of the year. We have been the subject of many visits from the Office of Nuclear Security post-9/11, with some £60K of improvements and some ½ man year of effort being deployed. We are undergoing quinquennial review of our discharge authorisations by the Environment Agency.

With very much a strengthened team of people now in place, we are proceeding to increase efficiency, and increase throughput. Results so far have been promising. When I took over at the reactor centre in 1997, the daily rate was only about 25% of where it should be. We are working with the users to recover our costs, and we thank those who have stayed with us. This is necessary to secure a strong future.  I am still keen to introduce a Supporter’s Club; it just needs a brave organisation to come forward and offer to help set one up that we can take to you all.

Training has been an area of success, with the reintroduction of live reactor training within Imperial College, and the production of a two week training course for the NII themselves. We are looking at several opportunities to provide support to Russia and the Former Soviet Union.  We are also discussion training with two other organisations in support of a new awareness of the security aspects of nuclear energy and radiation.

We are about to decommission the fast neutron facility known as NISUS, due to the need to return the driver mechanism.

We are attempting to restart a research programme within the reactor team. Cross-section measurement and evaluation is one area. We are also looking for funding partners to help us produce a new Cyclic Activation System as well as a new prompt gamma system.

Contact

s.franklin@ic.ac.uk

 

The Duo-Sphere neutron detector

Jack Roskell

The duosphere is an instrument developed by BAE Systems Marine to enable the characterisation of relatively low dose neutron fields in a submarine environment.

It uses the ratio of counts outputs from 127mm and 208mm diameter spheres to derive the correction factors for neutron instrumentation for the impact of the neutron spectrum on detector response. Rather than attempting to produce an instrument with a response that matches the variation of dose equivalent rate with energy over the whole neutron spectrum, the approach adopted was to plot the detector sensitivity against the ratio of the response of the two detectors for a selection of realistic neutron spectra (largely from the SPKTBIB catalogue).

It was found that to the 95% confidence level the actual sensitivities of the detectors used (i.e. Leake detector, detectors with 127mm and 208mm moderator spheres) were within approximately 20% of the value given by the fit for all of the spectra used to derive the fit (over 200). Field comparisons of duosphere predicted response corrections with those predicted from TNS measurements and Monte Carlo calculations have shown good agreement.

It is concluded that the duosphere approach provides a method of evaluating field correction factors for neutron instrumentation. In principle it could be applied to any neutron instrument or monitoring system provided that the variation of response with energy is known. The duosphere is sufficiently sensitive to allow accurate field correction factors to be determined in low dose rate neutron fields (within twenty minutes in a field of 1µSvh^-1 for a typical submarine spectrum.

 

The Siemens EPD-N² neutron/photon dosimeter

Jonathan Coleman, Siemens Environmental Systems Limited

The EPD^â-N² is a new electronic personal dosemeter for estimating the neutron and photon components of personal dose equivalent, Hp(10). The radiological characteristics have been evaluated and field testing of prototypes and pre-production units has been carried out for many months. In this presentation, the following aspects are presented:

• General characteristics
• Photon energy response
• Neutron energy response
• Neutron response in several workplace fields
• Other aspects of the radiological performance
• Points of interest from field trials

jonathan.f.coleman@siemens.com
Siemens Environmental Systems Limited
Sopers Lane
Poole
Dorset
BH17 7ER
UK

 

Testing and calibration of Euratom Under Water Neutron Coincidence Counters at NPL

Adrian Tolchard, A.N. Technology Ltd.

Fresh uranium-plutonium mixed oxide (MOX) fuel is widely used in light water reactors in Europe and Japan.  It is important for the EURATOM inspectors to verify the plutonium (Pu) content of this fuel for international safeguards purposes.  An Under Water Coincidence Counter (UWCC) has been designed for the measurement of Pu in MOX fuel assemblies prior to irradiation in a power reactor.  The UWCC uses high efficiency ³He detectors to measure the neutron emissions from spontaneous fission and induced fission in the fuel assembly.  The inspectors use the UWCC to measure MOX assemblies either in air or under storage pond water.

The neutron counting rate is analysed using one of the new generation shift register electronics such as the Advanced Multiplicity Shift Register to analyses the pulse stream for singles and reals (doubles) time correlation to determine the ²⁴⁰Pu effective mass per unit length of the fuel assembly.  The purpose of this brief talk is to introduce the concept of the UWCC and describe the characterisation and calibration tests carried out at the Centre for Ionising Radiation Metrology at the NPL using a range of neutron sources.  Aspects of the use of the UWCC for in-plant inspections will be mentioned briefly.

The UWCC consists of two, stainless steel clad neutron detection panels (termed ‘forks') connected to a water-tight back-plate which can be arranged in one of two possible configurations around either the Boiling Water Reactor (BWR) or Pressurised Water Reactor (PWR) type fuel-element assembly.  The detection system consists of eight, 7.5 atmosphere ³He tubes (four per fork) embedded in a polyethylene moderator and wrapped in cadmium sheet.  The entire assembly is enclosed in a watertight stainless steel housing.  The housing is sealed with an O-ring.  The purpose of the Cd lining is to make the detector package relatively insensitive to the effect of boron poison in the pond water and to reduce the effect of gamma ray pulse pile up.  The UWCC uses a dual-channel PDT-210A amplifier with one Amptek A-111 channel per 4 ³He tubes.  The detectors are cross-wired between the two forks and each channel collects signals from two detectors in each fork.  The amplifier output pulse is set to 50ns.  A signal OR box is used to connect the signals from the PDT pre-amplifiers to the AMSR or TCA and is used to provided the high voltage and low voltage (+5V) to the detectors.  The UWCC can be operated with the AMSR and the Integrated Neutron Coincidence Counting (INCC) software.

Using a range of neutron sources typically (²⁵²Cf) functional electronic tests were conducted to demonstrate the optimal working point for detector & preamplifier system with respect to high voltage and gain settings. Tests were conducted to

establish the counting rate produced by a ²⁵²Cf source (with certified activity) positioned at the centre of the detection cavity and the counting rate capability of the detector was demonstrated up to 0.25x 10⁶counts per second. The effective dead time of the system was determined and the necessary parameters determined.

The talk will include a brief discussion of the following:

• Measurement of the high voltage bias of the UWCC detectors

• Measurement of the high voltage bias plateau for the top and bottom ³He detector pairs of each panel of the UWCC detectors.

• The detector efficiency determined with the ²⁵²Cf source positioned centrally between the two prongs (panels) of the UWCC.

• The counting rate capability of the UWCC demonstrated at 2.5 x 10⁵ counts per second using ²⁵²Cf source 4774NC.

• The system dead time measured using two ²⁵²Cf sources (4774NC & 4775NC).

 

A.C. Tolchard,
A.N. Technology Ltd.,
Unit 6,
Thames Park,
Lester Way,
Wallingford,
Oxfordshire,
OX10 9TA

Tel: (01491) 824444
Fax: (01491) 832800

e-mail: tolchard.adrian@antech-inc.com

 

Testing and calibration of lithium gadolinium borate capture gated spectrometer in thermal and monoenergetic neutron fields at NPL

Andrew Willams, Nuclear Department, HMS Sultan

A composite scintillator consisting of lithium gadolinium borate (LGB) dispersed in a plastic scintillator matrix has been tested at NPL using monoenergetic fields created by the Van de Graaff accelerator and the thermal pile. A digital electronic system captures and analyses all light pulses occuring in the detector and determines there origin through pulse shape discrimination, displaying the results in four counters on an LCD. Pulses ascribed to proton recoils are subject to the further condition that they are followed, within a 10 ms time window, by a characteristic pulse from neutron capture in ⁶Li. This is known as capture-gated (or double-pulse) counting and further improves the rejection of gamma rays by the system. All pulses meeting these criteria are labelled as coming from fast neutrons and the pulse shapes are recorded on a data logging computer for off-line analysis. The thermal neutron fluence can be estimated simultaneously by counting captures in lithium/boron that are not coincident with proton recoil events, and also by counting captures in gadolinium. The fourth counting channel on the display shows the number of gamma ray events detected.

For the calibration, neutron energies chosen were 144, 250 and 565 keV, and 1.2, 2.5 and 5.0 MeV and irradiations were made with and without a shadow cone at source to detector distances of about 1 metre for both isotopic compositions of the detector. For the ⁶Li enriched version, the fast neutron detection efficiency was 1% at 2.5 MeV falling to 0.4% at 5 MeV. There is also a deterioration in the energy resolution with increasing energy.

A problem with the capture in gadolinium channel that had already been observed in initial testing was again present in these irradiations. Capture in gadolinium is characterised by a cascade of gamma rays totalling around 8 MeV in energy, of which about 2 MeV is deposited in the scintillator on average. High energy gamma rays present in the measurement environment can deposit a similar amount of energy and hence be mistaken for gadolinium capture signals. At NPL the source of the high energy gamma interference was the reaction of protons with fluorine in the LiF target used to produce the 144, 250 and 565 keV neutron fields.

 

Novel neutron logging methods to extend the life of North Sea oil wells

Alex pereira

Abstract

Obtaining petrophysical measurements or wireline logging data through casing is becoming more popular to assess reservoirs in both ageing fields and also in newly completed wells that have no open hole measurements.

Conventionally pulsed neutron measurements are used to identify reservoir fluids but Reeves new generation Compact tools and the Compact memory logging technique can be used instead.

Compact Memory Logging is a method for cable-less logging using slim (2 ¼ inch diameter) low-power tools that have responses equivalent to those of conventional sized tools. They can be deployed without the usual wireline surface equipment in a variety of ways that can very significantly reduce total logging time, when compared with more conventional deployment techniques, without sacrificing wireline-quality data. The short length of the tools also often allows access to wells when the surface facilities are restricted.

Two new techniques using Compact Memory Logging tools have recently been introduced to the UKCS as follows:

1) CNFI (Compact Neutron Fluid Identification)

identifies changes in formation fluids over a period of time to monitor reservoir performance. In this case, since we are logging the same well several times and comparing the logs, the porosity is usually considered invariant, and the Compact compensated neutron tool can be employed in a novel way. Monte Carlo modelling of the neutron response for a given formation shows that fluids can be idintefied in a known or measured porosity.

2) CNGI (Compact Nuclear Gas Identification)

identifies the fluids, usually gas, as a through casing formation evaluation. This is done by comparing 2 porosity logs, the Compact density and neutron, which respond differently to the fluids. Four associated data processing techniques may be used.CNGI may be run wher no openhole logs were available and/or to monitor changes in a reservoir over time.

Several recent log examples will be presented to illustrate these techniques.

 

Experimental validation of MCNP results for a novel design of neutron survey meter

Rick Tanner, David Bartlett, Chris Hill and César Molinos

NRPB, Chilton, Didcot, Oxon OX11 0RQ

 

Andrew Winsby, Malcolm Joyce

Engineering Department, Faculty of Applied Sciences, Lancaster University, Lancaster LA1 4YR

Presented by Rick Tanner

Neutron survey instruments based on moderation of fast neutrons and subsequent detection of the thermalized field tend to overestimate dose equivalent significantly in the intermediate energy region. A collaboration between NRPB, BNFL and NPL optimized and enhanced an original concept by Bartlett and Jones (NRPB) to produce a device for which the response was constrained to 0.5-2.0 times a calibration response. This device has seven detectors in it, one central ³He detector, and six photodiodes in contact with ⁶LiF discs located at a shallow depth in the moderator. The development to this stage was entirely based on modelling using MCNP.

A prototype of the instrument was constructed at NRPB to verify the results from the modelling. This was done using monoenergetic neutrons at NPL plus a thermal irradiation. Comparison with the modelling results showed broad consistency for the magnitude of the response, and confirmed the relatively small potential for systematic errors in workplace fields. Detailed comparisons between the measured and modelled responses showed some disagreement, however.

To understand the differences in the modelled and measured results, the MCNP model has been enhanced to make the geometry more realistic. This increased detail was not seen to make a significant improvement in the agreement between experiment and calculation, so the density of the polyethylene was measured. This was found to be less dense than expected, so the modelling was repeated using the measured density. The newly modelled results now show much improved agreement with experimental measurement, except for 15 MeV, where the difference may be caused by elastically scattered protons, and for thermal neutrons. The discrepancy for thermal neutrons remains the primary cause for concern.

The design of survey instrument has been enhanced by a further programme of modelling at the University of Lancaster, the improved design having a lower mass and superior response characteristic. This device has been constructed by BAe Systems and is now ready for experimental testing. During October it will be exposed at the Cadarache facility to a simulated PWR field (CANEL), bare radionuclide sources and a thermalized field (SIGMA).

 

Investigation of radiation doses at aircraft altitudes during a complete solar cycle: DOSMAX- A collaborative research programme

David Bartlett, NRPB

The DOSMAX project (European Commission RTD Programme: Nuclear Energy, Euratom Framework Programme V, 1998-2002, Contract N° FIGM-CT-2000-00068) continues the work of previously funded EU investigations into the radiation field in the Earth's atmosphere, particularly at altitudes frequented by commercial aircraft. Detailed studies have been completed during the minimum phase of the present solar cycle 23 and the present work seeks to complete investigations throughout the maximum phase between 2000 and 2003. The radiation field is produced by the interaction of galactic cosmic rays in the Earth's atmosphere, and the sun also plays an important role. The galactic component at Earth varies in intensity over the 11-year solar cycle and reaches a minimum when the sun is at its most active, as interactions with the solar wind deflect lower energy particles away from the atmosphere. During this phase an increased number of solar flares and coronal mass ejections are expected, with increased probability of solar energetic particle events which, on infrequent occasions, can produce significant enhancement of the radiation field at aircraft altitudes. In the DOSMAX project both low LET and high LET components of the field in aircraft are measured, using a wide range of active and passive detectors. Since the radiation varies with altitude, latitude and stage of solar cycle, the help of many airlines has been enlisted in order to cover the maximum range of measurements possible, over a three-year period.

 

Recent developments in the CAA/ MSSL/ NPL/ VAA collaboration measuring cosmic radiation doses in aircraft

Graeme Taylor, NPL

Typical figures for air crew exposure are: ~2 mSv per year for (predominantly) short-haul crew and ~4 mSv for (predominantly) long-haul crew. Compare these values to the CIDI figures for classified radiation workers, whose average exposure was only 0.8 mSv for the year 2000. Given that there are roughly 40,000 people in both ‘industries’ in the UK, total air crew exposure far outweighs the total exposure of all other classified radiation workers in the UK.

The Cosmic Ray Dosimetry Project is a collaboration between Virgin Atlantic Airways (VAA), the Mullard Space Science Laboratory (MSSL, part of University College London), the Civil Aviation Authority (CAA) and NPL, and is part-funded by the Particle Physics and Astronomy Research Council, although each organisation contributes additional funding in some way, shape or form. In NPL’s case this is via the National Measurement System Policy Unit, funded by the DTI.

Although other groups are working in this area (for example, the DOSMAX collaboration between scientific institutions), the VAA/MSSL/CAA/NPL collaboration is quite distinct in being an industrial/academic/regulatory body collaboration. This, amongst other benefits, gives us easy access to flights. To date measurement data has been recorded for over 450 flights, primarily on VAA, but with some interesting additions: a couple of BA Concorde test flights, a Lear Jet flight and a couple of CAA flights. In addition to this, instruments are currently with Air New Zealand and Air Emirates, both of whom will give us valuable information for flights in the southern hemisphere.

The instrument of choice for these measurements is the tissue-equivalent proportional counter (TEPC), or more specifically, the Hawk TEPC, which is a self-contained suitcase-sized unit that logs data every minute and which can keep going for weeks at a time. The TEPC has a good ambient dose equivalent response for neutrons between ~60 keV and 600 MeV, which is more than adequate for cosmic ray neutron fields. They have also been calibrated in the simulated cosmic ray field at CERN.

Although we have been struggling to keep up with the enormous amount of data being generated we have started looking at comparisons between experiment and route dose codes such as CARI, EPCARD and SIEVERT. Initial comparisons show that the best agreement is with EPCARD. However, this may be due to the fact that, of the three codes, only EPCARD produces a value for ambient dose equivalent, and assumptions have to be made when scaling the TEPC measurements to effective dose (predicted by CARI and SIEVERT). Once these issues have been addressed, a fairer comparison should be possible.

With something like 18 months to go and 4 TEPCs available, it is possible that by the end of the project we will have data for in excess of 1000 flights. This will enable us to really put the predictive codes through their paces and find out whether some really are better than others, and hopefully why they are better. The end result could well be a predictive code better than any currently available.

 

Members present and apologies

Present:
Loretta Admans		University of Surrey
Joseph Awotwi-Pratt		University of Surrey
Lee Atkinson		University of Lancaster
David Bartlett		NRPB, Chilton
James Brushwood		HMS Sultan, Gosport
Jonathan Coleman		Siemens, Poole
Bill Croydon		Siemens, Poole
Bob D’Mellow		University of Lancaster
Ben England		AWE, Aldermaston
David Evans		Rolls Royce, Derby
Neil Foreman		Centronic, Croydon
Simon Franklin		Imperial College, Ascot
Robert Green		University of Lancaster
Mike Henesy		HMS Sultan, Gosport
Stuart Green		University of Birmingham
Bernard Hynes		AWE, Aldermaston
Laurence Jones		DRPS-DSTL, Gosport
Malcom Joyce		University of Lancaster
John Leake		Consultant
Trevor Lowe		BAE Systems, Barrow-in-Furness
Ian McGregor		Rolls Royce, Derby
Richard Mills		HMS Sultan, Gosport
Cesar Molinos		NRPB, Chilton
Brian More		University of Birmingham
Per Munck		Lund University Hospital, Sweden
Alex Pereira		Reeves Technologies, Loughborough
Jack Roskell		BAE Systems, Barrow-in-Furness
Roger Samworth		Reeves Technologies, Loughborough
Stephen Shannon		John Caunt Scientific, Oxford
Jon Silvie		BAE Systems, Barrow-in-Furness
Roger Stokes		DERA DSTL, Gosport
Rick Tanner		NRPB, Chilton
Adrian Tolchard		AN Technology, Wallingford
Filip Vanhavere		SCK-CEN, Belgium
Andy Williams		University of Surrey
Apologies were received from:
Neville Bainbridge		AWE, Aldermaston
Jen Barnes		Thermo Electron, Reading
Philip Beeley		HMS Sultan, Gosport
Nichola Chapman		Imperial College, Ascot
Dave Deas		AWE, Aldermaston
Matthew Healy		University of Cranfield
Shaun Hughes		AWE, Aldermaston
Bob Mason		Sherwood Nutech