NPL – 20 September 2001

 

Attendees:		John Makepeace	NPL (Chairman)
		Andrea Woodman	NPL (Secretary)
		Robert Barlow	BNFL
		John Bennett	DRPS
		Raj Bhanot	BNFL
		Trevor Birkett	AWE plc
		Derek Brazer	AWE plc
		Suzanne Calder	UKAEA
		John Caunt	JCS
		Stephen Collins	Amersham plc
		Chris Davies	URENCO
		Alison Dale	NNC
		Julian Dean	NPL
		Philip Doyle	Amersham plc
		Dominic Duggan	Munro Group
		Steve Evans	Amersham plc
		James Forde-Johnston	Harwell Instruments Ltd
		Norman Green	NRPB
		Richard Greenwood	AWE
		Mark Griffiths	Amersham plc
		Trevor Hatt	PerkinElmer
		Phil Hurst	British Energy
		B Jani	Jani Consultancy Services
		Will Johnston	UKAEA
		David Locke	Southern Scientific
		Ian Maither	RCD
		Fred Martin	BNFL
		Neil McAllister-Hewlings	AWE
		Robert Munro	UKAEA
		Trevor Nicholls	Lab Impex Systems
		Thomas Nobes	BNFL
		David Orr	NRPB
		Bob Otlet	RCD
		John Parnell	Imaging Research Solution Ltd
		Hilary Phillips	NPL
		Max Pottinger	BNFL
		Mark Rainbird	AWE
		Alex Reid	Harwell Instruments Ltd
		Gareth Roberts	AEA Technology
		Dale Robinson	BNFL
		David Ryden	Harwell Instruments Ltd
		Mike Scott	BIC
		Graham Shephard	Devenport Royal Dockyard Ltd
		Dave Simmons	Amersham plc
		Brian Syme	Harwell Instruments Ltd
		Malcolm Wakerley	DEFRA
		Jill Walker	RCD
		Stuart Yates	Addenbrookes NHS Trust

 

1. Chairman’s Welcome and Opening Remarks

The chairman, John Makepeace opened the meeting and thanked so many people for attending.  The majority of work carried out at NPL is funded by the DTI through the National Measurement System Policy Unit and it is a requirement of NPL to not only maintain standards but also to disseminate information to the user community.  Hence users’ groups have been formed as a focus for people from similar fields of work to get together to discuss and hopefully solve problems.  NPL is funded to provide conference rooms, secretarial support and will contribute to discussions but in no way claims to have all the answers.  The chairman explained that the Air Monitoring Users’ Group had been suggested some time ago at an IRMF (Ionising Radiation Metrology Forum) meeting as there seemed to be a large number of people interested in this field.  As with other similar groups it may be found necessary to form subgroups to deal with particular topics.   

2. Draft Terms of Reference

The draft terms of reference were given to which there were a number of comments.  Firstly that the word “both” should be removed from the Aims to make the sentence grammatically correct.  Secondly it was accepted by the forum that the word monitoring includes sampling.  With the list of Activities it was suggested that we should aim to make improvements in the field of air monitoring and that inclusion of the words “facilitating continuous improvement in assessment techniques” might clarify this.  With the hope of encouraging academia to the users’ group it was decided that members shall be “representatives of UK establishments or organisations actively involved in research and measurements in the field of air monitoring…”

3. Introductions and Nominations of Topics for Discussion

By way of getting to know each other, each member introduced themselves and described the work they are involved in and any special interest they have in the field of air monitoring.  Any further topics for discussion were suggested by the delegates.  It was notable that the delegates included a spread of both users and suppliers/manufacturers of air particulate monitors and gas monitors.

4. Invited Talk: Issues associated with particulates in air monitoring

Max Pottinger, BNFL

A number of the technologies involved in monitoring air particulates were described such as personal air samplers, portable and fixed air samplers and continuous air monitors.  Difficulties associated with measuring flow rate and air sample volume were described.  The problem of choosing the best filter media was illustrated and the commonly used glass fibre filters (GFAs) were compared with membrane filters, of which various studies have been made in France and America.  It would seem that a better spectrum may be obtained from a membrane filter along with a smaller pressure drop across the filter however GFAs still appear to be the favourite in the UK.  It was felt that it would be of benefit to have an expert on filter media give a presentation at a future meeting.  The difficulty with choosing appropriate self absorption factors was described and it was mentioned that other countries at least have a consistent way of dealing with this whereas the UK does not.  Further problems are experienced with detection levels and using detection efficiencies relevant to the nuclide present.  The standards publication ISO:11929 was referred to, this covers all ionising radiation measurements including decision thresholds and detection limits for air sample filters.  A further potential problem expressed was concerned with beta monitors being affected by varying gamma backgrounds.  This is not so much of a problem with fixed monitors but is with monitors which are moved around.  A sudden spike in the gamma background can set off the monitor alarm hence the need for optimising the monitors for angle of radiation.

5. Invited Talk: Air Sampling using Dry Bed Absorbers

Bob Otlet, RCD

An alternative to bubbling gas through collection bottles, containing substances such as ethylene glycol for tritium or sodium hydroxide for C14, for analysis was described.  The system uses solid, dry non-deliquescent absorbers instead of liquid absorbers and this is found to be far more practical in the workplace.  At RCD they have designed both dynamic samplers and passive samplers using this solid absorber.  In the case of  dynamic tritium sampling the equipment comprises three columns of silica gel through which the gas is pumped.  In the first column, tritiated water vapour is collected by the silica gel which dries the air.  In the second column the now dry air, possibly containing HT, passes through pre-humidified silica gel in order to re-wet it so that it is ready for conversion from HT to HTO via the wet proofed catalyst.  This catalyst operates with a very high efficiency at ambient temperatures.  The third column then acts in the same way as the first by collecting the HTO.  The sampler can be run over a few hours or for up to six weeks.  The HTO is retrieved from the silica gel for analysis either by vacuum extraction or by the equilibration of the silica gel.  A pure sample is produced with no quench which may be measured by liquid scintillation counting.

The passive sampler is good for environmental applications as no power is needed to drive it and there are no moving parts to become worn.  The sampler relies on the basic physics of diffusion, Ficks law in particular.  The diffusion of a gas through a tube with a concentration gradient is controlled by the concentration gradient, the length of the tube and the cross-sectional area of the tube.  The equipment employs a polystyrene bottle with a tube in it with a screen on top to avoid draughts which would cause Ficks law to break down.

Both the dynamic and passive samplers have been validated by various means and found to be in good agreement with bubblers.

6. Discussion Session

Self Absorption Factors

There was much discussion regarding the use of self absorption factors with filters as the value used could range from 2 to 10.  Concern was raised that these factors were being estimated with no justifiable reasons for choosing between them.  It was suggested that it could be useful to consult with specialists from the Alpha Spectrometry Users’ Forum for advice both on the use of self absorption factors and also on the metrology of the activity in the filters.  It was suggested that a working group might be formed with the task of producing a definitive statement on the use of self absorption factors.  It was decided that the topic would be approached in a questionnaire to be distributed to all members.

Radon compensation schemes

Problems are found with deconvoluting spectra when measuring radon and the users were generally uncertain of what compensations are in place in the instruments used. 

It was pointed out that there is an IEC standard (IEC61578: Radiation protection instrumentation – Calibration and verification of the effectiveness of radon compensation for alpha and/or beta aerosol measuring instruments –Test methods)  published in 1997, however it is a very complex document and difficult for any test facility in the UK to comply with it.  The document was also considered not to be good enough to be a British Standard.  The US and France produce real alpha particulates in order to test their monitors but there appears to be no such system in place in the UK.  The comment was made that the Alpha Spectrometry Users’ Forum also have difficulties in deconvoluting spectra even when alpha sources have been made to the best specification possible.  The view of the manufacturers was that it would be useful to have a set of questions likely to be asked by the customer in order to produce some kind of standard test verification method such that different monitors could be compared more easily.

Stack monitoring

The comment was made that a recent draft copy of an ANSI report for stack monitoring had moved away from the isokinetic sampling method which has always been the recommended procedure in the past.  The suggestion was then made that as long as the method used could be justified then there should be no real problems.

Uncertainties

There seemed to be confusion over what the main uncertainties are in air sampling data and therefore which uncertainties need to be measured carefully and which can be estimated.  Uncertainties can arise in a number of areas including the sampling of the air and volume size, the analysis of the data, counting statistics and sample to sample variation.  It was decided that the questionnaire would be used to determine how uncertainties are treated in different companies.  It was pointed out that standards do not exist at user levels therefore higher level standards must be diluted thereby creating even higher uncertainties.  Following the responses to the questionnaire it is anticipated that some members will be approached to form a subgroup with the aim of looking at how uncertainties are treated and giving guidance on how best to improve them.

Flow rate measurements

There was some concern over the accuracy of flow rate meters and the suggestion was made that an expert in flow rate measurements from the National Engineering Laboratory in Glasgow could be invited to the next meeting to give advice.  The popularity of this suggestion will be gauged by comments made in the forthcoming questionnaire.

Good Practice Guide

The problem of how best to test air sampling equipment in order to comply with Ionising Radiation Regulations was raised and it was suggested that this might be a good topic for discussion at the next meeting.  John Makepeace reported that NPL have the financial support in the  new programme for the preparation of a good practice guide associated with air monitoring starting in October 2002.  It was proposed that at the next meeting two subgroups will be formed to cover both gas monitoring and air particulate monitoring aspects.

Web-site and e-mail discussion group

An AMUG web-site will be produced on the NPL web-site (www.npl.co.uk) and minutes of meetings will be displayed there.  The forthcoming questionnaire will also be put on the website along with responses from the questionnaire unless otherwise requested.  An e-mail discussion group will be set up such that members can e-mail John Makepeace (John.Makepeace@npl.co.uk) or Liz Engel (Liz.Engel@npl.co.uk) who will then forward messages to the rest of group if considered appropriate.

7. Concluding Discussion

It was agreed that a questionnaire would be produced and sent to all members covering topics described above.  Members of the group would then be asked to form subgroups covering self absorption factors, uncertainties and the testing of air sampling equipment.  The questionnaire will also be used to form a register of facilities/services, relevant to air monitoring, provided by the various organisations represented at the meeting.  This register will then be displayed on the NPL website.

Overall the meeting was considered a success and the next meeting is due to be held in September 2002.