Wednesday 24th May 2000
National Physical Laboratory

Present:

Chairman: Dr M R Sené, National Physical Laboratory
Secretary: Dr V E Lewis, National Physical Laboratory

There were 33 other members present from 19 establishments including:

AEA Technology (Amersham)			AEA Technology (Harwell)
AEA Technology QSA (Harwell)			AEA Technology (Winfrith)
BAE Systems (Barrow-in-Furness)			BNFL (Sellafield)
BNFL Magnox (Berkeley)			Bristol General Hospital
DERA Rad. Prot. Service (Alverstoke)			DNST (HMS Sultan, Gosport)
DRaStaC (Aldermaston)			Guys & St Thomas Hospital Trust
Johnson Controls (Dounreay)			Kingston University
Mainance International			National Physical Laboratory
National Rad. Prot. Board (Chilton)			NE Technology
Northern Ireland Rad. Prot. Service (Belfast)			Sherwood-Nutec Consultancy
UKAS (Feltham)

A further 13 members, including some from a further six establishments, had sent apologies.

MAIN AGENDA ITEMS

Intercomparison of calibrations of surface contamination monitoring

Clare Scott (NPL) reported that the participants’ measurements had started at the beginning of April. There were fourteen participants plus another seven who wanted to join the exercise if time permitted. It was intended that the measurements would continue until December 2000 and it was not clear how many of the seven would be able to participate. The timetable had been sent to participants along with the protocol.

Intercomparison of calibrations of γ-ray monitors and neutron area survey monitors

Vic Lewis (NPL) reported on the third intercomparison of calibrations of portable gamma-ray dose-rate monitors. This had started two years previously; the wash-up meeting had been held in November 1999. The exercise had involved the circulation of four monitors around nineteen establishments. Summaries of the results were shown to the meeting and briefly commented on. All of the discrepancies observed previously except a couple had been accounted for.

The ¹³⁷Cs results for the Farmer and 600 cc ionisation chamber were mostly within a ± 5% band. The uncertainties in the kerma rates reported by the participants ranged from below 1.7% to 10%. The results for the Mini 6-90/MC20 using ¹³⁷Cs had a similar spread. The spread of values was larger for the calibrations with ²⁴¹Am and smaller for those with ⁶⁰Co. The Mini 900D results were not in such good agreement, probably because this was an analogue device. Again, there was a larger spread for the ²⁴¹Am results. The results for the Mini 6-80/MC71 were similar to those for the Mini 6-90/MC20 for ¹³⁷Cs.

The uncertainties reported for the monitor readings varied considerably for each monitor. Some participants quoted uncertainty values that were up to two orders of magnitude higher than those quoted by others. Some uncertainties were certainly underestimated, for example, the values quoted by a few participants for the GM counter-based monitors (Mini 6-80/MC71 and Mini 6-90/MC20) were significantly lower than those due to statistics.

Full details have been published in NPL Report CIRM 32. This concluded that the overall accuracy was satisfactory and that calibrating facilities could meet the requirements of NPL Good Practice Guide 14 (GPG14). The consistency of the ²⁴¹Am results was not as good as would be expected from the estimated uncertainties. The treatment and estimation of uncertainties was inconsistent between laboratories, and guidance was needed to improve this situation. The policy of disclosure of the ownership of results had not deterred participation.

The problems which had arisen over the treatment of background in the low dose-rate measurements were discussed. The total dose rate seen by the monitor (ie. dose due to the radionuclide source plus the background) should be termed the “true dose rate” instead of using this term for the dose rate due to the radionuclide source only. A discussion ensued over the appropriate way of correcting for the background (or the component of the monitor response that was due to background). The practice of measuring the response at say 0.5 µGy h^-1 did not make sense; for one thing it meant counting for a very long time to acquire adequate statistics for both the total field and the background correction. One should measure background every time (at low dose rates) because this was an indication of the instrument’s performance. Measurement of absolute background was not useful because it was necessary to know the response of the monitor under calibration to radiation of that energy, etc.

It was proposed that the next gamma-monitoring intercomparison should cover ISO narrow-series x-ray fields. There was some support from the meeting for expanding the scope of the next intercomparison, but it was considered impractical to increase the amount of measurement that had to be undertaken by participants. It was suggested that the next gamma-monitoring intercomparison should concentrate on improving the treatment of uncertainties.

Intercomparison of calibrations of neutron area survey monitors

Vic Lewis (NPL) reported that the second IRMF intercomparison of calibrations of neutron area survey monitors had been completed. The exercise involved calibrations of the Harwell 0949 and Studsvik 2202D monitors in fields produced by ²⁴¹Am-Be and ²⁵²Cf sources.

The normalised results for the ²⁴¹Am-Be calibrations were within bands of ± 1.5% and ± 1.7% for the Harwell 0949 and the Studsvik 2202D respectively. The normalised results for the ²⁵²Cf calibrations were within bands of ± 1.0% and ± 0.6%. for the Harwell 0949 and the Studsvik 2202D respectively, albeit with a smaller number of participants. Participants had produced typical uncertainty budgets. The reported expanded uncertainties were of the order of between 1.6% (under discussion) and 4.7% at the 95% confidence level. The range of calibration values was well within the estimated uncertainties.

Full details have been published in NPL Report CIRM 34. This concluded that the overall agreement and accuracy were satisfactory and that calibrating facilities could meet the requirements of NPL Good Practice Guide 14. The treatment and estimation of uncertainties was consistent between the laboratories.

The meeting considered that the three-year cycle for both the gamma-monitoring and the neutron-monitoring intercomparisons was appropriate. A shorter gap was certainly not feasible because the exercises took about two years between the initial planning and the production of the final report. Working parties to plan the next round of intercomparisons would be set up at the next meeting of IRMF with a view to starting the measurements in the summer of 2001.

Good Practice Guide on Practical Radiation Monitoring

This guide is intended to fill the gap between the calibration good practice guide and the ability of people in the field to use an instrument.

There would be six sections on -

• definition of the problem - assessing the aim of the monitoring;
• information gathering - available records, history of facility, etc,
• selecting instruments;
• monitoring strategies - training for surveyors, plan of work;
• monitoring techniques - how to get readings and what to record;
• interpretation - expressing data as meaningful radiation quantities.

Lynsey Husband (NPL) reported on behalf of Alan Edwards (DRaSTaC) that it was hoped to publish by the end of the year. A draft would be circulated for comment before then.

Good Practice Guide on The Testing and Calibration of Installed Radiological Protection Instrumentation

Max Pottinger (BNFL Magnox, Berkeley) reported that air monitoring equipment would not be included in this GPG. The guide would cover installed gamma monitors and contamination monitors (including frisking monitors, hand and foot clothing monitors and exit monitors). The testing included routine function checks, weekly tests, periodic tests and before-first-use tests. The GPG was based on documentation produced by BNFL Magnox and Nuclear Electric; guidance documentation from other companies would be welcome.

It was intended to produce a draft by the end of June for circulation to parties with an obvious interest. The deadline for comments would be August. In the meantime some of the proposed tests would be tried out. Reviewing and re-drafting would follow in September along with completion of the appendices which could be circulated for comment with a short deadline. It was hoped to have the final draft ready by mid-November, with publication in 2001.

The problem of choosing radionuclide sources for testing monitors was discussed. The sources had to be long-lived and have sufficient energy to penetrate into monitors. Those considered were ²⁴¹Am (with problem of low energy), ¹³³Ba (with ten year half life) and ¹³⁷Cs (with shielding problem). Discussion followed on how to achieve the appropriate dose rates for various instruments.

Good Practice Guide on The Testing and Calibration of Installed Radiological Protection Instrumentation

It was thought that there was still a need for guidance on air monitoring equipment - perhaps IRMF was not the appropriate forum for this subject. Air monitoring was a big issue involving various types of instrumentation. Existing guidance in this area was limited and a forum was needed to represent the users; the IRMF did not attract people in this area. This issue could be addressed within the next NMS programme if DTI thought it sufficiently important.

Working Party on Uncertainties in Radiological Measurements

Vic Lewis (NPL), reporting on behalf of Mike Woods (NPL), summarised the background to the present situation. Following the withdrawal of the NAMAS document on uncertainties in radiological measurements, NIS 0825, a working party had been set up in April 1996 with six members, three of whom had since left and been replaced. Examples of uncertainty budgets, along with explanations, had been produced in five selected areas - neutron calibration, surface contamination, source calibration, radiotherapy and x-ray and g-ray RPIs. The first three of these had been finalised. There had been little progress on the remaining two, which needed to be put into a format consistent with those of the first three. The recent gamma-monitoring intercomparison had demonstrated the need for guidance in that area.

The original intention was to publish a revised NIS 0825. The alternative route would be to publish an NPL GPG. Either would be acceptable to UKAS for the purpose of accreditation.

NMS Programme for Ionising Radiation Metrology

Martyn Sené (NPL) described the National Measurement System (NMS) which consists of a number of scientific programmes. The NMS was funded by DTI. and covered the provision and development of primary and secondary standards, dissemination and traceability of those standards, R&D into science and techniques for measurement, maintenance of calibration facilities and links with the International Measurement System. The NMS Ionising Radiation Metrology Programme includes three technical themes covering Dosimetry, Radioactivity and Neutrons. A fourth theme, Technology Transfer, ensured that the benefits are disseminated to the user community. This was done through promotional activities, user groups and fora such as IRMF, the organisation of intercomparisons, provision of workshops and training courses, the production of guidance documents and by work on national and international committees.

The present three-year programme would end in September 2001. The formulation of the next programme was just starting and would take about a year, with input coming from NPL, the user community, regulatory drivers and the MAC Working Group which was set up by DTI to advise on the programme content. A draft programme would be produced by the end of 2000 following the holding of a number of focus group meetings of people from different technical areas. This would go out for consultation prior to drafting the final programme for submission to DTI. The user community would be invited to comment on the proposed programme. The NPL needed names of appropriate people to invite to the focus group meetings in September.

The main justification for the programme would be its relevance to the needs of the user community. A key aspect of the process would be the involvement of the users, who would be asked to make their views known through fora such as IRMF or by writing directly to NPL. The NPL also needed to be told what activities were valued in the present programme.

Private Finance Initiative at the National Physical Laboratory

Martyn Sené (NPL) reported on the re-building of NPL. Nearly all activities would be moved into new accommodation in the next two years. The Centre for Ionising Radiation Metrology would have two new buildings, one being a ⁶⁰Co facility. The Linac would not be moving and the associated services would be little affected. The neutron accelerator facilities would not be moving, but the manganese bath facility would be moving sometime next year.

As a result of the moves of equipment, facilities and staff, various calibration and measurement services offered by NPL would not be available for different periods. Customers were asked to contact NPL for up-to-date information.

Type testing

Peter Burgess (NRPB) reported on the Helsinki IEC meeting that was concerned with instrument design. The most commonly-used IEC standard, IEC 846, had been revised. The improvements included the need to consider energy and polar response together - the energy response needed to be measured at different angles. The specification was for angles out to 45° but data were requested for larger angles. There would be an improved battery function test to determine an instrument’s resilience to battery effects and a better temperature shock test. Most of the existing standards covering electronic personal dosemeters would be combined into one document. This would take a much broader view and will incorporate guidance on the uncertainties. There would be a proper battery test and temperature shock test for neutron survey instruments.

Price of calibrations

There was a heated discussion on the price of calibrations for radiological instruments. The prices were said to be driven down to uneconomical levels by commercial pressures of competition. There was nothing to prevent some services from not adhering to GPG14, thereby lowering their costs and forcing others to lower theirs.

Next Meeting

It was agreed that the next meeting will take place on Wednesday 15 November 2000.

Vic Lewis
Secretary, IRMF
Centre for Ionising Radiation Metrology, NPL (vic.lewis@npl.co.uk)

The above report is a summary of the minutes of the meeting. These have been sent to all members attending the meeting and those who had previously expressed an interest in attending IRMF meetings.

Anyone with an interest in the metrology of ionising radiation who wishes to attend or learn more about IRMF meetings and activities should contact the Secretary.

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