Wednesday 15th May 2002
National Physical Laboratory

Present:

Chairman: Martyn Sené, National Physical Laboratory
Secretary: Clare Scott, National Physical Laboratory

There were 41 other members present from 23 establishments including:

Addenbrookes Hospital		Amersham plc
AWE		BAE Systems Ltd
BNFL, Berkeley		BNFL, Sellafield
DERA Radiation Protection Service		HMS Sultan
John Caunt Scientific		Johnson Controls
Kingston University		Lab Impex Systems
National Physical Laboratory		New Cross Hospital
NIRMPA		NRPB
RRPPS		RWE NUKEM
Serco Assurance		St Thomas’s Hospital
Thermo Electron Corp.		UKAS
UKAEA		Urenco
Velindre Hospital		Warship Support Agency, Foxhill

Ten members from a further two establishments had sent apologies.

Agenda Items

Neutron Monitoring Comparison

Vic Lewis (NPL) reported that the current exercise involved the circulation of a Mark 7 NRM and an NM2 monitor around five laboratories for calibration in fields produced by ²⁴¹Am-Be and ²⁵²Cf sources. Two laboratories would calibrate the monitors in 2.7 MeV monoenergetic neutron fields in a bilateral arm of the comparison.

The exercise had progressed to schedule until a monitor received some damage whilst in transit. Additional checks at NPL showed the damage to be superficial. There had however been further minor problems that added to the delay. Two sets of results had been received to date and two further sets of measurements had been completed. It was hoped that the rest of the measurements and the final checks at NPL would be completed in July/August, enabling the analysis to be completed and the draft report circulated in September. The post-completion workshop would follow and the finalised report would be produced by the end of the year.

Gamma-radiation Monitoring Comparison

Vic Lewis (NPL) summarised the main features of the exercise. This involved the circulation of three monitors (Electra with MC20 probe, Mini-Rad 1000, EPD Mk2) for calibration in fields produced using ¹³⁷Cs, ²⁴¹Am and ⁶⁰Co sources at various target and optional dose rates.

The measurements began in September 2001 and good progress had been made with the first six of the fifteen participating laboratories. To date, ten participants had reported results, three more had completed measurements but were yet to submit results and a further two had not made measurements. It was expected that the measurement phase would end in July. Following the analysis of results, the draft report would be circulated in September in good time for participants to examine before the next IRMF meeting in November. It was intended to hold the wash-up meeting in the afternoon of the day of the IRMF meeting. The final report was anticipated by the end of 2002.

Vic Lewis showed a summary of the results, expressed as deviations from the mean for each monitor and field combination. The figures for both the Electra and Mini-Rad 1000 had greater spreads of results for the lower dose rates. A comparison at common dose rates indicated that the spreads for the Electra were slightly greater than those for the Mini-Rad 1000, and those for the ²⁴¹Am fields were greater than those for ¹³⁷Cs fields. The results for the EPD were adjusted to a common set of air kerma to personal dose equivalent conversion coefficients. Some of the calibrations were made on-phantom and others were free-in-air. For the latter, some participants had applied (different) correction factors while others had not applied any. In order to demonstrate the phantom effect the results were adjusted by setting the correction factor to unity.

A summary of the uncertainties reported for the air kerma rates showed considerable variation between participants with several somewhat greater than the lower values of around 2 – 3%. However, the uncertainties reported for the monitor readings tended to dominate those for air kerma rate. For some conditions the largest uncertainties were a factor of five greater than the lowest. This was due to participants adopting different methods of estimating the uncertainty associated with a fluctuating indication and different assumptions made about their probability distributions.

The interim conclusions for the comparison were that the reported calibrations were generally within ±10% and that guidance was needed on the treatment of uncertainties. The work and cooperation of the participants was gratefully acknowledged.

Comparison of surface contamination monitoring

Clare Scott (NPL) reported that there had been little progress since the previous IRMF meeting. The workshop that had been held to view and discuss the results was considered to have been very useful, and had enabled the participants to share their experiences. All that remained was the publication of the full report on the comparison.

Techniques for Monitoring Gamma Surface Contamination

Robin Mather (BNFL, Berkeley) described investigations into consistent techniques for calibrating gamma surface contamination monitors in compliance with GPG 14.  Two methods were used; the first was based on plotting a linear cumulative response graph from detector profile data and applying a correction factor to contiguous portions measurements made on 95% of the response area, as described in GPG 14, and the second used a detector profile to determine the radius of an annulus that encompassed the whole detector response and performing contiguous portions measurements on this area.  When the first method was used it was determined that the response correction factor was inaccurate by at least 15%.  The annular method appeared to work better than the first method however since it required measurements in 170 source positions for a G2 probe using ⁶⁰Co it was concluded that this was impractical.

Robin proposed an alternative method and asked the IRMF members for comments.  This method involved performing one-off contiguous portions measurements using source areas of 150, 900 and 2250 cm², calculating the ratios between these and extrapolating to determine a result for 1000 cm².  A reduced measurement area could then be used in future and a correction applied to determine the expected response to a 1000 cm² source area.

Robin concluded that clarification was required on the appropriate technique to be used for gamma monitoring.  It was hoped that field trials could be performed to test the validity of his suggested method and GPG 14 would be modified to incorporate the findings when it is next reviewed.

Peter Burgess (UKAEA) agreed that there was no logic in going beyond 1000 cm² because the gamma response goes on indefinitely (as a logarithmic relation with distance).  He agreed that the advice in GPG 14 needed to be reviewed.

John Wardle (AWE) reported that measurements made on ²⁴¹Am using contiguous portions were in agreement with a mathematical model. Other members thought that, although this was the case at 60 keV and below, higher energies would prove problematic. Martyn Sené thought that it should be possible to model for ⁶⁰Co for the limited number of detector sizes in use.

GPG29, The Examination, Testing and Calibration of Installed Radiological Protection Instrumentation

Clare Scott (NPL) reported that this Guide was published and available from the NPL e-store. There had been some problems with typographical errors but these had been resolved after reprinting.

Good Practice Guide 30, Practical Radiation Monitoring

Tony Richards (Leeds) briefly described the contents of the guide. It had chapters on -

• gathering information
• monitoring strategies
• instrument types
• monitoring techniques
• quantities and units to be used for reporting

and appendices on –

• different types of detectors and their uses
• typical record sheets
• estimation of surface contamination
• beta and alpha contamination
• failure modes for monitors

The guide was aimed at everybody – from the experienced RPA and the person carrying out the measurements to the newcomer. It was emphasised that this was guidance.

A draft version had been circulated to IRMF members and several other groups for comment.  The Working Group was grateful for the comments that had been received and that were extremely valuable.  The guide was being finalised before being sent to the printers by the end of summer. Particular thanks were due to Lynsey Keightley (NPL) for her hard work.

Trevor Birkett (AWE) commented that guidance for tritium monitors was needed but was not included in GPG 14 or GPG 29. Peter Burgess (UKAEA) stated that tritium monitors had been specifically excluded. Martyn Sené said that thismatter would be high on the agenda of the next meeting of theAir Monitoring Users Group who had been asked to look into producing a GPG.

Some discussion took place about whether the next version of the Guide should be circulated for comment, it was decided by the members that there had already been sufficient opportunity to comment and it would not be circulated again.

GPG on Treatment of Uncertainty in Radiological Measurement

Vic Lewis (NPL) described the aims, scope and progress of the GPG on uncertainties. The first section was a summary of the principles contained in M3003. Three other sections covered uncertainties in surface contamination monitoring, neutron area monitoring and photon dosimetry. Experience gained in the IRMF comparisons on gamma and neutron monitoring along with uncertainty budgets from UKAS-accredited laboratories had been particularly useful. The inclusion of a section on uncertainties in radioactivity measurement was unlikely.

First drafts had been prepared and circulated for comment to a small number of experts. A number of actions had been identified; in particular the guidance on assessing the uncertainty for a fluctuating monitor reading needed to be progressed. It was intended to produce a consultation version to be put on the IRMF website for members to comment on. (Note: The IRMF website is within the NPL website.)

Roger Worrall (UKAS) reported that UKAS had announced an amnesty on the requirement of section 4.2 (c) of M3003 for the resolution of the instrument being calibrated to be included in the evaluation of the overall uncertainty. Expressing the uncertainty in terms of two components by separating out that due to resolution was acceptable to UKAS. As commented earlier, there were all sorts of problems in determining this component, but experience had shown that eye-averaging of wobbling needles gave a reasonable estimate and in digital displays the superfluous digits could be ignored. It would be necessary to include a statement on the calibration certificate on whether or not the resolution of the instrument under calibration is included in the evaluation and also a definition of resolution should be included in a footnote or other appropriate place. In cases where the resolution was the dominant uncertainty, this separation would demonstrate that the uncertainty due to the laboratory was relatively low.

From June 2002 Eddie Veater will be taking over from Roger Worrall in the Radiological area.

GPG 34, Radiometric Non-Destructive Assay

Vic Lewis (NPL) reported that the second consultation version of GPG 34 had been put on the NPL website in January. It was freely downloadable in order to encourage comments from the widest possible area of the user community. To date there had been over 1200 hits on the site with some 415 downloads.

Forthcoming Events

The following meetings and courses were announced:

• Radiotherapy Standards Users Meeting, July 2002 at NPL
  (contact Rebecca Nutbrown  020 8943 6473)
• Neutron Users Club, 1 October 2002 at NPL    
  (contact Peter Kolkowski  020 8943 6855)
• Liquid Scintillation Users Forum, September 2002 at NPL
  (contact Andrea Woodman  020 8943 6435)
• Air Monitoring Users Group September 2002 at NPL
  (contact John Makepeace  020 8943 6480)
• Uncertainties in Gamma Spectrometry Workshop, 2002 at NPL
  (contact Dagmara Tyler  020 8943 6085)

Other Business

Neutron Emission from ²⁴¹Am Photon Sources

John Simpson (RWE NUKEM) reported that neutron dose rates of a few μSv.h^-1 were produced by curie-strength ²⁴¹Am sources commonly used for photon calibration.

Contact Beta Dose Rate

Peter Burgess (UKAEA) discussed the measurement of beta dose rates from small areas of contamination (eg. blobs on fingers) using a device such as an ionisation chamber in contact with the surface. Measurements at NRPB and Birmingham had shown that a factor of about 100 was necessary to obtain the dose rate. This was far higher than that (5 to 10) applicable to larger area sources. A report would be published.

Photon Reference Standards

John Wardle (DRaStaC) reported on the sale of photon reference standards by AWE and the attempts to increase their uptake, particularly by the medical sector. For some uses for which the desired radionuclide was not available, similar sources could be supplied along with a software algorithm as an alternative. Members were asked to contact AWE if they had need for photon reference sources. Denise Delahunty (RRPPS) said that RRPPS had been trying to disseminate such sources for years but with little success.

Testing Damaged Instruments

Keith Simmons (Warship Support Agency) raised the issue of testing damaged instruments, following a problem experienced with an α-probe that did not fail safe. Routine testing would not reveal this effect. Peter Burgess said that destructive type testing was not appropriate because it was not reproducible.

Next Meeting

The next meeting will take place on Wednesday 20 November 2002.

Clare Lee (née Scott)

Secretary, IRMF
Centre for Acoustics and Ionising Radiation
National Physical Laboratory
Teddington
Middx  TW11  0LW

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

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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