Wednesday 26^th May 2004
National Physical Laboratory

Present:

Chairman:		Steven Judge,		National Physical Laboratory
Secretary:		Clare Lee,		National Physical Laboratory
		Nasser Baghini		Imperial College Reactor Centre
		Graham Bass		National Physical Laboratory
		John Bennett		DERA RPS
		Derek Brazer		AWE
		Peter Burgess		UKAEA
		Wes Case		Amersham
		Robin Crosse		Thermo Electron
		Bill Croydon		Thermo Electron
		Julian Dean		National Physical Laboratory
		Denise Delahunty		RRPPS, Birmingham
		Geoff Druce		AWE
		Heather Fitzgerald		Radiological Protection Institute of Ireland
		David Gallacher		St Thomas’s Hospital
		Paola Hayes		HMS Sultan
		Richard Jenkins		BAE Systems Ltd
		Simon Jerome		National Physical Laboratory
		Steven Judge		National Physical Laboratory
		Gwe-Yan Kim		Korea Food and Drug Administration
		Don Mackenzie		Johnson Controls
		Andy Main		Johnson Controls
		Duncan McClure		NRPB, Chilton
		Jan McClure		NRPB, Chilton
		Lee Packer		AWE
		Martin Palmer		National Physical Laboratory
		Max Pottinger		BNFL Instruments
		Mike Renouf		BNFL
		Tony Richards		Consultant, Leeds
		Geraint Roberts		NRPB, Chilton
		Sue Ryan		New Cross Hospital
		Joanne Shaw		NRPB, Leeds
		Jon Silvie		BAE Systems Ltd
		Keith Simmons		MoD Nucleonic Calibration QP Committee
		John Simpson		RWE NUKEM, Winfrith
		Rick Tanner		NRPB
		Klaus Thieme		AEA Technology, Germany
		Russell Trueman		DERA RPS
		Eddie Veater		UKAS
		Jon Wardle		AWE
		Eliot Williams		UKAEA
		Tudor Williams		National Physical Laboratory
		Mike Woods		IRMC
		Marek Ziolowski		AEA Technology

 

27.1 Welcome and announcements

On behalf of NPL, Steven Judge welcomed members to the Twenty Seventh Meeting of IRMF, especially Dr Thieme and Dr Kim from overseas.  Steven announced that Martyn Sené had been promoted to Director of the Quality of Life Division and had consequently delegated responsibility for the IRMF to Steven.  Steven thanked Martyn for his work for the IRMF.

27.2 Apologies for absence

Apologies had been received from –

Graham Beard		Mainance International Ltd
Tim Dollery		Mainance International Ltd
Shaun Marriott		BAE Systems Ltd
Bob Mason		Sherwood Nutec
Sharan Packer		HSE
Martyn Sené		National Physical Laboratory
Graeme Taylor		National Physical Laboratory
Tudor Williams		National Physical Laboratory

 

27.3 Approval of Minutes of Twenty Sixth Meeting, 19 November 2003

The Secretary reported that she had not received any comments on the minutes prior to the meeting. However, the members present requested the following amendments:

• Section 26.4 should read ‘HSE were organising a draft proficiency test’ (Jon Wardle AWE);
• Section 26.11(d) should read ‘ISO 11929 Determination of the Lower Limits of Detection and Decision Threshold for Ionising Radiation Measurements and is produced in eight parts.’ (Tony Richards);
• Section 26.7 should read ‘Geoff Druce (AWE) added that he felt strongly that Personal Air Samplers should not be included and GPG14 would be a suitable location for EPDs.’ (Geoff Druce).

The Secretary agreed to amend the minutes featured on the IRMF website.  The Chairman approved the remainder of the minutes after it was agreed that they were an accurate record of the meeting.

27.4 Matters arising

Tony Richards (Leeds) reported that IPEM were interested in taking part in a comparison of X‑ray qualities and they were going to provide him with details of the appropriate contact person.

Tony Richards also stated that the BSI committee, NCE2, were looking to amend ISO4037 to incorporate a table of relevant uncertainties for conversion factors (see 26 minutes 26.11(c) for details of the issue). Max Pottinger (BNFL Instruments) added that this standard indicated no uncertainty on the conversion factor was needed for monoenergetic radiations, however the committee felt this was incorrect. Jon Wardle (AWE) said that he needed uncertainty values for narrow and wide series conversion factors too; Jan McClure (NRPB) informed him that these were both in the standard and were ± 2% (k=1). Eddie Veater (UKAS) apologised for not writing to the UKAS accredited labs to inform them that a compliance statement on the certificate about the conversion factor uncertainty not being accounted for, would be satisfactory; he had believed that this issue would have been resolved before this meeting.

Secretary’s notes: Eddie Veater wrote to the accredited laboratories concerned on 22 June 2004, a copy of this letter can be found at the end of these minutes; Tony Richards has supplied an update of the situation regarding ISO 4037, this can also be found at the end of these minutes.

27.5 Surface Contamination Monitoring Comparison

Clare Lee (NPL) reported that 9 organisations had registered to participate in the exercise. Clare had not had time to approach the manufacturer for the loan of a DP8 for use in the comparison and proposed to eliminate this instrument in the interests of starting and completing the measurements as quickly as possible. The measurement phase was expected to begin in the next few weeks.

There was some discussion about substituting the DP6 for the DP8 as this had been included in the previous exercise and could demonstrate ongoing competence. It was also suggested that if only two instruments were circulated, participants could be asked to perform uniformity tests on some of their wide area sources and report the results (see section 27.8 for background information).

Secretary’s note: Robin Crosse (Thermo Electron) said it was still worth contacting his company about the loan of a DP8, there may be one available.

27.6 Gamma Monitoring Comparison

Duncan McClure (NRPB) reported that planning of the next exercise continued and registration would take place at the next meeting.

27.7 Review of Good Practice Guide 14

Clare Lee (NPL) informed the meeting that a comments submission form had been added to the IRMF website. She asked the members to publicise the review to colleagues and to submit their own suggestions for amendments, requests for extensions to the scope etc.

Mike Woods (IRMC) asked the working group to consider adding something about the effect of uncertainties when comparing calibration results to type test data for a particular instrument; this was not included in the first edition of the guide. According to UKAS document Lab 12 ‘The Expression of Uncertainty in Testing’, only the first data point on the chart below constitutes a ‘pass’ (within ± 30% of type test data). Note that if a 95% confidence level is quoted, the uncertainties plotted must be at k=2.

Jon Wardle (AWE) commented that uncertainties could be large as ionisation chambers could fluctuate significantly at dose rates of the order of 2.5 mSvh^-1. Russell Trueman (DERA RPS) pointed out that GPG14 stated sufficient readings should be taken to establish a mean indication with suitable accuracy (± 10% standard deviation of the mean); as well as avoiding the possibility of a wildly fluctuating instrument passing a calibration, this gave the QP added confidence in the results of calibrations that had been supervised. There was further discussion about whether a similar requirement should be made of contamination monitors.

Mike Renouf (BNFL) asked if it would be possible to put a bulletin board and members area on the IRMF website where members could discuss the proposed revisions to GPG14. Steven Judge (NPL) said he believed this was not possible with the NPL IT system but Clare agreed to check.

27.8 Issues With Wide Area Sources

In response to two requests to discuss issues related to wide area sources, the Secretary had invited Isotrak (the manufacturer of the sources under discussion) to attend the meeting. Duncan McClure (NRPB) and Denise Delahunty (RRPPS) gave presentations of the problems they had experienced. Klaus Thieme (AEA Technology) responded to these issues and gave a presentation of the general production process of the sources.

The nature of NRPB’s problem had related to the uniformity of new sources recently purchased; they believed the measured uniformity to be greater than the ± 10% quoted on the calibration certificate. After finding this, he had also examined NRPB’s older source and found some of those also to be non-uniform. Duncan acknowledged that the technique used to determine their uniformity estimate had not been in exact compliance with ISO 8769.

View Duncan McClure's presentation

Denise explained that RRPPS owned a ⁹⁰Sr wide area source that had recently developed staining around the top surface of the backing plate; they had also been having difficulty getting this source calibrated. The source had been sent to NPL but measurements in a windowless proportional counter had been unstable and could not be certificated (Clare Lee suggested this may be due to the source no longer conducting). The source had been successfully calibrated later at AWE using a windowed proportional counter (no requirement for conduction).

View photograph of RRPPS source

There was some discussion about the possibility of the staining being due to the adhesive seeping out from beneath the active surface; this would also explain the non-conduction observed. Klaus Thieme and the other members were unable to offer any further potential explanations or advice.

In the general discussion that followed Clare Lee (NPL) stated that some of the sources NPL had purchased in 1996 for the contiguous portions project had been non-uniform. John Simpson (RWE Nukem) said that he had experienced problems with ¹⁴C and ²³⁴U sources but ³⁶Cl had been ok. Jon Wardle (AWE) said that their alpha and beta source uniformities were fine. Jan McClure (NRPB) was concerned that Isotrak certificates were routinely issued with a blanket uniformity statement of less than ± 10%. Jon Wardle (AWE) queried whether the 10% requirement was at k=1 or k=2 as ISO 8769 was not specific.

Secretary’s note: the uniformity of NPL’s sources was between 10 and 12% for ⁹⁰Sr and ¹⁴C and ²³⁴U was 23%. The ²⁴¹Am, ⁵⁷Co and ¹³⁷Cs sources purchased had all been <10%.

Klaus Thieme (AEA Technology) relayed the specifications for wide area sources as published in ISO 8769-1 and outlined the manufacturing process used by AEA Technology. In particular, the uniformity of sources was determined using a position sensitive windowed proportional counter. Klaus displayed the uniformity results that had been obtained by AEA Technology for the NRPB sources using the ISO definition; all data had fallen within specification. Klaus added that no customer complaints regarding source non-uniformity had been received in 40 years.

View Klaus Thieme’s presentation

Mike Renouf (BNFL) commented that it may be more appropriate for customers to order sources according to the characteristics they are interested in, rather than ordering ‘Class 2’ sources. Klaus responded that most customers order sources outside the specification of the ISO standards, usually < 2000 s^-1 and < 150 cm². Steven Judge (NPL) added that guidance varies within the countries of the EU, for example, in Germany the source area is chosen to match that of the detector and so, to a first order, the uniformity of the source does not matter. Steven asked Klaus to consider making uniformity data available to UK customers. Peter Burgess (UKAEA) added that this would promote customer confidence. Steven also asked if it would be possible to produce more uniform sources. Klaus said that the production process for ⁵⁵Fe had been changed; they would now be made by dispensing activity to the source rather than the anodizing technique, although more costly this would yield more uniform sources.

Max Pottinger (BNFL Instruments) asked Klaus if AEA Technology had produced coloured air sampler sources; Max had some from the mid-90’s. Klaus offered to check if Max supplied him with the source identifier.

David Thomas (NPL) asked if ³H sources were manufactured from anodised aluminium and if they were stable. Klaus confirmed this technique was used and the only stability problems had been observed with ¹⁴C.

27.9 A Semi-Empirical Model for Surface Contamination Monitoring

Steven Judge (NPL) reported that he had begun work on a mathematical model for surface contamination monitoring. This had been done in response to the finding that GPG30 contained some errors. More work on the mathematical model was required but members were invited to comment on the work so far. Steven was aiming to publish an NPL Report by the end of September.

View Steven Judge’s presentation

Denise Delahunty (RRPPS) commented that Steven’s model was quite complicated and it would need to be made simpler for the radiation protection audience. Steven responded that the model would get even more complicated and so would be published in the style of a scientific paper; this would then be simplified for inclusion in GPG30. Mike Woods (IRMC) added that once the complicated maths had been performed, it would be possible to generate generic responses for particular sets of instruments. Peter Burgess (UKAEA) suggested that instrument manufacturers should use this to supply response data for a range of radionuclides. Jon Wardle (AWE) said that AWE had produced a model to generate ^99mTc responses from instrument responses to standard reference sources but no-one had ever made use of it; Mike felt that this model had been too complicated for general use. It was the general feeling of the meeting that instrument manufacturers should commission determinations of instrument response to the standard ISO reference sources as well as some commonly used radionuclides as part of the Type Test, and publish the results in the instrument manuals.

Max Pottinger (BNFL Instruments) asked what the outcome had been of the GPG30 review meeting held after the IRMF meeting on 19 November 2003. Clare Lee (NPL) informed him that the use of ‘efficiency’ in GPG30 had been withdrawn, as had Appendix 6 containing a worked example. Once the modelling work was complete, GPG30 would be revised as necessary; the pdf version on the NPL website reflected the changes.

27.10 Any Other Business

(a) Enhanced Technical Underpinning of Portable Radiological Instruments at AWE

Geoff Druce (AWE) reported that AWE had undertaken a review of its current holdings of portable instrumentation and its future requirements.

View Geoff Druce’s presentation

Keith Simmons (MoD Nucleonic Calibration QP Committee) said that the MoD was going through a similar process, they had begun with the RPAs detailing their requirements, and instruments were then matched to these.

(b) Cable Lengths

Peter Burgess (UKAEA) reported that he and Reg Bosley (RWE Nukem) had made some preliminary studies of the effect of changing cable lengths. Using an Electra 1A and DP6 probe they had increased the cable length (the cable type had been consistent) and observed that the operating voltage had only changed by 3 V per metre of cable. They had therefore concluded that it was unnecessary to tweak the HT if only changing between a 1 m and 2 m cable for example.

(c) Dual Phosphor Probes

Peter Burgess (UKAEA) outlined some work he had done on the effects of threshold settings for DP6AD probes with Electra ratemeters. Dual phosphor probes, mostly DP6s, were in use throughout UKAEA. The major problem with these probes was that their response to ⁶⁰Co was low and also not particularly stable. Where ⁶⁰Co was a significant part of the fingerprint, UKAEA had been unable to trust the probes for clearance measurements and, hence, the measurement was often repeated using a detector with a better beta response such as the BP19.

The standard setting for the Electra used a beta threshold of 100 mV and an alpha threshold of 1.5 V. Pulses less than 100 mV were ignored and assumed to be noise, pulses between 100 mV and 1.5 V were taken as betas and pulses over 1.5 V were taken as alphas. The usual process for setting up dual phosphor probes was the beta channel dip method:

(1) Set ratemeter to beta channel;

(2) Wind down the HT to a low level, 600 V or so;

(3) Connect the dual phosphor probe;

(4) Place on top of an alpha emitting source, generally either ²³⁸Pu or ²⁴¹Am;

(5) Perform a plateau measurement by increasing the HT in big steps until the ratemeter starts to respond and then moving to much smaller steps until the minimum in the count rate is found; this then becomes the operating potential.

Peter explained that the basic physics behind this procedure was that at low HTs the gain of the photomultiplier is too low and the alphas appear in the beta channel. As the HT is increased, the gain increases and the count rate in the beta channel increases as more of the alphas exceed the beta threshold. As the HT is increased further, the alphas increasingly move into the alpha channel, causing the beta channel count rate to fall. At some point, the count rate in the beta channel starts to rise again. The assumption often was that this is background and was essentially noise, but Peter had never been entirely convinced. Inevitably, there were other radiations present, such as the gamma radiation from ²⁴¹Am at 60 keV, but there was nothing obvious from ²³⁸Pu. An alternative method was to set the HT just below the point at which ⁹⁰Sr + ⁹⁰Y beta radiation does not quite enter the alpha channel.

These methods were compared with the results from setting up a probe as a beta contamination monitor only, by selecting the alpha + beta channel and then performing a classic plateau, looking at the response to a nuclide at the low end of the energy range and comparing count rate and background against HT.

It was concluded that the revised settings gave a better contamination response and would allow UKAEA to be confident of ⁶⁰Co detection. However, the sensitivity to low energy gammas was increased considerably in the beta channel and further investigation should be carried out.

27.11 Forthcoming Events

The Chairman presented the following list of forthcoming events.

• Neutron Users Club meeting, NPL, 7 October 2004
  Contact Peter Kolkowski, 020 8943 6855
• Airborne Radioactivity Monitoring Users Group meeting, NPL, 9 November 2004
  Contact Andrea Woodman, 020 8943 6435
• Practical Course in Reference Dosimetry, NPL, January 2005
  Contact Rebecca Nutbrown, 020 8943 6473

Jan McClure (NRPB) informed the meeting that their Instrumentation course for June 2004 was fully booked but they were hoping to run a second course in 2005.

Paola Hayes (HMS Sultan) announced their Nuclear Instrumentation Calibration course to be held at HMS Sultan between 4 and 22 October 2004, further information was available from Jon Taylor, tel. 023 92 546051.

It was also announced that an SRP workshop on clearance monitoring would be repeated.

27.12 Next Meeting

The next meeting will be held on Wednesday 24 November 2004 in the Module 16 conference facility. As usual, the meeting will commence at 10.30 am, with coffee served from 10.00 am.

Clare Lee

IRMF Secretary
Quality of Life Division, NPL

ACTIONS

Secretary
Amend minutes of 26^th meeting.

GPG14 Review Working Group
Consider effects of uncertainties in pass/fail criteria.

Secretary
Investigate viability of bulletin board and members area on IRMF website.

Tony Richards
Determine the appropriate IPEM contact person to discuss a comparison of X-ray energies.

Eddie Veater
Write to UKAS labs about compromise position for uncertainties in conversion factors on calibration certificates.

John Simpson, Peter Burgess and Tony Richards
Continue dialogue with the NCE2 committee regarding the use of and origin of uncertainties for conversion factors stated in ISO 4037.

Members representing calibration facilities
Send additions and amendments for IRMF Catalogue to the Secretary.

All members
Inform the Secretary of suggestions for presentations on technical topics for the next IRMF meeting before the end of October 2004.

Statement on Uncertainty of Conversion Factors in ISO 4037-3

ISO 4037-3 recommends that the conversion coefficients from air kerma to the operational quantities for monoenergetic radiations given in certain tables should be regarded as having no uncertainty. However the mono energetic radiations from Cs-137 and Am-241 given in ISO 4037-3 should be treated as having an uncertainty associated with them.

Members of the BSI committee at the meeting of ISO TC85 recently held in Buenos Aires raised this inconsistency.

The conclusion reached was that:

“In practice the spectra from sources like Cs-137 and Am241 are not truly mono-energetic but include scattered radiation particularly from the encapsulation. It is therefore recommended that the monoenergetic data be used for these radioisotopes but that an uncertainty is included. This aspect should be made clear at the next revision of the standard.”

As this will not be changed until the next revision of the Standard ISO 4037-3 the UK must continue to use the information given and therefore the information given in such documents, as GPG 49 for the time being remains unchanged. However it may be possible to put a note in GPG 49 to this effect.

Tony Richards

June 2004

UKAS letter to accredited laboratories

At a number of UKAS visits to radiological calibration laboratories, assessors have raised non conformities regarding the inclusion in the uncertainty budget of a component of 2% for the conversion of air kerma to dose equivalent quantities. This requirement and the 2% figure were detailed in ISO 4037 Part 3, Para 4.1.2.

Some laboratories have chosen to challenge the content of ISO 4037 and have presented technical comments concerning the appropriateness of the general figure of 2%, which does not appear to be adequately justified in the standard. This matter was an agenda item at the Ionising Radiations Metrology Forum (IRMF) meeting held on Wednesday 19 November 2003 at NPL.

At that meeting I said that although I appreciated and broadly followed the valid arguments put forward UKAS and its technical assessors could not vary the requirements of internationally agreed standards. The only way to address this matter was to seek clarification from the working group responsible for this standard.

Two of the members of the IRMF present at the meeting were also members of the working group responsible for ISO4037 and agreed to take this to the working group and seek clarification. This was discussed again at the Ionising Radiations Metrology Forum (IRMF) meeting held on Wednesday 26 May 2004 at NPL. However the working group responsible for ISO4037 were still considering this matter.

In the interim UKAS advises that until clarification is obtained laboratories carrying out calibrations involving conversion of air kerma to dose equivalent quantities may either:

1. Modify their uncertainty budgets to include the 2% figure detailed in the standard.
2. State on the certificated that the uncertainty quoted did not include the 2% figure for the conversion of air kerma to dose equivalent quantities detailed in ISO 4037 Part 3, Para 4.1.2.

Yours sincerely

Eddie Veater

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