Minutes of the Twenty-Fourth IRMF Meeting
Wednesday 20th November 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 | |
| BAE Systems Ltd | BNFL, Sellafield | |
| DERA Radiation Protection Service | European Commission | |
| HMS Sultan | HTSL | |
| Johnson Controls | Lab Impex Systems | |
| National Physical Laboratory | New Cross Hospital | |
| NRPB | RRPPS | |
| RWE NUKEM | Serco Assurance | |
| Sherwood Nutec Consultancy | Siemens Environmental Systems | |
| St Thomas’s Hospital | Thermo Electron Corp | |
| UKAS | UKAEA | |
| Warship Support Agency, Foxhill |
Nine members from a further three 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. In addition, 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. As a result of this, the main exercise did not finish until July 2002. Vic was unable to show a complete set of results since discussions with participants were ongoing, however he reported that a spread of 9.4% had been observed in the results for the Mark 7 NRM with 241Am-Be and 16% for 252Cf. With the NM2 spreads of approx 10 % and 11% had been achieved for the 241Am-Be and 252Cf respectively. Initial analysis had suggested the large range of results for 252Cf could be due, partly, to one of the participants using a source that had not been calibrated for eleven years. Since the 250Cf component in the 252Cf has a longer half-life it would affect the effective half-life of the source and introduce errors of the order of 4‑5%. Vic commented that the spreads are higher than in the previous exercise. Further discussion would be held with the participants and a wash-up meeting would take place (possibly after the next IRMF meeting). Vic thanked the participants for their work on this exercise.
Gamma-radiation Monitoring Comparison
Duncan McClure (NRPB) reported that in previous IRMF meetings, the Working Group for this comparison had been asked to consider the inclusion of X-ray qualities and passive dosemeters. After some discussion it had been concluded that these should not be included this time, but it was acknowledged that the matter should be considered by the IRMF, perhaps as a separate exercise in the future. The instruments chosen for the present exercise were the Electra with MC20 probe, the Mini-Rad 1000 and the EPD Mk2. These were circulated for calibration in fields produced using 137Cs, 241Am and 60Co sources, at various target and optional dose rates. Duncan reported that overall there had been few problems. The results were generally very consistent, although the reported uncertainties differed widely.
Vic Lewis (NPL) presented a summary of the results and informed the meeting that the data would be discussed in detail at a wash-up meeting to be held following the main meeting. In general there had been large variations in the uncertainties estimated by the participants for the dose equivalent rates and for the monitor responses. The largest uncertainties quoted were some five times greater than the smallest. A draft report had been circulated to participants prior to meeting. The final report was expected to be published early in the New Year.
(Secretary’s note: The full report was published as NPL Report CIRM 55, December 2002)
Mike Woods (IRMC) voiced his concern that participants were quoting ranges as uncertainties. He pointed out the UKAS guidance had been publicly available for a significant time and uncertainty budgets should be prepared in a consistent and correct manner. Vic Lewis responded that the situation was not satisfactory and the forthcoming Good Practice Guide 49, dealing with uncertainties in radiological measurement addressed this problem specifically. Eddie Veater (UKAS) said he would write to all UKAS assessors and UKAS accredited laboratories to draw their attention to the UKAS announcement made by Roger Worrall at the last IRMF meeting, as it appeared some were not already aware of this.
Secretary’s extract from IRMF minutes 23: 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.
Duncan McClure and Vic Lewis thanked the instrument manufacturers for the loan of the instrumentation used in this exercise and acknowledged the efforts of the participants.
Comparison of surface contamination monitoring
Clare Scott (NPL) announced that the report on the comparison exercise had been published and copies were available at the meeting. Clare asked the attendees to consider joining a working group to begin planning the next exercise in the Yew Year. Volunteers for the working group were:
John Bennett (DRPS)
Peter Burgess (UKAEA)
Jan McClure (NRPB)
Clare Scott (NPL)
The first meeting would be held around February 2003.
Good Practice Guide 49, Treatment of Uncertainty in Radiological Measurements
Vic Lewis described the aims, scope and progress of the GPG on uncertainties. The document contains a foreword, glossary, references and technical sections on protection level photon monitoring, neutron area survey monitoring and surface contamination monitoring. Definitions and guidance on the use of Type A and Type B uncertainties, probability distributions, sensitivity co-efficients, divisors, confidence levels and coverage factors are included. Additional guidance on the handling of dominant uncertainties and the calculation of coverage factors for non-ideal situations was also incorporated. Vic went on to describe the example calculations that were prepared for photon and neutron monitoring.
Mike Woods (IRMC) reported that the section on surface contamination monitoring included example uncertainty budgets for the calibration of wide area reference sources and for the calibration of survey instruments. Mike demonstrated the use of partial differentiation to determine sensitivity co-efficients for use in uncertainty budgets.
Good Practice Guide 30, Practical Radiation Monitoring
Tony Richards (Leeds) announced that this Guide had been published. Copies were available at the meeting from the secretary. Tony thanked the other members of the working group for their valuable contribution to the production of the guide.
Realistic Neutron Fields
Graeme Taylor (NPL) outlined the problems that arose because neutron fields can vary considerably from one facility to another and also within each workplace. This was illustrated using typical spectra showing dose equivalent contribution against energy for a gas cooled reactor or AGR (very soft above 1 MeV), a transport flask (still reasonably soft) and fuel processing and source fabrication sites (much harder).
Calibration of area survey monitors was generally carried out using 241Am-Be or 252Cf fields where the dose equivalent spectrum is relatively hard. If an instrument calibrated in this way were used around an AGR, anomalous results could be produce due to the differences in the spectra. This was because an ideal instrument whose ambient dose equivalent response was constant with energy had not yet been manufactured. For such an instrument, the spectrum in which it was calibrated would not matter. However, such a development was unlikely to happen and therefore, care needed to be taken when interpreting measurements made in any field that was different to the calibration field. This became more important when the deviation from ideal of the instrument’s response is greater.
Although realistic neutron field facilities already existed in Germany and France (based on various source and moderator combinations), the spectra produced were still quite hard (with appreciable H*(10) contributions in at 4 - 5 MeV or higher). Clearly, these were not ideal for calibrating instruments to be used in soft spectrum situations.
In response to the lack of suitable facilities, NPL had developed the existing Van de Graaff accelerator facilities to generate a softer realistic neutron field. This employed a thick Al-Li target (based on aerospace alloy with 1.5% Li by weight) positioned at the centre of a 40 cm diameter D2O sphere. Because the energy of the beam from the Van de Graaff could be varied, a range of realistic neutron fields (with maximum neutron energies selectable from 0.6 to 2 MeV) could be produced depending on the beam energy used.
NPL had decided to investigate the use of an intermediate field with a maximum neutron energy of about 1 MeV with a dose equivalent rate of around 44 μSv h-1 and a mean neutron fluence-to-dose equivalent conversion factor of 20 pSv cm2 (one-fifth that of a D2O-moderated Cf source). There were some issues to be resolved (eg. the exact amount of light water in the D2O).
Measurements had been compared with predictions by Monte-Carlo modeling. The realistic field generated was found to be a little harder than the AGR spectrum, but it compared well with that of the transport flask. It should provide a good test for instruments that are to be used in soft neutron fields. The NPL system was considered to be a test facility rather than a calibration facility. It was planned to make this system available early in the New Year to perform measurements for outside organizations, starting with neutron spectrometry and following on with radiation protection instrumentation.
[Secretary’s note: Unfortunately, problems with the Van de Graaff accelerator itself has resulted in the facility being unavailable until Summer 2003 at the earliest.]
Forthcoming Events
The following meetings and courses were announced:
- Combined meeting of Gamma Spectrometer and Alpha Spectrometer Users Fora, 3 September 2002 at NPL
(contact Simon Woods 020 8943 6424) - Practical Course in Reference Dosimetry, 28-31 January 2003
(contact Rebecca Nutbrown 020 8943 6473) - Radionuclide Calibrators User Forum, 31 January 2003
(contact Michaela Baker 020 8943 6579) - Liquid Scintillation Users Forum, April 2003 at NPL
(contact Andrea Woodman 020 8943 6435) - Environmental Comparison Workshop, April 2003 at NPL
(contact Denise Woods 020 8943 6520)
Secretary’s notes:
The Environmental Comparison has since been organized for 8 May 2003.
Other Business
MoD Nucleonic Calibration Qualified Persons Committee
Keith Simmons (Warship Support Agency) informed the meeting that this committee was a tri-service group covering radiation detection and monitoring instruments used in support of UK Government requirements. The instruments capability range spans general Health & Safety to specialist full war threat detection and monitoring. The committee comprised 10 defence calibration facilities, some of which were represented at the IRMF.
The objectives of the committee included: the provision of a technical quality forum to establish best practice within the defence calibration industry; to facilitate the exchange of technical information; to produce nucleonic calibration protocols adopting best practice; to co-ordinate effort in determining problems with and the shortcomings of instruments and selecting new instrumentation; to implement MoD audits and comparison exercises.
Sponsorship of JSP-425 would be transferred from the Director of Health and Safety to NCQPC. This would allow amendments to be made more easily. The document would be updated and extended to include audits and comparison exercises as part of a quality system. Compliance with JSP-425 was not expected be as onerous as UKAS accreditation but it should ensure a consistence in quality between calibration sites. A separate manual would be published that would detail all HP instruments and protocols in use within MoD.
900V Geiger Tubes
Peter Burgess (UKAEA) relayed an incident involving the purchase of a 900V Geiger system that had been procured to replace an old gamma alarm. A few mSvh-1 dose rate had been thought to be appropriate for the environment in which it was to be used, so the new instrument was successfully tested for these conditions.
Peter had queried whether these instruments would respond properly over the whole dose rate that they could reasonably encounter (realistically Svh-1 in emergency situations). Peters experience of the most commonly used 500V Geiger instruments was that when dose-rate is increased to a very high level, the pulse size did not fall below around 20% of that observed at low dose rates, it is therefore possible to set a threshold with confidence that all pulses will be collected. However, 900V Geiger tubes are not always reliable.
Peter had commissioned additional overload tests and found that the instrument alternately displayed overload and the dose rate, however the indicated dose rate actually fell as the real dose rate was increased. It was observed that the pulse height and current dropped off causing the Geiger to fail catastrophically. This was not acceptable for the use intended. Modifications of the instrument using a Mini Instruments detector resulted in a satisfactory overload test.
Peter concluded that it was essential to perform a realistic assessment of the maximum possible dose rate an instrument could encounter and check the instrument performs well to at least that level; instruments should not be moved into different dose rate environments without re-assessing the maximum dose rate and ensuring the instrument is still suitable. It was Peters belief that 900V Geigers are guilty until proven innocent!
Keith Simmons asked what should be done when only a limited calibration is possible due to the lack of facilities for achieving very high dose rates. Mike Woods advised that calibration laboratories need to be clear with their customer to ensure the customer understands the limitations of the calibration and hence the use of the instrument. Mike recommended obtaining a written acknowledgement of this understanding from customer. John Bennett added that it is the responsibility of the customers RPA to ensure that an instrument is fit for the intended use.
Thermo Electron Corporation
Jen Barnes (Thermo Electron) announced that Saint-Gobain Crystals and Detectors had become part of the Thermo Electron Corporation on 1 July 2002. Jen reported that the Mini instruments part of the company would be relocated from Burnham-on Crouch to the Beenham site at the end of the year. Beenham had already begun producing and testing the Mini products in preparation for the transfer of the business. Some Burnham staff would move to Beenham but also new staff had been recruited and were undergoing training.
In response to a barrage of questions, Jen stated that:
- Mellissa was still the service manager;
- Roy Sheppard would be retained by Thermo;
- the telephone number hadn’t changed but e-mail addresses had;
- there was an ongoing process to produce a new website but in the mean time the website would be redirected from the old address;
- repair facilities should not be affected - an additional volume of work had been anticipated and recruitment and training undertaken should cover this;
- it was hoped that the lead time on spares would improve when they were able to hold larger stocks;
- it was hoped that the ‘make to order’ policy for new instruments would be abolished; and,
- she was not aware of any priority being given to US rather than UK orders.
Next Meeting
The next meeting will take place on Wednesday 21 May 2003.
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.