Minutes of the 8th Liquid Scintillation Users Forum Module 16, NPL 7 May National Nuclear Laboratory. Babcock Marine-Rosyth Ltd

Transcription

1 Minutes of the 8th Liquid Scintillation Users Forum Module 16, 7 May 2009 Attendees: Simon Jerome Arzu Arinc (Chairperson) (Secretary) Ben Ahern Rob Allott Eleanor Bakhshandeiar Andrew Boschier Keith Bradshaw Jane Caborn Philippe Cassette Lorraine Currivan John Davies Alan DuSautoy Ronald Edler Maria Garcia-Miranda Chris Gilligan Terry Gingell Chris Hardy Peter Harris Robert Huggett Marty Johnson Steven Judge David Lambert Etienne Letellier Olivia Marsden Andy Pearce Gary Prior Charlie Stewart Simon Temple Klaus Thieme Richard Wilson G E Healthcare Environment Agency Veterinary Laboratories Agency AMEC NUCLEAR UK National Nuclear Laboratory CEA RPIR Beta-Gamma Training PerkinElmer DSTL Babcock Marine-Rosyth Ltd Zinsser Analytic USAFSAM Magnox Veterinary Laboratories Agency UKAEA AWE Scientifics Ltd DSRL DPM Solutions Ltd Nuclitec GmbH Westlakes Scientific Consulting Apologies: Pete Burgess Nuvia

2 1. Chairman s welcome / previous minutes and actions arising Simon Jerome introduced himself and welcomed members to the eighth meeting. A straw poll showed which nuclides the participants were interested in and measured routinely: H-3, C-14, P-32, S-35, Cl-36, Fe-55, Ni-63, Kr-85, Sr-90, Tc-99, Pm-147 and Pu-241. A couple of participants mentioned using Cerenkov, gross beta and gross alpha counting. The minutes of the seventh meeting were accepted as a true record. No actions were raised from the previous minutes. 2. Invited talk: Determination of Carbon-14 in environmental level, solid reference materials Jane Caborn, NNL There is no suitable reference material for C-14 in more complex matrices, i.e. organically bound C-14. A comparison exercise was organised to identify suitable C-14 reference material. The aim was also to assess the statistical differences between techniques, levels of precision, sensitivity, practical advantages and disadvantages of each technique. C-14 is naturally occurring nuclide in environment, above ground nuclear test of 1955 and 1963 increased the amount of C-14 in biosphere and atmosphere. Four different materials were chosen for the comparison, three IAEA carbon series reference material and one mix of environmental biota sample. Homogeneity and possible degradation of the standards with time was checked analysing the C-13 content using a mass spectrometry technique. Four different methods used were direct counting, wet oxidation, combustion furnace and combustion coupled to conversion to benzene. Zeta and z-scores were used to identify discrepant laboratories. Combustion coupled to conversion to benzene is very specialised and gives a very low limit of detection compared with other techniques Combustion technique gives second best limit of detection and has the advantage of allowing simultaneous C-14 and H-3 measurements. All techniques successfully analysed the C-14 content. Liquid scintillation counter used significantly impacted on the limit of detection achieved. The biota sample has been demonstrated to be a viable QC material. A shortage of certified reference materials exists impacting on the ability to demonstrate that the techniques are fit for purpose. A participant commented that some of the backgrounds were higher than expected, and that possibly some background reducing options, such as low level count mode or high sensitivity count mode, may not have been used. A question regarding the standardisation of the vial was asked as C-14 sticks to glass vials. Jane answered that the laboratories were asked to do the measurements the same way they would do their routine analysis, so no parameters were fixed. 3. Invited talk: The TDCR method in LSC Philippe Cassette, CEA, France The TCDR technique is an absolute measurement method which doesn't require the use of a standard to calculate the activity. The model used to calculate the detection efficiency the Free parameter model - is based on the statistical description of the light emission. The mean number of photons emitted after absorption of an energy E by the liquid scintillator can be

3 calculated using the semi-empirical formula given by Birks, which is dependent on E, the electron stopping power and `Birks parameter' kb. The ionisation quench effect is stronger for radiation with high stopping power as recombination effect becomes more probable than radiative transfer. In a system with three photomultiplier tubes (PMTs), the free parameter model is calculated using the Triple to Double Coincidence Ratio (TDCR). In a real TDCR system, the three PMTs are all different, these differences have to be taken into account in the calculations. The kb parameter can be deduced by experimentation as for the correct kb value the calculated activity should always remain the same with decreasing efficiency. Monte Carlo simulation is the method used to calculate the uncertainty on the measurement. In the 300SL commercial TDCR counter from Hidex, the TDCR value is only used as a quench indicator. Measurements are being carried out at the moment to see if it could be used for the full TDCR method. The TDCR method is a mature LSC standardisation technique which is suitable for the standardisation of pure beta and some electron-capture emitters. For the moment, the use of the TDCR technique is limited to in-house build three PMT counters, but this could change if the commercially available TDCR counters are found to be suitable for the application of this technique. The next international LSC conference will be organised in Paris from the 6-10 September A participant asked if this method could be used for alpha emitters. Philippe commented that the TDCR method wasn't necessary for alpha emitters as the efficiency for an unquenched sample is almost one. Only a small correction of a few tenths of a percent is required because of the wall effect. A participant asked what size of PMT is used? In the traditional counter, two inch diameter Burle/RCA 8850 PMTs are used which are high gain with the first dynode of this PMT coated with gallium phosphide giving a very high emission of the first dynode and a good resolution on the single photon electron peak. 4. Invited talk: Necessary Quality Control for LSC Cocktail Production Ronald Edler, PerkinElmer The PerkinElmer Groningen site has three production groups: Microplates, luminescence reagents and LSC cocktails and supplies. The quality control of LSC cocktails starts with checking that the raw material is within acceptable parameters (colour, LSC background, luminescence, efficiency..). On some LSC cocktails, the short expiry date is quite often due to the fact that the luminescence reaches an unacceptable level after 2 to 3 years. K-40 contamination of nonylphenolethoxylates (NPE), an important component of LSC cocktails, is an issue at the moment as low-cost potassium catalysts are used in the production of NPEs. A reformulation of the LSC cocktails is necessary as a result of the environmental ban on the use of NPE which are endocrine disruptors in the aquatic environment. Alcohol ethoxylates are considered as an alternative to NPEs, but they are quite often prepared from bio-chemicals which contain a higher concentration of C-14. Changes in legislation and the world chemical market requires continuing support and efforts in LSC counting. As a result of the increasing K-40 and C-14 content in the raw materials, PerkinElmer has decided to create a new LS cockail for low level activity measurements, Ultima Gold u-llt which guarantees the presence of no K-40 and C-14 from bio-chemical sources in this cocktail.

4 A participant asked if it was possible to use the new Ultima Gold u-llt cocktail with an oxidiser. Ronald thinks it is possible in principle to use the new cocktail with oxidisers, but the use of a co-solvent to increase the uptake for C-14 is necessary. The reason for changing the composition of PicoFluor 15 with non-npe chemicals was questioned as it is already not a safe cocktail. It was replied that NPEs are banned from many countries, and as a consequence of the ban the production levels have dropped and some NPEs are produced only once a year. This creates some logistics issues as it required storing enough NPE from the production of specific cocktails for a whole year. A participant inquired as to why transition away from NPEs started with PicoFluor 15 rather than Ultima Gold. The reply given was that different types of NPEs are required for different cocktails and in the case of PicoFluor 15 the NPE wasn't produced anymore. It was enquired if there was a policy on cocktail names when the chemical composition of the cocktail changes. Usually there is an additional letter, e.g. UG LLT becomes UG u-llt but if the change is not significant then no modification is done to the name. A participant further suggested to change the name of the cocktail when the atomic composition changes as it can disturb measurements. However, unfortunately inter-batch changes are unavoidable due to the use of industrial products in the fabrication process which inevitably vary from one shipment to the next. The aim is to keep the efficiency, background and luminescence levels the same from one batch to the next rather than the atomic composition. 5. Invited talk: MCERTS for radioanalysis of waters Rob Allott, Environment Agency MCERTS is the Environment Agency's Monitoring CERTification Scheme. The aim is to promote the production of quality monitoring data. The sampling and chemical testing of the water, principally effluents, is a part of the MCERTS scope. This new scheme was necessary as the water quality industry is moving towards operator self-monitoring of their effluents. The various benefits of the MCERTS include the reliability of the data, gives clear guidance on what performance level is expected, indicates importance of quality in testing, provides confidence in the system of operator self-monitoring and limits the number of independent check monitoring. The MCERTS for radio-analysis of water will apply to contractors working for the Environment Agency and to Nuclear sites and their contractors. This MCERT is based on ISO 17025, but contains more details regarding the selection and validation of methods, performance targets (bias, precision), analytical quality control and reporting. The key issues are: Which radionuclides and groups to include in the standard, method validation, decision threshold and detection limits, performance criteria. Once the MCERT standard is finalised it will be available as an electronic copy on the Environment Agency website. The accreditation to ISO will be necessary in order to have accreditation to the MCERTS. A participant asked if it this new MCERT standard will be prescriptive similarly to the UKAS standard. Rob replied that only the method validation is prescriptive otherwise one could choose whatever method they wish. A participant enquired into the timescales. Originally the first draft was expected to be ready by end 2009, but it had been delayed.

5 6. talk: Approaches to measurement uncertainty evaluation Peter Harris Peter provided a list of useful documents for the evaluation of measurement uncertainty is given: GUM from BIPM, M3003 from UKAS, GPG 11 and SSfM documents from (see slides for more details). Uncertainty of measurement is a quantitative statement of one s knowledge of the quality of a measured value. Uncertainties can come from the item being measured, the measuring instrument and the environment. To evaluate the uncertainty on a measurement the knowledge of the input quantities and the model of the measurement are required. The model on the measurement can be a mathematical formula, a computer program or a numerical procedure that takes the values of the input quantities and delivers a value of the output quantity. Probability distributions are defined by their expectation and standard deviation. Examples of distribution include rectangular, Gaussian and the t-distribution. Type A uncertainty refers to uncertainty on the input quantity that is evaluated by statistical analysis of a series of values and type B refers to any other approach of evaluating the uncertainty. Uncertainty propagation can be done via analytical, conventional and numerical methods. Monte Carlo is a numerical method based on propagation of discrete representations of the distributions for the input quantities through the measurement model. In the Bayesian approach, there is a distinction between the measured quantities and the quantities about which prior information is available. This method usually requires the use of numerical methods. Are there any software programs to help implement the GUM? There are many software packages out there such as GUM workbench, BMTA spreadsheet for conventional uncertainty calculation, Add-ons to Excel for Monte Carlo calculations. 7. Any other business Glenis Tellett introduced the new Knowledge and Innovation centre which provides business accommodation for companies who rely on measurement for competitive advantage. Companies will get access to 's expertise, measurement and test facilities. Alan DuSautoy explained to the participants how the Radioactivity project formulation will be produced in future to improve impact and be more responsive to the UK needs by having a rolling programme formulation that will be carried out each year. Any feedback or new ideas on the programme formulation are welcome. has planned to organise a liquid scintillation training course and aims to hold it at the end of 2009 or early in Arzu Arinc Secretary, LSUF