5^th September 2001

National Physical Laboratory

Attendees:			Mike Woods	NPL (Chairman)
			Andrea Woodman	NPL (Secretary)
			Matt Ashworth	DRPS, Alverstoke
			Bob Barber	Addenbrookes Hospital, Cambridge
			Mike Barton	British Energy, Sizewell B Power Station
			Janet Bell	British Nuclear Fuels, Morr Raw
			Jennifer Campbell	UKAEA, Caithness
			Julian Dean	NPL
			Elizabeth Howie	British Energy, Hunterston B Power Station
			Simon Jerome	NPL
			Lynsey Keightley	NPL
			Carole Neal	AWE
			Bob Otlet	RCD, Wantage
			Jackie Pates	Lancaster University
			Andy Pearce	NPL
			Joanne Robson	NRPB, Cookridge
			Graham Shephard	Devenport Royal Dockyard Ltd, Plymouth
			Bill Stirton	British Energy, Hunterston B Power Station
			David Thornbury	AWE
			Karen Verrall	Berkeley Centre, Gloucestershire
			Jill Walker	RCD, Wantage
			Phil Warwick	Southampton Oceanography Centre
			N Westall	BNFL, Gloucestershire
			David Whithorn	AWE
			Larraine Young	NRPB, Glasgow

1. Chairman’s Welcome and Opening Remarks

Those present at the forum were welcomed by Mike Woods, the Head of Science for the Radioactivity Group in the Centre for Ionising Radiation Metrology at NPL. He explained the purpose of NPL by giving some insight into the history of the laboratory and described the work done in particular by the Centre for Ionising Radiation Metrology. He went on to describe the new three year NMSPU (National Measurement Systems Policy Unit) programme for NPL, more details of which can be found on the following website.

http://www.dti.gov.uk/nms/prog/old/index.htm

One of the main changes to the programme from previous years is an increased emphasis on knowledge transfer. This covers the organisation of intercomparisons and workshops with various hospitals and other user communities in the UK; the provision of training courses, good practice guides and users’ fora such as the Liquid Scintillation Users’ Forum. More details of these can be found on the NPL website, http://www.npl.co.uk

2. Draft Terms of Reference

Mike Woods went through the terms in details and they were accepted by the members of the forum. It should however be noted that the terms can still be changed if required in the future. He stressed in particular that the forum should be for the benefit of the users rather than NPL.

3. Introductions and Nominations of Topics for Discussion

By way of getting to know each other, each member introduced themselves and described the work they are involved in and any special interest they have in the field of liquid scintillation measurements. Any further topics for discussion were suggested by the delegates.

4. Invited Talk: “Recent Developments in Liquid Scintillation Counting”
Andy Pearce, NPL

This PowerPoint presentation can be viewed on the LSUF website.

The work involving liquid scintillation which is carried out at NPL was described and the concept of the CIEMAT/NIST efficiency tracing method was introduced. Andy Pearce had attended the LSC2001 conference which was held in Karlsruhe, Germany in May and he described the topics which were presented and discussed at the conference. He also attended the International Committee for Radioactivity Measurements (ICRM) conference held this year and described the topics discussed there, namely stability problems with alkaline matrices, ionisation quench, the wall effect, developments in CIEMAT/NIST in Germany and the LS working group web page:

(http://www-dta.cea.fr/damri/FR/Pages/Radioactivité/Scintillation/icrm1.htm)

Following this talk some interest was expressed in the CIEMAT/NIST method and so Andy Pearce gave another presentation in order to try to explain the technique. This presentation can also be viewed on the web page.

5. Additional Talk: “CIEMAT/NIST – What is it?”
Andy Pearce, NPL

This is a popular technique internationally for standardising mainly pure beta emitters because it does not require specialised equipment, only a standard liquid scintillation counter. The technique was developed by CIEMAT and NIST, the standards laboratories in Spain and the USA respectively.

A Poisson model is used to calculate the detection efficiency for a nuclide as a function of a ‘free parameter’, the amount of energy required to be deposited in a scintillator in order to produce one electron at the photocathode. This free parameter is therefore related to the amount of chemical quench in the sample. The relationship between the free parameter and the external standard quench factor given by the LS counter is determined using a tritium standard. These are then used to produce a quench curve for the nuclide of interest.

In practice, a set of standard tritium vials and a set of sample vials are prepared with identical chemistry and increasing quench. The external standard quench factor for each tritium vial is inputted into a computer programme to give the free parameter values. These are then inputted into another programme which gives the theoretical detection efficiency for the nuclide of interest and therefore the activity can be calculated.

CIEMAT/NIST is used as a rapid method to produce secondary standards of pure beta emitters. It involves knowledge of the spectral shape but this information is easily obtained. Initially the technique was developed for pure beta emitters but has now been extended by E. Günther at PTB, the German national standards laboratory to include most radionuclides with uncertainties ranging from 0.4 to 2 % depending on the energy of the nuclide.

6. Invited Talk: “Performance Testing of Selected Liquid Scintillators”
Julian Dean, NPL

A project was carried out at NPL with the aim of deriving sample uptake and efficiency data for four specific liquid scintillators with a range of nuclides and matrices. The scintillators used were Readysafe, Ultima Gold AB, Uniscint BD and Unisolve 100. Seven standard nuclides were studied with activities of 1 to 2 kBq and at least one matrix per nuclide was studied.

The maximum sample uptake was determined by adding the matrix solution in 0.5ml increments to 10ml of the scintillator until the solution went cloudy or unstable.

The detection efficiency and merit value (detection efficiency × proportion of aqueous phase in vial) were then measured for each nuclide in each matrix and each scintillator with 0.5g, 1.5g and the maximum load of matrix. In each case 0.1g of sample nuclide solution was used and all vials were made up to 18g with scintillator. All vials were measured on a Wallac Rackbeta 1219 and a Canberra Packard TriCarb.

It was observed that measurements using both liquid scintillation counters gave similar detection efficiencies. There was however found to be some conflict with the manufacturers’ data on maximum sample uptake. It was determined that no scintillator was superior to the others in all cases although it appeared that Ultima Gold AB did perform best overall for the nuclides and matrices studied. The project generated useful data for planning dilution checks at NPL and for deriving activity concentrations for some nuclides.

It was suggested that future work could include other scintillators, nuclides and matrices; a study of temperature effects, count rate stability, variations between batches of scintillators; a study of micellar size and micelle formation and other physical properties. It was also thought that it might be useful to pool the information into one database to be made available to the wider user community.

Comments

The delegates generally felt that the NPL study which looked at pure samples alone was not particularly useful to them as they tend to use matrices such assoil. The chairman suggested that as the individual users are likely to have done their own similar experiments it may be useful bring the information together into one database accessible to all. The point was raised however that it would be very important that the sample preparation procedure was described accurately in order to exactly reproduce the same matrix in another laboratory.

At least one other user had found problems with sample uptake and that specifically Ultima Gold AB has recently been accepting less than half of the amount suggested by the manufacturer. The manufacturers response to this had been to suggest that the water used was too pure!In contrast NPL had found Ultima Gold AB to be taking in more than the manufacturers proposed. The comment was also made that Ultima Gold LLT is the same formulation as Ultima Gold AB although may be less expensive.

It was felt that there is a need to study quenching in more detail as having a very variant sample composition can have a large impact on quenching and different sample impurities do not cause the same drop in detection efficiency.

7. Invited Talk: “Problems associated with Low Level Beta Counting by LSC”
Phil Warwick, Southampton Oceanography Centre

The liquid scintillation work carried out at the Southampton Oceanography Centre was described. In particular the importance of good chemical separation and sample preparation techniques in order to obtain stable sources without contamination or high backgrounds was expressed. A number of examples of different sample preparation techniques were explained for the analysis of nuclides such as Fe55, I129, Ni63 and Ra226 among others. Counting methods used at the Oceanography Centre include Cherenkov counting, a/b discrimination and gross a, gross b counting.

A particular problem experienced at the laboratory was described in detail, this involved the measurement of tritium samples and the observation of high count rate spikes in the data obtained. Samples were measured for 10 × 50 minute counting cycles and in one or two of these cycles count rates of up to ten times the expected count rate were observed. Discussions with the counter and cocktail manufacturers did not solve the problem. The manufacturers suggested a number of possible causes which included the presence of chemiluminescence, contaminated cocktail or vials, electronic noise, cosmic ray interference, static on the vials among many others. All of these were ruled out.

The effect has now been observed on four different counters and at three different sites using different cocktails and vials. It has also been seen when measuring Pu241. All counters have been thoroughly tested by the manufacturers and found to be in good working order. The samples have also been sent to RCD who saw the spikes but have not seen the effect on their own samples. It was suggested by Bob Otlet at RCD that the anti-coincidence systems used to remove cosmic ray interference are not 100% efficient and so this could possibly be the cause after all but does not explain why RCD have only seen the effect with samples sent from Southampton.

8. Discussion Session

A large number of topics were discussed. A brief summary of each topic is given below.

Register of facilities/services

The usefulness of having a register of facilities/services, relevant to liquid scintillation counting, provided by the various organisations which could be published on the web page was discussed and it was decided that a draft questionnaire would be produced and circulated to delegates for comment by NPL. It was emphasised that information would only go on the register with full permission from the delegates.

Training

The question was raised over whether there are any liquid scintillation courses run other than the Loughborough course run by Peter Warwick. Most organisations currently use this course along with in-house training.This topic is to be included in the questionnaire.

Saturation problems at high count rates

Jill Walker at RCD has observed that at count rates above 4000 cps an increase in activity does not give a linear increase in count rate. This was obviously not a unique problem to RCD although most delegates were only involved in low level counting. It was generally thought that the stated deadtime of the equipment was not accurate. Work undertaken at NPL had found, by counting a short lived nuclide with various window settings, that the deadtime also depends on the portion of spectrum chosen. It was suggested that we raise the question of deadtime accuracy to instrument manufacturers at a future meeting. Andy Pearce (NPL) is to collate data from the user community illustrating problems observed at high count rates and prepare a report to be presented to manufacturers.

Low count rates – background reduction techniques

There are a number of ways to reduce the background such as by the use of anti-coincidence shields, extra lead shielding and by reducing the width of the counting window. It was found that a certain amount of efficiency had to be lost in order to reduce the background and in general it was a case of optimising to get the best situation all round. It was noted that with the Quantilus counter the use of pulse shape analysis gave no advantage in reducing background due to the large amount of lead shielding already in place. A further effect mentioned was that an increase in acidity of a sample gave an increase in background, this was unlikely to be due to any radioactive contamination of the acid. Another comment made was that glass vials are by far better than polythene vials, especially for tritium which will diffuse out of polythene. It was thought that it would be useful to include the question of background reduction techniques in the questionnaire in order to give more information about the methods used and typical background levels achieved at each laboratory.

Gross α, gross β and spectral deconvolution

The question was asked whether liquid scintillation counting would be more efficient than gas proportional counting for confirming that a particular water source has gross a activity of less than 40 Bq/m³. The general consensus appeared to be that liquid scintillation would require more in terms of sample preparation time. The discussions identified who has experience in the areas of gross a, gross b counting and spectral deconvolution. Also discussed was the digital overload technique used in a few of the organisations present.

α/β discrimination

It was decided that this topic should be left for now to be covered in detail at a future meeting.

Low Level Standards

The problems associated with diluting currently available high activity standards down to levels more appropriate to low level counting were raised and the viability of a “consensus” standard was discussed. Simon Jerome at NPL explained that it is the responsibility of NPL to meet the UK’s needs for standards but only if the demand is really there and that customers are willing to pay for it. It was agreed that the questionnaire to be circulated would cover individual needs for standards so that the overall demand could be judged.

Sample Preparation

Delegates expressed an interest in a conference organised four years ago by Simon Jerome (NPL) on “Rapid Radiochemical Methods for Routine and Emergency Situations”. Simon is to provide those interested with a copy of the conference proceedings. It was felt that more useful discussion could be held between individual members perhaps through an e-mail discussion group. It was suggested that the methods used described on the questionnaire would identify users with common interests.

Environmental tritium/OBT

It was announced that NPL is to produce OBT standards as part of the coming programme.

Presence of Manufacturers at future meetings

It was generally felt that it would be useful to have the counter/cocktail manufacturers present for part of a future meeting so that questions could be put to them with regards to variations between cocktail batches, comparisons of different counters etc.

Surface Contamination Monitoring

In general smears are taken, soaked in scintillation cocktail for 24 hours and then assayed assuming a pickup of 10 %. However it was noted that the 10 % pickup assumption is just an estimate and concern was raised over whether more accurate pickup rates should be derived and for different nuclides. It was also stated that surface monitoring alone is not adequate for consignment of waste offsite and that in these cases destructive analysis must be done.

Good Practice Guides

It was decided that discussion of the requirement, if any for good practice guides concerning liquid scintillation counting should be left for a future meeting, however interest in this will be covered in the questionnaire.

Web-site and e-mail discussion group

An LSUF website will be produced on the NPL website (www.npl.co.uk) and will provide details such as the register of facilities, recommended textbooks. It is also intended to set up an e-mail discussion group between the delegates, however any e-mails will have to go through a postmaster to be filtered before being passed onto the rest of the group. All those wanting to be part of this discussion group should indicate this on the forthcoming questionnaire and provide their e-mail address.

MDAs/Uncertainties

There appeared to be some question over what exactly is meant by MDA (minimum detectable activity) and how it is calculated. Therefore it was agreed that a subgroup would be formed with the aim of firstly looking at all uncertainties related to liquid scintillation counting and then coming up with recommendations for how to tackle MDAs.

9. Concluding Discussions

It was agreed that a questionnaire would be produced to cover those topics described above and sent to all members. It is probable that some members of the forum would then be asked to form subgroups to cover MDAs and produce good practice guides. Comments made on the questionnaire with regards to saturation problems will be collated by Andy Pearce (NPL) for presentation to counter manufacturers.

Those present felt that the forum had been useful. Many topics had been raised and these could now be expanded on in future meetings and discussions via e-mail. The next forum is to be arranged for September 2002.

10. Actions arising from the meeting

APW (NPL) – to produce a questionnaire to be sent to all members of Forum.

AKP (NPL) – to collate information and prepare report on saturation problems to be presented to counter manufacturers.

SMJ (NPL) – to provide those interested with a copy of the “Rapid Radiochemical Methods for Routine and Emergency Situations” conference proceedings.

APW (NPL) – to arrange for relevant information to go on the web page.

APW (NPL) – to set up an e-mail discussion group.

Andrea Woodman

Secretary, LSUF