7/27/2019 Development and Testing of the Liquid Scintillation Counter Incorporated in a Prototype Modular Tritium and Carbo

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QueensPrinterandControllerof

HMSO

,2013

.10673/1013

www.npl.co.uk

Development and Testing o a PrototypeModular Tritium and Carbon-14 Gas

Environmental Monitoring System1Hilary Phillips, Marc Parisot2, Julian Dean1, Lauren Perrie1 and John Sephton1

1National Physical Laboratory, Teddington, United Kingdom2Grenoble INP-Phelma

email: hilary.phillips@npl.co.uk

Future work will include generation o low level standards o14C and 3H at NPL, which will be used or controlled active testing o the system. This

will be perormed using the NPL HTO conversion rig to generate HT gas rom tritiated water, and the NPL14

C conversion rig to generate14

C02 rom asodium bicarbonate solution. These gases will be standardised by internal gas proportional counting, transerred to cylinders and pressurised withdiluent gas to achieve the required activity concentrations, comparable to those expected to be measured by the AERGMS.

Abstract:Increasing quantities o radioactive waste are being placed into storage acilities. Many o the waste products contain organic materials whichmay undergo degradation leading to the release o tritium and carbon-14 species into waste containers and potentially into the environment.Monitoring or radioactive gas release is required or environmental regulatory compliance and or radiation protection o acility workers.Research is currently being undertaken at NPL as part o the EMRP MetroRWM project to improve and automate environmental samplingequipment or tritium and carbon-14 species. An innovative modular system is being developed which will lead to the introduction o anon-site small scale system capable o gas collection, liquid scintillation sample preparation and measurement. This paper outlines theevaluation o the liquid scintillation system that has been perormed to date using radioactive spiked samples o trapping medium o activityconcentration similar to that envisaged in service.

AcknowledgmentsThe authors grateully acknowledge unding provided by the National MeasurementOce o the United Kingdom Department or Business, I nnovation and Skills, throughthe Acoustics and Ionising Radiation Metrology Programme. This work is supported bythe European Metrology Research Programme (EMRP). The EMRP is jointly unded bythe EMRP participating countries within EURAMET and the European Union.

An initial prototype o the AERGMS wasconstructed and the easibility o the designdemonstrated. A revised modular prototypeis under construction or eld trial evaluation.The Automated Environmental Radioactive GasMeasurement System (AERGMS) will consist o:

A time-integrating gas-in-air samplingmodule based on gas bubbler technology

Automated modules or uid handlingand sample preparation

A Liquid scintillation counting module(NE LSC2 based)

Automatic data transeral to a remotecontrol centre

The NE LSC2 counter is a dual photomultipliertube (PMT), dual measurement channel liquidscintillation counter. The NE LSC2 counter isoperated using dead time units and scalars oreach o the PMTs and a PC or data acquisition.

The operating voltage (900V) is supplied by adedicated power supply.

Evaluation trials o NE LC2 perormance have indicated thatchemiluminescence (a phenomenon that occurs when a chemical reactionoccurs between the sample and the uor molecules in the scintillationcocktail) and quench will efected the measurements perormed usingthe AERGMS.

Three methods o minimizing chemiluminescence have been considered;increasing the lower measurement channel threshold to preventluminescence count detection; addition o a delayed coincidence countingcircuit to enable removal o the chemiluminescence count rate or the

introduction o a measurement delay time. A delay will be introducedbetween sample preparation and mixing in the AERGMS to enable thedecay o spurious luminescence pulses.

Tritium and carbon-14 liquid scinitillation vials containing sodiumhydroxide exhibited a signicant reduction in detected count rate whenmeasured on both the NE LSC2 and commercial counters due to quench.Whilst the level o quench in a scintillation sample is evaluated usingthe tSIE index in commercial liquid scintillation counters, no method isavailable or the NE LSC2 counter.

Nuclide NE LSC2 Efciency

Unquenched Quenched

3H 50% 30%

14C 70% 87%

Quench has caused the 14C response in the NE LSC2 counter to occur in the

tritium channel (A).

14

C activity will be determined in channel B, and thecount rate rom 14C in channel A estimated based on trials using standardAERGMS operating conditions. Correction o the total counts in channel Awill enable determination o the 3H count rate present.

The activity concentration o3H and 14C species in the sampled air streamwill be derived rom these count rates using the volume o air sampledduring gas trapping.

Evaluation o scintillant perormed andOptiphase HiSae 3 identied as suitable.(High fash point. Compatible with alkaline

samples necessary or14

CO2 stability)

Initially single trapping medium or CO2

and HTO envisaged but preliminary testshave indicated that separate media will be

required. Testing is on-going.