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Evaluation of the mean ionisation energy (W) of gas mixtures used in the NPL primary gas counting system

H C Phillips*, J P Sephton, J C J Dean and L C Johansson

National Physical Laboratory, Hampton Road, Teddington, Middlesex, TW11 0LW, UK

Author email address 24/26pt

IntroductionThe National Physical Laboratory (NPL) has well-established facilities for radioactivity measurements of beta emitting gases, such as 3H and 85Kr and more recently 11C, by internal proportional counting (Makepeace et al., 1994; Phillips et al., 2010; Marouli et al., 2007; Marouli et al., 2008; Marouli et al., 2010).

A Monte Carlo simulation based on the PENELOPE code has been developed to determine the corrections for counting losses during the standardisation of 11C or 14C as CO2 in P10 (Stanga et al., 2002; Baró et al., 1995; Mori et al., 1998). It is important to validate the Monte Carlo based loss correction technique. It is intended to incorporate this technique during the calibration of the NPL PET transfer instrument.

As an initial stage in this process it is necessary to determine the mean energy required to generate a single ion pair (W value) in the gas mixture 3% carbon dioxide in P-10. The W value of P-10 is well known, Alkhazov (1967), for example, reported a value of 26.0 eV. The introduction of gases such as CO2 and N2 will significantly alter the value of W. Values of W have not been published for the three or four part gas mixtures commonly used in the NPL gas counting systems.

Basic principlesThe determination of W depends on the measurement of :

• theDCcurrentthroughtheproportionalcounter when operated in ionisation chamber mode (gas gain K is unity)

and

• thecountratedeterminedduringoperationin the proportional counter mode.

The steady current flow across a proportional counter operated in ion chamber mode, I, is given by the following equation:

where N is the count rate, e the electronic charge, and E the mean beta particle energy.

W measurementGas production:

• NaH11CO3 + HCl = 11CO2 + H2O.

• 11CO2 dried using cardice trap

• 11CO2 transferred to counting system and mixed with counting gas (Marouli et al (2010)).

The current produced in the medium length counter used in ionisation mode was recorded at intervals of 10 minutes as the 11C decayed.

The rapid decay of 11C enables the transition from the measurement of ion current in ion chamber mode to activity in proportional counter mode without the need for gas dilution.

When the current dropped below 5 pA the current measurement was discontinued and the standardisation of the gas using the counting system in proportional mode commenced.

Plateaux were obtained with measurement periods of 2, 5, 7 and ultimately 10 s over the voltage range 2.6 to 2.8 kV. The activity concentration of 11C within the counting system was determined based on the response all three counters. The activity in the medium length counter was calculated and used in conjunction with the ionisation chamber data to determine the W value for the gas mixture.

The activity derived from internal gas proportional counting was confirmed by measurements of 11C in an aliquot of the NaH11CO3 solution performed using a secondary standard re-entrant ionisation chamber that had been previously calibrated using absolute counting techniques (Woods et al., 2002).

Determining the ion chamber operating voltage

A gas mixture of composition 3% inactive CO2 in P10 (90% Ar 10% CH4) was used to determine the ion chamber operating voltage.

The counter anode voltage was varied between 0 and 2.7 kV, with an external 226Ra solid source (3.7 MBq) positioned at the medium counter mid-length. The current induced in the counter was measured on an electrometer connected to a laptop. A plot of current against voltage was obtained and used to determine the ionisation chamber region of the proportional counter. The medium length counter was found to have an ionisation chamber operating voltage in the region of 1.6 kV. This compares well with empirical considerations (Knoll, 2000).

Counting EquipmentThe counting equipment has been described previously Makepeace et al., 1994; Marouli et al., 2008; Phillips et al., 2010.

The count rate was measured over a 100V wide range towards the end of the counting plateau and used to determine the activity present at the reference time. A correction for counting losses below the threshold was made using the energy spectrum from an MCA.

Results and conclusionsThe W value for the gas mixture 3% CO2 in P-10 was determined as 30 eV ± 3 eV. (k=2)

This value will be used in the calculation of counting losses using the Monte Carlo simulation when 11C or 14C are standardised as CO2 in P-10.

Comparison of these counting losses with those experimentally determined using the MCA loss estimation technique will be performed in a future study.

Further studies are envisaged to enable the use of the simulation when 85Kr in nitrogen or 3H in nitrogen are standardised in the NPL internal gas proportional counting systems.

Reaction Flask

Funnel Vacuum

Gauge

Vacuum

Gauge

Pressure

Gauge

Pressure

GaugeVacuum

LinesVacuum

Line

Rig

Taps

Condensing

Traps

Cardice TrapKnown Volume

Mixing Pump

Proportional Counters

Pre-amplifiers

Amplifiers

Timing Device

PC

Electronic

Connections

Rig Pipework

HV SUPPLY

COMPUTER

PROPORTIONAL

COUNTER

HV OPTICAL

ISOLATOR

ELECTROMETER

UNINTERRUPTABLE

POWER SUPPLY

AC

supply

GROUND GROUND

GUARD

RS232

link

Connection to case

Connection to case

Disconnected

from AC mains

HIGH VALUE HV RESISTOR

I = NeE W