Proportional Counters

DOE-HDBK-1129-99

Ionization Chambers -- Tritium decays to 3He by the ejection of a beta particle. The beta

particle generated by the decay of tritium ionizes the surrounding gas. The number of ions

produced due to the loss of energy of the beta particle is a function of the type of gas. A

sample of gas is collected in the ionization chamber and the ionization current is measured.

The resulting chamber ionization current is proportional to the quantity of tritium in the gas.

The larger the measuring chamber volume the higher the output current and the easier it is to

measure. However, as the volume of the chamber increases, the longer it will take to get an

accurate measurement. Modern electronic systems have solved most of the problems

associated with measuring small ionization currents in small volumes and as a result, the

volume of the ionization chambers has been reduced over the years from 50L down to 1 or 2L.

Most tritium measuring instruments have an ionization chamber.

Proportional Counters -- Gas proportional counters are also used to measure the amount of

tritium contained in a gas. A sample of the gas to be monitored is mixed with a counting gas

and passed through a proportional counter tube where the pulses caused by the decay of

tritium are counted. Proportional counter monitors can be used for most gas monitoring

applications and are also available to measure surface contamination.

Scintillation Crystal Detectors -- Scintillation detector systems are used to measure the total

mole percent of tritium in a sample of gas independent of the chemical composition of the

tritium in the gas (HT, DT, T2, and CHxTy). A sample of the gas is introduced into a

measurement chamber at low pressure, generally less than a few torr. The chamber contains

a scintillation crystal, which is exposed to the tritium as it decays. The light pulse produced in

the scintillation crystal is either counted or is used to produce a current, which is proportional to

the mole percent tritium contained in the gas sample. Crystal scintillation detection is generally

used to measure the mole percent of tritium in gases containing high concentrations of tritium.

Mass Spectrometer -- Magnetic sector, quadrupole, and drift tube mass spectrometers are

used as analytical tools to measure the individual components that make up the gas being

measured. Mass spectrometers are generally used for the purposes of assay and

accountability or for scientific purposes. A sample of the gas to be measured is introduced at

low pressure (a few microns) into a chamber and ionized. The ions produced are then

measured by a means that discriminates on mass. The number of ions produced at each mass

is measured and is proportional to the partial pressure of the component in the gas sample.

The sophistication of the measurement systems varies greatly from facility to facility through

out DOE. Light isotope, drift tube, mass spectrometers require a large capital investment and

require a skilled staff to operate, and, in some cases, may not be cost-effective. All DOE

tritium facilities do not require a light isotope drift tube mass spectrometer. Quadrupole mass

spectrometers and crystal scintillation detectors are much less expensive, but still require

operation by knowledgeable well-trained personnel. The DOE assay and accountability

requirements and regulations do not currently reflect this difference in sophistication and cost

and currently place the same requirements on small as well as large-scale operations.

Liquid Scintillation Counters -- Due to the need to measure the removable tritium on surfaces

and in the body water of workers, almost all tritium facilities are equipped with or have access

to a liquid scintillation counter. If the scintillation counter is not available on site the service can

generally be purchased from a local firm. Liquid scintillation counters are used to measure the

quantity of tritium on surfaces, in liquids, and in dissolved samples. For removable surface

contamination measurements, a wipe of the surface to be measured is taken using dry paper