Composition Measurements.

DOE-STD-6003-96

This section will discuss methods for both categories. The measurements techniques for

tritium can be grouped in the three general areas: composition measurements, thermal mea-

surements, and tritium concentration measurements.

Composition measurements determine the actual concentration determination for each

atomic/molecular species. This method can be used for gases only. Thermal methods

(calorimetry) rely on the radioactive heat of decay of tritium. For 1 g of tritium ~0.333 W is gen-

erated by decay. The temperature increase or heat generation is measured. Calorimetry can be

used for tritium in any form: solid, liquid, or gas. The only radioactive material present must be

tritium because other radioactive materials will contribute to the thermal properties of the

sample. The final method determined the total tritium concentration by the measurement of the

products or the effects of the products of the radioactive decay. The beta particle can cause

scintillation effects or ionization effects. These effects can be measured and the concentration of

tritium determined. The following methods are used or proposed to be used for measurement of

tritium: Pressure/Volume/Temperature/Composition (PVTC), using either a mass spectrometer

or laser RAMAN spectrometer for the composition measurement; Beta scintillation counter; Self-

assaying tritium storage beds; Scintillation Counting; and Ion Chamber.

Most of the techniques discussed here are batch samples, however some techniques can

be used for "on-line/real time" measurements.

a. Composition Measurements. PVTC measurement is used for measurement of

gaseous samples only. A representative sample of the gas is taken. The gas that is to

be measured must be mixed well. The volume, pressure, and temperature must be

measured accurately. The temperature is difficult to measure accurately because of

temperature gradients caused by the heat of decay of tritium. The composition of the

gas in the sample is then measured using a mass spectrometer or a laser RAMAN

spectrometer.

The mass spectrometer will measure all gas species. A high-resolution mass spec-

trometer is required to distinguish between different molecules with the same mass

number. For example HT and D2 have the same mass number, but must be sepa-

rated to determine the tritium concentration. All species that can contain tritium must

be measured. This includes, water as HTO, methane as C(H,D,T)4, ammonia as

N(H,D,T)3, etc. The sum of all the species containing tritium can then be determined.

If the approximate gas composition is unknown, the use of the mass spectrometer

may be difficult.

The laser RAMAN spectrometer is a relatively new system that can be used to mea-

sure molecular concentrations in a gas mixture. The sample is placed in a cell with

optical windows. The laser excites the rotational or vibrational atomic levels in the gas

molecules. The light emitted as the excited levels decay back to the ground state is

detected using a photodetector system. The measurement is absolute in that the fre-

quency spectrum of each molecule is unique. The intensity is proportional to the

amount of gas present. The disadvantages of the RAMAN method are that the amount

of inert gases cannot be determined. Common inert gases at a fusion facility are the