Off-Gassing Measurements

Tritium Primer

DOE-HDBK-1079-94

TRITIUM MONITORING

Such monitoring probes are used to survey areas quickly before more careful monitoring

by smears, or to monitor the smears themselves while in the field.

The probe must be protected carefully from contamination. When monitoring a slightly

contaminated surface after monitoring a highly contaminated one, contamination of the

probe can be an immediate problem. Placing a disposable mask over the front face of

the probe can reduce, but never eliminate this contamination, particularly if the tritium

is rapidly outgassing from the surface. Sensitivity of the instrument depends on many

factors, but should be about 103 to 104 dpm/cm2.

For highly contaminated surfaces (>1 mCi/100 cm2), a thin sodium iodide crystal or a

thin-window GM tube can be used to measure the characteristic and continuous x-rays

(Bremsstrahlung) emitted from the surface as a result of the interaction of the beta

particle with the surface material.

Off-Gassing Measurements

Off-gassing can be measured using one of two methods. The simplest method is to

"sniff" the surface for airborne tritium using a portable or fixed tritium monitor. The

most reliable method, however, uses a closed-loop system of known volume and a flow-

through ionization chamber monitor. By placing the sample inside the volume and

measuring the change in concentration over time, tritium off-gassing rates can be

determined accurately on virtually any material. The initial off-gassing rate is the

required value because the equilibrium concentration may be reached quickly in a closed

volume, especially if the volume is small because of recontamination by the airborne

tritium.

The uptake of tritium from off-gassing materials is difficult to predict. Off-gassing

tritium that is readily measured indicates contaminated equipment that should not be

released for uncontrolled use.

Liquid Monitoring

Liquid is almost universally monitored by liquid scintillation counting. The liquid must be

compatible with the cocktail. Certain chemicals can degrade the cocktail. Others may retain

much of the tritium; still others result in a high degree of quenching. In addition, samples that

contain peroxide or that are alkaline may result in chemiluminescence that can interfere with

measuring. Such samples should first be neutralized before counting. Chemiluminescence and

phosphorescence both decay with time. Phosphorescence, activated by sunlight or fluorescent

lighting, decays in the dark in a few minutes (fast component) to several days (slow component).

Chemiluminescence, the result of chemical interaction of sample components, may take days to

decay at room temperatures, but takes only hours to decay at the cold temperatures of a

refrigerated liquid scintillation spectrometer. Distillations may be necessary for some samples.

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Tritium