Tritium Air Sampling
For measuring ultra-low levels of tritium in air, it is common to use passive sampling techniques utilizing a small pump to draw a controlled amount of air through a series of vials which traps the airborne tritium. These passive samplers are a simple, low-cost method of measuring low concentrations of tritium for stack compliance or worker safety. However, they do not provide real-time measurement and are typically left to sample for periods of up to one week. After sampling is stopped, the collection vials are removed and the contents assayed using liquid scintillation counting to determine the radioactivity collected. Knowing the collection flow rate and the results of the scintillation assay, the average sample activity can be calculated for the period of time over which the sample was collected.
Overhoff’s Model TASC-HTO-HT is a high-quality and reliable tritium air sampler. It has been in service for over 23 years and has been carefully engineered and refined over time so that it is easy to use, reliable, and consistent. The sample holder tubing is made of 304/316 stainless steel which is cut, bent into shape, and brazed directly into sample holder caps which hold the sample collection bottles. The whole assembly is leak tested, sandblasted for a uniform finish free of burs and oxidation, and finally electroplated for a bright nickel finish. Special care is taken to select air diffuser stones which optimize the diffusion of air into the collection vials while preventing unwanted water from bubbling over and getting into the piping. Air flow is regulated by a highly accurate and stable mass flow meter.
The TASC-HTO-HT uses two sets of cascaded collection vials to ensure virtually 100% collection efficiency. First, the sample air is filtered by a high efficiency HEPA filter (99.99% at 0.1 microns) to remove dust and other particulates. Next, HTO (tritium oxide) is directly trapped in the first set of vials, while the HT (elemental tritium) fraction of airborne tritium is converted into HTO by means a tube furnace and trapped in the second set of vials. The tube furnace operates at 455-475 °C (851-887 °F) and has a temperature display indicator on the front panel.
Available in two versions:
- Standard flow rate: 100 scc/min, with choice of 6x20mL collection vials or 4x60mL collection vials
- Sensitivity for 3H: 10-9 µCi/ml (10-9 Ci/m3) for 7-day period at 100 ml/min
- High flow rate: 300 to 1,000 scc/min, flow rate adjustable via front panel potentiometer, with 4x250mL collection vials, includes totalized flow display
- Sensitivity for 3H: 10-9µCi/ml (10-9 Ci/m3) for 24-hour period at 1 L/min
Standard front panel indicators include: flow rate display (scc/min), elapsed time (0.1 to 999.9 hours with programmable reset), temperature display (°C), and low flow alarm visual indicator. The high-flow version (300 to 1,000 scc/min) has a front panel flow rate adjustment potentiometer and includes an additional front panel display for total flow (flow rate x elapsed time).
Tritium and Carbon-14 Air Sampling
The Model TASC-HTO-HT-C14 is the same as the TASC-HTO-HT but also includes an additional panel for C-14 collection. After the sample has been oxidized and the HTO and HT have been collected in the vials, the sample air is pumped through a series of desiccant columns to dry the air and remove all the tritium. After the desiccant columns, the air passes through a 15ml collection tube filled with granular sodium hydroxide (Ascarite) for collecting C-14 (CO and CO2).
Note: We have found solid sodium hydroxide (20-30 mesh) to the best C-14 collection medium for long-term system performance. It is possible to use a liquid C-14 collection medium (liquid NaOH) to capture the C-14 directly, but this compound is highly caustic.
Analysis of the C-14 concentration is obtained by acidification of the Ascarite media in a closed distillation system, which releases the CO2 to be captured downstream in a liquid scintillation cocktail in countable form. Testing has shown the distillation yield to be 95±5%. Dividing the counting efficiency by the elapsed flow rate will yield the C-14 concentration.
Optional Configuration for Model TASC-HTO-HT-C14:
-Adjustable flowrate 50-250 ml/min using 4x60ml collection bottles
-Adjustable flowrate 50-500 ml/min using 4x250ml collection bottles
Note: For measuring ultra-low levels of tritium in real-time, continuously, see Models 593 and 593.2.