Process Tritium Monitors: Measuring High Concentrations of Tritium

To measure high concentrations of tritium, Overhoff offers a line of tritium process monitors that employ ionization chambers with a small volume ranging from 10cc to 200cc and closely-spaced large-diameter electrode configurations to minimize nonlinearities.  Wire-grid, gold-plated electrodes are used to reduce or eliminate the effects of tritium plate-out contamination on the wall of the chamber.

In air ionization chambers, the assumption is made that all ions, both positive and negative, are separated from each other as they progress to the electrodes (wall and collector element) of the ionization chamber.  As the activity of tritium increases within a chamber, so does the population density of generated ionization pairs.  With increasing ionization pair density, the likelihood that some of the ions of opposite polarity will collide and neutralize will then increase.  The ionization current that is actually obtained is then less than what it would be if there were no recombination.  Recombination can only be prevented by ensuring that the electric field is sufficiently high and relatively uniform throughout the ionization chamber.  Overhoff’s special electrode and chamber design helps to reduce this effect.  Overhoff also uses a Kanne ionization chamber design which employs a tri-axial arrangement to enhance uniform field distribution and give a linear response all the way up to pure tritium.

Plate-Out, Contamination from Tritium Oxide (HTO)

Tritium oxide will occlude on most metal surfaces.  The walls of ionization chambers exposed to high tritium activity will gradually absorb tritium, which will migrate into the metal and contaminate the chambers.  Tritium located at the surface will emit beta particles into the volume of the ionization chamber and will then produce an ionization current even though the ionization chamber is filled with otherwise inert atmosphere which will cause an error in measurement.

Overhoff’s solution to reduce or eliminate HTO plate-out contamination is simply to reduce the true surface area of the chamber by replacing the solid walls with a fine wire mesh.  We call this mesh our “Wire-Grid” Ionization Chambers and it constitutes a phantom wall, with chamber properties identical to a solid wall design.  The phantom wall wire mesh absorbs all beta particles, and all secondaries from the solid wall of the chamber so that the wall effect is gone.  Improvement in plate-out is now simply the ratio of the surface of a solid wall as compared to that of the wire mesh.  A reduction of at least three orders of magnitude (1,000x) is possible using our wire-grid ionization chambers.  Additionally, we also offer gold-plating which plates the exposed surfaces of the ionization chambers with gold to further reduce HTO contamination.

Chamber sizes include 10cc, 50cc, and 200cc volume, are made out of stainless steel, and are typically remotely mounted from the electronics.  These chambers are generally supplied as a single chamber for measuring high concentrations, but can be built with matching gamma compensation chambers.  Includes detachable electrometer housing, allowing you to detach the electrometer from the chamber so you can easily clean, remove any moisture buildup, or decontaminate the chamber.

Our most common chamber size for measuring high concentrations of tritium is the 50cc chamber.  A 10cc chamber is used for measuring ultra-high concentrations of tritium.

Includes two CF (ConFlat) vacuum flanges (2.75” diameter), with a ¼“ VCR fitting on each flange.  Only metallic parts are in contact with the process gas, except for the special electrical insulator which is made of high-purity alumina ceramic instead of the standard PTFE.  The seals are OFHC (oxygen free high conductivity).  The BNC feed-through on one CF flange is for mounting of the detachable electrometer module.  The SHV-5 feed-through is provided on the second CF flange for the high voltage connection.  The electrode construction is from 0.035 inch diameter stainless steel wires fusion welded together to reduce HTO contamination.  The electrodes are also gold-plated to further reduce effects from HTO contamination.  Pressure tested to 200 psig and helium leak tested down to 1E^-9 atm-cc/sec at a differential pressure of 1 atm.

50cc Ionization Chamber

Volume: 50cc measuring, 100cc port to port

Dimensions: 2.75” (70mm) diameter (not including sample line connections) x 4.8” (122mm) long (not including hardware and connectors)

Weight: 3.7 lbs (1.7 kg)

Electrometer

Dimensions: 1.50” (38mm) high x 1.63” (41mm) wide x 3.70” (94mm) long

Weight: 0.7 lb (0.3 kg)

Typical current measurement range: 5E^-13 to 1E^-8 Amperes, which equals a tritium measurement range of 0.01 to 199.99 Ci/m³ (single-range Model 311).  Other measurement ranges all the way up to pure tritium are available.

Visit our modular configuration tritium in air monitors page or contact us for more details.