Ionization Chambers

Overhoff Technology Corporation (OTC) offers a wide variety of ionization chambers for the measurement of airborne tritium and other radioactive gases. 

OTC has designed and built ionization chambers of many different sizes and configurations, each of which was optimized for a specific application, from measuring low-levels of tritium in room, stack, and effluent air monitoring applications to measuring high-concentrations of tritiumin glove boxes, fusion reactors, nuclear pharmacies, process piping, storage beds, and recovery systems, as well as challenging environments such as nuclear power plants.  OTC ionization chambers span all sensitivities, from 10^-7 Ci/m³ to pure tritium, and can also be used to selectively measure tritium, or only its oxide, even in the presence of other radioactive gases.  OTC ionization chambers are designed for easy assembly and maintenance. 

Ionization Chambers:

An ionization chamber is an electrically closed vessel containing an internal electrode.  An electrical field is applied between the wall of the chamber and the electrode so that the ionization produced by radiation is collected in the form of a current.  In a linear ionization chamber, the current is proportional to the internal radioactivity and is essentially independent of the chamber’s electric field potential. 

Interchangeability:

All OTC ionization chambers have been designed so that the electrometer preamplifiers are located in a cavity in the mounting flange. Measurement (tritium) calibration is located directly at the ionization chamber-electrometer module so that, in general, any ionization chamber-electrometer module will function in combination with any main electronics cabinet, and still maintain instrument calibration. It is therefore possible to use one or several different ionization chamber-electrometer modules with any given main electronics system.

Non-specific Response:

Ionization chambers respond not only to the airborne isotope inside the chamber, but will also respond to ionization produced inside the chamber by external gamma, x-rays and cosmic ray fields.

Gamma Compensation:

To overcome undesirable effects due to external gamma fields, OTC tritium monitors can be supplied with compensating ionization chambers. Here, a second ionization chamber of identical dimensions is used to cancel the effects of external radiation upon the measuring ionization chamber. Additional gamma radiation suppression by means of lead shielding can be supplied.

The best gamma compensation is provided using four chambers arranged in a cruciform geometry, which provides nearly perfect gamma compensation regardless of photon energy, flux gradient, or flux direction.   Utilizing four 2L chambers will provide virtually no offset in gamma fields all the way up to 30 mR/hr. 

Linearity:

OTC ionization chambers are designed to be highly linear. At high radiation activity levels, it becomes increasingly probable that an ion and an electron will recombine, will be lost and not form part of the ion current collected by the electrode. Special electrode and chamber designs help to reduce this effect.

Kanne Design and Ionization Trap:

Most OTC ionization chambers utilize the Kanne configuration, where the ionization chamber is surrounded by a closely spaced second chamber.  The volume between the two surfaces serves as an ionization trap.  To prevent build-up of debris in the ionization trap, or within the active volume of the ionization chamber, it is normal to use a high efficiency dust filter ahead of the ionization trap.

Configuration:

Design of every ionization chamber system has been optimized both for performance as well as for economy.  Low level ionization chambers are at least 2 liters in overall volume, are often lead shielded and in a gamma compensation (dual) configuration.  High level ionization chambers are generally small and employ closely-spaced large-diameter electrode configurations to minimize nonlinearities.  Glove box ionization chambers employ perforated walls for direct intrusion into the glove box, thereby eliminating the need for pumps and plumbing.

Construction and Materials:

Most OTC ionization chambers are secured to a massive flat “baseplate” which serves not only as a mounting structure, but which also houses the electrometer.  Calibration of the entire system is rendered directly at the electrometer via trimmer potentiometers accessible in the side of the ionization chamber mounting flange.  This permits interchange of ionization chamber modules without loss of instrument calibration.  Dual ionization chambers consist of an identical pair of chambers, mounted on either side of the baseplate which houses the electrometer.  Stainless steel or aluminum is commonly selected, although copper and brass can also be used.  Insulators generally are chosen to be inert to radiation, but the insulator for the ionization collecting electrode is almost always (except for ultra-high levels) chosen to be fabricated from PTFE.  All commonly used inch or metric fittings and hose barbs can be supplied.

Chamber Models:

2 Liter Ionization Chamber

Single or dual, mounted onto 7” square baseplate.  Total wetted volume is 2,000 cc with an active internal volume of 1,600 cc, and a volume of approximately 400 cc in the ionization trap.  Supplied with two Swagelok or hose barb fittings. 

200 mL, 50 mL, and 10 mLIonization Chambers

Single or dual, mounted onto 7” square baseplate. These chambers are generally supplied as a single chamber for measuring high concentrations, but are sometimes built with matching gamma compensation chambers. Supplied with two Swagelok fittings and includes helium leak testing to ensure there are no potential leaks. 

Perforated Wall Ionization Chambers

Available in a nominal 2 liter and a 200 mL configuration, these ionization chambers have perforated walls, allowing free passage of surrounding atmosphere into the chamber.  Pumps are no longer required for these ionization chambers.  Suitable for area or glovebox monitoring, these chambers should be covered with light tissue to act as a dust filter when they are exposed to particulate laden air. 

Tritium Specific Measurements

Specific tritium only measurements are made possible through the use of drying systems or permeation tubes which segregate tritium oxide from all other radioisotopes. Use of such separation methods permit measurement of tritium in any of its forms: HT, HTO, or total tritium.

Wire-Grid Plate-Out Proof Ionization Chambers

Contamination from plate-out HTO is reduced by up to 1000x through use of a specially designed wire-grid phantom wall ionization chamber which replaces the regular inner chamber in a standard Kanne design. Tests have shown that an improvement of up to three orders of magnitude is reached using this design.  These chambers can also be gold plated to further reduce plate-out from tritium.  These features are essential when measuring high concentrations of tritium to prevent contamination of the detector over time. 

OTC manufactures a wide variety of single, dual, or multiple ionization chambers in sizes from 10 mL to 2 liters.  Ionization chambers are available in versions with wire-grids (plate-out proof), perforated walls, or as otherwise requested.