Frisch Grid Ionization Chamber

This example of a Frisch Grid ionization chamber was constructed in 1959 at Oak Ridge Associated Universities (then the Oak Ridge Institute of Nuclear Studies). It was designed by the physicist Elizabeth Rona who had previously worked with Marie Curie, Stefan Meyer, Otto Hahn, Lise Meitner and George de Hevesy.

Frisch grid chambers are still used today for alpha spectroscopy, but they have mostly been replaced by semiconductor detectors (e.g., surface barrier and PIP detectors).

An alpha-emitting sample was placed on the brass plate (located beneath the chamber in the photo) and then inserted into the chamber.

Since the size of an electronic pulse from the chamber is proportional to the energy of the alpha particle that produced the pulse, the alpha particle energy can be determined by measuring the pulse size. Because the energies of alpha particles are characteristic of the radionuclides that emit them, the Frisch Grid chamber can be used to distinguish the pulses produced by different radionuclides.

The above photograph (ca. 1959) shows the Frisch Grid chamber in use. P-10 gas was used as the detector fill gas even though this gas is normally used with proportional counters rather than ionization chambers. The pulses are being sorted according to size with the 20-channel pulse height analyzer located on the right side of the photo.