Well-type Condenser Ionization Chamber Built by Carl Braestrup
The photograph to the right shows a well-type condenser ion chamber built by Carl Braestrup (ca. 1940s). The chamber is shown connected to a string electrometer - information about the electrometer can be obtained by clicking here.
well-type (reentrant) chamber provides close to a four pi counting
geometry since the sample is effectively positioned inside the chamber at
its physical center. The sample, usually a liquid in a test tube, is
counted inside a well-like depression at the top of the chamber wall
Prior to the introduction of the sample, the chamber would be charged so that a potential was created between the inner and outer electrode. This was accomplished with a string electrometer of the type seen in the photograph. When the sample was placed inside the well, the emitted radiation ionized the air in the chamber and reduced the charge. A measurement was then performed of the time it took the electroscope fiber to move a specific number of divisions. Using a calibration that was appropriate for the radionuclide being measured, this time was related to the sample activity.
As seen in the diagram to the left, a sample positioned in the well is also inside the tubular inner (central) electrode.
The plastic example in the photograph is probably a model that Braestrup used to illustrate the construction details. Feitelberg (1949) described the Braestrup well chamber as being constructed from brass.
similar diagram of a chamber of this design, attributed to Braestrup, can
be seen in NBS Handbook 80 (NCRP Report No. 28), “A Manual of
Feitelberg, S. "Standardization of Radioactive Iodine" Science May 6, 1949, p. 456.
Donated by Richard Mooney
B. Braestrup (1897 – 1982) was born in
Copyright 1999, Oak Ridge Associated Universities