Condenser Ionization Chamber
Built by Carl Braestrup for Measuring X-ray Emissions from Television Sets |
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The two condenser ion chambers shown here were
constructed for Carl Braestrup and Richard Mooney at
New York
’s
Francis
Delafield
Hospital
. They were designed to measure the low-level x-ray emissions from the
picture tubes of television sets. Condenser chambers provided the required
sensitivity because the ionization could be integrated over an extended
period of time. The first version of the chamber, built in 1947, employed
three flat parallel electrodes with the central electrode fabricated from
nylon gauze made conductive with a coating of aquadag. The later version,
shown here, employed an aluminum rod as the central electrode. This
increased the sensitivity by reducing the system’s capacitance, although
it also resulted in a non-uniform electric field. The plastic chamber wall
is coated on the inside with aquadag to make it conductive. The larger of
the two chambers is 12" in diameter and 4" thick while the
smaller unit is 6" in diameter and 4" thick. The windows are
made of thin (£
0.5 mm) polystyrene.
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At
the time of the study, the International Commission on Radiological
Protection (ICRP) recommended that the exposure rate on any accessible
surface of a television set be limited to 2.1 mR/hr. As a result of their
measurements, Braestrup and Mooney concluded
(1959) that “a permissible radiation level of 0.5 mr per hour 5
cm from any accessible surface of the equipment [a value under
consideration] appears reasonable and will not require any changes in most
existing sets.” This
recommendation was later adopted by the NCRP and ICRP and it became the
regulatory limit currently specified in 21 CFR 1020.10 by the FDA. |
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Donated by
Richard Mooney
References
Braestrup, C. B., and Mooney, R. T. X-ray Emission from Television
Sets, Science 130: 1071-1074, Oct. 23, 1959.
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| Carl
B. Braestrup (1897 – 1982) was born in
Copenhagen
Denmark
. He worked as an
engineer at Bell Telephone Laboratories and Picker X-ray
Corporation, instructed at the Post Graduate Medical School of
Columbia University, and served as the Director of the Physics Laboratory
of the New York City Department of Hospitals. He was one of the first in
the 1930’s to develop a film badge that would account for the energy of
the radiation and was one of the first ten persons certified in x-ray
physics by the Radiological Society of North America. An internationally
recognized expert on shielding design, he developed a standardized
approach to the shielding design for medical and industrial x-ray
facilities that formed the
basis of the NCRP 49 methodology. Together with Harold Wyckoff, he
coauthored the classic textbook Radiation Protection (1958).
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