Radiological Survey Meter
CH-720
FCDA Item Number CD V-720 (Model 1)
Military Designation IM-123/PD
The following is from the Instruction Manual
The Chatham CH-720 was designed specifically to fulfill the requirements of the Civil Defense High Range
Radiological Survey Meter CD V-720. Utilizing three ranges of 5, 50 and 500 roentgens per hour full scale,
it measures gamma and x-ray radiation fields from 0.5 to 500 roentgens per hour and detects beta particle
radiation even in the presence of such fields. An air-filled ionization chamber serves as the radiation detecting
element. The chamber has a thin window which permits the transmission of beta particles into the chamber
volume, thereby making detection possible. With the window covered by a movable shutter the instrument
responds only to gamma or x-radiation. When the shutter is placed in the open position, the beta window is
exposed and the unit also responds to beta radiation.
The accuracy of the instrument is within ± 15% of true dose rate throughout a temperature range from -20º F
to +125º F. It is also within ± 15% for photon energies from .08 MeV to 1.2 MeV with the beta shutter in
either the open or closed position. It is unaffected by altitude changes up to 25,000 feet.
Top View
Left: IM-123/PD Serial Number 11841
Right: CD V-720 Serial Number 4466
Side View
Bottom View With Beta Shutters Open
Inside View Of Case Bottom
Note that the ion chamber stays with the lower part of the case.
Uses 1 "D" size 1 ½ volt battery and 2 "B" size (Neda Type 215) 22 ½ volt batteries.
This unit is all cast aluminum.
Inside View Of Meters
Government cost in 1969: $23.30
But already obsolete in 1963 ...
Government Procurement - Fiscal Year 1955 - 1985 113,231