CD V-720
Survey Meter
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CD V-720 Survey Meter

CD V-720 Family
This is the CD V-720 family. From left to right in the photo: Chatham CD V-720 Model 1, Victoreen Model 2, Landers Frary and Clark Model 3 and a Victoreen Model 3A. Click photo to see larger.

The CD V-720 is one of the early issue Civil Defense instruments. I haven't been able to find an exact issue date for the first CD V-720s. The first 720's were originally issued in the mid-1950's with the CD V-710. The Victoreen Model-2 is the only CD V-720 that I know of that was manufactured with a plastic case. The early versions of the CD V-720 (Chatham and Victoreen Model-2) were declared obsolete and went through disposition like the entire family of CD V-710s. Many of the early CD V-720s that are found today have had the labels defaced and the "CD" logos marked over. This was done during the disposition process.     

The CD V-720 is a high-range radiological survey meter that can discriminate between Beta and Gamma Radiation but only measures Gamma. The range of the CD V-720 is 0-500 Roentgens per hour. The sliding cover on the bottom of the meter case blocks Beta particles when closed allowing only Gamma radiation to pass through the detector. When the cover is opened this allows Beta particles to pass through the very thin metal membrane on the bottom of the CD V-720's Ion Chamber resulting in detection of beta radiation. See picture below.

Inside CD V-720
In this photo the holes on the bottom of the Ion Chamber can be seen. The bottom of the CD V-720 Ion Chamber has a thinner metal (.006 inch) on the bottom. This thin material allows the Beta particles to pass through the thinner metal bottom and then be detected by the chamber. The Victoreen 720s (seen here) have a thick aluminum plate with holes glued to the bottom of the chamber to reinforce it around the thin area and for protection.

Inide Chatham 720
This is the inside of the Chatham CD V-720. Click photo to see larger.
As far as I know Chatham only made a CD V-720 and CD V-700 in the early days. These Chatham meters are really unique. Note the plastic strap battery holders and the rounded, cast case design. This meter has the sliding metal cover on the bottom like all 720s. Also worth noting is that the ion chamber is mounted inside the removable bottom of the meter and has a cable that plugs into the top electronics section when the bottom is installed.