CD V-720 Ion Chamber Survey Meter (ca. 1956-1962)
The CD V-720 is a high range (0-500 R/h) ionization chamber that is also capable of responding to beta radiation. According to the Handbook for Radiological Monitors, it “is designed (1) for ground survey (2) for use in fallout monitoring stations and community shelters, and (3) as an interim aerial survey instrument. It will be used by the monitor for the major portion of survey requirements in the period immediately following a nuclear weapon attack.”
The CD V-720 employs the standard two controls for an ionization chamber: a range switch and a zero adjust. The sealed air-filled chamber has a thin window (located on the bottom of the case towards the front). The latter is protected and supported by a metal plate perforated with holes (ca. 0.5” diameter). For gamma measurements, the beta shield is positioned over the window. The shield is slid towards the back of the case when it is necessary for the instrument to respond to betas. Note that the instrument is only calibrated for gammas.
The oldest reference I have found for the CD V-720 is the 1958 reprint of the 1955 Technical Bulletin TB-11-20 Radiological Instruments for Civil Defense.
Here is the Bulletin's description of what the CD V-720 was intended to be: "A high-range beta-gamma survey meter is required for use by highly qualified monitors if it becomes necessary to make measurements in areas where extremely high-level contamination exists and for making high level beta radiation measurements (see Figure 4). This instrument will have three scales, 0-5, 0-50 and 0-500 r/hr gamma and will have a discriminating slide to permit the measurement of gamma only or of gama [sic] and beta radiation. The range of beta sensitivity will be several thousand reps per hour. OCDM Standard Item Specification CD V-720."
Note that the reprinted Bulletin references the OCDM even though the latter didn't exist in 1955—this particular change was made at the time of reprinting. The photo above right was used to illustrate the CD V-720 in TB-11-20. The text on the meter indicates that it is an "Experimental Model" produced by the Victoreen Instrument Company for the Federal Civil Defense Administration.
The earliest reference I have to a production version of the CD V-720 is the Chatham Electronics advertisement in the September 1956 issue of Nucleonics that reads: "Model CH-720 Survey Meter. Beta-gamma discriminating ionization chamber survey meter. Three ranges... 0-5, 0-50, 0-500 r/hr full scale. Sliding shield excludes beta. Complies with F.C.D.A. specifications CD V-720 for high range Civil Defense survey meters." The model number uses a CH designation, but the photo shows an instrument very similar to the Model 1 with the CD logo on the side.
The example pictured above is a Chatham Electronics Model 1. The military version produced by Chatham was the IM-123/PD.
The case of the instrument shown below, a Victoreen Model 2, is solid plastic with the exception of the metal sliding beta shield on the bottom.
This has to be one of the worst designed ion chambers of all time. The metal clips used to connect the upper and lower portions of the case are held in place with a cheesy plastic connection—just like the latch on the Victoreen CD V-710 Model 5. It is almost a miracle that some of these instruments have survived unbroken. What you typically encounter are repair jobs.
Even the handle is easily snapped off.
In the environment of a post-nuclear strike where the CD V-720 was intended to be used, finding the duct tape and glue to repair the thing might not be easy.
The following statement comes from a June 1959 revision of a 1955 Civil Defense Technical Bulletin: "To provide a capability for meeting the various requirements of civil defense operations and training, OCDM has developed the following types of instruments:... "A high-range survey meter to measure and discriminate between beta and gamma radiation in areas where high dose rates exist (CD V-720)." The accompanying photo in the bulletin appears to be the Victoreen Model 2.
Since an OCDM Advisory Bulletin dated January 8, 1959 refers to the Model 2 but not the Model 3, the Model 2 must have been introduced prior to 1959, and it is likely that the Model 3 was first produced in 1959 or later.
The Model 3 shown above right has the civil defense logo removed and a US-CE-C sticker on the other side.
In September 1985, FEMA sent a notification to the maintenance and calibration facilities that the Landers, Frary and Clark Model 3 CD V-720 was obsolete (as well as the Chatham Model 1s and Victoreen Model 2s).
To a large extent, the CD V-720 was superceded by the CD V-715 since the latter had an additional low-end scale (0-0.5 R/h) as well as the same three scales employed by the CD V-720. The other major difference between the two instruments was that the CD V-715 lacked the CD V-720’s beta detection capability.
Range: 0-5, 0-50, 0-500 R/h
Size: ca. 4.5" x 8" x 4"
- Chatham CD V-720 Model 1: One 1.5 volt D cell and two 22.5 volt
- Victoreen CD V-720 Model 2: Two 1.5 volt D cells and one 22.5 volt
- Landers Frary & Clark CD V-720 Model 3: Two 1.5 volt D cells and one 22.5 volt
- Victoreen Model 3: Two 1.5 volt D cells
- Victoreen CD V-720 Model 3A: Two 1.5 volt D cells
Temperature range: -20 to +125 degrees F
Materials: Both metal and plastic cases
Manufacturers: Chatham Electronics, Landers, Frary and Clark, Victoreen
Models: 1, 2, 3, 3A
Estimated dates of first production:
- Model 1 - ca. 1956
- Model 2 - ca. 1957, 1958
- Model 3 - ca. 1959, 1960
- Model 3A - ca. 1961
Approximate Cumulative Procurement, Inventory and Distribution of CD V-720s*
*The numbers in the above table should be considered approximate. I compiled them from data in the Annual Statistical Reports of the OCDM, OCD and DCPA. By "procured," I mean delivered by the manufacturer to the OCDM, OCD or DCPA. "Inventoried" means stored in a Federal (rather than state) warehouse available for distribution. "Distributed" means sent to the end user. The latter primarily means the states, but also various federal agencies and even foreign governments. The number of procured instruments may be greater than the combined number of inventoried and distributed instruments for a variety of reasons: some may have been sent back to the manufacturer, some may have been disposed of, the numbers might be incorrect, etc. In some cases, I assumed that the procured number was the same as the number ordered.
Instruments donated by the Iowa Dept. of Public Defense courtesy of Frank Klier and Brian Lewis, the Northern Ohio Chapter of the Health Physics Society courtesy of John Wills, and the Federal Emergency Management Agency courtesy of Carl Siebentritt.
- Office of Civil and Defense Mobilization. Radiological Instruments for Civil Defense. Technical Bulletin TB-11-20. September 1955 (Reprinted October 1958).
- Office of Civil and Defense Mobilization. Radiological Instruments for Civil Defense. Technical Bulletin TB-11-20. September 1955 (Revised June 1959).
- Office of Civil and Defense Mobilization. Interim Procedures for Maintenance of Radiological Instruments. Advisory Bulletin No. 229. January 8, 1959.
- FEMA, Radiological Instruments: An Essential Resource for National Preparedness, CPG 3-1/September 1986.
- Chatham Electronics Instruction Book and Maintenance Manual for CD V-720 Model 1.
- Victoreen Instruction and Maintenance Manual for CD V-720 Model 2.
- Landers, Frary & Clark Instruction and Maintenance Manual or CD V-720 Model 3.
- Victoreen Instruction and Maintenance Manual for CD V-720 Model 3A.