The CD V-700 is a Geiger counter employing a probe equipped with a Geiger–Müller tube, manufactured by several companies under contract to United States federal civil defense agencies in the 1950s and 1960s. There were substantial differences between various models and manufacturers during the CD V-700's production. Many of the earlier units required the use of now-obsolete high-voltage batteries, and were declared obsolete by the end of the 1970s. Later models use between two and five of the standard size D battery.
Tens of thousands of these units were distributed to US state civil defense agencies. Even though large numbers have been sold off as surplus to civilian users, many remain in use with first responders and state emergency management agencies today.
Manufacturers and models
- Nuclear Research Corporation, NRC Model 1, 1954,1955.
- Victoreen Instrument Co., CD V-700 Model 2, 1956.
- International Pump And Machine Works Inc., CD V-700 Model 3, 1957.
- Chatham Electronics, CD V-700 Model 3, 1957.
- Universal Atomics, CD V-700 Model 4, 1957,1958.
- Anton Electronics Labs Inc., CD V-700 Model 5, 1959.
- Anton Electronics Labs Inc., CD V-700 Model 6, 1960.
- Lionel Electronic Laboratories, CD V-700 Model 6, 1960.
- Victoreen Instrument Co., CD V-700 Model 6, 1960.
- Victoreen Instrument Co., CD V-700 Model 6A, 1961.
- Victoreen Instrument Co., CD V-700 Model 6B, 1962.
- Electro-Neutronics Inc., CD V-700 Model 6B, 1962.
- Lionel Electronic Laboratories, CD V-700 Model 6B, 1962.
- Victoreen Instrument Co., CD V-700 Model 6A Extended Range Model.
- Victoreen Instrument Co., CD V-700 Model 6B, Extended Range Model.
The thin sidewall Geiger-Mueller tube enclosed within the brass probe body detects beta radiation and gamma radiation with the detecting probe's beta shield open, or gammas only when the shield is closed. The brass body of the probe has an energy-compensation effect on the readings taken from the probe, reducing the over-reporting of events from low-energy gamma radiation as can be encountered with non-compensated GM probes.
A small number of CD V-700s was modified by the addition of a much larger end-window probe to detect alpha radiation in addition to betas and gammas and which operated at considerably lower voltages, resulting to changes in the high voltage electronics. These units were redesignated as the CDV-700M and were issued on the basis of six units per state emergency management agency. Their primarily intended use was for detecting and cleaning up low-level radioactive contamination in state RADEF calibration laboratories, though they could be used to detect alpha radiation in the field, if needed.
The CD V-700 is used to detect low levels of radiation, 0 mR/h to 50 mR/h. High-radiation fields can saturate the Geiger tube, causing the meter to read a very low level of radiation (close to 0 R/h), thus inducing the user to believe it is safe when it is not.
The CD V-700 also came with a "check source", a bit of a radioactive isotope under a sticker on the side of the unit. The isotope varied with the maker; depleted or natural uranium was common, though the Instruction and Maintenance Manual for the Lionel model 6B indicates that a "Radium D+E beta source" with an approximate half-life of 22 years is present under the nameplate. This produced, at the time of installation on the machine, about 1-2 mR/hr adjacent to the source, a value which was clearly visible on the analog meter as well as audible via the headphones that accompanied the units. Measured with the probe's beta window uncovered and the probe directly in contact with the operational check source, this is about 100 times normal sea-level background levels of radiation, and similar to the near-field from a uranium oxide-glazed red Fiestaware saucer. The level drops back to background levels a few feet from the source. Since the half-life of the different operational check source materials used varied and since all CD V-700's were manufactured prior to 1964 and are now approaching a half-century old, the amount of radiation emitted from the source may be much lower than when the unit was originally issued. Therefore, accurate calibration via the source cannot be assured.
The CD V-700, as a true Geiger counter, is capable of measuring ambient background levels of gamma radiation and detecting the presence of beta radiation in the environment, and thus can be used to detect such common low-level radioactive artifacts as uranium-doped marbles, Fiestaware plates and radium watch faces. This differentiates them from other civil defense radiation meters such as the CD V-715, CD V-717 and CD-V-720, which are ion chamber meters that can measure gamma radiation levels far above (up to 500R/h) what the CD V-700 can (up to 50mR/h). Conversely, the ion chamber units are so insensitive to low-levels of gamma radiation that no legally exempt radiation source can make them register at all.
The CD V-700s were usually packaged in various combinations with the high-level ion chamber units listed above in kits designated CD V-777. In their original Cold War application, the sensitive CD V-700s would be saturated and thus rendered useless by the high radiation levels expected after an exchange of nuclear weapons. In that setting, it would be the ion chamber units that received most of the use. The CD V-700s main purpose was as a peacetime training instrument and for use in checking food and shelter entrances for low levels of fallout contamination. Later, as the Cold War mission wound down, many CD V-700s were re-purposed as instruments for first responders to use at the scene of a radiological incident.
As the CD V-700s produced for the US government are now at least 45 years old, they are being slowly replaced by more modern instrumentation in US federal and state government stockpiles and are being sold off as government surplus. This makes them inexpensive and very common in the US for use by uranium prospectors, physics teachers, radiation hobbyists and others interested in detecting and measuring ionizing radiation.
Victoreen produced their Model 496 in the 1980s and many were purchased by the United States Department of Energy. The 496 used a modified version of the same casing as the CD V-700 Model 6Bs that Victoreen had produced in the 1960s. If the decal on top of a 496 is removed, the markings of the older civil defense meter can be found cast into the top cover. However, the 496 features a number of improvements over any of the older civil defense versions, including modernized electronics, a BNC connector or MHV connector for external probes, a built-in speaker with on-off knob, a built-in battery test circuit, and a meter face graduated in Counts Per Minute for use with various probes. Other 490-series Victoreen models of the same era shared similar physical characteristics but varied considerably in their features.
Ludlum "CD V-700 Model 7"
Not a Geiger counter per se, the CD V-700 Model 7 is a kit to upgrade and modernize the Lionel Model 6B, Lionel/Anton Model 6, or Electro Neutronics 6B with electronics derived from a recent version of the Ludlum Model 3 Geiger counter. The kit includes a new main electronics board with rotary switch, a connector for a detachable external probe, a wiring harness and switches to add new functionality and improved reliability to aging CD V-700s. The upgrade requires substantial modifications to the case. The State of Florida Department of Public Health has modified a large number of their CD V-700s to this standard.