I've worked in labs and had experience with Geiger counters, so I jumped at the chance to buy this one at a yard sale.

_{photo by @qiyi}

After some internet searches, I was able to use the serial number to find more about it. It is a 1962 Victoreen model V-720R. It is designed to detect very high levels of x-rays and gamma radiation. Instead of a hand-held probe, the detector is a 7.5 cm-diameter x 1 cm-thick disc inside the metal box.

The R in the serial number means this unit was retrofitted from the previous model (V-720). The V-720 had a sliding cover on the detector with holes to allow beta particles to reach the detector. The V-720R was designed to reduce background from beta radiation and detect gamma rays (which penetrate the thin metal casing). This would have been used to screen for extremely high levels of radiation following a nuclear attack.

The following is quoted from an excellent article by Oak Ridge Associated Universities describing this and other models:

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 [while the V-720R is shielded from 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.”

It is in perfect shape, and it was stored without a battery so there is no corrosion. The battery check works when a battery is inserted. I have not been able to check for detection because of a compelling safety issue: This model was designed for very high levels of radiation, and I wouldn't want to be around to see it detect anything!

The scale goes from 0-0.5 R/hr (Roentgens per hour) at the lowest range setting up to 500 R/hr at the highest. There are many different units for measuring radiation levels and it can get confusing.

The Roentgen takes into account bodily exposure and is thus a measure of radiation dosage. The reader is referred to the Wikipedia entry for Roentgen for a full explanation, but to put it in perspective the median lethal dose, fatal to 50% of people in 2 to 12 weeks, is 450 R! (source)

I'd like to add this table copied from that same source:

Roentgens Effects

• 15 R
  Smallest effect detectable by statistical study of blood counts of a group of people.
• 50 R
  Smallest effect detectable in an individual by laboratory methods.
• 75 R
  Smallest dose causing vomiting on day of exposure in at least 10% of people.
• 100 R
  Smallest dose causing loss of hair after 2 weeks in at least 10% of people.
• 200 R
  Largest dose that does not cause illness severe enough to require medical care in over 90% of people.
• 450 R
  Median lethal dose, fatal to 50% of people in 2 to 12 weeks.
• 600 R
  Severe sickness due to gastrointestinal tract damage, survivors unlikely.
• 2000-10000 R
  Death in minutes-to-1-day due to central nervous system damage.

References

https://www.orau.org/ptp/collection/civildefense/cdv720.htm
http://daretoprepare.com/NBC/Roentgen.chart.html