IM-75/PDR-18A Scintillator Survey Meter (ca. 1951-1955)

IM-75/PDR-18A Gamma Scintillator

The instrument shown here is an IM-75/PDR18A survey meter that was produced by Tracerlab Inc. in the 1950s.

The NAVSHIPS 94200.5 Directory of Radiac Equipment (ca. 1960) states: "The AN/PDR-18, -18A, -18B are portable instruments used to detect and measure high intensity gamma radiation on a direct reading meter... The detection of gamma radiation in this instrument is accomplished by means of a sensitive phosphor element. Gamma radiation impinging on the phosphor causes it to fluoresce and illuminate a photomultiplier tube."  

The AN/PDR-18, which first came out in 1949, is an absolutely fantastic instrument. It is one of the very earliest, if not the earliest, gamma scintillation survey instruments to go into production. The scintillating crystal in the AN/PDR-18A is approximately 1" x 3/4" x 1/2" in size and is coupled to a "side-on" photomultiplier tube (an RCA 931A). It is not hygroscopic, so it's definitely not NaI, and because it is crystalline, it can't be a plastic scintillator. The AN/PDR-18 clearly uses some type of organic crystal. In 1950, Tracerlab focused on producing two types of scintillators: plastic and stilbene. For this reason, I am guessing that the scintillator is stilbene.            

IM-75/PDR-18A Gamma Scintillator

The photo at right shows the inside of the front end of the instrument. The small black rectangular crystal housing (indicated by the white arrow) is in its normal position, mounted on the front side of the housing for the photomultiplier tube.

All three versions of this instrument the PDR-18, PDR-18A and the PDR-18B were identified in the 1975 version of the Naval Electronic Systems Command List of Radiacs in the Navy Program as being obsolete.  

A distinctive feature is its method for changing scales. The switch for changing the scales was connected (by a chain much like that used on a bicycle) to a wheel underneath the meter face on which the different scale ranges were indicated. Each range was indicated in a unique color. When the range switch was turned, the wheel to which it was connected also turned. This caused the range seen on the meter face to change. The PDR-18 had four selectable scales: 0-0.5 (yellow), 0-5.0 (orange), 0-50 (magenta), and 0-500 (red) R/h. The meter face could be illuminated by a light that was activated by the circular black button on the tip of the handle.

IM-75/PDR-18A Gamma Scintillator
IM-75/PDR-18A Gamma Scintillator

Range: 0-0.5 (yellow), 0-5.0(orange), 0-50 (magenta), and 0-500 (red) mR/h

Size: ca. 10” x  5 1/4” x  4 1/2” (excluding handle) 

Weight: 8.5 pounds

Batteries: Six 1 ½-volt D batteries. BA-2030 batteries recommended for low temperature operation.

Manufacturers: PDR-18 Westinghouse Electric, contract date of 1949; PDR-18A Tracerlab Inc., contract date of 1951; PDR-18B Westinghouse Electric, contract date of 1951.

The example shown here was repainted by the previous owner. The original color was a somewhat lighter gray.


  • Campbell, D.C., Radiological Defense, Vol. IV Armed Forces Special Weapons Project, Foreword dated January 1950.
  • Naval Electronic Systems Command List of Radiacs in the Navy Program P-9670-1, February 1975.
  • Directory of Radiac Equipment NAVSHIPS 94200.5, no date, ca. 1961-62.
  • Defense Atomic Support Agency, List of Military and Civil Defense Radiac Devices. DASA 1243 revised, 1966.