RCL Mark 1 Model 70, Mark 1 Model 71 and the Model 10306 Annular GMs (1950s)

RCL Mark 1 Model 70 71 10306 GM Tube

In the late 40s and early 50s, prior to the development of liquid scintillation counters, the main options for counting liquid samples were the use of dipping counters, Marinelli beakers, and the annulus tubes of the sort shown here. An annulus (jacketed) tube consists of a central thin-walled glass GM tube surrounded by an outer glass jacket (which served the same role as a Marinelli beaker). The water to be analyzed was fed into the lower of the two openings on the jacket. The sample might remain stationary in the annulus or it could flow continuously through the tube during the count. Annulus tubes were used for analyzing gas samples as well as liquid samples.

The photograph to the right shows (from left to right): a Model 10306, a Mark 1 Model 70 and a Mark 1 Model 71. All three were manufactured by Radiation Counter Laboratories of Chicago and Skokie Illinois. According to the accompanying Certificates of Examination, the Mark 1 Model 70 was tested in January of 1954, and the Mark 1 Model 71 was tested in January of 1955. The Model 10306 probably dates from the late 1950s. RCL produced several other jacketed counters that were similar to these, e.g., the Mark 1 Model 72 and the Model 10315.

The electrical connection to the central anode and the connection to the cathode can be seen in the photograph below left.

The end of the central anode (a fine vertical wire) can be seen projecting from the top of the silver cathode in the photograph below right.

RCL Mark 1 Model 70 71 10306 GM Tube
RCL Mark 1 Model 70 71 10306 GM Tube


  • Model 70 - 5 1/2" high, 3/4" diameter
  • Model 71 - 6 1/2" high, 1 1/4" diameter
  • Model 10306 - 6 1/4" high, 1 1/4" diameter

Wall: 30 mg/cm2 borosilicate glass

Gas: Neon with amyl acetate quench

Cathode: Inner surface of glass wall coated with silver

Operating voltage: 850 to 1000 volts


  • Mark 1 Model 70 - 3 prong
  • Mark 1 Model 71 - 4 prong
  • Model 10306 - 4 prong


  • AEC Radiation Instrument Catalog, page date March 1, 1949, page BG-2A3A.
  • Atomic Energy Commission Supplement 2 for the Radiation Measuring Instruments catalog RIB-8, July 1, 1954, page BG-5H.