Low Geometry Alpha Counter (1952)

Low geometry alpha counters, or Logacs (pronounced low jacks) were first used in the late 1940s. This particular example was built by Mary Lou Curtis who contributed to the development of the original Logacs. She used such devices to count high activity plutonium samples in the 1950s and 1960s at Mound Laboratory in Ohio. To my knowledge, Logacs were never manufactured commercially.  

The cylindrical aluminum extension chamber (on the bottom of the proportional counter in the photograph below left) was manufactured by Mound. The actual counter, a gas flow system employing methane gas, was manufactured by Nuclear Instrument and Chemical Corporation.

Low Geometry Alpha Counter (LOGAC)
Low Geometry Alpha Counter (LOGAC)

In the photograph below left, the sample holder has been removed from the extension chamber. The sample was held in place beneath the circular opening at the top of the holder by a spring-loaded clip.

Low Geometry Alpha Counter (LOGAC)
Low Geometry Alpha Counter (LOGAC)

A thin metal plate is located below the counting chamber at the top of the extension tube, just above the source. According to the sample activity, the plate would have one or four openings through which the alphas would pass. The alpha source could be positioned in the extension tube in one of two positions: 5 cm away from the counter (high) or 10 cm away (low).

The count rate, after correction for coincidence losses, was multiplied by a geometry factor to calculate the sample activity. The geometry factors for the standard Logac extension chamber, used with activities in the range of 105-109 dpm, are indicated on a piece of paper taped to the tube. Another extension chamber (not in the collection), ca. 6 feet long, was used to measure activities four orders of magnitude greater.

Donated by Mound Laboratory courtesy of Carl Bishop.


  • Curtis, M.L., Heyd, J.W., Olt, R.G., and Eichelberger, J.F. Absolute Alpha Counting. Nucleonics 13, May 1955, pages 38-41.
  • Curtis, M.L. and Heyd, J.W. Absolute Alpha Counting I: Determination of Backscattering Factors and Ranges. MLM-834, April 1953.
  • Curtis, M.L. Standard Low Geometry Chamber. Memo. May 23, 1950.