Model 121 "Drill Hole" (1955 - 1960)

The Precision Radiation Instruments Model 121, also known as the "Drill Hole," was a Geiger Mueller survey instrument designed for making measurements in drill holes as deep as 1000 feet. In almost all respects, it was similar to the Model 120, also known as the "Drill Hole." The Model 121 Drill Hole came with three detachable probes: the end window GM (with removable protective cover), a side window GM with sliding beta shield, and a special probe (3/4 " diameter) with a fifty-foot cable for subsurface measurements. Only one in this collection is the side window GM.

The assigned date (1955 - 1960) is based on the copyright date of 1955 for the Model 121's Manual.

Detectors: side window GM with sliding beta shield
         end window GM tube (30 mg/cm2)
         Special subsurface GM probe (3/4" diameter) with fifty-foot cable

Range: 0-0.04, 0 -0.2, 0 - 2 and 0 - 20 mR/h. It could be calibrated to read out as per-cent uranium.

Dimensions: 3.5" x 4.5" x 6.5"
Ser. No.: 21P 1546
Weight: 6 pounds
Batteries: two 1.5 volt (D cell), one 45 volt (Eveready 455) and one 67.5 volt (Eveready 467) batteries.
Leather case and strap
Headphones
Radium calibration source 0.9mR/h

Operation and Maintenance Manual for Model 121 "Drill Hole" Geiger Counter. August 1955.