A Portable Geiger-Müller Counter

Abstract

While all of the material presented here is not new, the original circuit was published in a physics journal (1) to which few radiologists have access, and some important changes have since been made. While the principle involved in a counter for field use is the same as that in the more complicated laboratory instrument, the practical design must differ greatly, as simplicity, ruggedness, and freedom from connection to power lines must be maintained without sacrifice of sensitivity. The above requirements appear to be well met in the little instrument described here, as it has been successful in the recovery of much radium, not only by the author but by Dr. George Henny of Philadelphia, Dr. Kenneth Corrigan of Detroit, and others. As shown in Figure 1, the instrument is completely contained in a steel box measuring approximately 5 1/2 × 6 × 3 inches, which is carried in a canvas case having a pocket for the headphones. Thus the hands of the operator are left free for carrying a stop-watch and tally counter with which to record the number of impulses per minute. The sensitivity is the same as that of other counters described by the author. "Sensitivity" is arbitrarily defined as the ability to double the normal count when 50 mg. of radium are located at a distance of about 212 feet, this count being directly proportional to the amount of radium and inversely proportional to the square of the distance. With a brief description, the diagram (Fig. 3) should be self-explanatory. The old type Ford spark coil normally has one end of the secondary winding grounded to the primary. This end must be removed from the primary and brought out by a well insulated lead to the plate and screen of the rectifier tube. This end of the secondary is easily located by fluoroscopy. Proper polarity of the primary to the battery must be determined by trial, since the secondary current is partly unidirectional and, to use the weaker alternation of the cycle, will result in sparking across the tube socket. In the compact arrangement shown here, the coil is removed from its wooden box and insulating compound, but some trouble can be saved if a slightly larger metal box is used to house the instrument. The size of the instrument has been considerably reduced by the use of a 1T4 for rectifier and another for amplifier instead of the larger tubes. Despite their miniature size they stand up under operation and even intentional abuse. The choke coil in series with the filament of the amplifier tube almost entirely prevents the sound of the vibrator in the phones. This choke consists of the core from any miniature transformer, wound with about six ohms (DC resistance) of No. 28 wire.