About our Radiation Counter

Our radiation counter employs a type 712, Geiger-Mueller tube whose specifications are given below. It sits on top of an office bureau in the newly refurbished, forced-air, basement of an office building in the Oakland section of Pittsburgh PA.

The output from the tube is interfaced directly to the RX data pin of serial port, A, on a 68040 NeXT computer. (Steve, I can't hold out much longer for the MAC OS 10!) Pulses from the counter cause RS-232 framing errors which are detected by the software. (The serial port baud rate is set to 38400 -- the highest allowed by the hardware).

I do not know how this method of counting pulses affects the tube's effective dead time, but at very low count rates -- typically, around fifteen counts per minute -- it shouldn't matter. (If it goes off the chart, I am outtahere.)

The "instantaneous" radiation level shown on the previous page is actually a five minute average. The calibrated reading (micro-roentgens per hour) is calculated as average number of counts per minute times (100/105).

The roentgen scale measures radiation dosage in terms of number of ions produced in air. One roengen is the amount of radiation that produces one coulomb of ions in one cubic centimeter of dry air at zero degrees celsius at standard atmospheric pressure.

Most of the radiation detected in our office is probably due to decay products of radon gas clinging to dust particles that settle near the counter. Western Pennsylvania is one of the country's radon "hot spots."

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Type 712 tube

Window

Wall

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