Geiger Counter, § II

❦ I needed a Geiger–Müller tube to add to the Geiger counter board.  The tube has low pressure gas in it between a high voltage anode and cathode.  When a particle of ionizing radiation passes through it it hits a gas atom and creates a + and - charged particle pair by knocking off an electron.  The pair race towards opposite ends of the charged tube.  Along the way they accelerate enough to knock other ions loose, which knock even more loose and create a cascade of charged particles.  This amplified signal registers in the circuit and is what allows single particles to be detected.  I found a J305 beta/gamma tube for sale for $31 by a company in China.  When the package arrived it had been checked by security.

❦ This tube can not detect alpha particles (helium nuclei), which are stopped by the glass.  But it can detect beta (high energy electron) and gamma (high energy photon) particles.  To try it out I temporarily connected it to the geiger counter shield which was plugged into the micro-controller underneath, which was connected to my computer.  I uploaded a program found here (link).  ☞ Note the conversion factor must be set correctly in the program for the model of tube you are using. 

❦ It clicked every few seconds and in the picture it registered 12 counts in the last minute (CPM, counts per minute), which, for this tube's sensitivity, corresponds to 0.0974 micro-Sieverts per hour of background radiation.