schematic diagram of my geiger counter

  
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I found some relatively cheap ex-army radiation detectors at an army-dump shop. One of them appeared to be suitable to even detect the (usually low) background radiation levels: A Frieseke & Hoepfner FH40T Geiger counter (fitted with a FHZ76V energy-compensated geiger-mueller tube), sensitive to ³ (gamma) radiation and ² (beta) radiation over 0. 25MeV.
schematic diagram of my geiger counter - schematic

The FHZ76V tube actually contains a Valvo 18550 tube, which is equivalent to Centronics ZP1320, Mullard Mx164 and LND-713 (found in this Probe Selection Guide and here ) The specs of the ZP1320 tube claim a sensitivity of 9cps/mR/h for Cs-137 (540cpm/mR/h). For `normal` background (0. 025-0. 045mR/h) this results in a counting rate of approx. 10-20cpm. Where I live, I measure values varying between 4cpm up to 25cpm. This variation is caused by the randomness of the decay of radioactive elements. The unit R in this text means Roentgen, a depricated unit of radiation exposure. Nowadays it is better to use S. I units. The Gray (Gy) and Sievert. The official conversion between Roentgen and Gray is: I`ve built a PIC16F628-processor based interface / pulse-counter, that counts the pulses and converts them to mR/h values and transmits them out of an RS232 port. This interface is then connected between the Geiger counter and a small PC, running Linux. On the PC, a simple script runs that reads the values from the RS232 port (one measurement value every 111seconds) and stores the entries in an RRD database and the graphs are made with rrdtool. ± (alpha) decay is helium nucli being released, (beta) decay is electrons ( ²-) or positrons ( ²+) and ³ (gamma) decay is electromagnetic radiation (like X-rays). I am now on the lookout for a device that can detect alpha radiation too. But the current situation in Fukushima has stirred up...



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