About this blog

About this blog

During the recent events in Fukushima - Japan, it soon became clear that the authorities are not very informative to civilians regarding radiation exposure values. Authorities seem to be witholding information, perhaps to avoid panic.??

So I got the urge to be able to detect and measure radiation by my own, especially since I live within a 15km radius from the NPP of Borssele and a 30km radius from the four reactors of Doel NPP, Belgium.

Browsing the internet, 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 γ (gamma) radiation and β (beta) radiation over 0.25MeV.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:

1 R = 8.77 mGy
1 Gy = 115 R

For sake of simplicity, in our calculations we simply use 1R = 10mGy and 1Gy= 100R. And so is 10µR = 0.1µSv.
This approximation is good enough for this experiment.

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.

There are 3 types of radiation:
α (alpha) decay is helium nucli being released, (beta) decay is electrons (β-) or positrons (β+) and γ (gamma) decay is electromagnetic radiation (like X-rays).
This Geiger-Mueller tube is only sensitive to β and γ radiation. The calibration is only correct for the γ radiation (662keV) emitted from Cs-137 .

I am now on the lookout for a device that can detect alpha radiation too. But the current situation in Fukushima has stirred up the market (crazy prices, run out of stock) for detection devices so I better wait until better times.

Wednesday, June 8, 2011

Disassembly and repair of FH40T geigercounter

After the malfunctioning of my FH40T geigercounter, it became clear that the probable cause was a gradual loss of High Voltage. Probably due to bad capacitors (loss of capacity or leak) in the voltage doubler / rectifier stage.
To reach the components concerned, the circuit boards need to be removed from the FH40T's housing.
To make removal possible, several wires need to be unsoldered.
To document the situation, to be able to connect all wires after repair of the circuit, I took this series of  pictures:

I've replaced all three 0.025µF / 400V capacitors, a 0.01µF / 250V capacitor and the three 100µF / 4V / 6V electrolytic caps (the meter averaging caps and supply filtering cap) too, as they probably have lost their capacity due to drying out.
After re-assembly of the unit, it seems to be working again, so the operation succeeded ! H.V. measured 500V at the FHZ55/500 stabilizer tube and it's glowing softly.


  1. Thank you for the high quality images. To contribute a little, here is the circuit diagram:

    Yours, katze

  2. Hello
    I see with pleasure that the "Probe Selection Guide" is useful:)
    I hope to update the document ...
    I also got on ebay an SI8B , I want to connect to a DOM410.
    I'll try as soon as possible.

  3. Hello LABX,
    yes I've found your "Probe Selection Guide" very useful. Thank you! Keep up the good work.

  4. Hi
    I updated the document.
    I added other usual tables.