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.

Thursday, May 26, 2011

2011-05-26 21h CEST - Measurement briefly interrupted by a malfunction in the FH40T

Goes, 2011-05-26 22:19 CEST:
Tonight around 21h, the readings from the FH40T suddenly ceased. No more clicks were detected. After some initial testing, no immediate cause for this malfunction could be found, the FH40T detection electronics seemed dead but fortunately the G.M. tube was still OK.

After an interruption of about an hour, I was able to continue the measurements with a spare (D.I.Y) counter circuit that I had already built, using the same G.M. tube FHZ76V.

I''ll try to repair the FH40T in the near future, or otherwise I'll try to build new electronic circuit and integrate an LC-Display into the FH40T housing. Something which I already had planned to do, but hesitated to destroy a functional Radiation detector.


  1. One of my 2 FH40T also died a few weeks ago.
    The GM tube also still ok.
    Maybe the HV transformer or bad caps?
    yes lets take it apart now!
    In both my FH40T there was and is a an annoying 2400Hz tone from the HV circuit which might be due to bad Caps.
    There is rubbish inside the analog galvano meter as well which prevents it from moving sometimes. Good idea to replace with LC-display and your counting circuitry.
    The audio-connector better be replaced as well it is a source of unwanted cracks sometimes(and trigger events) now in the audio-signal (gold-plated mini audio-jack?).


  2. Normally, military equipment like the FH40T are made to last almost forever. If the HV circuit is still ok it would be not a big deal to repair the detection part. Normally the pulses are decoupled with a small capacitor and can easily be brought to a 5V level with a zener diode and then be fed into the microcontroller.

    Watching the "new" values, they are slightly higher then before. Maybe you are more to the right of the plateau now, with a higher voltage level.

    I suggest to put the radiation graphs on the intro part of your page. visitors would be happy to see them immediatly.

    Did you recieve my email concerning the SI8B tube datasheet?


  3. To Fred: Thanks for your feedback.
    I've measured the HV (with a 100MegOhm load of a Multimeter with "high impedace" (the well-known 90Meg series resistance):
    HV is still present but way too low (240Volt) (far under the starting voltage of the tube).
    The annonying 2400Hz is also audible but much weaker than it was before. So I suspect some component in the HV circuit has gone bad, probably a Cap.

    I'm doubting whether to try and repair or to dump the circuit (find the circuit diagram first) and build it all new (and digital).

  4. To Katze: thanks for your feedback. It seems that the HV is at fault (see my other response).

    About the "new values": The tube is not in the (external probe) but completely in the open, and it mounted close to a (slightly opened) window.

    Thanks for the email about the SI8B, your info was very welcome :-)

    regards, RadGoes