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.

Sunday, April 24, 2011

Schematic diagram of my Geiger counter - computer interface

I got request to publish the interface circuit that I'm using to connect my FH40T Geiger Counter to the PC.
The circuit only existed in my head and on breadboard, so I had to find a software that let me draw the schematic diagram quickly. After some Googling, i decided to try the free software ExpressSCH.

Here is the drawing (click on the image to view full size or save as...):

I will soon publish the source code for the PIC16F68A, but it should be tidied a bit.first.
It uses a modified version of the code published by Emmett Kyle found here.

My mod to the software enables outputting measured counter values to the RS232 port for datalogging, and also adjusting timing values to cater for varies Geiger Tube sensitivities.


  1. understood from the manual that 320CPM(Imp/min) is equivalent to 0.5mr/hr when FHZ76 is in use.
    So you can compare the 2 scales and find out if the source was a gamma or betha emittor or combined of course. However the 0.5rm/hr scale is not linear strange enough maybe....
    the small r in mr is Rad or Rem most probably and indeed 1r is equivalent to .01 Sv or Gy.
    Did not find anything about Roentgen units in the manual are you sure?
    Based on the background radiation in Tokyo my readings are between 5 and 15 cpm according nulleffect spec which is about 78 nSv and 234nSv. Quite realistic until now I think.

  2. Fred, thanks for your comment.
    I was puzzled at first about which units the R means. Until I found some datasheets of the (equivalent) tubes, where the Roentgen unit was mentioned.
    As i understand it, when measuring Gamma radiation it makes no difference and you can interchange rad rem and Roentgen (except for decimal point position).

  3. Yesterday I was able to confirm my readings with the FH40T at a seminar held by Berkeley Nucleonics about their Isotope Spectrometer SAM 940 here in Tokyo.
    We could use a small Cs-137 standard source to calibrate the SAM-940 but I took the chance to check my FH40T and also compair it with readings from a handheld Inspector (medcom.com).
    BTW I use GPL Speclab software on a netbook(only Windows or wine4linux) which reads and analyses the sound-clicks and has a Counter/trigger module build-in now.
    So actually your interface is not needed but anyway thanks for sharing this extra option!
    Use the the factor 15.625 to convert CPM(Imp/min) to microSievert e.g. 10CPM = 156 nanoSv or 0.156 microSv.

    SpecLab by Wolfgang Buescher, you can get here:
    GeigerCounter with SL, Beta-version download :

    I write you in English because we should share this with anybody hoewel ik ook een Nederlander ben.

    cheers and all the best from Tokyo!

  4. Can you share the source code for the pic ?