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.


Note:

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

Radiation in Goes - Live graphs

Latest graphs. 

Update 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 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.
 
SI8B CPM measurement








Last month's radiation graph, normal background levels. 
Sun 04 sept at midnight: Raindrops leakage on GM tube and electronic circuit causing a spike and dropout of measurement. Fixed next morning.
Tue 28 june: Heavy rain and thunderstorms caused huge peak.
A lightly decreasing trend since the beginning of my measurements can be seen in the month graph. Did we experience Fallout from Fukushima before my measurements started, which is now slowly decreasing due to the half-life of the nucleides ?

Images kindly hosted by dead-men.de
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1 comment:

  1. katzeJuly 14, 2011 at 11:50 AM

    I asked myself for a while, why thunderstorms always cause you these peaks - because natural radiation should not be influenced by thunderstorms and only to a small degree by rain.
    My guess is that the flashes cause ticks in your counter. Try the following: Put your device close to a Halogen or "Leuchtstoffroehre"-Lamp and switch it on and off several times.
    I finished my selfmade counter and thought it would be rather robust and not vulnerable to disturbances - but I tried the scenario described above with my two desk lamps, and they induce large errors in the reading.

    Let me know if my guess applies in your case, too.

    Greetings, katze

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