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.

Saturday, May 21, 2011

Tests with SI-8B / СИ8Б Pancake GM tube

I did some experimental measurements with my new SI-8B / СИ8Б Pancake G.M. tube/probe, to verify it's functional and to prove it's able to detect any alpha radiation.

(Update 2011-06-07 22:30 CEDST): more measurement results and photos added.

Ua=400V Ra=4.7MΩ
  1. Background radiation, indoor..
    90-126cpm, average 105cpm (with or without aluminium Beta-shield on probe).

  2. Fertilizer (N-P-K), 100grams of granulars in a zip-bag laid on top of the probe:

    with Beta-shield: 134-149cpm, average 141cpm.
    without Beta-shield: 475-548cpm, average 505cpm.

  3. Am-241 Source from Smoke detector:

    with Beta-shield: 7-12cps (420-720cpm).

    without Beta-shield, just a piece of paper between Am-241 source and probe (blocking alpha): 13-25cps (780-1500cpm).

    without any shielding: 150-450cps (9000-27000+ cpm).

  4. Wolfram-Thorium (red color band, 2% Thorium, 2.4mm thickness) Tungsten welding electrode:
    A length (8cm) of WT20 electrode positioned across the SI-8B window:
    sample1:
    with Beta-shield: 253-280cpm, 268cpm average.
    without Beta-shield: 521-543cpm, 530cpm average.

    sample2:
    with Beta-shield: 239-285cpm, 261cpm average.
    without Beta-shield: 536-556cpm, 544cpm average.

    Both lengths of WT20 electrode positioned side-by-side across the SI-8B window:
    with Beta-shield: 356-427cpm, 389cpm average.
    without Beta-shield: 941-983cpm, 963cpm average.

  5. 14C 60µC test source (Beta radiation, 0.156476MeV), in TTL6109a
    "Strahlenspurgerat". Measured with the SI-8B held close to the mica window of the ionisation chamber, with the mode switch in the "R" position (so the 14C test source is exposed into TTL6109A's ionisation chamber).

    with Beta-shield: 102-143cpm, 116cpm average
    without Beta-shield: 3924-4360cpm, 4142cpm average

cpm = counts per minute
cps = counts per second

Photo's of experimental counter setup with SI8B, my HV inverter circuit and my Pulse-Counter / RS232- interface / LCD:
Test setup with SI8B, HV inverter and PIC pulse counter / LCD, powered by 4 NiMh coin cells
Close-up of the SI-8B mounted on a Pringles can.
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4 comments:

  1. JohanJune 18, 2011 at 5:33 PM

    I also have a pancake tube, they are really sensitive and able to detect alpha particles.

    I have a SI-13B and a SI-12B. The SI-13B is of typical pancake tube size, the 12B is quite small. When you hold a Am-241 source in front of it you can easily see the "fall off" of alpha particles if you move the source further away from the tube.

    I saw you tube on ebay too, always watching those Russian tubes for sale :)

    ReplyDelete
  2. AnonymousJune 24, 2011 at 7:34 PM

    Hello,

    Do you happen to know how Russian SI8B compares in sensitivity to LND pancake sensors?

    According to LND, sensitivity of their pancake sensors to gamma radiation is 60 cps/mRh. SI8B states 400 cps/mRh. I find it difficutl to unerstand why US made sensors of roughly the same size as SI8B would be 6-7 times less sensitive. I wonder if there is some confusion in units, etc.

    If you have any information on that, that'd be much appreciated,
    Jenya

    ReplyDelete
  3. radiation-goesJune 25, 2011 at 1:45 PM

    Hello Jenya, thanks for your comment.
    As for the specs of the SI8B i'm not sure if the gamma sensitivity of "400" is really in CPS. It could very well be CPM but that would be extremely low, it is unclear to me.
    I have not been able to check the value as I have no calibration (gamma) source. I can only compare it's sensitivity to my FHZ76V tube - the SI8B is roughly ten times more sensitive.

    The big difference between the LND pancakes and the SI8B could lie in the effective surface, the SI8B has 30cm2. How does that compare to the LND's ?
    RG

    ReplyDelete
  4. katzeJune 27, 2011 at 5:41 PM

    Hello, I´m not absolutely sure about the following but it seems to make sense:

    The Manufacturer states "Sensitivity to Gamma Radiation: 400 Pulses/μR"

    That would mean: "400pulses/hour=1µR/hour" and that would result in 6,66cpm.

    1µR = 0,0084µGy and so 1µGy = 119,047619047619µR (in Air)

    So we have 0.0084 µGy/h = 6,666cpm
    leading to 1µGy/h = 793.65cpm

    Realistic Background Radiation would be between 0.1 and 0.2 µGy/h
    0.1 µGy/h = 79.365cpm
    0,2 µGy/h = 158.73cpm

    With Cap, I get around 127cpm that would be 0.16 µGy/h,
    without, it is a little less with 123cpm, that would be 0,155 µGy/h

    Maybe it helps ;-)

    ReplyDelete

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