Roy and Niels

Roy and Niels

Sunday, October 20, 2013

I Just Bought a Dosimeter

I recently bought a cheap personal dosimeter by the Ukrainian company SOEKS. The best time to buy these is, when there has *not* recently been a nuclear disaster - prices are more reasonable then. I went for their "Defender" model, since it is capable of measuring accumulated dose, and I do not need fast counting provided by those devices with two GM-tubes.

The SOEKS Defender dosimeter.
Ok, I said "dose", but in fact I am not sure what it measures, it should be the operational quantity "personal dose equivalent", but it could also be something completely different (see ICRP 103). In fact I would not be surprised if they do some sort of "dose equivalent" calibration following ICRP 26 protocol from 1971. Many still confuse this.). Just for now, I'll just call it "dose", well knowingly that this is wrong.

It is the first time I order stuff from Ukraine, but shipment was really professional. They even email you a picture of your parcel with your address on, before they ship it. Contrary to some of the reports, I had no trouble with customs either. It was directly delivered to my institute address with no fuzz. Good thing!

Unboxing it, I found a nifty small gadget, which fits in a pocket, It comes with rechargeable AAA batteries (SOEKS brand, these people are really dedicated!), which can be recharged via the USB connector. I might have preferred AA batteries, as these simply perform much better than AAAs. But still, nice that you can always switch to normal batteries if you are in the field, run out of power and have no time to recharge.

One thing I really miss though, is a loop for attaching a lanyard. :-/

Switching it on, I immediately measured the normal background radiation levels here in Aarhus, around 0.12 µSv/hour.

One of the first things I tried out, was to take it along a trip to Turkey (yeah, the NUFRA2013 conference), to see what dose I'd receive during the flight.

I left it turned on while my cabin luggage was X-rayed. I did this three times, and each of these measurements gave 0.8 µSv.

On board the airplane I could measure a background level of roughly 2 µSv/hour, triggering the "Dangerous radiation" warning of the dosimeter. The threshold for the warning can be adjusted by the user. The 2 µSv/hour is probably a very conservative measurement anyway, as the device does not count the contribution from neutrons.

Video documentation of that flight (at roughly 10 km altitude):

Dose rate increased rapidly when we came above 8 km altitude. Under 8 km, it was pretty much just the background radiation. For the 4-5 hour trip from Antalya to Amsterdam I measured a total dose of 8.19 µSv. (This is 10 times the amount my luggage received when it was X-rayed. Very interesting.)

Arrived at Shiphol, Amsterdam from Antalya. Flight time 4 hours 47 minutes. Accumulated dose 8.19 µSv.
At our institute I can test the dosimeter at several sources. None of them are calibrated in any way, it is merely to see some signal. The strongest sources we have at the institute are couple of Americium-Beryllium sources for neutron generation. Whenever you have neutron fields you also have plenty of gammas, and if the sources is packed in a hydrogen rich moderator material there are also energetic protons (recoiling hydrogen nuclei).

The AmBe source sown below is locked up in a safe, but even when the safe is closed, you can see an elevated photon background in a meters distance:

Elevated background radiation near the neutron cabinet.

Inside the safe, the AmBe source is located in a paraffin moderator. The source is used for various activation experiments, where you can lower the materials to be activated into the two plastic tubes.

The AmBe neutron source is hidden in the paraffin block. Through the tubes you can immerse various materials for neutron activation analysis.

Here, I recorded the highest dose rate recorded so far, 320 µSv/hour. Again, this is probably not a very correct measure, since that area is flooded with neutrons as well. How neutron interact depends strongly on the target material.

320 µSv/h. Occupational limit is 20 mSv/year, public limit is 1 mSv/year, so let's quickly close the door again.

We have several dosimeters at the institute, so I tried to compare the elevated background with a calibrated device, the quite common and more pricey "Gamma scout" dosimeter:
Comparison of the SOEKS Defender against a Gamma Scout. In fact this was the best agreement I ever saw between the two devices.
They both measure the same background to approximately 0.7 µSv/hour, but the agreement varied a lot depending where in the room I was. Mostly, I noticed a considerable underestimation of the dose reported by the SOEKS device, sometimes up to a factor of 2. Probably this is just the precision you can get, it's good enough for my (non-work related) purposes. If you want high-precision monitoring, you need professional equipment which is well calibrated, and that is certainly a different price tag.

I also have a little Japanese silver medallion. I activated it by leaving it in on top of the AmBe paraffin block. Neutrons generate Ag-110 and Ag-108 isotopes with T_1/2 of 26 sec and 2.4 min, respectively. I could follow the cooling down of the medal with the dosimeter:

Japanese silver medallion activated by neutrons. After 15-30 minutes it cools down to normal background levels.

Various sources for student exercises.

Anyway, I still hope to find time to realize my Chernobyl visit, will certainly bring this handy device with me. :)


P.s.: Not really related, I recently came across a very old video of how I in my young ages zapped a CCD detector using a medical linear accelerator. The chip suffering in the video is an engineering grade CCD47-20 by EEV (E2V / Marconi), which was intended to fly on a satellite mission. I later also irradiated one with protons. (Sorry, video is a polyglot mix of German and Danish.)

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