Monazite (CE) from Madagascar

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Hi guys,
This weekend I tested a nice sample of Monazite (CE) which contains Thorium.
The sample is pretty active, 14000 CPM on the Geiger Counter and most importantly more than 1000 CPS of gammas according to my GS spectrometry kit. Coming from the 2 CPS of my last Trinitite sample it was nice to deal with a spectrum where peaks weren't hiding...

All expected peaks from Th232 are there, so nothing special to say about them, with the possible exception of the 99 keV peak from Ac228 which is new to me, I never detected it before therefore I was a bit skeptical. In the end I called it, but I am still not convinced so I put a question mark there.

Anyway, since the spectrum itself is pretty straighforward I want to focus on the quantitative analysis. I just received a new detector, the Mirion PDS 100G, a spectroscopic dosimeter, capable of giving you the equivalent dose and the spectrum of the detected radiation.
That provides a good chance to check the quantitative analysis of my tests with another detector. I will post a review of the detector shortly in the appropriate forum section, so here I will just focus on the results.

The first bunch of images attached are the usual ones, the spectrum in counts per bin and energy per bin, a couple of photos of the sample, the read on my geiger counter and the quantitative results.

Last images (the ones at the bottom) show results from the PDS. The background spectrum is not particularly clear, but it was recorded with the sole intent of having a value for the equivalent dose, so I stopped the recording as soon as the figure was stable (after 2997 seconds) and the peaks weren't fully formed yet.
The average value of the equivalent dose from the background is reported in the spectrum screen, but with only two decimal digits. You can see the third decimal digit in the photo of the detector display taken right after the measure was over (it's 0.096).
The Monazite spectrum is much clearer, given the higher activity. It was recorded over 3996 seconds.
Background spectrum is showed in logaritmic view, while Monazite spectrum is in linear view,
At contact with the detector the sample gave a peak read around 1.60 µSv/h. For the long read I put the sample at a few millimetre from the detector, in an attempt to have a similar distance between the sample and the crystals of both the PDS and the GS.

My usual analysis led to the following results:
- Average Dose from the background 0.106-0.108 µSv/h
- Average Exposure from the sample + background = 1.045 µGy/h
- Average Exposure from the sample = 0.96 µGy/h
Assuming a weighting factor equal to 1 the Gy can be converted in Sv with a ratio 1:1.

Results according to the PDS 100G
- Average Dose from the Background = 0.096 µSv/h
- Average Dose from the sample + background = 1.19 µSv/h
- Average Dose from the sample = 1.094 µSv/h

The difference is in the region of 10-15%, which is not bad I think since every device has its own margin of error, and there are several uncertainties in my home-made method, like the weight of the crystal which is a factor in the results from calculations, or the actual distance between the sample and the crystal itself, the latter being irrelevant in the background measurement, but playing a significant role in the sample measurement. Without knowing the exact position of the crystal in both detectors is hard to be sure the two measurements were actually done at the same distance.
Repeating the test at bigger distance, which I will certainly do, will make this last uncertainty less important, but for now these results might be an indication that probably past dose assessments weren't off by a mile.

Have a nice Sunday.

Massimo

[stamasd]

Very nice. I wish I could get a sample of monazite, however I've been unsuccessful so far. I have to make do with thorium mantles.

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Very nice. I wish I could get a sample of monazite, however I've been unsuccessful so far. I have to make do with thorium mantles.

It might sound strange but the "hottest" thorium source I tested so far is the back of this Pentax lens viewtopic.php?f=5&t=519, more than twice as hot as this Monazite sample.

[stamasd]

That is "hot" indeed, but I have enough sources of refined thorium (lantern mantles, a whole box of them, over 100pcs); I'm looking for a source of natural, unrefined thorium.

[pilgrim]

Massimo,
excellent work, in the end your homemade analysis were very accurate.
The PDS seems to work very well ;-)

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The PDS seems to work very well ;-)

Indeed! Phase 1 is comparing the results from the PDS with those from the GS, once I am confident the device works fine I will go for outdoor spectra and equivalent dose assessments and I think a short trip to Orvieto (where background radiation should be quite higher than the Italian average) will be in order.