Smoke Alarm - Americium 241

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Go-Figure
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Smoke Alarm - Americium 241

Post by Go-Figure » 30 Jun 2019, 09:00

Hi guys,
Today I tested a smoke alarm containing 0.9 microcurie of Am241 so unsurprisingly Am241 is what I found.
The smoke alarm was bought on Ebay and the package clearly says what's inside.

The radioactive material is inside the little ionisation chamber so among the interactions registered by the Geiger Counter (roughly four times the background) there were likely no alphas.
I tested the object at several distances: 0.5 cm - 1.0 cm - 2.0 cm. These are the net distances between the housing of the scintillator and that of the chamber.
Background was measured and then subtracted.
Knowing the activity, another comparative exposure assessment attempt was in theory possible, but without knowing the exact shape and position of the source inside the chamber a theoretical prediction would probably involve too much uncertainty besides that which already has to start with.

The spectrum is pretty simple. only Am241 shows up. Given its half life of 432 years there's been no time yet to accumulate enough Neptunium 237 for my scintillator to detect it. It might be interesting to test an old one, in order to see if some Np237 is at least visible.

For this measurement I calibrated with a composte background, meaning natural background plus some Cs137, using four calibration points (the two usual from Cs137 + one from Pb214 and K40). I've been doing this for a while now and it keeps the calibration in good shape on a wide range on energies, while calibrating with just Cs137 meant that in the past K40's peak was often 25-35 keV off. That's not the case now, but in this one I have a +2-2.5 keV drift all across the spectrum. Not a big deal...albeit having to do with a source whose peaks are all located below 60 keV probably focusing on calibrating low energies as good as possible would have been a wiser choice this time around.

There's a funny thing going on in the quantitative analysis. In my tables the energy value (and therefore the exposure value) for the sample is not obtained subtracting the background value from the "sample+background" value, all the exposure values are calculated indipendently from the corresponding spectrum. This leads sometimes to small discrepancies, meaning the the sum of the exposure from the sample spectrum + exposure from the background spectrum doesn't exactly match the value of the exposure from the sample+background spectrum, but until today difference was always very small. Here the difference appears to be larger, particularly for the spectrum recorded at closer distance.
Maybe having all the energy of the sample concentrated in just a couple of peaks at very low energy made the background subtraction feature working less good than usual. I checked the calculation twice and didn't find any mistake, so far at least.

Anyway, the activity is there, but given the low energy of the peaks the level of exposure doesn't match the background even at very small distances.

The spectrum is presented in both Counts per Bin and Energy per Bin view, although this makes very little difference this time for obvious reasons. The spectrum shown below is from the measurement at closest distance.

So that's it for the smoke alarm, on to the next one! (well...next weekend).

Massimo
Attachments
Smoke Alarm@0.5Net - ID - 4 Hours - BG Subtraction - Counts x Bin - No Shield - 0.020 Clean - 29-06-19.png
Smoke Alarm@0.5Net - ID - 4 Hours - BG Subtraction - Energy x Bin - No Shield - 0.020 Clean - 29-06-19.png
DSC05537-RC.jpg
DSC05542_FR.jpg
DSC05554-R.jpg
DSC05563-R.jpg
Spectrum Analysis @ 0.5 Net - 29-06-19-001.jpg
Spectrum Analysis @ 1.0 Net - 29-06-19-001.jpg
Spectrum Analysis @ 2.0 Net - 29-06-19-001.jpg
Last edited by Go-Figure on 30 Jun 2019, 18:15, edited 3 times in total.

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pilgrim
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Re: Smoke Alarm - Americium 241

Post by pilgrim » 30 Jun 2019, 15:32

Hi Massimo, good work as always.
If it helps, this is my spectra recorded with the same source using a Victoreen LEG with a 1,5 x 0,5" NaI(Tl) crystal.
c-Am241-Victoreen-10000s.jpg
The peaks are 13.9, 17.7, 23.5 (sum peak of 20.8 e 26.4) and 59.6 keV
Daniel, Italy

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Go-Figure
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Re: Smoke Alarm - Americium 241

Post by Go-Figure » 30 Jun 2019, 18:20

pilgrim wrote:
30 Jun 2019, 15:32
Hi Massimo, good work as always.
If it helps, this is my spectra recorded with the same source using a Victoreen LEG with a 1,5 x 0,5" NaI(Tl) crystal.
c-Am241-Victoreen-10000s.jpg
The peaks are 13.9, 17.7, 23.5 (sum peak of 20.8 e 26.4) and 59.6 keV
Interesting that you have a few low energy peaks which I don't see, while the 59.5 peak is much less prominent.
I wonder is this is due to the size of the detector or the setting of the software (like the filter) or a combination of both.

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pilgrim
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Re: Smoke Alarm - Americium 241

Post by pilgrim » 30 Jun 2019, 19:03

I don't know but I think it is due to the detector efficiency at low energies...
In this case the advanced filter works as a zoom, to make PRA showing energies from 0 to 200 keV
Daniel, Italy

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Go-Figure
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Re: Smoke Alarm - Americium 241

Post by Go-Figure » 30 Jun 2019, 21:01

pilgrim wrote:
30 Jun 2019, 19:03
I don't know but I think it is due to the detector efficiency at low energies...
Of course. I wonder if it's all due to the size of the detector.

Conor Whyte
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Re: Smoke Alarm - Americium 241

Post by Conor Whyte » 02 Jul 2019, 07:17

I find with my tiny CsI/Na detector that I can see the lowest energies very well for Am241. Visible when the right voltage setting ~1310V for the smaller tube, shows the lower energy 13.92KeV, 17.8KeV and 26.5KeV gamma peaks quite well.. I am however unable to see those energies below 20KeV well with my larger Scionix detector from iRad labs. anything from ~20KeV and up is visible to this detector to about 3MeV. (Still experimenting)

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Go-Figure
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Re: Smoke Alarm - Americium 241

Post by Go-Figure » 02 Jul 2019, 08:43

Conor Whyte wrote:
02 Jul 2019, 07:17
I find with my tiny CsI/Na detector that I can see the lowest energies very well for Am241. Visible when the right voltage setting ~1310V for the smaller tube, shows the lower energy 13.92KeV, 17.8KeV and 26.5KeV gamma peaks quite well.. I am however unable to see those energies below 20KeV well with my larger Scionix detector from iRad labs. anything from ~20KeV and up is visible to this detector to about 3MeV. (Still experimenting)
Thank you Conor.
I guess we can conclude that in gamma spectroscopy size matters, but not always the bigger the better.

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