Possible to detect Plutonium with HPGe detectors?
I know that BRAWM has stated that they are not able to measure Plutonium with the detector they have. However, I was wondering if this article might be of use for experimental setup of the HPGe detector to directly measure PU-239.
The source in this experiment is measured in nCi, so the amounts if in the air might be much smaller and therefore make this impossible, but I thought I would forward it along anyway.
http://www.phys.ufl.edu/~coldwell/Fittery/robfit/reno/INMN/inmm5.html
Thanks for the link to that
Thanks for the link to that paper! As you guessed, this is not possible to do in our situation.
If our detectors were 10 cm away from a 98 nCi Pu-239 source, we would also be able to see the source in the right conditions, especially given the background suppression they used. As you suspected, the problem is that any Pu-239 in the air would be so tiny that it would be impossible to detect.
To show why this would happen, let's assume that Pu-239 has a similar activity to the maximum activity the EPA measured for U-238, which was 2E-4 pCi/m^3. I should note that the EPA would have detected Pu-239 long before this activity level, and they still have not detected it.
Converting to Bq/L, this quantity is 7E-9 Bq/L. Per day, we pump about 3,800,000 liters through our filters. This would be 0.027 Bq of Pu-239 in our air filters for each day of filtering. This is 7E-4 nanocuries, or 1/140,000 of the strength of the source used in that paper.
Not every radioactive decay of Pu-239 produces a gamma-ray. In fact, very few decays produce gamma-rays. The only gamma-ray line that we could hope to detect with our detectors in any quantity from Pu-239 is its 129 keV line, emitted 6 times in every 100,000 decays.
I did a quick calculation, and assuming 100% efficiency we would need to count an air sample for over 250 days in order to detect a single 129 keV photon. It gets even worse -- if you look at any of our spectra, the region around 129 keV is where our background continuum peaks. So there is a hopeless signal-to-noise problem here.
In contrast to gamma-rays, every single radioactive decay of Pu-239 produces an alpha particle. Like gamma-rays, alpha particles are emitted at specific energies that are tracers of the specific isotope. The natural background from alphas is also quite small. Therefore, performing alpha spectroscopy is really the most sensitive way to check for Pu-239, since very few isotopes emit alphas. We're taking data right now with an alpha spectrometer system and will publish the limits we calculate from it.
Mark [BRAWM Team Member]
Americurium
I am curious then about these results from France for americurium 241.
http://balisescriirad.free.fr/Resultats_%20Japon/Resultats_analyses_19_0...
Can you also detect this americurium?
Is this americurium the by-product of the plutonium that may have been released to the air? Or was it created in the reactor and was released as a result of the meltdown?
Thanks for your answers. Enjoy learning about all this.
We would be able to detect Am-241
In contrast to Pu-239, Am-241 has a very strong gamma-ray signature along with its alpha decay -- a 59.5 keV gamma emitted in 36% of all decays. We have seen no evidence for Am-241 in any of our measurements so far.
Am-241 is created in reactors as a result of neutron capture on other elements, such as uranium and plutonium. Since it requires neutrons to form, it would only be made in a reactor, where there are lots of neutrons.
Mark [BRAWM Team Member]
Thanks so much for your
Thanks so much for your answer. I'm curious though - are there instruments that could detect Americurium at such low levels as the they are reporting as the amount under which they are not detecting it?
There are three answers I
There are three answers I can think of:
(1) Using the same equipment for longer time. (They are presumably using germanium detectors like we are.) There is always a tradeoff with detectable limits -- in order to lower your limits, you need to spend more time making the measurement. Because of the way statistics works, you generally need to take a measurement for 4 times as long in order to bring the detectable limit down to 1/2 of what it was. So with any system, you can almost always push the limit down as far as you wish, but time ends up being the issue.
(2) Performing alpha spectroscopy. Am-241 is an alpha emitter. You should get better signal-to-noise with an alpha spectrometer compared to a germanium detector.
(3) Mass spectroscopy is a very advanced tool that allows you to determine very tiny amounts of elements by their mass and charge. Here, the cost of testing and the difficulty in sample preparation become issues.
Mark [BRAWM Team Member]
I was wondering if you are
I was wondering if you are considering doing at least one or two tests with mass spectroscopy say with a milk sample just to see what you get. I know my daughter used mass spectroscopy in her undergraduate chemistry class at the University of North Florida, so it must be available at Berkely? I realize that students are probably using for their coursework right now, as I recall, she only had a certain amount of time allowed to run her tests.
No americium 241.
What that chart shows is that the level of americium detected is lower than their minimum detectable level. Here is one of those bad Google translations of the lab notes at the bottom: "When looking for the radioactive element is absent or if its activity is too low to be detected and / or quantified is the limit of detection (LOD) is calculated and published. The result of analysis is expressed as "Cesium 137: <0.02 mBq/m3". In this example, the laboratory CRIIRAD ensures that the activity of cesium-137 is strictly less than 0.02 mBq/m3.:"
americurium
Here is the French:
Lorsque l'élément radioactif recherché est absent ou si son activité est trop faible pour être détectée et-ou quantifiée, c'est la limite de détection ( LD) qui est calculée et publiée. Le résultat de
l'analyse est exprimé sous la forme "Césium 137 : < 0,02 mBq/m3 ". Dans cet exemple, le laboratoire de la CRIIRAD garantit que l'activité du césium 137 est strictement inférieure à 0,02 mBq/m3. En
revanche, on ne peut savoir si l'activité réelle est de 0,01 mBq/m3 ou de 0,0001 mBq/m3. Cette limite de détection dépend de nombreux paramètres : volume d'air prélevé, géométrie de comptage,
durée de comptage, ce qui explique la variabilité des limites de détection obtenues.
As I speak French, I didn't need to use Google to translate. What this says is that when a radioactive element is absent or too weak to be detected and or quantified, the limit of detection is published. They express the results in the form of "Cesium 137 < 0.02 mBq/m3", then they guarantee that the activity of cesium 137 is strictly less than this amount, but the amount may be 0.01 mBq/m3 or it could be 0.0001 mBq/m3. The limit of detection depends on a number of parameters, the length of counting, the volume of the air, the geometry of the counting and that explains the variability in the limits of detection.
I agree that it may be lower than the limit of detection as stated above, but they did find it! If it exists in the air in whatever amount, then did it come from the plutonium in the reactor or from plutonium in the air or from their own nuclear power plants? Americurium does not exist naturally - so where are these small amounts coming from? Were they detecting these amounts before Fukushima?
No, they did not find Am-241
The previous poster had been correct to say that they have not detected Americium-241. They are reporting their detectable limits (we use the term "MDA" or Minimum Detectable Activity). This does not mean they have found Am-241 -- it means they have ruled out any higher activity to a high degree of certainty.
Mark [BRAWM Team Member]
I'm not certain that I
I'm not certain that I understand your last sentence. Could you explain what you mean exactly by "higher activity"? also, the French groups explanation of detectable limits seem to disagree with the way you explain it. They say explicitly that they guarantee that the amount won't be greater than the MDA, but that it could be .01 mBq/m3 or .0001 mBq/m3 or even 0. This is different than saying there is no Americurium at all.
You caught me in technical
You caught me in technical speak. "Activity" is a measure of how radioactive something is. Another way to say my earlier sentence is "This does not mean they have found Am-241 -- it means they have ruled out any levels of it above their detectable limit."
By giving the detectable limit, they are saying that there is not more Am-241 than that level. If there were more, they would have seen it. My explanation was consistent with their explanation.
You are right to point out that this is different from saying that there is no Am-241 at all. However it is also wrong to say that they have detected it, which was implied in the earlier post.
Mark [BRAWM Team Member]
bump
bump