Po-210. Comparison of Gamma and Alpha spectrometry

[Sergey_KGB]

Hi Everybody!
Just have read in the Report into the death of Alexander Litvinenko
https://www.gov.uk/government/uploa...493860/The-Litvinenko-Inquiry-H-C-695-web.pdf

Following Mr Litvinenko’s death, measurements of polonium 210 were taken using tissue samples from Mr Litvinenko’s lung, spleen, kidneys and liver, using gamma ray spectrometry. These tests showed raised levels in each of the organs, with the highest result (49,000Bq per g of tissue) in the kidney sample and the lowest (3,500Bq per g of tissue) in the lung sample.

I'm very surpised to see that namely Gamma Spectrometry was used while only 1 in 100,000 Po-210 decays results in emission of Gamma photon. Alpha spectrometry is much more (100,000 times) accurate.
What do you think about it?
It is purely scientific question. Political consideration should be discussed elsewhere.

Suppose that we analyze 1g of kidney. Then Alpha Specrometer would give about 49,000 counts per second (in ideal case). While Gamma spectrometer would have to detect only 1 gamma photon per 2 seconds maximum and it would be hard to separate such low gamma stream from gamma photons from other sources (including background emission).

 

[Jeff Rosenbury]

My understanding was they detected the alpha particles. I suspect the article is in error.

I seem to recall the KGB being very surprised that the hospital was able to detect alpha particles. They assumed the alpha particles were undetectable which was why they choose polonium. Fortunately, once detected the trail of polonium allowed the assassin's exact path to be traced, and the reactor the polonium came from to be found and dated.

I will try to refrain from the obvious political comments out of some shred of respect for the OP (puts KGB in his screen name?) and more for the community at large. Yet I can't refrain from saying, "Murder is bad."

 

[mfb]

Gamma rays are much easier to detect. Alpha radiation is stopped within very short distances (~50 micrometers), so only a thin surface layer contributes to measurements, and even there (and in the air between source and detector) some particles lose energy which makes spectroscopy challenging - you don't get nice peaks in the energy spectrum.

 

[Sergey_KGB]

My understanding was they detected the alpha particles. I suspect the article is in error.

I seem to recall the KGB being very surprised that the hospital was able to detect alpha particles. They assumed the alpha particles were undetectable which was why they choose polonium. Fortunately, once detected the trail of polonium allowed the assassin's exact path to be traced, and the reactor the polonium came from to be found and dated.

I will try to refrain from the obvious political comments out of some shred of respect for the OP (puts KGB in his screen name?) and more for the community at large. Yet I can't refrain from saying, "Murder is bad."

Yes, my late Father was a KGB colonel (military counter-intelligence) but I myself absolutely unconnected to KGB, FSB and so on. I'm just a private person.
KGB addition to my real name is just for fun.
My interest is purely scientific one.
It is not just an article but Final report and it is an offcicial document of HM government.
Gamma spectroscopy (usage of this term) is not a mistake because Alpha spectroscopy is also mentioned in the report (though only once).

Witness A1 was one of the principal scientific experts who gave evidence to the Inquiry. She is an expert in nuclear physics who spent 34 years working for the AWE in Aldermaston; she now works for another nuclear establishment in the UK.

A1 stated that in the light of laboratory tests using both alpha and gamma spectrometry, she was absolutely confident in the conclusion that the alpha radiation discovered at the multiple scenes had been caused by polonium 210

So samples gathered in different locations were tested also using Alpha spectrometry. But suprisingly samples from the dead body were investigated using only Gamma spectrometry.
Also I would like to comment this strange statement in the Final report

...uranium 232, which has an alpha output indistinguishable from that of polonium 210

I'm very puzzled by this statement because
Po-210 emitts alpha particles with energy 5.30438 MeV
As for U-232 then as I'm aware energies of its alpha emission are
With probability 70% - 5.32024 MeV difference 15.86 KeV
With probability 30% - 5.26348 MeV difference 40.09 KeV
So U-232 produces 2 lines on the alpha spectrum while Po-210 only one. So it is highly unprofessional claim that alpha output of U-232 is 'indistingushable' from one for Po-210. What is typical energy resolution for Alpha spectrometers?
The best ones have energy resolution 11 KeV or less.
Thus Po-210 and U-232 has own quite distiguishable alpha 'fingerprints'.

 

[Sergey_KGB]

Gamma rays are much easier to detect. Alpha radiation is stopped within very short distances (~50 micrometers), so only a thin surface layer contributes to measurements, and even there (and in the air between source and detector) some particles lose energy which makes spectroscopy challenging - you don't get nice peaks in the energy spectrum.

Yes, it is much easier to detect Gamma rays. But if samples that allegedly contain Po-210 are tested then Gamma spectroscopy is useless from my point of view.
It is my point and I would like to hear learned opinion of experts (I'm not an expert in this area at all).

 

[Jeff Rosenbury]

An official government report might be misleading on methods. Just what detection methods are available is a closely guarded secret.

 

[Sergey_KGB]

An official government report might be misleading on methods. Just what detection methods are available is a closely guarded secret.

According to the report both Gamma and Alpha spectrometry are avilable and were used. Thus there is nothing to hide.

 

[mfb]

The best ones have energy resolution 11 KeV or less.

For particles entering the detector. How many layers of atoms do the alpha particles traverse before they lose an average energy of 15 keV? How can you distinguish uranium alphas from polonium alphas if you don't know the energy loss as precise as that?

 

[e.bar.goum]

I'm not understanding why using gamma techniques would be seen as "useless".

As mfb says, quire rightly, the resolution is much better with gamma rays, and further, to do alpha analysis, you'd be wanting to do all the testing in vacuum. Which is tricky with biological samples. Otherwise, you'd certainly not get 11 keV resolution due to scattering.

The decay of 210Po is well known, so if you see the appropriate characteristic gamma rays, you know how much 210Po is in the sample. Of course, doing both alpha and gamma analysis is good to be sure, but you should be able to get the same answer with gamma spec alone.

 

[Vanadium 50]

Most of the problems have been identified. To a good approximation, all alpha sources are about 5 MeV. It's been brought up that the alphas will range out if the layer is too thick, but there is an additional problem: as the alphas lose energy (a random process) it becomes harder and harder to measure differences in energy between two nuclei. One can say "Ah, just use a thin source", but a thin source is not a massive source, so the number of decays goes way down - you don't get much signal. And there is background: Po-210 is a radon daughter.

 

[Sergey_KGB]

For particles entering the detector. How many layers of atoms do the alpha particles traverse before they lose an average energy of 15 keV? How can you distinguish uranium alphas from polonium alphas if you don't know the energy loss as precise as that?

Anyway, Po-210 has only one line on Alpha spectrum while U-232 has 2 lines.
Indeed, due to cause outlined by you, they are not lines exactly but rather Gaussian-shape curves (very sharp however). Po-210 would produce one such a curve but U-232 two with amplitudes in proportion 7:3 (according to probabilities of respective energies). I propose this pictue as an illustration

 

[Sergey_KGB]

I'm not understanding why using gamma techniques would be seen as "useless".

As mfb says, quire rightly, the resolution is much better with gamma rays, and further, to do alpha analysis, you'd be wanting to do all the testing in vacuum. Which is tricky with biological samples. Otherwise, you'd certainly not get 11 keV resolution due to scattering.

The decay of 210Po is well known, so if you see the appropriate characteristic gamma rays, you know how much 210Po is in the sample. Of course, doing both alpha and gamma analysis is good to be sure, but you should be able to get the same answer with gamma spec alone.

I wrote about the cause previously. Only one in 100,000 Alpha decays of Po-210 causes emission of Gamma photon with energy 803KeV.
So if we have a sample with Po-210 activity of 49,000 Bq/g then
10mg would produce 490 emissions of Alpha particles per second but
1kg is required to produce 490 Gamma photons per second.
But 1kg sample could contain a lot of other Gamma emitters with energies close to 803KeV (also due to enrgy loss)
Due to high penetrating ability of gamma rays it is impossible to exclude background gamma emission.
By contrast penetrating ability of alpha particles is extremely low. Thus backgrand emission is in fact excluded.

 

[e.bar.goum]

I wrote about the cause previously. Only one in 100,000 Alpha decays of Po-210 causes emission of Gamma photon with energy 803KeV.
So if we have a sample with Po-210 activity of 49,000 Bq/g then
10mg would produce 490 emissions of Alpha particles per second but
1kg is required to produce 490 Gamma photons per second.
But 1kg sample could contain a lot of other Gamma emitters with energies close to 803KeV (also due to enrgy loss)
Due to high penetrating ability of gamma rays it is impossible to exclude background gamma emission.
By contrast penetrating ability of alpha particles is extremely low. Thus backgrand emission is in fact excluded.

As everyone has been pointing out in this thread, the activity is not the issue. Low count rates are dealt with all the time in nuclear physics. You can integrate for an arbitrary amount of time. Further, the energy loss of gammas in material is negligible compared with alpha particles. Background characterization is standard. You can gate out other emitters by doing coincident gating. Further still, you'd just use a shielded detector! Gamma ray spectroscopy is an extremely common, extremely reliable, robust technique.

V50's point about alpha straggling in the sample is a good one.

Look, clearly both techniques were used, but you shouldn't rubbish gamma spectroscopy because you aren't aware of the sensitivity of it.

 

[Sergey_KGB]

Most of the problems have been identified. To a good approximation, all alpha sources are about 5 MeV. It's been brought up that the alphas will range out if the layer is too thick, but there is an additional problem: as the alphas lose energy (a random process) it becomes harder and harder to measure differences in energy between two nuclei. One can say "Ah, just use a thin source", but a thin source is not a massive source, so the number of decays goes way down - you don't get much signal. And there is background: Po-210 is a radon daughter.

I would agree with you if alpha activity of a sample is relatively low. Precise Alpha spectrometry in this case is not possible. For exact spectrum we need very thin source but in this case stream of alpha particles would be too weak to make measurements.
But in our case it is claimed that samples contained big amount of Po-210. Reportedly parts of kidney had alpha activity 49,000 Bq/g. A sample before Alpha spectrometry has to be dried. Thus activity of dried sanmple could be more (3-4 fold). Anyway no less than 100,000 Bq/g. Thus 10mg distributed on a metal plate as a thin cover would produce no less than 1000 alpha particles per second. Only part of this stream would be detected by sensor. But anyway alpha stream is massive enough to make pricise measurements.

 

[Sergey_KGB]

As everyone has been pointing out in this thread, the activity is not the issue. Low count rates are dealt with all the time in nuclear physics. You can integrate for an arbitrary amount of time. Further, the energy loss of gammas in material is negligible compared with alpha particles. Background characterization is standard. You can gate out other emitters by doing coincident gating. Further still, you'd just use a shielded detector! Gamma ray spectroscopy is an extremely common, extremely reliable, robust technique.

V50's point about alpha straggling in the sample is a good one.

Look, clearly both techniques were used, but you shouldn't rubbish gamma spectroscopy because you aren't aware of the sensitivity of it.

As I pointed out previously I'm not an expert. So I have some simple questions addressed to true experts.
- what is a typical size of a sample during Gamma spectrometry?
- what is minimal stream of gamma photons to (photon/sec) required by Gamma spectrometers?
- what is energy resolution of Gamma spectrometers?

 

[e.bar.goum]

As I pointed out previously I'm not an expert. So I have some simple question addressed to true experts.
- what is a typical size of a sample during Gamma spectrometry?
- what is minimal stream of gamma photons to (photon/sec) required by Gamma spectrometers?
- what is energy resolution of Gamma spectrometers?

1. It Depends. Anything from micrograms to tonnes.

2. Many fewer than one count a second. Some detectors used for dark matter searches or neutrinoless double beta decay measurements talk about photons per *year*. If you count for long enough, you see a peak.

3. For germanium, typically 1-2 KeV. You can do better, you can do worse, it depends.

 

[mfb]

Anyway, Po-210 has only one line on Alpha spectrum while U-232 has 2 lines.
Indeed, due to cause outlined by you, they are not lines exactly but rather Gaussian-shape curves (very sharp however). Po-210 would produce one such a curve but U-232 two with amplitudes in proportion 7:3 (according to probabilities of respective energies). I propose this pictue as an illustration

You might be able to get this picture in a mass-spectrometer-like environment: a few atoms in a vacuum. You will never get it with realistic biological samples.

https://radwatch.berkeley.edu/AirFilterResults has more realistic spectra. Note the width and the long tails of every peak.

 

[Sergey_KGB]

Many thanks for interesting information.
I have found a description of standard, accredited method to conduct a test fo Po-210 using alpha spectrometry, including very low concentrations.
https://wiki.ceh.ac.uk/display/~jc.mora@ciemat.es/polonium-STUK

Method used for matrices: Environmental, Water, Terrestrial, Biological, Foodstuff, Industrial products, Swipe filters

Minimum detectable activity (MDA) is 0.6 mBq/L for one litre water and 180 hour counting.

There exists well known method (using alpha spectroscopy) to detect Po-210
So there is a logic question - why well known, very precise method was not used but instead Gamma spectrometry was used?
How reliable could be the result of Gamma spectroscopy in the case with Po-210?
Are mistakes possible?

https://en.wikipedia.org/wiki/Polonium

The baseline urinary excretion of polonium-210 in healthy persons due to routine exposure to environmental sources is normally in a range of 5–15 mBq/day.

Daily amount of urine is about 1 Liter or slightly more (or less). So healthy person has about 10 mBq/L in urine or 0.01 Bq/L or 0.00001 Bq/ml.
Now let's look at this document. 22.11.06 when mr.Litvinenko was alive, British nuclear laborstory (AWE) estimated concentration of Po-210 in his urine.
https://www.litvinenkoinquiry.org/files/2015/04/INQ007656wb.pdf

Urine 22.11.06 Measurement AWE 825 Bq/ml

Just compare 825 and 0.00001 - 80 millions times higher.
Now let's look at the Final report
https://www.litvinenkoinquiry.org/files/2015/04/INQ007656wb.pdf

The blood and urine samples arrived at AWE in the early evening of 21 November. Tests were conducted overnight...
...the results of the urine tests at AWE, which revealed that polonium was present in Mr Litvinenko’s urine, were discussed. However, it was thought that this reading was an anomaly caused by the plastic bottle in which the sample had been stored

How it is possible to explain concentration 80 millions times higher than normal level by 'plastic bottle'?
Just after the death of mr.Litvinenko, Italian journalist Mario Scaramella (with whom he met) was tested for Po-210
https://en.wikipedia.org/wiki/Mario_Scaramella

On 3 December Italian Senator Paolo Guzzanti was quoted after speaking with Scaramella by phone, saying health officials had told Scaramella the dose of polonium he had received is usually fatal.
...
Latest news inform that he was only exposed to minute traces of polonium

By the way still results of mr.Scaramella tests (initial and others) are not made public and it appears that the initial test contained a mistake.

 

[Dale]

So there is a logic question - why well known, very precise method was not used but instead Gamma spectrometry was used?

That is an historical question which could only be answered by those who performed the test. It seems as though the technical questions have been answered, so we will close this thread for now.