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Trinitite II

Dear Reader,

I have reexamined the gamma spectrum from the trinitite, and I have some news for my loyal readers. What I did was to look at someone else’s gamma spectrum of trinitite and then try to match peaks.

Here is the spectrum

trinitite9

Gamma spectrum of trinitite

What we can now see are two peaks (51.7 and 129.3 keV) which are due to the gamma emissions from plutonium-239. Also we can see a set of three lines due to uranium L lines X-rays.

We might ask why are we seeing uranium x-rays coming from a sample which contains so little uranium. One explanation which I think is very reasonable is that the alpha decay of the plutonium-239 forms uranium-235 which is formed in an electronically excited state. The uranium-235 then undergoes a rearrangement of the electrons to form the X-rays. This has been observed by others during XRF studies on plutonium metal.

This is further evidence that the sample contains the radionuclides which should be expected from the trinitiy test. So now I have managed to prove that the sample contains plutonium.

As the sample also contains americium-241 I think it would be reasonable to next make an attempt to find the lines for neptunium X-rays. These could be a further sign that the sample contains americium. I can not think of any other alpha emitters which will be present in large / moderate or even less than tiny amounts in the trinitite.

I will have to think further about the sample.

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Trinitite

Dear Reader,

Recently I purchased off eBay a small lump of trinitite, now I had been warned that a lot of fake trinitite is being offered for sale. So I choose to take the step of examining the sample with gamma ray spectroscopy.

In less than a minute I had been a peak at 668 keV which could either be due to either 214Bi (665 keV from the beta branch) or 137Cs (662 keV from 137mBa) was seen. This peak suggested that some radioactivity was present in the sample. I did a quick check at 609 keV. The line at 665 is emitted during a small fraction (1.46) of beta decays of 214Bi, while the 609 keV photons are emitted by 46.1 of all decays. As a result it is clear that the sample contains some man made radioactivity.

trinitite1a

Next I looked at the low energy end of the spectrum, here is a log log view to allow you to see this part of the spectrum better. I found a strong peak at 66 keV. I suspect that this is 59.5 keV peak for americium, keep in mind that the energy calibration of the detector is a little off. It was over reporting the energy of the 137mBa, so it is not totally unreasonable for it to over report the energy of the 241Am. As americium is associated with plutonium this is a good sign that the rock is a true lump of trinitite.

trinitite2

I then looked for some of the other lines of this americum nuclide, I looked for 99 and 103 keV photons. I found peaks at 99, 101 and 105 keV. This suggests that some peaks were in this expected range. Maybe it could be americium present. At 81 keV we should expect a peak for 133Ba, in our spectrum we see peaks at 81.7, 83.8 and 87.7 keV.

Also at 128 keV the spectrum contains a peak which could be due to the 122 keV line from 152Eu.

trinitite3

The spectrum also contains at 1414 keV a line which could be due to the 1408 keV emission from 152Eu. Also this nuclide will emit at 964, 444 and 245 keV. In the high energy part of the spectrum we can also see a line at 1466 keV which corresponds to the 1461 keV emission of 40K (decaying into 40Ar).

trinitite5

In our spectrum we see a line at 969 keV which can be matched with the 964 keV emission of 152Eu.

trinitite6

We can go further into the problem, in the range of 400 to 500 keV it is hard to decide if a peak is present. The signal to noise ratio is too bad in this range.

trinitite7

Now if we try again in the range of 200 to 300 keV range, we can see a line at 251 keV which is a possible match to the 245 keV.

trinitite8

The section of the spectrum between 300 and 400 keV shows peaks at 358 and 362 keV one of which could be the 356 keV line for 133Ba.

trinitite4

I think that after seeing this evidence that we can come to the conclusion that the rock sample came from a place where a nuclear fission event occurred, so it is likely to be real trinitite.

We will come back to this later, what I hope to do next is to try to estimate the way in which the efficiency of the detector changes as a function of photon energy. We will try to match the different lines from different radionuclides to the graph.

Thorium fueled reactors

Dear Reader,

It has come to my attention that the thorium based fuel cycle is being discussed in magazines such as “Chemistry World” which is the magazine of the Royal Society of Chemistry. As with all technology it is important that we have a honest and reasonable debate about it.

One attractive thing about the thorium fuel cycle is that it tends to form less of the transuranium elements such as plutonium, one idea for a nuclear fuel would be to make a mixture of thorium and plutonium dioxides. The idea is that the plutonium will provide the seed fuel while new fuel can be made from the thorium. Natural thorium  (232Th) can be converted by thermal neutrons into  233Th which will decay via  233Pa into  233U.

In many ways a thermal reactor is better than a fast one, I assume that many of my readers have heard of the term “fast breeder”, the idea of a fast breeder reactor is that it uses fast neutrons to make more fuel than it consumes. Commonly a fast breeder is fueled with a mixture of  238U and something fissile such as  235U or  239Pu. The reason why a fast neutron spectrum is better is that thermal neutrons can cause fission of 239Pu but the fission to capture ratio for fast neutrons is more favoring fission than capture. The capture (nγ reaction) of neutrons with 239Pu tends to form a neutron poison (240Pu) which is activated further to form 241Pu which undergoes beta decay to form minor actinides such as 241Am and even curium. These minor actinides can be a right royal pain. Another problem is that in a thermal reactor the formation of 236U by the nγ reaction of 235U can occur, the 236U is long lived and can be activated further to make short-lived 237U which can decay into 237Np. The 237Np can then form by another capture reaction 238Np which does a beta decay into the house of horror bugbear isotope of plutonium (238Pu). It is interesting that while the greens complain about the “evils of plutonium” they never seem to mention the fact that a lot of plutonium formed in power reactors is more alpha active than pure 239Pu. They seem to be trapped in their thinking by the long half life of the nicest plutonium isotope, 239Pu is not very radioactive gram for gram when compared with many other things such as radium.

As the 239Pu undergoes less activation and more fission in a fast reactor it is a logical choice for making and using plutonium, but on the other hand a fast reactor is bad for the thorium based fuel cycle. Here the desired outcome is neutron capture by natural thorium. The intended reactions are the neutron activation of 232Th to form 233Th (t½ 22 min) which decays by beta decay to 233Pa (t½ 27 days) which in turn undergoes a beta decay to 233U (t½ 159200 years). While 233U can be used for both reactor fuel and bombs, it is interesting to note that it is normally contaminated with some 232U. The decay of 232U forms high energy gamma emitters which will increase the dose rate near the 233U, this could make bomb and fuel fabrication more difficult.

The unwanted reaction in the thorium containing reactor is the n,2n reaction on 232Th to form 231Th, the 231Th then does a beta decay to get to long lived 231Pa. The next neutron capture then forms 232Pa which decays into 232U.

Some of the daughters of 232U (208Tl and 212Bi) emit very high energy gamma rays (up to 2.6 MeV) which will be much more troublesome than the gamma rays from 241Am which is commonly found in plutonium which has been allowed to age for some years. The majority of the gamma rays from 241Am are much lower in energy (60 and 33 keV) are much lower in energy and thus can be shielded against with a lead apron (circa 1 mm Pb) or a sheet of glass attached to a glove box. To attenuate the 208Tl gamma rays a very thick layer of shielding would be required making glovebox work impossible unless the glove box worker is willing to incur a large hand dose and happens to look rather like Mr Tickle of the Mr Men.

The key thing to understand is that a slow or thermal neutron has too little energy to do the n,2n reaction on the natural thorium. While a thermal neutron is able to do the neutron capture which we want. With some luck we can consider some reactor designs which reduce the formation of 232U.

More uranium glass

Dear Reader,

I have been in the second hand shops today of Lidköping, I have managed to find some more uranium glass. The glass does glow a nice green / yellow when you expose it to UV light. I will have to make a radiometric examination of the glass to determine if it is uranium glass or some other glass which has similar optical properties.

I think in future I will carry a UV torch in my pocket when I am out and about, here is a photo of the glass under normal light.

Mark's new uranium glass

Mark’s new uranium glass

Here is a photo when the only light is UV light from my UV torch.

My new uranium glass with UV light, notice it is glowing green

My new uranium glass with UV light, notice it is glowing green

I will examine the glass as soon as possible, then I will post the results of my tests.

Candlestick II

Dear Reader,

I took my candlestick to work and I quickly found it was radioactive, it was emitting beta particles according to a quick check with a contamination meter. As it was emitting that nice yellow/green light when exposed to UV light and it was emitting beta particles I quickly decided it was genuine uranium glass.

The next step in the characterization of the candle stick was to use gamma spectroscopy on it, now before we get going I would like to point out that gamma spectroscopy is not a press the button and get the result type of machine. For those of you who are proper traditional chemists / scientists you will be aware that for a new type of sample it is very hard with most machines to create a method with a spectrometer where you just put in the sample and press go before getting the final answer.

One of the problems is the issue of self adsorption, for the lower energy gamma lines many of the photons will never escape from a large sample. The ideal sample for gamma spectroscopy would be a tiny spec (a point source) which would be at a well defined distance from the detector.

The candlestick is anything but well defined in distance from the detector and it is far from being a point source. I did not want to melt it down to make a lump with a more simple shape so I decided that we should measure it in its native form.

One of my questions about the candle stick was “is the uranium a depleted uranium, or is it a natural uranium which is likely to predate the nuclear age ?”

I reason that as DU is less valuable than natural uranium it would be the logical uranium to use if you were making a uranium glass candlestick in the 1950s or later. But if it was a more early candlestick then it would be more likely to have a natural isotope signature for its uranium.

We need to consider three uranium isotopes

238U which is the bulk of natural uranium, this does not have any useful gamma lines but its daughter (234Th) which emits gamma rays, as the half life of 234Th is short when compared with the age of the candle stick it can be treated as an extension of the radioactive decay of the parent 238U. 70% of the 234Th will decay to the meta stable state of 234Pa (234mPa). It is important to note that the 234Pa (both forms) give a forest of gamma lines (hedgehog spectrum).

Nuclide Half life Decay mode Main gamma lines
238U 4.468 x 109 years alpha No gamma
234Th 24.1 days beta 63.3 (4.8 %), 92.4 (2.8 %) and 92.8 (2.8 %)
234mPa 1.17 minutes beta 258.3 (0.73 %), Hedgehog spectrum
234Pa 6.7 hours beta Hedgehog spectrum

 

If the uranium had been a depleted uranium then I would expect that almost all the 234U and 235U would have been removed. As the 234U has a long half life it serves to block the decay chain of 238U if the sample is not old on a geological time scale.

I reasoned that by looking for the decay products of 234U that I could test the hypothesis that the uranium was a prenuclear age natural mixture of isotopes.

This uranium will decay to form a long lived radium (226Ra) which will then slowly on the timescale of the candlestick’s age decay further.

234U –> 230Th –> 226Ra –> 222Rn –> 218Po –> 214Pb

Nuclide Half life Decay mode Main gamma lines
234U 245500 years alpha No gamma
230Th 75380 years alpha 67.7 (37 %)
226Ra 1600 years alpha 186 (3.6 %)
222Rn 3.8 days alpha No gamma
218Po 3.1 minutes alpha No gamma
214Pb 26.8 minutes beta 242 (7.4 %), 295 (19.3 %), 352 (37.6 %),
214Bi 19.9 minutes beta Forest of lines
214Po 0.1643 ms alpha No gamma

 

The 214Pb will decay by beta emission to form 214Bi and then 214Po which then decays to form 210Pb. As after 226Ra no nuclide has a half life longer than a few days until you reach 210Pb we can treat these decays as extensions of the radium decay if we make a kinetic model of the candlestick.

The fissile 235U does have a useful gamma emission of its own, this can be used to confirm if the uranium was natural or depleted.

It will decay by alpha emission according to the following mechanism.

Nuclide Half life Decay mode Main gamma lines
235U 703800000 years alpha 109 (1.5 %), 144 (11 %), 163 (5.1 %), 186 (57 %), 205 (5%),
231Th 25.52 hours beta No gamma
231Pa 32760 years alpha Forest of lines
227Ac 21.773 years beta No gamma
227Th 18.72 days alpha Forest of lines

 

I hope to now be able to go through the spectrum and then hunt for lines, I recall that the 186 keV line for 235U was present. So far I think the uranium is from before the nuclear age.

Uranium glass

Dear Reader,

For some time I have been looking for some uranium glass, this is a type of glass which contains uranium. because of the uranium it is green in colour. The glass also fluorescent, when exposed to UV light it emits green light. I was in a second hand shop today and I spotted some green glass, I happened to have a long wavelength UV light in my pocket so I did a quick test of the glass with UV light. As the 20 SEK candlestick emitted green light I bought it.

I have made a short film of the candlestick being exposed to UV light. I hope to upload to youtube soon a film of the candlestick in a dark room being exposed to a UV light which I turn on and off.

20 SEK (circa £ 2) candlestick made of uranium doped glass

20 SEK (circa £ 2) candlestick made of uranium doped glass

I also hope to take the glass candlestick to work and measure its radioactivity. When I do I will post the details in another blog post.

Microscope and more on the islands of stability

Dear Reader,

I bought a microscope today at Lidl, it was a German microscope (BioLux Bresser). When I get the chance I will try it out and tell you people what I can see. The nice thing is that this microscope has a USB camera which comes with it.

But before I get onto that, here are some pictures of the island of stability which I plotted. I have ploted the neutron and proton numbers on the horizontal scales and on the verical scale the log (base ten) of the half life in seconds. I have ignored all nuclides which have half lives shorter than 1 second.

Here are the pictures.

island 1 island 2 island 3 island 4 island 5 island 6 island 7 island 8 island 9 island 10 island 11 island 12

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