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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.

It is back in action

Dear Reader,

It has come to my attention that the space probe which landed on the comet has started working again. I have to ask the question why did the probe lack a RTG (RadioThermal Generator) based on something such as Pu-238 or Am-241. If it had been equipped with such a nuclear battery then it would have been able to operate without a need for sunlight.

While these battery packs are not very PC, I am aware that some of these radioactivity powered generators have survived launch accidents. If they are well designed then even in the event of a rocket blowing up on the lauch pad then no threat is posed to the general public.

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.

Police act in cobalt-60 robbery case in Mexico

Dear Reader,

It appears that the police in Mexico have identified six people who they believe are connected with the van robbery which caused the large cobalt-60 source to go missing. One of these people has been reported to have shown some signs of radiation exposure.

Without being privy to all the evidence it is impossible for me to make an estimate of the person’s radiation dose and thus workout what is likely to happen to the person. But assuming that they survive then I am sure that authorities will be strongly considering some criminal charges against them.

While I hold the view that the unauthorised possession of a large radioactive source should be treated with great gravity, I asked my legal advisor if the hijackers should be charged with a violation of the nuclear / radioactivity laws of Mexico or not. My advisor told me that unless the state can show that the criminals knew that they were stealing a dangerous radioactive object then they should not be charged with anything other than normal robbery. My advisor commented that to commit the radioactivity crime that they needed to be aware of what they were doing was something different to a normal robbery.

Well while we are on the subject of cobalt-60 I would like to point out something to you, now firstly it is not a pure gamma emitter (gamma emission is always linked to beta or alpha decay or some other process which forms a nucleus in an excited state). The cobalt-60 undergoes a beta decay to form an excited state of nickel-60, this excited state then normally emits 2 gamma photons to drop down into the stable ground state of the nickel-60.

These two photons are emitted at the same time. In a typical gamma semiconductor spectrometer many of the photons from the sample are lost, they fly away because they miss the detector. The spectrometer can be made more sensitive by making the distance between the detector and the sample smaller, or by making the geometry closer to a perfect sphere which surrounds the sample. In the ideal world you might think that you could have a 4π geometry where no matter which way a photon fly from the sample it will be detected.

But there is a problem, if the two photons are captured by the detector then the spectrometer will register an event with the total energy of both photons, this will create a false peak with a higher energy than either of the two real photons. What happens inside the detector crystal is that the energy of the photon is used to form free electrons and holes in the block of semiconductor. The block of semiconductor is often a giant diode which is reverse biased with about 3000 volts. When the event occurs the charge carriers (holes and electrons) allow some charge to pass through the crystal. The spectrometer measures the amount of charge which flows during each event. The more energy deposited in the crystal by an event the greater the number of charge carriers produced by the event.

A second problem is that if the gamma photon undergoes a Compton scattering event where it delivers some of its energy to an electron then it can change direction and then leave the crystal. In this way the photon can deliver only a fraction of its energy to the crystal. This effect can result in a broad peak in the lower energy part of a spectrum when a high photon energy gamma emitter is present in the sample. Thus it can be very hard to measure a low energy gamma emitter like americium-241 when a high energy gamma emitter such as cobalt-60 is present. One solution is to use anticoincidence counting. I will write about this soon.

Americium II

Dear Reader,

I have chosen to write about americium, I had noticed that no posts entitled americium existed on wordpress. Americium is one of my personal bests in terms of Z (number of protons). The element which the highest Z I have ever seen is plutonium, the highest Z I have been in the same room as is californium while the highest Z I have ever used in an experiment which I did myself is americium.

Americium is a element which can not make up its mind, in some ways it is like a lanthanide. It has a very insoluble fluoride and it extracts with bis-2-ethylhexyl hydrogen phosphate into alkane diluents while on the other hand it thinks it is a transition metal. It is more able to bind to nitrogen donors than a lanthanide and it is isoelectronic with uranium(0).

Uranium(0) complexes of cyclopentadienyl ligands are able to bind to carbon monoxide in a similar way to a transition metal. Thus uranium(o) is like a transition metal.

Also americium(III) is coloured, it is pink in colour while the lanthanides are almost perfectly colourless. The formation of coloured metal cations is very much a transition metal thing. For example copper(II) in water is blue while nickel(II) in water is green. Iron(III) is a funny one, in acidic water it is a pale purple but in water it undergoes hydrolysis to form a dark brown complex.

I may well write some more about americium in the near future.

Americium

While the general public get both excited and concerned about plutonium, some of the other actinides are equally important. Americium because of its higher stability of the +3 oxidation state has chemistry which is very different to plutonium.

I have seen predictions which suggest that the americium-241 either released from Chernobyl or formed in the environment as a result of the beta decay of the plutonium-241 released by the accident will become the radionuclide of greatest importance near Chernobyl after the cesium-137 has decayed away.

Fukushima will be a different matter as far less of the americium or plutonium in the fuel was released during the accident.

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