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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.

New gamma camera design

Dear Reader,

Having an academic interest in nuclear accident chemistry I search the literature every now and then for articles which mention “Fukushima”, I saw one which caught my interest it was about an idea which I think is truly interesting. It is about the age old problem of how do we see radiation.

Now two easy to imagine gamma camera’s exist, these are the pin hole type and the gamma camera with lots of holes, each hole has a well collimated detector at the bottom of it. These gamma cameras will require plenty of heavy lead shielding to operate and collect nice pictures. When the gamma energy is low (such as Am-241 or I-131) it will be possible to make these machines but when the gamma energy is much higher (Cs-137 or Co-60) it will be very hard to build these gadgets as the gamma rays need thick layers of lead to stop them.

Here is the most simple design the pin hole camera which uses a small hole to make the image appear.

Pin hole camera

Pin hole camera

The second design is the array of holes, this will work as long as the holes are much longer than their diameter. Also it will work better with low energy gamma emitters as they are easier to stop in the shielding. If the maker of the camera is clever there are some things that they can do to improve the image such as moving the camera around to reduce the effect of the grid of holes on the picture. In the following diagram it should be clear that while the red gamma ray can reach the thick black detector plate the blue and purple rays are blocked by the lead in the shielding / holes array.

Gamma camera design two

Gamma camera design two

The Compton effect camera works in a different and much smarter way, it uses something known as Compton scattering of gamma rays and two detector arrays. The idea is that when a gamma ray scatters off an electron it changes direction and at the same time loses some energy. At a bare minimum what is needed is an energy dispersive detector at the back of the camera and an ordinary detector at the front of the camera.

The geometry of the Compton camera

The geometry of the Compton camera

The classic formula for Compton scattering is

λ’ – λ = (h/mec).(1- cos θ)

We can rearrange and alter it a little to get

cos θ = 1 – [(c2 me)/E’] + [(c2 me)/E]

cos θ – 1 = [(c2 me)/E] – [(c2 me)/E’]

(cos θ – 1) / (c2 me) = 1/E – 1/E’

(c2 me) / (cos θ – 1) = E – E’

(c2 me/h) / (cos θ – 1) = v – v’

(cos θ – 1)(h / c2 me)  = (1/v) – (1/v’)

(cos θ – 1)(h / c me)  = (c/v) – (c/v’) = λ’ – λ

(cos θ – 1)  = (λ’ – λ)/(h / c me)

cos θ = 1 + (λ’ – λ)/(h / c me)

θ = cos-1 {1 + (λ’ – λ)/(h / c me)}

θ = cos-1 {1 + (λ’ – λ)/(h / c me)}

Now that algebra was fun, to digress the other day I speculated what would happen in a world where children were banned from doing maths and were forced to play video games and do facebook all day at school. I suspected that some children would rebel by forming illegal underground maths clubs where at clandestine meetings they would study geometry and calculus. Maybe they would pass around maths textbooks behind the bike shed or in the woods, some lads might hide a cache of maths books in their bed rooms out of reach and sight of their mothers. Just imagine the shock and horror of a woman when she discovers her 15 year old son is hanging around fully clothed with an immoral maths freak girl who is doing Laplace transformations, or maybe her son has fallen in with the bad of the bad Fourier transformers.

But back to the real world

If we assume that we have a monochromatic gamma source such as the 137mBa formed from 137Cs then we will have a original gamma energy (E) of 662 keV (1.0606 x 10-13 J), as we know the electron rest mass and the speed of light we can from the energy of the photon after scattering work out the angle it was scattered through.

If the Compton camera is used to image when the background is high or when the source emits photons with several different energies then the front detector also needs to be an energy dispersive detector. For example if we were to image a X-ray source or 192Ir source then we would need both detectors to be energy dispersive. We also have the advantage if both detectors are energy dispersive that we will also get a gamma spectrum from the object. This could be an advantage if two different sources are present in the field of view of the camera.

Here is a graph of the energy of the product photon as a function of the scattering angle.

Scattered photon energy as a function of scattering angle

Scattered photon energy as a function of scattering angle

For those of you who like log scales here is the graph with a log scale for the y axis

Graph of energy of scattered photon as a function of scattering angle for four different original gamma photons

Graph of energy of scattered photon as a function of scattering angle for four different original gamma photons

What happens in Compton scattering is that the photon scatters off an electron, the electron gains some of the energy of the photon. As the gamma photons have much more energy than the electrons it can be regarded as gamma photons bouncing off stationary electrons. As the electron takes some of the energy away from the photon the scattered photons have lower energies than the original photons.

What happens in the camera is that by measurement of the energies of the events in the two detectors the angle change of the photon in the first detector is measured. Then as we know the relative positions of the two events in the two detectors we know the angle of the scattered photon. This allows us to create a cone which will include the location of the original source. Here is a crude sketch I have made of the operation of the Compton camera.

The Compton camera is in operation

The Compton camera is in operation

What happens is that the camera will have a computer in it which trys to recreate the original image, it will for each photon event create a curved shape. By adding the data for different events it will be able to establish what the original image (where the gamma source was). This type of camera can be used for a range of tasks which include medical and industrial applications.

Serious nuclear reactor accidents

Dear Reader,

I have recently published a review article about some of the chemistry of a serious nuclear accident, this is in a new journal named “Cogent Chemistry”. For those of you who do not know what “Cogent” means, it means “Something which appeals to the intellect or one’s powers of reasoning”. I have to confess I had to use a dictionary to look it up.

Now I am waiting for some feed back on this article, I am wondering what will come to me. It is important to note that such a review article is as politically neutral as possible. The role of a review in science is not to act as propaganda for either the nuclear industry or the antinuclear sector. In some ways the antinuclear sector seems as much of an industry as the nuclear industry. Some people seem to make being “antinuclear” as much of a full time job as some of the spokespersons hired by the nuclear sector. I am aware of some “interesting” behaviours which both pro and antinuclear zealots have. I will always refuse to name the zealots from either side, so please do not ask me for a list of them.

Some of the antinuclear zealots might even harm the environmental movement, if a person appears to be a foaming at the mouth antinuclear (anti-GMO, or anti-you add the name of a technology or industrial activity) they may start to appear to be an unreasonable person with a clear axe to grind. They may lose credibility and they might also start to tell lies (be dishonest, economic with the truth call it what you like) as a result of this “crying wolf” people will start to ignore the whole of the environmental movement so when another (and genuine) issue appears people will ignore it thinking “yet another scare story”. I am aware of people making claims of effects which are impossible, I also suspect that some people make up statistics / data and I also see dishonest tricks of argument such as “appeal to authority”, and “abusive ad hominem”. The latter is when a person’s character is attacked as a method of undermining an argument. For example consider one version of “ad hominem” known as “poisoning the well”. Someone might argue their electricity bill is wrongly calculated, and that how can you trust a worker from wicked utility company when their state that  2 + 2 = 4, thus the bill is wrongly calculated.

I assume that most of my readers have a GCSE in maths or some other similar basic maths qualification so they should know that 2 +2 = 1 + 3 = 5 -1 = 6 – 2 = 7 – 3 = 8 /2 = 4

Now while I am sure that none of my readers (antinuclear, pronuclear or otherwise) would want to argue that working for a particular company renders you unable to count. But I have seen some people from both sides of the debate use these types of tricks.

One use of a related trick was an incident where a meeting regarding a renewal of a license for a nuclear forensics / research site was being discussed. At one point the subject of radioactivity levels was being discussed, one activity was smaller than that due to potassium in a normal person. One person became angry and was saying “how dare you say that you know nothing about me and my body”. I think that this is a rather silly reason to become upset and angry (example of poisoning the well) as I can tell you that if you lost most of the potassium from your body you would die of a heart attack. Hypokalemia is the term for low blood potassium which when taken to an extreme results in a heart attack.

On the other hand some of the lunatic pronuclear lobby (One man who works in a national nuclear research centre calls them “atomheads“) are likely to harm the nuclear industry (and all other sectors which use radioactivity) by overselling nuclear technology and doing questionable things to prop up the image of their favourite industry. Promises of “electricity too cheap to meter”, nuclear powered cars (like the Ford Nucleon) and other outlandish things will result in disappointment. Also some statements which were later shown to be wrong on safety issues will in the long run do a lot of harm, during the three mile island accident a series of ambiguous and contradictory statements were made which in the long run did a lot of harm to the reputation of nuclear power. In terms of the flow of information to the public the three “big” reactor accidents were very different. Three mile island had a series of statements made to the media / public by the utility / the state, the Soviet Union tried to suppress the news about Chernobyl (that failed big time when workers at a Swedish plant arriving for work were found to be contaminated) while Fukushima was the internet age nuclear accident where lots of organisations were racing to post news and updates (some of which contained or were based on bad data). But a discussion of the way in which the early information was released for these accidents will have to wait for another day and another post.

I suspect that the article will enlighten and entertain the reasonable people from both sides of the nuclear / antinuclear debate rather vexing them. However both the most hardline opponents and staunch supporters of nuclear power will find the article disagreeable. The most antinuclear zealots will hate it as it fails to paint the picture of hopeless total doom which they so want while the pronuclear zealots will hate it as it discusses some of the things which can go wrong. Those who are protruth and proenlightenment should have no problem with it.

Uranium glass again and how to make a radiometric measurement I

Dear Reader,

I have checked most of the uranium glass with a geiger counter, the geiger counter uses a tube which is known as a Geiger-Muller tube which is a gas filled high voltage discharge tube which uses an avalanche effect to increase the number of free electrons and ions formed in the tube after the absorption of radiation by the gas inside the tube. It is important to understand that GM tubes are not all born equal, it is possible through careful design to optimise a tube for an application. I borrowed a GM tube based device to check my uranium glass, this first device was a bit of a disappointment.

It has a tube with a thin mica end window and it has some beta sensitivity, but it is not very sensitive. It was intended as a gamma / beta detector which has a full scale reading of 1000 rem per hour. I think that such a device is the tool of choice when dealing with a high dose rate event. It would be very suitable for nuclear warfare use assuming that the fragile GM tube survives the bomb detonation, it could be very useful when dealing with a industrial radiography accident such as a lost source or a radiotherapy accident which involves a lost source. This detector has a rate meter for GM tube events which goes as low as 1 count per second.

With this high dose rate meter I could not get any reading from my uranium glasses, except for a very dark green one which gave 2 counts per second, the background in my house was 1 count per second. It is hard to work out if 2 cps is different to 1 cps as radiation from radioactive decay and cosmic rays is occurring randomly.

So I then tried a different GM tube based device, I choose one which is optimised for looking for low to moderate levels of beta emitting radioactive contamination. Note while from personal experience I know it works for carbon-14 it will never work for tritium. The device is a “Radiation Alert Inspector” made by S.E. International INC (Summertown Tennessee). This device can give the count rate or it can be set to record the number of counts over a given time.

As I was dealing with very weak sources I choose a counting time of five minutes, I measured the background in my house three times. The results for the background were 248, 224 and 263 counts. The total number of counts in these three determinations of the background count rate were 735 counts, which makes the count rate in my house to be 0.82 counts per second, the ESD on this count rate is 0.03 counts per second which is about 3.7 %. This ESD is based on the number of events observed. So based on this count number we should expect 245 events (give or take 9 events) in five minutes.

I measured a glass object which does not fluoresce when exposed to UV light, I got a total of 224 counts in five minutes. The difference between the count number for this object and the background is 21 counts, the sum of the ESDs is 24, so this difference is unlikely to be significant in a statistical sense.

I then went and measured a green glass milk jug which fluoresces nicely with UV light, this gave a count of 580 events. The difference between this count number and the background count number is a staggering 335, while the sum of the ESDs is 33, as the difference is ten times the sum of the ESDs it is very real which suggests that the milk jug does contain something which is radioactive.

The lowest count number I got in five minutes on uranium glass was 441 on a pale green thin blown glass vase, this value still suggests that it is radioactive. The difference between the background and the sample is 196 and the sum of the ESDs is 30. This is still very convincing.

Now while I have done quite a trivial experiment I would like to ask other people who are considering doing independent radiation measurements to up their game a bit. I sometimes see data shown on the internet where the people making the measurements do not explain their experimental method fully or state the number of counts which they use to estimate the dose rate or radioactivity level. For example Greenpeace have been using NaI spectrometers in Japan, they used a thing called a “Georadis RT-30” which is a nice bit of kit. The only problem is that they did not give full details of how they obtained dose rates with these machines.

While I know that a NaI spectrometer will never give as good energy resolution as a high purity germanium detector this type of NaI detector can distinguish between different radionuclides (based on the gamma photon energy), what I would like Greenpeace to report are the gamma spectra and all the details such as the counting time, details of the dead time correction. This would allow the contribution of Cs-137 to the dose rate to be separated from the gamma rays from the uranium decay chains.

I would also like spectra obtained at different distances from a known cesium-137 source at different distances. This could be used to calibrate the spectrometers. I would also like to see the spectra obtained using natural uranium, natural thorium and uranium ore samples. This would allow me to see how well the machine is able to separate the signals from the different gamma photons.

When I write gamma spectrum I always mean a table or chart of counts per channel against channel number. The Greenpeace NaI spectrometer has 1024 channels so with some luck it should be able to separate the photopeak for Cs-137 (662 keV) from all the other gamma photons or at least allow a partial cleanup of the data.

Now I will not pretend that Greenpeace are neutral regarding the question of “should the world have nuclear power ?”. I know that they are opposed to nuclear power, the fact that they are opposed to it does not either disqualify them from commenting on nuclear issues or make them more trustworthy. I hold the view that if Greenpeace put in extra effort into their radiometric measurements then in the long run it will be good for them and the rest of society.

Firstly it would make their results more trustworthy, people would be more willing to accept their results as true.

Secondly it would avoid problems such as “I will not trust it until I have checked to see if another explanation exists for their observation”. For example if an antinuclear activist claimed that hot spot exists in Aberdeen (Scotland) as a result of a discharge from a Scottish nuclear reactor, then I would want to know that they had not been fooled by the high gamma background due to the rocks in Aberdeen (Granite). One way of proving to me that a high gamma level was not due to the granite is to show me the gamma spectrum.

In recent times the higher background radiation levels on some beaches in the western part of the USA have provoked great excitement. The radiation has been blamed by some on the Fukushima accident, however a close examination of the site indicates that the radiation is coming from daughters of uranium / radium rather than cesium 134 or cesium 137. As the Fukushima event released mainly cesium and iodine this radionuclide signature is not reasonable for the beach.

A person or group which has a track record of making hasty statements will carry much less weight than a group which takes its time and makes sure that its statements are correct.

The problem with making a statement which is quickly shown to be false is that the person or group which made the statement will lose credibility, so by taking additional care to improve the quality of the work which is behind a statement then in the long run you will be more persuasive. I will get onto another point about radiometric measurements soon.

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

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.

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