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Magnum opus

Dear Reader,

Over christmas I wrote a quasi magnum opus which was a review of the chemistry of serious nuclear accidents, it has now been published at Cogent Chemistry. It is free to read as it is in an open access journal.

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.


Dear Reader,

It has come to my notice that a series of slugs have slithered into Vara, to my great displeasure these brutes have chosen to invade my garden. I have had to do some research work to enable me to respond in the correct manner to these animals. For those of you who do not know this the slugs are a pest which has been accidentally introduced into Sweden, I did some research and discovered a few interesting things about them.

Firstly slugs hate caffeine, in large doses it is fatal to them while in small doses it puts them off their dinner. I have to admit while I find the slugs repulsive I do not wish them all dead. I would rather that they went off into the forest where they can do little harm.

Caffeine is one of those molecules which has been characterised again and again by crystallography, one example is the magnesium bromide dicaffeine solid (M.B.Cingi, A.M.M.Lanfredi, A.Tiripicchio, G.Bandoli, D.A.Clemente, Inorg.Chim.Acta , 1981, 52, 237). Below is the molecular structure of caffeine in all its glory.

Crystal strucutre of caffeine

Crystal structure of caffeine, note it shows bond lengths and not bond orders

I have a rather long and personal history with caffeine, one summer as an undergraduate I devised a total synthesis of the drug only to be disgusted to discover that the book I got the structure from had an error. As a result I was trying to devise a synthesis of a related compound (Isocoffeine). I can not recall all the details of my isocaffeine synthesis but I do recall that it would have used MIC (methyl isocyanate) for one of the steps (Not a nice substance !). To put it bluntly I felt decaffeinated by the experience !

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.

Alternative fuel for RTGs

Dear Reader,

I was reminded by a reader that a shortage of Pu-238 exists, for those of you who are not in the know. Pu-238 is a proper radioisotope of Pu. In comparison Pu-239 is quite low in terms of activity per gram. As a possible replacement for Pu-238, Am-241 has been suggested.

For other applications other radionuclides such as Sr-90 have been used in RTGs, this is what the Soviets used for a lot of the RTGs which were used at remote lighthouses or substations in the middle of nowhere.

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.

Good luck to her

Dear Reader,

It has come to my attention that Elizabeth Wurtzel has got married, I say good luck to the young lady. I hope it works out well for her, in case you do not know she is a woman known for her rather shocking autobiographical writing about things like depression. I have read two of her books and they were interesting and shocking in places.

Sadly I have also discovered that she has had some health trouble (breast cancer), I hope that her treatment has cured her and that she has a long and healthy life.


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