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Organic forms of radioactive iodine

Dear Reader,

Some time ago me and Emma celibrated the start of a project on radioactive iodine with some interesting soup. We have now come to the end of the project, I would say that it has been an interesting journey.

Myself and Emma together with Diana have done something interesting, we tested a hypothesis which in someways is a rather frightening one. Imagine a world where the standard method of measuring radioactivity in workplaces and the environment is defective in such a way that it underreports the radioactivity.

This is a world where a false sense of security would exist, a world where people are unwittingly exposed while being reassured. The centre of the hypothesis relates to the fact that a chemical reaction between methyl iodide and an additive (normally DABCO) is needed to fix methyl iodide into a charcoal. Without this reaction the methyl iodide tends to be able to escape from the charcoal. Normally charcoals and other gas capture systems are tested (quailified) with methyl iodide.

Many organic iodine compounds such as ethyl iodide and chloromethyl iodide which can form during a serious nuclear accident react more slowly than methyl iodide with DABCO. Thus as they persist longer in the charcoal pad in their unchanged form they have a greater potnetial to reenter the gas phase and thus escape from the charcoal. This sluggish chemcial reaction casts doubt on the ability of a methyl iodide quailified charcoal to intercept things like ethyl iodide.

What we did was to do a series of tests on charcoals which have shown that the non methyl iodide organic iodines which we have tested with the charcoal absorb either as well or better onto the charcoal than methyl iodide. This indicates to us that the standard charcoal may well work just as well as sampling devices for non methyl iodide radioactive organic iodine. We would rather go to larger scale tests before givng a final answer, but the intial tests look very promising.

These results also suggest that gas masks (respirators) based on DABCO loaded charcoals will be able to remove these alternative organic iodines from air. This is further good news.

If you want to read the first paper we have published on the subject then here is a the share link which will work for the next 49 days.  The paper is

E. Aneheim, D. Bernin and M.R.S.J. Foreman, Affinity of charcoals for different forms of radioactive organic iodine, Nuclear Engineering and Design, 2018, 328, 228-240.


Gamma spectroscopy under different conditions

Dear Reader,

While working on the project for SSM I had to make some measurements by means of gamma spectroscopy using a high purity germanium detector, note that it is not a high purity geranium detector. Being a main group chemist I do occasionally allow myself a joke. One such joke among main group chemists regards the fact that germanium sounds a little like a flowering plant which is popular in the UK.

What happens in a high purity germanium detector is that gamma photons enter a crystal of germanium which has a high voltage (2 to 3 kV) applied to it. When the photon deposits energy in the crystal this produces charge carriers (free electrons and holes). As a result of the event charge flows through the crystal, the electronics measures the amount of charge, and from that it is possible to work out the amount of energy which was deposited in the crystal during the event.

This sounds all very simple and easy but in real life it is not easy, for example some energy can enter the crystal in the form of the photon and some of this energy can then escape from the crystal as a result of Compton scattering or pair production.

Also the photons from the radioactive decay event must be able to enter the detector, also they have a finite probability of being able to interact with the crystal. The issue of penetration and overpenetration add a new layer of complexity, it causes the sensitivity of the detector to change greatly as a function of the energy of the photons.

Also the radioactivity can generate secondary radiations, for example beta particles which strike high Z (atomic number) materials can generate characteristic X-rays and braking radiation (Bremsstrahlung). During my efforts to make some measurements I created what I think is a good set of spectra which show off some of these effects.

My first problem was that I wanted to verify that I had iodine-131, I choose to put some methyl iodide which contained a lot of radioactivity into the detector. I knew that putting the vial into the detector would overload the detector, so I used a trick to cut down the gamma flux. I put the sample inside a lead pot. Or lead pig, this greatly reduces the photon flux near the vial. I knew that the activity calibration already made for that detector with a different geometry would not work, but all I wanted to do was to verify the identity of the radionuclide.

In some ways I got a better answer than the one which I would have had if I had used an unsheilded vial with less activity in it, I was more able to observe the higher energy gamma photons from the radionuclide. This was because the lower energy photons from the radioactivity were strongly attenuated by the lead pot.Here are the gamma spectra for iodine-131 recorded under different conditions.

Firstly here is the gamma spectrum for the radioactive iodine in a LSC vial.



I have shown both a view of the graph with a log and a linear scale on the y axis. Now here are the graphs for the radioactive iodine inside a lead pot.



What you should be able to see is that the high energy end of the spectrum is now more important, the moderate energy gamma line for I-131 at about 300 keV is much weaker, as a result the higher energy gamma lines are now more important in the spectrum.

It is also interesting that when we look at the low energy end of the spectrum that the lead pot changes things greatly.


What we have here are some x-ray peaks and a low energy gamma line. It is important to understand that the beta decay of the iodine forms a xenon atom in an excited state. The capture of an electron by the xenon atom can result in the generation of photons. In this case it is k line photons. The radioactive iodine also emits gamma rays with about 80 keV energy. Please keep in mind that the energy calibration of the spectrometer is a bit off, it tends to over report the energy of the events.

The lead pot was responsible for the generation of some lead K x-rays. What I suspect happened was that the gamma rays from the radioactive iodine with 300 or more keV interacted with the pot and generated plenty of fast moving electrons. This would have been by both the photoelectron and the Compton effects. Those electrons which were generated close to the surface of the lead pot were able to excite lead atoms, these lead atoms then emitted x-rays, here you can see the K alpha and K beta lead x-ray photons. The lead pot was able to screen out the 80 keV photons from the radioactive iodine so the X-ray generation must have occurred on or near the outer surface of the lead pot. One interesting experiment would be to put a pure high energy beta emitter such as strontium-90 inside a range of lead pots and then count these with the HPGe gamma spectrometer.

If I was to have covered the outside of the lead pot with a layer of copper then these lead x-rays would have been attenuated. One option for getting the best shielding out of a lead (or DU) object is to cover the object with copper, then aluminium and then finally plastic. The idea is that the secondary radiations from the lead (fast electrons and X-rays) will be captured in the copper. The copper being a lighter element is less able to generate x-rays from the electrons and it will also offer some attenuation of the lead x-rays. The lower energy x-rays generated in the copper will then be attenuated by the aluminium. Finally the very low energy x-rays generated in the aluminium will be captured in the plastic.

It is interesting that the lead shielding for the gamma spectrometers are lined with copper sheet to try to reduce the formation of secondary radiations by the action of cosmic rays (and other background radiation) and the radiation from the sample in the spectrometer on the lead shielding. It is also interesting to note that photographic film is more sensitive to high energy gamma rays if it is placed in contact with a thin lead sheet.

During the Fukushima event some people claimed that workers at the site were issued with lead sheets to make their dosimeters less sensitive, under some conditions by wrapping a dosimeter in lead sheet it would be possible to make it over report a dose of gamma rays. A lot will depend on the energy of the gamma rays, while a thin lead sheet will make a dosimeter under-report things like x-rays from a dental radiography set (70 kV tube) it will make the dosimeter over report the dose from a high energy gamma emitter such as cobalt-60. The old fashioned film badges which we used to use back in 2000 for gamma dose measurement of people had a series of metal filters designed to allow the badge to make better measurements of gamma photons with different energies.

Note that before you put anything into a gamma spectrometer it is important to wrap it in a plastic bag. If you have ever been a responsible dog owner you will know how to pick up dog poo (or other objects) using a plastic bag in such a way that you never touch the dirty side of the plastic bag. Put your hand into the bag, pick up the object and then turn the bag inside out and tie it shut. I did this with the lead pot containing the radioactive iodine and then just to be sure I bagged it a second time. I reason that no harm can come from using a second bag, and just in case something went wrong during the first bagging the second bag added while the singly bagged object was on a clean bench would reduce the potential for the transfer of radioactive contamination yet further.

My second problem was that I wanted to get a quench curve for liquid scintillation counting with large chemical amounts of methyl iodide present, any of my readers who are organic chemists will know how methyl iodide is a toxic and very volatile liquid which is difficult to pipette out. While my one of favorite pipettes (200 microlitre) is unable to measure out 20 or 50 microlitre volumes of methyl iodide a 10 microlitre pipette can dispense 5 microlitres it as the diameter of the tip is much smaller. But for 10, 20 and 50 microlitres I used a gas tight syringe designed for gas chromatography work.

Rather than trusting my pipetteing with this volatile liquid, I choose to use the gamma spectrometer to check my volumes. By making a measurement of the number of counts per second I was able to make an independent measurement of the radioactivity in the LSC vials. I will save the results of this for another day.

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


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.


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.


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.


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.


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).


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


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.


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.


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.


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 SSM project, Lets make banish radiation accidents to science fiction.

Dear Reader,

It has come to my attention, and I imagine many of my readers attention, that radiation accidents feature on a regular basis in science fiction. Many science fiction tales have at their core some form of radiological misadventure for example the incredible hulk and spider man both were “normal” men until they were subjected to a dose of radiation or bitten by a radioactive spider.

I am sure that my readers will be glad to know that governments, university academics and other groups are working towards a world where horrid radiation accidents are only found in science fiction. As part of this effort the Swedish radiation protection authority (SSM) are supporting work in the Nuclear Chemistry unit at Chalmers. In this work the effects of chloride contamination on the release of fission products from fuel and the capture of organic forms of radioactive iodine onto charcoals is being investigated.

The chloride contamination issue became of interest during the Fukushima accident, in this accident sea water was used to cool the stricken nuclear reactors for some time. If the reactors had dried out a second time then nuclear fuel could have been exposed to molten salt. Some of my readers will be aware of the fact that metal chlorides tend to be more volatile than metal oxides. For example copper chloride is more volatile and is able to enter a flame where it generates green light. The effect of dichloromethane on the flame coming from a brass butane torch can be seen in the following film.

It is reasoned that if a similar process is able to increase the mobility of a fission product then a salt addition could increase the threat posed by a reactor accident to the public. Also this study could offer an insight into an accident in a chloride based pyro-reprocessing process using a molten salt such as a NaCl/KCl eutectic.

The other area relates to both sampling and personal/collective protection. One of the methods of measuring radioactive iodine in air is to use a Maypack filter. This is a device through which a large volume of air is sucked. After sucking the air the Maypack is dismantled and the different parts measured for radioactivity. It is possible to determine what chemical form the iodine was in when it reached the Maypack. Aerosols, elemental iodine, HOI and organic iodine all collect in different parts of the device. Methyl iodide is known to react with charcoals which are impregnated with chemcials such as potassium iodide and DABCO. However if an organic form of iodine which is unable to react quickly with KI and DABCO was to enter the charcoal pad in the filter then it could revapourise and be lost from the charcoal pad. As the organic iodine fraction is often a major part of the radioactive iodine released from a nuclear plant this could result in an underestimation of the amount of radioactive iodine released from a plant.

Also if the organic iodine compound is more able to pass through charcoal pads in respirators then it could cause the protection factor of a respiratory to be lower than that predicted by tests using methyl iodide which seems to be the de facto standard alkyl iodide used in nuclear safety experiments. We hope in this project to test the hypothesis that the DABCO loaded charcoal is able to capture all the organic forms of iodine which can form during an accident. This work is of interest to the ISO organization (ISO TC94 SC15 WG2&WG3 PG3 JTG CBRN) as the ISO organization is currently considering a standard for protective clothing for use in the nuclear / radioactivity sector.

Now while nuclear / radiological accidents are clearly not “fun” and are not joking matters, it is important to understand that the nuclear chemistry section at Chalmers is not a “Fun Free Zone” (FFZ). Years ago I read a chemistry laboratory manual which commented “Chemistry should be done while sober but not while somber”. A trip to a chemistry department should not have to be sad and unhappy, even those who toil in the nuclear chemistry section are allowed to share a joke, well that is as long as it is in good taste and on an acceptable subject.

Here are some of the people who work on nuclear accident chemistry eating gazpacho soup, now you might ask what is so funny about gazpacho soup. Well to understand the cultural significance of this soup to me you need to know of Red Dwarf.

Emma on the left and Mark on the right, both are enjoying some soup.

Emma on the left and Mark on the right, both are enjoying some soup.

One classic British comedy which is well known to people of my generation is Red Dwarf which is about the last man left alive in the universe, again in the first episode this sci-fi story features a radiation accident which I would rank being at least. INES level five (based on the death of the whole crew of a spaceship). Maybe it might be an INES level of six based on the large number of deaths.

One of the central figures (Rimmer) who is a rather disagreeable fellow died in the first series before returning as a hologram. His last words were “gazpacho soup” which relates to the event which he credited with ending his career. He sent the cold soup back in disgust in front of the ship’s captain demanding that it be heated up. In fact the pompous, officious and cowardly Rimmer was doomed to failure because of other matters.

He spends many hours with a more laid back man in this science fiction comedy often being the subject of many jokes. For years I was in blissful ignorance thinking that gazpacho soup was something invented by the script writers of Red Dawrf, while as a postdoc on a busniess trip to Spain years ago I saw it on the menu of a restaurant. I then tried it partly out of curiosity and partly as it felt funny to be eating the substance that troubled Rimmer so much.

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.

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.

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