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Asked by anon-381928 to Adam W on 26 Jan 2024.
Question: what is neutron scattering?
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Adam Washington answered on 26 Jan 2024:
I guess first I should explain scattering. On one level, scattering is something that sighted people do every day. Light comes out of a light bulb, scatters off of a piece of paper, and hits the retina of your eye. Your brain then processes this information to tell you that there’s a piece of paper in front of you. On some parts of the paper, there’s ink that absorbs the light and stops it from scattering. Since no light scatters from that part of the paper, your brain interprets it as black. By comparing the parts with scattering to the parts without, you start making out shapes, then letters, then words.
However, not all scattering involves light bouncing off of something. Instead, the light might just bend. Imagine an empty glass of water. You can look at the glass and know that it is empty because you can see straight through it. Now imagine that we’ve added some water into the glass. The water is clear, just like the air that was in there before. However, you can “see” the water because it bends the light that comes in, changing how things appear. Looking at these small changes in angle allow us to see more things than just looking for the big bright bits where light bounces off.
Scientists extend this idea of scattering to look at things that we can’t see. We can take a beam of x-rays and watch how it bounces off of a sample to get an idea of the structure of the sample within. This is the experiment that Rosalind Franklin performed to find the double-helix structure of DNA. The x-rays scatter off of these molecules because the x-rays around the same size as the distances between the atoms.
So that covers scattering, but not neutron scattering, so I should explain a bit about neutrons. When you go to the doctor’s office and they want to look inside you, they might take an x-ray, allowing them to see past all the squishy stuff to your bones. However, what would you do if you wanted to see inside of a crab? If has its skeleton on the outside, so how would you see the squishy stuff on the inside?
The denser a material is, the more an x-ray will scatter. The relationship for neutrons, however, is much more complicated. Certainly light materials, like hydrogen, scatter neutrons heavily while other dense materials, such as aluminium, barely scatter the neutrons at all. As a result, it’s possible to use neutrons to see light materials that x-rays cannot see.
Of course, this power comes with a price, or else everyone would be doing neutron scattering. The relative “brightness” between an x-ray source and a neutron source is about the same as the difference between a birthday candle and the sun. The x-rays will always be much faster and at a higher resolution than the neutron instruments, but it doesn’t matter how fast the x-rays can measure if the x0rays don’t see your sample!
Comments
Martin M commented on :
Adam has given a nice explanation that I would find hard to add much to.
Andrew M commented on :
Neutron scattering does have possibly my favourite unit – the Barn (b). It’s not named after some ancient measure or some famous scientist, but from the saying “you couldn’t hit the broad side of a barn”. The more Barns an atom has, the easier it is to hit.
Edd B commented on :
Most neurons are in the nuclei of atoms and don’t do very much except being heavy. Neutron scattering is most common at a national science facility which makes a high energy neutrons in a beam to look at things. These neutrons can come from a nuclear reactor (like the ILL) or a spallation neutron source (Rutherford Appleton Laboratory in the UK) where neutrons are chipped out of a big atom like tungsten by firing protons in like a snooker break.
Scattering is one of the things which can happen when radiation interacts with a solid. When a beam of neutrons hits a sample most go straight through, some are absorbed (how many depends on what atoms are present), others change direction (elastic scattering) and the rest change direction and speed/energy (inelastic scattering). So neutron scattering is an effect but it also describes a group of scientific characterisation methods.
Elastic neutron scattering can be used to look at the sizes of things, this is Small Angle Neutron Scattering (SANS) because the neutrons only change direction a little bit. If you look at the neutrons which are scattered a bit more you are looking at the Bragg scattering, this gives information on long range atomic order so why diamonds and graphite are different. If you look at all the neutrons which have been scattered especially the ones which changed direction a lot (like 90 degrees from their original direction) and you perform some clever maths called a Fourier Transform they you can directly see how close atoms are to one another. The graph is called a Pair Distribution Function (PDF) This allows us to look at the structure of liquids and glasses even though they are not crystalline. This is the sort of science I do and there are matching X-ray techniques (SAXS, X-ray Bragg and X-PDF.)
Inelastic neutron scattering is where the sample gives energy to the neutrons or takes some away. Because energy comes in set size portions (quantized) then we can measure the energy before and after the scattering to see what sizes of energy are in a material. The easiest for me to explain is plain INS (inelastic neutron scattering) which gives information on bond strengths like infrared or Raman spectroscopy. X-rays can’t do this but X-ray absorption is more interesting than neutron absorption.
Tina-Jaine H commented on :
Adam has given a good explanation, so I’ll build from there. A few people have talked about using neutron scattering for imaging purposes, but my interest in neutron scattering is how it affects the reaction in a nuclear power station. Neutrons and their behaviour are absolutely key in a nuclear reactor.
Most neutrons spend all their time in the nuclei of atoms. Generally, it takes a nuclear reaction to send them moving freely outside an atom. There are also a small number of radioactive materials which naturally emit neutrons, but these are rare.
In a nuclear reactor, which is the business part of a nuclear power station, the fuel is uranium 235, which is capable of fissioning (i.e. splitting roughly in half), if it gets a poke of energy by being hit by a neutron at the right speed. This releases a lot of energy, which is what itâs all about.
When a uranium atom fissions, the 2 smaller atoms it forms tend to have too many neutrons. This is because elements at the bottom of the periodic table have more neutrons per proton than elements higher up the periodic table. You can see this if you have a look at a periodic table, and calculate the neutron to proton ratio of a few elements near the top, the middle and the bottom of the periodic table. Itâs because in the bigger nuclei, the repulsive electromagnetic force of the positive charge from so many protons is bigger, and could tend to push the whole thing apart. They need much more neutrons add more of the weak nuclear force which attracts nuclei to each other, to balance the repulsion.
Uranium is down the bottom of the periodic table, as almost the biggest atom, so when it is split in half it makes 2 elements around the middle of the periodic table. It tends to split into 2 slightly uneven âhalvesâ, so you get a range of medium sized atoms like rubidium, strontium, yttrium, indium, iodine, xenon, caesium. These are called fission products. These would all be more stable with fewer neutrons, so some neutrons are released in the reaction – typically 2 – 3 neutrons per fission. Even so, the fission products end up with too many neutrons, and thatâs what makes them radioactive.
The neutrons which are released are useful though, because they can go on to trigger the next fission and keep the reaction going. But first we have to manage the number of neutrons and their speed and. If every one of those neutrons went on to trigger another fission, the rate of reaction would grow very rapidly. It could get out of control and cause a serious accident. And triggering the reaction requires the neutrons to be moving slower than they are initially.
There are a few things a neutron might do when it encounters any material.
1) It can go straight through the spaces around the nuclei (all material is made up mostly of empty space, and the electrons and nuclei take up a tiny amount of space).
2) It can bump into the nucleus, and bounce off like a snooker ball. This is scattering.
Or
3) it can get absorbed into the nucleus, which usually triggers some reaction.
Controlling the number of neutrons:
For this we want to absorb enough neutrons somewhere other than the fuel, to leave exactly 1 neutron per fission to go on to trigger another fission. We use control rods made of boron to absorb the neutrons. Boron 10 can absorb neutrons into its nucleus and form lithium 7 and an alpha particle (a helium 4 nucleus with no electrons). The alpha particle will soon get slowed down by scattering, enough to capture some electrons and become a stable helium atom, so itâs not a problem in a reactor. There are many control rods, which insert into gaps between the fuel rods. We have to carefully adjust the position of the control rods to ensure they absorb exactly the right amount of neutrons to keep the reaction rate steady. The control rods can be lifted up to reduce the degree of contact between the rods and the fuel and increase the reaction rate, or fully inserted to shut the reaction down.
Controlling the speed of the neutrons:
This is where scattering is more important. A neutron which hits a uranium atom while going too fast (i.e. right after it has been emitted from a fission reaction) will generally scatter, and not be absorbed or trigger a fission reaction. It’s like the neutron needs to spend a bit more time in close proximity with the uranium nucleus to trigger the reaction. The fast neutrons don’t stay close enough for long enough.
The technical way to express this is that uranium has an extremely low âfission cross sectionâ for fast neutrons, and a much much higher fission cross section for slower neutrons. Conversely, uranium has a much higher âscattering cross sectionâ for fast neutrons, than for slower neutrons. You can think of cross section as the area you would have to hit to cause the interaction you are interested in â either fission or scattering in this case. This is the property measured in Barns, which Andrew mentioned. It has units of area, but you can also think of it like the probability of that interaction occurring. So itâs EXTREMELY unlikely that a fast neutron would trigger a fission.
So we need to slow the neutrons down, to a speed we refer to as âthermal neutronsâ, and we can use scattering off other materials to do that.
When a neutron scatters with any material, the nucleus of that material will also bounce to some degree, depending on its mass. The lighter the nucleus the more it bounces, and more energy is transferred from the neutron to the nucleus.
Imagine firing a snooker ball at a 10m tall round rock. Compared to the snooker ball, the rock is practically immoveable, so it wonât move, i.e. it wonât gain any kinetic energy or momentum. The snooker ball would bounce off with almost as much kinetic energy as it started with. But when you fire a snooker ball at another snooker ball the same mass, they both noticeably change their speed, the 1st one slows down a lot. More kinetic energy is transferred from the 1st ball to the 2nd when they are a similar mass, than if the 2nd ball was much heavier. This is a consequence of the combination of conservation of energy and conservation of momentum. The total energy of the 2 objects must be the same before and after the interaction, and the total momentum must also be the same before and after. But kinetic energy is proportional to velocity squared, whereas momentum is proportional to velocity. The maths of this means more energy is transferred when the 2 objects are a similar mass, compared to when the 2nd object is higher mass.
We use this fact to slow the neutrons down to the optimum speed to trigger the fission. So the fuel is surrounded by water, which contains a lot of hydrogen – the smallest atom. A nucleus of hydrogen has the same mass as a neutron, so when they bounce off each other the water molecule is given some kinetic energy, and the neutron loses some.
It takes many of these scattering interactions to slow each neutron down to the optimum speed to cause fission. We call this process moderation â it moderates the speed of the neutrons, but itâs all about scattering.
It is possible to use other light weight atoms/molecules as a moderator, but water is particularly useful because itâs also a good coolant, for taking the heat out of the reaction and transferring it to the turbine where it can be used to generate electricity.
Water has another useful property here. Like most liquids, water expands as it gets hotter. Expansion makes the molecules further apart, and creates more space in between. This means that the neutrons spend longer moving around in the free space, and donât scatter off water molecules as often. The longer the neutrons are whizzing around, the more chance of them getting absorbed by the control rods and being removed from the process. So the hotter water slows down the moderation process as well as reducing the number of available neutrons, and effectively slows down the nuclear reaction.
But, reducing the reaction rate also reduces the temperature in the reactor, so there is a feedback loop which helps to keep the temperature stable in the intended range. It’s called a negative temperature coefficient of reactivity â when temperature increases, the reactivity decreases.
Reactivity is the ratio of number of reactions from 1 generation to the next. If the neutrons released from 1 fission reaction lead to exactly 1 more fission reaction, then the reactivity = 1, and the reactor is in a steady state. This is what we want in normal operation. If reactivity is less than 1, the reaction rate (fissions per second) is reducing. If itâs more than 1, the reaction rate is increasing.
Some older designs of nuclear reactor, including Chernobyl, had graphite for the moderator, and had a positive temperature of coefficient of reactivity, so the reaction increased as temperature increased. This property made the Chernobyl accident, which involved the reactor overheating, much worse.
Iâm a nuclear safety engineer, and itâs vital I understand all these aspects of physics to help me predict how a reactor behaves in different conditions, especially possible accident scenarios.