Scientists at ISIS create neutrons by bombarding a target with high energy protons.
The target is usually buried deep inside a protective bunker.
But today we get to see it…
Martyn Bull: “This is target station two’s experimental hall, and the big blue bunker in the centre is where the target is, the neutron target. In this target station is tiny, it’s about this big, about the size of a packet of biscuits.
This tiny tiny object receives high energy proton beam 48 kW of power dumped into this thing liberates all its neutrons and we have to cool it with water as well, to stop it melting.”
But luckily we visited the day a new target arrived for installation…
Leslie Jones: “This is our neutron spallation source. It’s the target for our second target station at ISIS. We are actually quite privileged to see this- you wouldn’t normally be able to see this whilst it’s in service, once it’s installed, never to be seen again apart from in the remote handling cell.
So, we’ve actually got a very special instrument here made from heavy metals. The main core is made of tungsten and then it’s been clad in tantalum. Both these materials are suitable for us to get neutrons from. So when it’s in a horizontal position, and that would be how we normally have it installed (you can see how heavy it is as I’m wincing a little bit as I’m doing this!). Here it is in situation there- we would have a manifold which would control the cooling water and then from this direction (towards the front face of the target), we would have a proton beam striking the front. Now the water comes along the top and over the front face and back down there, in two directions. That’s extremely important for this to keep all that energy cool, and to keep the material cool so we don’t get any problems with the operation there.
Meanwhile, in the middle, after the beam has been hitting this, around about this area here (points to a part of the target on the top, quite close to the front face) the core temperature is going to get up to about 800Oc. So you can imagine there’s a lot of water that goes over the surface, to keep that keep that at around 50oc and take all that heat away during operation. This is housed inside what we call a beryllium reflector, which is keeping as many neutrons around the area that we can, because they are coming at all directions. We only want them to come out at a certain direction on this plane, where they are going to pass through some moderators. Now they kind of slow the neutrons down to an energy we can use, direct them to a suite of instruments that the scientists are going to use for analysing things. As soon as it starts hitting here (points to the front face), because this is a suitable source of neutrons, this material, we will start getting them, but mainly it is sort of 40-50 mm back from there where most of the energy is going to be dumped. It’s basically, we’re throwing kind of lots of energy at it, and as it hits the dense material, it’s going to tail off getting all the way through, so most of it gets dumped at the front.
The main core is tungsten, and the cladding on the outside is tantalum. Ideally, the scientists would have like us to make it out of tungsten, because that would give the most neutrons. Tantalum is a little bit less, maybe only 10% less, but obviously they always want the most! Unfortunately we’ve found- there is evidence to show that if we just use tungsten in a water-cooled environment, with radiation the water would wear some of the tungsten away, so we get deposits of radioactive tungsten in the water system which we don’t want. However, if we clad it in tantalum, tantalum is very inert, and doesn’t dissolve in even many strong acids, so it is much more suitable for keeping everything under control.
Very quickly we are going to build it up into a proper target module, with a manifold and all the thermocouples fitted on there. Once it’s in operation, it’s going to see beam for quite a long time now. It’s actually going to be producing the neutrons that scientists are going to use for their experiments, so it’s why it’s such a key piece of equipment. It’s going to be doing that for a period of 2-3 years, and so we’ll be monitoring it all the time, to make sure we’re keeping it to the right temperature and all the parameters are kept in place so it’s going to have a good long service life.”
“And then what?”
Leslie Jones: “Then, unfortunately it has to be retired, and it’s going to be a radioactive piece of equipment, which means it needs to go into careful storage.”