When complete in 2019, the €1.48bn European Spallation Source (ESS) will be the most powerful source of neutrons in the world. With construction expected to start in 2013, and the facility fully open by 2025, the ESS will produce neutrons by accelerating protons in a linac to 2.5 GeV before smashing them into a seven-tonne target. The neutrons will then be cooled by a moderator and sent to 22 experimental stations to be used by researchers to probe the structure and physical properties of a wide range of solids, liquids and gases. The ESS will specialize in long wavelength, or “cold”, neutrons that suit experiments on large-scale structures such as polymers and biological molecules.

But one big problem for those designing the ESS is that this process of “spallation” will deliver so much energy – the proton beam will have a power of 5 MW – that the temperature of the target will jump by more than 100 °C in just 2.8 ms. Indeed, as the target becomes radioactive it will produce a decay heat of 35 kW even when there is no proton beam. Researchers at the ESS are therefore designing a proton target that can not only generate copious amounts of neutrons, but also be able to handle these extreme heat conditions.

A neutron-rich material makes for a good proton target and ESS bosses are currently investigating two different options – a lead bismuth eutectics (LBE) alloy or tungsten. LBE is solid at room temperature but at the ESS’s operating conditions becomes liquid, which is similar to another possible target material – mercury. On the other hand, tungsten is solid up to 3000 °C and has a very high density of 19.25 g cm^–3, giving it a high neutron yield. The material also has the advantage of longevity, with a life-span of three years or more, compared with six months for a lead–bismuth target.

As Physics World went to press, the ESS board was expected to decide which target to use, with tungsten the clear favourite having got the thumbs up from the ESS’s science advisory board in July. Indeed, tungsten is also already used at other neutron-scattering facilities including ISIS in Oxfordshire, which has a solid tungsten target about the size of a house brick. “The material is really the best you can pay for,” says Ferenc Mezei, head of the ESS’s target division. “There are other materials, iridium for example, that have a higher density but they are much more expensive.”

More power

There are some challenges to implanting a target given the heat created by the ESS’s huge proton-beam power of 5 MW, which will be around 20 times greater than that at ISIS. One proposed solution is to make the target rotate once every three seconds so that only a certain part is hit by the proton beam at any one time.

One design for the ESS’s target is to use a disc – 2.5 m in diameter and 13 cm high – made up of a solid inner hole and an outer ring. The beam will hit the disc edge-on, first encountering the outer layer, which is made up of around 10,000 small rods of tungsten each about 12 cm high and 1.5 cm in diameter. The beam then travels through the inner solid tungsten where it will lose energy so fast that it does not actually reach the centre of the disc. The advantage of using rods in the outer ring, rather than solid tungsten, is that in taking most of the proton beam, they distribute and reduce the stress of the whole target during its rapid rise and decrease in temperature.

The disc is made to rotate so that the 5 MW beam will be distributed such that a section of the disc sees – on average – about the same power density as that at ISIS. This allows that part of the target to cool down by 100 °C in around 3 s before it comes in contact with the beam again.

The sheer size of the target, and its activation, means that researchers cannot build a full prototype to test such high beam powers. However, in designing the ESS’s target, researchers can take solace from the fact that the technology has been tested before, for example at ISIS’s muon facility, which uses a small rotating graphite target to produce muons – heavier cousins of electrons.

“Rotating targets are around,” says Mezei. “So we are confident that this kind of technology will work”.

• You can download a PDF of the October 2011 Physics World Big-Science Supplement here.