Producing neutron beams by the spallation method involves a complex engineering infrastructure to accelerate particles to high energies. Each section of the ISIS machine is looked after by a highly skilled and experienced team.
The process starts with an ion source to produce negative hydrogen ions (two electrons bound to a proton, H-) which are extracted and injected into a chain of linear accelerators. Then stripped of the electrons by a thin piece of aluminium oxide foil, the bare protons enter a circular accelerator 50 metres across called a synchrotron.
During acceleration they go round 10,000 times before being kicked out of the synchrotron towards the targets where neutrons and muons are produced.
Originally based on Russian technology, the ISIS ion source works by generating a discharge plasma in hot hydrogen gas fed with caesium vapour. This creates the H- ions which offer the easiest way of feeding particles into the synchrotron accelerator.
“We are now the leading operators and developers of this type of ion source,” says Dan Faircloth, head of the ion source section. The ion sources last for between 20 and 30 days before they have to replaced. “Keeping the source in optimum condition is often called ion sourcery,” says Dan. “We have to maintain the discharge so as to produce as many H- ions as possible. If you switch off the ion source and then start it up again, its behaviour changes. They are fickle beasts, which is what makes the ion-source technicians superstitious. They don’t like to talk about a source because they think it can hear you,” he jokes.
A collection of 10 sources, which are made at ISIS, is kept ready to go. “We are building up a database of operating conditions and failure modes so we can improve reliability,” says Dan.
Protons on target
Neutron production also depends upon the target systems being kept in good condition. Although a target lasts up to five years, the surrounding liquid-methane moderators (for TS1), which slow down the neutrons, do degrade and have to be changed every other run cycle. Because the whole system becomes radioactive, changing the moderator requires a remote handling cell.
“Special suits and breathing apparatus are required while we equip the cells with tools and cameras,” says Dave Haynes of the Targets Section. “Changing the moderator is complicated but we’ve done it so many times that it is routine,” notes Dave.
Setting up and monitoring the accelerator
Maintaining the machine performance is essential to providing optimum experimental conditions for users. The job of Dean Adams and his team is to set up the accelerator for each user run. The whole facility works under extremely tight tolerances. The proton beam must remain very stable, which means that, for example, power supplies must be stable to within 0.01 per cent. Any loss of beam as a result of it hitting the accelerator sides will directly affect the quality of the final experimental beams, as well as damaging the machine.
“We provide on-call 24-hour support for any operational problems,” says Dean. "We establish where the problem is and call out the relevant crew to fix it.”
Sarah Whitehead and Tony Kershaw of the Diagnostics Section monitor the location of every proton pulse via beam monitors situated along the synchrotron. “If there are three consecutive pulses that are outside the tolerances, we switch the machine off,” says Tony. “We aim for at least 90-per-cent availability of the machine for over the 40 to 50 days of user time,” he adds.
Staying In control
The routine running of the accelerator is monitored from the ISIS Main Control Room by a duty officer and three assistants working in three daily shifts, 365 days a year.
Duty officer Tom Noone says to maintain the machine at maximum efficiency requires experience and is a bit of a ‘black art’. “You know how the machine should run and I might give it a ‘tweak’ from time to time depending on how it’s running.” he says.
One of the team regularly tours the site carrying out routine checks on components such as the moderators. “We are all electronics engineers so if a piece of equipment goes off in the middle of the night, we will attempt a first fix before calling in a specialist”, adds Tom.
Neutrons for the chop
Once the neutron beams are generated, they must be in the right energy range for a particular experiment. This is achieved by using rotating devices called choppers which block unwanted neutrons. ISIS uses several kinds of chopper, the simplest being a blade which sits just beyond the target and rotates at the same frequency as the neutron pulses. It eliminates high-energy, fast-moving neutrons while letting the bulk of them through. Further down the beamlines are disc choppers with slots that allow neutrons in selected energy bands to pass, whilst other choppers might be set up to allow only every other pulse through.
Tim Carter designs, develops and maintains the chopper drive-systems including new designs for TS-2. “ISIS is leading the way in chopper development. We have dramatically improved their reliability – essential for achieving good experimental results”, he comments.
Each neutron instrument consists of many components, each of which is looked after and developed by groups within ISIS. For example, ISIS develops its own neutron detector arrays, which comprise mainly gas and scintillating devices. The latter consist of material that emits light when neutrons impinge on it. The light is then transmitted via thousands of optical fibres encoded in a position-sensitive way.
Brian Holland, who works on detector designs at ISIS, has been involved in upgrading the detectors. “Scintillators are a bit of a speciality here; they are massive and require very careful work but are great fun to build,” he enthuses.
Next: The heart of the journey