The UK Neutrino Factory Design

This poster shows the main systems that constitute the neutrino factory. Four areas are highlighted in which the UK will perform key technology demonstrations within the next five years.

Proton Driver Front-End Test Stand (FETS)

The neutrino factory complex could be built up in stages: an outline plan exists to incrementally upgrade the ISIS accelerator at the Rutherford Appleton Laboratory (RAL), supporting both the Neutrino Factory and enhancements to the current neutron and muon science (aligning with the UK Neutron Strategy technology case). The Neutrino Factory itself can be upgraded to a high-energy machine known as the muon collider, which would extend capabilities across the whole of particle physics, surpassing the LHC at CERN in several areas.

The basis of a Neutrino Factory is a powerful (4–5MW) pulsed proton accelerator, known as a

proton driver

. It starts with an H − ion source followed by components for low-energy beam transport and acceleration (LEBT, RFQ). An important operation known as

beam chopping

must also be performed at low energies: it produces clean gaps in the beam that allow injection into a circular accelerator without any beam loss, which at these power levels would produce unacceptable radioactivity.

Proton Power Neutron Science Neutrino Physics Fundamental & Higgs Physics H − Ion Source LEBT: L ow E nergy B eam T ransport 150kW ISIS 1MW  2.5MW

 5MW ISIS MW Upgrade ESS-class Machine Neutrino Factory RFQ: R adio F requency Q uadrupole Beam Chopper Muon Collider Left: staging scenario leading to a UK Neutrino Factory and possibly a muon collider.

Right: plan view of the new accelerators laid out to match the area of the RAL and Harwell site.

180MeV H − Linac The FETS team is designing the LEBT and RFQ and actively pursuing an ion-source research programme and a chopper development project. Together, these will make the FETS: a full-scale prototype of the initial segment of the proton driver, soon to begin construction at RAL.

Other Applications The proton driver for a next-generation neutron spallation source is very similar to that for the Neutrino Factory and FETS meets the specification for both projects. The technique of beam chopping will find applications in nearly all high-power proton accelerators, including those for nuclear waste transmutation, materials testing (for example IFMIF, which supports the ITER fusion experiment) and safe subcritical nuclear power plants known as energy amplifiers.

FETS RFQ FETS chopper Achromat for removing beam halo Proton bunches compressed to 1ns duration at extraction • Mean power 5MW • Pulsed power 16TW Target enclosed in 20Tesla superconducting solenoid Stripping Foil ( H − to H + / protons ) Two Stacked Proton Synchrotrons (boosters) • 1.2GeV

• 39m mean radius • Both operating at 50Hz Solenoidal Decay Channel (in which pions decay to muons ) RF Phase Rotation Two Stacked Proton Synchrotrons (full energy) • 6GeV • 78m mean radius • Each operating at 25Hz, alternating for 50Hz total Beam Fast chopping section Slow chopper and beam dump

FFAG Electron Model of Muon Acceleration (EMMA)

An new kind of accelerator called a

non-scaling FFAG

has been devised for muon acceleration before the storage ring. FFAGs, or

Fixed Field Alternating Gradient

accelerators, achieve similar goals to synchrotrons but with fixed magnetic fields. This removes a major limitation for the neutrino factory as muons, which decay in roughly 2.2

m s, must be accelerated quickly and powerful magnets can only be varied slowly. The term ‘non-scaling’ here means that cheaper dipole and quadrupole magnets can be used instead of the original, custom-shaped ‘scaling’ FFAG magnets.

Despite these advantages, a non-scaling FFAG has never been built before. A scaled-down machine called EMMA, using electrons instead of muons, could soon demonstrate the technology at Daresbury Laboratory, where an existing electron accelerator will be able to provide its input beam.

Other Applications FFAGs have the potential to lower the cost or increase the performance of a variety of accelerators. Proton and ion versions of the machines have been suggested as compact sources for next-generation cancer radiotherapy. Larger versions could be competitive with synchrotrons as proton drivers.

(produces pions from protons ) Proton Beam Dump FFAG I (3-8GeV) Muon Cooling Ring

Solid Target R&D Programme

Colliding the proton beam with a dense target is currently the only known way to produce enough muons for the Neutrino Factory. This deposits 1MW of heat into the small volume of the target, requiring engineering beyond the current state of the art. The UK is investigating the use of a simpler solid target to complement liquid target studies at CERN and elsewhere. Solid targets may reach temperatures of 2000°C, at which the response to thermal shocks induced by the proton pulses is largely unknown.

To test the candidate materials’ resilience over the target lifetime, the UKNF targetry team is producing thermal shocks by passing a fast current pulse through a wire of the material. The wire may be heated to the temperature of interest and the pulses repeated at 50Hz to put it under similar conditions to the target of a real proton driver.

Solenoidal Muon Linac to 3GeV (other technologies possible)

Muon Ionisation Cooling Experiment (MICE)

The muon beam must be ‘cooled’, or reduced in size, to fit inside the accelerators downstream. This can be achieved by a technique known as

ionisation cooling

.

The principle of ionisation cooling is to pass muons travelling in a range of directions through a material or

absorber

whose constituent atoms they ionise, losing energy in the process. Momentum is lost in the direction of travel, but then replaced only in the forward direction by the electric field in an

accelerating cavity

placed after the absorber. Thus transverse momentum is consistently removed, producing a well collimated beam of muons that can be focussed to a smaller size downstream.

While muon cooling is theoretically possible, it has not been tried in practice and the technical obstacles are considerable: for instance, the best absorber material is liquid hydrogen, which must be contained and cooled to cryogenic temperatures. Therefore the MICE Collaboration will build a short section of an ionisation cooling channel at RAL, using muons from the ISIS accelerator to measure the cooling effect to an accuracy of 0.1%, in order to predict the performance of the full neutrino factory cooling channel.

FFAG II (8-20GeV)

R109

Near Detector FFAG III (20-50GeV) Muon Decay Ring ( muons decay to neutrinos ) Entire MICE beamline [ 900 –1000 m below ground ]

Physics Motivation

The recent discovery that neutrinos have mass has invalidated the current

Standard Model

new theory is required, with the potential to explain on a deeper level why the particles of nature are as they are, but the only experimental data that can inform such a theory is data that contradicts the Standard Model. Hence neutrino mass measurements are one of the few windows onto these new laws of nature.

of particle physics. A When neutrinos were believed to be massless, their interaction with other leptons (via the was predicted to occur in a straightforward manner, where However, with the introduction of mass, the states previously believed to be fundamental turned out each to be a mixture of new states n 1 , n 2 and n 3 n e interacted with e, . As these parts have different masses, their quantum wavefunctions will become out of synchronisation over time, making the mixed neutrino appear to n m with

oscillate

oscillation wavelength measured will be a direct indicator of the neutrino masses.

m

weak nuclear force

and n t with t . so that it interacts with types of lepton other than the one it was formed with. This oscillation occurs as a function of L/E, the distance to the detector divided by the neutrino energy. As higher-energy neutrinos are much easier to detect, the Neutrino Factory places detectors as far away as practical from the source to let these oscillate fully. The ) Absorber Accelerating cavity Cut-away absorber showing the liquid hydrogen containment

Far Detector 2 Neutrino Factory Far Detector 1