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22l of liquid hydrogen; a milestone for MICE and a cross-campus success for STFC

The international Muon Ionization Cooling Experiment (MICE) on ISIS at RAL has established stable operation passing a muon beam through a 22l volume of liquid hydrogen. Operating with this volume of liquid hydrogen is a major technical challenge and is the culmination of several years work by personnel from Technology Department (RAL and DL), PPD and ISIS.

Muon beams are produced from the decay of pions captured from the debris that arises when a proton beam strikes a target. When produced in this way, an intense muon beam occupies a large volume in three-dimensional “physical space” and a large volume in six-dimensional “phase space”; such a beam is said to have a large emittance.

Muon beams of low emittance have been proposed as the basis of powerful facilities for the study of fundamental particles. To create such beams requires that the muon-beam emittance be reduced very quickly; in a time short compared to the lifetime of the muon (2.2 microseconds). At production, a bunch of muons from the beam occupies a volume comparable to that occupied by a water melon and is highly divergent. To accelerate and manipulate the beam requires that the size of the bunch transverse to its direction of motion be reduced such that, after cooling, the bunch occupies a volume comparable to that of a cucumber.

The MICE collaboration seeks to demonstrate that ionization cooling can deliver the required cooling effect. In an ionization-cooling channel the beam is passed through a material (the absorber) in which it loses energy and is subsequently accelerated. Liquid hydrogen is expected to give the best cooling performance. A systematic study of the factors that determine the ionization-cooling performance of a section of accelerator that contains an absorber composed of liquid hydrogen is therefore a critical step in the successful execution of the MICE experiment.

The MICE liquid-hydrogen absorber is filled by condensing hydrogen gas. The condensation rate is such that it takes roughly one week to fill the 22l volume of the absorber vessel. The vessel was filled for the first time late in the evening of the 25th September 2017. This success was the culmination of years work by personnel from Technology Division (RAL and DL), PPD and ISIS. The absorber vessel was designed and built at KEK in Japan. The detailed design of the 120micron thick aluminium windows was carried out by the Project Engineering Group at RAL and the University of Oxford. The windows were fabricated in the USA.

To make the system work required a full system-engineering approach that was guided by M.Hills, A.Nichols, S.Watson, M.Courthold, T.Bradshaw and later by V.Bayliss and J.Boehm all of Technology Department at RAL and M.Tucker of PPD. The critical control system was delivered by P.Warburton from the Electrical Engineering Group in Technology Department at DL. The DL Control Systems and Safety Interlocks Group made critical contributions to the integrated safety-engineering approach that was adopted from the outset and delivered through a close collaboration between personnel from Technology and ISIS Departments. Personnel from the Particle Physics Department provided the “mission need” and M.Tucker, supported by S.Balashov, delivered the critical vacuum and gas-handling systems.

For the MICE collaboration establishing stable operation with liquid-hydrogen is a major milestone that allows the critical study of the factors that determine the ionization-cooling effect to be carried out. For STFC the successful completion of the liquid-hydrogen system demonstrates the strength of the cross-campus, inter-disciplinary collaboration necessary to deliver such a technically-demanding project.

Plot showing liquid hydrogen filling the MICE absorber.

Spokesman’s Update

Liquid-Hydrogen System

Last week, the absorber, condenser and associated pipe work were cooled to operating temperature.  Over the weekend, Josef Boehm, Mark Tucker and Phil Warburton brought the H2 liquefaction system into operation.  Liquid is now being condensed steadily and the absorber fill has begun.  It is estimated that around 1.5 l of liquid has been accumulated so far.  The status (“shifter” and “expert” panels) is shown below.

This is an important step!  The LH2-fill rate is a little slower than expected making it necessary to revise the block-diagram run plan.  The revised plan will be presented at tomorrow’s Operations Meeting.  The likelihood is that it will be necessary to advance “LH2 empty” data taking to exploit the time during which the absorber is filling.

Data Taking: Cycle 2017/02

Under the leadership of Paolo Franchini, MOM for the first part of the Cycle, the experiment has been brought into operation.  The readout has been tested successfully.  Melissa Uchida has devised, and implemented, the waveguide-swaps for the tracker.  A first re-calibration of the VLPCs has been carried out, a second calibration will take place today.  Over the weekend, David Adey refreshed the spare AFE boards; the boards are “live spares” for the tracker readout.  Finally, Alan Bross and Sandor Feher have brought the spectrometer solenoids up.  The magnets have been run at the currents required for the first configuration that will be used in the Cycle.

Decay Solenoid

The decay solenoid has been recommissioned.  It has been shown to be operational.  At present there is a gremlin in the interlock chain that defeats remote operation.  The fault is being addressed.

Conferences: COOL17 and NuFACT17

The new material presented at the recent video conferences has been finalised.  MICE talks at COOL17 (Dimitrije Maletic, Melissa Uchida) will take place at COOL today.  The contributions to NuFact (Francois Drielsma, Chris Hunt, John Nugent and Jaroslaw Pasternak) will follow next week.

Liquid-hydrogen System Commissioning

The third and final test of the liquid-hydrogen system using neon was successful. The test ended last Thursday (03Aug17) and the system was allowed to warm up over the weekend. The warm-up has been slow, indicating excellent thermal installation.

The outstanding work that must be carried out before the final safety inspection includes the completion of the remedial work on the hydrogen-quench line, the installation and inspection of ATEX-rated fittings in the LH2 shed on the roof of then MICE Hall and the completion of the safety-related paper work. In addition, the leak testing in the hydrogen-gas panel has to be repeated to document the (low and satisfactory) leak rates. The liquid-hydrogen team is working steadily to address the last remaining issues.

The final safety tour is scheduled for 17Aug17. Assuming the system is signed off as safe to operate, the commissioning of the system with hydrogen will commence as soon after the tour as possible.

Introducing MICE

The international Muon Ionization Cooling Experiment (MICE) is a high energy physics research experiment based at Rutherford Appleton Laboratory in Oxfordshire, U.K., with collaborators from institutes across the globe.

Muons are fundamental particles just like electrons, however they are heavier and they are unstable – they decay into photons (the particles which make up light), electrons and another type of fundamental particle known as neutrinos. The feynman diagram for this decay is shown below. Muons decay extremely quickly; on average they only live for 2.2 millionths of a second (2.2μs) before disappearing.

The feynman diagram for muon decay.

Now, neutrinos, one the particles which muons decay into, are very interesting fellows. If you were hosting a dinner party for fundamental particles, they would be top of your list. In particular neutrinos interact with normal matter and its mirror counterpart part, anti-matter differently (what physicists call charge-parity asymmetry). Understanding how differently could help us solve a fundamental problem in particle physics – why there is so much more matter in the Universe than anti-matter.

In order to study neutrinos and how they behave, we need to first generate a beam of them. There are number of ways to do this using particle accelerators. The way which lets you best understand the neutrino beam  is to make a beam of muons, and to wait while the muons decay, producing neutrinos. A facility which generates neutrinos using this method would be known as a Neutrino Factory.

Muons themselves are quite easy to make if you have a particle accelerator handy. You start off with hydrogen gas and heat it up to a very high temperature. When the hydrogen gets hot enough, the electrons orbiting the hydrogen atom nuclei (which is just one proton) detach themselves, and you have protons and electrons moving freely in a state known as a plasma. Using electric fields, protons can be pulled out of this plasma and accelerated to high energy by more electric and magnetic fields. Once they are traveling fast enough, this proton beam can smashed into a solid target (we use titanium) which generates a shower of subatomic particles, including another type of unstable particle known as pions. Finally, pions in turn quickly decay into muons.

Sounds simple enough, I hear no one saying. In practice however there are a few more complications. The muon beam which comes from pion decay is very spread out and divergent. Imagine trying to direct the water from a hosepipe through a house window. Easy enough if the pressure is high, but what if you partly cover the nozzle with your finger and the water is spraying everywhere? That is a very rough picture of what muon beams are like when they are first created – it is very hard to use them effectively unless we can make the beam more like the water from a hosepipe without a finger on the nozzle – narrow and all travelling in the same direction.

The purpose MICE of is to show that we can take a beam of muons and then shrink this beam’s size and divergence (known as emittance) to get the sort of well behaved beam we would need to do neutrino science. Normally, when accelerator physicists want to shrink the size of a particle beam, they used magnetic fields to push the particles into a smaller configuration. Muons are more tricky however, because they decay so very quickly – by the time magnets would have shrunk the beam, all the muons would be gone!

MICE is testing a new way of shrinking emittance, called ionization cooling. The idea is to send the muons through a material (typically hydrogen or lithium hydride), causing the muons to lose energy as they interact with the material (knocking electrons off the atoms, ionizing them). After the muons have lost energy in this manner they can be given energy back, via radio frequency electric fields, but only in the direction of the beam. Thus the muons lose energy in all directions, but get back get it back only in the direction we want them to travel, causing the beam to shrink.

Ionization Cooling – muons lose energy in all directions in a low atomic number absorber. Energy is then restored only in the direction of the beam via radio frequency (RF) electric fields. This shrinks the beam emittance.

MICE is built, commissioned and presently taking data on the first part of this process, passing a muon beam through a low atomic number material.

As off August 2017, lithium hydride running is done, and we are preparing the experiment for liquid hydrogen running in September and October. Results will be published soon, so watch this space for more updates from the research frontier!

For more information on MICE, including our publications, and live data plots, see