Future Muon Sources - University of Huddersfield

Report
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Future
Muon
Sources
Professor Bob Cywinski
Dean of the Graduate School
Special Advisor (Research)
International Institute for Accelerator Applications
University of Huddersfield
Future Muon Sources, University of Huddersfield, 12/13 January 2015
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Future
(Surface) Muon
Sources
Professor Bob Cywinski
Dean of the Graduate School
Special Advisor (Research)
International Institute for Accelerator Applications
University of Huddersfield
Future Muon Sources, University of Huddersfield, 12/13 January 2015
Surface muons for MuSR spectroscopy
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Physics
Materials
Magnetism
Superconductivity
Surfaces
Fundamental physics
Polymers
Semiconductors
Hydrogen in
metals
Chemistry
Molecular dynamics
Oxides
Muonium
Biology
Proteins
Currently there are 300-400 muon beam users world-wide, with 255 signed-up
members of the International Society for MuSR Spectroscopy (ISMS)
Future Muon Sources, University of Huddersfield, 12/13 January 2015
Muon facilities world-wide
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TRIUMF
ISIS
PSI
JPARC
Continuous Beams
Pulsed (50Hz) Beams
Continuous Beams
Pulsed (25Hz) Beams
Future Muon Sources, University of Huddersfield, 12/13 January 2015
What do we need?
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At all existing facilities, muon production is a sub-optimal compromise determined through
consultation with other users of the proton drivers (symbiotic, parasitic, or complimentary?)
Question: What do muon beam users want ?
Answer:
Orders of magnitude more muon intensity
and smaller muon beam dimensions
Why?
At current positron count-rates (up to 40kHz) a typical spectrum from a typical sample
(of a few cm2) will take ~30min to collect with reasonable statistics
Parametric studies (as functions of temperature, magnetic field,
pressure and/or sample concentration) can take days
Studies of small (mm2) samples (eg single crystals) can take
even longer
Low energy muon studies of surface phenomena can take weeks
Future Muon Sources, University of Huddersfield, 12/13 January 2015
Designing the future
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The Accelerator
Linac, synchrotron, cyclotron, FFAG ?
Energy, current, frequency ?
Protons or other particles ?
Pion production target
Material (graphite, beryllium, nickel, composite..)
Geometry, volume, size
Beam conditioning
Collection geometry, beam optics, cryogenic cooling,
pulse shaping
Cost
Stand alone facilities or shared accelerator beams?
Future Muon Sources, University of Huddersfield, 12/13 January 2015
Pion production
H ig h
E n e rg y
P ro to n
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C a rb o n o r
B e ry lliu m
N u cle i
N e u trin o
P io n
 = 26 ns
4.1 MeV
+
+

M uon
p
S
S
p
S =0
Future Muon Sources, University of Huddersfield, 12/13 January 2015
Pion production
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Single pion production (threshold 280MeV)
Double pion production (threshold 600MeV)
Future Muon Sources, University of Huddersfield, 12/13 January 2015
Proton Energy ?
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Surprisingly our simulations show that
higher energy protons do not necessarily
produce more (surface) muons
A peak in muon production rate is
observed just below 500 MeV
Increasing energy produces more
pions in the forward direction and well
outside the momentum range likely to
be used by a decay beam
Future Muon Sources, University of Huddersfield, 12/13 January 2015
Proton Energy ?
Surface muon production normalised
to proton energy
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Surface muon production normalised
to number of interacting protons
Future Muon Sources, University of Huddersfield, 12/13 January 2015
Pulsed or CW operation?
Synchrotron operation at 50Hz (eg ISIS) is
inefficient for MuSR – it provides a measuring
window of 20ms whilst only 20µs (ie 10τm) is
needed (duty cycle =0.1%)
At a pulsed source the positron count rate
(~40kHz at ISIS) is limited only by detector
deadtime effects. Significant increases in
countrate can therefore be achieved by
increasing source intensity
Tw(ns)
Relative µSR asymmetry
The finite proton pulse width (eg 80ns at ISIS)
limits the dynamic response of a µSR
spectrometer at a pulsed source. There are no
such limitations in CW
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1.0
25
50
0.5
100
0.0
0
20
40
60
80
100
Transverse field, mT
Future Muon Sources, University of Huddersfield, 12/13 January 2015
Pulsed or CW operation?
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At a CW source only one muon can be
allowed in the sample at a time. Long time
beam-borne backgrounds are generally
significantly higher than at a pulsed source
(but can be reduced by the Muons on
Request technique)
Conventional muon spectrometers at PSI
and TRIUMF already count at 25-40KHz.
This is the maximum rate possible with
CW operation and is governed by the
muon lifetime.
However significant increases in muon
beam intensity are important for small
samples and LE muons
Muons on request (MORE) at PSI
Future Muon Sources, University of Huddersfield, 12/13 January 2015
Production targets
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Pion production targets should produce a high
yield of pions and muons
Pion production rates are approximately
independent of atomic number, although the
production of other particles (neutrons,
gammas) increases with Z. Low-Z materials
minimize proton scattering
Particle/target interactions should generate
little heat and targets should dissipate heat
easily
Monolithic targets are not necessarily the best
design – surface to volume ratio should be
maximised, whilst the target size should be
kept small
PHYSICAL REVIEW SPECIAL TOPICS - ACCELERATORS AND
BEAMS 17, 034701 (2014 – Bungau et al)
Future Muon Sources, University of Huddersfield, 12/13 January 2015
Possibilities
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Gains in muon beam intensities can be made at existing facilities by
improving target geometry and composition, and muon collection but:
(i) proton energies, repetition rate and current are fixed
(ii) the fraction of protons taken by the pion production
target, although negotiable, is also fixed (eg 4% at ISIS)
x10
The greatest gains necessitate construction of a fully stand-alone facility:
(i) Proton energy ~400-500MeV, current 0.5-1mA
(ii) Optimal target geometry, thickness and material
A dedicated proton driver unconstrained by parasitic uses of the proton beam will
enable precise tailoring of beam/target assemblies, allowing smaller proton/muon
beams, and more efficient pion/muon collection and will also facilitate the
implementation of multiple muon production targets.
x10
(Additionally an optimised pulsed muon source should have a repetition rate
approaching 10kHz and a pulse width of ~30ns)
Future Muon Sources, University of Huddersfield, 12/13 January 2015
http://vimeo.com/19475801
The way forward?
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ESS
5MW
Future Muon Sources, University of Huddersfield, 12/13 January 2015
The way forward?
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The foundation stone for the 1.7b€ European Spallation Source has
just been laid after more than 25 years of design, redesign and
political campaigning by 5000 European neutron beam users
There is much we can learn from the ESS (and SNS and J-Parc) campaigns
We need to build a strong science case, emphasising “impact” and the
roles that muons can play in the “Grand Challenges”
We need to engage the wider muon community (fundamental physics,
imaging, etc)
It may also be beneficial to engage with Fermilab and Brookhaven
National Laboratory. Both have recently held workshops which
have focussing upon muon production and MuSR facilities
Future Muon Sources, University of Huddersfield, 12/13 January 2015
Future Muon Sources 2015
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Session 1: Muon production and accelerator technologies
Session 2: Specialised beams
Session 3: Condensed matter µSR / New Techniques
Session 4: Update and outlook from the Facilities
Session 5: Novel applications of muons
Future Muon Sources, University of Huddersfield, 12/13 January 2015
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Future Muon Sources, University of Huddersfield, 12/13 January 2015

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