Muon Targets

1
Muon Targets

• J.S. Lord
• ISIS

2
The ideal target

• Gives a high yield of muons or pions
• Produces few unwanted particles
• electrons/positrons, neutrons, scattered protons,
  gamma rays
• Small source area
• to allow full intensity in a small sample
• phase space cannot be compressed!
• Generates little heat, and/or can dissipate any
  heat easily
• Low residual activity

3
Nuclear Physics

• Protons from beam hit protons (or neutrons) in a
  target nucleus
• Single pion production (strong nuclear
  interaction)
• p p ? p n ?
• Threshold 290 MeV, useful at 500-1000 MeV
• Double pion production
• p p ? p p ? ?-
• Threshold 600 MeV, useful at gt1000MeV
• Cross sections
• 4 of 800MeV beam interacts in 1cm C (not all
  giving pions)
• transmitted protons lose 4.5MeV

4
Nuclear Physics

• Subsequent decay of pions to muons by the weak
  interaction
• ? ? ? ?
• Pion lifetime 26ns
• Muon spin polarised (in rest frame of pion)
• Muon momentum 29MeV/c (energy 4.1MeV) in rest
  frame of pion
• Surface beam pions decay at rest in the target
  (always forward decay)
• Decay beam pions decay in flight in the beamline
  (forward or backward decay)

5
Nuclear Physics

• High energy electrons can also be used to create
  pions or muons
• e- ? ? ? ? ?-
• Yields less than for equivalent proton beam
• e- ? ? ? ? ?-
• Muons produced directly like this will not be
  polarised!

6
Target materials

• Pion production roughly independent of Z
• Other effects increase with Z
• Neutron production (spallation)
• Gamma production (bremsstrahlung)
• Beam scattering and energy loss
• Cooling
• ISIS 6kW nuclear interactions, 1kW beam losses
• High melting point and thermal conductivity
  preferred (C3800K, Be1560K, not Li454K)
• Low thermal expansion, high strength
• Higher density preferred for a compact bright
  source

7
Target location

• Primary target
• any size of target
• beam stop once protons slowed below threshold
• Intermediate target in extracted beam
• Transmitted beam re-focused onto another target
• Must keep beam loss and scatter low
• Target in re-circulating beam?
• Thin with low energy loss,
• Beam scraper in accelerator (MICE target at ISIS)

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8
Surface muon target

• Pions produced where proton beam intersects the
  target
• Wide range of momentum, all directions
• Stopping power stronger at low momentum
• Range of 100 MeV/c pion in C is 25 mm, 50MeV/c is
  2mm
• Stopped pion distribution roughly matches proton
  spot
• Range of 29MeV/c muon in C is 0.73 mm
• Typical beamline uses muons within 0.2 mm of
  surface
• Muon yield increases more slowly than thickness

9
Decay muon target

• Pions must escape the target
• 150MeV/c pion can escape through 80mm of carbon
• Yield will increase linearly with target mass in
  beam
• For small source
• edge-on view of thin target
• Upstream or downstream with well focused proton
  beam
• Downstream angles often preferred
• highest pion energies available
• contaminated with scattered protons etc
• need half-quadrupoles or septum magnets
• 90 angle good for positioning first quadrupole
  or solenoid collector
• large solid angle available

10
Muon Collection

• Quadrupoles
• ?0.03 sr on MuSR
• Solenoid elements
• ?0.135 sr on ?E4 (LEM)
• 1.0 sr on Dai Omega
• Target inside solenoid
• ?2? possible
• Need equally large convergence at instrument too
• or large sample spot

11
Pulsed Source Timing

• Muon pulse is convolution of
• Finite proton pulse width
• 26ns contribution from pion lifetime (surface
  muons)
• Spread of times of flight in beamline, may be
  comparable
• Much smaller effects in decay beam
• 3ns pion lifetime spread for 100MeV/c backward
  muons

12
Target designs

• Edge cooled plate

• Rotating wheel

Beam
Copper base plate
Cooling water
13
Example targets in use

• ISIS targets on rack (3-10mm)
• Target plates at 45 to beam
• Water cooled base plate
• Muons or pions extracted at 90 to proton beam

14
Example targets in use

• PSI target M (rotating, 5mm)

15
Example targets in use

• PSI target E (rotating, 40mm)
• Proton beam edge-on through material
• Muons extracted at 10 and 90
• Pions extracted at 5 and 175

protons
16
Example targets in use
Muons to instrument

• Dai-Omega target at KEK
• surface muon beam
• optimised for conical acceptance of the instrument

Protons
17
Example targets

• Edge cooled target for use at J-PARC
• Protons incident at 90
• Muons or pions extracted at about 60 and 120