Secondary Particle Production and Capture for Muon Accelerator Applications S.J. Brooks, RAL, Oxfordshire, UK ([e-mail])

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Secondary Particle Production and Capture for
Muon Accelerator ApplicationsS.J. Brooks, RAL,
Oxfordshire, UK (s.j.brooks_at_rl.ac.uk)
Transmission of positive muons to the end of the
phase rotator for various target materials. The
main results in this paper derive the optimum
proton driver energy for a target of tantalum,
mercury or copper, 1cm in radius with lengths
given by the table below. Targets are simulated
using MARS15 then the resulting particles are
tracked through the UKNF decay channel and phase
rotator using the code Muon1. This is done for
each energy and material, and the muons leaving
the channel with the desired energy of 18023 MeV
are counted in units of muons per proton.GeV on
target, which is actually proportional to the
muon yield rate for a fixed power.
Schematic of the current UK neutrino factory
design (under study). Our design includes
several unique features such as a solid,
rapidly-moving target, and split extraction of
pulses from the main proton synchrotron to
alleviate thermal shocks in the target. This
proton machine could be realised for instance via
staged upgrades of ISIS at RAL. The area of
interest in this poster and paper is the front
end highlighted in blue.
R109
Near Detector
RFQ (Radio Frequency Quadrupole)
LEBT (Low Energy Beam Transport)
H- Ion Source
FFAG II (8-20GeV)
Beam Chopper
FFAG III (20-50GeV)
180MeV DTL (Drift Tube Linac)
Achromat for removing beam halo
Muon Decay Ring (muons decay to neutrinos)
To Far Detector 1
FFAG I (2-8GeV)

• Two Stacked Proton Synchrotrons (full energy)
• 6GeV
• 78m mean radius
• Each operating at 25Hz, alternating for 50Hz
  total

To Far Detector 2
Target enclosed in 20Tesla superconducting
solenoid (produces pions from protons)
Proton Beam Dump
RF Phase Rotation Transforms longitudinal phase
space as shown in the diagram (right).
Solenoidal Decay Channel (in which pions decay to
muons)
Stripping Foil (H- to H/protons)

• Two Stacked Proton Synchrotrons (boosters)
• 1.2GeV
• 39m mean radius
• Both operating at 50Hz

Muon Linac to 2GeV (uses solenoids)
Muon Cooling Ring

• Proton bunches compressed to 1ns duration at
  extraction
• Mean power 5MW

• Pulsed power 16TW

The pion distribution does not change radically
enough with proton energy to affect the optimal
lattice. Comprehensive optimisations of all
parameters of the decay channel and phase rotator
were conducted on a distributed computing
network. Two independent runs, one starting with
secondary particles from a 2.2GeV proton beam and
the other from a 10GeV beam, each produced an
optimal lattice after several months. The
table above shows their yields and what happens
when they are run on each others beams in fact,
the lattices are almost identical, as shown by
the parameter graphs below, so not specialised
for pions coming from one energy of beam.
The current phase rotator captures one muon sign,
with the other falling between RF buckets. The
effect on the opposite sign can be seen in the
blue particles in the longitudinal phase space
diagram above. This means that when all the
muons are counted, as seen in the diagram below,
only a few of the negative muons are in the
correct energy band, but the target always
produces both signs anyway so there is no way of
getting rid of these.
Analysis of how beam loss mechanisms change as
proton energy varies. For higher primary beam
momentum, even the more forward-directed
secondary particles can have a transverse
momentum sufficient to be outside the channels
acceptance. Here we see the higher energies
experiencing losses further down the decay pipe
from smaller-angled particles.
Relative performance of a channel for capturing
negative particles. By changing the phases of
the RF systems by 180 negative particles will be
treated analogously to positive ones in the
optimised channel. The graph below shows the
difference in performance between the two.
Proportion of muons leaving the channel in the
correct energy band. This seems to be
functionally independent of the proton energy, so
although losses may redistribute in the early-mid
decay channel, by this stage they are simply
proportional to yield as proton energy varies,
thus the phase rotator decouples from the proton
energy issue.
Primary energy (heat) deposition in rod. Proton
beams deposit heat directly in the target as well
as by the particle reabsorption mechanism shown
in red (top figure). The graph above shows how
much power out of a 5MW beam would be converted
directly into heat in the target. This is one of
the driving factors of the solid target design so
it is fortunate that this optimum of minimum
heating (8GeV) coincides with the optimum of muon
capture in the phase rotator.