The 14th Symposium on Accelerator Science and Technology, Tsukuba, Japan, November 2003

THE PRISM PROJECT
M. Yoshida for the PRISM working group,Department of Physics, Graduate School of Science, Osaka University
Abstract
PRISM (Phase Rotated Intense Slow Muon source) is a
project to produce a pure and high brightness muon beam
at low energy. PRISM aims primarily at searches for
lepton-flavour-violating processes, such as µ-e conversion
in a muonic atom. Energy spread of the PRISM muon
beam can be narrowed by phase rotation at a FFAG(Fixed Field Alternating Gradient synchrotron) ring. The
PRISM-FFAG ring with RF (Radio Frequency) cavities
of a high electric field gradient, such as of exceeding
200kV/m, is being constructed at Osaka University. The
construction of the PRISM-FFAG phase rotator will becompleted by JFY2007. And by using it, phase rotation ofa muon beam will be experimentally demonstrated.

extracted from the 50-GeV PS in about 10nsec time width
are injected to a target. The target would be made of, suchas, graphite or other materials. Muons produced frompion decays are injected to a phase rotator, PRISM-FFAG,to obtain a high quality muon beam.
PRODUCTION TARGET
PRIMARY PROTON

MATCHING
SECTION SOLENOID

PROTON BEAM DUMP

CAPTURESOLENOIDINTRODUCTION
The evidence of neutrino oscillation is strongly
indicated by the Super-Kamiokande and many otherexperiments. Although the phenomenon suggests newphysics beyond the standard model of particle physics,
naive extension with the modified massive neutrino sector
would accommodate neutrino mixing and oscillation.
However, the observation of lepton flavour violation
processes in charged leptons would definitely indicatenew physics, such as supersymmetric (SUSY) models orextra-dimension models. Lepton flavour violation has notbeen observed yet. It is considered that the muon systemis the best to search for lepton flavour violation.
A µ−e conversion phenomenon in a muonic atom is oneof the lepton flavour violating processes with muons. An
electron with energy of 105MeV, which is emitted in a
substance as a result of the conversion, is experimentallysearched. A negative charged muon beam of low energy
with narrow energy spread is required for the search toimprove the electron energy resolution.
In order to obtain such a muon beam, the constructionof PRISM [1] (Phase Rotated Intense Slow Muon source)is planned. It aims at mainly searches for lepton flavour
violation by using a high quality muon beam. In PRISM,muon momentum is arranged to 68 MeV/c (±3%) byphase rotation. Five or more order of high sensitivity thanthe present experimental limit [2] on the µ−e conversion
is expected in PRISM project.

PRISM OVERVIEW
Phase rotation is performed to produce a muon beamwith narrow energy spread. In order to perform phaserotation, PRISM consists of a Fixed Field Alternating
Gradient synchrotron (FFAG) ring with high gradient RF(Radio Frequency) cavities, of exceeding 200kV/m.
It is proposed to construct PRISM with the J-PARC
50GeV proton synchrotron (50-GeV PS) [2]. Protons

INJECTION SYSTEM

5M

FFAG RING
RF CAVITYEJECTION SYSTEM

Fig.1: Schematic layout of PRISM
A schematic layout is shown in Fig. 1. As seen in Fig.1,
PRISM consists ofa pulsed proton beam (to produce a short pion pulsedbeam),a pion capture system,a pion decay and muon transport system (in a longsolenoid magnet of about 10 m long), anda phase rotation system.
In the pion-capture system, the pions produced
backward from the target are collected with the solenoid
magnets of about 10 Teslas. Pions decay to muons in the
succeeding decay region, and muons are transported to
the PRISM-FFAG to make phase rotation. A muon beam
after phase rotation is extracted for experiments. The
characteristics of the muon beam obtained by PRISM areas follows.
• Muon beam intensity: 1011−1012/sec (Proton beam
intensity of 1014/sec is assumed)• Central momentum: 68 MeV/c.
• Momentum spread: ±3% (after phase rotation)

The 14th Symposium on Accelerator Science and Technology, Tsukuba, Japan, November 2003

Figure 2: Horizontal phase space of the PRISM-FFAG by Monte-Carlo simulation. The left-hand upper figure shows an
initial phase space at an injection, and the right-hand lower shows the one after the fifth turn. Phase space is plotted ateach turns only for muons which survive in the fifth turn. Muon was injected with the momentum of 54.4MeV/c,
61.2MeV/c, 68.0MeV/, 74.8MeV/, and 81.6MeV/c, and with a time spread of ±5 nsec.

PION CAPTURE SYSTEMFFAG AS A PHASE ROTATOR

Since it is necessary to stop muons in target material ina µ−e conversion experiment, PRISM aims to produce alow energy muon beam. Therefore, it is necessary tochoose low energy pions generated at the proton beamtarget. The 50GeV protons from the J-PARC accelerator
are considered to be used in PRISM.
The expected-pion momentum distribution wassimulated using MARS and GEANT3.21. The transverse
momentum distribution of the pion generated at the targetof graphite has a peak around 100 MeV/c. Although thetotal momentum of the generated forward in the beamaxis is distributed from 200 MeV/c up to over 400 MeV/c,
the backward pions has a peak at 120 MeV/c and has
smaller amount of a high energy tail, which may cause abackground in a µ−e conversion experiment. Therefore,
the backward pions are collected for PRISM.
In order to capture pions as many as possible at thetarget, a solenoid magnet with large solid angle would beused. In order to optimize the magnitudes of a solenoid
magnetic field, the production and capture of pions at thetarget are simulated using GEANT3.21. It turns out thatthe more muons can be extracted from the pion decay andmuon transport system, if a solenoid magnetic field islarger. It is expected that 0.2 pions can be captured per
proton with a configuration of magnetic field of 6T and
the inner bore radius of 10cm of a magnet.
In order to obtain a higher magnetic field, it isexamined whether a superconductive solenoid magnet canbe used in PRISM. Extensive studies of the pion capturesystem are underway.

In a phase rotation, slower particles are accelerated andfaster particles are decelerated by high gradient RF
electric fields. Assuming the initial muon beam has amomentum spread of 68MeV/c±30% and a time spread of±5ns for each momentum region, the final momentum
spread of muon beam can be reduced to less than ±3% byphase rotation.
An option to use a linear accelerator has been
considered to be a phase rotator. However, since themomentum spread of the incidence muons is large, a
relatively low frequency RF, ranging 1−10MHz, must beused. In this case, a total length of a linear accelerator isexpected to be 150m long beam line.
On the other hand, when a circular accelerator is used,
number of magnets can be reduced and a total cost can be
kept down. Recently, FFAG has been demonstrated
experimentally to accelerate protons at KEK. It is also
suitable for the PRISM phase rotation ring, since (1) it has
synchrotron oscillation, which is indispensable to phaserotation, and (2) it has large transverse acceptance andmomentum acceptance to allow high intensity.
The current design of the PRISM-FFAG ring is shownin Fig.1. A radial type FFAG with eight D-F-D triplet
magnets is considered. The diameter of the ring is about
10m. Two straight sections are used for injection andextraction systems. A total of 12 RF cavities are installed
to the remaining straight sections.
In order to perform phase rotation efficiently, the highgradient electric field, exceeding 200kV/m, is required on
RF. An RF wave shape has been studied with simulations.
It shows that a saw-tooth shape could only yields
narrower energy spread than a sinusoidal shape does.

The 14th Symposium on Accelerator Science and Technology, Tsukuba, Japan, November 2003

Then, a saw-tooth shape RF of 5 MHz will be made withthe combination of different RFs of higher harmonics
frequencies of sinusoidal-shape.
A detailed design work to optimize the PRISM-FFAGparameters is now going. Experimental demonstration ofhigh gradient RF and the PRISM-FFAG phase-rotator
would be planned in Research Center for Nuclear Physics(RCNP) in Osaka University in a five-year program until
JFY2007. The current parameters of PRISM-FFAG areshown in Table 1.

out and the surviving rate is about 10-18. The result of thesimulation shows that PRISM-FFAG has a very large
horizontal acceptance of more than about 10,000π mm
mrad and a vertical acceptance of more than about 2000π
mm mrad. In this simulation, a smaller gap at high-energy
side of the magnet limits the vertical acceptance. This
problem can be solved with a flat yoke of the FFAG ring.
A flat-gap FFAG magnet system with trim coils is being
designed for PRISM, and tested by JFY2004.

Table 1: Parameters of PRISM-FFAGA neutrino factory [4] is considered as futureapplication of PRISM. Since a FFAG ring has large
acceptance, muons can be accelerated, without muon
ionization cooling, up to 10 GeV or more. At a neutrino
factory, neutrinos are produced from the muons decaying
in a muon accumulation ring. If muons of high energy
decay in the straight section of the ring, neutrinos areboosted to the forward direction, yielding an extremely
high intensity neutrino source. At a neutrino factory, thediscovery of new phenomena such as CP violating in theneutrino sector is expected, as well as determination of
the neutrino mass hierarchy.

FUTURE PROSPECT WITH PRISM
ParametersValues

Central Energy

20MeV

Number of Sector

8k value

5Transition Gamma

2.45

Average Radius

5m

B@F/B

0.435/0.0717 T

F/2 angle

0.037 radD angle0.031 radCONCLUSION

F/2 bend angle

26 degree

Packing f

0.18

Tune(H/V)

3.404/2.641

The PRISM project aims to construct a high intensity
muon source by using phase rotation. The muon beamfrom PRISM is expected to have low energy of about 68
MeV/c with a narrow spread, such as ±3%.
The Letters of Intent for the PRISM project have been
submitted to J-PARC. From the review committee, strong
encouragement with high grade (A+) of the scientificevaluation on PRISM has been obtained. We further are
strongly requesting the early realization of theexperimental search for µ−e conversion with PRISM.
Regarding the PRISM-FFAG ring, high gradient RF
cavities and the FFAG ring are being constructed. Phaserotation of muon beam will be demonstrated in our 5-year
program by JFY2007.

To study that the PRISM-FFAG ring has sufficiently
large acceptance, Monte-Carlo simulation was performed.
GEANT3.21 was used for tracking to take into accountmuon decays, scattering in iron yoke, and so on. The
injected muons are assumed to have a time spread of ±5ns
at each momentum, which was predicted by thesimulation for the pion decay and muon transport system.
A FFAG magnet produces a magnetic field B at radius R
as
B = B0 ( R / R0 ) k(1), where R0 is the radius of the central momentum and B0 is
the magnitude of magnetic field at R0, and k is the fieldindex. A 3-dimensional magnetic field of the FFAGmagnets was calculated by using TOSCA and applied tothe simulation code.
Figure 2 shows the result of the simulation. It is seen inFig.2 that the closed orbits of muons with five different
momenta are located in five different radii, where thelower momentum tracks are at smaller radius. The muons
injected with various momenta come into an orbit of thecentral momentum as phase rotation goes. After 5 turns,
all muons come to the central momentum and the orbit. Itis noted that most of pion contamination in a beam decay

REFERENCES[1] “The PRISM Project −A Muon Source of theWorld-Highest Brightness by Phase Rotation−“,Letter of Intent to the J-PARC 50GeV Proton
Synchrotron Experiments, (LOI-24).
[2] “Results of the SINDRUM II Experiment”, P.Wintz,
Proceedings of the First International Symposium on
Lepton and Baryon Number Violation (LeptonBaryon 98).
[3] The Joint Project Team of JAERI and KEK, KEKReport 99-44, JAERI Tech. 99-059[4] “A Study of a Neutrino Factory in Japan (Nufact-J) “,
Letter of Intent to the J-PARC 50GeV Proton
Synchrotron Experiments, (LOI-27).