The 9th Symposium on Accelerator Science and Technology, Tsukuba, Japan 1993

Concept of a Muon Profile Monitor with Scintillating Fibers
Yoshihiko Mizumachi and Takahiro Sato
KEK, National Laboratory for High Energy Physics
Oho, Tsukuba, Ibaraki 305, JapanI. INfRODUCTION

magnetic field B is not uniform. Hence, very stable anduniform magnetic field will be formed by the use of asuperconducting dipole magnet and high quality pole faces.
Design value of field uniformity is about 1 ppm after carefulshimming work. However, if there are some inhomogeneity
left, we must estimate the effective field seen by the beam not
to deteriorate the accuracy of the experiment. For that purposea muon profile monitor is very useful and it can even relax therequirement for the magnetic field uniformity.
One particular features of this storage ring is its very low
beam intensity, of the order of 2 x 1QA4 at most. Hence, thesignal level of the stored beam from a popular pickup electrodewill be of the same order as thermal noise. (Farley has
proposed to measure the moment of beam distribution withmany electrodes around the beam. In order to improve SINratio of each pickup electrode signal he also proposed tosynchronously sample the electrode signal and accumulate. His
method requires many revolutiotimes and does not give
instantaneous information about the beam distribution.) Somf
other high gain detection scheme is necessary to make realtime beam observation. In this respect, application oftechniques of high energy physics experiment will be usefulsince they often deal with the detection of a single particle, thelowest beam intensity. Scintillation counter is a goodexample. It consists of a scintillator and a high gain
photomultiplier (PMT). Beam distribution, or beam profile,can be measured with a set of a scintillating fiber array andphotomultipliers. The light yield from each fiber will be
proportional to the number of particle passing through it.

A muon storage ring of 3.094 GeV /c momentum is being:onstructed at BNL under the collaboration among US,
German, Russian and Japanese high energy physics
laboratories and universities. It will be used for themeasurement of muon anomalous magnetic moment.[!]Fig. I shows the structure of the storage ring. It consists of360° contimuous guiding magnetic dipole field and four
vertically focusing electrostatic quadrupoles and as a whole
it forms a weak-focusing system. There is no RF system init.
ESQ-1

ESQ-2

ESQ-3

II. SCINTILLATING FIBER PROFILE MONITOR

Fig.2 shows the concept of the fiber profile monitor. A
muon excites scintillation light in a fiber and the light isguided in the fiber up to a PMT cathode.

Fig.l Muon storage ring
In the past, muon g-2 experiment has been performed at CERN
three times and the final one reached the precision of 7 ppm.[2]New experiment aims at the precision of 0.35 ppm. In the g-2
experiment we detect the decay electron inside the ring. The yield
has a modulation frequency m a as the difference between thespin precession frequency m s and the cyclotron frequency m c ofcirculating polarized muon beam due to the parity violation.

~~r=M=a=g=n=e=t=P=ol=e====t:=::::):___ _ n~e-1!-o~:_I •("· ~!1...---==-------.J-(m a= m s- m c = (eB!mc)a"~WindowOpt.

Fiber

where a is the anomalous magnetic moment defined as a =(g - 2)/2 and g is the gyromagnetic ratio of a muon definedby the ratio of the magnetic moment J1 and the spins as J1 /s=(eh /4pmc)g.

Owing to the momentum and spacial spread of stored muon
beam, m a of the beam particle will have spread if theFig. 2 Schematic of a scintillating fiberprofile monitor

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8000

Output of the PMT's are put into ADCs and stored in amemory. Relative intensity of each fiber indicates the beam
distribution over the aperture of 90 mm diameter with avertical fiber array and a horizontal array. Even for a very
short life of the stored beam (64 JlS) it will be possible tomeasure the profile several times during a single storage andfollow the time evolution of the muon profile. Since thedetection efficiency of each channel is not uniform (due tothe variation of photomultiplier gain and opticaltransmission efficiency), calibration of sensitivity isnecessary. For this purpose, fiber array frame will berotated by right angles so that each fiber sees the common.
part of the beam.
The fiber array is expected as a diagnostic tool at thecommissioning of the ring. If the electrostatic quadrupole isturned off, the horizontal profile monitor at the 180°
position from the inflector will indicate the beam
momentum error. Further, another horizontal monitor atthe 270° position of the injection point is used to adjust thekicker strength in the case of muon injection.
In this fiber monitor scheme, the interaction between thefiber material and muon beam is of concern in two aspects.
One is the muon emittance growth due to multiple
scattering and the other is radiation damage of the fiber.
III. SIMULATION STUDY

Most serious problem in a fiber monitor is the destructive
effect of the fiber material on the stored beam due tomultiple scattering. Initial thought was that muon makes
only a few thousand revolutions during its life time and thescattering by very sparcely arranged material would not
make fatal effect. The scattering of the fiber array in thepresence of weak focusing is estimated by the simulation.
In the simulation, we used a random number generator andcreated particles which have gaussian distribution in almost
all of the variables as horizontal and vertical beam position,horizontal and vertical beam divergence and in momentum.
The effect of the fiber array is represented by the change ofangle of motion at the location of the fiber by the quantity
when a particle hit a fiber. Energy loss (or momentumchange) was not taken into account but but will benecessary to include in the future simulation. The averageof the angle change is given by the following semiempirical formula,

7000
6000

...
Q)

..c

E 5000:::sz~.~

4000

-...

as 3000

c..

2000
1000

rs
400

200

0600

1000

800Revolutions
Fig. 3 Decrease of muon due to multiple scattering byfibers. Natural muon decay is not included.
loss is small but the destructive effect is noticeable evenwith the thinnest fiber (0.3 mmdiameter) available. Hence,the profile measurement can be made in a test run only andthe fiber must be drawn back during physics experimentbecause the scattered particles make serious background forthe decay electron measurement.
Fig.4 shows the horizontal profile after 1000 revolutions
without a fiber and in the presence of horizontal and verticalfiber monitors of 0.3 mm fibers over 10 em range.

....(1)300.c
E 2so::lZ200,!!150

8= (13.6/{Jcp)(x/X) [1 + 0.038In(x/X)](,):;: 100....

where p and f3c are the momentum (in MeV/c) and thevelocity of muon and x!X is the thickness of the scattering
material in radiation lengths and x is the thickness of thescattering material. Scintillating fiber is mainly made ofpolystyrene, for which X is about 0.45 m. For the fiber
diameter 0.3 mm, 8 = 0.085 mrad. Scattering angle at eachpassage through a fiber is also given by random numbers. We
assume circular aperture limit of 45 mm radius. Fig. 3 shows
the decrease of muon number during the storage. Large
decrease of muon in the first few revolutions is due to thescraping by the aperture. In the absense of fibers, later beam

caa. 50
0-60-40-20

020

46(Horizonta l Position (mm)Fig.4 Simulation for the horizontal distribution of muons
after 1000 revolutions at the location of profile monitor-371-

Reliable result would be obtained in the early period of;torage while the scattering effect is not so much
1ccumulated. However, we see that a relative profile pattern is10t very much different from that for the original profile even1fter 1000 revolutions. This seems to exhibit the~ffectiveness of fiber profile monitor.

161412(I)'E::1IV. PRELIMINARY BEAM TEST OF A PHOTOMULTIPLIER

1000

Two kinds of scintilating fiber profile monitor
configuration has been considered. One is to use a single
multianode PMT and the other is to use independent sngle
PMTs. We made preliminary tests for both scheme in thesecondary beam line of KEK 12Ge V PS where the we havepion/muon beam flux of about 2000 particles in 1.5 sec
spill time. In the first test geometry, 1 mm diameter fiber
arrays are connected to a multianode photomultiplier,Hamamatsu H4139. Anode output voltage for a single
passage of 2 Ge V/c particle is about 40m V. The signal
level is sufficient but cross talk was observed among themultianode channels. This is not an adequate feature for
profile measurement. In the second scheme, 1 mm fibers areconnected to individual PMTs, Hamamatsu R657.

4Fig. 6 Pulse height distrbution of PMT signal

V. REFERENCES[1] "Muon g-2 Design Report AGS821" BNL, July 1992[2] J.Baily et al., Nucl.Phys. B 150, 1 (1979)

ISCINHIPMTGATE

pilmu beam

1.8 GEVIC

Fig.5 Schematic of the scintillating fiber beam test
Configuration of the signal detection is shown in Fig. 5.
Pulse height distribution of the output is shown in Fig. 6
From the mean value and the width of the distribution we canapproximately estimate the number of photoelectron yield at
the photocathode if normal distribution is assumed. It ranges
around three. Since the instantaneous beam intensity in themuon ring is much higher, scintillating fiber profile monitor
looks feasible.

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