The 12th Symposium on Accelerator Science and Technology, Wako, Japan 1999

SECONDARY EMITTION MONITOR WITH ELECTRON MULTIPLIER
M KANAZAWA. K.ICHINOHE*, T.NAKAJIMA*, A.KOMIYAMA*, and S.SATO
National Institute of Radiological Sciences, Chiba, Japan*Accelerator Engineering Corporation, 2-13-1 Konakadai, Inage-ku, Chiba 263, Japan

Abstract
To monitor the spill shape, secondary electron monitor
was developed, where electrons are multiplied with anelectron multiplier. The output is further amplified with acurrent amplifier. Regarding experiences in the previous
R&D of the monitor, improved monitor in several points
was installed in the beam line. In this report the installed
monitor and its tested performances are presented.

1 INTRODUCTION
In the operation of the synchrotron, many type of beam
monitors are important to tune the machine. In the beamtuning of slow extraction, a beam spill monitor is important
to know the time structure of the extracted beam.
Monitoring this beam signal, we can see that the extracted
beam is good or not in its time-shape for an irradiation ofcancer treatment. When we have started the operation of theHIMAC synchrotron, a thin plastic scintillator was using.
Though the monitor could be used in the daily operation,
there are several points that are better to be improved. These
are followings:
a. Effect of multiple scattering is large in the case of
proton or low energy heavy ion beam.
b. The plastic scintillator has damage quickly with beamhitting, which coloured brown. This damage makes
position dependence of the beam, which produce
smaller pulse at the damaged place. To suppress thiseffect under a certain value, we must replace theplastic scintillator frequently.

2 MONITOR
As a source of secondary electron, a polypropylene filmof3J.UI1 thickness courted with 151J.g/cm2 aluminium is used.
In the prototype monitor for a beam test, aluminium foil witha thickness of 2J.UI1 and a surface area of 1Ox 1Ocm2 wasused. Though we could use this thin aluminium foil without
any broken, it required special attention to attach on amonitor flame. In the case of polypropylene, its handling iseasy. Concerning the effect of the multiple scattering, thepolypropylene film of 3J.liil thickness has less effect ofmultiple scattering. In figure 1, the mean square angles ofthe multiple scattering are shown, which are calculated withthe following formula;
62 = Z2(Es/pcl3it1Xo .
(1)where Es = 15MeV and tis the thickness of the scatterer, Xo
is one radiation length of the scatterer in glcm2 , and Z is thecharge, p(MeV/c) and cl3 are the momentum and velocity ofthe beam. In the case of the polypropylene the effect of themultiple scattering is smaller than the aluminium foil byfactor of two. With this thin film, effect of emittance growthis acceptable even in the case of low energy of lOOMeV/u.
Considering these two advantages of easy handling and small
multiple scattering, we have chosen to use the polypropylene
film.

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a. Monitor the beam spill shape on an oscilloscope.
b. Use for the feedback of the extracted beam to obtainflat beam spill.
c. Monitoring the beam intensity for machine operationand for an experiment.
In the design of this beam monitor system, above
requirements were considered to fulfil.

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u"iTo improve these situations, we have made an R&D of asecondary emission monitor. In this test monitor, secondary
electrons emitted from an aluminium foil are amplified with
an electron multiplier (selatron, EME-2061c)[1]. Afterseveral beam tests, we found that the monitor of this type
was possible to use for a machine tuning in the daily
operation, and a refined monitor was installed in the beamline for daily use. In the machine operation this monitor isused for the flowing different usage:

0. 2- ecinti llator]

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0.2..

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100200300<,..

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Figure 1: Comparison of multiple scattering angles of heavy
ion with e/m=0.5 between three cases of0.2mm scintillator,
2J.UI1 aluminium, and 31J.m polypropylene.
Yields of secondary electron were measured [2], and theratio of the electron emission ranges from 0.4 to 1 withcarbon beam. The energy range between 100 and 400
MeV/u. The beam intensity of carbon is 3x108 pps fortreatment in HIMAC, corresponding current of thesecondary electron is less than 48pA This current value is
too small to directly observe the spill shape in theoscilloscope even if a current amplifier with a high input-486-The 12th Symposium on Accelerator Science and Technology, Wako, Japan 1999

impedance is used. To observe the beam spill with theemitted secondary electron in an oscilloscope, the electrons
are amplified with an electron multiplier of aHAMAMATSU R5150-10. (After several experiences, wehave decided to use this electron multiplier instead of theselatron) An· input aperture of the electron multiplier isnarrow, the emitted secondary electrons are focussed on theinput aperture. For this purpose the polypropylene declined
45 degree to the beam line, and the electrodes are arranged
to .make desired electric field for focussing as shown inFigure 2.
The output current is further amplified with a current
amplifier. In this amplifier, there are four gain settings thatcan be selected with remote control. There are three
outputs, that are isolated each other.

downstream of the beam line. As seen in figure 5, measured
data show linear dependence on the beam intensity in thewide intensity range with some offset value. This linear
dependence is convenient to use as an intensity monitor. In
the case of the selatron as the electron multiplier, thedependence was not linear[3]. This is one reason why weuse the R5150-1 0 as the electron multiplier though we havestarted the test with the selatron.

10'

Q

10'

010'

0c

1000

00100..

~01010'

10'

10'

10°

Be ... intensity(ppp)

Figure 3: Output counts ofthe VFC vs beam intensity.
To use this monitor as an intensity monitor, dependence
on the beam position should be small. In figure 9, output
counts are shown as a function of the beam position. If weuse the selatron, the variation of the output was smallenough. In the case of the R5150-10, the variation of output
was large. This is the left problem that should be improved
to use as an intensity monitor.

Signal .·. current2000• • • ••

~015000(~0Figure 2: Monitor system.

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3 BEAM EXPERIMENTS
The monitor was installed at the downstream of the firstquadrupole magnet in the beam transport line from thesynchrotron. The profile monitor is just in front of thismonitor, which is used to know the beam position.
As a spill shape monitor, we can use with as low beam
intensity as 106 pps. In the observed spill signal, we canclearly see the ripple of 777Hz due to frequency modulationin RF-KO extraction.
To use as an intensity monitor, we have used a VFC(Voltage Frequency Converter). The output counts of thisconverter are proportional to the integrated output voltageof the current am~lifier. Changing the beam intensity widelyfrom 105 to 10 pps, output counts of the VFC aremeasured. In each measurement the beam intensities werecalibrated with parallel plate ionzation chamber that was atI 0•

Horizontal

VerticalI5000-10-5010

S.... poaition (IIIII)

Figure 4-1 : Position dependence of the monitor outputwith the selatron.

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The 12th Symposium on Accelerator Science and Technology, Wako, Japan 1999
7000

CY

0
5000
4000

Horizontal~ Vertical0.. • aII

3000
2000

4.5-Q.9I•0 0• 0 (•..

~ rooo .---t--~---=-+--l..l---1--~
l1500

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1000

=---;---~---+---4--~~~

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Beam poe it ion (mm)Figure 4-2: Position dependence of the monitor output withthe electron multiplier ofR5150-10.

To check the long-term stability, we have measured theoutput counts with same beam intensity. The monitor system
is used with same condition. In the first case where theoutput currents of the electron multiplier were around 0.5j.t.A
with the beam intensity for therapy. The output counts weredecreased to half value after one month (see Figure 5-l)which were similar between the selatron and the R5150-1
electron multiplier. In general the gain of the electron
multiplier will decrease if we increase the integrated output
charge of it. Considering this tendency, we have samemeasurement with lower gain setting of the electron
multiplier with order of2. In this case the gain of the current
amplifier is increased with order of two to have same output
voltage of the current amplifier. The obtained result isshown in figure 5-2. Though there were sudden drop of theoutpu_t, the gain was not decreased rapidly like the high gain
expenment.
Though there are several problems in this monitor, wehave started to use in the experiment to measured the beam
int~ity. During the experiment of short period, beam
posttton at the monitor will be same. In this condition, themonitor can be used as an intensity monitor with goodaccuracy.

0Time (h)Figure 5-2:Trend of monitor output with low gain of theelectron multiplier (R5150-l 0).

4 DISSCUSSION
The secondary electron monitor with the electron
multiplier can be used one year without serious problem.
Though there are gain drops at several times, we can use themonitor with increments of the high voltage of the electron
multiplier. In our experiences, the gain drop was occurred
when we broke the vacuum or peak beam intensity was high.
T~ere is beam position dependence of the output current
wtth the R5150-10, whose effect was not strong with theselatron of Murata. One possible reason is a groundelectrode in front of the first dynode of the R5150-10.
Removing this ground plate or its effect, we will test theposition dependence of the output current. Though there areseve~al problems that should be improved, we could use thismomtor about one year. From this fact, it will be possible touse this monitor with the electron monitor of the R5150-l 0
in daily operation and in the experiment where the beam
intensity must be monitored.

ACKNOWLEDG EMENTS
The authors are grateful to the operating crew ofHIMAC from AEC. They also express their thanks to themembers of Accelerator Physics and Engineering division atNIRS .

4. 5-1. 5.•cJ0..
0JREFERENCES

Q"J0[I] Technical note of the Mur~ta manufacturing Co. Ltd.
[2] M.Sudou et al., Proc. 9 Symp. On Accel. Sci. andTech., Tsukuba, (1993)351[3] S.SATO et al., Proc. llth Symp. On Accel. Sci. andTech., Hyogo, (1997)258
Figure 5-l:Trend of monitor output with high gain of theelectron multiplier (R5150-10).

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