JAERI-Conf 95-021

PA44
Development of a Beam Pulse Monitor for the JAERI A VF
Cyclotron
Susumu OKUMURA, Mitsuhiro FUKUDA, Ikuo ISHIBORI ,Takashi AGEMATSU, Watalu YOKOTA, Takayuki NARA,Yoshiteru NAKAMURA, and Kazuo ARAKAWA
Advanced Radiation Technology Center, Japan Atomic Energy Research Institute,Takasaki, Gunma, Japan
Abstract

Signlll

A beam pulse monitor has been developed for timing
information about pulsed beams produced by the JAERI AVF
cyclotron. This monitor provides fast timing signals forbeam pulses of proton and heavy ions using a micro-channel
plate, which detects secondary electrons and photons yieldedby interaction between the beam and a target. In addition tousing a wire target, we adopted a thin aluminum foil target forincreasing the amount of electrons and photons. The monitor has been tested in 45 MeV proton and 260 MeV neon
beams.

~~JrrZ~'r~

1. Introduction
The JAERI AVF cyclotron (K=llO) accelerates various
ion beams from proton to xenon in a wide range of energies
mainly for materials science experiments. The extracted
beams from the cyclotron are pulsed at the same frequency as
the RF of the cyclotron and thinned out at an interval(l~s-lms) by a beam chopping system for time-of-flight
experiments and time-resolving analyses.
The timing information on the beam pulse is needed fortuning parameters of the chopping system to a required con"dition of the beam pulse. Use of a trigger signal produced bydetection of the beam pulse allows higher time-resolution
experiments than that of a signal originated from the RF
because of fluctuation of phase difference between the beampulse and the RF. We have a plan in which the cyclotron
beam energy will be measured by a time-of-flight methodwith fast timing counters to calibrate an analyzing magnet.
For these purposes, a beam pulse monitor, detecting thebeam pulse with high time resolution and high sensitivity,
has been constructed and tested using 45 MeV proton and
260 MeV neon beams.
2. Beam Pulse Monitor
The beam pulse monitor provides a fast timing signalof the beam pulse by detecting secondary electrons and photons emitted from a target inserted into the beam 1l. A structure of the monitor is shown in Fig. 1.
The monitor allows installation to a small port 98 mm
in diameter and permits transmission of a beam 40 mm indiameter through the inside of the monitor. The monitor isdriven by a stepping motor to adjust the relative positions
between the targets and the beam.
Wire targets is widely used in beam diagnoses for theirconvenience, but it is difficult to provide enough secondary
electrons and photons produced by interaction between a2mm
50mm

Fig. 1 A beam pulse monitor with a hollow structure,
constructed from insulator rods and electrodes, has twotargets and a MCP. The targets consists of a foil and awire, stretched between a set of electrode away from eachother for independent use. The left figure indicates thatthe targets are arranged not to disturb a flow of electrons
from the wire target.

wire target and high-energy light ion beams. Therefore, afoil target, having a large interaction area with the beams,
has been adopted for detection of high-energy light ion
beams. A wire target was also mounted for detection of lowenergy heavy ion beams so as to minimize the degradation ofthe beam.
The foil target is a 3 mm thick aluminum-foil strip with5 mm width, and the wire target is a tungsten wire 0.3 mm indiameter. The foil and the wire targets are placed in front of
the MCP at a distance of 50 mm and 94 mm, respectively.
The foil and the wire targets ·are stretched in parallel with theMCP and at a right angle to the beam direction. They arepositioned away from each other in the beam direction so as
not to disturb a flow of the secondary electrons from the wiretarget. The foil target is tilted at an angle of 45 degrees to the- 150-

JAERI-Conf 95-021

beam axis for efficient detection of secondary electrons andphotons emitted from the surface of the foil.
Secondary electrons are collected into a micro-channel
plate (MCP, F4655-10, Hamamatsu photonics Ltd.) througha slit using an electrostatic field. The operational voltage ofthe MCP is -2.4 kV, a recommended maximum voltage.
The collection field for secondary electrons, biased up
to -6 kV relative to the entrance of the MCP, is produced withseven sets of electrodes. The electrodes are spaced at 20 mm
by insulator rods of Al2 0 3 • Each electrode is connected withneighbor ones by dividing resistors to form a uniform electrostatic field. A floating high-voltage power supply with
two outputs is used for adjustment of the collection field independently from the MCP voltage. One of the outputs
serves a potential of the MCP and the other serves a potential of the top of the electrode relative to the MCP.
The entrance face of the MCP, and the surfaces of thewire and the foil targets were evaporated with Csl for optimizing the emission of secondary electrons 2'.
The monitor was installed in a chamber at the end of thebeam line, where the pressure is around 10-4 Pa.
The output signal from the MCP was amplified by a fastpreamplifier (Ortec VT120) and fed into a constant fraction
discriminator (Ortec 935) to produce fast timing signals. A
time to amplitude converter (fAC) was started by the MCP
signal and stopped by the RF signal with a divided rate. The

output of the TAC was fed into a CAMAC analog to digitalconverter (ADC) and analyzed on a personal computer with adata taking system, "K-max".
3. Results and discussion
Time spectra for 45 MeV Wand 260 MeV 20 Ne 7+ beams
at a collection voltage of- 4 kV are shown in Fig. 2 and 3.
Each spectrum shows the time structure of the beam having
one or two peaks in this case. Difference between the peakwidths measured with each target indicates that the time resolution using the foil target is superior to the one using thewire target.
The expansion of the peak width for the wire target
suggests interference with the secondary electrons emitted
from the wire target. A flow of secondary electrons from thewire target may be disturbed and widen in time when passing
through the field near the foil target.
At a collection voltage of 0 V, the peaks of the spectra
appear at faster positions on time axis than the peaks of thespectra at collection voltages of more than a few kV. An
example of the peak shift is shown in Fig. 4.
The results of the test suggest that the main particles
collected into the MCP are photons at 0 V and electrons at -4
kV. Adjustment of the collection voltage allows the selection of the particles detected with the MCP.

r---"""!""---~--"""!""----..

800

400

r---"""!""---~--"""!""----..

Foil target

Foil target·················r-···············-r············ ··}'wfi'M"'

640

"S

=: : ::::::::::::::::J::::r:::::::::::::::r.............
1----j_1.0
8·: : :. :::.r::.: .r. ;1.::::. : :.2 DS.................t................. j..~...., 11

F'WHM -r-

480

5lDS

U,o ............. 1~3 DS ...... ,.............. -~,.0 .................

r................r. . . . . . .r. . . . . . . .

l200

Chaiinel

487.5;_Tim~"l~~ch~el ~Time. . . . . . . .r...!
. . . . . . .r'target·· . .Wire

400\.

.

...... 300'd, ....... FWHM ........... ~ ............. ;............. ..

uu ..

3.6 DS ---j5

8200

24100

450
200

250

Channel ~Time400

497.5

Cha'iinel~TimeFig. 3 Time spectra measured with the foil target andthe wire target at the collection voltage of - 4 kV for260 MeV 20 Ne7• beam. The values of FWHM wereevaluated on the assumption of the Gaussian distribution.

Fig. 2 Time spectra measured with the foil target andthe wire target at the collection voltage of- 4 kV for 45
MeV W beam. The values of FWHM were evaluated on
the assumption of the Gaussian distribution.

- 151-

JAERI-Conf 95-021

000=
6u-500
400

200
100

200260•ooChaHnei

400

Tinie~

Fig. 4 Time spectra measured with the foil target atcollection voltages of 0 and - 4 kV for 45 MeV H•
beam. An average of the time differences between thecorresponding peaks of them is 5.0 ns, which is inagreement with a calculated value of 5.6 ns, the difference between the traveling times of electrons and photons.

4. Conclusion
We have developed a beam pulse monitor which provides a fast timing signal for light and heavy ion pulsed
beams produced by the JAERI AVF cyclotron'using a MCP.
An aluminum foil and a tungsten wire can be used for producing electrons and photons originated from the beam pulse. In
this arrangement of the targets, the foil target is superior to
the wire one in time resolution.
References1) M. Kase, T. Kawama, T. Nakagawa, N. Inabe, I. Yokoyama, A. Goto andY. Yano, ''Development of a Beam
Phase Monitor with a Micro-Channel Plate for the RIKEN
Ring Cyclotron", Proc. 9th Symp. on Ace. Sci. andTech.,Tsukuba,Japan, 1993, pp. 474-476.
2) H. L. Seifert, D. J. Vieira, H. Wollnik and J. M. Wouters,"Increased Secondary Electron Yield from Thin Csl Coatings", Nucl. Instr. and Meth., A292, pp. 533-534, 1990.

- 152-