The II th Symposium on Accelerator Science and Technology, Harima Science Garden City, I997

Parallel Plate Ionization Chamber for the Medical-use Heavy-ion Beams
Eriko URAKABE, Makoto INOUE, Yoshihisa IWAS HITA, Mitsutaka KANAZAWA*,Toshiyuki SHIRAI, Takashi SUGIMURA, Masahiro TADOKORO**, Masatsugu NISHI**,Akira NODA, Masami TORIKOSHI*, Koji NODA*,and Yuzo FUJITA***
Nuclear Science Research Facility, Insutitute for Chemical Research,Kyoto University, Gokanosho, Uji, Kyoto 611, JAPAN*National Institute of Radiological Sciences, 4-9-1, Anagawa,Inage, Chiba, Chiba 263, JAPAN**Power & Industrial Systems R&D Division, Hitachi Ltd.,7-1-l, Omika, Hitachi, Ibaragi 319-12, JAPAN***High Energy Accelerator Research Organization, Midori, Tanashi, Tokyo 188, JAPANAbstract..-----H.V.
rt;==~~ Cu"ent Signals

We have developed a Parallel .Plate Ionization
Chamber (PPIC) in order to measure the time structure of
the slow-extracted beam intensities. The series of tests
shows that the PPIC can be a useful beam monitor forthe time-structure measurements of beam intensity. We
also try to use PPIC as a profile monitor. The
preliminary results of this type of PPIC are described.

1Introduction

The radiation therapy using the ion beams has
merits of higher dose concentration. The spot scanning
method is considered to be outstanding to effectively use
this merit. We are planning to test this method at
HIMAC (Heavy Ion Medical accelerator in Chiba)[1].
Intensity of beams slowly extracted from asynchrotron, in general, varies in time mainly due to thecurrent ripple in the synchrotron magnets. The speed ofspot scanning can be controlled to realize the accurate
dose distribution. In this system, the intensity of theslow-extracted beams should be measured at the upstreamof the irradiation point in non-destructive way.
As a beam monitor for this purpose, a Parallel
Plate Ionization Chamber (PPIC) is chosen from lessdisturbance to the beam and its wide dynamic range. The
performance of the PPIC [2] for the intensity
measurement is investigated with C6+ beams extracted
from HIMAC. We have also developed such a PPIC as
can be used for profile monitor using the strip electrodes.
The preliminary test results of the profile monitor aredescribed in this paper.

2Experimental Setup

The PPIC is composed of three plane electrodes asshown in Fig. 1. These electrodes are made of 3 J.UTI-thickaluminized polypropylene film (270 Jlg/cm2 for thepolypropylene and 20 Jlg/cm2 for the aluminium). The
gap distance between these electrodes is adjusted with theFig. 1 The cross sectional view of the PPIC.
Preamplifier
xl,xlO,xlOO
SOQ Cable driver amplifier'l'UDIIIOD

Fig. 2 The trans-impedance amplifier system. It consistsof 10/100 MQ pre-amplifier and x 1, x 10, x 100 Cable
driver amplifier.

spacers.
In the set of experiments the C6+ (290 MeV/u)beams slow-extracted from synchrotron with RF-KO
method [3][4] at around 11/3 resonance are measured.
Corresponding to the repetition rate of the frequency
modulation of RF-KO field, we can see the beam ripple.
The frequency usually used for therapies is 777 Hz, andchanged up to 3 kHz to study the time response of PPIC
system.
Before the time structure measurement, wemeasured the plateau characteristics under the various-308-

The lith Symposium on Accelerator Science and Technology, Harima Science Garden City, 1997

beam intensities. The picked up signals from the PPIC
were integrated during 10 spills in order to reduce thestatistical errors. Because of the fluctuation of C6t beam
intensities, we normalized the output signals of the PPICby that from the reference monitor. We select theHIMAC ion chamber which is calibrated with Faraday
cup as the reference monitor.
The charges from the PPIC were converted into
voltage signals with current amplifiers and observed by adigital oscilloscope (HEWLETT PACKERD 54540A)
triggered by the. timing signal of beam extraction. The
trans-impedance amplifier system is shown in Fig. 2.
The pre-amplifier with high gain was designed for the fastcomponent measurement of the PPIC signals. The
absolute gain of the amplifier can be selected from 1 M,.
10 M, 100 M, and 1 GQ in order to realize a wide:
dynamic range of detector system. We compared PPIC
signals with those of the 0.2 mm-thick plastic
scintillation counter which has been used as a "Ripple
Monitor" at just after the extraction point to investigate
the time response of the PPIC.

for that with the intensity 1.7 x 106 pps.
We conclude that the dynamic range covered withthe PPIC is from the almost maximum intensity of C6+
beam at HIMAC (2 x 109 pps) to about 106 pps or
somewhat wider.

3.2 Time Structure Measurement
We have measured the time structure of the RFKO extracted beam intensity with PPIC. The time
structure of the C6t beams measured with PPIC (4.2 mm
gap distance) and Ripple Monitor is shown in Fig. 4.
The frequency modulation of RF-KO is 3 kHz, and we
can see the beam ripple of corresponding frequency.
Figure 5 shows the frequency spectra. Both detectors havethe peak at 3 kHz and its harmonics in the frequency
spectra. These components come from the repetition

3 PPIC Results

3.1 Detector Performance~The plateau curves of the PPIC for the C6t beams(1.3 x 109, 1.9 x 108, 1.7 x 107 and 1.7 x 106 pps) are:shown in Fig. 3. The charge signal from the PPIC
normalized by that of the reference monitor should be
independent of the variation of the beam intensity. ThePPIC (3 mm gap distance) needs applied voltage of 300
V to reach the plateau region for the beam with intensity
1.3 x 109 pps. On the other hand, only 50 Vis sufficient=·~~~~~~~~~~~~~~ ~
1.57

8.575

8.585

8.58'"'

O.S95

8.59

Time [s]Fig. 4 Time structure measured by the PPIC (4.2 mm)with the 758.8 Torr-air (upper) and the Ripple Monitor(lower). The beam intensity is 5.7 x 108 pps C6+ (290MeV/u) and the frequency modulation rate ofRF-KO field
is 3kHz.
-100~

10 r····+··l,-·+······1·······+····,.·!······+······!!······+·····+····-.1U

1 Wt···+·tr·+-····-lr·······i··-lit+····· ..,·,·:·i·'<T·Ie

IS::p.. 8.1
1118.10.01

1100
10Applied Voltage M

1000Fig. 3 The plateau curve of PPIC (3 mm gap distances)with 759.6 Torr-air for the beams of C6t (290 MeV/u).
The frequency modulation of RF-KO is 777 Hz. The
ordinate is the ratio between the output charge from thePPIC and the reference monitor.

01z8.01

37'Freqency [kHz]45s911:;:di~

....
=~

Fig. 5 Frequency spectra of the signals from the PPIC
(4.2 mm) with the 758.8 Torr-air (upper) and the Ripple
Monitor (lower). The repetition rate of the frequency
modulation ofRF-KO field is 3kHz.

--309-

The 11th Symposium on Accelerator Science and Technology, Harima Science Garden City, 1997

rate of the frequency modulation of the RF-KO field.
Because the plastic scintillation counter is fast enough torespond the beam change in kHz order, it is concluded
that our PPIC with air and its electric circuit achieve thekHz order time response.

14f-············+·················i··············~ 3 f-············+·················!···············

4 Construction of Multi-Strip PPICco.

g 2 1-··············+················ii···········We have developed the other type of PPIC that has
the multi-strip electrodes in order to measure the profileof heavy ion beams. Figure 6 show the new multi-strip
PPIC (MuSIC). The MuSIC has strip electrodes for thebeam proflle projected horizontal and vertical direction.
The Music also has plane type 60 x 120 mm electrodes
in order to measure the time structure of total beam
intensity. The multi-strip electrodes are aluminizedpolypropylene, the surface aluminium of which is cut
into many stripes with laser. Each strip width is 2 mm (minus 15 J.liD for cutting line width). These electrodes
are fixed on the glass-epoxy frame and precisely set on
the aluminum frame with guide pole and spacer.
Atmospheric-pressure air is chosen as the countergas in the MuSIC because the response of air is fastenough to catch up with the beam ripple. And 4 mm gap
distances is selected considering the SIN ratio and thevoltage to reach the plateau region.
--t·a

1 f-·············+·················!·······;;• -f------1----""'l""-1 La.............L...L...........a...i...L.L..........~....................................................................~-100
1030-30-20
Holizontal position [mm]Fig. 7. The preliminary results of C6+ (290 MeV/u)beam profile. The beam intensity is 4 x 108 pps. The
applied voltage of MuSIC is 500 V.
(Yokogawa DL1500L). The preliminary result of theprofile measurement is shown in Fig. 7. The ordinate isthe voltage of each strip signals, and the abscissa is theposition for projection direction.

Acknowledgement
The authors would like to express their gratitude
to Mr. M. Hori for the fruitful discussion about thecharacteristics of the PPIC. They appreciate the help
given by Mr. H. Tonguu, A. Morita and M. Shibata ofKyoto univ. They would also like to thank to Yokogawa
Electric Co., Minotos Co. and the crews of AEC for theexcellent supports to the present experiments. Theseexperiments have been carried out as a Research Projectwith Heavy Ions at NIRS-HIMAC.

ReferencesFig. 6 The newly developed multi-strip PPIC (MuSIC).

5 Result of MuSIC
The horizontally projected profile of C6+ (290MeV/u) beam is obtained from 30 strip electrodes. The
pre-amplifiers with a gain of 10 MQ convert current
signals the voltage signals. These signals aremultiplexed, and amplified by programmable gain
amplifier, then measured with a digital oscilloscope[1] Y. Hirao et al., " Heavy Ion Medical Accelerator
in Chiba -A Design Summary and Update-", NIRS-M89, HIMAC- 001, December 1992
[4]E. Urakabe et al, "Parallel Plate Ionization
Chamber for studying the Time Structure of Slow
Extracted Beam", Beam Science and Technology., NSRF,ICR, Kyoto univ.,vol. 2, 1997, P 23-29[3] M. Tomizawa et al., " Slow Beam Extraction at
TARN II", Nucl. Instr. and Meth. in Phys. Res. A326
(1993) 399-406[4] K. Noda et al., " Slow beam Extraction by aTransverse RF Field with AM and FM ", Nucl. Instr. andMeth. in Phys. Res. A374 (1996) 269-277-310-