A BEAM PRO~LE MONITORING SYSTEMFOR THE BIO-MEDICAL IRRADIATION
Akira Ito, Cyclotron Unit
The Institute of Medical Science, The University of Tokyo
4-6-1 Shirokanedai, Minato-ku, Tokyo 108
ABSTRACT
A multi-wire ionization chamber system for monitoring the charged
particle beam profiles was designed and constructed to meet theThe system isrequirements for the bio-medical applications.
composed of the dual 16 channel multi-wire ionization chambers,
electrometer amplifiers, control logic, CRT display and theTheinterface to the computer through the CAMAC modules.
orthogonal beam profiles both in horizontal and in vertical canbe monitored, simultaneously with irradiating the bio-medical
samples.
Introduction
The high energy charged particle beams are recently used for thestudy of radiation biology and medical applications. However,the charged particle beam from the accelerator has, in general,
So, thespatial distributions of intensity.
inhomogeneous
spatial distribution of the beam must be monitored and controlled
to give the uniform dose distribution on the irradiated area.
Fbr most of the biological and medical irradiations, the required
sqmewhere around 1 Gy/min ( aboutrangesintensitybe~10 protons/em sec or 3 pA/cm~ ). This intensity level is far
beyond the capability of the multi-wire proportional countersystem, but it is still too weak for most of. the scintillation
devices and for the secondary emission scanner or the scanningwire technique. So, it is quite important to develop the beamprofile monitor system for this intensity range.(l)svstemMulti-wire chamber beam rofile monitor·. 1 shows the block diagram of the charged particle beamsystem using the multi-wire ionization
monitoring
profile16 channeldual
The system is composed of thechamber.
multi-wire chamber, electrometer amplifiers, local control logic
and CRT display or the CAMAC interface for the computer control.
The multi-wire chamber for both vertical and horizontal has only
the thickness of 2 em, so that it is easy to house it at any
place in the beam line. The central electrodes are made of gold
The 16 collecting
covered tungsten wires of 30 pm diameter.
electrodes and two guard electrodes were wired with 5 mm spacing,
providing the 8 x 8 em J, sensitive area. The chamber is designed
to integrate the charge collected in the central electrode rather
It isthan to count the pulse from the individual particle.
operated either as an ionization chamber filled with air or as aproportional chamber flowing the PR gas. The charge from eachwire is integrated in the feedback capacitance by a low offset
The integration
current ( 0.1 pA) IC operational amplifier.
time interval is remotely controlled. The output voltage fromeach current integrator is then fed to .the sample and hold IC.
The 16 channel output signals are multi-plexed continuously,generating the stable beam profiles on the CRT display in theThese procedures can also be controlled by thecontrol area.
CAMAC modules under computer program.

-157-( Experimental Area )( Control Room )Horizontal

2ch CRT

NIM
16ch
\Amp

Local
Controlr;:::=~ Logic

\Hv

tria.

~]

\mp Verticald~

orCAMACBias (0...·3kV)

Fig.l Block diagram of the beam profile monitoring system
Beam profiles for the bio-medical irradiation
The performance of the system was first tested with the Co-60
gamma ray field to check the uniformity of the individual chambercell. Then, the system was tested with the 52 MeV proton beamfrom the INS FM cyclotron. The proton beam was extracted in air
after shaping the beam spot to 2.5 x 2.5 em;:. by a slit system.
Fig. 2 shows an example of the proton beam profile. The upper
curve shows the vertical profile and the lower represents thehorizontal one.
In this case, the chamber was operated as anionization chamber filled with air. Fig. 3 shows an example ofthe vertical beam profile of the pion beam ( 150 MeV/c ) at then-M channel of KEK. The beam intensity was so weak ( about 10pions/burst ) , that the chamber was operated as a proportionalchamber.

Fig.2 Profiles of 52 MeV proton Fig.3 Vertical profile of 1(-beam
Reference(1) Cuperus,J and Morgado,R., A MULTI-WIRE CHAMBER SYSTEM FORHEAVY ION BEAM MONITORING AT THE BEVALAC, IEEE Trans. on Nucl.
Sci., Vol.NS-22, No.3, pp.l561-1563, 1975.
-158-