Assembly and method for monitoring the lateral position of a beam of ionizing radiation

A transmission ion chamber assembly for detecting the lateral displacement of an ionizing beam has two ion chambers located one behind the other along the beam axis. Each chamber contains two electrode plates and a spacer which separates and electrically insulates the electrodes from each other. The electrodes of each chamber are inclined with respect to each other along a preselected measuring axis. The axes of both chambers are perpendicular to the beam axis and to each other. In operation, the collector electrodes furnish current signals which depend upon the position of the beam along the associated measuring axis. With orthogonal chambers, i.e. measuring axes, the beam position in a plane perpendicular to the beam axis can be determined and aligned, e.g. via beam steering coils surrounding the beam.

BACKGROUND OF THE INVENTION 
The invention relates to a device and a method for monitoring a beam of 
ionizing radiation and, more particularly, to a beam steering system 
including a transmission ion chamber assembly for detecting the lateral 
position of such a beam. 
In various applications, especially in radiotherapy, it is necessary to 
monitor the actual beam position and to correct any deviations from the 
intended beam position. To do this, a number of devices--called 
transmission ion chambers--have been developed. 
For example, Medical Physics 11 (1984), pages 105 to 128, section VI.B., 
discloses a multi-chamber construction of five parallel plates alternately 
carrying high voltage (polarizing) electrodes and ion-trapping 
(collecting) electrodes. Each collecting electrode is divided into four 
sectors such that four distinct laminar collecting volumes are designed. 
By summing and subtracting the current signals of specific sector pairs, 
the beam dose and position are measured. For correcting misalignments, the 
position signals are used to energize beam steering coils grouped around 
an electron beam. 
In European patent No. 40589, there is described a modified chamber 
assembly which, in relevant part, varies from the above mentioned chamber 
in that it uses only one collector electrode split up into eight segments. 
In all of these chamber arrangements, the beam position is determined by 
sensing the differences between signals derived from different beam areas. 
The chamber must therefore be struck by a large beam cross-section, i.e. 
placed downstream of all the beam widening (and weakening) elements. As a 
result, the signal and in particular the signal to noise ratio are weak 
and require a sophisticated signal processing system to obtain an 
acceptable sensitivity. Moreover, the chamber cannot be utilized in 
instances where the radiation field is built up by scanning rather than 
spreading the beam. Further, because the chamber assembly contains a 
multitude of electrodes and provides a plurality of signals, it is 
mechanically complicated and large numbers of electronic components are 
required to produce it. 
It is an object of this invention to provide a transmission ion chamber 
arrangement which generates strong signals with a relatively low noise 
level. 
It is another object of this invention to provide a versatile transmission 
ion chamber assembly, capable of handling different kinds of beams (for 
example, electron or x-ray beams) and beam diameters (for example, swept 
or diffused beams). 
It is a further object of this invention to provide a transmission ion 
chamber assembly which is simple and inexpensive to produce. 
It is yet another object of this invention to provide a mechanically and 
electronically simple system for monitoring and correcting the position of 
an ionizing beam. 
It is still another object of this invention to provide a simple and 
accurate method for measuring and correcting lateral misalignments of an 
ionizing beam. 
It is yet another object of this invention to improve upon known beam 
steering systems of this type. 
SUMMARY OF THE INVENTION 
According to one broad aspect of the invention, a transmission ion chamber 
assembly for detecting the lateral displacement of an incident beam of 
ionizing radiation includes two chambers through which the beam passes, 
one after the other. Each chamber has an upstream base wall, a downstream 
base wall and a sidewall spacing both base walls; and contains a pair of 
electrodes, a collector electrode and an high voltage (HV) electrode. Both 
electrodes are separated from each other by a distance which varies 
gradually along a chamber-specific measuring axis and, preferably, over 
the entire electrode extension along this axis. When both chambers are in 
place, their measuring axes form angles with the beam path as well as 
between each other. Means are provided to extract from the collector 
electrodes current signals in dependence upon the beam position along the 
associated measuring axis. 
The invention is based on the following effect. Each ion chamber contains 
gas which is ionized when hit by a beam of ionizing radiation. Gas 
electrons will migrate to the positive electrode, and the gas ions will be 
trapped at the negative electrode. The amplitude of the ion current, which 
is normally used to provide the signal, depends upon a variety of 
parameters, essentially the kind of ionizing radiation, the type of the 
filling gas, the gas pressure, the voltage drop between the electrodes and 
the irradiated gas volume. Since the full beam is intercepted by the 
electrodes, the cross-section of the irradiated gas volume corresponds to 
the beam cross-section and is therefore virtually constant. Thus, the 
current signal is basically a function of the distance between the 
electrodes and in practice, varies roughly linearly with it. Consequently, 
if, as here, the electrode distance varies along a certain axis, the 
signal amplitude reflects the beam position along this axis, so that by 
using two crossed chambers the beam position in a plane perpendicular to 
the beam axis may be detected. 
In a preferred embodiment, the distance between the collecting electrode 
and the HV electrode varies linearly across the entire length of these 
electrodes, and the measuring axes extend perpendicular to the beam axis, 
as well as to each other. 
One precondition for an accurate measurement is, as already mentioned, that 
the beam can strike the cell electrodes with its entire cross-section, 
regardless of the degree of its misalignment. This means that the 
electrodes are, preferably, at least twice as large as the beam 
cross-section. Yet, the chamber may still be more compact than 
conventional types, since it can be placed just downstream of the first 
beam-widening element. 
All parts of the chamber assembly according to the present invention can be 
easily built and assembled. In the simplest case, the two chambers contain 
electrode plates, i.e. substrates coated with conductive layers, and share 
one electrode plate. 
In cases where the monitored beam is produced by an electron beam, each 
current signal is typically processed such that it energizes a pair of 
steering coils located at opposite sides of the electron beam. To this 
end, there is, according to another aspect of the invention, provided a 
signal processing unit in which each current signal is first amplified in 
an amplifier and than subtracted from a reference signal in a comparator. 
The reference signal is furnished by a reference signal source; its value 
corresponds to the amplitude the amplified signal would have if the beam 
were centered. The comparator output signal, which is indicative of the 
beam misalignment along the associated measuring axis, is fed into a 
driver unit which controls the current through the pair of steering coils. 
According to a further aspect of the invention, there is provided a method 
for detecting the lateral displacement of a beam of ionizing radiation. 
The first step of this method is to direct the beam through two activated 
transmission ion chambers, each containing a collecting electrode and a 
high voltage electrode. The distance between both electrodes varies 
gradually along a chamber-specific measuring axes, and both axes are 
perpendicular to the beam axis as well as to each other. Then, current 
signals are derived from the collector electrodes. These signals 
correspond to the beam position along the associated measuring axis. This 
method could be expanded to correct beam misalignments by simply 
transforming each current signal into an error signal which is indicative 
of the beam displacement along the associated measuring axis, and by using 
this error signal to energize a pair of beam steering coils surrounding 
the beam. 
The foregoing and other objects, features and advantages of the invention 
will be apparent from the following more particular description of a 
preferred embodiment of the invention, as illustrated in the accompanying 
drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
For the sake of clarity, parts essentially known per se are depicted 
schematically. Throughout the drawings, like elements are designated with 
the same numerals. 
FIG. 1 shows a LINAC with a bending magnet 1 which projects an electron 
beam 2 through a window 3 along a beam axis 4. The beam, which is actually 
pulsed with a pulse length of several usec and a pulse repetition rate of 
a few milliseconds, has a diameter of about 1 millimeter and comprises 
electrons in the range of 10 MeV. After passing window 3, beam 2 hits a 
target 5 which produces an x-ray beam. This beam is shaped in a shielding 
block 6 and in a jaw system comprised of two pairs of opposite jaws 7, 8 
and 9. Between target 5 and shielding block 6 there is disposed a chamber 
assembly 10. 
Chamber assembly 10 comprises, as shown in FIGS. 2 and 3, three plates 11, 
12 and 13. These plates are spaced from each other by spacer rings 14, 15. 
The outer plates are tilted against the central plate so that their 
distance varies linearly along measuring axes 33 and 34, respectively. 
Both axis are, as can be seen from the figures, orthogonal to each other 
in a plane perpendicular to the beam axis 4. Outer plates 11, 13 are 
coated on their inner sides with conductive layers serving as collecting 
electrodes 16, 17, and the central plate 12 carries on each side a 
conductive layer each serving as a high voltage electrode 18, 19. 
Each part of chamber assembly 10 can be made of conventional material. The 
plates may consist of a polyester known under the trademark Capton and may 
have a thickness of several mil. The electrodes may be sputtered gold 
layers and may be circular. The cell defined by the Plates and spacer 
rings may be filled with air. Such an assembly has a very low 
self-absorption for gamma rays as well as electron beams. To prevent the 
polyester foils from being bent, i.e. the distances between corresponding 
electrodes from being altered, by atmospheric temperature and/or pressure 
changes, the cells communicate with the environment through (not shown) 
openings in the spacer rings. Any fluctuations in these parameters are 
electronically compensated. The means by which this is achieved is known 
to persons skilled in the art and has therefore not been shown. 
The chamber assembly is part of a feed-back system for correcting the beam 
position. In this system, which is outlined in FIG. 4, the HV electrodes 
18, 19 are connected to a high voltage source 20. The signals which are 
derived from the collecting electrodes 16, 17 and represent the amount of 
ions produced by one of the beam pulses, are amplified in amplifiers 21, 
22 and then subtracted from reference signals in comparators 23, 24. The 
reference signals are furnished by reference signal supplies 25 or 26. The 
value of the reference signals equals the amplitude the amplified signals 
would have in case the beam were aligned. They depend upon the actual beam 
intensity which is independently measured and communicated to the supplies 
25, 26. The difference between the amplified signal and the reference 
signal controls, via a driver stage 27, 28, the current in a pair of 
steering coils 29, 30, 31 and 32 which are located on opposite sides of 
the electron beam 2 before it is diverted in the bending magnet 1. This 
current creates at the beam a transverse magnetic field which exerts a 
correcting force perpendicular to the beam axis and the field direction. 
It is varied such that the difference signal disappears, i.e. the beam 
becomes aligned. 
Having thus described the invention with particular reference to a 
preferred form thereof, it will be obvious to those skilled in the art to 
which the invention pertains, after understanding the invention, that 
various changes and modifications may be made therein without departing 
from the spirit and scope of the invention as defined by the claims 
appended hereto. For example, an electron rather than an x-ray beam could 
be monitored. In this case, the incident beam is much more intensive but 
also spread over a larger solid angle so that the gas could, if necessary, 
easily be kept below its saturation region, for example by lowering the 
high voltage. Further, in some instances it is preferable to have the 
distance between corresponding electrodes vary according to a non-linear 
function or with a profile having an extremum at the ideal beam position. 
The latter does not require a reference signal and affords a simple 
measurement of the overall beam intensity. It is also possible to use the 
electrons rather than the ions of the ionized gas for generating the 
current signal.