CA1067624A

CA1067624A - System for monitoring the position, intensity, uniformity and directivity of a beam of ionizing radiation

Info

Publication number: CA1067624A
Application number: CA245,763A
Authority: CA
Inventors: Rene Boux
Original assignee: CGR MEV SA
Current assignee: CGR MEV SA
Priority date: 1975-02-07
Filing date: 1976-02-06
Publication date: 1979-12-04
Also published as: DE2604672C2; JPS51102800A; GB1558601A; CH588086A5; DE2604672A1; FR2300414A2; FR2300414B2; NL7601175A; JPS5823704B2

Abstract

A SYSTEM FOR MONITORING THE POSITION, INTENSITY, UNIFORMITY AND DIRECTIVITY OF A BEAM OF IONIZING RADIATION.

Abstract of the Disclosure A monitoring system transparent to ionizing radiation, designed for measuring the position, intensity, uniformity and directivity of a radiation beam, this monitoring system comprising, in one embodiment, three superimposed ionization chambers respectively provided with three electrodes, one of which ionization chambers being equipped with an elec-trode whose area is smaller than the areas of the two other electrodes,such last areas being substantially equal to the cross-sectional area of the radiation beam. Circuits for processing the signals furnished by the different electrodes are associated with comparators and with a safety system making it possible to stop the beam source in the event that "threshold" values are exceeded, or to control said beam.

Description

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The present invention relates to an improvement in a monitorin~ system for monitoring the position, intensity, uniformity and directivity of an ionizing radiation beam, this monitoring sys-tem, which comprises ionization chambers of the kind described by the present applicant in Canadian Patent No. 995,827, delivered on August 24, 1976, to R. Boux, relates more particularly to the ion-collecting electrodes arranged in these ionization chambers, the new electrodes arrangement allowing to obtain same results with a more simple structure and more simple processing circuits than those corresponding to the above-mentioned patent.
According to the invention, a system for monitoring the position, intensity, uniformity and directivity of an ionizing ra-diation beam issued from a radiation source, comprising at least two ionization chambers each of said ionization chambers comprising one electrode, the electrode of one of said ionization chambers having an area less than the area of the electrode of the other ionization chamber, said area of said electrode of said other ionization chamber being substantially equal to the cross-sectional area of said radiation beam, said electrode of the other ioniza-tion chamber, at least, comprising a plurality of electrically con-ducting elements which are electrically insulating from one another, said eiectrodes being respectively associated with processing cir-cuits for processing electrical signals furnished by said electro-des, said processing circuits controlling a safety system and said safety system controlling said source of said radiation beam.
Preferred embodiments of the present invention will be hereinafter described with reference to the accompanying drawings, in which:
- Fig. 1 illustrates a monitoring device comprising two ionization chambers in accordance with the invention;

- Fig. 2 schematically illustrates two electrodes of the kind used in said two ionization chambers in accordance with the . . .

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invention;
- Fig. 3 illustrates two embodiments of the distribution of the radiation intensity along a diametral axis of said electro-des;
. - Fig. 4 schematically illustrates a comparator circuit associated with these electrodes;

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~ s 5, G and 7 schelncl~ically i~ lustrate three other embodiments Or a system in accordance with the invention ;
- Figs ~, 9 and lO respectively illustrate a diagram Or processing circuits and ~wo comparator circuits which make it possible to monitor the position~ intensity, unifor-mity and directivlty Or an ionizing radiation beam.
Fig. l illustrates a first example of two ionization chambers employed in a monitoring system in accordance with the present invention, These ionization chambers l and

2, respectively possess two circular electrodes Eol and Eo2 constituted, for example as Fig. 2 schematically illustrates, by a frame C carrying a sheet of polyethylene terephthalate known by the trade mark of "MYLAR", upon which there has been deposited by vaporization under vacuum, a thin metallic film which is transparent for the ionizing beam. The electrode E
has a diameter dl substantially equal to the diameter of the ionizing beam whilst that Eo2 has a diameter smaller than dl.
There areas, respectively SO1 and So2 are such that :

S ~ S
ol / o2 In operatio~, the ionizing fluxes ~l and ~2 respectively intersecting the electrodes Eol and Eo2 are proportional to the currents Iol and Io2 picked off by the electrodes E
and Eo2. If the flux is uniform, then the equation :
ol ~ (l) So2 corresponding to :

I Ol ~ _ A~ Io2 (2) o2 is satisfied.
It is to note that the electrodes located within these ionization chambers, having upper and lower metallised transpa-rent walls (not shown in the figures), are suitably biased bya substantially identical bias, which is a negative bias by ; respect with these walls.

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In Fig. 3, graph (a) represents the variation in the current Iol and therefore in the f lux ~ol~ along a diametral axis xx of the electrode Eol, for a uniform and properly centred radiation beam, whilst in said same figure the graph (b) illustrates the variation in the current Iol through said electrode Eol when the radiation beam is non-uniform (in the considered case the beam is denser at its centre than at its periphery). The currents Iol and Io2 are then no longer proportional to the areas S0l and So2 of the electrodes Eol and Eo2, and the equation (2) becomes :

Iol < o2ol _ Io2 (3) .., ' A circuit (Fig. 4) comprising two comparators Cp and Cpo2 enables the voltages V0l and Vo2 corresponding to the currents Iol and Io2 picked off at the electrodes E
and Eo2, to be compared, so that the safety system Ss is triggered if :

V0l ol - V > Vthreshold ( ) VthreshOld being a threshold voltage of given value, which depends upon the operating parameters or the characteristics of the irradiation device.
The ionization chambers 1, 2, which make it possible to control the intensity, uniformity and directivity of the radiation beam, can be associated with another ionization chamber (not shown in Fig. 1) equipped ~or example with probes or comprising a known type of split electrode enabling ` 30 the centering of the beam $o be monitored. In a second '! embodiment, the device in accordance with the invention comprises two ionization chambers respectively equipped with ~, .

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two circular split electrodes El and E2 shown schematically in Fi~. 5. Two insulating strips 3 and 4 (formed of "MYLAR"
not carrying the metallising) diametraLly split the electrode El into the elements ell and el2, and the electrode E2 into two other elements e21 and e22 (Fig. 5) these insulating strips 3 and 4 being arranged at 90 to one another. The electrode El has an area Sl substantially equal to the cross-sectional area of the ionizing beam, whilst the electrode E2 has a smaller area S2.
In operation, the elements ell, el2 and E21, e22 f .the electrodes El and E2 respectively pick up currents il~
and i21,.i22. The ionizing radiation fluxes ~1 and ~2 respect-~: ively intersecting the electrodes El and E2 are proportional ~o the currents Il = ill + il2 and I2 i21 22'picked off by the electrodes El and E2 If the flux is uniform, then the equation :
~, . S
~1 = 52 - ~2 (1 ) corresponding to :

:~ 20 Sl ~
- Il = ~~ S I2 (2') ~ :
is satisfied.
'. . In operation, if we consider the most unfavourable case of an non-uniform radiation beam having an eccentricity d by respect with the axis of the insulating strip 3 of the electrode El, such that :

d ~ 2 1_ . dl and d2 being the respective diameters of the electrodes El and E2, then a comparison of the currents Il and I2 which are pic~ed off, provides the following information :

l < 1 - ~ I2 (6) , 2 (7) i21 ~ i22 (8) The inequalities (6) and (8) then give rise to the operation of an alarm or safety system which stops the radia-tion beam.
In the case of an uniform beam which is eccentric, then the conditions :
ill = il2 ( 9 ) i21 = i22 tlO) Il ~ 51 I2 (11) ~ ~

are obtained. The inequality (11) causes the safety system to halt the emission of the radiation beam.
However, it should be pointed out that a centred, non-uniform beam, produces at the outputs of the monitoring circuits relationships identical to those (9), (10), (11), As in the former case, the inequality (11) produces operation of the safety system and consequent halting of emission of - 20 the radiation beam. In the embodiment which has just been described, the reliability of operation of the device is ;~
therefore assured, but no indication is given of the defect which has developed in the radiation beam:
In two other preferred embodiments (Figs 6 and 7), the defects which the beam has developed are indicated.
Fig. 6 schematically illustrates two electrodes E3 and E4 utilised in the system in accordance with the invention.
The electrode E3 comprises four elements e31, e32' e33 and e34 and the electrode E4 comprises a single element e4.
In operation, measurement of the current I3 = i31 + i32 + i33 + i34 makes it possible to monitor the ; flux ~ that is to say the radiation dose which is proportional : ~ ~ 6 1~6762~

to the current I3, this flux ~ likewise being monitored by the second ionization chamber furnishing a current I4 = 4 - I3 . Moreover -: \
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- centring of the beam is obtaineA if:
i31 - i32 = i33 -= i34 (12) - uniformity of the beam is satisfactory if:
I3 = k S I4 (13) k being a coefficient close to 1, taking account of the safety standards which are imposed.
If the radiation beam is a scanning beam, monitoring of the centring of the beam can be achieved by associating with the ionization chamber equipped for example with the electrode E3 which has four elements e31, e32, e33, e34, a centring device of the kind described by the present applicant in Canadian Patent Application No. 228,685 filed on June 6, 1975, which centring device may also be associated with two electrodes such as El and E2, each having two elemenbs, or better still with the two electrodes E5 and E6 of a monitoring device in accordance with the present invention as depicted in Fig. 7. This embodiment comprises three ionization chambers 5, 6 and 7. The electrodes E5 and E6 substantially have the same diameter as the radiation beam and each comprise two ele-ments eSl, e52 and e61, e62 respectively. The electrode E7 of smaller diameter, has only a single element, E7.
The elements e51, e52 of the electrode E5, those e61, ~; e62 of the electrode E6 and the element e7 of the electrode E7 res-pectively pick up the currents i51, i52; i61; i62 and i7-In the case of a non-uniform beam, this non-uniformity being due, for example, to the absence of a correcting filter or to some defect in the beam scan function, the relationships:
i7 ~ k(i51 ~ i52) (13) i7 > k(i61 ~ i62) (14) are obtained. These inequalities (13) and (14) lead to trigger a safety system which interrupts the operation of the radiation source, or controls the ionization beam.

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Fig. 8 ~chematically illustrates an embodiment of the circuits for processing the signals picked off by the 5' 6~ E7. Amplifiers A51, A52 produce voltages proportional to the current i51, i52. The operational amplifier As5 produces the voltage Vs5 proportional to the sum of the currents i51 + i52. The variable resistor R51 makes it possible to calibrate the measured value so that this sum represents the dose rate measured by the chamber 5, for a given ~cale.
The operational amplifier AD5 furnishes a voltage VD5 proportional to the difference of the currents i51 - iS2.
A variable resistor R52 makes it possible to compensate any slight dissymetry which might exist between the two elements e51 and e52 of the electrode E5. Similar elements have been indicated by corresponding references in the processing circuit belonging to the electrode E6. The amplifier Ao7 produces a voltage V7 proportional to the current i7 picked off by electrode E7, and therefore to the dose rate.
~ne voltages VS5, Vs6 and V7 are applied respectively to error-detection circuits comprising comparators Cpl, Cp2, Cp3, Cp4, Cp5, Cp6 as Fig. 9 shows. The resistors Rl to R7 make it possible to adjust the values of the signals applied to the comparators, to take account of mechanical inaccuracies in manufacture.
Since the area of the electrode E5 is larger than that of the electrode E7, the voltage Vs5 is higher than the voltage Vs7 in a ratio S7 namely the ratio of the areas S7 and S5 of the electrodes E7 and E5. The resistors Rl, R2 and R3 have resistances such that the voltages V7 satisfies the double inequality :
Rl / R2 + R3 Rl + R2 + R3 S5 ~ 7 < R1 + R2 + R3 55 ~;:

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If this double ineq~ality is not satisfied, the comparator Cpl or Cp2 (depending upon the imbalance) will produce a positive output voltage triggering the safety system . Ss, or a signal K.
; The comparators Cp3 and Cp4 trigger the system Ss ; if the dose rate exceeds a given value VMax. Whilst the comparators Cp5 and Cp6 trigger the safety system if there is any disagreement between the voltages Vs5 and Vs6 and therefore - between the dose rates measured by the electrodes E5 and E6.
In another embodiment shown in Fig. 10, the currents i51, i52, i61, i62, i7 respectively picked off by the elements of the electrodes E5, E6, E7 are applied to a system constituted by a multiplier M and an analogue-digital converter CA associa-ted with a co~puter Cl, this system carrying out checks on the aforesaid conditions and controlling a safety system Ss which ."~., .. makes it possible to stop the radiation beam source if these .: , conditions not met or control the radiation beam in order to obtain the desired characteristics.
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Claims

The embodiments of the invention in which an exclusive property or privilege is claimed, are defined as follows:

1. A system for monitoring the position, intensity, uni-formity and directivity of an ionizing radiation beam issued from a radiation source, comprising at least two ionization chambers each of said ionization chambers comprising one electrode, the elec-trode of one of said ionization chambers having an area less than the area of the electrode of the other ionization chamber, said area of said electrode of said other ionization chamber being subs-tantially equal to the cross-sectional area of said radiation beam, said electrode of the other ionization chamber, at least, comprising a plurality of electrically conducting elements which are electri-cally insulating from one another, said electrodes being respecti-vely associated with processing circuits for processing electrical signals furnished by said electrodes, said processing circuits con-trolling a safety system and said safety system controlling said source of said radiation beam.

2. A system as claimed in claim 1, wherein said electro-des each comprise two semi-circular elements separated from one another by an electrically insulating diametral strip, the insula-ting strip of the first electrode being disposed at right angles to that of said second electrode.

3. A system as claimed in claim 1, wherein said first electrode is split into four elements substantially of quadrantal shape and said second electrode being constituted by a single element.

4. A system as claimed in claim 1, said system comprising three ionization chambers, two of said chambers being respectively provided with electrodes having substantially the same size than the cross-sectional area of said radiation beam, said electrodes being respectively provided with two semi-circular elements sepa-rated from one another by an electrically insulating diametral strip, said insulating strips being disposed at right angles one from another, the third chamber having an electrode constituted by a single element, said third electrode having a smaller size than the size of electrodes of said two chambers.

5. A system as claimed in claim 1, wherein each of said electrodes is associated with a processing circuit for processing the signals furnished by said elements of said electrode, said pro-cessing circuits comprising:
- a comparator circuit making it possible, in respect of each electrode, to compare the sum and the difference of the signals furnished by said elements, with predetermined threshold values;
- a safety system making it possible to control the ra-diation source when the measured signals have values which differ from said threshold values.

6. A system as claimed in claim 4, wherein said electro-des of said two chambers are respectively associated with proces-sing circuits comprising:
- a comparator circuit making it possible, in respect of each electrode, to compare the sum and the difference of the si-gnals furnished by the elements of said electrodes, with predeter-mined threshold values;
- another comparator making it possible to simultaneously compare the signal furnished by the electrode of said third chamber with the sum of the signals furnished by the elements of both two chambers;
- a safety system making it possible to control the ra-diation source when the measured signals have values which differ from threshold values.