Note: Descriptions are shown in the official language in which they were submitted.
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The present application is a division o~ Canadian
patent application number 300,237 filed March 31, 1978.
This invention relates to a X-ray irradiation head
for panoramic irradiation.
The present invention concerns a X-ray irradiation
head for panoramic irradiation using a beam of charged and
accelerated particles. The irradiation head has a longitudinal
axis along which the particles propagate in the absence of
deflections. The head comprises an electromagnetic deflection
device for deflecting the charged particles and a target
having a surface producing X-rays. This surface is a surace
of revQlution the axis of which coincides with the axis of
the irradiation head. The deflection device separates the
paths of the particles fEom the axis in such a way that the
particles impinge upon the target. The deflection device
comprises a multipolar magnetic device, and a feed circuit,
to deflect the beam of charged particles in its entirety and
to make it converge on the target in a small impinging zone.
The magnetic device is fed by a feed circuit such that the
magnetic field rotates about the axis of the irradiation head,
the mean defl~ction of the deflected beam being variable and
the impinging zone moving correlatively on the target. The
multlpolar magnetic device is a quadripolar magnetic device.
The windings of pole pieces of the quadripolar magnetic
device are fed in pairs and in series by two sinusoidal
currents in quadrature delivered by the feed curcuit for
periodic scanning of the target by the beam of particles.
For a better understanding of the invention and
to show how the same may be carried into effect, reference will
be made to the following description and the attached drawings
among which:
- Fig. 1 shows the emission lobes corresponding to
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a conventional target (PRIOR ART);
- Fig. 2 shows the emission lobes corresponding to
a target according to the invention;
- Fig. 3 shows a third embodiment of the irradiation
head according to the invéntion, the panoramic irradiation
being effected by scanning of the target by the beam of
accelerated electrons.
The emission of high-energy X-rays by means of a
linear accelerator is obtained in the following manner :
electrons are supplied by a thermoemissive cathode under an
electrical field of from 30 to 50 Kv for example. These
electrons are then accelerated in an accelerating section of
the UHF type (a few thousand Mhz) by hign-power pulses (a few
megawatts) lasting several microseconds and striking the
target which is generally a tungsten pellet. When an electron
strikes the target, it is suddenly decelerated, giving rise
to the emission of an electromagnetic radiation. In addition,
electrons of the beam cause the ionisation of certain atoms
of the target and the migrations of the electrons on the various
layers of the atoms cause photons to be emitted. The intensity
of the radiation depends upon the heating voltage of the cathode
and the directivity of the radiation emitted depends upon
the energy of the X-rays emitted. In Fig. 1 which corresponds
to the prior art, a tungsten target 1, under the impact of the
beam of electrons, emits in a radiation lobe 2 with electrons
having an energy of 2 MeV, in a radiation lobe 3 with electrons
having an energy of 5 MeV and in a radiation lobe 4 with
electrons having an energy of 10MeV ; the narrowest lobe 4
corresponds to a more directional radiation.
Fig. 2, schematically slows an embodiment in
accordance with the invention. A tungsten target 5 frustoconical
in shape emits a radiation lobe 6 under the impact of the
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deflected beam of electrons e . The deflection system for
the beam of electrons is such that the impact zone describes
on the target a circle centred on the axis of the non-deflected
beam and the end of the radiation lobe also describes a circle
of radius R in the plane orthogonal to the axis of the accel-
erated beam. Another position of the radiation lobe is shown
in dotted lines in the drawing.
The irradiation head according to the present
invention and shown in Fig. 3 enables the entire beam of
electrons to be deflected so that it strikes the target on an
impact zone of small surface area. This impact zone is
capable of being displaced on the target either in steps, in
which case images of each of the points of the object to be
analysed are obtained, or continuously, in which case the
impact zone scans the target at a rhythm determined by the
frequencies of the currents applied to the deflection coils:
The cover of the irradiation head 20 shown in Fig. 3
comprises a vacuum envelope 21 in which travels a beam 22 of
accelerated electrons. The tungsten target 25 is frustoconical
in shape. In this embodiment, the accelerated beam of
electrons is deflected in its entirety relative to the axis
of the incident beam of accelerated electrons and is made to
rotate about that axis by means of a quadripolar magnetic
~device 40 of which the wiring diagram is shown in the part of
the drawing showing a section along X through the irradiation
head shown in the first part of the drawing. A cuxrent Il
delivered by a current source flows through the windings of
the poles 41 and 42 between the points 1 and 2 whilst a current
I2 delivered by the current source flows through the windings
of the poles 43 and 44 between the points 3 and 4. The
currents Il and I2 are sinusoidal currents of pulsation
phase-shlfted by 2
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The poles 1 and 2 create an instan.taneous induction
Bx = BOX sin ~ t along the axis X, soX being the ~aximum
amplitude of the induction along the axis X.
Under the effect of this induction, the beam of
accelerated electrons undergoes a deflection x alo~g the
axis X : x = k Box sin ~ t.
The poles 3 and 4 create an instantaneous induction
By = Boy cos ~ t along the axis Y, boy being the maximum
amplitude of the induction along the axis Y. Under the
effect of this induction, the beam of accelerated electrons
undergoes a deflection ~ along the axis Oy of amplitude
y = k ~oy cos ~t. k is a constant which depends upon the
distance travelled by the beam after having left the in.teraction
space of the magnetic field up to the target. The centre of
the impact zone of the beam on the target in a plane orthogonal
to the axis of the irradiation head describes a circle
if the components of the induction along X and Y, BOX and
Boy, are equal or an ellipse i these components are different~
Accordingly, alternating currents of the type in
question make it possible for example for the impact zone to
describe a circle on the.tungsten target, in which case the
axis of the radiatlon lobe generates a frustum With the same
arrangement, i~ the currents Il and I2 are adjusted to
constant values, the beam of electrons is deflected from
its path along the axes X and Y of constant values and remains
fixed providing the currents Il and I2 do not change value.
An arrangement such as th~s enables structures in which it is
possible to introduce the irradiation head to be examined
point by point. By increasing the current flowing through
the exciting coils, the beam of electrons is deflected
through a larger angle and it is possible, by selecting
the shape of the poles and the shape of the vacuum envelope
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in such a way that the beam of electrons is not blocked by
the walls of the en~elope and does in fact strike the target,
to obtain a X-radiation lobe of which the axis is perpendi-
cular to the axis of the incident beam of accelerated
electrons, in which case, irradiation is radial relative to
the axis of revolution of the irradiation head.
A panoramic irradiation, by scanning of the target
of revolution, is obtained with an arrangement such as this,
the beam of electrons rotating about the axis of the irradiation
head.
The invention is not limited to the embodiments of
the irEadiation head described and illustrated.
In particular, the targets described above were in
the form of a frustum or~spherical cap. These forms are by
no means the only forms and it is possible to use a target
in the form of a cylinder of revolution. However, if the
radiation emitted is to be homogeneous for different azimuths,
it is important to ensure that the directions of incidence
of the electrons on the corresponding zones of the target are
not too different. The forms illustrated in the drawings
enable the target to be bombarded in quasi-normal directions,
- irrespective of the azimuth.
In addition, in the description of the scanning
mode, the currents passing through the two electromagnets
were described as having the same pulsation. If the currents
have different pulsations, it is still possible to scan the
target although, in that case, the figure described is a
Lissajous figure.
Finally, this deflection device described is the
simplest to use for obtaining the required results (deflection
of the entire beam and scanning to include a solid angle).
Deflection devices using several pairs of poles-~ay be used
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.
to rotate the beam of charged particles about the axis of
the irradiation head. For example, to obtain periodic
scanning of the target of revolution, it is possible to use
three pairs of poles respectively fed by currents phase- ;
shifted by 3 or, generally, n pairs of poles fed by
currents phase-shifted by -n creating a rotating magnetic
field.
This irradiation head may be used in a portable
accelerator for borings in petroleum exploration, for medical
radiography or in metallurgy for examining plates, joints or
hollow parts.
,
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