Operating Manual

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The linear accelerator (linac)
The energy levels mostly used for linacs (linear accelerators) are 4 MeV and 8 MeV. Linear
accelerators can be constructed for one or two energy levels.
In the travelling-wave linac, the acceleration of electrons from a heated filament to very
high energies results from the electrons “riding” a high-frequency (3-10 MHz) electromag-
netic wave travelling in a straight line down an acceleration tube (the hollow guide).
The electrons are bunched into pulses at a frequency of a few hundred pulses per second.
The target, which the electrons strike to generate X-radiation, is at the opposite end of the
main wave guide of the filament assembly. This is a transmission type target from which
the radiation beam passes in a straight line.
The X-ray output from a linear accelerator is many times higher than from a Bètatron of the
same energy. An 8 MeV linac with a 2 mm diameter focal spot can deliver a radiation dose
rate of 30 Sv/minute at 1 metre distance from the focus. Small light-weight portable linacs
of 3 MeV capacity can have outputs of 1.5 Sv/minute at 1 metre distance.
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The intensity of radiation is increased by double-phased rectifying and varying degrees of
smoothing. At very low voltage ripple these sets are considered constant potential (CP)
equipment.
In the latest types of tank sets the mains frequency is first converted to a high frequency
alternating current and only then transformed upward, which makes it easier still to
smooth the ripple. At very high frequencies, up to 50 kHz, smoothing is hardly necessary
anymore and such X-ray sets can be called CP systems. Additional features may be built in,
for example an automatic warm-up facility, see note below. This type of circuitry with
advanced electronics leads to a higher degree of reliability and significant space and weight
reduction compared with earlier power supply systems. As a result of the various improve-
ments that have gradually been implemented, present day (high frequency) AC X-ray sets
perform as well as true CP sets.
Note: Because of the high vacuum prevailing inside the X-ray tube, it carefully has to be warmed-up
after a period of rest. During rest the vacuum deteriorates. This warm-up procedure has to be done
in accordance with the supplier’s instructions, to prevent high voltage short-circuiting which might
damage the tube or render it useless.
5.3 Megavolt equipment
The equipment described in sections 5.1 and 5.2 is used to generate X-radiation up to
approximately 450 kV. However, sometimes higher energy levels are needed.
Several types of equipment have been built to operate in the 1 MeV to 16 MeV range. In
industrial radiography almost exclusively Bètatrons or linear accelerators (linacs) are used.
Operating high-energy X-ray installations requires (costly) safety precautions.
The Bètatron
The Bètatron is an electron accelerator, which can produce X-radiation in the 2-30 MeV
energy range. The electrons are emitted into a round-sectioned donut shaped glass
vacuum tube, as shown in figure 5-5. After several millions of revolutions the electrons
reach maximum energy and are deflected towards the target. On the target, part of the
electron energy is converted into a tangentially directed beam of X-radiation.
To obtain a reasonably high radiation intensity, most Bètatrons have been designed to ope-
rate in the 10-30 MeV energy range, as these voltages achieve maximum conversion rate of
electron energy into radiation. Even so the output of Bètatrons is usually small compared
to linacs. Transportable low energy Bètatrons (2-6 MeV) have been built, but these gene-
rally have a low radiation output, which limits their application.
One advantage of Bètatrons is that they can be built with very small (micromillimeter)
focal spots. A disadvantage is that with these very high energy levels the X-ray beam is usu-
ally narrow, and the coverage of larger film sizes is only possible by using increased sour-
ce-to-film distances. The extended exposure times required can be a practical problem.
Fig. 5-5. Bètatron
X-rays
1. Ring-shaped accelerator tube
2. Anode
3. Cathode filament (emitting electrons)
4.&5 magnetic fields
6. Coils
7. Auxiliary winding
1
2
3
6
5
7
7
4