L’ AFFAIRE A RAGON

The terminology used to describe the various types of accelerator irradiator (sometimes also called machine source irradiators) is analogous to that used for radionuclide source irradiators. The primary radiation in machine irradiators is by electrons accelerated to high energies. Other accelerated particles are not used in food irradiation. In such irradiators, the electron beam emerging from the accelerator can be used for treatment of food. Alternately, this electron beam can be converted into X rays (also called bremsstrahlung) through the use of a conversion target. The electrons are incident on high atomic number target material (the most commonly used converters are tungsten and tantalum), and the difference in the kinetic energies of the incident electrons and the decelerated electrons emerges in the form of photon energy.

Two basic types of accelerator (based on the physical mode of particle acceleration) are in common use,

(a) The steady current type (e.g. Dynamitron and van de Graaff), where the accelerating high voltage is actually present in the facility;

(b) The pulsed beam type (e.g. LINAC and Rhodotron), where microwave energy in a resonant cavity is converted into acceleration and relativistic mass augmentation of the electrons.

For both cases, continuously moving conveyors are utilized to achieve a more uniform lateral dose distribution in the direction of the product motion. The dose distribution in the lateral direction perpendicular to the direction of motion of the product is usually made more uniform by scanning the beam electromagnetically.

With beam scanning, the scan frequency and the conveyor speed must be co-ordinated so as to ensure that the entire process load is evenly irradiated. For the same reason, in a pulsed beam accelerator, the pulse repetition rate as well as the scanning frequency and the conveyor speed must be co-ordinated. These considerations also apply if the electron beam is converted into bremsstrahlung (X rays). Some irradiators are designed to give a very broad beam of electrons striking an extended converter.

This converter may be as large as an isotope plaque source so as to achieve dose uniformity by source overlap. The electronic control of the accelerator together with on-line measurements of the electron flux render information which can be used for feedback and real time control of the dose distribution [71–73].

3.4.2.1. Stationary type accelerator irradiators

The only well defined example of a stationary type accelerator irradiator was the pot anode X ray tube system used for the demonstration of fish irradiation which is no longer in use [74]. In this system, the product was placed within the irradiation chamber (inside the pot) and was irradiated by bremsstrahlung from all sides simultaneously. Upon completion of the irradiation the X ray generator was turned off, the irradiated product was removed and a new batch inserted.

3.4.2.2. One direction one pass irradiators

In this concept the process load is moved through the electron beam in a direction of motion that is perpendicular to the direction of the beam. The effect of the motion in this type of irradiator is the same as that for radionuclide source irradiators, i.e. flattening of the lateral dose distribution in the process load in the direction of the motion. With 10 MeV electrons, for example, only one sided irradiation is necessary, provided that the thickness of the process load is restricted to ensure adequate depth–dose uniformity. For high dose applications, it is sometimes necessary to subject the product to more than one irradiation cycle. Dose distributions in the lateral direction perpendicular to the direction of product motion are flattened by employing beam overlap and/or by using side scatter plates (Fig. 21). A two sided irradiation may be performed in order to increase the usable thickness of the treated process load (see Section 4.3.3.2 for more discussion). However, maximum permissible thickness is again limited by the physical range of the electrons to ensure required dose uniformity.

FIG. 21. Sequence of irradiation in a one direction, one pass electron irradiator with beam overlap and scatter plates as illustrated. A process load passes through the irradiation zone from the position ‘in’ to the position ‘out’.

Scatter plate

Beam scanner

In Out

This kind of design is most common with industrial irradiation facilities, where the transport system may vary from conveyor belts, to carriers on several types of rails and even to bulk flow of particulate materials. It provides great flexibility as, in principle, each process load could be irradiated according to its specific treatment procedure by adjusting the beam parameters and transport characteristics by use of computer controls.

3.4.2.3. Two direction two pass irradiators

In this type of irradiator, the process load may also be shifted along the scan direction after one pass through the radiation zone, and then moved in the opposite direction for the second pass through the radiation zone. This is achieved, as illustrated in Fig. 22 by passing the process load through the beam from ‘in’ to the end of the first conveyor, from where it is shifted onto the second conveyor. It finally returns through the beam to ‘out’, completing a second pass; the irradiation is one sided, however. This procedure increases the lateral dose uniformity in the process load.

The two direction two pass irradiator giving two sided irradiation uses an identical product flow scheme as in the irradiator described above, except that the process load is turned through 180° on its second pass, resulting in an improved depth–dose distribution. The two types of two direction two pass irradiator are feasible concepts with both machine sources, electrons and X rays. This design

FIG. 22. Irradiation sequence in a two pass electron irradiator. After irradiation on one conveyor, the process loads are moved (but not turned) to the second conveyor and then passed along a parallel path through the other half of the scanned beam. A process load passes through the irradiation zone from position ‘in’ to position ‘out’.

Beam scanner

Elevation In

Out

principle when adapted to the special requirements of radiation processing of wires, cables, tubes and hoses is expanded to multipass irradiators, where the product is threaded in a rather large number of loops through the radiation zone.

3.4.2.4. Beam scanning and dose uniformity

As mentioned earlier, the electron beam is scanned to improve the dose uniformity across the width of the conveyor. Normally the beam is scanned in one of two ways,

(a) It goes to and fro across the conveyor, resulting in an overlap of the trace at each end of the scan (as shown in Fig. 23(a)),

(b) It goes in one direction across the conveyor followed by a very rapid fly-back, resulting in an even trace of the beam without any overlap zone (as shown in Fig. 23(b)) [72].

FIG. 23. Lateral beam path patterns on the top surface of a process load for a scanned beam electron irradiator, with simultaneous conveyor movement and beam scanning: (a) scanning to and fro; (b) scanning in one direction with an extremely fast return. (Note: The rectangles represent the trace of the beam spot on the product, see text.)

Conveyor movement

Square beam

Useful width

To and fro

Fast fly-back beam scan

Beam scan

(a)

(b)

1 2

1 2

For convenience, a square cross-section of the beam trace on the surface of the process load is assumed in these figures. In real cases the beam spot profile must be measured carefully; usually it has an elliptical shape with axis lengths between 2 and 10 cm. The dose distribution within the beam cross-section is generally bell shaped and the contour line at 50% of the maximum dose is used to define the beam cross-section.

Continuous beams: In most continuous beam accelerators, the scan frequency is high enough (100–300 Hz) to ensure that, for any given conveyor speed, multiple overlap of successive scan traces is achieved resulting in lateral dose uniformity.

Many facilities of this type — especially for high dose applications — utilize to and fro scanning; the wedge in the beam trace on both ends of the scan does not give rise to significant overdoses. The conveyor speed, of course, must not be set so high as to result in a gap between the successive traces.

Pulsed beams: In pulsed beam accelerators, the pulse repetition rate is usually limited to ≤300 pulses per second. This places limits on the scan frequency that can be used, which, in turn, imposes limits on conveyor speeds. Dose uniformity can be obtained only by the proper harmonization of scan frequency, pulse repetition rate and the conveyor speed to achieve optimum contiguity of successive pulses irradiating the product.