L’ARC

Before the irradiation facility can be used for commercial purposes, it should be thoroughly characterized [76, 77]. Since the dose absorbed by the product is affected by various parameters, relations between the dose and these parameters should be determined over the full operational range of the parameters. These parameters include source strength and source arrangement, conveyor speed or dwell time, multipass mode, irradiation geometry and bulk density of the process load. For a machine source, there are also other parameters that are important, such as beam current, beam energy, beam spot, scan width and scan frequency.

4.3.2.1. Radionuclide source irradiators

The process equipment, including the radiation source, conveyor mechanisms, safety devices and ancillary systems, should be tested to verify satisfactory operation within the design specifications. All the equipment should also be calibrated at regular intervals, including irradiator cycle timers or conveyor speed and weighing equipment.

The dose delivered to the product in an irradiator depends strongly on either the selected dwell time or conveyor speed, and it is most frequently used to control the dose to the product. Dose also depends on the bulk density of the process load.

Delivery of the same dose to a product takes longer as the bulk density increases.

These relationships should be established during facility qualification; this understanding is of practical help during process qualification and the operation of the facility. For this purpose, process loads with either real products or dummy products may be used. The bulk density of the dummy products should be chosen to be the same as, or as close as possible to, the mean bulk density of the products that are expected to be irradiated at the facility. The dosimeters are placed, by preference, in locations where the minimum dose is expected. The data should then be analysed using regression analysis to obtain the relationships between the variables. Some examples are given in Figs 24 and 25. It can be seen from Fig. 24 that dose depends linearly on dwell time; however, the intercept on the y axis suggests that in some

FIG. 24. Minimum dose as a function of dwell time; each data point is the mean of three dosimeter films per dwell time setting. Linear regression of the data results in A = 4.86 ± 0.40 (kGy), m = 1.15 ± 0.03 (kGy/unit time); where A is the sum of the transit dose and the shuffle dose, and m is the slope of the regression line. The dashed lines are 95% confidence limits.

Dwell time (arbitrary units)

Dose (kGy)

0 5 10 15 20 25 30

40

30

20

10

A B

FIG. 25. Dwell time as a function of bulk density of the process load for a ⁶⁰Co irradiator (0.5 MCi). In this example, the product receives a minimum dose of 1 kGy.

1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1

0 5 10 15 20 25

Dwell time (min) Product density (g/cm3)

irradiators there is a finite value for transit doses. In such cases, dose is not directly proportional to the dwell time. Figure 25 shows a typical relationship between the product bulk density and the dwell time to give the same dose [78].

4.3.2.2. Accelerator irradiators

Characterization of an accelerator irradiator would also include measuring the mean energy of the electron beam, beam spot profile and scan width [54]; information about the last two parameters helps to ensure that the dose is uniformly delivered on the surface of a process load. The penetration of the electrons depends on the beam energy. It is measured by determining the depth–dose distribution along the beam axis in a reference material, usually water or polystyrene. Figure 26 gives a typical depth–dose distribution which is generally measured by exposing several thin film dosimeters at different depths in the reference material. The thickness designated r₅₀ (half-value depth in water) can be used for estimation of the mean electron beam energy based on the following relationship (ASTM Standard E1649 [54] and Ref.

[79]):

E_mean = 2.33r₅₀ (3)

FIG. 26. Depth–dose curve for 10 MeV electrons in water, where the entrance (surface) dose is 100%. The various ranges are identified as: r_max is the depth at which the maximum dose occurs, r_optis the depth at which the dose equals the entrance dose, r₅₀is the depth at which the dose equals half of the maximum dose and r₃₃is the depth at which the dose equals a third of the maximum dose.

Depth in water (cm)

Relative dose (%)

r_max r_opt r₅₀ r₃₃ 140

120 100 80 60 40 20

0 1 2 3 4 5 6 7

For the example of Fig. 26, E_meanis calculated to be 10.6 MeV for r₅₀= 4.53 cm (of water).

For the measurement of the beam spot, strips or sheets of film dosimeters are useful. For certain types of facility, this information is necessary to ensure that the subsequent pulses overlap as the beam is scanned. The scan width may be conveniently measured by placing several small dosimeters or strips of dosimeter film along the scan direction. This information is necessary to ensure that the radiation zone covers the lateral size of the process load expected to be irradiated.

Figure 27 shows a typical dose distribution along the scan width (direction perpendicular to the conveyor motion) [54].

Similar to the characterization with radionuclide source irradiators (Section 4.3.2.1), the relationships of conveyor speed with dose and bulk density should also be established. For electron facilities, quite often the pulse frequency (pulse repetition rate, number of pulses per second) is increased to deliver higher doses instead of changing the conveyor speed. This gives a larger flexibility to such irradiators.

FIG. 27. A typical dose distribution along the scan direction for an electron irradiator. The scan width may be defined as the width at some defined fractional level (90% in this example) of the average maximum dose [54].

Width

4.3.2.3. Dosimetry systems

The necessary dosimetry systems should be carefully selected on the basis of the criteria listed in Section 2.3. A facility should have at least one routine dosimetry system; however, it is advisable to have two in the case of unexpected problems. In addition, some facilities have a reference dosimetry system; however, it is not absolutely necessary. The selected dosimetry system(s) should be well characterized as discussed in Section 2.4.2. Additionally, the accuracy of the dose measurements depends on the correct operation of the analytical equipment used to measure the dosimeter response. It should be regularly calibrated and its performance checked periodically to ensure that it is within specifications.