Beam spot measurements on A 400 keV electron accelerator

Ra~a't. rsyJ. Ctumt. 1979, Vol. 13, pp.I--4.

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BEAM SPOT MEASUREMENTS ON A 400 keY ELECTRON ACCELERATOR ARNE MILLER Accelerator Department, Rise National Laboratory, DK-4000 Roskilde, Denmark (Received 3 July 1978) Abstraet--A line probe is used to measure the beam spot radius and beam divergence at a 400 keV ICT electron accelerator, and a method is shown for reducing the line probe data in order to get the radial function.

LOW ENERGY electron accelerators are used in radiation processing for several applications °~ mainly involving polymeric materials. Many of the processes initiated by radiation are strongly dose rate dependent, and it can therefore be advantageous to know the instantaneous dose rate in a particular radiation process. This involves knowledge of factors such as beam current, energy, speed of material transport underneath the beam, and--if the beam is scanned--scan width and frequency, and beam spot size where the beam hits the material. Most of these parameters are readily available, but the size of the beam spot can be hard to determine. Film dosimetry methods are ruled out as the scan cannot be stopped due to possible overheating of the output window, particularly at high output currents. This paper describes the use of a line probe for the determination of the size of the scanned beam spot and the evaluation of the data, in order to get the radial function from the line probe data. The measurements were performed at Ris~'s 400keV ICT electron accelerator. The beam is scanned in two directions, x: 3cm, 50Hz and y: 100 cm, 200 Hz and the maximum beam current is 50 mA. When no products are being irradiated, the beam is absorbed in a water-cooled aluminum beam catcher. The accelerator is locally shielded with lead and it is equipped with a conveyor for the passage of material underneath the electron beam. The free height above the conveyor is about 5 cm and the distance from conveyor to output window about 10cm. Two sets of lead shutters prevent exposure of operating personnel, but makes the access to the beam area rather difficult. The line probe that was used consisted of a

0.4 cm dia and a 50 cm long copper wire suspended under the beam at a right angle to the y-scan. The arrangement was so that the height of the probe above the conveyor could be adjusted. The current that was picked up by the probe was lead to ground via a variable resistor ( - 1 0 0 f l) and the voltage signal across this resistor was read on an oscilloscope. The line probe was first placed at the center of the y-scan, and the signal picked up is shown in Fig. I. The beam current was 40 mA, but different currents down to 2 mA were tested without any observable change in curve shape. The scan frequency was 200 Hz and the width was 100 cm, this means that the beam movement during the 100~s per division at Fig. I was 4 cm. Measurements were made with the probe placed at three different heights above the conveyor: 2.2 cm, 3.7 cm and 5.2 cm. Each point along the curve of Fig. 1 is the integral value along the line probe, and it does not therefore, represent the radial function. Methods have been suggested,~2~ however, to reduce line probe data to radial functions, and they can be applied here. Programs already available for reduction of data from holographic measurements °~ were modified and applied to this problem. Figure 2 shows the radial functions of the beam spot at the three different heights. The curves are normalized to the maximum reading. The difference between the curve shape of radial function and the line probe data is shown in Fig. 3. It was assumed that axial symmetry existed in order to apply the methods for reducing line probe data. The symmetry was tested by placing a small probe (5 x 5 mm2) in the beam area and recording the relative readings at different distances from the

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ARNE MILLER

Fig. I. The signal picked up by the line probe ~ith the probe placed at the center of the scan and at a height above the conveyor of 5.2 cm. Horizontal: 100~s per division (4 cm per division). Vertical: 20 mV per division. r t ~ l T T l l l l l ~ l ; i T ~

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Fig. 2. The radial function of the beam spot as derived from the line probe data. a: 2.2 cm above conveyor, b: 3.7 cm above conveyor, c: 5.2 cm above conveyor.

Beam spot measurements ~n a 400 keV electrofi accelerat6r I

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Fig. 3. Normalized curves showing the difference between the line probe data and the radial distn'bution. I

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Fig. 4. The d/vergence of the electron beam, approximated by straight lines. maximum reading. The results are shown at Fig. 2 as the circled dots, and a fairly good agreement is demonstrated. The increasing width of the beam spot as the probe is moved away from the output window of the accelerator indicates the divergence of the beam. Figure 4 shows the radius of the beam spot at various current levels (50 and 10%) for the three different heights above the conveyor. Straight lines through these points converge at a point that fits well with the actual distance between output window and conveyor (10cm). It is not expected, however, that the beam will diverge as a straight line, but such an approximation is probably close enough under these conditions. From the figure it can be seen that the full width at half maximum of the beam spot at the conveyor is 10 cm.

When the probe is moved off center the curve is distorted as would be expected. Figure 5 shows the signal when the probe is placed 30cm off center. Because the beam spot is no longer axial symmetric the line probe data can not be reduced as simply as above, but the off axis measurements seem to indicate that the beam is still a cone with a top angle of about 60". This means that the beam spot area at 30 cm off axis is increased by about 35%, as the beam is scanned with a distance from scanner magnet to window of - 100 cm. This method can be used to measure beam spots of scanned DC electron beams with a reasonable accuracy. The factors that can give rise to errors are mainly: (a) Backscatter from the probe itself, giving rise to a smaller signal, and from the beam catcher giving rise to a larger signal. The former

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ARNE MILLER

Fig. 5. The signal picked up by the line probe with the probe placed 30 cm off center and at a height above the conveyor of 2.2 cm.

effect will only be a relative factor on the signal while the latter gives only an insignificant amount;~4)(b) The signal is not a true picture of the beam spot at a point, as it is the beam that moves over the probe and not vice versa. The small distortions however, that are produced in this way are judged, only to have a minor influence on these measurements.

REFERENCES 1. Proceedings of the First International Meeting on Radiation Processing, Radiat. Phys. Chem. 1977, 9. 2. O. H. NES'rORand H. N. Or.SEN, SIAM Rev. 1960, 2, 200. 3. A. MILLER and W. L. MCLAU6HLXN, Nucl. Instrum. Meth. 1975, 128, 337. 4. RALPHW. DRESSEL, Phys. Rev. 1966, 144, 332.