Proceedings of IBIC2012, Tsukuba, Japan

MOPA04AN ELECTRON BEAM PROFILE MONITOR FOR THE RACE-TRACKMICROTRON
S.D. Dhole∗ , V.N. Bhoraskar, N.S. Shinde
Department of Physics, University of Pune, Ganeshkhind, Pune, India
B.J. Patil, Department of Physics, Abasaheb Garware College, Karve Road, Pune, India

In electron irradiation experiments on materials suchas semiconductors, solar cells etc., an uniformity and thecharge distribution in the electron beam is very important.
Therefore, an electron beam current monitor and its electronic system have been designed and built to measure thedistribution of a beam current either in the horizontal orvertical direction along with the beam dimensions. To obtain X-Y beam profile, a special type of Faraday Cup wasdesigned which mainly consists of charge collecting electrodes made up of thin copper strips. Each strip having dimensions 0.5 mm wide, 4 mm thick and 20 mm long were
fixed parallel to each other and separation between themwas ∼ 0.5 mm. This multi electrode Faraday was mounted
at the extraction port of the Race Track Microtron, where 1
MeV electron beam allowed to fall on it. The beam characterization in the form of current and uniformity were measured. The current from each strip were measured using anelectronic circuit developed based on the multiplexing principle. The uniformity of the beam can be measured with anaccuracy of 10%. The minimum and maximum dimensions
which can be measured are 3 mm and 15 mm respectively.

INTRODUCTION
The MeV energy range electron beams have gained importance due to their applications in space technology andtailoring of device parameters. In several cases, approximate beam dimensions are required for controlled irradiation. The electron beam delivered by an electron accelerator called ’Race-Track Microtron’ [1, 2] has pulse duration 1.6 s with repetition rate 50 PPS. Normally, at 1 MeV
electron energy, the beam size obtained is around 6 mm ×4 mm. The beam dimension can further be increased byusing scattering mechanism which is required to irradiate
samples of large dimensions. The dimension of the beam
may vary due to several factors such as variations in themagnetic field configuration, electron beam injection conditions, microwave cavity field, etc. Similarly, when thebeam is passed through a bending magnet, the beam shapeat the magnet exit is very much different from that at themagnet entrance. It is not always possible to know the uniformity of the electron beam using scintillator and therefore
it was thought appropriate to design and fabricate a beam
current monitor, which can provide intensity distribution
over the beam area at different points along horizontal and∗ sanjay@physics.unipune.ac.invertical directions. Such a monitor along with an electronicsystem has been designed, fabricated and put into operationin the laboratory.

EXPERIMENT
Multielectrode Faraday Cup
This consist of charge collecting electrodes in the formof thin copper strips. Such fifteen numbers of copper strips,
each of 0.5 mm wide, 4 mm thick and 20 mm long were
fixed parallel to each other with adhesive on a Perspex
sheet. The separation between two strip was kept ∼ 0.5mm. The thickness of copper strip was calculated on thebasis of range of 1 MeV electron in copper [3]. Therefore,
the thickness of copper strip was chosen such that all theincident electron can stop inside the strip. The assembly
was enclosed in an aluminum box having length 50 mm,
width 50 mm and depth 30 mm. For charge measurement,each copper strip was connected to a BNC connector. Frontside of the box is closed with a 50 µm aluminized mylar
to provide shielding against external electrical noise. This
beam profile measurement assembly was kept in the electron beam line and each of the BNC connector was connected to an electronic circuit specially made for this monitor.

Electronic System
An electronic circuitry [4, 5, 6, 7, 8, 9] has been designed, built and assembled to obtain visual display of thecharge distribution of an electron beam on a CRO as well as
on a digital display. The block diagram of the circuit used
for this work is shown in Figure 1, whereas the circuit details are shown in Figure 2. Eight strips of the beam current
monitor were connected to multiplexer of type 4051B. This
multiplexer is 8 to 1 type multiplexer, which has a facilityof eight independent input bits or channels, three control
bits and one output bit. This is also sometimes called asdata selector, because output bit is depends on the input
data bit that is selected. For instance, if the control signal
ABC is 000, then the information from the first copper stripof the Faraday cup is transmitted to the output. However, ifthe control signal ABC is change to 111, then the information from the eight strips is transmitted to the output. Fromthe circuit one can see that the multiplexer can be operated
in the mode of transmit information, which is available atone of the eight inputs to the corresponding output terminals, sequence of which can be preset through a programmeISBN 978-3-95450-119-9
Beam Charge and Current Monitors

1c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○AbstractMOPA04

Proceedings of IBIC2012, Tsukuba, Japanc 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○Figure 1: Block diagram of the beam profile monitor with electronic system.
fed to its control signal. In this case, the control signal programme is made in such a way that, in sequence, the input
information from the first to eight strips is transmitted tothe output terminal. The control signals to the multiplexer
is provided by using decade counter IC 7490, which wasused in mod-8 counter system. The counter forms a three
flip-flop in this mode and has eight discrete states. It counts
through a street binary sequence from 000 upto 111 andthen back to 000. This decade counter is driven by a simple clock generator using 555 timer, a TTL compatible integrated circuit, that can be used as an oscillator to provide
a clock waveforms. It is basically a switching circuit thathas two distinct output levels. As a result, the circuit oscillates and the output is periodic, rectangular waveforms.
The circuit is said to astable mode. In the present circuit,the frequencies of the oscillator as well as the duty cycle
are accurately controlled by two external resistor (RA andRB) and a single timing capacitor (C). The time varies froma few milliseconds to a few seconds and duty cycle varies
from 10 % to 90 %. To match the output of the decadecounter IC7490with multiplexer control signal, an inverter
type IC7404 was used.
At the time of operation, the working of decade counterand identification of the control signal was also monitored
through light emitting diodes (LED’s) by connecting separately to the output terminal of the decade counter. Theseterminals of the counter are connected to a D/A converter
made of using R-2R ladder resistive network, which converts a particular data or signal form counter to an analog voltage. With increase in the output of decade counter,
the DAC output voltage also increases in magnitude thatis from 0 to 4.37 V in steps with 0.625 V and the current

form 0 to 0.4227 mA in step with 0.0625 mA. This analogvoltage, however, used to shift the oscilloscope beam in thehorizontal direction. The multiplexer output was connected
to one of the channels of the CRO through a pulse amplifier mode of IC 355 and IC 741. The gain of the amplifier
was adjusted as per the intensity of the electron beam. The
circuit performance has been found satisfactorily and withthis it was possible to observe beam profile on a storage
oscilloscope. A detail circuitry of the beam profile is also
shown in Figure 2.

EXPERIMENTAL DETAILS,MEASUREMENTS AND DISCUSSION
To measure intensity distribution in the beam, the monitor was mounted at a distance of 5 cm away from theextraction port of the Microtron. Position of the monitor
along the beam axis was adjusted remotely, so that electron
beam could cover all the eight copper strips. The outputsof multiplexer was connected to the channel 1 of a digital
oscilloscope type No. PM 3350 whereas, the DAC output
was connected to the external trigger of the oscilloscope.
The charge collected by each copper strip was proportional
to the beam intensity in that region. The beam profile pattern was observed on the oscilloscope. In that pattern, theamount of charge collected by a particular strip was indicated by the Y-amplitude, whereas, the strip number wasdefined by the X position of the oscilloscope spot. Thetime for which the current can be recorded from a strip waskept variable in the range from 1 ms to 10 s. In this way,
the whole profile of the beam could be seen on the oscilloscope. When the signal is fed to an A/D converter, it wasISBN 978-3-95450-119-92Beam Charge and Current Monitors

MOPA04

Figure 2: Detailed electronic circuitry of the beam profile monitor.
also possible to record the charge collected by a strip indigital number. This system, gives total charge in one plane
only. In order to get charge distribution in the perpendicular plane, the profile monitor was rotated by 900 and theexperiment was also repeated. Figure 3 gives charge distribution of a 1 MeV electron beam in X and Y planes respectively. From this graph, it was found that the intensityof the electron beam is large at the center and gradually
decrease with an increases in X position. This indicatesthat the concentration of electrons in the beam is less at
the edges as compared to that in the central position. The
amplifier sensitivity was adjusted such that the total number of electrons in the beam could be as low as 5 × 1010
electrons/cm2 . This monitor can indicate beam dimension
upto 15 mm either horizontal direction or vertical direction.
10X 10
2.9
2.8

22.5

e /cm

2.6-2.7

2.4
2.3
2.2
0

12

3

4

567

Copper Strip Number

Figure 3: Nature of charge distribution of the electron beam
after travelling a distance of 5 cm in air.

CONCLUSIONThe beam profile monitor has been designed and used
to study the beam shapes at the entrance and exit of themagnetic analyser and also beam focusing properties ofa quadruple lenses. The uniformity of electron beam canalso be checked by using this monitor. Uniformity of thebeam plays an important role in irradiation experiments anddamage induced studies in semiconductor materials and devices.

REFERENCES[1] V.B. Asgekar, et al. 1980, ‘Single cavity 8 MeV race track
microtron’, Pramana 15 (5), 479-493.
[2] V.N. Bhoraskar, 1988, ‘The microtron: a recirculating electron accelerator’, Indian J. Phys. 62A (7), 716.
[3] ESTAR, ‘NIST Database for stopping power of electrons invarious targets’.
[4] D.F. Stout and M. Kawfman, ‘Handbook of operational amplifier circuit and design’.
[5] J. Milliman and C. Halicies, 1985, ‘Integrated electronics:
Analog and digital circuits and systems’, McGraw Hill Kogaksha Ltd. International student edition.
[6] A. Malvino, ‘Electronic principle’, McGraw Hill book company, 1989, international edition.
[7] Linear IC Manual 1988, National semiconductor.
[8] TTL data bank 1978, Fairchild camera and Instruments corporation.
[9] Intersol IC manu, Hot ideas in CMOS.

ISBN 978-3-95450-119-9
Beam Charge and Current Monitors

3c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○Proceedings of IBIC2012, Tsukuba, Japan