Proceedings of IBIC2012, Tsukuba, Japan

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PULSED ELECTRON BEAM CURRENT AND FLUX MONITOR FOR THE
RACE-TRACK MICROTRON
B.J. Patil, Department of Physics, Abasaheb Garware College, Karve Road, Pune, India
Shahzad Akhter, N.S. Shinde, V.N. Bhoraskar, S.D. Dhole∗ ,Department of Physics, University of Pune, Ganeshkhind, Pune, India

In electron irradiation experiments on the materials, atrue current of the electron beam is to be known to calculate the electron fluence received by the sample. Therefore,a pulsed electron beam current and flux monitor alongwith electronic system for an electron accelerator called
Race-Track Microtron has been designed and developed.
The sensing device used was a ferrite core having suitable
number of turns of copper wire wound around it, throughwhich the electron beam was passed without loss in the intensity. With an appropriate developed electronic circuit,the instantaneous value of the induced voltage was measured which in turn provides value of the electron beampulsed current. The total charge passed through the ferrite
core per unit time was therefore recorded and an integrated
value of the total charge in a given period could be derived.
This system can be used to measure the electron flux in therange from 108 electron/cm2 to 1016 electron/cm2 . Moreover, this system has been used successfully in a few electron irradiation experiments where the knowledge of theelectron fluence received by the sample is required.

INTRODUCTION
Charged particle accelerators deliver particle beams either in the pulse or continuous mode. In case of continuous
beam current, combination of Faraday cup and current meter can serve the purpose of flux measurement. However,in case of pulsed electron beam, analog meters are not sensitive to the small beam pulse width which may vary fromnanoseconds to microseconds. In such cases, integrationof the charge collected over a known period is required toobtain particle fluence received by the target.
The Microtron, an electron accelerator of the Universityof Pune is operated in a pulse mode with pulse width 2 µs
and pulsating rate variable in the range 50 pps to 200 pps.
The energy range can be set in two ranges 0.5 to 1 MeV
and 6 to 8 MeV. For many applications, samples are exposed with electrons and fluence level (e− /cm2 ) is required
to be known with accuracy around 1%. In this laboratory,
for irradiation experiments, a Faraday cup is being used tomeasure electron fluence received by the sample. In thismethod, a conducting plate made of graphite or aluminumis placed in the beam path and the charge collected is mea∗ sanjay@physics.unipune.ac.in

sured by a current integrator. The thickness of the plate
is kept much more than the range of the electrons. However, all the incident electron do not flow to the integrator because a fraction of the incident electron are lost due
to backscattering. Secondary emitted electrons also reducethe charge reaching the current integrator. Due to this problem, it was difficult to estimate fluence level by measuring
the charge received by the sample. To avoid this problem aninduction type current transformer[1, 2, 3] and pulse integrator have been designed and fabricated to measure pulsed
electron beam current and hence fluence received by a sample.

DESIGN PRINCIPLE OF INDUCTIONMONITORThe measurement of pulse current using ferrite core isbased on the principle of current transformer[2]. If idealtransformer conditions are assumed i.e. unity coupling
without core losses and winding reactances much greaterthan respective primary and secondary resistances, the ratio of primary and secondary current inversely proportional
to the turns ratio
ip
Ns=(1)
isNp
Where ip = primary current, is = secondary current, Ns =number of secondary turns, Np = number of primary turns
In many cases, while the monitoring of primary current
requires a voltage output from the secondary, the secondary
current passes through load resistance RL
Vs = i p .

NpRLNs(2)

Considering Ns =50, Np =1, then ip =is × 50
Vs = is RL ∴ is =Vs = ip

ip50

(3)
(4)

Relation 3 gives the magnitude of voltage Vs directly proportional to the primary beam current.

DESIGN PRINCIPLE OF THE CURRENT
TRANSFORMER
The current monitor consists of a ferrite core type
T45HP3 having OD 46 mm, ID 28.6 mm and width 13
ISBN 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

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Proceedings of IBIC2012, Tsukuba, Japanc 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○mm. The permeability and the flux density of the ferrite
core as supplied by the manufacture are 2300 Henries/mand 3900 Weber/m2 respectively. Winding of nearly 1000
turns of fine insulated copper wire of 40 SWG are provided
around the core. The measure value of the inductance andresistance of the core are 67 mH and 150 Ω respectively.
When electron bunch passes through the ferrite ring, pulse
voltage is generated directly proportional to the number ofelectrons in the bunch. The details of the system are shownin Fig. 1 and induced pulse is shown in Fig. 2.

Figure 1: Induction beam current monitor.

is added in the peak detector to get recovery time variablein the range 0.01 to 0.1 second. DC voltage was given toan amplifier having variable gain. The amplifier DC output
was fed to a 3 and 1/2 digit A/D converter. An IC7107 is
used to convert the analog signal into digital form, magnitude of which can be directly seen on display. Instantaneous value of the pulse current of the electron beam wastherefore observed in terms of voltage.
Second stage of this circuit is an integrator. In this, aprecession amplifier was used to increase the amplitude of
the DC level, which was fed to a precision voltage to frequency converter. In this way, by measuring the frequency
accurately, small variation in the input voltage and hence inpulse current can be observed. The IC used is type LM331
with 100kHz full scale ±0.03 nonlinearity with overall
conversion accuracy around 0.1 %. Output of this converter
was fed to a multiplexed counter MM74C926 and its carryout terminal was used to cascade the counters. On the display, the pulse current can be seen in digital form. The system was calibrated using a re-entrant type Faraday cup inwhich all the backscattered and secondary emitted electron
were also collected. The total electron charge of the Faraday cup was measured by a separate commercially available current integrator having based level of 10−8 coulomb.
Facilities such, presetting (1 sec to 104 sec) and present
count (108 ) were also incorporated.
The time scaling factor was used to obtain a number ofcounts in a fixed time period. For this purpose, a 60 Hz frequency signal was obtained using a crystal oscillator type
IC5369, having frequency 3.579 MHz. The 60 Hz signal
is given to a decade counter type 7490. The circuits areused in different mode such that time period from 1 second to 104 seconds can be set. The outputs of the counters
are connected to multiplexed switch and the selected timemode pulse is further used to gate and latch signals of thecounter 74C926. In this way for any period from 1 second
to hundreds of seconds, the total integrated charge value interms of count can be obtained on the display.

PRECISION AUTO DOSE CONTROLSYSTEM
Figure 2: Photograph of induced pulse from induction
monitor.

DESIGN OF THE PULSE INTEGRATORAs the machine pulse beam current is available in therange 0.1 mA to 10 mA, the output voltage of the induction current monitor was small in amplitude. Thepulse voltage was therefore amplified, integrated and thengiven to an A/D converter and subsequently to a counter[4, 5, 6, 7, 8, 9, 10]. Block diagram of the pulse integrator isshown in Fig. 3. A FET input preamplifier was used to amplify the induced current pulse. The amplifier output wasfed to ultra low pulse peak detector. A damping resistor R

Further to control either irradiation period of electrondose, an additional feedback circuit was designed and fabricated. A separate counter was used to set the time using
time scaling circuit and counts from V/F converter. Thecounter can be set to work either in time mode or in countmode through mode selector switch. The output of thiscounter is given to (A input) magnitude comparator which
compares the data (B input) in counter with the data set
on a thumbwheel BCD switch and generates three outputs
A < B, A > B and A = B. The IC74LS85 is used as
a comparator in the present case to set the counts or time.
The output of this comparator (A >B) is used to control
mastercontrol triggering circuit of the modulators of theMicrotron. After completion of the desired dose or irradiation time, the circuit puts off the trigger pulse generator.

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Figure 3: Block diagram of the pulse integrator used with ferrite induction current monitor.
In this way the electron beam is stopped from coming outof the machine.

CALIBRATION OF THE MONITOR
For the calibration of the induction current monitor, there-entrant type Faraday cup as mentioned earlier was usedto collect all the electrons of the beam. The output fromthis Farady cup was connected to charging condenser (tantalum, 1 µF) through 50 ohm co-axial cable and thus theintegrated charge was measured by digital multimeter perunit time. Simultaneously, the signal from the induction
monitor was fed to the pulse current integrator circuit madein this laboratory. The charge shown by the condenser wasfound to vary linearly with that shown by the present induction current integrator system. In this way, the pulse
output of the induction monitor was calibrated in terms ofthe charge collected by the re-entrant Faraday cup calibration curve is shown in Fig. 4. It is observed from the figurethat the flux increases with increase in the induced voltage.

Figure 4: Calibration curve for the induced current monitor.

CONCLUSION
The current monitor consists of a ferrite core having OD
46 mm, ID 26.6 mm, width 13 mm and wound 1000 turns
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TUPA06

Proceedings of IBIC2012, Tsukuba, Japan

copper wire around it. When electron beam pass throughthe axis of the core, induced pulse generated which is directly proportional to the electron beam current. Further
this pulse is given to the pulse integrator designed indigenously where the induced voltage pulse shows directly proportional to the flux of the electron beam.

REFERENCES[1] Manca J.J. and Froelich H.R. 1976 Ferrite induction orbit
current and beam position detectors for a variable energy
race track Microtron. Nucl. Instr. Meth 136,249-253.
[2] Anderson J.M. 1971 Review of Sci. Instr. 42(7)[3] Bebs L, Ovadia J, valassis J, 1959 ’ Review of Sci. Instr.
29(11)[4] David f. stout and Milton kawfman ’Handbook of operational amplifier circuit and design’[5] Milliman J and Halicies C. 1985. Integrated electronics:
Analog and digital circuits and systems, McGraw Hill Kogaksha Ltd. International student edition.
[6] Malvino a 1989 Electronic principle, McGraw Hill bookcompany, international edition.

c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○[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
[10] CMOS logic data book 1988 National semiconductor.

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4Beam Charge and Current Monitors