PULSED ELECTRON BEAM CURRENT AND FLUX MONITOR FOR THERACE-TRACK MICROTRON

AbstractIn electron irradiation experiments on the materials, a

true current of the electron beam is to be known to calcu-late the electron fluence received by the sample. Therefore,a pulsed electron beam current and flux monitor along-with electronic system for an electron accelerator calledRace-Track Microtron has been designed and developed.The sensing device used was a ferrite core having suitablenumber of turns of copper wire wound around it, throughwhich the electron beam was passed without loss in the in-tensity. With an appropriate developed electronic circuit,the instantaneous value of the induced voltage was mea-sured which in turn provides value of the electron beampulsed current. The total charge passed through the ferritecore per unit time was therefore recorded and an integratedvalue 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. More-over, this system has been used successfully in a few elec-tron irradiation experiments where the knowledge of theelectron fluence received by the sample is required.

INTRODUCTIONCharged particle accelerators deliver particle beams ei-

ther in the pulse or continuous mode. In case of continuousbeam current, combination of Faraday cup and current me-ter can serve the purpose of flux measurement. However,in case of pulsed electron beam, analog meters are not sen-sitive 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 µsand pulsating rate variable in the range 50 pps to 200 pps.The energy range can be set in two ranges 0.5 to 1 MeVand 6 to 8 MeV. For many applications, samples are ex-posed with electrons and fluence level (e−/cm2) is requiredto 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 plateis kept much more than the range of the electrons. How-ever, all the incident electron do not flow to the integra-tor because a fraction of the incident electron are lost dueto backscattering. Secondary emitted electrons also reducethe charge reaching the current integrator. Due to this prob-lem, it was difficult to estimate fluence level by measuringthe charge received by the sample. To avoid this problem aninduction type current transformer[1, 2, 3] and pulse inte-grator have been designed and fabricated to measure pulsedelectron beam current and hence fluence received by a sam-ple.

DESIGN PRINCIPLE OF INDUCTIONMONITOR

The measurement of pulse current using ferrite core isbased on the principle of current transformer[2]. If idealtransformer conditions are assumed i.e. unity couplingwithout core losses and winding reactances much greaterthan respective primary and secondary resistances, the ra-tio of primary and secondary current inversely proportionalto the turns ratio

ipis

=Ns

Np(1)

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 currentrequires a voltage output from the secondary, the secondarycurrent passes through load resistance RL

Vs = ip.Np

NsRL (2)

Considering Ns=50, Np=1, then ip=is × 50

Vs = isRL ∴ is =ip50

(3)

Vs = ip (4)

Relation 3 gives the magnitude of voltage Vs directly pro-portional to the primary beam current.

DESIGN PRINCIPLE OF THE CURRENTTRANSFORMER

The current monitor consists of a ferrite core typeT45HP3 having OD 46 mm, ID 28.6 mm and width 13

B.J. Patil, Department of Physics, Abasaheb Garware College, Karve Road, Pune, IndiaShahzad Akhter, N.S. Shinde, V.N. Bhoraskar, S.D. Dhole∗,

Department of Physics, University of Pune, Ganeshkhind, Pune, India

Proceedings of IBIC2012, Tsukuba, Japan TUPA06

Beam Charge and Current Monitors

ISBN 978-3-95450-119-9

337 Cop

yrig

htc

2013

byJA

CoW

—cc

Cre

ativ

eC

omm

onsA

ttri

butio

n3.

0(C

C-B

Y-3.

0)

mm. The permeability and the flux density of the ferritecore as supplied by the manufacture are 2300 Henries/mand 3900 Weber/m2 respectively. Winding of nearly 1000turns of fine insulated copper wire of 40 SWG are providedaround 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, pulsevoltage 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.

Figure 2: Photograph of induced pulse from inductionmonitor.

DESIGN OF THE PULSE INTEGRATORAs the machine pulse beam current is available in the

range 0.1 mA to 10 mA, the output voltage of the in-duction 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 am-plify the induced current pulse. The amplifier output wasfed to ultra low pulse peak detector. A damping resistor R

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 outputwas fed to a 3 and 1/2 digit A/D converter. An IC7107 isused to convert the analog signal into digital form, mag-nitude of which can be directly seen on display. Instanta-neous 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 ofthe DC level, which was fed to a precision voltage to fre-quency converter. In this way, by measuring the frequencyaccurately, small variation in the input voltage and hence inpulse current can be observed. The IC used is type LM331with 100kHz full scale ±0.03 nonlinearity with overallconversion accuracy around 0.1 %. Output of this converterwas fed to a multiplexed counter MM74C926 and its carry-out terminal was used to cascade the counters. On the dis-play, the pulse current can be seen in digital form. The sys-tem was calibrated using a re-entrant type Faraday cup inwhich all the backscattered and secondary emitted electronwere also collected. The total electron charge of the Fara-day cup was measured by a separate commercially avail-able current integrator having based level of 10−8 coulomb.Facilities such, presetting (1 sec to 104 sec) and presentcount (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 fre-quency signal was obtained using a crystal oscillator typeIC5369, having frequency 3.579 MHz. The 60 Hz signalis given to a decade counter type 7490. The circuits areused in different mode such that time period from 1 sec-ond to 104 seconds can be set. The outputs of the countersare 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 secondto hundreds of seconds, the total integrated charge value interms of count can be obtained on the display.

PRECISION AUTO DOSE CONTROLSYSTEM

Further to control either irradiation period of electrondose, an additional feedback circuit was designed and fab-ricated. A separate counter was used to set the time usingtime 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 whichcompares the data (B input) in counter with the data seton a thumbwheel BCD switch and generates three outputsA < B,A > B and A = B. The IC74LS85 is used asa comparator in the present case to set the counts or time.The output of this comparator (A >B) is used to controlmastercontrol triggering circuit of the modulators of theMicrotron. After completion of the desired dose or irra-diation time, the circuit puts off the trigger pulse generator.

TUPA06 Proceedings of IBIC2012, Tsukuba, Japan

ISBN 978-3-95450-119-9

338Cop

yrig

htc

2013

byJA

CoW

—cc

Cre

ativ

eC

omm

onsA

ttri

butio

n3.

0(C

C-B

Y-3.

0)

Beam Charge and Current Monitors

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 (tan-talum, 1 µF) through 50 ohm co-axial cable and thus theintegrated charge was measured by digital multimeter perunit time. Simultaneously, the signal from the inductionmonitor 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 in-duction current integrator system. In this way, the pulseoutput of the induction monitor was calibrated in terms ofthe charge collected by the re-entrant Faraday cup calibra-tion 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.

CONCLUSIONThe current monitor consists of a ferrite core having OD

46 mm, ID 26.6 mm, width 13 mm and wound 1000 turns

Proceedings of IBIC2012, Tsukuba, Japan TUPA06

Beam Charge and Current Monitors

ISBN 978-3-95450-119-9

339 Cop

yrig

htc

2013

byJA

CoW

—cc

Cre

ativ

eC

omm

onsA

ttri

butio

n3.

0(C

C-B

Y-3.

0)

copper wire around it. When electron beam pass throughthe axis of the core, induced pulse generated which is di-rectly proportional to the electron beam current. Furtherthis pulse is given to the pulse integrator designed indige-nously where the induced voltage pulse shows directly pro-portional 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 energyrace 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 opera-tional amplifier circuit and design’

[5] Milliman J and Halicies C. 1985. Integrated electronics:Analog and digital circuits and systems, McGraw Hill Ko-gaksha Ltd. International student edition.

[6] Malvino a 1989 Electronic principle, McGraw Hill bookcompany, international edition.

[7] Linear IC manual 1988, National semiconductor

[8] TTL data bank 1978, Fairchild camera and Instruments cor-poration.

[9] Intersol IC manu, Hot ideas in CMOS

[10] CMOS logic data book 1988 National semiconductor.

TUPA06 Proceedings of IBIC2012, Tsukuba, Japan

ISBN 978-3-95450-119-9

340Cop

yrig

htc

2013

byJA

CoW

—cc

Cre

ativ

eC

omm

onsA

ttri

butio

n3.

0(C

C-B

Y-3.

0)

Beam Charge and Current Monitors