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A current transformer and gated integrator for measurement of weak currents from pulsed accelerators
NUCLEAR
INSTRUMENTS
AND
METHODS
97
(I97I) 319-32I; ©
NORTH-HOLLAND
PUBLISHING
CO.
A CURRENT TRANSFORMER AND GATED I N T E G R A T O R FOR M E A S U R E M E N T OF WEAK CURRENTS F R O M P U L S E D ACCELERATORS H. FEIST, M. K O E P and H. R E I C H
Physikalisch-Technische Bundesanstalt, Braunschweig, Germany Received 19 August 1971 A non-intercepting beam current monitor and a gated integrator are described which allow to measure peak currents down to about 1 l~A from the PTB 5.8 MeV microtron. The monitor
differs from similar circuits by using only one winding on a mu-metal core and by a special current-voltage converter of 40 m.Q input impedance.
1. Introduction
In this paper an induction monitor and a gated integrator for use at the 5.8 MeV microtron of the PTB are described which allow to measure pulse currents down to 1 /~A and to integrate those pulses.
The choice of a suitable beam monitor in electron accelerator experiments is sometimes difficult. Ionization chamber monitors as the secondary emission chamberl-¢), placed between source and target, disturb the primary beam by scattering and bremsstrahlung production. The reading of monitors placed behind the target depends on target properties and the energy of the beam. Induction monitors or current transformers 5) traversed by the primary beam avoid these disadvantages. But up to now they are developed for pulse currents of more than 1 mA only [see e.g. refs. 6-8]. In the pAregion diffÉculties arise from amplifier noise and high frequency disturbances present at the accelerator installations.
2. Current transformer and amplifier
The secondary current is through the load resistance R of a current transformer is proportional to the primary current (beam current) ip only, if the inequality
(1) is fulfilled [see ref. 5]; ~ is the duration of the beam pulse and L the inductance of the transformer coil. If the leakage inductance of the coil is negligible, iSis given by
current I current-vo,ltogeconvener tr°nsf°rmel
(2)
iS = ip/N,
pulse omplifier
o+14V
i ~12::~nP:'R...~IIk~21 ~.k-~ .2I !0k~l2.~7 7 k~l27k~.351k ~I2.2k~_~2 ks21 I~ current
1
6"0"I I
I I u],,5o: 15o I I
i
v
Fig. 1. Current transformer and amplifier circuit. The current-voltage conversion is 20 mV//~A. Fine adjustement is performed by the potentiometer P. The dimensions o f the mu-metal core are in m m 28 0 x 52 0 × 25, band thickness 0.05 mm.
319
320
H. F E I S T et al.
where N is the number of turns. As the current is should be as large as possible for the detection of weak beam currents, we chose N = 1. In order to satisfy inequality (1) as well, a transformer core of large cross section (dimensions see text to fig. 1) and high initial permeability (mu-metal) was used. The amplifier was designed as a current-voltage transformer with very low input impedance (fig. I). The input stage is similar to commercial ac current probes (see e.g. Hewlett Packard type 456 A). Its low impedance is achieved by feedback of the amplified and inverted signal from the collector of transistor T2 via resistor R1 and by the common base circuit of transistor T1. Capacitive coupling by C1 is preferred to dc coupling because it simplifies the amplifier's design [Gardiner et al.8)]. Connecting C1 with the grounded base of T1 reduces the hf-pick-up compared to connecting it with the emitter. The disadvantage of using capacitor C1 is that, together with the coil inductance L and the input impedance R, it forms an RLC network with small damping which causes low frequency base line oscillations at the amplifier output. The oscillations are suppressed by the inductance L1 and by suitable RC-couplings between the amplifier stages, so that the amplifier acts as a highpass. Current transformer and amplifier are placed into the same aluminium box with holes for the beam. The feeding leads into the box are supplied with filter elements; grounding loops are thoroughly avoided. Investigation of amplifier characteristics and rough calibration of the monitor are done by the aid of two additional turns on the transformer core fed by rectangular pulses of 1/~s duration corresponding to the beam pulses of the microtron. The monitor shows the following characteristics:
a) Pulse shape. The slope of the top of output pulses amounts to about 5% per/as independent of the amplitude. Partly this value is due to imperfect fulfilment of inequality (1), partly to the RC-couplings and inductance LI of the amplifier. The top slope does not influence the results of relative charge measurements, because the pulse duration of the accelerator is constant. I f desired, the slope can be reduced by enlargement of the transformer core cross section or of the number N of the turns. A disadvantage in the latter case is the reduction of is proportional to N - 1, but on the other hand L is increased proportional to N 2 in inequality (1). b) Smallest measurable pulse amplitude. Fig. 2 illustrates the response of the monitor showing a pulse current of I0 #A from the microtron. Pulse amplitudes of about 1/~A - somewhat distorted by noise - are yet measureable. A better signal to noise ratio can be obtained by connecting in parallel the single-turn
Fig. 2. Pulse f r o m the linear amplifier (fig. 3) produced by a IO/~A electron b e a m pulse from the microtron (horizontal 0.5 #s/div., vertical 0.5 V/div.).
m
l inp
7 linear amplifier
I
emitter ' 'lso~ follower
. ~ thermostat
T1
1.6k~ll 15k~ll
,
.
-6v.-LV
I
I
I .
lllkcLII
IC
-
output
' Schmi~"-~ t I
I
I
O.~2pF
_
.15ko~
uni-
vibrator
n
I; 7_r3j, s ~ _ 0 . 8V - - - - _ t oscilloscope
---J
1
.sv
15v
gate pulse I
trigger pulse accelerator
from
0
T1 ' 2N2905 T2: 2N221g
Fig. 3. Gated integrator. T h e dotted line s u r r o u n d s the parts of the circuit which are placed into a thermostat.
CURRENT
TRANSFORMER
windings of several cores traversed by the beam to the same amplifier input. This is due to the fact that the signal amplitudes add up linearly, while the noise amplitudes are increasing proportional to the square root of the number of (identical) cores. It should be emphasized, that the slope of the pulse top is not changed in such an arrangement.
3. Gated integrator Integration of the monitor output voltage pulses over long time intervals is done by a simple gated integrator (fig. 3) which differs from known circuits 7'9'1°) by the voltage-current converter and the device for discharge. Transistor TI simultaneously acts as voltage-current converter and gate. The gate is opened by a trigger pulse from the accelerator and permits charging of the integration capacitor C during the radiation time of the microtron only. Thus the integration of noise during the intervals between beam pulses is avoided. The trigger pulse drives TI just to the border of conductivity. When, at this moment, a voltage pulse is given to the emitter of T1, the capacitor will get an amount of charge proportional to this pulse. For stable operation some parts of the circuit are placed into a thermostat which keeps the temperature constant within _+0.1 °C. The temperature can be controlled by observation of the voltage drop at the thermistor R. The voltage supplies are stabilized against temperature variations by reference elements. When the electrode of the capacitor C connected to T1 and T2 reaches a potential of about - 2 V, a Schmitt-trigger operates giving a pulse to a counter and opening transistor T2 for discharge of C. The discharge is stopped when the potential has reached - 6 V. The potentials --2 V and - 6 V mark the hysteresis of the Schmitttrigger which essentially depends on the values of resistors only and shows no strong temperature dependence. More than 100 pulses of any height are necessary before discharge takes place; thus the uncertainty of charge collection is smaller than 1%. The discharge time amounts to 0.2 ms, the time between beam pulses is 2 ms or more so that the charge collection of one pulse only is incomplete. 4. Testing procedures The stability of the main parameters of the integrator was often controlled by the aid of a pulse generator. The voltage-current conversion and the linearity of the integration showed fluctuations in the order of 0.1% "~nly. The zero adjustement changed in a range which
AND
GATED
INTEGRATOR
321
corresponds to 0.5% at 10 pA and to 0.1% at 50 pA beam pulse current. After its checking the integrator was used for testing the transformer and the amplifier circuit. Voltage pulses of 1/~s duration were at first fed to the integrator directly, then to the "test and calibration input" of the transformer. The potentiometer P (fig. 1) in the latter case was adjusted so that the counting rate of the integrator had the same value in both cases. In a pulse current range from 5 pA to 200 pA deviations from the linearity and variations of the stability were found to be smaller than 0.1%. The upper limit of the current range can easily be extended into the mA-region by reducing the gain of the amplifier.
5. Applications For the measurement of cross sections of direct electron-positron production by electrons from the 5.8 MeV microtron of the PTB 1~) an electron beam completely free of photons was required because of the high pair production cross section of photons. The non-intercepting beam current monitoring system developed here was suited as the current amplitudes amounted to about 50 pA. Combining current transformer and Faraday cup measurements in a manner similar to that described by Pruitt ~2) and by Hague et alJ 3) the monitoring system was calibrated absolutely and employed to measure Faraday cup errors to well within 0.1%. Details of this procedure are described elsewhere14). References 1) G. W. Tautfest a n d H. R. Fechter, Rev. Sci. Instr. 26 (1955) 229. 2) V. J. Vanhuyse, E. D. W a t t e c a m p s , R. E. van de Vijver a n d G. J. Vanpraet, Nucl. Instr. a n d Meth. 15 (1962) 59; V. J. Vanhuyse and R. E. van de Vijver, Nucl. Instr. a n d Meth. 15 (1962) 63. 3) B. Planskoy, Nucl. Instr. and Meth. 24 (1963) 172. 4) C . J . K a r z m a r k , Rev. Sci. Instr. 35 (1964) 1646. 5) R. Berg~re, E. Delezenne a n d A. Veyssiere, Nucl. Instr. and Meth. 15 (1962) 327. 6) L. Bess, J. Ovadia a n d J. Valassis, Rev. Sci. lnstr~ 30 (1959) 985. 7) j. L. Menke, Nucl. Instr. a n d Meth. 64 (1968) 61. s) S . N . Gardiner, J . L . Matthews and R . O . Owens, Nucl, Instr. and Meth. 87 (1970) 285. 9) H. K l e s s m a n n a n d D. Petrick, Int. Elektron. R u n d s c h a u 21 (1967) 45. 10) W. R. Hardy, R. Yager a n d J. Shewchun, Nucl. Instr. and Meth. 77 (1970) 331. 11) H. Feist and H. Reich, Z. Physik 239 (1970) 437. 12) j. S. Pruin, Nucl. Instr. and Meth. 92 (1971) 285. a3) j. F. Hague, R. C. Jennings a n d R. E. R a n d , Nucl. Instr. and Meth. 24 (1963) 456. 14) H. Feist and M. K o e p , to be published in: PTB Mitteilungen.
INSTRUMENTS
AND
METHODS
97
(I97I) 319-32I; ©
NORTH-HOLLAND
PUBLISHING
CO.
A CURRENT TRANSFORMER AND GATED I N T E G R A T O R FOR M E A S U R E M E N T OF WEAK CURRENTS F R O M P U L S E D ACCELERATORS H. FEIST, M. K O E P and H. R E I C H
Physikalisch-Technische Bundesanstalt, Braunschweig, Germany Received 19 August 1971 A non-intercepting beam current monitor and a gated integrator are described which allow to measure peak currents down to about 1 l~A from the PTB 5.8 MeV microtron. The monitor
differs from similar circuits by using only one winding on a mu-metal core and by a special current-voltage converter of 40 m.Q input impedance.
1. Introduction
In this paper an induction monitor and a gated integrator for use at the 5.8 MeV microtron of the PTB are described which allow to measure pulse currents down to 1 /~A and to integrate those pulses.
The choice of a suitable beam monitor in electron accelerator experiments is sometimes difficult. Ionization chamber monitors as the secondary emission chamberl-¢), placed between source and target, disturb the primary beam by scattering and bremsstrahlung production. The reading of monitors placed behind the target depends on target properties and the energy of the beam. Induction monitors or current transformers 5) traversed by the primary beam avoid these disadvantages. But up to now they are developed for pulse currents of more than 1 mA only [see e.g. refs. 6-8]. In the pAregion diffÉculties arise from amplifier noise and high frequency disturbances present at the accelerator installations.
2. Current transformer and amplifier
The secondary current is through the load resistance R of a current transformer is proportional to the primary current (beam current) ip only, if the inequality
(1) is fulfilled [see ref. 5]; ~ is the duration of the beam pulse and L the inductance of the transformer coil. If the leakage inductance of the coil is negligible, iSis given by
current I current-vo,ltogeconvener tr°nsf°rmel
(2)
iS = ip/N,
pulse omplifier
o+14V
i ~12::~nP:'R...~IIk~21 ~.k-~ .2I !0k~l2.~7 7 k~l27k~.351k ~I2.2k~_~2 ks21 I~ current
1
6"0"I I
I I u],,5o: 15o I I
i
v
Fig. 1. Current transformer and amplifier circuit. The current-voltage conversion is 20 mV//~A. Fine adjustement is performed by the potentiometer P. The dimensions o f the mu-metal core are in m m 28 0 x 52 0 × 25, band thickness 0.05 mm.
319
320
H. F E I S T et al.
where N is the number of turns. As the current is should be as large as possible for the detection of weak beam currents, we chose N = 1. In order to satisfy inequality (1) as well, a transformer core of large cross section (dimensions see text to fig. 1) and high initial permeability (mu-metal) was used. The amplifier was designed as a current-voltage transformer with very low input impedance (fig. I). The input stage is similar to commercial ac current probes (see e.g. Hewlett Packard type 456 A). Its low impedance is achieved by feedback of the amplified and inverted signal from the collector of transistor T2 via resistor R1 and by the common base circuit of transistor T1. Capacitive coupling by C1 is preferred to dc coupling because it simplifies the amplifier's design [Gardiner et al.8)]. Connecting C1 with the grounded base of T1 reduces the hf-pick-up compared to connecting it with the emitter. The disadvantage of using capacitor C1 is that, together with the coil inductance L and the input impedance R, it forms an RLC network with small damping which causes low frequency base line oscillations at the amplifier output. The oscillations are suppressed by the inductance L1 and by suitable RC-couplings between the amplifier stages, so that the amplifier acts as a highpass. Current transformer and amplifier are placed into the same aluminium box with holes for the beam. The feeding leads into the box are supplied with filter elements; grounding loops are thoroughly avoided. Investigation of amplifier characteristics and rough calibration of the monitor are done by the aid of two additional turns on the transformer core fed by rectangular pulses of 1/~s duration corresponding to the beam pulses of the microtron. The monitor shows the following characteristics:
a) Pulse shape. The slope of the top of output pulses amounts to about 5% per/as independent of the amplitude. Partly this value is due to imperfect fulfilment of inequality (1), partly to the RC-couplings and inductance LI of the amplifier. The top slope does not influence the results of relative charge measurements, because the pulse duration of the accelerator is constant. I f desired, the slope can be reduced by enlargement of the transformer core cross section or of the number N of the turns. A disadvantage in the latter case is the reduction of is proportional to N - 1, but on the other hand L is increased proportional to N 2 in inequality (1). b) Smallest measurable pulse amplitude. Fig. 2 illustrates the response of the monitor showing a pulse current of I0 #A from the microtron. Pulse amplitudes of about 1/~A - somewhat distorted by noise - are yet measureable. A better signal to noise ratio can be obtained by connecting in parallel the single-turn
Fig. 2. Pulse f r o m the linear amplifier (fig. 3) produced by a IO/~A electron b e a m pulse from the microtron (horizontal 0.5 #s/div., vertical 0.5 V/div.).
m
l inp
7 linear amplifier
I
emitter ' 'lso~ follower
. ~ thermostat
T1
1.6k~ll 15k~ll
,
.
-6v.-LV
I
I
I .
lllkcLII
IC
-
output
' Schmi~"-~ t I
I
I
O.~2pF
_
.15ko~
uni-
vibrator
n
I; 7_r3j, s ~ _ 0 . 8V - - - - _ t oscilloscope
---J
1
.sv
15v
gate pulse I
trigger pulse accelerator
from
0
T1 ' 2N2905 T2: 2N221g
Fig. 3. Gated integrator. T h e dotted line s u r r o u n d s the parts of the circuit which are placed into a thermostat.
CURRENT
TRANSFORMER
windings of several cores traversed by the beam to the same amplifier input. This is due to the fact that the signal amplitudes add up linearly, while the noise amplitudes are increasing proportional to the square root of the number of (identical) cores. It should be emphasized, that the slope of the pulse top is not changed in such an arrangement.
3. Gated integrator Integration of the monitor output voltage pulses over long time intervals is done by a simple gated integrator (fig. 3) which differs from known circuits 7'9'1°) by the voltage-current converter and the device for discharge. Transistor TI simultaneously acts as voltage-current converter and gate. The gate is opened by a trigger pulse from the accelerator and permits charging of the integration capacitor C during the radiation time of the microtron only. Thus the integration of noise during the intervals between beam pulses is avoided. The trigger pulse drives TI just to the border of conductivity. When, at this moment, a voltage pulse is given to the emitter of T1, the capacitor will get an amount of charge proportional to this pulse. For stable operation some parts of the circuit are placed into a thermostat which keeps the temperature constant within _+0.1 °C. The temperature can be controlled by observation of the voltage drop at the thermistor R. The voltage supplies are stabilized against temperature variations by reference elements. When the electrode of the capacitor C connected to T1 and T2 reaches a potential of about - 2 V, a Schmitt-trigger operates giving a pulse to a counter and opening transistor T2 for discharge of C. The discharge is stopped when the potential has reached - 6 V. The potentials --2 V and - 6 V mark the hysteresis of the Schmitttrigger which essentially depends on the values of resistors only and shows no strong temperature dependence. More than 100 pulses of any height are necessary before discharge takes place; thus the uncertainty of charge collection is smaller than 1%. The discharge time amounts to 0.2 ms, the time between beam pulses is 2 ms or more so that the charge collection of one pulse only is incomplete. 4. Testing procedures The stability of the main parameters of the integrator was often controlled by the aid of a pulse generator. The voltage-current conversion and the linearity of the integration showed fluctuations in the order of 0.1% "~nly. The zero adjustement changed in a range which
AND
GATED
INTEGRATOR
321
corresponds to 0.5% at 10 pA and to 0.1% at 50 pA beam pulse current. After its checking the integrator was used for testing the transformer and the amplifier circuit. Voltage pulses of 1/~s duration were at first fed to the integrator directly, then to the "test and calibration input" of the transformer. The potentiometer P (fig. 1) in the latter case was adjusted so that the counting rate of the integrator had the same value in both cases. In a pulse current range from 5 pA to 200 pA deviations from the linearity and variations of the stability were found to be smaller than 0.1%. The upper limit of the current range can easily be extended into the mA-region by reducing the gain of the amplifier.
5. Applications For the measurement of cross sections of direct electron-positron production by electrons from the 5.8 MeV microtron of the PTB 1~) an electron beam completely free of photons was required because of the high pair production cross section of photons. The non-intercepting beam current monitoring system developed here was suited as the current amplitudes amounted to about 50 pA. Combining current transformer and Faraday cup measurements in a manner similar to that described by Pruitt ~2) and by Hague et alJ 3) the monitoring system was calibrated absolutely and employed to measure Faraday cup errors to well within 0.1%. Details of this procedure are described elsewhere14). References 1) G. W. Tautfest a n d H. R. Fechter, Rev. Sci. Instr. 26 (1955) 229. 2) V. J. Vanhuyse, E. D. W a t t e c a m p s , R. E. van de Vijver a n d G. J. Vanpraet, Nucl. Instr. a n d Meth. 15 (1962) 59; V. J. Vanhuyse and R. E. van de Vijver, Nucl. Instr. a n d Meth. 15 (1962) 63. 3) B. Planskoy, Nucl. Instr. and Meth. 24 (1963) 172. 4) C . J . K a r z m a r k , Rev. Sci. Instr. 35 (1964) 1646. 5) R. Berg~re, E. Delezenne a n d A. Veyssiere, Nucl. Instr. and Meth. 15 (1962) 327. 6) L. Bess, J. Ovadia a n d J. Valassis, Rev. Sci. lnstr~ 30 (1959) 985. 7) j. L. Menke, Nucl. Instr. a n d Meth. 64 (1968) 61. s) S . N . Gardiner, J . L . Matthews and R . O . Owens, Nucl, Instr. and Meth. 87 (1970) 285. 9) H. K l e s s m a n n a n d D. Petrick, Int. Elektron. R u n d s c h a u 21 (1967) 45. 10) W. R. Hardy, R. Yager a n d J. Shewchun, Nucl. Instr. and Meth. 77 (1970) 331. 11) H. Feist and H. Reich, Z. Physik 239 (1970) 437. 12) j. S. Pruin, Nucl. Instr. and Meth. 92 (1971) 285. a3) j. F. Hague, R. C. Jennings a n d R. E. R a n d , Nucl. Instr. and Meth. 24 (1963) 456. 14) H. Feist and M. K o e p , to be published in: PTB Mitteilungen.