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An accurate low current measurement circuit for heavy iron beam current monitor

Chao-Yang Zhou a,b, Hong Su a,, Rui-Shi Mao a, Cheng-Fu Dong a, Yi Qian a, Jie Kong a

a Institute of Modern Physics, Chinese Academy of Sciences, Lanzhou 730000, Chinab Graduate University of the Chinese Academy of Sciences, Beijing 100049, China

a r t i c l e i n f o

Article history:

Received 8 December 2011

Received in revised form 19 January 2012

Available online 16 March 2012

Keywords:

Faraday cup

Low beam current monitor

Picoammeter

Gated integrator

a b s t r a c t

Heavy-ion beams at 106 particles per second have been applied to the treatment of deep-seated inoper-

able tumors in the therapy terminal of the Heavy Ion Research Facility in Lanzhou (HIRFL) which islocated at the Institute of Modern Physics, Chinese Academy of Sciences (IMP, CAS). An accurate low cur-

rent measurement circuit following a Faraday cup was developed to monitor the beam current at pA

range. The circuit consisted of a picoammeter with a bandwidth of 1 kHz and a gated integrator (GI). A

low input bias current precision amplifier and new guarding and shielding techniques were used in

the picoammeter circuit which allowed as to measure current less than 1 pA with a current gain of

0.22 V/pA and noise less than 10 fA. This paper will also describe a novel compensation approach which

reduced the charge injection from switches in the GI to 1018 C, and a T-switch configuration which was

used to eliminate leakage current in the reset switch.

2012 Elsevier B.V. All rights reserved.

1. Introduction

Beams of heavy-charged particles like carbon ions have been

applied to the treatment of deep-seated tumors[1]in the therapy

terminal of the HIRFL at IMP, CAS [2]. A beam current monitor sys-

tem was used in clinical experiments to ensure the treatment plan

and patient safety. Detectors in the monitor system, which register

a secondary signal induced by the beam, like scintillator or gas

chambers, are sensitive to the released energy rather than the

charge. A beam current monitor (BCM) was needed to calibrate

these detectors.

Beam current is normally measured by making use of an iso-

lated Faraday cup (FC) as a beam dump [3,4]. Beams applied to

the treatment are at 106 particles per second which equates to cur-

rents less than 1 pA. Measurements of this low beam current usu-

ally suffer from poor accuracy. Additionally, the beam current is

modulated because of the operating modes of the accelerator. Thus

there needs to be a balance between accuracy and bandwidth of

the measurement electronics in order to accurately measure the

beam current. Systems previously developed for measurement of

currents in this order of magnitude are limited in their bandwidth

to some ones of Hertz[5].

This paper contains a description of an accurate low current

measurement circuit for the BCM system. The circuit was imple-

mented with two sections. The first was a picoammeter with a

bandwidth of 1 kHz, and the other was a GI [6] with a variable inte-

gration time. This paper will describe the structure of the picoam-meter based on noise analysis and introduction of guarding and

shielding techniques used in sub-picoamp current measurement

[7]. It also contains a description of the charge injection compensa-

tion and leakage current elimination techniques which were used

in the design of the GI.

2. Low current measurement circuit in the beam monitor system

Fig. 1shows the block of the heavy ion beam current monitor

system. The accurate low current measurement circuit consists of

a picoammeter and a GI. The current signal from the FC, which

was pulsed for 3 s a period of 20 s, was converted into a voltage

signal through a transimpedance amplifier, the output of which

was integrated by the GI in order to measure the charge of the

beam. Outputs of the two sections were acquired by a data acqui-

sition system based on PXI. The signals were processed by the local

computer, and the current from the FC and the beam charge were

remotely monitored.

Compared to conventional current integrators [8], which inte-

grated the current from the FC by a GI directly, the two separated

parts of this novel circuit avoided many disturbances like leakage

current of the feedback capacitor and charge injection of switches.

In practice, due to the high sensitivity of the picoammeter, the two

parts of the circuit were implemented on two different printed cir-

cuit boards and assembled in one metal box which was installed

locally to the FC system.

0168-583X/$ - see front matter 2012 Elsevier B.V. All rights reserved.http://dx.doi.org/10.1016/j.nimb.2012.01.033

Corresponding author. Tel.: +86 9314969365.

E-mail address:suhong@impcas.ac.cn(H. Su).

Nuclear Instruments and Methods in Physics Research B 280 (2012) 8487

Contents lists available at SciVerse ScienceDirect

Nuclear Instruments and Methods in Physics Research B

j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / n i m b

http://dx.doi.org/10.1016/j.nimb.2012.01.033mailto:suhong@impcas.ac.cnhttp://dx.doi.org/10.1016/j.nimb.2012.01.033http://www.sciencedirect.com/science/journal/0168583Xhttp://www.elsevier.com/locate/nimbhttp://www.elsevier.com/locate/nimbhttp://www.sciencedirect.com/science/journal/0168583Xhttp://dx.doi.org/10.1016/j.nimb.2012.01.033mailto:suhong@impcas.ac.cnhttp://dx.doi.org/10.1016/j.nimb.2012.01.033

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3. The design of the picoammeter with ultra low noise

3.1. Noise analysis of the picoammeter

Since the ammeter with an active I/V converter does not have

the shortcomings of the ammeter with a passive I/V converter

(i.e. too large input resistance and a shunt voltage), a feedback

structure was selected for this picoammeter.

The most sensitive range of the picoammeter depends on the

minimal value of the current, which can be transformed into thestandard voltage with acceptable accuracy. To determine this min-

imal value, noise produced by the electrometric amplifier, scaling

resistor, circuit of the measured current, structural elements of

the picoammeter and cables were analyzed in this paper. A sche-

matic of the picoammeter circuit with indicated sources of the

most important noise sources is shown inFig. 2.

If we take into consideration disturbances shown inFig. 2, the

output voltage of the picoammeter can be described by Eq.(1)

Uo IsRf It Ina IbRf 1 Rf=RsUoa Una UnR 1

where: Ibis the input bias current of the amplifier; Ina is the instan-

taneous value of the input noise current of the amplifier; It is the

current coming from constructing and circuit elements (i.e. leakage

of insulators, cables; printed circuit board) in transmission; UnR isthe instantaneous value of thermal noise voltage of the scaling

resistor; Una is the instantaneous value of input noise voltage of

the amplifier; alsoUoa is the input offset voltage of the amplifier.

3.2. Structure of the picoammeter

As can be seen inEq.(1), a low bias current, a low noise current,

a low noise voltage and a low offset voltage are needed in order to

minimize the noise figure of the picoammeter. In order to address

those issues, we developed a novel picoammeter whose block dia-

gram is shown inFig. 3.

Fig. 3 shows an I/Vconverter with two cascaded stages that pro-

vides a total open-loop gainA 400. A new precision amplifier

with guaranteed bias current smaller than 20 fA and typical value

3 fA was used as the input stage, A1, to meet the tight design spec-

ification related to the input bias current. For the second stage, A2,

a wide bandwidth (145 MHz) amplifier was used.

Due to the stable open-loop gain of this two-stage circuit, an

offset voltage compensation circuit which introduced an offset to

the inverting input of A1 worked well. Such a circuit would have

been difficult to implement in single stage DC coupled amplifier.

With this circuit, the output offset voltage could be adjusted to less

than 2 mV. The guard generator amplifier, A3, was used to drive

the input guard ring.

The two-stage I/Vconverter was followed by A4 which includedstages of gain, filter and buffer. The precision OPA, A1, had a very

low voltage noise, but a current noise up to 10 fA=ffiffiffiffiffiffi

Hzp

. The filter

limited the bandwidth of the circuit to 1 kHz to reduce the current

noise. The last stage was a buffer amplifier. It generated an output

signal that was split into two paths. One was fed into DAQ system

for current curve display, and the other was fed into GI for

integration.

For measurement of currents less than 1 pA, the transimped-

ance, G, which was equal to the feedback resistor, Rf, at low fre-

quency, must be in the range of 1012 X. Resistors with high

resistance value and at the same time with small temperature

resistance coefficient and voltage resistance coefficient are difficult

to obtain and very expensive. For this reason the single feedback

resistor,Rf, was replaced by a T-resistor configuration which could

achieve a current gain as high as 0.22 V/pA with 100 MX resistors.

3.3. Guarding and shielding techniques

Noise coupled by external sources could be reduced with guard-

ing and shielding techniques. Guarding is an important technique

in sub-picoamp current designs. A guard is a low impedance point

in the circuit that is at nearly the samepotential as the high imped-

ance lead being guarded. On the PCB layout in Fig. 4, the guard cre-

ates an equal potential zone around the input pins, preventing

external leakage currents from coupling into the input circuit.

Fig. 4shows the general configuration of the input traces. All

the sensitive feedback components were located within the perim-

eter of the guard ring. Pin 2 and 7, which were non-connected pinsof the amplifier, A1, were connected to the guard traces to insulate

Fig. 1. Block diagram of the heavy ion beam monitor system.

Fig. 2. Schematic diagram of the picoammeter including the dominant sources ofnoise.

Fig. 3. Block diagram of a picoammeter with a two-stageI/Vconverter.

C.-Y. Zhou et al. / Nuclear Instruments and Methods in Physics Research B 280 (2012) 8487 85

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the input terminals from the other pins. The solder mask was also

removed from this area to reduce surface charge accumulation. The

guard generator was a buffer amplifier which drove the input

guard traces to the same potential as the inverting input node.

In order to shield the circuit from electromagnetic interference,

the entire circuit was enclosed within a metal shield box and the

input signal was connected to the FC using a triax cable. The inside

shield of the triax cable was connected to the guard terminal of the

picoammeter circuit so that the signal conduct and the inside

shield were at the same potential. This approach eliminates theconsiderable charge injected by traditional coaxial-cables.

4. The design of the new GI

To get the total charge of the beam, the output of the picoam-

meter was fed into the GI, the schematic of which is shown in

Fig. 5.

The input signal voltage Vi is converted into current via the

resistorRSand integrated by the feedback capacitor Cf. The output

voltageVo is given by

Vo 1

CfRs

Z T0

Vitdt V0 2

Since the input voltage Vi, is proportional to the current fromthe FC, the GI output Vo, represents the total charge of the beam

for a given duration T. In practice, because the 3 s beam on time

was too long for such a circuit, the beam current was integrated

with intervals varied from 100 ls to 1 ms.

The charge injection compensation and leakage current elimi-

nation techniques were implemented in the GI section.

4.1. Charge injection compensation

DMOS switches SD5400 were chosen as switches in the GI.

When a transient voltage excursion appears at the gate, there will

be an injection of electric charge into analog path via the gate-to-

drain and the gate-to-source capacitances. This will cause errors to

the output. The network indicated by the dotted line inFig. 5wasused to compensate this charge injection.

When the state of switches used in the GI is changed, certain

quantity of charge will be injected into the inverting input via

the capacitor, CC, by the compensation network. This is opposite

to the charge injected by switches. Since it is known that the

amount of charge injected is equal to the product of the voltage

excursion amplitude on the component and its capacitance, gener-

ating a product equal but opposite to each other can make a perfect

compensation between the capacitorCC

and switches in the GI.CCwas chosen to be 10 pF. Thus by changing the amplitude of the

voltage excursion on CC, the error of the output voltage caused

by charge injection was reduced to less than 2 mV.

4.2. T-switch configuration for leakage current elimination

While the GI is in integration or hold state, the reset switch

has to be kept off. However the effect of leakage current in the GI

reset switch will cause errors to the output in the long duration of

these states. To eliminate this effect, the single reset switch was re-

placed by a T-switch configuration shown in Fig. 6.

The leakage current of DMOS switch is composed of the source/

body reverse leakage current and the drain/source subthreshold

current. Since the source/body reverse leakage current can be neg-ligible in SD5400, the drain to source subthreshold current be-

comes the main factor of the off current. If the source and

body are at the same potential, the drain/source of a MOS transis-

tor in the deep subthreshold region of operation is given by

IDS IDOW

L eVGS=nVT1 eVDS=VT 3

In Eq.(3),IDSis the drain to source current of the transistor, IDOis the saturation current, VGSis the gate to source voltage, VDSis the

drain to source voltage, W/L is the ratio of width and length of the

transistor, andVTis the threshold voltage of the transistor. As Eq.

(3)shows, even forVGS= 0, the only way to establish a zero switch

leakage current is to set VDS to 0 V. To maintain the VDS of the

DMOS switch at virtual ground, a T-switch configuration shown

inFig. 6was used to replace the single reset switch. The T-switch

configuration was composed by two DMOS switches, S1, S2, in ser-

ies and a grounded DMOS switch, S3, attached to the node between

the two switches. When the two DMOS switches in parallel with

the integration capacitor are off, the third switch, S3, is on. In this

configuration, VDSof the switch connected to the inverting input of

the GI is maintained at 0 V, and very little leakage current flows

through this switch.

5. Measurements

A complete set of measurements was performed in the lab in or-

der to characterize AC and DC performances of the circuit. Addi-

tionally, the long-term stability was evaluated. The 100fA-resolution AC and DC Current Source, Keithley Model 6221,

Fig. 4. PCB layout of the input guard traces.

Fig. 5. The schematic of the GI with a network for charge injection compensation.

Fig. 6. A T-switch configuration as the reset switch.

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was selected to supply input current signal to the picoammeter

with a model 7078 triax cable. The outputs of the two sections of

the circuit were both fed into a 14-bit NI PXI-6133 ADC card which

was controlled by the DAQ software developed using LabWindows/

CVI.

Specifications of the picoammeter and the GI are shown in

Tables 1 and 2. Parameters such as conversion gain, linearity error

and noise were measured when the current source was in DC mode

and filtered. The sample interval of the ADC was 100 ls. All DC test

results presented here, including linearity of the two sections

shown inFigs. 7 and 8, were based on the average of more than

100 K points which were more than 10 s. In the measurement of

the GI, the charge integrated each sample was equal to the productof the source current and the integration time, which was a bit less

than the sample interval. The 3 db bandwidth of the picoamme-ter was obtained when the current source operated in AC mode.

6. Summary

The results of a laboratory test of a low current measurement

circuit which demonstrate that it had a high accuracy and long-

term stability were presented. Because of the circuit design tech-

niques used when implementing the guard and shield circuit de-

signs, the performance of the two sections should work very well

when it is used as part of a BCM system.

Acknowledgements

This work was supported by National Program on Key Basic Re-

search Project of China (973 Program) (No. 2010CB834204), the

National Natural Science Foundation of China (No. 10735060)

and Important Direction Project of the CAS Knowledge Innovation

Program (No. KJCX2-YW-N27).

References

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Table 1

Specifications of the picoammeter.

Parameter Value

Full scale output 2.5 V

Current conversion gain 0.22 V/pA

Linearity error