The Proportional Ion Chamber and its Corresponding Circuit
Shao Beibei, Zhao Yan, Tang Yueli*
Engineering Physics Dept. Tsinghua University
E-mail: BBSHAO@Thinker.ep.tsinghua.edu.cn*Institute of High Energy Physics Academia Sinica Beijing 100084

Abstract
High pressure electric current type ionization chambers
have been used for pulsed radiation field measurement ofaccelerators for long time. The ion chamber like
proportional tubes may improve the sensitivity and lower
the cost of the ion chambers system. An amplify factor of
1,000 to 10,000 is introduced in the proportional area.
With which the sensitivity of the tube is much higher than
the ion chamber. At the ion chamber area, it fits the needsof very high radiation field.
As the corresponding circuit, a low cost high sensitivity
current digitizer has been designed and tested. It can
respond to the weak current of 1 x 10 -13 A and has nicelinearity between 5 x 10 -13 to 2 x 10-7 A. The system covers
108 order of radiation field.
According to the level of radiation field, the high voltage
on the tube is switched between proportional area and ionchamber area. A micro controller is employed for read out
and the switch control. The system can be used foraccelerators pulse radiation monitor and can also be usedfor beam loss measurement.
1 The detector

voltage plus. Electric current signal is extracted from thewall of the chamber. The High Voltage can be adjusted
from 0 to 2000V with steps of every 50V between 0 to1000V and every 100V between 1000V to 2000V.
1.2 The testing of the detector
To test the detector with radiation source, a radiation
source of 7mCi Ra226 is at the distance of 0.5 m. Fig 2 andfig 3 shows the relationship between High Voltage andsignal current at ionization area and proportional area. We
can see the detector has a good linearity in proportional
area and while in the ionization area, the curve is flatenough.
1.00E-11

1.00E-12

1.00E-13

0100

200
300
H.V.(v)

400

500

600

1.1 The structure of the detector
The Xe gas filled proportional ion chamber is made by Dr.
Li Jiyuan in another Institute in Beijing. The thin copper
tube chamber has a gilded tungsten filament in diameterof 5 µm for central pole. A protection ring on the isolatoris used for grounding the detector. The detector has a size of
220 mm long and 50 mm in diameter. The sensitive volume
is approximately equal to:

1.00E-07
1.00E-08
1.00E-09
I(A)V = πr 2 × h × 0.8 ≈ 7 × 10 −4 m 3 .

Fig. 2 The function curve of detector at theionization area.

1.00E-10

proportional
ion chamber

1.00E-11
1.00E-12600

Fig. 1 The proportional chamber test circuit
The isolation resistance of the isolator is higher than 1014Ω.
The detector is tested with a electrometer. The test circuit
shows as fig. 1. The central wire is connected to high

800

1000 1200 1400 1600 1800 2000
H.V.(v)

Fig.3 the function curve of the detector at theproportional area

1.3

The sensitivity at different areas

The sensitivity of the detector is tested also with above
instrument and circuit by moving out the radiation source.
At the background level, the radiation is roughly equal to
0.13µSv/h. In the ionization area, a 2x10-14 A current
signals was observed, when the high voltage was around
300V. In the proportional area, the current signals were
1.2 x 10 –13A, 1.0 x 10 –12A and 1.2 x 10 –10 A, when High
Voltages were at 1000V, 1600V and 2000V.
The measurement of the proportional ion chamber showsfrom 200V to 400V, it has excellent ion chambercharacters. While as the high voltage over 800V, typical
proportional tube characters are measured as Fig3. If gives
very nice linearity and sensitivity

Fig 5 shows the two stage amplifiers. The first stage is acurrent integration circuit. The key components of thecircuit is low bias current amplifiers and low leakage
current capacitor and diodes. The LMC662 from NationalSemiconductor was selected for the first stage amplifier.
The main characteristics are as follows:
lVoltage gain  126dB
lInput offset voltage  3mV
lOffset Voltage drift  1.3µV / °C
lInput bias current  2fA

2 The digitizer
2.1 Principle of the digitizer
Digitizers, sometimes it is called I/F converter . Normallya charge bump circuit or current integration circuit is used
for digitizing the very weak current. The principal of thecircuit is show on Fig. 4:

Fig 5 The two stage amplifiers
2.3 The discharge circuitThe fig. 7 shows the discharge circuit. An amplify factorof 1,000 is introduced be the second stage amplifier. Whenthe signal output from the amplifier reaches the thresholdof the D type flip plop, a discharge pulse, regulated by theclock circuit, is generated. The pulse introduce through theR10. The discharge pulse injects to the integrate amplifier
though the D2 and R2.

Fig4: The circuit principle of I/F converter
The electronics circuit block diagram of the digitizer
showed as Fig 6.
2.2 The key components
Fig7: Discharge circuitEDischarge
CircuitA

Current
Integration

Amplify·Gain=1000

B

DSETQDCLRZero AdjustingRegulation

QCClockregulator
Fig6: The block diagram of the digitizerFshows as Fig. 8.
Acknowledgement
4 The read out instrument
A Digital Data Logger (DDL) is used for system read out.
The original idea of DDL is from Radiation protectiongroup at CERN. It was developed in the earlier of 1980th.
The microprocessor Intel 8080 was used for DDL. In themiddle of 1980th, when The Beijing electron positron
Collider and the synchrotron radiation facility in Hefei wasbuilt, we designed our own DDL for radiation monitoringsystem. Motorola micro processor 6809 was employed in
the DDL. During the upgrade project of the above twoaccelerators in the earlier of 1990th, a new DDL wasdesigned and built with MC68HC11. The performances of
the DDL are improved grately.
5 Testing and result
The complete system has been well tested. When
HV=1300V, local radiation background level = 0.13µSv,
the system gives 41 count/min. It means the sensitivity ofthe tube and the digitizer is good enough. While measuringwith radiation source 564mCi 137 Cs and at the distance 1.5,2, 3m, the linearity is also excellent. The testing result

We would like to thank Dr. Li Jiyuan, who made the very
nice detectors for us.
In fact the most system testing was be done in theInstitute of High Energy Physics. The Dr. Li jianping andmany members in his group, such as Mr. Liu Liefu, Liu
Shudong, supported us strongly to all of the project.
References[1]NRC INDUSTRIES, DIVISION OF NUCLEARRESEARCH CORPORATION, IP-100 Operation andMaintenance Manual(V1),(V2)[2]Keithley Instruments,Inc, Low Level Measurements,1993 4th edition[3]National Semiconductor Corporation, Operational
Amplifiers Databook,1993 Edition[4]Li Jian-Ping, Tang Yue-li, Shao Bei-Bei, Intelligent
Environmental Neutron and Gamma Monitoring System,Chinese Physics, Vol.9, No.2, April-June 19889

1000000
Count/min

100000

1.5m

2m

3m

H.V.

10000
1000

50
15025035045055065
075
085
095
0
1050
1150
1250
1350
1450
1550
1650

100Fig8. Testing with 564mCi 137 Cs and at the distance 1.5, 2, 3m