Instrument Based on Variance-Covariance Technique for Measurement in
Pulsed Radiation Field
I. Almási (a), E. Anachkova (b,d),
T. Bartha (a),
Dr. Katalin Erdélyi (a), A.M. Kellerer (b,c)
and H. Roos (b)
(a) MicroVacuum Ltd., H-1147
Kerékgyártó u.10. Budapest, Hungary
Institut der Universitä t München, Schillerstrasse 42, D-80336 München, Germany
(c) Institute für
Strahlenbiologie, GSF Forschungszentrum für Umwelt und Gesundheit, Postfach 1129, D-85758
(d) Institut für
Strahlenhygiene, Bundesamt für Strahlenschutz, Postfach 1108, D-85758
A portable instrument has been developed for determination
of the dose average lineal energy yD and the dose equivalent in terms of the
variance-covariance method. For the variance-covariance technique simultaneous
measurements in two detectors and two independent channels of signal processing are
needed. The precise determination of the fluctuation of the energy deposition requires
high precision current measurement. The signal processing technique and the control
software were optimized for the measurement in pulsed radiation field.
In this contribution we are going to present electronic
tests with different pulse parameters. Measurement results at high dose rate provided by
medical 60Co source are demonstrated. Some preliminary experimental results in
pulsed photon and electron fields are also presented.
The variance-covariance method (1) based on the
determination of the fluctuation of the energy imparted in the tissue equivalent
proportional counter provides a technique for determination of the dose average lineal
energy yD and the dose equivalent. A portable, fully battery operated device
was developed in terms of variance-covariance technique and some experimental results at
medium doserate 137Cs field was presented (2). This device provides the
possibility of connecting different TEPCs or ionisation chambers. It can communicate with
computers via RS 232 interface, and the evaluation program can be modified and adapted to
The electronic charge, proportional to the energy imparted
over specified time interval in the two channels of the detector is measured by two fully
symmetrical, low noise, switched electrometers. Each electrometer is connected to an A/D
converter with 20-bit resolution. The sampling time is programmed by the control unit. In
the control unit the control functions and the evaluation program are stored in an EPROM.
The operating parameters and the detector parameters are put on an EEPROM and can be
modified via RS 232 interface. The electrometer readings are stored in a RAM and can be
read out via RS 232 interface. The operating parameters need to be set through the control
buttons on the top panel of the instrument. The measured results are displayed on 8
character LCD display.
The following basic parameters can be adjusted to the
- Sampling frequency: from 2 Hz to 1000 Hz
- High voltage: from 200 V to 1300 V with a step of 50 V
- Number of sampling intervals: 500, 1000 and 2000
- Detector parameters:
- Gas multiplication factor
- Mean chord length
- Air-kerma calibration factor
Electronic tests were made by using test signals with
different pulse height from 1V to 5 V through resistance of 1 Gohm (providing current
pulses of 1-5 nA), with 3 ms pulse width and 5 ms repetition time (simulating the detector
signal in medical accelerator field).
Figure 1. demonstrates the linearity of
the output signal in these operating conditions.
Figure 1. LINEARITY TEST (test pulse
amplitude - output voltage) Parameters: pulse width: 3ms, pulse repetition time: 5ms,
sampling frequency: 10 Hz
We also tested the electronics with pulses that have
different pulse width while the repetition time was chosen to have the same average
current. The results are shown on Figure 2. This test demonstrates the performance of the
resolution of the pulse width.
Figure 2. LINEARITY TEST (test pulse width
- output voltage) pulse amplitude: 5 V, sampling frequency: 10 Hz
Measurements in radiation field
Measurements were made at high dose rate 60CO
field by using 2-channel TEPC detector. The detector was filled with CH4 gas at
50 mbar (simulated diameter 150 nm). The TEPC detector was biased at 400V. Our results are
shown in Figure 3 and in Figure 4. We interpreted the
results in relative units in order to demonstrate the appropriate operation through a
broad range of operation parameters.
Figure 3. Normalised output rate at
different sampling frequency
(normalised to output at 100 Hz), 60Co source
Figure 4. yD at different sampling
frequency 60Co source (8,4 Gy/h)
Preliminary measurements were made at medical linear
accelerator (type NEPTUN) in photon field and in electron field as well. The results for
sampling frequency in the range from 10 Hz to 100 Hz are demonstrated in Figure 5
and Figure 6.
Figure 5. Photon exposure with accelerator
(9MV, 3 Gy/min, 250 pulse/s, pulse width 3ms)
The problem at such measurements at medical accelerator is
to find the optimal parameters for the complicated case of high dose rate, narrow pulse
width and pulse repetition time of the order of 5 ms. From one hand the sampling interval
should be long enough to get some pulses, from the other if the sampling interval is too
long the deposited energy is too large to see the fluctuations of energy imparted, which
are the base of the variance method.
Figure 6. Electron exposure with
accelerator (10 MeV, 3 Gy/min, 100 pulse/s, pulse width 3ms)
In the view of our presented results we are going to
continue our research by optimizing the sensitivity of the detector and the operating
parameters of the electronic unit for the pulsed radiation field at linear accelerators
for therapeutic applications.
A.M. Kellerer and H. H. Rossi, Radiat, Res., 1984, 97, 237
I. Almási, E Anachkova, T. Bartha, Dr. Katalin Erdélyi, A.M. Kellerer and H.
Roos, in Microdosimetry, an Interdisciplinary Approach, eds. D. Goodhead, P.O'Neil and
p.353, Royal Society of Chemistry, 1997.
The authors thank Mr. Béla Pazonyi (National Oncology
Institute, Budapest, Hungary) for the assistance he has provided in the measurements at
the linear accelerator and at the high dose rate 60 Co source.