PUBLICATION #2

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

^(b) Strahlenbiologisches 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 Oberschleisheim, Germany

^(d) Institut für Strahlenhygiene, Bundesamt für Strahlenschutz, Postfach 1108, D-85758 Oberschleisheim, Germany

ABSTRACT

A portable instrument has been developed for determination of the dose average lineal energy y_D 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.

Introduction

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 y_D 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 special applications.

Electronic unit

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 measurement conditions:

Operation parameters:

• 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

MEASUREMENTS

Electronic tests

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 (8,4 Gy/h).

Figure 4. y_D 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.

REFERENCES

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 H.-G.Menzel, p.353, Royal Society of Chemistry, 1997.

ACKNOWLEDGEMENT

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.