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Use of the variance-covariance method in radiation protection


Measurements in the field of a diagnostic X-ray tube have been performed with a 2 pulse generator. The beam was filtered with 1 mm of aluminium. The twin detector consists of 2 cylindrical proportional counters. The plastic detector walls have a thickness equivalent to 13 mm of tissue. The detector currents are integrated on capacitors. The voltage at the capacitors is digitized and the results stored on a computer. Calibration of the proportional counters was performed with an americium-241 alpha source. As a side product to this the Townsend coefficients for methane based tissue equivalent gas have been determined for a broad range of reduced field strengths. In further measurements variations of the dose averaged lineal energy, y d, during the 10 ms time interval of the high voltage pulse of the X-ray tube have been determined.

In a theoretical analysis the inherent possibilities of the variance covariance method for suppression of noise and electric pickup have been examined. Several types of disturbances have been considered. Preliminary measurements have been performed in the photon and electron fields of a 20 MV linear accelerator. Tocope with the high dose rate 2 improvements of the instrumentation are necessary. Increased sampling frequency will reduce the dose per sampling interval and smaller detectors will reduce the current delivered. In the experimental system, the signal processing has been improved by the implementation of faster and more accurate analogue to digital conversions (ADC) and the mechanical device for pressure stabilization in the gas flow system has been replaced by an electronic pressure control. In addition the gas flow is now adjusted by a mass flow control.

Work is being done to transform a system designed for variance covariance measurements in pulsed therapeutic X-ray and electron fields into one appropriate for measurements in continuous cobalt-60 beam, regarded as an intermediate step before measuring in diagnostic X-rays. The proportional counters were found to be unsuitable for continous beam measurements so it has been decided to apply new detectors. Ionization chambers and charge integration have been chosen to achieve a more satisfactory calibration. 2 Keithley 617 programmable electrometers together with a computer controlled circuit with relay switches are designed to investigate the charge from both chambers in separate integrators in simultaneous intervals. The electrometers are connected to the computer via a bus and new software has been developed to control the integration and handle the collected data. A prototype twin chamber has been used for a general testing of the system. After testing it was decided to change the integration method to avoid interference on the signals from the relay switches. This can be done without major changes of the system. A new pair of cylindrical detectors has been designed and manufactured together with a vacuum housing which can be used for several chamber designs. Testing of the chambers has started.

4 cylindrical ionization chambers have been constructed for the measurement of absorbed dose in water for X-ray beams. 2 detectors have been made with tissue equivalent plastic and 2 with air equivalent plastic. 4 X-ray radiation beams have been investigated. The photon spectral distributions have been investigated with a compton spectrometer and the 90 degree scattered radiations from a small scatter were measured with a hyper pure germanium detector. From the measurements the spectra at the position of the scatter was reconstructed. The measured spectras were in good agreement with expected spectra. Variance covariance measurements were planned to determine the dose averaged lineal energy and to use this as an indication of the radiation quality. 2 types of electrometers, one with a resistor in the feedback circuit and the other with a capacitor, were designed and tested. The electrometers have been calibrated over a one year period. A computer program has been written for the variance covariance method which allows automatic measurements with both types of electrometers. A Monte Carlo code has been installed for simulation of the measurements.
Reductions of the dose limits in radiation protection and changes of the quality factor for densely ionizing radiations will require increased precision in area monitoring and in personal dosimetry. Tissue equivalent proportional counters are increasingly employed for this purpose. However, their routine use in radiation protection requires the variance-covariance method which is an extension of the pulse height determination in two ways: it is not restricted to radiation fields of extremely low dose rate and, unlike the variance methods, it is applicable in the time varying radiation fields that are frequently encountered in radiation protection practice.


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