Browsing by Author "Urquhart, DF"

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    The Australian Commonwealth standard of measurement for absorbed radiation dose
    (Australian Atomic Energy Commission, 1978-09) Urquhart, DF; Johnson, EP; Badger, WS
    As an agent of the Commonwealth Scientific and Industrial Research Organization, the Australian Atomic Energy Commission (AAEC) is responsible for the maintenance of the Australian Commonwealth standard of absorbed dose. A graphite calorimeter was designed and built at the AAEC Research Establishment to provide such a standard for the dose absorbed in carbon from y- and X-ray beams with maximum photon energies above 500 keV. The calorimeter measures the dose averaged over the volume of a thermally insulated absorber disc of diameter 20 mm, thickness 3 mm and mass 1.7 g. The absorber and associated adiabatic and temperature controlled jackets are contained within a small graphite phantom of diameter 15 cm. The depth to the centre of the absorber is adjustable between 2.1 and 10 cm. By using a modified vacuum chamber window, the minimum depth can be reduced to 5 mm for use with high energy electron beams. The calorimeter has been in operation for more than 12 months and no change in its response to a 60Co source has been detected in that time. The precision of dose measurements is ± 0.3 per cent at a dose rate of 4.2 mGy s-' (25 rad min-'). Graphite and water phantoms have been made to enable portable working standards of absorbed dose in carbon and water to be calibrated against the primary standard.
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    Calibration and operation of the AAEC working standard of measurement for the activity of radionuclides, Part 1 - the measurement system
    (Australian Atomic Energy Commission, 1986-05) Urquhart, DF
    A pressurised 4 pi gamma ionisation chamber is used by the AAEC as a working standard of measurement for the activity of radionuclides. The instrument has a high degree of stability and the uncertainty in its activity measurements is only marginally greater than that of the primary standards used in its calibration. This report details the method of calibration for a set of known standardised isotopes and extends the usage of the ion chamber to other isotopes by an interpolation technique. Particular attention is paid to eliminating changes in calibration due to instrumental variations with time. The measurement system is straightforward in operation giving best results for gamma emitting isotopes with energies above 200 keV and minimum activity of the order of 1 MBq.
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    Exposure calibration of a thimble chamber by calorimetric and ionimetric methods.
    (Australian Atomic Energy Commission, 1973-08) Urquhart, DF; Johnson, EP; Badger, WS
    A Baldwin-Farmer thimble chamber, fitted with a perspex build-up cap was calibrated against an absolute graphite cavity chamber to provide an interim working standard of exposure for 137Cs and 60Co photons. The calibration factor (roentgens per coulomb) at 60Co energy was 2% higher than an NPL. 2MV calibration for this chamber, carried out in the UK by the National Physical Laboratory. The thimble chamber, fitted with an aluminium build-up cap, was also used to make a comparison between the graphite cavity chamber and an aluminium micro-calorimeter which is to be used as an interim absorbed dose standard. Exposure calibration factors for the thimble chamber derived from calorimeter measurements were in good agreement with calibration factors derived from cavity chamber measurements, at both 137Cs and 60Co energies.
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    Standardisation of absorbed dose by means of an aluminium calorimeter
    (Australian Atomic Energy Commission, 1974-11) Urquhart, DF; Badger, WS; Johnson, EP
    An aluminium calorimeter was used to determine an absorbed 'dose in water' calibration factor for a thimble ionisation chamber for Co gamma radiation. Several distinct steps were involved in the calibration. An absorbed 'dose in aluminium' calibration factor for the chamber was measured to determine the effect of depth on this factor; this calibration factor was used to transfer experimentally the measured dose in aluminium to dose in a water phantom at depths of 5 cm and 7 cm for a range of field sizes. A build-up factor (to describe the radiation quality) in the water phantom was calculated as a function of depth and field size. Using these steps, an absorbed dose in water calibration factor for the thimble chamber was measured over a limited range of radiation quality. Two further experiments were made to test the overall calibration procedure. The dose in water measurements were used to determine the G-value for ferric ion production by irradiating ferrous sulphate dosimeters for two or three field sizes at depths of 5 and 7 cm respectively. The measured G-values were compared with those obtained by other workers using calorimetry. The long-term stability of the calorimeter was observed over a period of about 14 months, during which time no change was detected in the calorimeter.
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    Standardisation of radioisotopes at Lucas Heights by 4πβ and 4πβ-γ coincidence counting, 1964
    (Australian Atomic Energy Commission, 1964-02) Urquhart, DF
    The principles of 4πβ and 4πβ-γ coincidence absolute counting methods are discussed, with particular reference to the corrections required. The practical aspects of source preparation, instrumentation, instrumental adjustments, and counting procedure are described in some detail. Examples are given of the standardisation of several commonly used isotopes and the results obtained are compared with standards obtained from overseas laboratories. The examples include T1-204 which was standardised in this laboratory while participating in an international intercomparison of this isotope.