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A circuit is described for the direct comparison of the magnitudes of two ionization currents. The circuit principle embodies a capacity potential divider, the scale of which may be calibrated in actual values of the ratio of the two currents. A number of applications of the circuit to ? ray and X-ray dosimetry are indicated, followed by a detailed account of its application to the measurement of radon seed strengths.

The dose comparator apparatus (Kemp, 1946) has been used to measure backscatter factor and percentage depth dose for various field sizes at 50 cm. and 40 cm. F.S.D., using radiation of approximately 1·0 mm. Cu H.V.L. The results are in fairly good agreement with values given by Quimby and Laurence (1940), but there are considerable deviations (up to 30 per cent.) from the figures given in the Mayneord and Lamerton Survey (1941) for large field areas, the latter giving lower backscatter factors and higher percentage depth dose values at large depths. These deviations are shown to be mainly due to the weight given in the Mayneord and Lamerton Survey to results obtained in wax phantoms.Various factors affecting measurements in a phantom have been investigated. These factors include the effects of phantom material, of using a large ionization chamber, or a chamber with a heavy metal stem, and of applicator construction. A comparison of results with the same applicator used on different X-ray sets is also given.It is concluded that measurements with the dose comparator apparatus are free from any significant errors.

Dosage calculators for both unscreened and screened linear radium sources are described. The calculator for unscreened sources may be used for a small number of sources in tandem, including arrangements in which the sources have different line densities. The calculator for screened sources incorporates an electrical method of integration, and takes into account both obliquity and nonhomogeneity of the radiation. It may be used for one source only, or for a small number of sources in tandem provided the sources possess the same line density.

As part of a programme directed towards the optimization of the Laboratory's calibration services—including the calibration techniques themselves—it was decided over five years ago to embark on the development of a secondary standard exposure meter. The aim was to produce an instrument whose specifications and performance would meet to the full the requirements of a secondary standard, and around which an optimized calibration service could be built (Kemp, 1970a, b). The following is a preliminary account of the outcome; a fuller description will be published in due course. It must be emphasized that no attempt has been made to produce an all-purpose instrument, since a basic proposition in the underlying philosophy is that by definition a secondary standard should have the single function of serving as the link between the primary standards and the multiplicity of field instruments.The instrument is comprised of three basic units, namely, the ionization chamber, the measuring circuit, and the stability check source.The design objectives were:(1) long-term stability of a high standard;(2) an effectively level response over as wide an X-ray energy range as possible;(3) the achievement of (1) and (2) without the necessity of adjustment during or after the manufacture of the chamber.The following are the main aspects of the design concerned with the attainment of these objectives:(1) the use of high-purity, high-density extruded graphite as the cap material (Barnard et al., 1964; Kemp and Barber, 1956).

An account is given of a series of precision experiments to investigate cut-off effects in a parallel-plate type of free-air chamber, and the relationship of these effects to the chamber geometry is analysed. The discussion is extended to enable estimates to be made of the errors associated with other parallel-plate chambers. In conclusion, it is shown that deficiencies in the geometry of the standard chamber in use at present at the National Physical Laboratory in England have led to errors approaching 3 per cent in the realisation of the röntgen at ordinary deep-therapy qualities. The corresponding error in the National Bureau of Standards chamber in the United States is estimated to lie between 1 and 2 per cent.

The design of a twin-chamber X-ray intensity comparator is described, which employs a circuit given in an earlier paper (Kemp, 1945). The instrument may be employed with equal facility in work on both therapeutic and diagnostic tubes. Its application to such measurements as those of percentage depth doses, cross-field and along-the-beam intensity distributions in air, half-value layers, dosage rates, and small quantities of X radiation in röntgens are considered in detail, and an example of its use outside the sphere of ordinary radiological measurements is also given.

An apparatus is described which enables X-ray dose distributions in air or in a water phantom to be explored automatically. Apart from its applications in the investigation of routine treatments, the apparatus greatly facilitates the assessment of specialised techniques such as those employing asymmetrical beam filtration. The speed of operation is such that isodose lines can be plotted at the rate of 2·5 to 3·0 cm. per minute, enabling an average set of curves to be obtained in less than two hours. No subsequent paper work is involved.