The NACP electron chamber is one of three parallel-plate chambers recommended for use in the UK. Measurements with this chamber type have indicated a problem in determining the recombination correction. This is due to a variation of the ionization current I with polarizing voltage V which deviates from the accepted Boag theory. It is shown that there is a chamber-dependent threshold voltage below which the NACP chamber follows the Boag theory. Above this voltage the chamber should be used with caution, although it is still possible to correct for the dependence of the chamber response on the dose per pulse. The existence of such deviations from theory demonstrates the usefulness of the 1/I against 1/V plot and the limitations of the Boag two-voltage analysis. Values for the initial recombination and the coefficient of general recombination are measured for several NACP chambers. It is shown that from these one can derive a value for the effective plate separation and the collector radius of each chamber. Differences in the behaviour of NACP chambers manufactured by Scanditronix and Dosetek are discussed and the implications of free-electron collection are considered.

During 1998, NPL plans to introduce the world's first absorbed dose calibration service for electron beam radiotherapy. The service will be based on the primary standard graphite calorimeter, and will enable the direct calibration of electron ionization chambers, without reference to air kerma standards. This calibration is a two-step process. Firstly, a set of NACP-designed parallel-plate reference chambers have been calibrated against the calorimeter over the last few years. These chambers are then used to calibrate user chambers by direct comparison in a water phantom under standard conditions. This paper describes the calibration of the reference chambers against the calorimeter and the derivation of absorbed dose to water calibration factors (with an estimated uncertainty in this calibration of +/-1.50% at the 95% confidence level).

Over several years the National Physical Laboratory (NPL) has been developing an absorbed dose calibration service for electron beam radiotherapy. To test this service, a number of trial calibrations of therapy level electron beam ionization chambers have been carried out during the last 3 years. These trials involved 17 UK radiotherapy centres supplying a total of 46 chambers of the NACP, Markus, Roos and Farmer types. Calibration factors were derived from the primary standard calorimeter at seven energies in the range 4 to 19 MeV with an estimated uncertainty of +/-1.5% at the 95% confidence level. Investigations were also carried out into chamber perturbation, polarity effects, ion recombination and repeatability of the calibration process. The instruments were returned to the radiotherapy centres for measurements to be carried out comparing the NPL direct calibration with the 1996 IPEMB air kerma based Code of Practice. It was found that, in general, all chambers of a particular type showed the same energy response. However, it was found that polarity and recombination corrections were quite variable for Markus chambers-differences in the polarity correction of up to 1% were seen. Perturbation corrections were obtained and were found to agree well with the standard data used in the IPEMB Code. The results of the comparison between the NPL calibration and IPEMB Code show agreement between the two methods at the +/-1% level for the NACP and Farmer chambers, but there is a significant difference for the Markus chambers of around 2%. This difference between chamber types is most likely to be due to the design of the Markus chamber.

This paper describes the development of a robust and portable calorimeter for use in clinical electron and photon beams. Although intended for therapy-level dosimetry, the new calorimeter can also be used for high-dose measurements at industrial facilities. The system consists of a front end (the calorimeter itself), means for thermal isolation and temperature control, and a measurement system based on thermistors in a dc Wheatstone bridge. It was found from investigation that the heat transfer between components was significant. The restrictions on the design placed by the requirement for portability led to higher heat transfer than was desirable. Much effort was put into thermodynamic modelling of the system and determining the heat transfer coefficients. Effort was also focused on the development of a temperature control system sensitive enough to allow measurements of temperature rises of the order of 1 mK. The control system responds to the calorimeter, phantom and air temperatures and maintains the temperature of the calorimeter to within +/-0.2 mK over several hours. Initial operation at the National Physical Laboratory (NPL) in x-ray and electron beams from the NPL linear accelerator shows that the system is capable of measurements of 1 Gy at 2 Gy min(-1) with a random uncertainty of +/-0.3% (1 standard deviation). Operation in 60Co at a doserate of 1 Gy min(-1) has also been achieved with a similar uncertainty. It is intended to test the calorimeter 'in the field' during 2000.

Dosimetry protocols recommend that ionization chambers used in radiation therapy be pre-irradiated until they 'settle', i.e., until a stable reading is obtained. Previous reports have claimed that a lack of pre-irradiation could result in errors up to several per cent. Recently, data collected for a large number of commonly used ion chambers at the Institute for National Measurement Standards, NRC, Canada, have been collated and analysed, with additional data contributed by the National Physical Laboratory, UK. With this data set, it was possible to relate patterns of ion chamber behaviour to design parameters. While several mechanisms seem to contribute to this behaviour, the most obvious correlations implicate the type of insulator surrounding the central collector electrode, the extent of collector electrode shielding and possibly the area of the insulator exposed at the base of the active air volume. The results show that ion chambers with electrode connections guarded up to the active air volume settle quickly (approximately 9 min) and the change in response is small (less than approximately 0.2%). For ion chambers where the guard connection surrounding the central collector does not extend up to the active air volume, settling times of 15-20 min and an associated change in response of up to 1% are typical. For some models of ion chambers, the irradiation rate may also play a role in settling behaviour. Settling times for the ion chambers studied here were found to be independent of beam quality.