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A TLD system is described which permits the measurement of absorbed X-ray doses in a water phantom, together with the quality of the incident X-ray beam. The system has been developed for dose and energy intercomparison studies between centres with deep therapy X-ray machines. A distinction has been made between deep therapy treatments and treatments at or near the surface. For beam qualities with a HVL above 1-1 mm Cu, the deep therapy region, measurements have been performed at 5 cm depth in water using CaF2: Mn and LiF (TLD 700). For beam qualities with a HVL of 0-11-1-4 mm Cu, these qualities include the superficial therapy region, measurements with LiF dosemeters at 2 and 10 cm depth in water have been carried out. The uncertainty in the absorbed dose value for X-ray beams with a HVL of 0-11-3-0 mm Cu amounts to +/- 4%; the uncertainty in the determination of the effective energy of the incident beam is +/- 3 to +/- 7%.

Intercomparisons of absorbed doses and dose distributions for X-rays with effective energies 80-1 30 keV, using mailed LiF thermohminescent dosemeters, have been carried out as a part of a co-operative programme of research in Europe on late effects of radiation. The studies have demonstrated a number of discrepancies in the absolute dosimetry. In only six out of fourteen institutes, did the absorbed dose agree t o 5% relative to the standard value; with regard t o the dose distribution, in o d y five out of thirteen institutes were mice irradiations performed in conditions of uniform irradiation. The results of the intercomparisons demonstrate the need for standardization of the X-ray dosimetry if a meaningful comparison of bioIogicaI results is to be achieved.

A mouse-sized test phantom containing TL dosemeters was exposed to X-rays having a HVL value of 1.4-2.8 mm Cu. Dependence of the response of 7LiF dosemeters on the HVL of the incident beam resulted in an increase of TL response per rad in muscle for dosemeters positioned at the central axis of the phantom compared with the response of similar dosemeters exposed free-in-air. An increase of 7.3% in TL response per rad in muscle was found over the HVL range. The spread in effective energy inside the phantom was small and the dose distribution may be estimated directly from the TL response of 7LiF dosemeters at the entrance, central and exit position in the phantom.

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