Small circular beams of synchrotron radiation (0.1 mm and 0.4 mm in diameter) were used to irradiate ionization chambers of the types commonly used in radiotherapy. By scanning the chamber through the beam and measuring the ionization current, a spatial map of the dosimetric response of the chamber was recorded. The technique is able to distinguish contributions to the large-field ionization current from the chamber walls, central electrode and chamber stem. Scans were recorded for the NE 2571 Farmer chamber, the PTW 30013, IBA FC65-G Farmer-type chambers, the NE 2611A and IBA CC13 thimble chambers, the PTW 31006 and 31014 pinpoint chambers, the PTW Roos and Advanced Markus plane-parallel chambers, and the PTW 23342 thin-window soft x-ray chamber. In all cases, large contributions to the response arise from areas where the incident beam grazes the cavity surfaces. Quantitative as well as qualitative information about the relative chamber response was extracted from the maps, including the relative contribution of the central electrode. Line scans using monochromatic beams show the effect of the photon energy on the chamber response. For Farmer-type chambers, a simple Monte Carlo model was in good agreement with the measured response.

Purpose: To map the spatial response of four solid‐state radiation detectors of types commonly used for radiotherapy dosimetry.Methods: PTW model 60016 Diode P, 60017 Diode E, 60018 Diode SRS, and 60019 microDiamond detectors were radiographed using a high resolution conventional X‐ray system. Their spatial response was then investigated using a 0.1 mm diameter beam of 95 keV average energy photons generated by a synchrotron. The detectors were scanned through the beam while their signal was recorded as a function of position, to map the response. These 2D response maps were created in both the end‐on and side‐on orientations.Results: The results show the location and size of the active region. End‐on, the active area was determined to be centrally located and within 0.2 mm of the manufacturer's specified diameter. The active areas of the 60016 Diode P, 60017 Diode E, 60018 Diode SRS detectors are uniform to within approximately 5%. The 60019 microDiamond showed local variations up to 30%. The extra‐cameral signal in the microDiamond was calculated from the side‐on scan to be approximately 8% of the signal from the active element.Conclusions: The spatial response of four solid‐state detectors has been measured. The technique yielded information about the location and uniformity of the active area, and the extra‐cameral signal, for the beam quality used.

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