Sources of error in the detector.

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Sources of error in the detector.

The factors contributing to possible errors in the measurement of diffraction intensities are summarised below along with estimates of the size of errors.

The calibration image formed by illumination with a uniform flood field typically displays variations in sensitivity of about due to inhomogeneities and imperfections in the plate surface. Use of the image file as a correction to subsequently measured flood field images demonstrated that this correction procedure introduced an uncertainty which was on average much less than the statistical noise in the data.

The intrinsic noise of the image plate scanner as measured by the manufacturers is ADC units. (One ADC unit is approximately equivalent to one absorbed photon.) This value is very small when compared to the dynamic ranges of the detectors, for the MAR scanner and for the RED scanner. The dynamic range of the image plate scanners are limited by the number of bits used by the ADC unit.

The X-ray absorption efficiency of the phosphor is for X-ray energies below and falls off to only with photons.

The linearity of the image plate phosphor has been measured by Amemiya et al [2] for X-rays over six orders of magnitude. The response was found to be linear to within a relative error of 0.05 over the range from 8 to photons/pixel. At higher levels of exposure however the measured intensity falls off gradually. The response is however linear within the dynamic range of the detectors.

Gwyndaf Evans
Fri Oct 7 15:42:16 MET 1994