The dynamic range of the X-ray diffracted signal is usually more than six orders of magnitude for material science studies. As these requirements cannot be satisfied by the available detectors, a new detector using hybrid pixel technology has been developed by the BM2 beamline (D2AM/CRG) in collaboration with the CPPM/IN2P3, a laboratory participating in CERN detector development.

In the design of the X-ray Pixel chip with Adaptable Dynamics (X-PAD), we aimed for low noise, high dynamic range and fast read-out. The first prototypes were designed to use the diodes from the Delphi experiment leading to modules containing 6000 pixels of 330 x 330 µm2. These have been tested using synchrotron radiation on the BM2 beamline.

The dynamic response measurement results are shown in Figure 129. Low count rates of about 0.01 photons/pixel/s have been measured and high count rates near 106 photons/pixel/s have been reached. It should be emphasised that neighbouring pixels are not disturbed by loss of linearity in one pixel, which is not the case for CCDs where blooming appears.

Fig. 129: Counted photons on 2 neighbouring pixels versus incident flux.

In SAXS experiments, most of the results arise from scaling laws. This requires a large dynamic range. Figure 130 corresponds to a 400-second exposure of silver behenate. During the test of the XPAD detector, the beam stop used with the CCD settings was shifted upwards allowing us to record high intensities near the direct beam, i.e. at very low scattering vector. We measured up to 7 orders of diffraction with an intensity scale of 4 orders of magnitude. The white spots correspond to untunable pixels.

Fig. 130: SAXS on silver behenate recorded at 20 keV.

A test experiment using the diffraction high-resolution settings was also carried out: the reciprocal space zone around an intense five-fold axis of CdYb icosahedric quasicrystal was studied.

A small oscillation allowed integration of the Bragg peaks (Figure 131a) and the detection of some diffuse intensity. However the knowledge of the diffuse scattering requires mapping the reciprocal space obtained through fixed snap shots as shown in Figure 131b. The diffuse scattering associated with the out-of-Bragg condition peak in the middle is revealed and its extension in reciprocal space can now be defined.

Fig. 131: Bragg peaks and diffuse scattering of CdYb icosahedral quasicrystal at 20 keV. a) oscillation, b) fixed, inserts are zooms using a gray scale shifted by 2 orders of magnitude.

As technological improvements enable us to reach 100-150 µm2 with similar or enhanced performance, smaller pixel sizes will become available. Using modular technology, a large detector with a typical size of 106 pixels is expected for 2004.

Principal Publication and Authors
J.-F. Bérar (a,c), L. Blanquart (d), N. Boudet (a,c), P. Breugnon (b), B. Caillot (a,c), J.-C. Clemens (b), P. Delpierre (b), I. Koudobine (b), C. Mouget (c), R. Potheau (b) and I. Valin (b), J. Appl. Cryst. 35, 471-476 (2002).
(b) CPPM-IN2P3, Marseille (France)
(c) Laboratoire de Cristallographie, CNRS, Grenoble (France)
(d) LBNL, Berkeley (USA)