Medipix 2 in X-ray diffraction

https://doi.org/10.1016/j.nima.2007.01.175Get rights and content

Abstract

The Medipix 2 detector is a detector with a high dynamic range and, in principle, no intrinsic detector noise. This makes the detector perfectly suitable for use in X-ray diffraction experiments.

For X-ray diffraction systems used in materials science, it is common to use X-rays with an energy of 8 keV. When the Medipix 2 read-out chip is bump-bonded to a 300μm silicon sensor it is able to detect these X-rays with a high efficiency.

The Medipix 2 detector has been integrated into a PANalytical X’Pert PRO X-ray diffraction system. Details about this integration will be discussed and measurements performed with this system will be shown.

Introduction

In the past few years, the Medipix 2 collaboration [1] has developed a pixelated single photon counting read-out chip with 256×256 pixels and a pitch of 55μm [2]. Bump-bonding this chip to a 300μm silicon sensor makes a detector capable of detecting 8 keV X-ray radiation with a high efficiency. It turns out that the minimum threshold level of such a detector is in the range of 3–4 keV. With an energy resolution of about 20% for 8 keV X-rays this means that the detector is perfectly suitable for use in X-ray diffraction experiments.

With the successful introduction of the X’Celerator detector [3] in 2001, PANalytical entered the area of 1D solid state position sensitive detectors. This gives a very good solution for many diffraction applications. The aim to improve on specific properties is reached by the new generation of pixel detectors. Since the Medipix 2 collaboration was, at that time, already working on a 2D pixel detector, PANalytical has joined this development.

PANalytical has been an industrial partner of the Medipix 2 collaboration since the year 2001 with the aim to incorporate the Medipix 2 detector in their X-ray diffraction equipment. In the current product line of PANalytical the Medipix 2 detector will be used as a 0D and 1D detector. The reason for this is that for many applications 0D or 1D detection is sufficient. As an outlook, additional applications exploiting the full 2D capabilities are under development.

Using the Medipix 2 detector in 0D or 1D mode will result in a detector with an enormous dynamic range for certain applications which will be demonstrated for high resolution and reflectivity measurements. In many cases, beam attenuators are not needed anymore.

Section snippets

Technical aspects

In many diffraction experiments, data needs to be measured over a large angular range. With an active length of 14 mm, the Medipix detector gives excellent data over a limited angular range. In order to cover large ranges we use the detector in scanning mode. In this situation the detector is moved continuously and acquisitions are taken each time the detector has moved a pitch. The resulting acquisitions are added taking into account the movement of the detector (called “shifted addition”). The

Charge sharing

One of the things one should be aware of when using a Medipix 2 detector as a 1D or 0D detector is that it will not be able to measure ALL the incoming photons. The reason for this is charge sharing [5].

The number of photons counted with various setting of the lower level discriminator is shown in Table 1. [6].

From Table 1 we can conclude that the number of photons measured diminishes rapidly with increasing lower threshold. To get good performance for 8 keV the lower level should be set

Stability

A key property of a detector is that is should be stable. To test this we made a large series of scans with a powder diffractometer. A typical setup of such a measurement is shown Fig. 2. In our setup, no special optics were used to reduce the background in the experiment, the main interest was to see if the Medipix detector could measure the patterns in a reproducable way. The result for (a part of) a thousand of such scans is shown in Fig. 3. From the type of measurements shown in Fig. 3 we

More examples

One of the areas where a Medipix 2 detector can play an important role is reciprocal space mapping (see Ref. [7] for more details on this technique) for semiconductor devices. In Fig. 8, Fig. 9 the measurements of a (2 2 4) reflection of a sample with a GaAs substrate are shown. Fig. 8 is measured with the X’Celerator, Fig. 9 with the Medipix 2 detector. With the Medipix 2 detector we clearly can see two different peaks. With the X’Celerator this is not the case. The difference is due to the fact

The future

The Medipix 2 detector is a perfect building block for many applications. Currently, the detector can be used as a 1D or 0D (scanning) detector in PANalytical X-ray diffraction equipment. However, the active area of one chip is small, about 2cm2. One obvious way to come to larger active areas is to tile chips together to form larger areas. This belongs to the contents of the RELAXD [8] project. The aim of this project is to come to large area detectors with a very short read-out time and

Acknowledgements

The development of the Medipix 2 detector for apply in X-ray diffraction would not have been possible without the support of the Medipix2 collaboration. Especially, we want to thank Michael Campbell, Xavier Llopart, Lukas Tlustos and Erik Heijne from CERN for their intense support during the project. We want to thank Jan Visschers and his group from NIKHEF, the Netherlands, for fruitful discussions and their contribution in the design of the read-out electronics.

References (8)

  • X. Llopart et al.

    Nucl. Instr. and Meth. A

    (2003)
  • See...
  • Philips Analytical, J. Appl. Cryst. 34 (2001)...
  • D.S.S. Bello

    Nucl. Instr. and Meth. A

    (2003)
There are more references available in the full text version of this article.

Cited by (18)

  • X-ray coherent scattering in metal physics

    2012, Comptes Rendus Physique
  • 10 years of the Medipix2 Collaboration

    2011, Nuclear Instruments and Methods in Physics Research, Section A: Accelerators, Spectrometers, Detectors and Associated Equipment
    Citation Excerpt :

    Many images have been taken which demonstrate that the potential of noise-free imaging is fully realised with the Medipix2 chip using Si sensors [60,139,153], high Z sensors [27] and gas as the detecting medium [6]. The Medipix2 chip is also being exploited for commercial X-ray materials analysis by PANalytical, Almelo, The Netherlands mainly because of the extended dynamic range provided by the single photon counting approach [11,32,76]. The single photon counting technique provides outstanding images at very low rates enabling high resolution phase contrast imaging with micro-focus X-ray sources [67] (see Fig. 9) and other pseudo-coherent X-ray sources [79].

  • A Gigabit per second read-out system for Medipix Quads

    2011, Nuclear Instruments and Methods in Physics Research, Section A: Accelerators, Spectrometers, Detectors and Associated Equipment
    Citation Excerpt :

    These attractive features are very useful in X-ray applications such as material analysis. PANalytical produces the first commercial product based on the Medipix2 chip, the PIXcel detector in their X-ray diffraction systems [1]. Although there is a lot of interest from various fields to use the Medipix chips, one common limitation is the small size of the Medipix chip.

  • Adding functionality to microchips by wafer post-processing

    2007, Nuclear Instruments and Methods in Physics Research, Section A: Accelerators, Spectrometers, Detectors and Associated Equipment
View all citing articles on Scopus
View full text