Comparison between Various Designs of Micropattern Detectors

Comparison between Various Designs of Micropattern Detectors

DOI: 10.4018/978-1-4666-6014-4.ch011
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In this chapter a comparison between various designs of micropattern detectors is given, describing their specific advantages and disadvantages, which finally determines the fields of their applications. It is shown that at low counting rates the maximum achievable gas gain is determined by the Raether limit, which is about 106-107 electrons, depending on the design. At high counting rates, the maximum achievable gain additionally drops due to the contribution of several other effects (e.g. avalanches overlapping in space and time). Typically, micropattern detectors have a position resolution of ~30 µm, energy resolution of ~ 20% FWHM for 6 keV X-rays, and a time resolutions of ~1 ns. Some advanced designs offer even better characteristics. The diversity of micropattern detectors makes them attractive for many applications. For example, in measurements requiring simultaneously excellent time and position resolutions, mutigap multistrip detectors can be used in high energy physics applications, and hole-type structures are advantageous for the detection of visible photons. In some commercial applications, where reliability and robustness are important, spark protected detectors with resistive electrodes could be useful.
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2. Gain Characteristics

2.1 Maximum Achievable Gains at Low Courting Rates

In spite of the great variety of micropattern detector designs described in the previous Chapters, one can clearly observed that their maximum achievable gains at low counting rates are comparable, ~104 when measured with 6 keV photons. This is almost 100 times lower than “classical” gaseous detectors like PPACs and MWPCs can offer.

As was shown through the previous Chapters, the maximum achievable gas gain of all micropattern detectors is limited mainly by two factors:

  • 1.

    By defects (tips, dirt, insertions) on electrodes and dielectric surfaced created during the production or during the detector handling

  • 2.

    In high quality detectors, free of defects, the breakdowns appear due to the Rather limit (Fonte, 1999).

The exact value of the Raether limit depends on the particular design and also on the gas mixture, gas pressure. Typically it is in the range of 106-107 electrons. The Raether limit increases almost linearly with the width of the avalanche gap (Peskov, 2001). For example for GEM it is ~106 electrons whereas for TGEM it is ~107 electrons. Hence, all small gap detectors (MSGC, GEM, MICROMEGAS etc.) have a fundamental limit in achieving a high gas gain.

One of the ways to increase the maximum achievable gain is to use cascaded detectors. In this case each amplification element operates at lower voltages so the role of defects is strongly suppressed. Due to the diffusion of ”hot” electrons exiting the preamplification structure the avalanche volume in the last multiplication stage becomes larger and this increases the Rather limit (Fonte, 1998). On the other hand, the Raether limit as function of the multiplication steps have a tendency for saturation. So in practice two-three steps configuration looks to be optimum (Peskov, 2001). This offers a good compromise between simplicity and reasonably high achievable gains. Therefore, many applications triple GEMs, triple TGEM/RETGEM are used or MICROMEGAS in combination with one or two GEMs used as a preamplification structure.

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