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manual-main fixed, detaildoc not, but for developers and in next release
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Dhanya Maliakal
@ -7,8 +7,8 @@ original version by: Nikos Drakos, CBLU, University of Leeds
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Jens Lippmann, Marek Rouchal, Martin Wilck and others -->
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<HTML>
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<HEAD>
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<TITLE>Which detector settings should I choose?</TITLE>
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<META NAME="description" CONTENT="Which detector settings should I choose?">
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<TITLE>In which X-ray energy range can I use the detector?</TITLE>
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<META NAME="description" CONTENT="In which X-ray energy range can I use the detector?">
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<META NAME="keywords" CONTENT="slsDetectors-FAQ">
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<META NAME="resource-type" CONTENT="document">
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<META NAME="distribution" CONTENT="global">
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@ -26,30 +26,30 @@ original version by: Nikos Drakos, CBLU, University of Leeds
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<BODY >
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<A NAME="tex2html299"
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<BR>
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<B> Next:</B> <A NAME="tex2html300"
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HREF="node16.html">How do I chose</A>
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<B> Up:</B> <A NAME="tex2html296"
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HREF="node14.html">Single photon counting detectors</A>
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<B> Previous:</B> <A NAME="tex2html290"
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HREF="node14.html">Single photon counting detectors</A>
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<B> <A NAME="tex2html298"
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<B> Next:</B> <A NAME="tex2html346"
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HREF="node16.html">What limits the maximum</A>
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<B> Up:</B> <A NAME="tex2html342"
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HREF="node14.html">General questions about detectors</A>
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<B> Previous:</B> <A NAME="tex2html336"
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HREF="node14.html">General questions about detectors</A>
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<B> <A NAME="tex2html344"
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HREF="node1.html">Contents</A></B>
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<BR>
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<BR>
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@ -58,95 +58,152 @@ original version by: Nikos Drakos, CBLU, University of Leeds
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<A NAME="CHILD_LINKS"><STRONG>Subsections</STRONG></A>
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<UL>
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<LI><A NAME="tex2html301"
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HREF="node15.html#SECTION00311000000000000000">MYTHEN</A>
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<LI><A NAME="tex2html347"
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HREF="node15.html#SECTION00311000000000000000">Sensors</A>
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<LI><A NAME="tex2html348"
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HREF="node15.html#SECTION00312000000000000000">Frontend electronics</A>
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</UL>
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<!--End of Table of Child-Links-->
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<HR>
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<H1><A NAME="SECTION00310000000000000000">
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Which detector settings should I choose?</A>
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In which X-ray energy range can I use the detector?</A>
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</H1>
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<P>
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The choice of the operation settings is very important in order to obtain good quality data.
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<BR>
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Normally slower settings will reduce the electronics noise and therefore it is possible to work at lower energies, but will saturate for high photon fluxes.
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<BR>
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On the other hand, faster settings will allow to work with higher photon intensities without pileup, but not to access lower energies because of an higher electronics noise.
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<BR>
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Therefore it is extremely important to chose adequate settings for the detector depending on the X-ray energy and expected maximum count rate.
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In the following is a description of the energy and intensity range coverd by the different settings for each detector.
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What limits the energy range in which the detector can be used is defined both by the sensors characteristics and the readout electronics.
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<P>
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<H2><A NAME="SECTION00311000000000000000">
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MYTHEN</A>
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Sensors</A>
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</H2>
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Most of the SLS detectors make use of silicon sensors.
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<P>
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Since silicon is a relatively light for hard X-rays the only limitation at high energies is the acceptable absorption efficiency that can be achieved in the sensors thickness.
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<BR>
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Figure <A HREF="#fig:effidet">2.1</A> shows the absorption efficiency as a function of the X-ray energy and detector thickness. Normally it is possible to use sensors up to 1 mm thick, while to achieve larger absorption thicknesses it is necessary tu assemble and control telescopic systems (possible up to a few mms).
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<BR>
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To achieve larger absorption thicknesses, the sensors can be oriented in edge-on configuration (in particular strip sensors). However in this case one should take into consideration the dead entrance window due to the cutting distance from the strips, which is normally several hundreds micron, or even up to mms and reduces the absorption efficiency at lower energies.
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<P>
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<DIV ALIGN="CENTER"><A NAME="fig:effidet"></A><A NAME="1066"></A>
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<TABLE>
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<CAPTION ALIGN="BOTTOM"><STRONG>Figure 2.1:</STRONG>
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Efficiency of a silicon sensor as a function of the sensors thickness and X-ray energy.</CAPTION>
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<TR><TD><IMG
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WIDTH="556" HEIGHT="537" ALIGN="BOTTOM" BORDER="0"
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SRC="img14.png"
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ALT="\includegraphics[width=\textwidth]{images/effiSiHardXRays2}"></TD></TR>
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</TABLE>
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</DIV>
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<P>
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In standard face-on orientation, the backplane of the sensor acts as the entrance window. It presents a think n+ doped layer, which is unsensitive to radiation and causes a loss of efficiency at low energies.
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Figure <A HREF="#fig:effiback">2.2</A> shows the absorption efficiency of the sensors for different backplane thicknesses at low energies.
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<BR>
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The exact thickness of the backplane for standard SLS sensors is not exactly known but should be about 1-2 <IMG
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WIDTH="14" HEIGHT="30" ALIGN="MIDDLE" BORDER="0"
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SRC="img15.png"
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ALT="$ \mu$">
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m.
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<P>
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<DIV ALIGN="CENTER"><A NAME="fig:effiback"></A><A NAME="1072"></A>
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<TABLE>
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<CAPTION ALIGN="BOTTOM"><STRONG>Figure 2.2:</STRONG>
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Efficiency of a silicon sensor as a function of the X-ray energy for different thicknesses of the backplane.</CAPTION>
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<TR><TD><IMG
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WIDTH="554" HEIGHT="538" ALIGN="BOTTOM" BORDER="0"
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SRC="img16.png"
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ALT="\includegraphics[width=\textwidth]{images/effiThinkBackplanes}"></TD></TR>
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</TABLE>
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</DIV>
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<P>
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However for lower energies, the main limitation is normally given by the noise of the frontend electronics (if single photon resolution is required).
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<BR>
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For higher energies it is also possible to use different sesnor materials as CdTe or Ge, although up to now they cannot provide the same signal quality as silicon.
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<P>
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<H2><A NAME="SECTION00312000000000000000">
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Frontend electronics</A>
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</H2>
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<P>
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Normally the user can follow these rules:
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The limitations on the energy range arising from the readout electronics come from the noise and from saturation.
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The electronic noise limits the minimum detectable energy for single photons, while saturation limits the maximum detectable signal either for single photons or in total.
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<OL>
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<LI>If the X-ray energy is lower than 8 keV the <I>High gain</I> setting should be used. Since it is a slow mode of operation it is necessary to take care that the maximum count rate is lower than 100 kcounts/s for all channels (use filters to reduce the beam intensisty).
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</LI>
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<LI>For energies higher than 8 keV, the <I>Standard</I> setting is normally fine if the count rate can be kept lower than 300 kcounts/s for all channels (use filters to reduce the beam intensisty).
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<UL>
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<LI>In <B>single photon counting detectors</B>, the minimum threshold cannot be set lower than 3-5 times the electronic noise.
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If the threshold is set at approximately half of the X-ray energy (see specific documentation about single photon counting detectors), the minimu detectable energy will be about 6-10 times the noise.
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<BR>
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In order to reduce the noise of the frontend electronics different settings can be chosen, but this puts a limit on the maximum incoming flux that can be detected without incurring in pileup (see specific documentation about single photon counting detectors). Figure <A HREF="#fig:mythensett">2.3</A> shows an example of the settings used for the MYTHEN detector for different energy ranges and fluxes.
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<BR>
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For state of the art single photon counting detectors, the minimum thrshold can be about 2-3 keV (details depend on the detector and can be further reduced using special settings).
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<P>
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Concerning saturation, this imposes a maximum value for the comparator threshold. Normally photons of higher energies can still be detected, but without resolution concerning the threshold energy and eventually losing spatial resolution.
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By changing the settings it is possible to increase the maximum threshold value (normally also noise increases in this case).
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<P>
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</LI>
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<LI>In case a larger count rate is required in order to keep the acquisition time shorter, the <I>Fast</I> setting must be selected. However the maximum count rate should never exceed 1 Mcounts/s for all channels.
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<LI>For <B>charge integrating detectors</B> the electronics noise puts a limit on the minimum detectable signal. Therefore if single photon resolution is required, the minimum detectable energy is defined as for single photon counting detectors at about 6-10 times the electronic noise. In case no single photon resolution is required, the electronic noise will put a limit on the sensitivity of the detector i.e. the total accumulated signal needs to be larger than 6-10 times the noise in order to be detected (also about 2-3 keV depending on the detector). It is important to point out that the acquisition time of charge integrating detectors is limited by the leakage current of the sesnors and the noise quadratically sums out. Therefore the signal for low energy photons should be strong enough to be acquired during single frames.
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<P>
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Concerning saturation, this sets a limit on the total number of photons acquired during the acquistion slot and is normally much larger than the energy released by single X-rays. Dynamic gain switching can strongly increase the dynamic range of the detector up to 10E+4 12 keV photons.
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<P>
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</LI>
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</OL>
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</UL>
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<P>
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<DIV ALIGN="CENTER"><A NAME="fig:settings"></A><A NAME="1073"></A>
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<DIV ALIGN="CENTER"><A NAME="fig:mythensett"></A><A NAME="1083"></A>
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<TABLE>
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<CAPTION ALIGN="BOTTOM"><STRONG>Figure 2.1:</STRONG>
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Plot indicating the reccomended choice of detector settings as a function of the X-ray energy and maximum count rate per channel..</CAPTION>
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<TR><TD>
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<DIV ALIGN="CENTER">
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<IMG
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<CAPTION ALIGN="BOTTOM"><STRONG>Figure 2.3:</STRONG>
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Settings to be chosen for the MYTHEN detector as a function of the X-ray energy and radiation intensity.</CAPTION>
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<TR><TD><IMG
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WIDTH="555" HEIGHT="284" ALIGN="BOTTOM" BORDER="0"
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SRC="img13.png"
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ALT="\includegraphics[width=\textwidth]{images/settings}">
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</DIV></TD></TR>
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SRC="img17.png"
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ALT="\includegraphics[width=\textwidth]{images/settings}"></TD></TR>
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</TABLE>
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HREF="node16.html">What limits the maximum</A>
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<ADDRESS>
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Thattil Dhanya
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2017-08-22
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2017-08-23
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