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<li class="toctree-l1 current"><a class="current reference internal" href="">The Basics of Data Reduction</a><ul>
<li class="toctree-l2"><a class="reference internal" href="#the-concept-of-tzero-and-tgood">The concept of t<sub>zero</sub> and t<sub>good</sub></a></li>
<li class="toctree-l2"><a class="reference internal" href="#the-concept-of-dead-time">The concept of dead-time</a></li>
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<div class="section" id="the-basics-of-data-reduction">
<span id="basics-of-data-reduction"></span><h1>The Basics of Data Reduction<a class="headerlink" href="#the-basics-of-data-reduction" title="Permalink to this headline">¶</a></h1>
<p id="index-0">This section explains</p>
<ul class="simple">
<li>the concept of t<sub>zero</sub> and t<sub>good</sub></li>
<li>the concept of dead time correction</li>
<li>how to set the detector calibration factor,</li>
<li>how to group data</li>
</ul>
<p>NOTE: detailed information about functionality contained under the Mantid Analysis GUI TABS -
Home, Grouping Options, Settings, Data Analysis and Results Table – will be referred to and can be
found in the hand-out Mantid 4 muons.</p>
<p>NOTE: from now on we will load data using the Muon Analysis interface</p>
<div class="section" id="the-concept-of-tzero-and-tgood">
<h2>The concept of t<sub>zero</sub> and t<sub>good</sub><a class="headerlink" href="#the-concept-of-tzero-and-tgood" title="Permalink to this headline">¶</a></h2>
<p>Although the timing origin for the muon response in the sample is when the middle of the
muon pulse has reached the sample, the good data region is not obtained until the entire
pulse has arrived at the sample. This time is defined as t<sub>good</sub>. t<sub>zero</sub> on the
other hand is the time at which the middle of the muon pulse reaching the sample. The difference
between t<sub>good</sub> and t<sub>zero</sub> is t<sub>good offset</sub>.</p>
<p>When using the Muon Analysis GUI, tzero and tgood are loaded from the NeXuS file having
been determined by the instrument scientist (NB: you will determine tzero from
experimental data later on) However, for the purpose of demonstration, we will aim to
broadly determine these parameters from the raw data and see how they compare with the
GUI entries.</p>
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<img alt="_images/TgoodTzero.png" src="_images/TgoodTzero.png" />
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<div class="section" id="the-concept-of-dead-time">
<h2>The concept of dead-time<a class="headerlink" href="#the-concept-of-dead-time" title="Permalink to this headline">¶</a></h2>
<p>After a detector has recorded a positron count there is a small time interval before it is able
to detect another count. It is possible, especially at the high data collection rates now
available on the muon instruments, that a positron will arrive within this interval and not be
recorded. Statistical analysis can be used to correct for this. A silver sample is used to
determine dead time values for each detector, the results of which are made into a dead
time data file. The NeXuS format stores this data internally. For further information about
the correction of detector deadtime see: Kilcoyne, RAL report RAL-94-080 (1994).</p>
<p>To explore the effect of dead time:</p>
<ul class="simple">
<li>Load data file EMU00034998.nxs. Make sure ‘none’ is selected for Dead Time Correction</li>
<li>Reload the data file but select “From Data File”. What happens?</li>
<li>Create a plot to show the effect.</li>
</ul>
</div>
<div class="section" id="the-detector-calibration-factor-alpha">
<h2>The detector calibration factor alpha<a class="headerlink" href="#the-detector-calibration-factor-alpha" title="Permalink to this headline">¶</a></h2>
<p>The detector calibration factor, alpha can be determined for any given detector pairing by
clicking on Guess Alpha (Grouping Options) and analysis of transverse field data.</p>
<ul class="simple">
<li>Using the GUI, load transverse field measurement MUSR00024563.nxs</li>
<li>Select the Grouping Options TAB</li>
<li>Notice that when you load a data file using the GUI, the default option for the
MuSR spectrometer is to GROUP (or add) all data in detectors 1-32 (a group
of detectors referred to as BCK) together. Similarly, data in detectors 33-64 (a
group called FWD) is summed.</li>
<li>To generate alpha click on Guess Alpha.</li>
<li>Reload the data file. What has happened?</li>
<li>You can rename groups fwd and/or bck and/or change the detectors to be summed.
Alternatively you can create additional grouping options</li>
<li>In the Group Table, create two new grouping options with names of your
choice that add every 5th detector (i.e. 33,38,43 ... )</li>
<li>Below ‘long’ enter a new Group Pair name for this new grouping option.</li>
<li>Click ‘Guess Alpha’.</li>
<li>Reload the data file BUT before you do choose your new Group Pairing option from
the Group / Group Pairing drop down list. What do you notice about alpha?</li>
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