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<title>Geant4模拟总结（二）</title>
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<h1 class="entry-title">
<a href="Geant4-sim-teach2.html" rel="bookmark"
title="Permalink to Geant4模拟总结（二）">Geant4模拟总结（二）</a></h1>
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<abbr class="published" title="2011-08-02T00:00:00">
二 02 八月 2011
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By <a class="url fn" href="./author/木大.html">木大</a>
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<p>In <a href="./category/物理.html">物理</a>. </p>
<p>tags: <a href="./tag/geant4.html">geant4</a><a href="./tag/模拟.html">模拟</a></p>
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<p>今天要讲的主要是DetectorConstructor的编写。</p>
<p>首先介绍下神马是DetectorConstructor。我们知道唱戏先得搭台子。创建编写DetectorConstructor类就是为模拟搭戏台舞台。比如你要模拟中子穿过晶体的过程。那么你首先得摆一块晶体。要摆一块晶体，除非你已经有一块形状尺寸材料都很理想的晶体了，然后你可以把它摆到某一个位置，放到位置后还可以再转一转方向什么的。否则，你就得自己去定义一块晶体出来，就像上边提到的，定义出晶体的材料、形状、尺寸。然后你就可以把这块晶体放到你想放置的位置了。</p>
<p><strong>首先</strong>，定义材料。比如你想用的材料是纯CsI材料。你要先定义Cs原子和I原子。你可以像这样来定义：</p>
<div class="codehilite"><pre><span class="n">G4Element</span><span class="o">*</span> <span class="n">elementCs</span> <span class="o">=</span> <span class="k">new</span> <span class="n">G4Element</span><span class="p">(</span> <span class="s">"Cesium"</span><span class="p">,</span> <span class="s">"Cs"</span><span class="p">,</span> <span class="mi">55</span><span class="o">.</span> <span class="p">,</span> <span class="mf">132.90543</span><span class="o">*</span><span class="n">g</span><span class="o">/</span><span class="n">mole</span> <span class="p">);</span>
<span class="n">G4Element</span><span class="o">*</span> <span class="n">elementI</span> <span class="o">=</span> <span class="k">new</span> <span class="n">G4Element</span><span class="p">(</span> <span class="s">"Iodine"</span><span class="p">,</span> <span class="s">"I"</span><span class="p">,</span> <span class="mi">53</span><span class="o">.</span> <span class="p">,</span> <span class="mf">126.90447</span><span class="o">*</span><span class="n">g</span><span class="o">/</span><span class="n">mole</span> <span class="p">);</span>
</pre></div>
<p>我们知道，上边的c++代码的语义是：第一行，创建一个指针elementCs，指向一个G4Element类的对象，并初始化该对象。构造函数中使用了四个参数，前两个字符串是该对象代表的原子的名称和简写。第三个参数是该原子的原子充数A，第四个是该原子的摩尔质量。这样，第一行就定义了Cs原子。同样的方法第二行代码定义了I原子。
之后，我们就可以用定义好的两种原子来定义纯CsI这种材料。代码：</p>
<div class="codehilite"><pre><span class="n">G4Material</span><span class="o">*</span> <span class="n">CsI</span> <span class="o">=</span>
<span class="k">new</span> <span class="n">G4Material</span><span class="p">(</span><span class="s">"CsI"</span><span class="p">,</span><span class="mf">4.51</span><span class="o">*</span><span class="n">g</span><span class="o">/</span><span class="n">cm3</span><span class="p">,</span> <span class="mi">2</span><span class="p">,</span> <span class="n">kStateUndefined</span><span class="p">,</span> <span class="mf">273.15</span><span class="o">*</span><span class="n">kelvin</span><span class="p">,</span> <span class="mf">1.0</span><span class="o">*</span><span class="n">atmosphere</span><span class="p">);</span>
<span class="n">CsI</span><span class="o">-></span><span class="n">AddElement</span><span class="p">(</span> <span class="n">elementI</span><span class="p">,</span> <span class="mi">1</span> <span class="p">);</span>
<span class="n">CsI</span><span class="o">-></span><span class="n">AddElement</span><span class="p">(</span> <span class="n">elementCs</span><span class="p">,</span> <span class="mi">1</span> <span class="p">);</span>
</pre></div>
<p>第一行代码，创建一个指针CsI，指向一个G4Material类的对象，并用一些参数来初始化该对象。构造函数中使用了六个参数。分别是名称、密度、几种成份（在这里是表示有两种原子）、状态（气态、液态、未定义等）、温度、压强。其中状态定义为未定，则Geant4可自行通过密度压强以及温度等参数计算其状态。后两行代码则表示这种材料的两种成份是什么。这里说明是前边我们定义的elementI和elementCs，并且原子个数比例是1：1。</p>
<p>经过这两段代码我们就定义出来可以后边使用的取名为CsI的材料。我们后边可以使用CsI这个指针来调用这种材料。</p>
<p>然后，我们还要定义晶体的尺寸和形状。比如我们要定义成立方体。立方体的类是G4Box。我们先用下边一行代码定义一个边长2.5cm的立方体。</p>
<div class="codehilite"><pre><span class="n">G4Box</span><span class="o">*</span> <span class="n">solidCsI_log</span> <span class="o">=</span> <span class="k">new</span> <span class="n">G4Box</span><span class="p">(</span><span class="s">"solidCsI_log"</span><span class="p">,</span><span class="mf">1.25</span><span class="o">*</span><span class="n">cm</span><span class="p">,</span> <span class="mf">1.25</span><span class="o">*</span><span class="n">cm</span><span class="p">,</span> <span class="mf">1.25</span><span class="o">*</span><span class="n">cm</span> <span class="p">);</span>
</pre></div>
<p>上边代码的四个参数分别是名称和立方体的半长、半高、半宽（对应x、y、z三个坐标）。</p>
<p>之后，我们把材料和形状合在一起，合成一个包含材料和形状信息的CsI晶体实体，我们叫它几何体(或者LogicalVolume)。代码：</p>
<div class="codehilite"><pre><span class="n">G4LogicalVolume</span> <span class="o">*</span> <span class="n">CsI_log</span> <span class="o">=</span> <span class="k">new</span> <span class="n">G4LogicalVolume</span><span class="p">(</span><span class="n">solidCsI_log</span><span class="p">,</span> <span class="sr">//i</span><span class="n">ts</span> <span class="n">shape</span>
<span class="n">CsI</span><span class="p">,</span> <span class="sr">//i</span><span class="n">ts</span> <span class="n">materical</span>
<span class="s">"CsI_log"</span> <span class="p">,</span> <span class="sr">//i</span><span class="n">ts</span> <span class="n">name</span>
<span class="mi">0</span><span class="p">,</span><span class="mi">0</span><span class="p">,</span><span class="mi">0</span><span class="p">);</span> <span class="sr">//</span><span class="n">position</span> <span class="n">in</span> <span class="n">self</span> <span class="n">coordinate</span> <span class="nb">system</span>
</pre></div>
<p>这样，CsI_log就已经是我们定义好的一块完整的晶体了。我们现在用下边的代码把它摆放到世界中: <br />
</p>
<div class="codehilite"><pre><span class="n">G4PhysicalVolume</span><span class="o">*</span> <span class="n">CsI_phy</span> <span class="o">=</span> <span class="k">new</span> <span class="n">G4PVPlacement</span><span class="p">(</span><span class="mi">0</span><span class="p">,</span>
<span class="n">G4ThreeVector</span><span class="p">(</span><span class="mf">0.0</span><span class="o">*</span><span class="n">cm</span><span class="p">,</span><span class="mf">0.0</span><span class="o">*</span><span class="n">cm</span><span class="p">,</span><span class="mf">0.0</span><span class="o">*</span><span class="n">cm</span><span class="p">),</span>
<span class="n">CsI_log</span><span class="p">,</span> <span class="sr">//i</span><span class="n">ts</span> <span class="n">logical</span> <span class="n">volume</span>
<span class="s">"CsI_phy"</span><span class="p">,</span> <span class="sr">//i</span><span class="n">ts</span> <span class="n">name</span>
<span class="n">World_log</span><span class="p">,</span> <span class="sr">//i</span><span class="n">ts</span> <span class="n">mother</span> <span class="n">volume</span>
<span class="n">false</span><span class="p">,</span> <span class="sr">//</span><span class="nb">no</span> <span class="n">boolean</span> <span class="n">operation</span>
<span class="mi">0</span><span class="p">);</span> <span class="sr">//co</span><span class="n">py</span> <span class="n">number</span>
</pre></div>
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