<html><body style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space; ">Hello Tom,<div><br><div><div>On Jul 30, 2008, at 6:17 AM, wokaoyan1981 wrote:</div><blockquote type="cite"><span class="Apple-style-span" style="border-collapse: separate; color: rgb(0, 0, 0); font-family: Helvetica; font-size: 12px; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: normal; orphans: 2; text-align: auto; text-indent: 0px; text-transform: none; white-space: normal; widows: 2; word-spacing: 0px; -webkit-border-horizontal-spacing: 0px; -webkit-border-vertical-spacing: 0px; -webkit-text-decorations-in-effect: none; -webkit-text-size-adjust: auto; -webkit-text-stroke-width: 0; "><div><div><div style="text-indent: 15.75pt; margin-top: 0cm; margin-right: 0cm; margin-left: 0cm; margin-bottom: 0.0001pt; text-align: justify; font-size: 10.5pt; font-family: 'Times New Roman'; "><span lang="EN-US">Thank you for your wonderful suggestions. I delete components behind collimator 1 and set a series of<span class="Apple-converted-space"> </span><span> </span>monitors instead . Parameters dist, xw and yh are set as the following:</span></div><div style="text-indent: 15.75pt; margin-top: 0cm; margin-right: 0cm; margin-left: 0cm; margin-bottom: 0.0001pt; text-align: justify; font-size: 10.5pt; font-family: 'Times New Roman'; "><span lang="EN-US">1: xw=<st1:chmetcnv tcsc="0" numbertype="1" negative="False" hasspace="False" sourcevalue=".06" unitname="m" w:st="on">0.06m</st1:chmetcnv></span><span style="font-family: 宋体; ">、</span><span lang="EN-US">yh=<st1:chmetcnv tcsc="0" numbertype="1" negative="False" hasspace="False" sourcevalue=".11" unitname="m" w:st="on">0.11m</st1:chmetcnv></span><span style="font-family: 宋体; ">、</span><span lang="EN-US">dist=<st1:chmetcnv tcsc="0" numbertype="1" negative="False" hasspace="False" sourcevalue="2" unitname="m" w:st="on">2.000m</st1:chmetcnv></span><span style="font-family: 宋体; ">（</span><span lang="EN-US">prototype</span><span style="font-family: 宋体; ">）</span><span lang="EN-US">;</span><span style="font-family: 宋体; "></span></div><div style="text-indent: 15.75pt; margin-top: 0cm; margin-right: 0cm; margin-left: 0cm; margin-bottom: 0.0001pt; text-align: justify; font-size: 10.5pt; font-family: 'Times New Roman'; "><span lang="EN-US">2: xw=<st1:chmetcnv tcsc="0" numbertype="1" negative="False" hasspace="False" sourcevalue=".08" unitname="m" w:st="on">0.08m</st1:chmetcnv></span><span style="font-family: 宋体; ">、</span><span lang="EN-US">yh=<st1:chmetcnv tcsc="0" numbertype="1" negative="False" hasspace="False" sourcevalue=".13" unitname="m" w:st="on">0.13m</st1:chmetcnv></span><span style="font-family: 宋体; ">、</span><span lang="EN-US">dist=<st1:chmetcnv tcsc="0" numbertype="1" negative="False" hasspace="False" sourcevalue="2" unitname="m" w:st="on">2.000m</st1:chmetcnv>;</span></div><div style="text-indent: 15.75pt; margin-top: 0cm; margin-right: 0cm; margin-left: 0cm; margin-bottom: 0.0001pt; text-align: justify; font-size: 10.5pt; font-family: 'Times New Roman'; "><span lang="EN-US">3: xw=<st1:chmetcnv tcsc="0" numbertype="1" negative="False" hasspace="False" sourcevalue=".08" unitname="m" w:st="on">0.08m</st1:chmetcnv></span><span style="font-family: 宋体; ">、</span><span lang="EN-US">yh=<st1:chmetcnv tcsc="0" numbertype="1" negative="False" hasspace="False" sourcevalue=".13" unitname="m" w:st="on">0.13m</st1:chmetcnv></span><span style="font-family: 宋体; ">、</span><span lang="EN-US">dist=<st1:chmetcnv tcsc="0" numbertype="1" negative="False" hasspace="False" sourcevalue="4.049" unitname="m" w:st="on">4.049m</st1:chmetcnv><span class="Apple-converted-space"> </span>(focusing to the M2Exit slit)</span><span style="font-family: 宋体; ">；</span></div><div style="text-indent: 15.75pt; margin-top: 0cm; margin-right: 0cm; margin-left: 0cm; margin-bottom: 0.0001pt; text-align: justify; font-size: 10.5pt; font-family: 'Times New Roman'; "><span lang="EN-US">4: xw=<st1:chmetcnv tcsc="0" numbertype="1" negative="False" hasspace="False" sourcevalue=".08" unitname="m" w:st="on">0.08m</st1:chmetcnv></span><span style="font-family: 宋体; ">、</span><span lang="EN-US">yh=<st1:chmetcnv tcsc="0" numbertype="1" negative="False" hasspace="False" sourcevalue=".13" unitname="m" w:st="on">0.13m</st1:chmetcnv></span><span style="font-family: 宋体; ">、</span><span lang="EN-US">dist=<st1:chmetcnv tcsc="0" numbertype="1" negative="False" hasspace="False" sourcevalue="10.649" unitname="m" w:st="on">10.649m</st1:chmetcnv><span class="Apple-converted-space"> </span>(focusing to the end of the collimator1)</span><span style="font-family: 宋体; ">；</span></div><div style="text-indent: 15.75pt; margin-top: 0cm; margin-right: 0cm; margin-left: 0cm; margin-bottom: 0.0001pt; text-align: justify; font-size: 10.5pt; font-family: 'Times New Roman'; "><span style="font-family: 宋体; "><br></span></div><div style="text-indent: 15.75pt; margin-top: 0cm; margin-right: 0cm; margin-left: 0cm; margin-bottom: 0.0001pt; text-align: justify; font-size: 10.5pt; font-family: 'Times New Roman'; "><span lang="EN-US">The simulation suggests: N increases remarkably while ERR decreases.<span class="Apple-converted-space"> </span></span></div></div></div></span></blockquote><div><br></div><div>Yes, this is indeed the case. Consider a given McStas detector bin. N neutron rays (not actual neutrons but neutron "rays", cf. chapter 4 in the manual) hit this bin.</div><div>The N neutron rays each have a given weight p_j, from which we determine the intensity (in units of neutrons/second) by Sum(p_j). ERR is nothing but the RMS error in units of neutrons/second, given that N rays of individual weight p_j hit the bin. So yes, ERR will decrease with increased N.</div><div><br></div><div>In summary, I and ERR is your intensity with errorbars while N describes your statistics (and hence defines the errorbars).</div><div><br></div><blockquote type="cite"><span class="Apple-style-span" style="border-collapse: separate; color: rgb(0, 0, 0); font-family: Helvetica; font-size: 12px; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: normal; orphans: 2; text-align: auto; text-indent: 0px; text-transform: none; white-space: normal; widows: 2; word-spacing: 0px; -webkit-border-horizontal-spacing: 0px; -webkit-border-vertical-spacing: 0px; -webkit-text-decorations-in-effect: none; -webkit-text-size-adjust: auto; -webkit-text-stroke-width: 0; "><div><div><div style="text-indent: 15.75pt; margin-top: 0cm; margin-right: 0cm; margin-left: 0cm; margin-bottom: 0.0001pt; text-align: justify; font-size: 10.5pt; font-family: 'Times New Roman'; "><br></div><div style="text-indent: 15.75pt; margin-top: 0cm; margin-right: 0cm; margin-left: 0cm; margin-bottom: 0.0001pt; text-align: justify; font-size: 10.5pt; font-family: 'Times New Roman'; "><span lang="EN-US">I don’t know which project to choose since the results differ in neutron intenstiy, divergence and distribution. See I, y_div, x_I in enclosure.</span></div></div></div></span></blockquote></div></div><div><br></div><div>I did not look at your instrument file in detail, so exuse me if I get something wrong. The table above suggests that you are experimenting with focusing from the source? Source focusing is only meant to reduce the solid angle of the produced neutrons, ie. it does not make sense to emit neutrons in full 4PI (as a real world neutron source) when we are only interested in neutrons that hit our guide entrance.</div><div><br></div><div>Beware however; If you reduce the solid angle so much that eg. the first guide element is under illuminated you WILL get faulty results, in terms of underestimated divergence and wrong intensity.</div><div><br></div><div>Will have a real look at your instrument file as soon as time allows.</div><div><br></div><div><br></div><div>Hope this helps,</div><div><br></div><div>Peter Willendrup</div><div><br></div></body></html>