RE: plotting of neutron fluence in flair

From: Vasilis Vlachoudis <>
Date: Tue, 6 Aug 2013 14:33:54 +0000

Hi Kristian,

Please give a look on the neutron lecture of any of our beginers courses.

<Xgeo> = geometric mean of the X axis = e^((logEi + lowE[i+1])/2) = sqrt(Ei * E[i+1])
Isolethargic units, comes out from the concept of the neutron lethargy xi ~= 2 / (A + 2/3)
which is the average logarithmic energy loss of a neutron for each collision with a target nucleus A.

As you can see xi is constant therefore you expect that the neutrons on average lose the same amount of energy
on any collision, therefore if you plot the differential of neutron flux per logarithmic energy (assuming that the capture
is negligible) you should get a flat spectrum in the range from 1eV to few 100 keV.
therefore dn/dlnE = Const <=> E*dn/dE = Const <=> E*Flux = Const

In brief, since the neutron moderation is logarithmic, the isolethargic neutron spectrum gives you immediately
a feeling in which energy region you have more/less neutrons

Best Regards
From: [] on behalf of Kristian Ytre-Hauge []
Sent: 05 August 2013 13:15
Subject: plotting of neutron fluence in flair

Dear Fluka experts,
When scoring neutrons with USRTRACK I believe the output in the tab.lis file is given in the units
[neut. cm-2 GeV-1 primary-1]
Plotting neutron fluence in flair has previously been discussed here and from the discussion I found these statements:
To plot normal fluence like a histogram choose:
X:Low [xl]
Y:Y (with steps)
To plot isolethargic fluence as histogram (logarithmic x-axis):
X = Low [xl]
Y = Y*<Xgeo> (with steps)
My questions are:
1) What are the advantages of an isolethargic plot, and when should it be used?
2) What are the proper units on the Y-axis for such a plot? In the flair tutorial («quick start guide») The fluence is given as: (dn/dlnE). If someone could explain how we arrive at these units it would be much appreciated.

Best regards,
Kristian Ytre-Hauge
Received on Tue Aug 06 2013 - 18:36:11 CEST

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