# What is " isolethargic energy spectrum"?

I read a paper about neutronics simulation calculation, and the source(neutron source) in that simulation is “a neutron beam with isolethargic energy spectrum”.
So, what is “isolethargic energy spectrum”? How can I define such a spectrum in geant4?

Thank you for any help.

Best regards,
jcren.

From this thesis (https://wiki.uib.no/particletherapy/img_auth.php/5/5c/Project_Thesis_-_John_Alfred_Brennsæter.pdf) an isolethargic spectrum is one which is presented in a certain way, sometimes also referred to as flux per unit lethargy. As stated in the thesis it makes the area under graph have a physical quantity when plotting the x-axis on a log scale.

As to defining one in Geant4, as it is merely a way of presenting a spectrum there is no one ‘type’ of spectrum, any spectrum can be converted to an isolethargic energy spectrum. Hope this helps.

Thank you, Katherine Krawec.
I appreciate your explanation and I once think in the same way as you. However, the reason why I asked this question is that I read a paper and the author defines isolethargic energy spectrum(as shown in the picture):

I also found its source is in another paper(New Standard Evaluated Neutron Cross Section Libraries for the GEANT4 Code and First Verification):

So, as you can see, the author intepreted the isolethegic energy distribution as “constant in logarithmic bins”. I’m wondering which quantity is constant and I have not found any clues to this question.

Can anyone help me ?
Thank you and best regards,
jcren.

Maybe it means that the E*(dΦ/dE) is a constant. But I still don’t know how to define it in Geant4.

Hello, I have encountered the same problem and do not quite understand “isolethargic energy spectrum”, have you solved this problem in Geant4 now?