Need help understanding my neutron energy spectra

I need some help understanding these results. I’ve done a simple d/Li stripping simulation.

I have 1e8 deuterons of 40 MeV hitting a lithium (G4_Li) target (yellow). The full thickness of the lithium volume is the same as the deuteron Bragg peak in lithium, ~19mm.
Behind it is a target volume (Purple) which I made (G4_Galactic) so I can note only the effects of the deuterons on the Lithium. The target volume is a sensitive detector and is also where I fill my histogram with the kinetic energy of neutrons entering the target volume on there first step.

Below is the same simulation run 3 times, each time with a different physics list show as the title of each histogram.

I believe I should be seeing a broad peak around 19-20 MeV, which is ~ half the deuteron energy, but I don’t see it. Am I missing knowledge/understanding, could there be something wrong in the simulation, or something else?
Also what is the smaller peak around 6 MeV for the INCLXX physics? and why is there such a large difference in the number of neutrons produced between the INCLXX physics and the other 2?

Thank you very very much for any help.


I had the same problem with my 28 MeV deuterons on Carbon :smiley:
What is the angular acceptance of your target volume?
Does it count every neutron leaving the Lithium?
The peak around 19-20 MeV is there for neutrons leaving in forward direction (~0°) - at least this is what i observed for carbon.


Hey thanks :slight_smile:
I didn’t know that the peak would reveal itself at ~0°.
The full angle, I think, the target makes is ~45°. I have to go back and look. I looked at the 2D neutron distribution, fit a Gaussian and decided on the angle that made 1stdev. This is an area I have to work on.

The neutrons counted are only the ones that enter the SD.
I’m gonna reduce the angular size of the target and see what happens.

Since we’re doing similar work, do you have any recommendations for a book (or any resources) on this topic?

Hey thanks again!

Hello again,

I don’t know any books about this but there are plenty of papers around this topic. The physics of neutron beams via a spallation/stripping process came around more than 50 years ago.
I recommend googling Buzzwords like ‘deuteron lithium neutron beam’ or ‘deuteron stripping in light elements’ and see if there is anything useful for you.
An often cited paper is ‘The Production of High Energy Neutron Beams by Stripping’ by R. Serber

but it is more about the angular distribution…

Best Regards!

I found that paper yesterday and it’s pretty good learning.

I’m running simulations now looking at neutrons a small angle away from the beam direction. I’ll likely post them to this thread soon. Not getting what I expected but I think it’s just my understanding at this point.

Again thanks for the help.
You’ve been great.

From a paper, it states that the broad neutron peak is ~15MeV which is what looks to be shown in my histograms below. I presummed the broad peak would be around 19 MeV, but that seems to be just my misunderstanding of the physics.

Final question here. The top-left plot at the top of the thread shows a small peak around 6MeV. I’m still not sure what is causing this. Anyone have a clue? I’m not sure if this is actual physics process or something else in G4.

Hi. Take a look at Figure 6 of “Measurement of Neutron Emission Spectra in Li(d,xn) Reaction with Thick and Thin Targets for 40-MeV Deuterons”, M. Hagiwara, T. Itoga, N. Kawata, N. Hirabayashi, T. Oishi, T. Yamauchi, M. Baba, M. Sugimoto & T. Muroga, Fusion Science and Technology, vol 48, Nov 2005, 1320-1328. In the top right box, the 0 degree neutron flux curve has a peak at ~17 MeV. In the 10 degree curve immediately below it, there is a similar peak but also a slight peak (almost an inflection) at ~5-6 MeV. This small peak is also seen on the 15 degree and possibly 20 degree curves (top right box). It may be due to an excited state in the residual nucleus - there is a broad level in 7Be at 10 MeV, but that is just speculation on my part.

Thank you very much!

This forum has been very helpful.