Mgdraw.f on (n,p) reaction events

Dear experts,

I would like to use mgdraw.f to record the ^{3}He(n,p)^{3}H reaction events of monoenergetic neutrons in a ^{3}He sphere (after this discusion). And I obtained the dump files as wish, but got troubled checking the dump file.

The follwing USDRAW was used to record informations (region 6 is the ^{3}He sphere region, covered by Stainless-steel shell and Polyethy regions):

      IF ( MREG .EQ. 6 ) THEN
     &                    KPART(1:NP), TKI(1:NP)
      END IF

The data in dump file was like:

6         300         113           8  0.93956563005071403     
6         300         263           8  0.93956563005071403                8   3.4885340343050838E-011
6         300         139           8  0.93956563001122295     
6         300          73           8  0.93956563010594019     
6         300          83           8  0.93956563002393911     
6         300          21           8  0.93956563010594019

Here are some points that I cannot understand well:

  1. Why in the KPART(1:NP) there was only neutron(8) without any proton(1) or triton(-4) ?
  2. Does LTRACK=263 means the neutron had been scattered 263 times before entering region 6 ?
  3. Why there is such a difference between the energy in ETRACK and TKI ?

Besides, could you please help me to select the (n,p) events with USDRAW ?
The .inp and mgdraw.f files are attached here:
pe3-test.inp (4.5 KB)
mgdraw_np.f (15.1 KB)

Thanks for your help,

Dear @Jungle,

For the moment, I reply your first question:

This was responded in your previous post by @fogallar. It is related to the way it is constructed the library for low energy neutrons (neutrons below 20 MeV). Due to the groupwise treatment there is no secondary particle produced in FLUKA, except neutrons and gammas. There are some exceptions, where charged products can be generated, but only for special materials such as 10B or 6Li (you can find them in the manual → LOW-NEUT, but I tell you already that 3He is not in the list)

For the rest of the questions, I do not have a defined answer for the moment, but I’ll reply as soon as I get the proper explanation.

Thanks for understanding,


  1. As @adonadon pointed out, only neutrons and gammas are explicitly generated in low energy (<20 MeV) neutron reactions. Charged particles, as written in the manual and anticipated by @fogallar in the former discussion, are not transported but their energy is deposited at the point of interaction (kerma approximation).
  2. Not exactly, it scattered 262 times (to move from generation 1, as primary beam particle, to generation 263), no matter where, before undergoing the reaction in region 6 you are scoring.
  3. ETRACK is the total energy (mass + kinetic energy) of the interacting neutron, TKI is the kinetic energy of the resulting neutron (moved to generation 264).

Based on 1, unfortunately no (n,p) event can be properly selected, unless considering as such those where no reaction product appears.

Dear @adonadon and @ceruttif,

Thanks for your detailed explanation, but there still are some points that I can not undersatnd well.

  1. I now confirm that the protons and tritons produced in (n,p) reaction would not be transported and their energy would be deposited at the point of interaction. But if they are not recorded in the KPART(1:NP), how could the RESNUCLEi card get scores like:
  A_min:           1  -  A_max:           5

  A:            3  9.9551762E-04 +/-   2.1123247E-03 %
  A:            1  9.9551762E-04 +/-   2.1123247E-03 %
  1. In another way, I had used the ENDRAW in mgdraw.f to record the energy deposite events. In the tests, 1000000 primariy neutrons × 5cycle × 1run was set. I got series of 7.63203541E-04 energy deposition events (^{3}He(n,p)^{3}H reaction releases about 0.765MeV) in the dump files and the number of PROTON and TRITON (in nuclei/pr) as shown below:
    I was wondering if the distinctions between RESNUCLEi and dump files are just statistical errors or something else.
| energy(GeV) | resnuclei   | dump1  | dump2  | dump3  | dump4  | dump5  |
| ----------- | ----------- | ------ | ------ | ------ | ------ | ------ |
| 2.86e-11    | 0.99551762  | 996216 | 996201 | 996293 | 996240 | 996187 |
| 2.5e-10     | 0.82390540  | 825488 | 825420 | 826206 | 825234 | 825066 |
| 2.5e-8      | 0.1517156   | 154707 | 154947 | 154282 | 154640 | 154986 |

In case of low energy neutron reactions, if one has asked for RESNUC scoring, the code samples the residual nuclei from the information embedded to this purpose in the neutron library. For 3He, this yields the protons and tritons you observe. However, such a scoring sampling is uncorrelated with the kerma sampling scored in ENDRAW, that’s why you find parallel results that do not match on an event-by-event basis but only statistically (being indeed the discrepancies a measure of the associated error).
This loss of event-by-event correlation is a limitation of the current groupwise treatment that is linked to the nature of the library data.

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