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Strange reaction.

  1. Mar 16, 2007 #1

    vanesch

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    Hello,

    can anybody help me out with the following (IMO) strange reaction:

    HE-4(N,P)H,SIG

    I'm looking for the thermal neutron reactions on He-4, and according to this site:
    http://www3.nndc.bnl.gov/exfor7/endf00.htm

    (enter target He-4, reaction n)

    the above reaction has a tabulated cross section well into the thermal and cold domain (~10^(-2) barn and up). The Q-factor for this reaction is something like 801 KeV.

    However, when I try to calculate the energetic balance, using the values given by

    http://www3.nndc.bnl.gov/nudat2/reCenter.jsp?z=1&n=2

    then this doesn't make any sense:

    neutron: Delta = 8.0713 MeV
    He-4: Delta = 2.4249 MeV
    proton: Delta = 7.289 MeV

    neutron + He-4 - proton = 3.2072 MeV

    and nothing containing a proton has such a low energy content, so this reaction is energetically not possible IMO.

    I suspect a He-3 contamination in the He-4 when these data were taken, but am I totally off and is this reaction real ?
     
  2. jcsd
  3. Mar 16, 2007 #2

    Astronuc

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    I don't see this reaction as possible.

    He-4 has a low cross-section, and it is more likely to scatter, than absorb a neutron. I tried a Q-calc with n and He-4, but it kept giving me n and H-4.

    I don't think n + He-4 -> H-4 + p is feasible, but

    n + He4 -> T + n' + p might be possible, but I believe that requires some threshold energy.

    He-3 certainly loves neutrons.
     
  4. Mar 17, 2007 #3

    vanesch

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    The point is: I wanted to use He-4 as a replacement for He-3 in a thermal neutron detector to find out what is the background of non-thermal neutron counts. But if there is a (small) cross section of He-4 for thermal neutrons, this screws me. Moreover the Q is comparable to the He-3 reaction, so I wouldn't be able to discriminate against it.
    This is why it annoyed me to find this tabulated cross section for He-4 (n,p) on the NNDC site. So do you think it is an error in their data tables ?
     
  5. Mar 17, 2007 #4

    Astronuc

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    I looked for information on n,p reactions for He-4 and could not find anything. What I did find it this -
    http://www3.nndc.bnl.gov/exfor7/servlet/E4sGetEvaluation?EvalID=16316&req=4999

    The cross section for n-absorption by He-4 is small (0.05 barn) for thermal neutrons, and I believe that is for n,g, although the above link has a statement - "THERE ARE NO GAMMA IN NEUTRON INTERACTION WITH HE-4 NUCLEI". The cross-sections for n,g by Kr and Xe are approximately 24 b for thermal neutrons.

    What neutron spectrum are you investigating? Thermal + epithermal?

    To discriminate from thermal neutrons, in the past I have used a box covered in Gadolinium which absorbed the thermal neutrons but allowed higher energy neutrons to enter. The box was filled with water and my colleagues and I used small buttons of Cd to activate by the epithermal neutrons slowing down to thermal energies in the water. It was a crude experiment.
     
    Last edited: Mar 17, 2007
  6. Mar 17, 2007 #5

    vanesch

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    Last edited: Mar 17, 2007
  7. Mar 17, 2007 #6

    vanesch

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    The problem is that we have to establish extremely low backgrounds in a flux of thermal neutrons (requirements of less than 1 count per minute on an area of > 3m^2 in a flux of thermal neutrons that can go up to 10^6/second - simply crazy requirements). Now, there are of course also cosmic neutrons, an entire gamma spectrum, epithermal and other at that low level. The "trick" I wanted to use was to fill the detector with He-4 instead of He-3, because as such, I am sure that I ONLY see non-thermal neutrons and other stuff. In other words, I'm sure I have a pure background measurement in real conditions of use.
    But if there is a reaction with thermal neutrons in He-4, then this trick won't work, because my main "background" will be in fact a signal.

    The point is that I don't seem to find any reaction n(thermal) + He-4 -> anything that can respect conservation of energy (except for elastic scattering).
     
  8. Mar 17, 2007 #7

    Astronuc

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    I believe that is correct.
     
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