Neutron Detection-Introduction


Now let’s turn our attention to the detection
of neutrons. As I’m sure you remember from our discussion
of special nuclear materials, these materials emit neutrons either by spontaneous fission,
alpha-n reactions, or by induced fission. Detection of neutrons is sometimes the best
choice for detecting these types of materials because free neutrons very rarely occur spontaneously. On the other hand, detecting neutrons is not
easy—they’re slippery little devils. As we talked earlier, they’re neutral, so
they do not interact directly with the electrons in matter. It seems at first glance that all of our tricks
that we’ve used so far will not work. However, we can adopt a different strategy. Since neutrons interact directly only with
nuclei, that’s our only choice. We have to somehow put nuclei with large interaction
probabilities into our detector and choose those nuclei so that when they do interact
with a neutron they produce one or more charged particles. Now, all of the tricks that we already know
for collecting the electrons from ionizing radiation can be used to detect neutrons. There are two types of neutron interactions
that we use to detect or measure properties of neutrons. One such reaction is elastic scatter, during
which the neutron transfers some of its kinetic energy to a nucleus. If enough energy is transferred, the recoiling
nucleus ionizes the material surrounding the point of the interaction. Fast neutrons pass the most energy during
the elastic scatter with low-Z nuclides. So low-Z materials are always used for recoil
detectors. Recoil detectors are no good for thermal neutrons,
because thermal neutrons, if you remember, only have kinetic energies only on the order
of 1/40th of an eV. Considering that ionizations take a least
a few eV, recoils from thermal neutrons don’t so anything. The second type of nuclear reaction that we
can use to detect neutrons is absorption reactions that produce charged particles. These charged particles can be protons, alpha
particles, gamma rays, or fission fragments, among others. Some reactions require some minimal neutron
energy, but most of the reactions that we use in neutron detectors are based on thermal
neutron reactions. In general, the cross section for nuclear
reaction goes up as the energy of the neutron goes down. If there are fast neutrons that we are trying
to detect, then the strategy, often, is to thermalize them so that they can interact
with our detector. Both fast and thermal neutron detector materials
can be solid, liquid, or gas. Let’s look a bit closer at what you buy
or lose for each type of detector. Here’s a graph of the cross sections for
helium-3, boron-10, and lithium-6. Uranium-235 and plutonium -239 have the same
shape for low-energy neutrons. This region, below a few eV, where the cross
sections are straight lines on log-log graphs, is called the 1 over V cross section region,
because the cross sections are proportional to the inverse of the neutron velocity. Notice that both of these axes are log scales,
so that I can gain factors of tens to hundreds to thousands in probability of interactions
if I slow down or moderate the neutrons before they enter the detector.

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7 Responses

  1. Not Available says:

    Awesome presentation! Thanks for helping with my own little presentation to the class tomorrow morning 🙂

  2. Sunita Dafauti says:

    Excellent explanation!!!
    Explained precisely… Great job!!!

  3. bipul himansu says:

    That was a very nyc explanation … keep doing new videos

  4. 滚子Rolla says:

    If a neutron generator or a gamma ray tool from the oilfield was started… could it effect the sensors around it?

  5. 滚子Rolla says:

    How far would they have the ability to influence scientific tools?

  6. 滚子Rolla says:

    I'm ignorant. I worked for one of the biggest Oilfield Service Companies on earth.

  7. 滚子Rolla says:

    I meant "ignorant" in a sad proud way… explain it???

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