NE406 Radiation Protection and Shielding

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Lesson 10 - Neutron interactions

In this lesson, like the previous, we are going to put our emphasis on the "big picture" of the neutron cross sections, concentrating on the effect of the cross sections in normal shielding situations.  This issue that I want to hit are:

1. The general structure of neutron cross sections.
2. The production of secondary gamma rays from neutron capture and inelastic scatter reactions.

General structure of neutron cross sections

There are four general ranges in the energy-dependent neutron cross sectional structures:

1. A low-lying (< 1 eV) thermal energy range in which most neutron absorption occurs.   The absorption cross section in this range tends to be inversely proportional to neutron velocity (i.e., 1/v).  The actual values of the thermal absorption cross section is extremely unpredictable, with certain elements -- for example boron, cadmium, and germanium -- providing extremely high thermal neutron absorption cross sections.
2. The energy range from the top of the thermal range (about 1 eV) to the first inelastic scattering threshold (which is isotope-dependent) is dominated by elastic scattering, except in the vicinity of resonances.  As was previously studied, the average fractional energy loss from elastic scattering is given by , where .  This results in fairly efficient slowing down for light nuclides, but inefficient slowing-down for heavy nuclides.
3. Above the energy threshold for the lowest level of inelastic scattering, this mechanism tends to dominate the slowing-down processes, especially for heavier nuclides.  This is true even if the cross section value for inelastic scattering is lower than elastic, because of the higher fractional energy loss associated with inelastic scatter (as studied previously).  The secondary neutrons that emerge from inelastic scattering reactions tend to be isotropic in the COM system due to the formation of a compound nucleus -- and the subsequent "forgetting" of the original direction of the incoming neutron.
4. For most nuclides a fourth range -- the resonance range -- is superimposed over the previous 3 ranges.  This range is characterized by many tall, thin cross section "spikes" that generally correspond to absorption reactions.  Although these resonances can result in a significant fraction of the neutron absorption, we generally do not depend on epithermal (i.e., above-thermal energy) resonances for shielding protection from neutrons because of the "gaps" between resonance that allow neutrons to stream through.

Production of secondary gamma rays from neutron interactions

As was previously emphasized, our general "strategy" in shielding -- for both neutrons and gamma rays -- is to slow down high energy particles and then capture them when they are slow.  For gamma rays, this strategy results in shields consisting of the heavy nuclides, which provide higher interaction coefficients for both Compton scattering (which is dependent on electron density) and photoelectric absorption interaction coefficients.  For neutrons, we generally combine low-mass isotopes (for efficient elastic scattering energy loss) with one of the high absorption isotopes.

Since many shielding designs have to stop BOTH gamma rays and neutrons, our shields tend to have a mixture of extremely high-Z materials (e.g., lead to stop gamma rays) and "doped" low-Z materials (e.g., borated water or polyethylene) to stop neutrons.  

One important consideration in the placement of these materials is the fact that neutron inelastic scattering and neutron capture generally is accompanied by the production of secondary gamma rays.  Therefore, we generally place our neutron shielding FIRST, so that this secondary gamma ray source occurs within our gamma shielding.

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