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:
- The general structure of neutron cross sections.
- The production of secondary gamma rays from neutron capture and inelastic scatter
General structure of neutron cross sections
There are four general ranges in the energy-dependent neutron cross sectional
- 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.
- 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.
- 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.
- 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
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