Biomedical Engineering Reference
In-Depth Information
through elastic scattering in matter. They diffuse about until captured by atomic
nuclei.
Neutrons of higher energies, up to about 0.01 MeV or 0.1 MeV (the convention
is not precise), are known variously as “slow,” “intermediate,” or “resonance” neu-
trons. “Fast” neutrons are those in the next-higher-energy classification, up to about
10 MeV or 20 MeV. “Relativistic” neutrons have still higher energies.
9.4
Interactions with Matter
Like photons, neutrons are uncharged and hence can travel appreciable distances
in matter without interacting. Under conditions of “good geometry” (cf. Sec-
tion 8.7) a narrow beam of monoenergetic neutrons is also attenuated exponen-
tially by matter. The interaction of neutrons with electrons, which is electromag-
netic in nature,
2)
is negligible. In passing through matter a neutron can collide
with an atomic nucleus, which can scatter it elastically or inelastically. The scatter-
ing is elastic when the total kinetic energy is conserved; that is, when the energy
lost by the neutron is equal to the kinetic energy of the recoil nucleus. When the
scattering is inelastic, the nucleus absorbs some energy internally and is left in an
excited state. The neutron can also be captured, or absorbed, by a nucleus, leading
to a reaction, such as
(n, p)
,
(n, 2n)
,
(n,
)
. The reaction changes the atomic
mass number and/or atomic number of the struck nucleus.
Typically, a fast neutron will lose energy in matter by a series of (mostly) elastic
scattering events. This slowing-down process is called neutron moderation. As the
neutron energy decreases, scattering continues, but the probability of capture by
a nucleus generally increases. If a neutron reaches thermal energies, it will move
about randomly by elastic scattering until absorbed by a nucleus.
Cross sections for the interactions of neutrons with atomic nuclei vary widely
and usually are complicated functions of neutron energy. Figure 9.4 shows the to-
tal cross sections for neutron interactions with hydrogen and carbon as functions
of energy. Because the hydrogen nucleus (a proton) has no excited states, only elas-
tic scattering and neutron capture are possible. The total hydrogen cross section
shown in Fig. 9.4 is the sum of the cross sections for these two processes. The
capture cross section for hydrogen is comparatively small, reaching a value of only
0.33 barn (1 barn =
α
)
,or
(n,
γ
10
-24
cm
2
) at thermal energies, where it is largest. Thermal-
neutron capture is an important interaction in hydrogenous materials.
2 Although the neutron is electrically neutral, it
has spin, a magnetic moment, and a nonzero
distribution
of electric charge within it. These
properties, coupled with the charge and spin
of the electron, give rise to electromagnetic
forces between neutrons and electrons. These
forces, however, are extremely weak. In
contrast, the neutron interacts with protons
and neutrons at close range by means of the
strong, or nuclear, force. The stopping power
of matter for neutrons due to their
electromagnetic interaction with atomic
electrons has been calculated (see
Section 9.12).
