Detection
[edit]
Main article: Neutron detection
The common means of detecting a charged particle by looking for a track of ionization (such as in
a cloud chamber) does not work for neutrons directly. Neutrons that elastically scatter off atoms can
create an ionization track that is detectable, but the experiments are not as simple to carry out; other
means for detecting neutrons, consisting of allowing them to interact with atomic nuclei, are more
commonly used. The commonly used methods to detect neutrons can therefore be categorized
according to the nuclear processes relied upon, mainly neutron capture or elastic scattering.[114]
Neutron detection by neutron capture
[edit]
A common method for detecting neutrons involves converting the energy released from neutron
capture reactions into electrical signals. Certain nuclides have a high neutron capture cross section,
which is the probability of absorbing a neutron. Upon neutron capture, the compound nucleus emits
more easily detectable radiation, for example an alpha particle, which is then detected. The nuclides 3
He
,6
Li
, 10
B
, 233
U
, 235
U
, 237
Np
, and 239
Pu
are useful for this purpose.
Neutron detection by elastic scattering
[edit]
Neutrons can elastically scatter off nuclei, causing the struck nucleus to recoil. Kinematically, a neutron
can transfer more energy to a light nucleus such as hydrogen or helium than to a heavier nucleus.
Detectors relying on elastic scattering are called fast neutron detectors. Recoiling nuclei can ionize and
excite further atoms through collisions. Charge and/or scintillation light produced in this way can be
collected to produce a detected signal. A major challenge in fast neutron detection is discerning such
signals from erroneous signals produced by gamma radiation in the same detector. Methods such as
pulse shape discrimination can be used in distinguishing neutron signals from gamma-ray signals,
although certain inorganic scintillator-based detectors have been developed [115][116] to selectively
detect neutrons in mixed radiation fields inherently without any additional techniques.
Fast neutron detectors have the advantage of not requiring a moderator, and are therefore capable of
measuring the neutron's energy, time of arrival, and in certain cases direction of incidence.
Sources and production
[edit]
Main articles: Neutron source, Neutron generator, and Research reactor
Free neutrons are unstable, although they have the longest half-life of any unstable subatomic particle
by several orders of magnitude. Their half-life is still only about 10 minutes, so they can be obtained
only from sources that produce them continuously.
Natural neutron background. A small natural background flux of free neutrons exists everywhere on
Earth.[117] In the atmosphere and deep into the ocean, the "neutron background" is caused
by muons produced by cosmic ray interaction with the atmosphere. These high-energy muons are
capable of penetration to considerable depths in water and soil. There, in striking atomic nuclei,
among other reactions they induce spallation reactions in which a neutron is liberated from the
nucleus. Within the Earth's crust a second source is neutrons produced primarily by spontaneous
fission of uranium and thorium present in crustal minerals. The neutron background is not strong
enough to be a biological hazard, but it is of importance to very high resolution particle detectors that
are looking for very rare events, such as (hypothesized) interactions that might be caused by particles
of dark matter.[117] Recent research has shown that even thunderstorms can produc
