1.6. General Properties and Sources of Particles and Waves 51
life of 433 years. It decays byαemission with a mean energy of 5.48MeV.Ifthis
isotope is mixed with beryllium-9, theα-particles interact with the beryllium nuclei
transforming them into carbon-13 in excited state. The de-excitation of carbon-13
leads to the emission of neutrons. Note the similarity of this process to that of the
(^238) Pu−Besource mentioned above.
One can essentially use anyα-particle source to make an (α−n) source. In
general, the (α−n) reaction can be written as
α+np+pX→np+2+p+3Y+n. (1.6.7)
Just likeα-particles, photons can also be used to stimulate nuclei to emit neu-
trons. A common example of suchγ-emitting nuclides is antimony-124.^12451 Sbemits
anumberofγ-rays, the most probable of which has an energy of around 603keV.
If these photons are then allowed to interact with beryllium nuclei, it may result
in the emission of a neutron. Antimony-beryllium neutron sources are commonly
used in laboratories. Such sources are sometimes referred to asphotoneutronsources.
Fusion Sources
Fusion is a reaction in which two light nuclei (hydrogen and its isotopes) are
forcibly brought so close together that they form a new heavier nucleus in an excited
state. This nucleus releases neutrons and photons to reach its ground state. The
fusion reaction can therefore be used to produce neutrons.
To initiate a fusion reaction, a fair amount of energy must be supplied through
some external means because the nuclei are repelled by electromagnetic force be-
tween the protons. This energy can be provided by several means, such as through
charged particle accelerators. The advantage of fusion sources over the spallation
sources is that they need relatively lower beam energies to initiate the fusion process.
These sources are also more efficient in terms of neutron yield. The fusion process
producing neutrons can be written as
(^2) D(d, n) (^3) He
(^3) T(d, n) (^4) He.
HeredandDrepresent Deuterium (an isotope of Hydrogen with one proton and
one neutron). T is another isotope of Hydrogen with one proton and 2 neutrons.
The nomenclature of the above equations is such that the first term in the brackets
represents the incoming particle and the second term the outgoing one. The above
reactions can also be written as
2
1 D+
2
1 D →
3
2 He+n
3
1 T+
2
1 D →
4
2 He+n.
Nuclear Reactors
Nuclear reactors produce neutrons in very large numbers as a result of neutron-
induced fission reactions
235
92 U+n→FF^1 +FF^2 + (2-3)n.