ASNT

PART 4 Introduction to the Neutron

The neutron is a subatomic elementary particle that is neutral and is capable of penetrating deep inside high atomic number materials. It was discovered by James Chadwick in 1932 (Chadwick 1932a; Chadwick 1932b). It has a similar mass to the proton, a nuclear spin of one-half, a magnetic moment (Table 1), and an electric dipole moment that is either zero or too small to be measured. X-rays are scattered by the electrons of an atom while the neutron is

scattered or absorbed by the nucleus of an atom. There are five major types of neutron interactions with matter: coherent elastic scattering, incoher- ent elastic scattering, coherent inelastic scattering, incoherent inelastic scattering, and absorption. All of these interactions contribute to the loss of transmission through an object during radiography. The first successful neutron radiograph was measured in 1935 by H. Kallmann and E. Kuhn with a small neutron generator (Kallmann 1948), just three years after the discovery of the neutron. Since then, the field of neutron radiography has expanded and diversified, and the capability is commonly available at commercial facilities and large-scale research facilities such as reactors and spallation neutron sources around the world. The broad scientific and diagnostic impact of the field of neutron radiography is discussed elsewhere in this book.

Table 1 Neutron characteristics

Charge

0

Rest mass

1.675 × 10 –27 kg 1.532 × 10 –18 m –9.662 × 10 –27 J.T –1 1.320 × 10 –15 m

Classical radius

Magnetic moment

Compton wavelength

CHAPTER 2

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Part 4