Chapter 15. pdf

Neutron Emission
The neutron tool emits high energy (4.5 MeV) neutrons from a radioactive source. They move very
fast, and their energy is related to their speed. They are called 
fast neutrons
. The neutron sources used
in logging are a mixture of two elements (i) a source of alpha radiation such as radium, plutonium or
americium, and (ii) beryllium-9. The alpha particles from the radium, plutonium or americium interact
with the beryllium-9 in an atomic reaction that produces carbon-12, a fast neutron and gamma rays.
(15.1)
15.2.2
Neutron Scattering
The fast neutrons interact with the nuclei of atoms within the formation. The interaction is a form of
elastic scattering involving the neutrally charged neutron and a stationary positively charged nucleus.
At each interaction (collision) the neutron looses some energy and slows down, and the nucleus of the
atom in the formation material gains energy. Such collisions occur with nuclei of 
ALL
elements.
However, the process of energy transfer (i.e., energy loss from the neutron) is most efficient when the
masses of the neutron and the nucleus are the same, and becomes much less efficient when the nuclei
of the formation material are more massive than the neutron. The neutron has approximately the same
mass as hydrogen nuclei (the lightest element). Hence the neutrons lose energy by elastic scattering
most efficiently by interaction with hydrogen nuclei, and lose energy much less efficiently by
interaction with more massive nuclei such as silicon or oxygen. Figure 15.2 shows the efficiency of
several elements in slowing down fast neutrons.
Figure 15.2
The fast neutron slowing efficiency of hydrogen, silicon and oxygen atoms as a function
of neutron energy for a clean sandstone, 
φ
= 0.15.
γ
+
+
⇒
+
n
C
He
Be
1
0
12
6
4
2
9
4