Neutron moderation

Neutron moderation reduces the kinetic energy of fast neutrons through repeated collisions in a neutron moderator. In a simple elastic collision model, hydrogen is especially effective because a proton and a neutron have nearly equal masses: the maximum elastic recoil energy fraction can approach one. The mean logarithmic energy decrement describes typical accumulated slowing, whereas the maximum transfer describes a single endpoint.

Slowing, absorption and escape compete. A material can remove neutrons by neutron capture before they become thermal neutrons, and some neutrons escape its boundaries. Describing a neutron moderator therefore requires more than collision kinematics. Near thermal equilibrium, target thermal motion can also give energy back to a neutron; the initially stationary target approximation must then be replaced.

A material used for neutron moderation, such as water or paraffin wax in the Fermi slow-neutron activation experiment.

Consider a nonrelativistic particle of mass $m$ undergoing an elastic collision with a stationary free target of mass $M=Am$, where $A\ge 1$. Assume the outgoing direction is uniformly distributed over solid angle in the center of mass frame; this assumption concerns the scattering angle distribution and need not hold for every target or energy. Let $\mu$ be the cosine of that frame's scattering angle. Transforming the outgoing velocity back to the laboratory givesThe uniform distribution of $\mu$ on $[-1,1]$ makes $x$ uniform on $[\alpha ,1]$. For $A>1$, integration yieldsFor equal masses, take the limit $\alpha \to 0$: $\xi =1$. The expected value of the fractional energy after a collision is instead $\mathbb{E}[x]=(1+\alpha )/2$. These are different averages: averaging a logarithm does not give the logarithm of an average.

For independent collisions with the same distribution, the logarithms of successive energy ratios add. ThusHere $E_{\mathrm{geom}}$ is the geometric-mean energy, and $n_{\log }$ is a characteristic collision count for reaching an energy scale $E_{\ast }$. It is not an exact first-passage expected value. Individual histories fluctuate, and neither the time between collisions nor absorption is specified by this model.

In an original chosen example, set $E_0=10^6\mathrm{eV}$ and $E_{\ast }=0.025\mathrm{eV}$. Approximate hydrogen by $A=1$ and carbon by $A=12$. ThenThese are illustrative model values, not measured collision counts from 1934. The energy scale marks where neglected thermal motion matters; the calculation does not assert continued cooling to zero energy.