Développement et optimisation d'un système de polarisation de ...

tel-00726870, version 1 - 31 Aug 2012
30 1.2. Ultracold neutrons
1.2 Ultracold neutrons
The neutron was discovered by Chadwick in 1932 while he was bombarding Beryllium with α
particles [39]. It is only stable inside the nucleus and may be extracted using fission or spallation
processes. As a free particle, it decays to a proton via the weak interaction with a rather long
lifetime (Sec. 1.2.2). Because of their neutrality and their long lifetime, fast neutrons with an
energy larger than 500 keV are commonly used to probe matter since they easily penetrate into
it. Once moderated (via collisions on light nuclei), their wave behavior appears. The associated
De Broglie wavelength is given by:
λn =
h
√ 2mnE
with E the neutron energy, mn the neutron mass and h the Planck constant. When λn becomes
larger than the distance between two atoms, neutron may be reflected by matter. This is because
the neutron does not interact with a single nucleus but with a set of nuclei. This phenomenon
was first formulated by Fermi in 1936 [40] and observed 10 years later [41]. A neutron with a
kinetic energy E and an incidence angle θ will be reflected on a surface if:
sin θ ≤
VF
E
where VF is called Fermi potential. It is a characteristic of the material, and is defined as:
VF = 2π¯h2
mn
with Ni the atom density of element i and bi the coherent scattering length of element i. This
potential is really small: a few hundreds of neV at most.
1.2.1 Definition
In 1959, Zeldovich [42] suggested the existence of very slow neutrons that may be fully reflected
from material surfaces and therefore trapped in vessels. In fact, Eq. (1.6), shows that θ ∈ [0, 2π]
if E < VF . Neutrons fulfilling this condition are called ultracold neutrons (UCN). Their energy
is below 300 neV, corresponding to velocities below 8 m/s, or, to temperatures close to 3 mK:
this is why they are called ultracold. The ability to be trapped makes the neutron a very
interesting probe for fundamental physics: it can be stored, manipulated and studied during a
time as long as its lifetime. The best limits on the neutron lifetime or neutron EDM come from
measurements with trapped UCN [3, 43].
1.2.2 Interactions
Neutrons are affected by the four fundamental interactions. A very interesting property is that
the typical energy associated to each force (except the weak force) have the same order of
magnitude. Typical potential energies for three of the four interactions are listed in Table 1.1.
The potential energies corresponding to typical experimental apparatus dimensions, applied
magnetic fields and coatings have the same order of magnitude as the kinetic energy of UCN.
i
Nibi
(1.5)
(1.6)
(1.7)