Ultracold Neutrons - Reflecting Materials

Reflecting Materials

Material: VF vC η (10−4)
Beryllium 252 neV 6.89 m/s 2.0-8.5
BeO 261 neV 6.99 m/s
Nickel 252 neV 6.84 m/s 5.1
Diamond 304 neV 7.65 m/s
Graphite 180 neV 5.47 m/s
Iron 210 neV 6.10 m/s 1.7-28
Copper 168 neV 5.66 m/s 2.1-16
Aluminium 54 neV 3.24 m/s 2.9-10

Any material with a positive neutron optical potential can reflect UCN. The table on the right gives an (incomplete) list of UCN reflecting materials including the height of the neutron optical potential (VF) and the corresponding critical velocity (vC). The height of the neutron optical potential is isotope-specific. The highest known value of VF is measured for 58Ni: 335 neV (vC=8.14 m/s). It defines the upper limit of the kinetic energy range of UCN.

The most widely used materials for UCN wall coatings are Beryllium, Beryllium oxide, Nickel (including 58Ni) and more recently also diamond-like carbon (DLC).

Non-magnetic materials such as DLC are usually preferred for the use with polarized neutrons. Magnetic centers in e.g. Ni can lead to de-polarization of such neutrons upon reflection. If a material is magnetized, the neutron optical potential is different for the two polarizations, caused by

where is the magnetic moment of the neutron and the magnetic field created on the surface by the magnetization.

Each material has a specific loss probability per reflection,

which depends on the kinetic energy of the incident UCN (E) and the angle of incidence (θ). It is caused by absorption and thermal upscattering. The loss coefficient η is energy-independent and typically of the order of 10−4 to 10−3.

Read more about this topic:  Ultracold Neutrons

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