Neutron filters for thermal or cold neutron beams

W. Van Dingenen, Serge Hautecler

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    The transmission of polycrystalline filters for cold neutrons and of single crystal blanks for thermal neutrons has been measured at 293° K and at 77° K.For polycrystalline filters the filtering efficiency and the transmission for cold neutrons depend on the grain size. The best result is obtained with cast be : at a preset transmission of 10-5 for thermal neutrons and of 10-4 for γ-rays, the transmission for cold neutrons has a value of 0.613. With the best sample of sintered Be of high density (1.85) a mean cold neutron transmission of 0.517 is measured for the same preset transmission for thermal neutrons and γ-radiation. A sample of BeO, with the same stopping power for thermal neutrons and γ's, has a transmission of 0.159 for neutrons with wave length > 4 A.
    The effective transmission of single crystal blanks for thermal neutrons is found to be very sensitive to the product of the mosaic spread and the reflectivity of the crustal. The number of single crustal blocks, contained in the blank, is les important. In this respect, MgO (magnorite) seems to be a very promising material, owing to the perfectness of its crustals. Its other characteristics (density, Debye temperature, free atom cross-section) are also reasonably good. For an equal epithermal neutron and γ-ray transmission of 10-5 the thermal-neutron transmissions at 77° K compare as follows:
    54 cm Mg0 : 0.383
    73.2 cm SiO2 : .013443.6 cm Bi : 0.031Owing to its large mosaic spread and reflectivity Bi single crystal blanks are not useful in the thermal region. They are however indicated for use - in combination with MgO - in the cold neutron range, because of highest neutron transmission and good stopping power for γ-rays.
    Original languageEnglish
    PublisherSCK CEN
    Number of pages38
    StatePublished - Oct 1963

    Publication series

    NameSCK CEN Reports
    PublisherSCK CEN

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