Absorption of neutrons is not usually a major problem, but there are some elements where absorption can be problematic and others where even minor levels of an element render a measurement impossible. Likewise, while not exactly absorption, incoherent scatter of neutrons renders then unable to contribute to diffraction and can be lumped in with absorption. Both should always be considered before applying for beam time. I recommend using the web calculation tool at NIST https://www.ncnr.nist.gov/resources/activation/ to estimate sample absorption and incoherent scattering. Note that the effective \(\mu \) value is the 1 over the computed “1/e penetration depth” for “abs+incoh” (absorption and incoherent scattering). For TOF experiments, consider the longest wavelength that the instrument uses (with the planned chopper settings).
As was discussed for x-ray diffraction in more depth in §5.5, with \(\mu R < 1.0\), absorption can be ignored. For \(1.0 < \mu R < 1.5\), experiments are feasible, if not ideal. For \(1.5 < \mu R < 2.5\), experiments are slow and far from ideal, but are feasible. For \(2.5 < \mu R\), you are likely wasting valuable neutrons. Unlike with x-rays, your options are more limited as to how to redesign experiments to avoid absorption/incoherent scattering. Changing composition to avoid a problematic isotope is expensive, but may be possible with a few light elements (H, B, Li) but is probably not possible further down the periodic table, but you should look up prices. Changing wavelengths is only possible to a minor extent, if at all. Changing the sample diameter is readily possible, but comes at the expense of increased data collection time, but is still preferred if it can bring \(\mu R \sim 1.0\). One other option to be considered with absorbing samples is use of a annular sample. This places a ring of sample around either a hollow or minimally absorbing/scattering central object. This places much more sample into the beam than container with a radius that matches the ring thickness, but the annular container will show a similar magnitude for absorption. Note that GSAS-II does not offer corrections for an annular sample container, so that correction will need to be applied to the measured intensities and their uncertainties before the data can be input to GSAS-II.
The neutron activation values available from the NIST web site will probably not be very accurate, without help from an experienced beamline scientist or health physicist to establish the dose, but will give an idea of which elements in your sample will become radioactive and for how long. One should assume that every sample coming out from a neutron beam is radioactive and should be handled accordingly, but longer-lived neutron activation will restrict what can be done with a material, after it has been used as a neutron sample.