Vampire off revenants—are common in vampire folklore. Garlic is a common example, a branch of wild rose and hawthorn plant are said to harm vampires, and in Europe, sprinkling mustard seeds on the roof of a house was said to keep them away. Other apotropaics include sacred items, for example a crucifix, rosary, or holy water. Vampires are said to be unable to walk on consecrated ground, such as that of churches or temples, or cross running water. Although not traditionally regarded as an apotropaic, mirrors have been used to ward off vampires when placed, facing outwards, on a door (in
Neutron cross section nuclear reactor. On the other hand, for very high energy neutrons (over 1 MeV), formula_9 can be neglected, and neutron cross section is approximately constant, determined just by cross section of atomic nuclei. However, this simple model does not take into account so called neutron resonances, which strongly modify neutron cross section in energy range of 1 eV–10 keV, nor threshold energy of some nuclear reactions. Cross sections are usually measured at 20 °C. To account for the dependence with temperature of the medium (viz. the target), the following formula is used: where "σ" is the cross section at temperature