  A formulary for the application of the Diosi-Penrose criterion to solids in
quantum superpositions is developed, which takes the solid's microscopic mass
distribution (resulting from its nuclei) and its macroscopic shape into
account, where the solid's states can differ by slightly different positions or
extensions of the solid. For small displacements, smaller than the spatial
variation of the solid's nuclei, the characteristic energy of the Diosi-Penrose
criterion is mainly determined by the mass distribution of the nuclei. For
large displacements, much larger than the solid's lattice constant, the solid
can be idealised as a continuum, and the characteristic energy depends on the
solid's shape and the direction of the displacement. In Diosi's approach, in
which the mass density operator has to be smeared, the solid's microscopic mass
distribution plays no role. The results are applied to a special single-photon
avalanche photodiode detector, which interacts as little as possible with its
environment. This is realised by disconnecting the detector from the
measurement devices when it is in a superposition, and by biasing the
photodiode by a plate capacitor, which is charged shortly before the photon's
arrival. For a suitable choice of components, the detector can stay in a
superposition for seconds; its lifetime can be shortened to microseconds with
the help of a piezoactuator, which displaces a mass in the case of photon
detection.
