Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Measurement of the \{220\} lattice-plane spacing of a $^{28}$Si crystal   The spacing of the \{220\} lattice planes of a $^{28}$Si crystal was measured
by combined x-ray and optical interferometry to a $3.5\times 10^{-9}$ relative
accuracy. The result is $d_{220}=(192014712.67 \pm 0.67)$ am, at 20.0 $^\circ$C
and 0 Pa. This value is greater by $(1.9464 \pm 0.0067)\times 10^{-9} d_{220}$
than the spacing in natural Si, a difference which confirms quantum mechanics
calculations. Subsequently, this crystal has been used to determine the
Avogadro constant by counting the Si atoms, a key step towards a realization of
the mass unit based on a conventional value of the Planck or the Avogadro
constants.

30
The categories of this paper are: q-alg
17
The categories of this paper are: q-fin
0
The categories of this paper are: hep-ph
31
The categories of this paper are: ao-sci
34
The categories of this paper are: chem-ph
16
The categories of this paper are: stat
33
The categories of this paper are: plasm-ph
8
The categories of this paper are: hep-ex
15
The categories of this paper are: nucl-ex
25
The categories of this paper are: patt-sol
22
The categories of this paper are: chao-dyn
37
The categories of this paper are: supr-con
24
The categories of this paper are: dg-ga
28
The categories of this paper are: alg-geom
14
The categories of this paper are: math-ph
9
The categories of this paper are: nlin