  The Back-n white neutron beam line, which uses back-streaming white neutrons
from the spallation target of the China Spallation Neutron Source, is used for
nuclear data measurements. A Micromegas-based neutron detector with two
variants was specially developed to measure the beam spot distribution for this
beam line. In this article, the design, fabrication, and characterization of
the detector are described. The results of the detector performance tests are
presented, which include the relative electron transparency, the gain and the
gain uniformity, and the neutron beam profile reconstruction capability. The
result of the first measurement of the Back-n neutron beam spot distribution is
also presented.
