Brown and Kontar (2005) review studies of "dips" in HXR spectra.  One possibilituy
is that the dips are caused by albedo. (Alexander & Brown, 2002, Kontar
KcKinnon & Brown 2004).  Albedo can cause a dip around 50 keV, as observed (Fig 7 in Brown &
Kontar) but directivity of photons has not been folded into the analysis.

Brown & Kontar 2005
Kontar, KcKinnon & Brown 2004 Solar Phys (in preparation?)
Alexander & Brown, Empirical correction of RHESSI spectra for albedo Solar Phys 210, 407, 2002.