Carbon density changed from 2.1 to 2.2 to match CXRO, CRC and RSC. The NIST
X-ray attenuation tables use 2.26; the Handbook of Mineralogy has 2.09-2.23.
The Neutron Data Booklet gave the value as 1.9-2.3, and 2.1 was chosen
from this range. The remaining density will continue to use values from the
Neutron Data Booklet, which cites CRC as the primary source.