Table of mass attenuation coefficients for various materials.

The atomic mass data came from (2008-4-27):
	http://en.wikipedia.org/wiki/List_of_elements_by_atomic_mass

The files under the directory "element" came from these NIST web pages:
	http://physics.nist.gov/PhysRefData/XrayMassCoef/tab3.html
after processing by my "nist-z1" script.
The first column is energy in MeV, second column is mass atten coef in cm^2/g.
That mac is "with coherent scattering" based on Xcom output.

The materials in directory "compound" (bone, etc.) came from these NIST pages:
http://physics.nist.gov/PhysRefData/XrayMassCoef/tab4.html

The soft tissue data came from
http://physics.nist.gov/PhysRefData/XrayMassCoef/ComTab/tissue.html

The bone data came from
http://physics.nist.gov/PhysRefData/XrayMassCoef/ComTab/bone.html

http://allmeasures.com/Formulae/
Al: 2700 kg/m^3 or 2.7 g/cm^3
I: 4930 kg/m^3 or 4.93 g/cm^3
Cu: 920 kg/m^3 or 0.92 g/cm^3
Pb: 11340 kg/m^3 or 11.34 g/cm^3

ICRU (1989), Tissue Substitutes in Radiation Dosimetry and Measurement,
Report 44 of the International Commission on Radiation Units and Measurements
(Bethesda, MD).

The tables give
	energy (MeV), mass attenuation coefficient (\mu/\rho),
	and mass energy-absorption coefficient, (\mu_en/\rho)
	(in cm^2/gram)

Here are the densities from:
http://physics.nist.gov/PhysRefData/XrayMassCoef/tab2.html

				Z/A	   I	  rho [g/cm^3]
ADIPOSE TISSUE (ICRU-44)	0.55579    64.8   9.500E-01
AIR, DRY (NEAR SEA LEVEL)	0.49919    85.7   1.205E-03
BONE, CORTICAL (ICRU-44)	0.51478   112.0   1.920E+00
BRAIN, GREY/WHITE (ICRU-44)	0.55239    73.9   1.040E+00
BREAST TISSUE (ICRU-44)		0.55196    70.3   1.020E+00
LUNG TISSUE (ICRU-44)		0.55048    75.2   1.050E+00
MUSCLE, SKELETAL (ICRU-44)	0.55000    74.6   1.050E+00
TISSUE, SOFT (ICRU-44)		0.54996    74.7   1.060E+00
WATER, LIQUID			0.55508    75.0   1.000E+00

lead:
http://physics.nist.gov/PhysRefData/XrayMassCoef/tab3.html
copper:
http://physics.nist.gov/PhysRefData/XrayMassCoef/ElemTab/z29.html
aluminum:
http://physics.nist.gov/PhysRefData/XrayMassCoef/ElemTab/z13.html

Some of the table have two identical energy values due to k-edges.
Previously I had perturbed those values to make the energy monotone increasing.
Instead, I modified the interpolator to deal with such jumps.