TYPE:	Photon mass attenuation coefficients and mass energy
	absorption coefficients for air (dry, near sea level).

FORM:	5 ascii columns
	log10(energy),log10(mu_en/rho),energy,mu/rho(cm2/gm),mu_en/rhA(cm2/gm)

SOURCE:	Downloaded from http://physics.nist.gov/PhysRefData/XrayMassCoef

	The following description comes from the NIST documentation:

	  "Tables and graphs of the photon mass attenuation coefficient 
	  mu/rho and the mass energy-absorption coefficient mu_en/rho
	  are presented for all of the elements Z = 1 - 92 and for 48 compounds
	  and mixtures of radiological interest. The tables cover energies 
	  of the photon (x-ray, gamma ray, bremsstrahlung) from 1 keV to 20 MeV.  
	  The values are taken from the current photon interaction database 
	  at the National Institute of Standards and Technology, 
	  and the mu_en/rho values are based on the new calculations 
	  by Seltzer described in Radiation Research 136, 147 (1993). 
	  These tables of  and mu_en/rho replace and extend the tables 
	  given by Hubbell in the International Journal of Applied Radiation 
	  and Isotopes 33, 1269 (1982)."

	The log10 columns have been added to facilitate interpolation since
	the data is well behaved in a log-log plot.
