atten  vers3.3
07-OCT-94  MJF
02-FEB-96  MJF
22-MAR-99  MJF

	spectral attenuation by multiple slabs of different materials

example:

	atten  << EOF
	N_materials
	mat_file thickness(cm)
	mat_file thickness(cm)
	mat_file thickness(cm)
	mat_file thickness(cm)
	.
	.
	EOF

directories/links:
        _materials: path to directory of material files
        _database: path to database directory

arguments:

	N_materials:	the number of material slabs considered
	mat_file:	file name for a specific material descriptor
	thickness:	material thickness in cm

	Note, if a mechanically collimating grid is to be used, 
	a negative value of thickness, corresponding to the 
	geometric blocking factor (black), should be input.  
	A dummy mat_file name must also be supplied, but
	the dummy file need not exist
	The value is equal to the fraction of area in the grid occupied
	by metal which totally stops the x-ray beam.
	The attenuation is computed as (1-black).
	Attenuation by the material filling the grid must be
	entered separately.

associated files:

	read:  spectra.tmp (see spect.tmp.doc for format)

	write: spectra.tmp (photons/mas/str-keV modified)
			   (list of mat_file thickness values appended)

method:	
	do i=1,N_materials
		do j=1,N_energies
			p_out(j) = p_in(j) * EXP(-thick(i)*sigma(i,j))
		endo
	endo

	The photoelectric cross sections are obtained from

        Biggs F, and Lighthill R,
        "Analytical Approximations for X-Ray Cross Sections",
        Part II, SC-RR-710507, Sandia Laboratories, Albuquerque, New Mexico, 1971.

	The incoherent (Compton, see sinc.doc) and coherent(see scoh.doc)
	cross sections are taken from

	McMaster WH, N. Del Grande NK, Mallett JH, and Hubbell JH,
        "Compilation of X-Ray Cross Sections", Sec. 2, Rev. 1,
        University of California, Livermore,
        U. S. Atomic Energy Commission Report UCRL-50174,  1969.

	The cross sections are converted to attenuation coefficients using
	the material information in the material files.

	The total cross section reported includes the coherent scattering
	cross section.