Use total cross-section for attenuation #801

Job	Run time
ubuntu-latest.openmpi.python-3.12	8m 36s
macos-latest.openmpi.python-3.12	9m 16s
windows-latest.msmpi.python-3.12	18m 58s
macos-latest.mpich.python-3.12	10m 27s
ubuntu-latest.mpich.python-3.12	7m 55s
	55m 12s

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