Atomic and nuclear properties of materials:
Erbium (Er)

Quantity	Value	Units	Value	Units
Atomic number	68
Atomic mass	167.259(3)	g mole-1
Density	9.07	g cm-3
Mean excitation energy	658.0	eV
Minimum ionization	1.168	MeV g-1cm2	10.59	MeV cm-1
Nuclear collision length	107.6	g cm-2	11.86	cm
Nuclear interaction length	186.1	g cm-2	20.53	cm
Pion collision length	131.2	g cm-2	14.48	cm
Pion interaction length	213.1	g cm-2	23.50	cm
Radiation length	7.14	g cm-2	0.7881	cm
Critical energy	8.73	MeV (for e-)	8.41	MeV (for e+)
Molière radius	17.36	g cm-2	1.915	cm
Plasma energy	55.32	eV
Muon critical energy	163.	GeV

For muons, dE/dx = a(E) + b(E) E. Tables of b(E): PS PDF TEXT
Table of muon dE/dx and Range: PS PDF TEXT
Explanation of some entries
Table of isotopes Warning: may not be current
x ray mass attenuation coefficients