Atomic and nuclear properties of materials:
Neptunium (Np)

Quantity	Value	Units	Value	Units
Atomic number	93
Atomic mass	[237.04817(2)]	g mole-1
Density	20.2	g cm-3
Mean excitation energy	902.0	eV
Minimum ionization	1.095	MeV g-1cm2	22.17	MeV cm-1
Nuclear collision length	118.5	g cm-2	5.850	cm
Nuclear interaction length	208.7	g cm-2	10.31	cm
Pion collision length	141.4	g cm-2	6.984	cm
Pion interaction length	235.0	g cm-2	11.60	cm
Radiation length	5.87	g cm-2	0.2897	cm
Critical energy	6.57	MeV (for e-)	6.33	MeV (for e+)
Molière radius	18.93	g cm-2	0.9349	cm
Plasma energy	81.22	eV
Muon critical energy	127.	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
Note: Since there are no stable isotopes, [atomic mass] is that of the longest-lived known isotope as of Jan 2007.
Table of isotopes Warning: may not be current