Atomic and nuclear properties of materials:
Neptunium (Np)

Quantity	Value	Units	Value	Units
Atomic number	93
Atomic mass	[237.0482]	g mole-1
Specific gravity	20.2	g cm-3
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
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 is no stable isotope, given atomic mass is that of the longest-lived known isotope as of Jan 2007.
Table of isotopes