Atomic and nuclear properties of materials:
Helium gas (He)

Quantity	Value	Units	Value	Units
Atomic number	2
Atomic mass	4.002602(2)	g mole-1
Specific gravity (20° C, 1 atm)	1.66E-04	g cm-3
Mean excitation energy	41.8	eV
Minimum ionization	1.937	MeV g-1cm2	3.222E-04	MeV cm-1
Nuclear collision length	51.8	g cm-2	3.117E+05	cm
Nuclear interaction length	71.0	g cm-2	4.269E+05	cm
Pion collision length	79.5	g cm-2	4.778E+05	cm
Pion interaction length	103.6	g cm-2	6.229E+05	cm
Radiation length	94.32	g cm-2	5.671E+05	cm
Critical energy	257.13	MeV (for e-)	252.23	MeV (for e+)
Molière radius	7.78	g cm-2	4.677E+04	cm
Plasma energy	0.26	eV
Muon critical energy	2352.	GeV
Boiling point @ 1 atm	4.220	K	-268.9	C
Index of refraction (@ STP, Na D)	35.0	(n-1)x106

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