The usual symbol for mass is m, and its SI unit is the kilogram. The SI standard of mass is a cylinder of platinum and iridium kept at the International Bureau of Weights and Measures near Paris and assigned, by international agreement, a mass of 1 kilogram.
Reactor Physics and Thermal Hydraulics:
J. R. Lamarsh, Introduction to Nuclear Reactor Theory, 2nd ed., Addison-Wesley, Reading, MA (1983).
J. R. Lamarsh, A. J. Baratta, Introduction to Nuclear Engineering, 3d ed., Prentice-Hall, 2001, ISBN: 0-201-82498-1.
W. M. Stacey, Nuclear Reactor Physics, John Wiley & Sons, 2001, ISBN: 0- 471-39127-1.