Water contains lots of hydrogen atoms, with mass of 1 atomic mass unit. (whatisnuclear.com)
Those values are similar to the previous standards but are typically less precise to take into consideration the variation in atomic masses. (scientificamerican.com)
The amount of x-ray energy absorbed during an x-ray beam's passage through a product is determined by the product's thickness, density and atomic mass number. (eaglepi.com)