P-32 is a reactor produced radionuclide, with a physical half-life of 14.29 days. It decays solely by beta-minus emission to S-32. The beta particles (electrons) emitted have a maximum energy of 1700 keV (average 694.7 keV). The beta particle loses its energy largely via collision with orbital electrons, leading to ionization or excitation. Only a small fraction of energy loss (0.4% in water) comes from direct reactions with the nuclei, leading to deflection and rapid deceleration of the beta particle, accompanied by the emission of the lost energy in the form of photon radiation ["braking radiation," Bremsstrahlung, (10)]. This becomes important when adequate shielding needs to be chosen for the handling of this isotope. The maximum range for the beta particle in air is 607 cm, in water 0.785 cm, with an average range in air of about 152 cm and in water of about 0.198 cm. The distances traveled in tissue are approximately equal to those traveled in water—average is 3 mm with a maximum range of 8 mm.
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