During nuclear fission, the nucleus of plutonium-239 splits into two smaller nuclei, releasing a large amount of energy and several neutrons. This process is typically initiated by the absorption of a neutron by the plutonium-239 nucleus.

Isotopes are atoms of the same element that have the same number of protons but different numbers of neutrons. This results in isotopes having the same atomic number but different mass numbers.

A small sample of plutonium-239 can be stored safely in a lead-lined container to shield against β-radiation. The container should be kept in a secure, controlled environment to prevent any leakage or exposure.

After 48000 years, which is 2 half-lives, the mass of plutonium-239 remaining would be:
Initial mass = 800 g
After 1 half-life (24000 years): 800 g / 2 = 400 g
After 2 half-lives (48000 years): 400 g / 2 = 200 g
Therefore, the mass remaining after 48000 years is 200 g.

1. Nature: β-radiation consists of high-energy electrons (or positrons), while γ-radiation consists of high-energy photons (electromagnetic waves).
2. Penetration: β-radiation is less penetrating than γ-radiation. β-particles can be stopped by a few millimeters of aluminum, whereas γ-rays require several centimeters of lead or concrete to be effectively shielded.

γ-radiation should be placed at the far right end of the electromagnetic spectrum, as it has the highest frequency and shortest wavelength among the types of electromagnetic radiation.