Radioactivity occur when an unstable nucleus changes to a stable configuration this is done by releasing some particles.
The Emission of energy in form of Ionizing radiation is Radioactive decay.
The time taking for a radioactive decay to occur is half life.
The emission of this material are usually in form of alpha particle, beta particle and gamma rays.
Radioactive material are said to be life threatening because of it adverse effect on humans. It could lead to mutation when exposed to excessively, abortion of foetus, cancerous cells growth.
Truman stated that his decision to drop the bomb was purely military. A Normandy-type amphibious landing would have cost an estimated million casualties. Truman believed that the bombs saved Japanese lives as well. Prolonging the war was not an option for the President.