Short answers, pulled from the story.
Americium was first synthesized in late autumn 1944 by Glenn T. Seaborg, Leon O. Morgan, Ralph A. James, and Albert Ghiorso at the University of California, Berkeley, using a 60-inch cyclotron. The discovery was part of the Manhattan Project and kept secret until November 1945.
Americium-241 is used in ionization smoke detectors because it emits five times more alpha particles than radium-226 and produces relatively little harmful gamma radiation. A typical detector contains just 0.29 micrograms of the element, which ionizes the air inside a detection chamber; smoke disrupts that ionization and triggers the alarm.
Americium is not made directly from uranium. Uranium-238 first captures neutrons to form plutonium-239, which then captures two more neutrons to form plutonium-241, and that isotope beta-decays over about 15 years into americium-241. A tonne of spent nuclear fuel contains about 100 grams of americium isotopes.
Americium-241 has a half-life of 432.6 years. It decays by alpha emission to neptunium-237, which has a half-life of about 2.14 million years.
Americium-242m has the largest cross-section for absorption of thermal neutrons of any known isotope, at 5,700 barns, giving it a critical mass of roughly 9-14 kg for a bare sphere. Its half-life is 141 years, and proposals exist to use it in compact hospital neutron sources, nuclear batteries, and spacecraft propulsion, though scarcity and high price currently block all such applications.
Americium is harmful because its radioactivity promotes cancer-cell formation in bones, liver, and gonads, where it can remain for decades. The biological half-life of americium-241 is 50 years in bone and 20 years in the liver. The most severe documented exposure was Harold McCluskey, who absorbed 500 times the occupational standard following a laboratory explosion but died at 75 from unrelated pre-existing disease.