Infantry in the Middle Ages heavy toll on the English invaders of their lands. Adapted by the English, the longbow was nevertheless a difficult weapon to master, requiring long years of use and practice. Even bow construction was extended, sometimes taking as much as four years for seasoned staves to be prepared and shaped for final deployment. A skilled longbowman could shoot 12 arrows a minute, a rate of fire superior to competing weapons like the crossbow or early gunpowder weapons. The nearest competitor to the longbow was the much more expensive crossbow or Arbalest, used often by urban militias and mercenary forces. It required
Pit (nuclear weapon) risk of predetonation and fizzle. Weapon-grade plutonium therefore has plutonium-240 content limited to less than 7%. Supergrade plutonium has less than 4% of the 240 isotope, and is used in systems where the radioactivity is a concern, e.g. in the US Navy weapons which have to share confined spaces on ships and submarines with the crews. Plutonium-241, commonly comprising about 0.5% of weapon-grade plutonium, decays to americium-241, which is a powerful gamma radiation emitter. After several years, americium builds up in the plutonium metal, leading to increased gamma activity that poses occupational hazard for workers. Americium should therefore be separated,