OK, I understand the idea of neutron bombardment of fissile material causing the nuclei of this material to split, thereby releasing more than 1 neutron, which then go on to bombard other nuclei, such that if more than 1 neutron per reaction ends up causing further reactions, the rate of reactions grow exponentially. I also understand that this very key chain factor (or whatever it's called) depends on the shape of the material body, as it is more likely that neutrons will escape a small body than a large body, and that for a given shape type (typically a sphere as it is the most efficient), there is a minimum size (or alternatively mass) that yields this chain factor as 1, which when using the mass as the parameter for a spherical shape is called the "critical mass" - and that a bomb is set up such that there are wedges of the sphere that are kept separate, and thus with a net chain factor of less than 1, keeping the bomb from prematurely exploding, and then for the detonation the wedges are carefully shot together creating a single spherical shape that has enough mass together than the chain factor goes above 1, thereby yielding a quick exponentially growing chain reaction. I also know that to assist with keeping neutrons from escaping there is a coat of material outside of the sphere that helps to reflect neutrons that might otherwise escape, thereby making it possible for a lower critical mass than what would need to be without such a coat. But looking at Pu-239, the natural radioactivity is alpha decay, not neutron decay (something that doesn't even seem to be a natural form of radioactive decay). So I could see a body of Pu-239 shooting off alpha particles, but where does that initial neutron come from? Is it that although the standard radioactivity is via alpha particle release, a small portion will actually be via neutron release? Is it that an alpha particle will sometimes react with a Pu-239 nucleus and release that first neutron? Is that cosmic rays or whatever that are everywhere on Earth are always generating a few neutrons? Is it that a bomb has some special neutron source?