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Well, I'll be honest my physics is not good. But I really wonder about radiation and other physical stuff.
When I search about it I see three types of ionizing radiation: Alpha, Beta and Gamma. But when I search about nuclear weapons I see that they work by launching a neutron to a u235. So I wonder how is this neutron launched? I know that fission occurs 2 or 3 neutrons. But you need a neutron to start fission, right?
Neutron radiation is what it sounds like - it is a type ionising radiation that consists of neutrons.
You are correct - to start a nuclear chain reaction in a fissile material such as $^{235}U$ there needs to be a source of neutrons. A large enough quantity (a critical mass) of fissile material can spontaneously start its own chain reaction triggered by neutrons produced by its own decay or from background radiation. A fission weapon needs to trigger the chain reaction at a precisely controlled time. This is done by either smashing two sub-critical quantities of material together to create a critical mass, or by using conventional explosives to compress a sub-critical mass until it becomes critical.
