A gorgeous sexy girl can be called a neutron when she does not have to pay for herself. The joke originates from
the double meaning of the noun "charge" - "electric charge" (
positive or negative) and "charge for services" (money).
As neutron has
no charge this word is used to define customers who do not pay for themselves, so they are charge-free, like neutrons.
A neutron (a
silicon baby) walks into the bar. The bartender says, "for you
no charge". - It means the girl gets all the
drinks for free as she attracts new male customers into the bar or the bartender is going to seduce her later on.