They are produced by supernovae, neutron stars, galactic centres, black holes and the like.

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Strong gravitational waves are thought to be produced by the collision of two neutron stars.

ECONOMIST: Fundamental physics

The device's primary source of neutron radiation was the radioactive element californium, but the stream of neutrons produced by the californium was multiplied by passing it through a lattice of highly enriched uranium U-235, whose nuclear fission released additional neutrons.

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