<P> Fast neutron detectors have the advantage of not requiring a moderator, and are therefore capable of measuring the neutron's energy, time of arrival, and in certain cases direction of incidence . </P> <P> Free neutrons are unstable, although they have the longest half - life of any unstable subatomic particle by several orders of magnitude . Their half - life is still only about 10 minutes, however, so they can be obtained only from sources that produce them continuously . </P> <P> Natural neutron background . A small natural background flux of free neutrons exists everywhere on Earth . In the atmosphere and deep into the ocean, the "neutron background" is caused by muons produced by cosmic ray interaction with the atmosphere . These high - energy muons are capable of penetration to considerable depths in water and soil . There, in striking atomic nuclei, among other reactions they induce spallation reactions in which a neutron is liberated from the nucleus . Within the Earth's crust a second source is neutrons produced primarily by spontaneous fission of uranium and thorium present in crustal minerals . The neutron background is not strong enough to be a biological hazard, but it is of importance to very high resolution particle detectors that are looking for very rare events, such as (hypothesized) interactions that might be caused by particles of dark matter . Recent research has shown that even thunderstorms can produce neutrons with energies of up to several tens of MeV . Recent research has shown that the fluence of these neutrons lies between 10 and 10 per ms and per m depending on the detection altitude . The energy of most of these neutrons, even with initial energies of 20 MeV, decreases down to the keV range within 1 ms . </P> <P> Even stronger neutron background radiation is produced at the surface of Mars, where the atmosphere is thick enough to generate neutrons from cosmic ray muon production and neutron - spallation, but not thick enough to provide significant protection from the neutrons produced . These neutrons not only produce a Martian surface neutron radiation hazard from direct downward - going neutron radiation but may also produce a significant hazard from reflection of neutrons from the Martian surface, which will produce reflected neutron radiation penetrating upward into a Martian craft or habitat from the floor . </P>

Where does a neutron have more mass inside or outside of the atomic nucleus