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Neutronium is a proposed name for a substance composed purely of neutrons.

The word was coined by scientist Andreas von Antropoff in 1926 ( before the discovery of the neutron itself ) for the conjectured " element of atomic number zero " that he placed at the head of the periodic table.

However, the meaning of the term has changed over time, and from the last half of the 20th century onward it has been used legitimately to refer to extremely dense phases of matter resembling the neutron-degenerate matter theorized to exist in the cores of neutron stars.

Science fiction and popular literature frequently use the term " neutronium " to refer to a highly dense phase of matter composed primarily of neutrons.