Read the text below and answer the question.
What Is a Neutron Bomb?
By Anne Marie Helmenstine, Ph.D., About.com Guide
(1) A neutron bomb, also called an enhanced radiation bomb, is a type of thermonuclear
weapon. An enhanced radiation bomb is any weapon which uses fusion to enhance the production
of radiation beyond that which is normal for an atomic device. In a neutron bomb, the burst of
neutrons generated by the fusion reaction is intentionally allowed to escape using X-ray mirrors and
an atomically inert shell casing, such as chromium or nickel. The energy yield for a neutron bomb
may be as little as half that of a conventional device, though radiation output is only slightly less.
Although considered to be 'small' bombs, a neutron bomb still has a yield in the tens or hundreds of
kilotons range. Neutron bombs are expensive to make and maintain because they require
considerable amounts of tritium, which has a relatively short half-life (12.32 years). Manufacture of
the weapons requires that a constant supply of tritium of be available. Neutron bombs have a
relatively short shelf-life.
(13) U.S. research on neutron bombs began in 1958 at the University of California's
Lawrence Radiation Laboratory under the direction of Edward Teller. News that a neutron bomb
was under development was publically released in the early 1960s. It is thought that the first
neutron bomb was built by scientists at the Lawrence Radiation Laboratory in 1963, and was tested
underground 70 mi. north of Las Vegas, also in 1963. The first neutron bomb was added to the U.S.
weapons arsenal in 1974. That bomb was designed by Samuel Cohen and was produced at the
Lawrence Livermore National Laboratory.
(21) The primary strategic uses of a neutron bomb would be as an anti-missile device, to kill
soldiers who are protected by armor, to temporarily or permanently disable armored targets, or to
take out targets fairly close to friendly forces. It is untrue that neutron bombs leave buildings and
other structures intact. This is because the blast and thermal effects are damaging much further out
than the radiation. Although military targets may be fortified, civilian structures are destroyed by a
relatively mild blast. Armor, on the other hand, isn't affected by thermal effects or the blast except
very near to ground zero. However, armor, and the personnel directing, it is damaged by the intense
radiation of a neutron bomb. In the case of armored targets, the lethal range from neutron bombs
greatly exceeds that of other weapons. Also, the neutrons interact with the armor and can make
armored targets radioactive and unusable (usually 24-48 hours). For example, M-1 tank armor
includes depleted uranium, which can undergo fast fission and can be made to be radioactive when
bombarded with neutrons. As an anti-missile weapon, enhanced radiation weapons can intercept
and damage the electronic components of incoming warheads with the intense neutron flux
generated upon their detonation.
(Taken from the website: http://chemistry.about.com/od/chemistryfaqs/f/neutronbomb.htm).
In the sentence, “An enhanced radiation bomb is any weapon which uses fusion to enhance the production of radiation beyond that which is normal for an atomic device”, the underlined relative pronoun which refers to the word