In order for nuclear weapons to function successfully it is necessary for the fission chain
reaction to initiate at the correct time. When the system becomes supercritical, a neutron is
necessary in order to begin the fission process. This "window" for successful neutron
initiation differs depending on the design of the weapon. Gun assembly devices stay in a
supercritical state during a relatively long time - a time which is sufficient for a background
neutron to initiate the fission chain reaction.
However with regard to implosion devices this neutron initiation window is much smaller,
because the interval during which the bomb is near optimum criticality is relatively short.
Although theoretically it is possible to initiate the fission chain reaction by means of a
singular neutron, it is an advantage for an initiator to at least emit several neutrons at
the optimum period, because it is possible to capture a singular neutron without causing fission.
A method for initiating the fission chain reaction is to use a continuous neutron emitter:
a material which has a high spontaneous fission rate, or an alpha emitter together with
beryllium. Although the neutron production method is stochastic, they are produced with a
specific average rate. As a result there will be in uncertainty with regard to the initiation
time, which in turn leads to a high degree of variability in the performance of the device, i.e.
An improved version of a continuous/spontaneous neutron emitter is that which can
produce a burst of neutrons at an exactly-defined time to in order to maximise the
performance of the device (yield) but at the same time to reduce variability. These so-called
"internal initiators" can be inside the device, or external designs, which are positioned
outside of the high explosive.
Polonium-Beryllium (Po-Be) initiators were employed in the first nuclear weapons. They had
the codename "Urchin". The neutrons which are necessary in order to initiate the fission
chain reactions are produced as a result of a mix of an alpha emitter, such as polonium, with
In order for this to be an improvement to a continuous neutron source, it is necessary for
the alpha emitter and beryllium to remain unmixed. This is apart from the moment when
neutrons are desired, when they are required to be rapidly mixed. Fortunately, of all forms
of ionising radiation alpha particles penetrate the least, and it is relatively easy to block
A suitable alpha emitter needs a compromise between high alpha particle activity in order to
guarantee the production of a sufficient quantity of neutrons, and a sufficiently long half life
in order to avoid frequent replacement of the initiator. As a result polonium-210 is an
obvious choice for alpha emitters.
The need for carefully timed, fast, efficient mixing is insured by means of the design of the
The initiator is placed at the center of the fissile mass, and it uses the arrival of the shock
wave to drive the mixing process. This insures that the entire mass becomes highly
compressed and emits neutrons where they can be most effective.
An alternative to the mixing of an alpha emitter with beryllium is to apply the high
temperatures and densities which it is possible to achieve near to the center of an implosion,
in order to ignite Deuterium-Tritium (D-T) fusion reactions. Very small quantities of
deuterium and tritium are necessary and they are found in a small high-pressure sphere at
the center of the fissile core.
The tritium in a DT initiator is radioactive and has a half-life of approximately 12 years. This is
a lot longer than polonium and than other potential radioactive alpha emitters which it is
possible to use in the Urchin initiators. Thus it can be stored during a longer period.
An alternative from a source of neutrons inside the nuclear device is when an external
neutron initiator (ENI) is positioned outside of the device and it is not necessary even to
place it near to the fission assembly. Bomb casings, cruise missiles and re-entry vehicles
permit the ENI positioning to be virtually anywhere in the weapon.
ENIs employ a miniature particle accelerator (this is often called a neutron tube). This
accelerates deuterium and tritium together to generate high-energy neutrons by means of a
fusion reaction. The tube contains an ion source and ion target at opposing ends in a short
vacuum tube. The application of a large current leads to the emission of ionized hydrogen
from the source. A large voltage then accelerates this in the direction of the target, where -
if there is sufficient energy - some of the deuterium and tritium ions undergoes fusion,
which generates high-energy neutrons.