sometimes called a neutron tube.
Neutron tubes have several components including an ion source, ion
optic elements, and a beam target; all of these are enclosed within a vacuum-tight enclosure. High
voltage insulation between the ion optical elements of the tube is provided by glass and/or ceramic
insulators. The neutron tube is, in turn, enclosed in a metal housing, the accelerator head, which is
filled with a dielectric medium to insulate the high voltage elements of the tube from the operating
area. The accelerator and ion source high voltages are provided by external power supplies. The
control console allows the operator to adjust the operating parameters of the neutron tube. The
power supplies and control equipment are normally located within 3–10 metres (10–30 ft) of the
accelerator head in laboratory instruments, but may be several kilometers away in well
logging instruments.
In comparison with their predecessors, sealed neutron tubes do not require vacuum pumps and gas
sources for operation. They are therefore more mobile and compact, while also durable and reliable.
For example, sealed neutron tubes have replaced radioactive modulated neutron initiators, in
supplying a pulse of neutrons to the imploding core of modern nuclear weapons.
Examples of neutron tube ideas date as far back as the 1930s, pre-nuclear weapons era, by German
scientists filing a 1938 German patent (March 1938, patent #261,156) and obtaining a United States
Patent (July 1941, USP #2,251,190); examples of present state of the art are given by developments
such as the Neutristor,[3] a mostly solid state device, resembling a computer chip, invented at Sandia
National Laboratories in Albuquerque NM.[citation needed] Typical sealed designs are used in a pulsed
mode[4] and can be operated at different output levels, depending on the life from the ion source and
loaded targets.[5]
Neutristor in an inexpensive vacuum sealed package ready for
testing
Ion sources
[edit]
Main article: Ion source
A good ion source should provide a strong ion beam without consuming much of the gas. For hydrogen
isotopes, production of atomic ions is favored over molecular ions, as atomic ions have higher neutron
yield on collision. The ions generated in the ion source are then extracted by an electric field into the
accelerator region, and accelerated towards the target. The gas consumption is chiefly caused by the
pressure difference between the ion generating and ion accelerating spaces that has to be maintained.
Ion currents of 10 mA at gas consumptions of 40 cm3/hour are achievable.[2]
For a sealed neutron tube, the ideal ion source should use low gas pressure, give high ion current with
large proportion of atomic ions, have low gas clean-up, use low power, have high reliability and high
lifetime, its construction has to be simple and robust and its maintenance requirements have to be
low.[2]
Gas can be efficiently stored in a replenisher, an electrically heated coil of zirconium wire. Its
temperature determines the rate of absorption/desorption of hydrogen by the metal, which regulates
the pressure in the enclosure.
