Usually the tamper is found next to the fissile core, and it reduces the rate of dissembling for
the rapidly expanded fissioning core, by comparison with an untamped device.
It is necessary for suitable tampers to have sufficient mass to provide the necessary inertia.
To minimize the size of the tamper, high density material is preferable. If the density of the
tamper is lower than the density of the fissile core, instabilities can occur during the
converging implosion. The degree of instability depends on many factors, which include
implosion strength, duration, symmetry, the quality and finish of the components and the
density difference between the tamper and the core.
To maximize the tamping, it must be in direct contact with the surface of the fissile core. At
the same time as the shock travels out approximately at the same speed as the rarefaction
wave when it travels in, if the tamper thickness equals the radius of the core, by the moment
when the shock reaches the surface of the tamper, all of the core will be expanding and no
more tamping effect will be obtained.
A tamper with a short x-ray mean free path arrives quickly in thermal equilibrium with a
fissioning core. The hydrodynamic expansion begins at the thermal boundary, rather than at
the interface between the tamper and core. This increases the distance over which the
rarefaction wave must travel to cause disassembly and gives an additional source of
Tamper materials of sufficiently high density can include:
- Osmium is the most dense metallic element and has a blue-gray color. It is a hard
metal but is brittle, and remains lustrous even at high temperatures. Because of its
hardness, brittleness, low vapor pressure (the lowest in metals of the platinum
group), and very high melting point (the fourth highest of all elements), it is difficult
to machine, form or work solid osmium. Generally osmium is thought to be the most
dense known element, it is a little denser than iridium.
- Iridium is a very hard, brittle, silver-white transition metal of the platinum family. It is
the most corrosion-resistant metal. It is the only metal which maintains good
mechanical properties in air at temperatures above 1600 oC.
- Platinum (metal) is silver-white, lustrous, ductile, and malleable. It does not oxidize
at any temperature, although halogens, cyanides, sulfur, and caustic alkalis corrode
it. The metal has an excellent resistance to corrosion and high temperature and it has
stable electrical properties.
- Rhenium is a silver-white metal and it has one of the highest melting points of all
elements (it gives way only to tungsten and carbon here). When it is annealed this
metal becomes very ductile and it can be bent, coiled, or rolled.
- Gold is the most malleable and ductile of all the metals and easily creates alloys with
many other metals. It is possible to produce these alloys to modify the hardness and
other metallurgical properties. Gold is a good conductor of heat and electricity and
reflects strongly infrared radiation. Chemically, it is not affected by air, moisture and
the majority of corrosive reagents, it is thus very suitable to serve as a protective
coating on other, more reactive, metals. However, it is not chemically inert.
- Tungsten is a steel-gray metal which is often brittle and hard to work. If it is pure, it is
possible to work it easily. One can work it by means of forging, drawing, extruding or
sintering. Of all the metals in their pure form, tungsten has the highest melting point,
lowest vapor pressure and the highest tensile strength. Tungsten has the lowest
coefficient for thermal expansion in any pure metal. The alloying of small quantities
of tungsten with steel strongly increases its toughness.
- Uranium is a silver white, weakly radioactive metal, which is a little softer than steel,
strongly electropositive and a poor electrical conductor. It is malleable, ductile, and a