nucleus (central mass) of the atom while the electrons orbit the nucleus much like planets around a sun. It is these particles that determine the stability of the atom.
Most natural elements have very stable atoms which are impossible to split except by bombardment by particle accelerators. For all practical purposes, the one true element whose atoms can be split comparatively easily is the metal Uranium. Uranium's atoms are unusually large, henceforth, it is hard for them to hold together firmly. This makes Uranium-235 an exceptional candidate for nuclear fission.
Uranium is a heavy metal, heavier than gold, and not only does it have the largest atoms of any natural element, the atoms that comprise Uranium have far more neutrons than protons. This does not enhance their capacity to split, but it does have an important bearing on their capacity to facilitate an explosion.
There are two isotopes of Uranium. Natural Uranium consists mostly of isotope U-238, which has 92 protons and 146 neutrons (92+146=238). Mixed with this isotope, one will find a 0.6% accumulation of U-235, which has only 143 neutrons. This isotope, unlike U-238, has atoms that can be split, thus it is termed «fissionable» and useful in making atomic bombs. Being that U-238 is neutron-heavy, it reflects neutrons, rather than absorbing them like its brother isotope, U-235. [See comment.]
U-238 serves no function in an atomic reaction, but its properties provide an excellent shield for the U-235 in a constructed bomb as a neutron reflector. This helps prevent an accidental chain reaction between the larger U- 235 mass and its 'bullet' counterpart within the bomb. Also note that while U-238 cannot facilitate a chain-reaction, it can be neutron-saturated to produce Plutonium (Pu-239). Plutonium is fissionable and can be used in place of Uranium-235 {albeit, with a different model of detonator} in an atomic bomb.
Both isotopes of Uranium are naturally radioactive. Their bulky atoms disintegrate over a period of time. Given enough time (over 100,000 years or more) Uranium will eventually lose so many particles that it will turn into the metal Lead. However, the process of decay can be accelerated in what is known as a chain reaction. Instead of disintegrating slowly, the atoms are forcibly split by neutrons forcing their way into the nuclei. A U-235 atom is so unstable that a blow from a single neutron is enough to split it and henceforth bring on a chain reaction (by releasing further neutrons). This can happen even when a (comparatively small) critical mass is present. When this chain reaction occurs, the Uranium atom splits into two smaller atoms of different elements, such as Barium and Krypton.
When a U-235 atom splits, it gives off energy in the form of heat and Gamma radiation, which is the most powerful form of radioactivity and the most lethal. [See comment.] When this reaction occurs, the split atom will also give off two or three of its 'spare' neutrons, which are not needed to make either Barium or Krypton. These spare neutrons fly out with sufficient force to split other atoms they come in contact with. [See chart below.] In theory, it is necessary to split only one U-235 atom, and the neutrons from this will split other atoms, which will split mor ... so on and so forth. This progression does not take place arithmetically, but geometrically. All of this will happen within a millionth of a second.
The minimum amount to start a chain reaction as described above is known as SuperCritical Mass. The actual mass needed to facilitate this chain reaction depends upon the purity of the material, but for pure U-235, it is 110 pounds (50 kilograms), but no Uranium is ever quite pure, so in reality more will be needed. [See comment.]
Diagram of a Chain Reaction
Diagram Outline
[1] - Incoming Neutron
[2] - Uranium-235
[3] - Uranium-236
[4] - Barium Atom
[5] - Krypton Atom
Uranium is not the only material used for making atomic bombs. Another material is the element Plutonium, in its isotope Pu-239. Plutonium is not found naturally (except in minute traces) and is always made from Uranium. The only way to produce Plutonium from Uranium is to process U-238 through a nuclear reactor. After a period of time, the intense radioactivity causes the metal to pick up extra particles, so that more and more of its atoms turn into Plutonium.
Plutonium will not start a fast chain reaction by itself, but this difficulty is overcome by having a neutron source, a highly radioactive material that gives off neutrons faster than the Plutonium itself. In certain types of bombs, a mixture of the elements Beryllium and Polonium is used to bring about this reaction. Only a small piece is needed. The material is not fissionable in and of itself, but merely acts as a catalyst to the greater reaction.
III. The Mechanism of The Bomb
An ordinary aircraft altimeter uses a type of Aneroid Barometer which measures the changes in air pressure at different heights. However, changes in air pressure due to the weather can adversely affect the altimeter's readings. It is far more favorable to use a radar (or radio) altimeter for enhanced accuracy when the bomb reaches Ground Zero.
While Frequency Modulated-Continuous Wave (FM CW) is more complicated, the accuracy of it far surpasses any other type of altimeter. Like simple pulse systems, signals are emitted from a radar aerial (the bomb), bounced off the ground and received back at the bomb's altimeter. This pulse system applies to the more advanced altimeter system, only the signal is continuous and centered around a high frequency such as 4200 MHz. This signal is arranged to steadily increase at 200 MHz per interval before dropping back to its original frequency.
As the descent of the bomb begins, the altimeter transmitter will send out a pulse starting at 4200 MHz. By the time that pulse has returned, the altimeter transmitter will be emitting a higher frequency. The difference depends on how long the pulse has taken to do the return journey. When these two frequencies are mixed electronically, a new frequency (the difference between the two) emerges. The value of this new frequency is measured by the built-in microchips. This value is directly proportional to the distance travelled by the original pulse, so it can be used to give the actual height.
In practice, a typical FM CW radar today would sweep 120 times per second. Its range would be up to 10,000 feet (3000 m) over land and 20,000 feet (6000 m) over sea, since sound reflections from water surfaces are clearer.
The accuracy of these altimeters is within 5 feet (1.5 m) for the higher ranges. Being that the ideal airburst for the atomic bomb is usually set for 1,980 feet, this error factor is not of enormous concern.
The high cost of these radar-type altimeters has prevented their use in commercial applications, but the decreasing cost of electronic components should make them competitive with barometric types before too long.
The air pressure detonator can be a very complex mechanism, but for all practical purposes, a simpler model can be used. At high altitudes, the air is of lesser pressure. As the altitude drops, the air pressure increases. A simple piece of very thin magnetized metal can be used as an air pressure detonator. All that is needed is for the strip of metal to have a bubble of extremely thin metal forged in the center and have it placed directly underneath the electrical contact which will trigger the conventional explosive detonation. Before the strip is set in place, the bubble is pushed in so that it will be inverted.
Once the air pressure has achieved the desired level, the magnetic bubble will snap back into its original position and strike the contact, thus completing the circuit and setting off the explosive(s).
The detonating head (or heads, depending on whether a Uranium or Plutonium bomb is being used as a
