A general trend among the elements is that the ratio of neutrons to protons in an atomic nucleus increases with the element's atomic number (the number of protons the nucleus contains, which determines which element it is). Heavier elements require relatively more neutrons to stabilize the nucleus. When the nucleus of a heavy element like uranium (atomic number 92) is split the fragments, having lower atomic numbers, will tend to have excess neutrons. These neutrons are shed very rapidly by the excited fragments. More neutrons are produced on average than are consumed in fission.
Fission is a statistical process. The nucleus rarely splits into pieces with nearly the same mass and atomic number. Instead both the size and atomic numbers of the fragments have Gaussian distributions around two means (one for the lighter fragment around 95, one for the heavier around 135). Similarly, the number of neutrons produced varies from zero to six or more, and their kinetic energy varies from 0.5 MeV to more than 4 MeV, the most probable energy is 0.75 MeV, the average (and median) is 2 MeV.
A breakdown of the energy released by fission is given below:
MeV
Kinetic energy of fission fragments 165 +/- 5
Instantaneous gamma rays 7 +/- 1
Kinetic energy of neutrons 5 +/- 0.5
Beta particles from product decay 7 +/- 1
Gamma rays from product decay 6 +/- 1
Neutrinos from product decay 10
TOTAL 200 +/- 6
All of the kinetic energy is released to the environment instantly, as are most of the instantaneous gamma rays. The unstable fission products release their decay energies at varying rates, some almost immediately. The net result is that about 180 MeV is actually available to generate nuclear explosions, the remainder of the decay energy shows up over time as fallout (or is carried away by the virtually undetectable neutrinos).
The principle issues that must be solved to construct a fission weapon are:
Keeping the fissionable material in a subcritical state before detonation;
Bringing the fissionable material into a supercritical mass while keeping it free of neutrons;
Introducing neutrons into the critical mass when it is at the optimum configuration (i.e. at maximum super criticality); Keeping the mass together until a substantial portion of the material has fissioned.
To explode, the bomb must first be imploded: compress a subcritical spherical fissionable mass (a ball of normal density uranium and other metals) with specially designed explosives. Implosion is the detonation of explosives on the outer surface, instead of the inner surface, which causes the detonation/shock wave to move inward. Once the shock wave is transmitted to the fissionable core it compresses the core and raises the density to the point of supercriticality which then leads to a great explosion. Essentially what is happening here is that the fissionable mass is crushed to a great density, and once the mass has reached that supercritical density it goes off.
There are four main problems that must be taken care of for an atomic bomb to explode. They are all related with creating a fission chain reaction:
The fissionable material must be kept in a subcritical state before detonation. The fissionable material must be brought into a supercritical state while keeping it free of neutrons. Otherwise most of the fissionable mass would be used up and it would not generate a large explosion (if any). The neutrons must be added to the critical mass when it is at maximum supercriticality, meaning at the most "explosive" point. This can be compared to releasing a rubber-band when it is fully stretched so it will travel with the most speed.
The fissionable mass must be kept together until a large amount of it has gone through fission, making it efficient. If the fissionable mass does not stay together, the fission reaction would immediately be stopped. When the atomic nuclei in the center of an atomic bomb, which is composed of fissile materials, are split, an enormous amount of energy is released as dangerously high levels of heat and radiation. This is done by when a single neutron strikes the nucleus of a fissile material such as uranium 235or plutonium239, two or three more neutrons are released. When those neutrons are ejected, enormous energy is released. The flying neutrons then hit other nuclei of the uranium and cause them to split in a similar manner, releasing more energy and neutrons. When this fission spreads, a huge amount of energy is generated instantaneously.
Thermal radiation in the form of soft X-rays are released by the fission primary (trigger). The photon gas fills the bomb casing, and produces tremendous pressure that implodes the secondary stage to very high densities. When the implosion shock reaches the center of the secondary, the fission spark plug is compressed. The energy released by the spark plug ignites the fusion reaction, leading to the main energy release in the weapon.
In this levitated pit design, the thin outer shell of plutonium is driven inward by the explosive charge at velocities of several kilometers per second. The impact of this shell on the center sphere of plutonium creates two very high pressure shock waves, one travelling in toward the center and one travelling out through the shell. These shocks, with pressures of several megabars (several million atmospheres) compress the plutonium to densities of 2.5-4 times normal. The collapse of the central sphere also compresses the fusion fuel in the center.
When the fission reaction has released enough energy to raise the temperature in the core to several million degrees K (a few hundred tonnes of TNT equivalent), a fusion reaction is ignited in the fuel. The deuterium + tritium fusion reaction:
D + T -> He-4 + n + 17.6 MeV
produces large amounts of extremely high energy neutrons (14.1 MeV). When one of these neutrons collides with a plutonium nucleus, it fissions and releases an additional 4.5 neutrons on average (compared to about 2.9 for fission induced by lower energy fission neutrons). Since each fusion neutron directly generates 180 MeV through fission, and another 810 MeV as a result of the second generation neutrons, the energy released by fusion driven fission greatly exceeds that produced by the fusion reaction itself.
Solving issues 1, 2 and 3 together is greatly complicated by the unavoidable presence of naturally occurring neutrons. Although cosmic rays generate neutrons at a low rate, almost all of these background neutrons originate from the fissionable material itself through the process of spontaneous fission. Due to the low stability of the nuclei of fissionable elements, these nuclei will occasionally split without being hit by a neutron. This means that the fissionable material itself periodically emits neutrons.
The process of assembling the supercritical mass must occur in significantly less time than the average interval between spontaneous fissions to have a reasonable chance of succeeding. This problem is difficult to accomplish due to the very large change in reactivity required in going from a subcritical state to a supercritical state. The time required to raise the value of k from 1 to the maximum value of 2 or so is called the reactivity insertion time, or simply insertion time.
It is further complicated by the problem of subcritical neutron multiplication. If a subcritical mass has a k value of 0.9, then a neutron present in the mass will (on average) create a chain reaction that dies out in an average of 10 generations. If the mass is very close to critical, say k=0.99, then each spontaneous fission neutron will create a chain that lasts 100 generations. This persistence of neutrons in subcritical masses further reduces the time window for assembly, and requires that the reactivity of the mass be increased from a value of less than 0.9 to a value of 2 or so within that window.
Simply splitting a supercritical mass into two identical parts and bringing the parts together rapidly is unlikely to succeed since neither part will have a sufficiently low k value, nor will the insertion time be rapid enough with achievable assembly speeds.
Assembly techniques only address issues 1 and 2, reconfiguring sub-critical masses rapidly into supercritical ones. The next problem is to make sure fission does occur when it is desired.
Since neutrons are generated periodically by spontaneous fission, one approach would be to hold the supercritical mass together after it is assembled until spontaneous neutrons start the reaction. This is at least possible for gun assembly, but it is unsatisfactory for implosion since the highly compressed pit begins expanding soon after the shock wave dies out. Even in a compressed pit the fission reaction takes about 250 nanoseconds, roughly the duration of the maximum compression. It is therefore important to initiate the chain reaction very soon after maximum compression is achieved, or even slightly before.
A better method is to have neutron generator whose operation is precisely synchronized with the assembly process. Three general mechanisms have been developed for this, all of which use charged particle reactions to generate neutrons.
The first type of generator to be invented relies on the fact that one of the neutrons in beryllium-9 is easily knocked loose. Occasionally if it is struck by an alpha particle, like those produced by some produced by some radioactive isotopes, a neutron will be released as a result of the collision:
Be-9 + He-4 -> Be-8 + n + He-4
This happens in only 0.008% of collisions, so a strong alpha emitter (like polonium-210) is required to achieve the neutron flux needed by an implosion weapon. A neutron generation rate of 10-100 million neutrons per second is needed to ensure the prompt initiation of the reaction, thus 100-1000 billion alphas per second are required (3-30 curies of radioactive material). The generator is located in the center of the pit. Clever designs (still classified in the US, though detailed descriptions now exist in the open literature) are needed to keep the alpha emitter and beryllium separate, but still allow the implosion process to bring them together rapidly. This type of generator was used in all of the early atomic weapons.
The major problem with the beryllium/alpha emitter generators is that the strong emitters used have very short half-lives (138.4 days for Po-210). Maintaining inventory of weapons thus requires continual manufacture and replacement of generators. Also, due to difficulties in precisely controlling the mixing of the beryllium and polonium it is difficult to control the initiation of the fission reaction accurately. These types of generators had a tendency to start the reaction later than optimum.
A similar approach is to use the implosion to initiate a neutron generating fusion reactions with tritium and deuterium. It may seem surprising that this can be made to work, given the well known fact that fission explosions are required to produce the temperatures that fusion reactions normally need. Three considerations overcome this obstacle. First, an exceedingly low rate of fusion is actually required. One neutron (and thus one fusion) every 10 nanoseconds is sufficient, a rate that is only some 10^-24 as fast as an actual fusion explosion would need. Second, implosions focus energy and can reach very high temperatures near the center. Theoretically the temperature at the center is infinitely high, but lack of perfect symmetry reduces this. Even so, a high precision implosion can reach temperatures of several hundred thousand degrees C. Third, the velocity of atoms in a gas or plasma is a statistical distribution. A very small portion of the atoms can greatly exceed the average energy. Thus enough atoms in the D-T mixture near the center can reach fusion energies to produce the required rate of neutron production. This type of implosion initiator is even more difficult to engineer than the Be/Po-210 type since the very high precision implosion is required to achieve the required symmetry. The major advantage is that the short half-life Po-210 is not needed.
A more sophisticated system is to use an electronically controlled particle accelerator called a pulse neutron tube. These generators use the deuterium + deuterium or deuterium + tritium fusion reactions to produce large amounts of neutrons. A very short surge of high voltage current accelerates a pulse deuterium or tritium nuclei to energies sufficient to cause fusion reactions, then slams them into a deuterium or tritium rich target. A short pulse containing millions of neutrons is produced. The timing of the pulse can be precisely controlled. Because of the large number of neutrons produced, the generator can be located anywhere in the weapon with assurance that a sufficient number will reach the pit. This is the initiator commonly used in most modern nuclear weapons.
Fusion reactions, also called thermonuclear reactions, are reactions between the nuclei of certain isotopes of light elements. If the nuclei collide with sufficient energy provided by heat in a bomb, or by a particle accelerator in the laboratory then there is a significant chance that they will merge to form one or more new nuclei with the release of energy. Different nuclei combinations have different inherent likelihoods of reacting in a collision at a particular temperature. The rates of all fusion reactions are affected by both temperature and density. The hotter and denser the fusion fuel, the faster the fusion burns. The fusion reactions used in bombs and power plant designs are simple, and extremely fast - which is essential since the fuel must be fully consumed within microseconds.
The splitting of atomic nuclei releases enormous energy. When a single free neutron strikes the nucleus of an atom of fissile (radioactive) material like uranium 235 or plutonium 239, it usually knocks two or three more neutrons free. Energy is released when those neutrons split off from the nucleus, and the newly released neutrons strike other uranium 235 (or plutonium 239) nuclei, splitting them in the same way, releasing more energy and more neutrons. This chain reaction spreads almost instantaneously. The atomic bomb (A-bomb) was a weapon of destruction that used the power released by the splitting of atomic nuclei.
When a critical mass is available, a chain reaction takes place instantaneously, releasing energy far beyond the capacity of ordinary explosives.
Bibliography:
Physics Research Assignment Manesh Sharma
