专业英语复习资料

O UR MATERIAL world is composed of many substances distinguished by their chemical, mechanical, and electrical properties. They are found in nature in various

physical states—the familiar solid, liquid, and gas, along with the ionic ―plasma.‖

However, the apparent diversity of kinds and forms of material is reduced by the

knowledge that there are only a little more than 100 distinct chemical elements

and that the chemical and physical features of substances depend merely on

the strength of force bonds between atoms.

We recall that radiant energy may be released by heating of solids, as in the wire of a light bulb; by electrical oscillations, as in radio or television transmitters; or by atomic interactions,

as in the sun. The radiation can be viewed in either of two ways—as a wave or as

a particle—depending on the process under study. In the wave view it is a combination

of electric and magnetic vibrations moving through space. In the particle

view it is a compact moving uncharged object, the photon, which is a bundle of

pure energy, having mass only by virtue of its motion.

A COMPLETE understanding of the microscopic structure of matter and the exact

nature of the forces acting is yet to be realized. However, excellent models have

been developed to predict behavior to an adequate degree of accuracy for most

practical purposes. These models are descriptive or mathematical, often based

on analogy with large-scale processes, on experimental data, or on advanced

theory.

An increase inthe temperature of the gas as a result of heating causes greater molecular motion,which results in an increase of particle bombardment of a container wall and thus

of pressure on the wall.

The electronic structure of the other elements is described by the shell model,

in which a limited number of electrons can occupy a given orbit or shell. The

atomic number Z is unique for each chemical element and represents both the

number of positive charges on the central massive nucleus of the atom and the

number of electrons in orbits around the nucleus.

The force of electrostatic repulsion between like charges, which varies inversely

as the square of their separation, would be expected to be so large that nuclei

could not be formed. The fact that they do exist is evidence that there is an even

larger force of attraction.

To disrupt a nucleus and separate it into its component nucleons, energy must be supplied from the outside. Recalling Einstein’s relation between mass and energy, this is the same as saying that a given nucleus is lighter than the sum of its separate nucleons, the difference being the binding mass-energy.

M ANY NATURALLY occurring and man-made isotopes have the property of radioactivity,

which is the spontaneous disintegration (decay) of the nucleus with the emission

of a particle. The process takes place in minerals of the ground, in fibers of

plants, in tissues of animals, and in the air and water, all of which contain traces

of radioactive elements.

Whereas the electron (sometimes called negatron) is a normal part of any atom, the positron is not. It is an example of what is called an antiparticle, because its properties are opposite to those of the normal particle. Just as particles form matter, antiparticles form antimatter.

The measurement of the activity of a radioactive substance is complicated by the presence of background radiation, which is due to cosmic rays from outside the earth or from the decay of minerals in materials of construction or in the earth. It is always necessary to measure the background counts and subtract them from those observed in the experiment.

N UCLEAR REACTIONS—those in which atomic nuclei participate—may take place spontaneously, as in radioactivity or may be induced by bombardment with a

particle or ray. Nuclear reactions are much more energetic than chemical reactions,

but they obey the same physical laws—conservation of momentum, energy,

number of particles, and charge.

The interpretation of nuclear reactions often involves the concept of compound

nucleus. This intermediate stage is formed by the combination of a projectile

and target nucleus. It has extra energy of excitation and breaks up into the

outgoing particle or ray and the residual nucleus.

In the case of reactions that produce a positron,however, either nuclear masses or atomic masses with the subtraction of the mass-energy required to create an electron-positron pair should be used, 0.0011 amu or 1.02 MeV.

When fast neutrons, those of energy of the order of 2 MeV, are introduced into a

medium, they make inelastic or elastic collisions with nuclei. On each elastic collision neutrons are deflected in direction, they lose energy, and they tend to

migrate away from their origin.

We include those particles produced by both atomic and nuclear processes and those resulting from electrical acceleration, noting that there is no essential difference between

X-rays from atomic collisions and gamma rays from nuclear decay; protons can come from a particle accelerator, from cosmic rays, or from a nuclear reaction in a reactor. The word

―materials‖ will refer to bulk matter, whether of mineral or biological origin, as well as to the particles of which the matter is composed, including molecules, atoms, electrons, and nuclei.

In addition to the X-rays as a result of transitions in the electron orbits, a similar radiation called bremsstrahlung (German: braking radiation) is produced. It arises from the deflection and resulting acceleration of electrons as they encounter nuclei.

Charged particles such as protons, alpha particles, and fission fragment ions are classified as heavy, being much more massive that the electron.

Unless the energy of the bombarding particle is very high and it comes within the

short range of the nuclear force, there is a small chance that it can enter

the nucleus and cause a nuclear reaction.

In the presence of a nucleus, a gamma ray photon disappears and two particles

appear---an electron and a positron. Since these are of equal charge but of opposite sign, there is no net charge after the reaction, just as before, the gamma ray having zero charge.

If neutron is captured by the proton in a molecule of water or some other hydrocarbon, a gamma ray is released.

The resulting deuteron recoils with energy that is much smaller than that of the gamma ray

but still is far greater than the energy of binding of atoms in the water molecule.Again dissociation of the compound takes place, which can be regarded as a form of radiation damage.

The energy of slow neutron can be neglected in calculation of fission energy, on the grounds that fission can be induced by absorption in U-235 of very slow neutrons. Only one natural isotope, 235U, undergoes fission in this way, while 239Pu and 233U are the main artificial isotopes that do so. Most other heavy isotopes require significantly larger excitation energy to bring the compound nucleus to the required energy level for fission to occur, and the extra energy must be provided by the motion of the incoming neutron.

It may be surprising that the introduction of only 6.5 MeV of excitation energy can produce a reaction yielding as much as 200 MeV. The explanation is that the excitation triggers the separation of the two fragments and the powerful electrostatic force provides them a large amount of kinetic energy. By conservation of mass-energy, the mass of the nuclear products is smaller than the mass of the compound nucleus from which they emerge.

If a neutron is absorbed by the nucleus of one atom of uranium and one neutron is produced, the latter can be absorbed in a second uranium atom, and so on. In order to sustain a chain reaction as in a nuclear reactor or in a nuclear weapon, the value of must be somewhat above l because of processes that compete with absorption in uranium, such as capture in other materials and escape from the system.

There is a possibility of using the extra neutron, over and above the one required to maintain the chain reaction, to produce new fissile materials. "Conversion" involves the production of some new nuclear fuel to replace that used up, while "breeding" is achieved if more fuel is produced than is used.

When two light nuclear particles combine or ―fuse‖ together, energy is released because the product nuclei have less mass than the original particles. Such fusion reactions can be caused by bombarding targets with charged particles, using an accelerator, or by raising the temperature of a gas to a high enough level for nuclear reactions to take place.

The fusion of two deuterons—deuterium nuclei—in what is designated the D–D reaction results in two processes of nearly equal likelihood.

The fusion reactions do not take place merely by mixing the reactants, because of the very strong force of electrostatic repulsion between the charged nuclei. Only by giving one or both of the particles a high speed can they be brought close enough to each other for the strong nuclear force to dominate the electrical force.

Energies in the kilo-electron-volt and million-electron-volt range can be achieved by a variety of charged particle accelerators. Bombardment of a solid or gaseous deuterium target by

high-speed deuterons gives fusion reactions, but most of the particle energy goes into electrostatic interactions that merely heat up the bulk of the target. For a practical system, the recoverable fusion energy must significantly exceed the energy required to operate the accelerator. Special equipment and processes are required to achieve that objective.

The most promising (有前途的) medium in which to obtain the high particle energies

that are needed for practical fusion is the plasma.

The plasma must be created and heated to the necessary temperature under the constraint (约束) of electric and magnetic fields. Such force s on the plasma are required to assure (保证) that thermal energy is not prematurely (过早地) lost. Moreover,the plasma must remain intact long enough for many nuclear reactions to occur, which is difficult because of inherent instabilities of such highly charged media.

A device that provides force s on charged particles by some combination of electric and magnetic fields and brings the ions to high speed and kinetic energy is called an accelerator.

Colliding high-energy beams of particles and antiparticles can create far more massive nuclear species than can simple ion bombardment of stationary targets. The reason is that a

high-energy charge expends most of its energy in accelerating new particles to meet momentum conservation requirements. In contrast, when a particle collides with an antiparticle, the momentum is zero, allowing all of the energy to go into new mass.

It has been predicted that as many as 50 neutrons can be produced by a single

high-energy (500 MeV) deuteron in spallation reaction. The large supply of neutrons can be used for a number of purposes: (a) physics and chemistry research; (b) production of new nuclear fuel, beneficial radioisotopes, or weapons tritium; and (c) burn unwanted

plutonium or certain radioactive waste isotopes.