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2. ELECTRON MICROSCOPE IN METALLURGY

(Для перевода со словарем)

Examination of an object with a microscope may reveal information about its composition or the manner in which it was made. However, for greater certainty confirming evidence from other types of Investigation usually is desirable.

For practical purposes optical microscope is limited to magnifications of perhaps 2,000 or 3,000 diameters. So, a microscope of a different type was developed. It became known as an "electron microscope", because its operation depends on streams of electrons instead of light rays. As distinct from ordinary optical microscope, it provides magnifications many times higher than can be obtained with the best optical instruments but it has disadvantages too. It is expensive and usually requires considerable skill and patience on the part of the operator.

The electron microscope is essentially a transmission instrument but pictures can be obtained by reflection if the electrons strike the specimen at an angle instead of from a perpendicular direction. However, because of this angle, the image is distorted considerably except for a narrow central strip. Use of the microscope in this way is likely to be limited to special problems.

Because the beam of electrons cannot pass through a piece of metal thicker than 1/100,000 centimeter, nearly all early work with the electron microscope in this field was done by a "replica" method. Some materials, such as the plastic, can be used to obtain a thin layer of substance - a replica - that produces faithfully the irregularities in the surface of a specimen and is relatively transparent to electrons. The replica can be. used instead of the original specimen in the electron microscope.

3. BASIC DESIGN CONSIDERATION OF A NUCLEAR ROCKET

(Для перевода со словарем)

The production of thrust in chemical rockets consists of raising the propellant to a high temperature and pressure in the combustion chamber, and then expanding it through a nozzle to obtain the maximum obtainable velocity. The same expansion process is used in nuclear rockets, but the method of heating the propellant gases is different. The high gas temperature obtained in chemical rockets results from the combustion of a fuel and oxidizer. These propellants are chosen with the objective of creating as high a

temperature as possible while maintaining a low molecular weight in order to maximize the exhaust velocity. In the nuclear rocket as distinct from chemical rocket, combustion of the propellant gases is not required to produce the high temperatures and pressures. Heat is generated by nuclear fission in a reactor and transferred to the propellant, thus eliminating the need to have both oxidizer and fuel.

To attain the objective of maximum specific impulses, it is desirable to select nuclear-rocket propellants with a small molecular weight. Hydrogen, the element with the smallest molecular weight, is theoretically the ideal propellant, from the standpoint of obtaining large exhaust velocities and therefore high specific impulses. While hydrogen does react with many materials at elevated temperatures, other low-molecular-weight elements present storage or use problems that exceed those associated with hydrogen.

The heat required to raise the propellant temperature is produced by fission. Fissionable elements such as uranium, thorium or plutonium are used as fuel. An atomic nucleus of a fissionable element is split when struck by a neutron. However, fission ч:ап be avoided by reflecting the neutron or allowing the nucleus to capture the neutron. The probability of both these actions occuring depends on the velocity of the neutron in relation to the type of nucleus and its velocity.

The velocity of the neutron is important to the process because it is a form of energy and fission is more apt to occur at certain energies than others. In general, the higher velocity a neutron possesses, the less likely fission will take place. Whenever fission does occur, neutrons are not the only particles which create heat. When a nucleus is split, it is split into fragments that possess large quantities of energy in varying velocities. Neutrons are also emitted with high velocities (50,000,000 feet per second), along with other nuclear particles (alpha and beta particles, gamma-rays and neutrinos). Energies of all these fission products are reduced by interaction with other nuclei. This interaction in turn generates heat.

Elements having the properties of being good reflectors and poor absorbers of neutrons are used to reflect and thus reduce the energy of neutrons. Materials (such as carbon) used for this purpose are called moderators. Although their main objective is to slow neutrons to the point where they are likely to cause fission, the heat generated by moderators is only a small part of the total heat generated.

КОНТРОЛЬНЫЕ ВОПРОСЫ

I. В какой функции выступают и как переводятся на русский язык следующие несвободн1>1е словосочетания (§ 62):

at the expense of (smth.), because of (smth.), apart from (smth.), on account of (smth.), with respect to (smth.), irrespective of (smth.), aside from (smth.), in (by) virtue of (smth.), previous to (smth.), with reference to (smth.), along with (smth.), in view of (smth.), in contrast to (smth.), as distinct from (smth.), prior to (smth.), combined with (smth.), owing to (smth.), with (in) regard to (smth.), contrary to (smth.), in spite of (smth.), except for (smth.), in conjunction with (smth.), according to (smth.), together with (smth.), thanl<s to (smtii.), in accordance with (smth.).

2. Как переводятся следующие несвободные словосочетания, группирующиеся вокруг слов so и as (§ 60, 61):

so as, so that, so ar, or so, and so on, and so forth, as to (smth.), as for (smth.), as follows, as yet, as soon as, as well as, * as well, as if, as though, as (so) far as, as (so) long as.

3. Как переводятся следующие несвободные словосочетания, группирующиеся вокруг слов date, time, part (§ 57-59):

up to date, out of date, at times, for the time being, in time, for the most part, in pa,rt, on the part of (smb.).

4. Как переводятся следующие несвободные словосочетания Ч§ 65):

in common, at length, by now, by then, on the whole, on the average, no longer, a great (good) deal, in turn, once more, at once, one another, each other.

5. Какую роль выполняет вспомогательный глагол to do в утвердительном предложении при наличии смыслового глагола (The atmosphere does offer protection...) (§ 98)?

6. Укажите способ перевода конструкций типа "it was not until 1958 that..." (упр. 11).

7. Какое значение имеют префиксы semi-, trans-, non-(упр. 17)?