Radioactivity
Radioactivity is a manifestation of nuclear instability. Unstable nuclei boil, erupt, emit particles, blow up, or transform themselves by some means, usually into another nuclear species. This process is called radioactive decay. We have already mentioned various aspects of the phenomena encountered.
The most common forms of radioactive decay involve the emission of α, β, or γ-rays. The energies of emitted particles are often of the order of Mev but higher and lower energies are also found. In the range of energies around 1Mev, γ-rays are more penetrating than β-rays, and β-rays are more penetrating than α-rays.
The decay process is describable in terms of a sign constant, called the decay constant. With one exception, this decay constant is unaffected by pressure, temperature, chemical composition, or other such factors that can be varied in the surroundings of radioactive nuclei.
Radioactive substances are often classified according to whether they occur on the earth in a radioactive state without further treatment, or whether they are produced by bombardment in man-made apparatus. The former are called "naturally" radioactive, the latter "artificially" radioactive.
As one might expect, naturally occuring radioactive substances are either very long-lived, or are the products of long-lived substances. Short-lived substances, unless replenished, have decayed, and so have disappeared from the earth. This raises interesting questions regarding the origin of matter. For example, to what extent are stable nuclei the products of radioactive decay? Or, what limits can study of natural radioactivity set on the age of the earth?
Artificially prepared radioactive substances are also of great interest. Their general availability has opened up whole new chapters in the exploration of matter.
Radioactivity is moreover, an aspect of one of the most fascinating manifestations of nature —the metamorphosis of one thing into something else - the creation of something new. We might be able to say a good deal concerning the transformation of the lump of sugar in the cup of tea; the emission of a quantum of light is perhaps less familiar, and the transformation of a nucleus is a process that we are just inventing the words to describe.
TEXT 6
How radar works
The design of a radar begins with consideration of its intended use, that is, the function to be performed by the radar as a whole. The uses generally divide into three categories:
1. Warning and surveillance of activity, including identification.
2. Aids to the direction of weapons, that is, gunfire control and searchlight control.
3. Observation of terrain echoes or beacons for navigation and control of bombing.
There is nothing mysterious or complex about radiolocation. It rests on the foundations of ordinary radio theory, and is a technique based on the transmission, reception, and interpretation of radiofrcquency pulses. Considered as a whole, it must be admitted that even the most elementary of radar equipment is difficult to visualize, but this is simply due to the fact that so many (normally) curious circuits and pieces of apparatus are gathered together under one roof. No particular circuit or detail of the equipment is in itself especially difficult to understand, and once the elements are known the complete assembly is no longer mentally unmanageable.
The word "radar" is derived from the phrase "radio direction-finding and range", and it may be more expressive than the older "radiolocation", or it may not. Finding the position of an aircraft or a ship by means of radio covers a very wide field of electronic application, covers, in fact, the whole area of radio direction-finding (R. D. F.) from the elementary bearing-loop to the principle of the reflected pulse which represents the latest principle of the technique. In this article the term will be used to cover only those methods of detection which depend upon the reflected pulse, the characteristic (by popular opinion) which distinguishes radar from all other methods oi position-finding in that no co-operation is required on the part of the target. We shall not dwell, therefore, upon the older and more familiar methods which depend upon the reception at two or more points of a signal transmitted by the body under location itself.
The actual equipments in use which employ the reflected pulse principle are greatly varied from the point of view of physical appearance, but their basic principles are the same.
First, let us tabulate and briefly analyse the problem to be met. The aim of radar is to find the position of a target with respect to a fixed point on the ground - say the position of an aeroplane or a barrage balloon with respect to the radar equipment situated in a field a mile or so away. Three quantities must be measured in order to define the position of the aeroplane or the barrage balloon: first, the slant range, the length of the most direct line drawn from the radar site to the target; second, the angle of bearing, i.e. which point of the compass the target occupies; third, the angle of elevation. Fig. 6 should make these points clear for you. When the target is an aeroplane, these three quantities are continuously varying so that the problem of position-finding is somewhat complicated by the fact that the radar equipment has to "follow" as well as find. In the case of barrage balloon, things are not quite so difficult, and the three important factors may be found at leisure.
Fig. 6
TEXT 7