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A BRIEF ACCOUNT OF RADIO-ACTIVITY CHAPTER I DISCOVERY OF RADIO-​ACTIVITY THF. ohject. or leondumoulin.nl hrief leondumoulin.nl~e is to give a simple account of the.
Table of contents

• A Brief History Of Radioactivity
• A Brief History Of Radioactivity | Hackaday
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• A brief account of radio-activity
• The nature of radioactive emissions

The gamma rays are very penetrating and are not deflected in the magnetic or electric fields. They have the least ionizing power and a very great velocity. The penetrating power of each type is complex and varies with the source, so the statements given are but generalizations.

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The alpha rays are projected particles which lose energy in penetrating matter. The rays are examined and measured in several ways: 1. By their action on the sensitive photographic plates. The use of this method is laborious, consumes time, and for comparative measurements of intensity is uncertain as to effect.

By electrical methods, using electroscopes, quadrant [Pg 16] electrometers, etc. These are the methods most used.

The alpha rays have been identified as similar to the so-called canal rays. These were first observed in the study of the X rays. When an electrical discharge is passed through a vacuum tube with a cathode having holes in it, luminous streams pass through the holes toward the side away from the anode and the general direction of the stream. They travel in straight lines and render certain substances phosphorescent. These rays are slightly deflected by a magnetic field and in an opposite direction from that taken by the cathode rays in their deflection. The rays seem to be positive ions with masses never less than that of the hydrogen atom.

Their source is uncertain, but they may be derived from the electrodes. The gamma rays are analogous to the X rays and are of the order of light. They are in general considerably more penetrating than X rays. For example, the gamma rays sent out by 30 milligrams of radium can be detected by an electroscope after passing through 30 centimeters of iron, a much greater thickness than can be penetrated by the ordinary X rays.

Is this power of emitting radiations a permanent property or is it lost with the passage of time? The first investigations of the activity of uranium and thorium showed no loss of intensity at the end of several years, and radium also seemed to show no decrease in its enormous activity. Polonium, however, was found to lose most of its activity in a year, and later it appeared that some radio-active substances lost most of their activity in the course of a few minutes or hours. A phenomenon called induced or secondary radio-activity was also observed.

Thus a metal plate or wire exposed to the action of thorium oxide for some hours became itself active. This induced activity was not permanent but decreased to half its value in about eleven hours and practically disappeared within a week. Similar phenomena were observed when radium was substituted for thorium.

In Crookes precipitated a solution of an active uranium salt with ammonium carbonate. The precipitate was dissolved so far as possible in an excess of the reagent, leaving an insoluble residue. This residue was many hundred times more [Pg 18] active, weight for weight, than the original salt, and the solution containing the salt was practically inactive. At the end of a year the uranium salt had regained its activity while the residue had become inactive. Another method of obtaining the same result is to dissolve crystallized uranium nitrate in ether.

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Two layers of solution are formed, one ether and the other water coming from the water of crystallization. The aqueous layer is active, while the water layer is inactive.

Similarly, by adding barium chloride solution to a solution of a salt of uranium and then precipitating the barium as sulphate, the activity is transferred to this precipitate. These experiments give proof of the formation and separation of a radio-active body by ordinary chemical operations. So, too, in the case of thorium salts a substance can be obtained by means of ammonium hydroxide which is several thousand times more active than an equal weight of the original salt.

After standing a month, the separated material has lost its activity and the thorium salt has regained it. Here, again, there is the formation, separation, and loss of a radio-active body. Now, these are ordinary chemical processes for the separation of distinct chemical individuals. The results, therefore, lead naturally to the conclusions: 1 it would seem that uranium and thorium are themselves inactive and the activity is due to some other substance formed by these elements; 2 this active substance is produced by some [Pg 19] transformation in those elements, for on standing the activity is regained.

This latter conclusion is startling, for it indicates a change in the atom which, up to the time of this discovery, was deemed unchangeable under the influence of such physical and chemical changes as were known to us. The search for new radio-active bodies and the study of their characteristics has been systematically and successfully carried on.

The bodies obtained in the above experiments were named uranium X and thorium X , respectively. Further, it became clear from the investigation of uranium minerals that radium, polonium, actinium, and ionium originated from uranium. From thorium minerals a body was separated called mesothorium, which was analogous to radium.

Both thorium and radium were found to give off a radio-active gas.

The first lost half of its activity in less than one minute. The second was more stable and lost half of its activity in about four days. The name radium emanation was given to the latter and it was found chemically and physically to belong to the class of monatomic or noble gases, such as helium, argon, neon, etc. In some cases the chemical action was determined and these new bodies were found analogous to well-known elements, as radium to barium, polonium to bismuth. The physical properties were investigated and, where possible, spectra were mapped and atomic weights determined.

In this way more than thirty new elements have been added to the list. These new elements are called radio-active elements, but it is an open question whether all atoms do not possess this property in greater or less degree. Certainly, it is possessed in varying degree by four of the old elements widely separated in the Periodic System, namely, uranium, thorium, rubidium, and potassium. The last two, while feebly active themselves, do not form any secondary radio-active substance so far as is known.

Only two of the elements, then, can definitely be said to go through these transformations. It is just possible that radio-activity may be found to be a common property of all atoms and of all matter. It is important to know how these new bodies were discovered and distinguished from one another. Two properties are relied upon. One is the nature of the rays emitted and the other is the duration of the activity. Of course, knowledge of the physical and chemical properties is also of great importance whenever obtainable.

The nature of the radiation is a distinguishing characteristic, though similarity here does not prove identity of substances. The rays may also differ in the velocity with which they are emitted by different radio-active substances. The duration of the activity is called the life period. This is absolutely fixed for each body and furnishes the most important mode of differentiating among them. It measures the relative stability and is the time which must elapse before their activity is lost and they, changing into something else, entirely disappear.

The measure usually adopted is the half-value period.

The nature of radioactive emissions

Two hypotheses are made use of:. That the activity of the matter so formed decreases according to an exponential law with the time from the moment of its formation. These hypotheses agree with the experimental results. The decrease and rise of activity, for example, of uranium and uranium X , and also of thorium and thorium X , have been measured, plotted, and the equations worked out. Manifestly, a state of equilibrium will be reached when the rate of loss of activity of the matter already produced is balanced by the activity of the new matter [Pg 22] produced.

This equilibrium and the knowledge of the rate of decrease in general will have little value if this rate, like chemical changes, is subject to the influence of chemical and physical conditions.