The Discovery of X-rays and Radioactivity

Since Millikan carried out his work a number of other methods have been developed for determining the charge of the electron, and its value is now known to about 0.001%. 

Several great scientific discoveries were made in a period of a few years, beginning in 1895. These discoveries made great changes in chemistry as well as in physics. X-rays were discovered in 1895, radioactivity was discovered in 1896, the new radioactive elements polonium and radium were isolated in the same year, and the electron was discovered in 1897. 

Soon after the discovery of x-rays the French physicist Henri Becque-rel (1852-1908) investigated some minerals containing uranium. He found that these minerals emit rays that, like x-rays, can pass through black paper and other opaque materials and expose a photographic plate. He also found that the radiation produced by the uranium minerals 

A simple electroscope. When an electric charge is present on the gold foil and its support, the two leaves of the foil separate, because of the repulsion of like electric charges. 

Marie Sklodowska Curie (1867-1934) then began a systematic investigation of Becquerel radiation, using the electroscope as a test. She investigated many substances, to see if they were similar to uranium in producing rays. She found that natural pitchblende, an ore of uranium, is several times more active than purified uranium oxide. 

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Soon after the discoveries of X-rays and radioactivity it was learned that radiation could cause changes in matter. In 1901 P. Curie found that when a radium source was placed on his skin, woimds were produced that were difficult to heal. In 1902 skin cancer was shown to be caused by the radioactivity from radium but S years later it was learnt that radium therapy could be used to heal the disease. Large radiation doses were found to kill fungi and microorganisms and produce mutations in plants. 

Experiments were under way to understand how electricity and matter interact when the discoveries of x-rays and radioactivity were announced. Scientists trying to understand this new phenomena radioactivity experimented day and night. Ernest Rutherford was one of the many. He tried to understand the nature of radioactivity and classified it into three basic types. While trying to find out more about radioactivity, he conducted his gold foil experiment that ultimately provided greater insight into the subatomic nature of the atom by discovering the nucleus. He also identified the proton present in the nucleus. 

What unexpected observations led to the discovery of x-rays and radioactivity Who discovered them and when ... 

In this chapter we consider some of the experiments that led to the conclusion that the atom is electrical in nature. Dalton s concept of a structureless atom provided no mechanism to explain these observa tions. These experiments, started over 150 years ago, also culminated in the discovery of X rays and radioactivity (Section 25.1). The spontaneous disintegration of naturally radioactive atoms into smaller particles contradicts the Daltonian hypothesis that atoms are unalterable. In turn, these discoveries inaugurated a more complete theory of the structure of atoms—the nuclear theory of the atom—and reaffirmed the atom as the unit of chemical changes. 

The dual discoveries of radioactivity and X-rays made possible the further discovery and identification of several new elements, such as radium and polonium, which needed to be accommodated, and thus provided further tests of the robustness of the periodic system and its ability to adapt to changes. Indeed, while it is the electron that is mainly responsible for the chemical properties of the elements, discoveries connected with the nucleus of the atom nevertheless have had a profound influence on the evolution of the periodic system. The exploration of the nucleus, along with further work on the nature of X-rays and radioactivity, led to the discovery of atomic number and isotopy, two developments that would together resolve many of the lingering uncertainties surrounding Dimitri Mendeleevs periodic system. 

In the early years of Theosophy during Blavatsky s lifetime, the modem science that most occupied the movement was, without doubt, the theory of evolution rather than Victorian discoveries in physics and chemistry.4 While Blavatksy in Isis Unveiled and Secret Doctrine grappled in a limited way with modem physics and chemistry, she engaged much more fully with the work of alchemists, especially that of Paracelsus. Her defense of alchemical transmutation was based not upon contemporary science—though she asked of transmutation, Is the idea so absurd as to be totally unworthy of consideration in this age of chemical discovery (Isis 1 503)—but rather upon the exalted reputation of medieval and early modem scientists and alchemists who claimed to have witnessed transmutation (1 503-504). Moreover, the major events that launched modem particle physics—the discoveries of X-rays, the electron, radiation, radium, and radioactive decay—all occurred after Blavatsky died in 1891. 

It is clear that, along with the discovery of x-rays in 1895, Roentgen also found the chemical action of ionizing radiation. He drew attention to the similarity of the photographic effect induced by light and x-rays. Application to medicine appeared very quickly, followed by industrial applications. However, this field of investigation remained nameless until Milton Burton, in 1942, christened it radiation chemistry to separate it from radiochemistry which is the study of radioactive nuclei. Historical and classical work in radiation chemistry has been reviewed by Mozumder elsewhere [1]. Here we will only make a few brief remarks. 

The scientific development of radiation chemistry is reviewed from the discovery in 1895 of x-rays and radioactivity by Roentgen and Becquerel through to the present. 

The purpose of this article is to review the development of radiation chemistry which began with the discovery of x-rays by Roentgen(l) in 1895 and shortly afterwards of radioactivity by Becquerel(2), which in both cases Involved the observation of chemical change in photographic plates and luminescence in certain phosphors. Clearly, in the space available, the review will be restricted and subjective, but will, it is hoped, give the general framework in which the subject has developed. 

Beginning with J.J. Thomson s discovery of the electron in 1897, developments came quickly. In 1911, Ernest Rutherford proposed the nuclear structure of the atom, and by 1920 he had named the proton and the neutron. All of this work was made possible by the discovery of X-rays in 1895, which allowed physicists to probe the atom, and by the discovery of radioactivity in 1896. The phenomenon of radioactivity destroyed the ancient concept of the immutability of the atom once and for all and demonstrated that one element could be transformed into another, thus in a sense achieving the goal that the alchemists had sought in vain. 

The discovery of cosmic ray produced 81Kr in meteorites [l]1 introduced a new method of high sensitivity measurements of 81Kr concentrations and cosmic ray exposure dating. The method consists of a direct measurement of both radioactive 81Kr atoms (Ty2 = 2.13 x 105y, [2] and of stable spallation Kr atoms by a... 

Roentgen s discovery of x-rays stimulated great interest in this new form of radiation worldwide. Antoine Henri Becquerel (1852-1908) accidentally discovered the process of radioactivity while he was studying x-rays. Radioactivity involves the spontaneous disintegration of unstable atomic nuclei. Becquerel had stored uranium salts on top of photographic plates in a dark drawer. When Becquerel retrieved the plates, he noticed the plates contained images made by the uranium salts. Bec-querel s initial discovery in 1896 was further developed by Marie Curie (1867-1934) and Pierre Curie (1859-1906). Marie Curie coined the word radioactive to describe the emission from uranium. 

The four main discoveries of X-rays, radioactivity, atomic number, and isotopy are examined in this chapter by following a roughly historical order, although it must be appreciated that there was much overkp among these four themes. 

X-radiation has been known from the very earliest days of nuclear science Rontgen named the rays in 1895, before Becquerel s discovery of radioactivity, and the classification of X-rays inevitably proceeded in a piecemeal fashion. Hence while Figure 1.18(a) is the sort of logical nomenclature one might pursue with today s hindsight, in practice this is only an approximation to the complicated real situation of Figure 1.18(c). Broadly, K , Kp, L and Lp fit the format more or less correctly, but the logical system fails even with some of these, and does so most decidedly with more distant transitions. 

Lind (1961) defines radiation chemistry as the science of the chemical effects brought about by the absorption of ionizing radiation in matter. It can be said that in 1895, along with X-rays, Roentgen also discovered the chemical action of ionizing radiation. He drew attention to the similarity of the chemical effects induced by visible light and X-rays on the silver salt of the photographic plate. This was quickly followed by the discovery of radioactivity of uranium by Becquerel in 1896. In 1898, the Curies discovered two more radioactive elements—polonium and radium. 

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