Physics

The Discovery of Radioactivity (continued)


Henri Becquerel's contribution

Henri Becquerel belonged to an illustrious family of scientists. His grandfather, Antoine Becquerel, born in 1788, was an important researcher of electrical and magnetic phenomena, having published a great treatise on the subject. Henri's father, Edmond Becquerel (1821-1891), was noted for his studies of ultraviolet radiation and the phenomena of phosphorescence and fluorescence. Especially from 1859 to 1861, he had studied calcium, barium, strontium, and others. Among the materials he studied included some uranium salts.

In his father's laboratory, Henri Becquerel developed his scientific training and carried out his first research - almost all on optics and many of them, from 1882 to 1897, on phosphorescence. Among other things, he studied the invisible (infrared) phosphorescence of various substances. In particular, he studied uranium salt fluorescence spectra using samples his father had accumulated over the years.

Nothing was more natural than Henri Becquerel's interest in X-rays and, more particularly, in Poincaré's conjecture and in the works of Henry and Niewenglowski. In fact, it simply seemed that in addition to being able to emit visible and infrared radiation, luminescent bodies could also emit X-rays. Becquerel decides to experiment on the subject. We will reproduce below the full text of Henri's first note on the subject, presented to the Academy on February 24, 1896 (two months after the discovery of the X-ray discovery):

"In previous meeting from the French Academy of Sciences, Charles Henry noted that by placing phosphorescent zinc sulfide in the path of the rays coming out of a Crookes tube, the intensity of the radiation penetrating the aluminum increased.

In addition, Niewenglowski found that commercial phosphorescent calcium sulfide emits radiation that penetrates opaque substances.

This behavior extends to various phosphorescent substances and, in particular, to uranium salts, whose phosphorescence has a very short duration.

With the double uranium potassium sulfate, of which I have some crystals in the form of a thin transparent crust, I performed the following experiment:

A Lumiére photographic plate is wrapped in two sheets of very thick black paper, so that the plate does not darken even when exposed to the sun for a day. A plate of phosphorescent is placed on the outside of the paper and exposed to the sun for several hours. When the photographic plate is revealed, the silhouette of the phosphorescent substance appears, which appears black in the negative. If a coin or perforated sheet metal is placed between the phosphorescent substance and the paper, the image of these objects can be seen in the negative.

The same experiments can be repeated by placing a thin sheet of glass between the phosphorescent substance and the paper; and this precludes the possibility of any chemical action by vapors that could come out of the substance upon being warmed by the sun's rays. It can be concluded from these experiments that the phosphorescent substance in question emits radiation that penetrates an opaque role to light and reduces silver salts. sensitize photographic paper".

Note that Becquerel knows the previous works of Henry and Niewenglowski and reproduces, without much change, the experiment of the second. It only tested one new substance - double potassium uranyl sulfate - confirming the Poincaré hypothesis.

The following week (March 2, 1896), d'Arsonval reportedly obtained radiographs using a fluorescent lamp and covering the objects to be radiographed with a fluorescent glass containing uranium salt. It concludes in this article that all bodies that emit greenish-yellow fluorescent radiation are capable of impressing photographic plates covered with light-opaque paper.

It is at this same session of the Academy that Becquerel presents a second note, which is commonly described as representing the discovery of radioactivity. Cortés Pla is one of those who makes this mistake, despite having read (and translated) Becquerel's articles: "A week later, on March 2, the Academy hears the result of further investigations that would immortalize Becquerel's name, as they describe the existence of a new phenomenon: radioactivity… " ref. 6, p. 32

In this second note, Becquerel continues the study of the effects produced by double potassium uranyl sulfate. The previous experiment varies, noting that the radiation emitted by this material is less penetrating than ordinary x-rays. Also note that penetrating radiation is emitted both when phosphorescent material is illuminated directly by the sun and when illuminated by reflected or refracted light. It also notes that even in the dark, the material studied sensitizes photographic plates (such as Niewenglowski calcium sulfide). Here is the transcript of this part of the article:

"I will particularly insist on the following fact, which seems very important to me and unaware of the mastery of the phenomena one might expect to observe. The same crystalline coverslips, placed next to photographic plates, under the same conditions, isolated by the same bulkheads, but without radiation excitation and kept in darkness, still produce the same photographic impressions. Here's how I was led to make this observation: some of the previous experiences were prepared on Wednesday, 26, and Thursday, February 27; and since, on these days, the sun appeared only intermittently, I kept the experiments I had prepared and placed the plates with their wraps in the darkness of a furniture drawer, leaving the uranium salt blades in place. As the sun did not reappear in the following days, I unveiled the photographic plates on March 1, hoping to find very faint images. On the contrary, the silhouettes appeared with great intensity. I thought at once that the action must have continued in the dark and prepared the following experience:

At the bottom of an opaque cardboard box I placed a photo card; then, on the sensitive side, I put a uranium salt coverslip, convex coverslip with the highest central part and that it touched gelatin only in a few points; Then, next to it, on the same plate, I placed another slide of the same salt, separated from the gelatin by a thin glass slide. After performing this operation, in the dark room, the box was closed, then placed inside another cardboard box and finally into a drawer.

I repeated the process with an aluminum foil-enclosed receptacle on which I placed a photographic plate, and outside, a coverslip of uranium salt. The set was enclosed in an opaque cardboard box and then in a drawer. After five hours, I revealed the plates and the silhouettes of the crystalline blades appeared in black, as in previous experiments, as if they had become phosphorescent with light. Regarding the coverslip placed directly on the gelatin, there was virtually no difference between the effects at the points of contact and the portions of the coverslip that were about one millimeter apart; The difference can be attributed to the different distances of the active radiation sources. The action of the coverslip placed on the glass was somewhat weakened, but the shape of the coverslip was very well reproduced. Finally, through the aluminum foil, the action was considerably weakened, but nevertheless it was very clear.

It is important to note that this phenomenon does not appear to be attributed to phosphorescent light radiations, since after 1/100 of a second these radiations become so faint that they are almost imperceptible.

One hypothesis that arises very naturally to the mind would be the assumption that these radiations, whose effects have a strong analogy with the effects produced by the radiation studied by Lenard and Roentgen, could be invisible radiations emitted by phosphorescence, whose duration of persistence was infinitely longer than that. that of the light radiations emitted by these substances. However, the present experiences, without being contrary to this hypothesis, do not allow it to be formulated. The experiences I am developing now may hopefully provide some insight into this new kind of phenomenon. ".

Note that there is almost nothing new in this "new kind of phenomenon." The only news is that invisible phosphorescence seemed to last much longer than visible phosphorescence (which was by no means contrary to what was known).

In another X-ray review article that same month, Raveau describes the studies of Charles Henry, Niewenglowski, Piltchikof, d'Arsonval, and Becquerel as all special cases of the phenomenon predicted by Poincaré and discovered by Charles Henry. .

The following week (09/03/1896), amid the usual quota of X-ray articles, Battelli and Gambasso study the role of fluorescent substances in increasing the effect of Roentgen rays. Troost studies phosphorescent zinc sulphide (blends) and repeats and confirms Charles Henry's observations, obtaining strong radiographic images by exciting phosphorescence by magnesium light. Troost also cites the works of Niewenglowski and Becquerel. In turn, Henri Becquerel presents a third communication. It states that the radiation emitted by the studied uranium salt is capable of discharging an electroscope (such as x-rays). It was natural to try to repeat with this radiation all sorts of experiments ever performed with Roentgen radiation to test whether they were the same or not. However, the main analogy that seemed to work in Becquerel's mind was another: the phenomenon was very similar to the invisible phosphorescence (which he had studied) in which there was emission of infrared radiation. Now infrared radiation is of the same nature as light and, contrary to what had been described in the case of x-rays, it is reflected and refracted. Becquerel studies the radiation of potassium uranyl sulfate and concludes that it is reflected on metal surfaces and refracted in ordinary glass. It is now known that this radiation does not reflect or refract on glass.

In the same article, Becquerel describes observations in which uranium salts continue to sensitize photographic plates even when phosphorescent material is kept in darkness for 7 days and notes: "Perhaps this fact can be compared to the indefinite conservation, in certain bodies, of the energy they have absorbed and emitted when they are heated, a fact about which I have already pointed out in a paper from 1891 about heat phosphorescence ". It is noted that Becquerel continues to rely on the phenomena he already knows, recognizing nothing fundamentally new in what he studies.

In the same article, Becquerel studies other phosphorescent materials. Some of them are uranium salts. With all of them the same effects are observed. With zinc sulfide, contrary to what Henry and Troost had observed, Becquerel sees no effect. However, Becquerel makes observations in the dark - and Henry and Troost had done experiments while zinc sulfide was lit. Other phosphorescent materials (strontium and calcium sulfide) are examined. The former has no effect in the dark. A sample of calcium sulfide that produced orange phosphorescence also has no effect, but two calcium sulfides with blue and greenish luminescence. "produced very strong effects, the most intense I have ever had in these experiments. The fact about blue calcium sulfide is in keeping with Mr. Niewenglowski's observation through the black paper. "

From our current knowledge, it is very difficult to understand how the effects described by Becquerel may have occurred. Radiations emitted by uranium salts, in fact, do not reflect or refract; and calcium sulphide should not emit radiation similar to uranium salts (and worse yet!). Either there were effects that cannot be explained by our knowledge, or Becquerel was mistaken in his observations - and in this case it may have been induced by his theoretical expectations to see nonexistent phenomena. Unless these experiments are repeated with the same materials he uses, it will not be possible, however, to exclude the existence of physical phenomena that are currently ignored and different from radioactivity.

Two weeks pass and Becquerel publishes new work (23/03/1896). It describes observations that some non-luminescent uranium compounds also produce the effects described above. Therefore, this invisible phosphorescence appears to be unrelated to visible phosphorescence or fluorescence. But it seems, according to Becquerel, to be really a case of phosphorescence, since he claims that radiation increases when crystals in the dark are exposed to sunlight or when illuminated by an electric discharge - again, the phenomenon described does not should happen, as far as we know. There is another curious observation in this article. Becquerel claims that the calcium sulphide samples, which had produced effects in the dark, no longer impressed photographic plates.
As we have seen, Becquerel believed that the radiation he studied was similar to light because it reflected and refracted, unlike X-rays. In his next article, he described experiments with thin tourmaline slides and claimed to have noticed polarization effects of his radiation (another strange result!). It also goes on to state that the effect becomes stronger when the material is excited by light (and repeats it in the following work as well).

It is now 7 weeks. Only then Becquerel presents new communication. Having observed that all uranium compounds (luminescent or not) emitted the same invisible radiation, Becquerel decided to test the metallic uranium. He takes a sample prepared by Moissan (a chemist who in that same year had isolated the metal) and finds that he also emits radiation. Now that could have shown that it was not a phenomenon of phosphorescence but something of another nature. But Becquerel concludes that this is the first case of a metal that has an invisible phosphorescence. It would be natural from there to investigate the existence of other elements that emit similar radiation, but Becquerel does not. After this work of May 18, he seems to be uninterested and leaves this study.

The first two years

As can be seen from the description so far, Becquerel's work has established neither the nature of the uranium radiation nor the subatomic nature of the process. His work, originated, like that of Charles Henry and others, by the Poincaré hypothesis, was just one of many, at the time, which yielded difficult to interpret results. Seen in the context of the time, it was research that had neither the impact nor the fruitfulness of X-ray discovery.

Few researchers have devoted themselves to the study of "Becquerel rays" or "uranium rays" until early 1898. On the one hand, the luminescent compounds of uranium (or metallic uranium) themselves were difficult to obtain. On the other hand, Becquerel seemed to have exhausted the subject. In addition, many other phenomena announced at the same time distracted attention and also pointed to delicate aspects of such studies.

In Japan, in 1896, Muraoka investigated whether certain luminescent worms were capable of emitting penetrating invisible radiation capable of sensitizing photographic plates. It seemed so, but the results were strange: the effect only came when the worms were kept moist and when there was a card between them and the photographic plate. It was later concluded that the effect was due only to moisture (since moist paper produced the same result). In the same year, some recently polished metal plates (zinc, magnesium and cadmium) were also sensitized to photographic plates. A US researcher, McKissic, reported the same year that many other substances appeared to emit Becquerel rays: lithium chloride, barium sulfide, calcium sulfate, quinine chloride, sugar, chalk, glucose and uranium acetate. Several other similar claims arose in the same period - almost all without foundation. All of this helped to confuse the situation.

In a subject review article published in 1898, Stewart described all sorts of works published at the time. It came to the conclusion (probably the most accepted at the time) that Becquerel's rays were short-wavelength transverse electromagnetic waves (like light) and that the emission process was a kind of phosphorescence. Repeats Becquerel's results concerning reflection, refraction and polarization of uranium rays and the increase in radiation intensity after exposure to light. It essentially adopts the same conception as Becquerel. It is true that in 1897 Gustave le Bon had repeated Becquerel's experiments and had not noticed any signs of reflection, refraction or polarization, but no one paid him any attention. Everyone thought it was a kind of ultraviolet radiation.

It can be said that from May 1896 to early 1898 this field of study became stagnant. The only new result during this time was that uranium radiation remained strong for months, although no light was received. Although Becquerel still claimed that light excitation increased emitted radiation, Elster and Geitel did not find this effect (which, of course, does not exist).

The discovery of new radioactive materials

In early 1898, two researchers independently came up with the idea of ​​trying to locate materials other than uranium that emit the same kind of radiation. The search was made in Germany by G.C. Schmidt and in France by Madame Curie. In April 1898, both published the discovery that thorium emitted radiation, such as uranium. The study method was not photographic but with the use of an ionization chamber, observing the electric current produced in the air between two electrified plates, when a material that emitted radiation between the plates was placed. This method of study was safer than using photographic plates, as these, as we have seen, can be affected by many different types of influences.

The radiation emitted by thorium was observed in all of its compounds examined, as occurred with uranium. It produced photographic effects and was a little more penetrating than uranium. Schmidt claimed to have observed the refraction of thorium rays (as Becquerel had previously done), but could not notice either reflection or polarization of the rays.

Marie Curie studied various minerals as well as pure chemicals. Not surprisingly, he noted that all uranium and thorium minerals emitted radiation. But noted a strange fact:

"All minerals that were active contain the active elements. Two uranium minerals - pechblenda uranium oxide and calcolite copper and uranyl phosphate they are much more active than uranium itself. This fact is very remarkable and leads us to believe that these minerals may contain a much more active element than uranium. I reproduced calcolite by Debray's process with pure products; this artificial calcolite is no more active than other uranium salts " .

In the same work, Marie Curie draws attention to the fact that uranium and thorium are the elements of greatest atomic weight (of which they were known). It also speculates about the cause of the phenomenon. Given the enormous duration of radiation, it seemed absurd at the time that all the energy emitted (which seemed infinite) could come from the material itself. Marie Curie assumes that the source would be external, meaning that all space would be permeated by a very penetrating, imperceptible radiation that would be absorbed by the heavier elements and reissued in an observable form.

The discovery of the effect produced by the thorium gave new impetus to the search for "Becquerel rays". Now it was clear that this was not an isolated phenomenon occurring only in uranium. Marie Curie gives this phenomenon the name "radioactivity":

"Uranic rays have often been called Becquerel rays. One can generalize this name by applying it not only to uranic rays, but also to toric rays and all similar radiation.

I will call radioactive substances that emit Becquerel rays. The name hyperphosphorescence that was proposed for the phenomenon seems to give me a false idea of ​​its nature. "

It turns out that Marie Curie was aware that this was a much more general phenomenon.

A few months after the discovery of the effect produced by the thorium, Marie and Pierre Curie will present a work of even greater importance. In the previous work, Marie Curie had suggested that the pitchblende might contain other, unknown radioactive material. She goes out of her way to try to isolate this substance. For this, it is dedicated to an analytical chemistry work, progressively separating the constituents of pechblenda, testing them by the electric method, in order to separate the radioactive fractions from the inactive ones. Firstly, starting from pechblenda which was two and a half times more active than uranium, the mineral was dissolved in acid. Then, hydrogen sulfide (H2S) was bubbled through the liquid, and several precipitated insoluble sulfides formed. The uranium and thorium remained dissolved. The precipitate was very active. By adding ammonium sulfide, arsenic and antimony sulfides (not active) dissolve. The residue goes through other separation processes. Finally, the active material is bound to bismuth and not separable from it by the usual processes. It was therefore no known element. Through fractional sublimation processes it was possible to obtain a material (still attached to bismuth) that was 400 times more active than pure uranium. The Curie couple suggests:

"We therefore believe that the substance we remove from pechblenda contains an unidentified metal, neighboring bismuth for its analytical properties. If the existence of this new metal is confirmed, we propose to name it polonium, the country of origin of a of us".

It cannot be said that the existence of a new element was in fact established. The supposed new metal behaved like bismuth and had no spectral streaks that could be noticed. There was therefore some skepticism about this discovery initially.
In an article written after the work on polonium, Marie Curie reviews her knowledge of the subject. It casts doubt on the existence of reflection, refraction and polarization of Becquerel's rays and denies, based on the studies by Elster and Geitel, the possibility of intensifying radioactivity by exposure to the sun. Marie Curie clearly defends the idea that radioactivity It is an atomic property.

At the last meeting of the 1898 Academy of Sciences, the Curies and Bémont presented a new paper. In it, they present evidence of a new radioactive element, chemically similar to barium, also extracted from pechblenda. In this case too, it was not possible to separate the new element from the known metal; but it was possible to obtain a material 900 times more active than uranium. Moreover, this time the spectroscopic analysis allowed to notice an unknown spectral ray. The authors of the article call this new element "radio" because it seems more radioactive than any other element.

Later Steps

There was still much to be understood. What were the radiations emitted: like X-rays or not? Until that time, it seemed so. Where did the energy released from these materials come from? Why are some elements radioactive and others not? None of this had been clarified. There was also no suspicion that radioactivity led to transformations from one chemical element to another. The name "radioactivity" existed, but the complex phenomenon to which we now call this name was not yet known.

The remaining story is long and rich. It cannot be described in detail here. The central purpose of this chapter was to show that Becquerel was far from establishing the existence of radioactivity as we conceive it today. Let us, therefore, only indicate some of the later episodes, to give an idea of ​​what was yet to be discovered.

The nature and diversity of radiation emitted by radioactive materials was gradually established. In early 1899, Rutherford noted two types of uranium radiation - one more penetrating and one easily absorbed. He called them a (less penetrating) and b. However, he imagined that both were different types of x-rays. In late 1899, Geisel observed that polonium radiations were deviant by a magnet. These rays could not, therefore, be X-rays. The Curie couple found that some rays were deflected by the magnet and some were not. The deflected ones corresponded to Rutherford's b-radiation. The sense of deflection showed that they were similar to cathode rays, ie, endowed with negative electric charge. Subsequently, the Curie couple observed, by electrical measurements, that this radiation actually carried a negative charge. Undeflected radiation has been identified as radiation a (which is actually slightly deflected for its large mass / charge ratio).

Becquerel, at this stage, did some studies on the deflection of these radiations. He tried to deflect radiation b through an electric field, but failed initially. This was achieved in 1900 by E. Dorn. In the same year, Villard discovered that non-deviant rays were of two types: a-rays (low penetrating) and other very penetrating rays, which were called "g rays." It was not until 1903 that Rutherford observed that radiation could be deflected electrically and magnetically, and then found to be positively charged particles. Only then did the notion of the nature of these three radiations become clearer.

Another aspect of radioactivity - the transformation of radioactive elements - has also slowly emerged. In 1899 Rutherford noted the existence of a radioactive emanation of thorium. Dorn found that radio also produced a similar emanation. After several months, it was found to be a new chemical element, gaseous (radon). This gas was being produced by the radioactive material. In addition, the Curies had noticed in late 1899 that radio could make nearby bodies radioactive. The following year, Rutherford discovered that the induced radioactivity was due to a deposit created by gaseous emanation. However, this deposit was not identical with emanation.

It was also found that emanation and deposit quickly lost their radioactivity, which proved to be a gradual atomic change. Following these and other studies, Rutherford and Soddy presented the theory of radioactive transformations in 5 articles published from November 1902 to May 1903. With these works, the outlines of the new view on radioactivity had already been established. Many aspects were clarified in the following years.

Final comments

Rather than diminishing Becquerel's role in discovering radioactivity, the purpose of this chapter was to show the great difficulty in establishing phenomena that are not theoretically expected. It is easy to observe what is predicted - indeed, as it turned out, one can observe what was predicted even when the prediction is false. Much harder is to see what goes against all expectations.

The in-depth study of such episodes should be part of the education of every experimental scientist, for the experimenter's stereotyped view lowers and trivializes experimental work — when, in fact, good experimental work is extremely difficult, creative, and thought provoking, provided have the courage to face phenomena in the laboratory that refuse to respect established theories.

Source: Institute of Physics page - UFRGS