Sfetcu, Nicolae (2024), Discovery of Astatine by Horia Hulubei, Cunoașterea Științifică, 3:3, 16-31, DOI: 10.58679/CS73411, https://www.cunoasterea.ro/discovery-of-astatine-by-horia-hulubei/

Abstract

The search for new chemical elements was a prominent scientific endeavor in the late 19th and early 20th centuries. This era saw the discovery of several elements such as radium, polonium, and radon by prominent scientists such as Marie Curie and Ernest Rutherford. Astatine was one of the missing elements from the periodic table, and its existence was predicted due to the periodicity of the elements, with iodine as its nearest stable neighbor. Scientists have sought to confirm its existence and properties. The discovery of astatine by Horia Hulubei and his collaborator Yvette Cauchois at the beginning of the 20th century is recognized by many scientists, although the authorship of this discovery was ignored by the Austrian radiochemist Friedrich Paneth, who was responsible for setting priorities in the discovery of new chemical elements. This article will delve into the historical background and scientific endeavors that led to the discovery of astatine by Horia Hulubei.

Keywords: astatine, Horia Hulubei, Friedrich Paneth, Yvette Cauchois, periodic table, halogens

Descoperirea astatinului de Horia Hulubei

Rezumat

Căutarea de noi elemente chimice a fost un efort științific proeminent la sfârșitul secolului al XIX-lea și începutul secolului al XX-lea. Această epocă a văzut descoperirea mai multor elemente, cum ar fi radiul, poloniul și radonul, de către oameni de știință proeminenți precum Marie Curie și Ernest Rutherford. Astatinul a fost unul dintre elementele lipsă din tabelul periodic, iar existența lui a fost prezisă datorită periodicității elementelor, cu iodul ca cel mai apropiat vecin stabil. Oamenii de știință au căutat să-i confirme existența și proprietățile. Descoperirea astatinului de Horia Hulubei și colaboratorului său Yvette Cauchois la începutul secolului al XX-lea este recunoscută de mulți oameni de știință, deși paternitatea acestei descoperiri a fost ignorată de radiochimistul austriac Friedrich Paneth, responsabil cu stabilirea priorităților în descoperirea noilor elemente chimice. Acest eseu va aprofunda contextul istoric și eforturile științifice care au dus la descoperirea astatinului de către Horia Hulubei.

Cuvinte cheie: astatin, Horia Hulubei, Friedrich Paneth, Yvette Cauchois, tabelul periodic, halogeni

CUNOAȘTEREA ȘTIINȚIFICĂ, Volumul 3, Numărul 3, Septembrie 2024, pp. 16-31
ISSN 2821 – 8086, ISSN – L 2821 – 8086, DOI: 10.58679/CS73411
URL: https://www.cunoasterea.ro/discovery-of-astatine-by-horia-hulubei/
© 2024 Nicolae SFETCU. Responsabilitatea conținutului, interpretărilor și opiniilor exprimate revine exclusiv autorilor.

Discovery of Astatine by Horia Hulubei

Nicolae SFETCU[1]

nicolae@sfetcu.com

[1] Researcher – Romanian Academy – Romanian Committee of History and Philosophy of Science and Technology (CRIFST), Division of History of Science (DIS), ORCID: 0000-0002-0162-9973

Introduction

The search for new chemical elements was a prominent scientific endeavor in the late 19th and early 20th centuries. This era saw the discovery of several elements such as radium, polonium, and radon by prominent scientists such as Marie Curie and Ernest Rutherford. Astatine was one of the missing elements from the periodic table, and its existence was predicted due to the periodicity of the elements, with iodine as its nearest stable neighbor. Scientists have sought to confirm its existence and properties.

Over fifty elements were discovered during this period, as technological advances enabled Mendeleev’s predictions to be confirmed (Thornton and Burdette 2010). The measurements of H. G. J. Moseley (1887-1915) established that only seven elements — 43, 61, 72, 75, 85, 87, 91 — between hydrogen and uranium remained unknown at the beginning of the 20th century (Moseley 1913) (Moseley 1914). Competing findings for all of Moseley’s „missing” elements thus emerged, and the validity of some claims remained controversial for decades.

Astatine, the rare and highly radioactive element with the chemical symbol „At” and atomic number 85, is one of the least understood and most enigmatic elements in the periodic table. Its rarity and radioactivity have made it a challenging subject of study. The race to characterize this element provides insight into how history often influences the course and credit for scientific discovery.

The discovery of astatine by Horia Hulubei and his collaborator Yvette Cauchois at the beginning of the 20th century is recognized by many scientists, although the authorship of this discovery was ignored by the Austrian radiochemist Friedrich Paneth, who was responsible for setting priorities in the discovery of new chemical elements.

This essay will delve into the historical background and scientific endeavors that led to the discovery of astatine by Horia Hulubei.

Horia Hulubei

Horia Hulubei played a significant role in the discovery of astatine. Born in 1896, Hulubei received his education and training in physics in France and worked closely with renowned scientists such as Marie Curie. He returned to Romania in the 1920s and founded the Institute of Atomic Physics in Bucharest, where he carried out revolutionary research.

Horia Hulubei (November 15, 1896 – November 22, 1972) is known for his contributions to the development of X-ray spectroscopy. He enrolled at the Faculty of Sciences of the University of Iasi, but interrupted his studies by joining the army when Romania entered the First War Mondial, as second lieutenant. General Henri Mathias Berthelot, head of the French military mission in Romania, sent him to France to an aviation school; upon completion of training, Hulubei fought as a pilot in the French Air Service on the Western Front, being seriously wounded. He was awarded the Legion of Honor (Frangopol 2012) (Golea 2018).

In 1922 he resumed his physics and chemistry studies, graduating in 1926 with magna cum laude (Frangopol 2012). In Paris, he obtained his doctorate at the University of Paris – Sorbonne, under the guidance of the Nobel laureate, Jean Baptiste Perrin (1870-1942) (Constantinescu and Bugoi 1998) with the thesis „Contribution to the study of the quantum diffusion of X-rays” (1933), the president of the commission being Marie Curie (Olariu, Stenström, and Hellborg 2005). He continued his research at the University of Paris, staying in contact with scientists such as Frédéric Joliot-Curie, Paul Langevin and Albert Einstein (Facultatea de Fizica Iași 2021).

Horia Hulubei. Credit B. Thornton and S. Burdette(Thornton and Burdette 2010)

In 1926 he worked with Jean Perrin to build an X-ray laboratory at the Sorbonne University. In 1928, they were joined by Yvette Cauchois, with whom she examined the radioactivity of radon in the hope of observing evidence of the presence of element 85 (Scerri 2013).

Hulubei in his lab. Credit: Wikimedia Commons

Between 1927 and 1938 he established the first laboratory of the structure of matter in Romania, in Iași (Facultatea de Fizica Iași 2021).

He was elected a corresponding member of the Romanian Academy in 1937, and a full member in 1946 (Academia Romana 2023). In 1948, he was stripped of his academic title (Stratan 1999), accused of having supported the Romanian government allied with Germany during the war. The intervention of the Nobel laureate Jean-Frederic Joliot-Curie was necessary to drop the charges (Constantinescu and Bugoi 1998) (Thornton and Burdette 2010), being rehabilitated in 1955 (Berindei 2008).

Between 1960 and 1970, Hulubei was a professor of atomic physics in the Department of Atomic and Nuclear Physics of the Faculty of Physics of the University of Bucharest. He was the founder and first director of the Institute of Atomic Physics (IFA) in Măgurele in 1949 (Buzatu 2010).

Currently, the National Institute of Nuclear Physics and Engineering in Romania was named after him, as „Horia Hulubei National Institute of Nuclear Physics and Engineering” (IFIN HH) (IFIN-HH 2023).

Hulubei claimed and published the discovery of new elements, „moldavium” in 1936 (Hulubei 1940) and „sequanium” in 1939, but later it was shown that the respective spectral lines did not belong to the new elements (Fontani 2006). Also, together with Cauchois and Sonia Cotelle, he conducted studies and experiments on polonium and neptunium (Byers and Williams 2006).

In 1936, Horia Hulubei together with Yvette Cauchois discovered element 85 by X-ray analysis, conducting further research and publishing further studies in 1939, calling it „dor” in 1945 (the astatine) (Hulubei and Cauchois 1939).

Astatine 85

The discovery and confirmation of astatine involved the efforts of many scientists and many years of work in Europe, India and the United States. The question of whether astatine exists was also explored. Although unstable in nature, astatine is nevertheless a fascinating and vital member of the periodic table.

 Astatine has the symbol At and atomic number 85. It is the rarest naturally occurring element on Earth, occurring only as a decay product of various heavier elements. In the pages of Time magazine in 1931, astatine was dubbed „the rarest, most fugitive thing on earth” (Time 1931). All isotopes of astatine are short-lived; the most stable is astatine-210, with a half-life of 8.1 hours.

(The electronic shell of astatine-85. Credit: DePiep/Wikimedia Commons, CC BY-SA 3.0 license)

The properties of astatine are not known with certainty (Greenwood and Earnshaw 1997), but it is seen as a heavier analogue of iodine and a member of the halogens. Its name was given by a team from the University of California, Berkeley (D. R. Corson, MacKenzie, and Segrè 1940), who obtained the first synthesis of this element in 1940, after the word ἄστατος (astatos) from ancient Greek, which translates as „unstable”.

Astatine is highly radioactive, probably being a black or metallic-looking solid with an unknown structure (Donohue 1982). One of its most disputed properties is the existence of diatomic astatine (At₂) (Takahashi and Otozai 1986).

Astatine is known to be able to form anions, and some metallic characteristics. It has an electronegativity of 2.2 on the revised Pauling scale – lower than that of iodine (2.66) and the same as hydrogen, and on the Allred-Rochow scale it is 1.9, lower than that of hydrogen (2.2) (Wulfsberg 2000).

Astatine has 41 known isotopes with mass numbers of 188 and 190–229. It is the rarest naturally occurring element, with only four naturally occurring isotopes (astatine-215, -217, -218 and -219) (Lavrukhina and Pozdni︠a︡kov 1970) which are continuously produced as a result of the decay of radioactive thorium and uranium ores.

Astatine was first produced by bombarding bismuth-209 with energetic alpha particles. Astatine-211 is the subject of ongoing research in nuclear medicine (Vértes, Nagy, and Klencsár 2003), with applications in the treatment of cancer. It preferentially (and dangerously) concentrates in the thyroid gland and tends to be absorbed by the lungs and spleen, and in radiocolloid form tends to concentrate in the liver (Lavrukhina and Pozdni︠a︡kov 1970).

The first attempts at discovery

When the Russian chemist Dmitri Mendeleev first published the periodic table of the elements in 1869, he left a blank space under the element iodine. Niels Bohr designated that empty space as the fifth halogen element, leading to its designation as „eka-iod” („a space below iodine”, in Sanskrit) (Ball 2002). Since then, many scientists have claimed to be the first to discover astatine – one of the most complex and fascinating histories of the discovery of chemical elements.

In 1922, Frederick Loring, working with numerical analysis, concluded that the element either did not exist or existed in very small amounts (Loring 1922). A few years later, in X-ray experiments with John Gerald Frederick Druce, they reported results suggesting the presence of the element 85 (Loring and Druce 1925) (Kostecka 2020)

Many other scientists attempted to isolate and characterize eka-iodine without success between the 1920s and 1930s in the United Kingdom, Germany, British India, Denmark, France, and Switzerland (Kostecka 2020).

As with all other discoveries, nationalistic biases prevailed in establishing the identity of the „discoverer”. Because of this, most sources give an incorrect account in declaring Corson, MacKenzie and Segrè as the true discoverers (Scerri 2013).

The first claim was in 1931, by the American physicist Fred Allison (1882-1974), of the Polytechnic Institute of Alabama (now Auburn University), who called it „alabamine” (with the symbol „Ab”) after the name of the institute. In 1935 he claimed to have discovered a new element in a sample of monazite sand. Later his discovery was rejected (Slack 1934), his scientific method of work not being accepted, a method of material analysis that he called the magneto-optical method (Thornton and Burdette 2010). In 1935, the American physicist H. G. MacPherson also showed that Allison’s findings were false due to the imperfections of his instruments (Scerri 2013) (Allison et al. 1931).

In 1937, the Indian chemist Rajendralal De of Dacca (now Bangladesh) claimed his discovery by isolating the serial equivalent of radium F or polonium-210 in the radium series, naming it „dakin”. Like Allison, he was using monazite sand for his research. De published an update of his work in 1947 and a review of his studies in 1962 (Thornton and Burdette 2010). It later turned out that his discovery was not astatine, and the identity of the element he found is currently unknown (Allison et al. 1931).

Discovery of the element Astatine 85

Hulubei’s collaboration with the French physicist Yvette Cauchois was instrumental in the discovery of astatine. Together they published a paper detailing their experiments, which involved using X-ray spectroscopy to identify the characteristic X-ray emissions of astatine. These emissions provided crucial evidence of the element’s existence and allowed them to confirm its place in the periodic table.

Both Horia Hululbei and Yvette Cauchois (1908-1999) studied with Jean Perrin, Hulubei in addition with Marie Curie. Using X-ray spectroscopy, they bombarded the radon with radiation and found an emission line where eka-iodine was expected to be. They published papers on this work in 1936, extending the work in 1939, and a former student of theirs was able to reproduce the results (Stratan 1999) (Hulubei and Cauchois 1939).

Hulubei suggested the name „dor” or „dorium” as an abbreviation for „longing for peace” in Romanian (Scerri 2013).

In 1944, Hulubei published a summary of the data obtained.

The instrument used in the experiments incorporated a curved crystal for splitting high-energy X-rays into a spectrum, which was then recorded on a photographic plate (Cauchois 1933) (later called the Cauchois spectrometer). Hulubei and Cauchois placed a sample tube of radon in the spectrometer and measured the characteristic X-rays produced when the daughter elements of radon were formed by radioactive decay (Thornton and Burdette 2010).

In 1934, they described a study in a paper entitled „Nouvelle technique dans la spectrographie cristalline des rayons y” (”New techniques in crystal gamma-ray spectrography”) (Hulubei and Cauchois 1934). Although eka-iodine is not mentioned in the article, Hulubei later cited this publication as the first time he saw its spectral lines (Thornton and Burdette 2010).

Hulubei himself describes the scientific methodology he used in the study of element 87, which is the same as that for element 85:

„I used the X-ray spectroscopic analysis method… I give some details on the experimental procedure I used:

„In order to obtain the desired spectrum, it is necessary to apply to the X-ray tube a voltage at least equal to its excitation voltage. Throughout the formation of the tube, i.e. as long as the substance to be analyzed was still emitting vapors, the voltage varies continuously, passing frequently below the excitation voltage, and this still, if the conditions in the tube allow us to reach this voltage. Meanwhile, the tube, not being able to emit the spectrum we are interested in, sends us a continuous background, which is disastrous for analysis; printing it on the photographic plate destroys the contrast and masks any faint emission corresponding to a possible rare element we would detect. Then, in the conditions of a mediocre vacuum, the incandescent filament evaporates considerably, the material of the filament is deposited on the anticathode and introduces a strongly absorbing substance in the path of the radiations to be analyzed; in addition, an intense continuous background is emitted by this matter (filaments, in general, are made up of heavy elements).”

„I worked with spectrographs based on the principle of focusing wide beams through curved crystals and which I built as suitable as possible for convenient and rapid manipulation in the case of chemical analyzes by X-rays…” (Hulubei 1940)

Horia Hulubei and Yvette Cauchois reported X-ray wavelengths for three eka-iodine spectral lines in radon emission spectra that were closely related to the positions predicted by Henry Moseley. In a paper published in 1936, they claimed to have observed a line at 151 X units, or siegbahns, exactly where the Kα1 line for eka-iodine was expected. In 1939, they reported two more X-ray lines consistent with the presence of eka-iodine and the predictions of Moseley’s law. These new experiments used higher resolutions than previous ones and included additional checks and balances, leading to greater confidence in the authors’ claims to have discovered the new element (Scerri 2013). In 1941, Manuel Valadares (1904-1982) repeated Hulubei’s work with a large sample of Rn and saw new characteristic lines of the element (Fontani et al. 2014). Manuel Valadares performed the experiments at the University of Lisbon in Portugal. confirming the presence of eka-iodine (Valadares 1941).

In the 1944 abstract, Hulubei included a description of six X-ray lines believed to be due to natural radioactive decay.

At a 1946 conference in Nice, Hulubei presented a summary of his work on element 85 (Hulubei 1944).

Other attempts to discover astatine

In 1938, Walter Minder (1905-1992), a Swiss radiochemist, was convinced that he had discovered eka-iodine, writing: „The beta decay of Ra-A certainly leads us to hypothesize the formation of element 85. For this reason, we suggest the helvetium name” (Minder 1940). Together with Alice Leigh-Smith (1907-1987) he published a paper in 1942, proposing as name for the element 85, „anglohelvetium” (Fontani et al. 2015).

In 1940, a group of researchers at the University of California, Berkeley, consisting of Dale R. Corson (1914-2012), Kenneth R. MacKenzie (1912-2002) and Emilio Gino Segrè (1905-1989), synthesized the element. by bombardment with bismuth alpha particles (D. R. Corson, MacKenzie, and Segrè 1940). In the article announcing their discovery, they also noted the possible existence of natural element 85 and cited the work of Hulubei and Cauchois in Paris, who allegedly observed the element (Scerri 2013).

In 1942, a Viennese radiochemist, Berta Karlik, and her assistant Gertrud Cless-Bernert claimed to have found the only natural isotope of element 85, supported by Stefan Meyer (1872-1949). Their work won the 1947 „Haitinger Prize” for chemistry for the discovery of element 85, proposing the name „Viennium” (Fontani et al. 2015) (Lykknes and Tiggelen 2019). In 1943, astatine was found as the product of two natural decay chains by Berta Karlik and Traude Bernert, first in the so-called uranium series and then in the actinium series (Karlik and Bernert 1943).

Confirmation and naming of astatine

The confirmation of the existence of astatine and its identification as a new element was a significant milestone in the history of chemistry. Its discovery filled a gap in the periodic table and made it the rarest naturally occurring halogen. The efforts of Hulubei and Cauchois not only confirmed the element’s existence, but also contributed to the understanding of its properties and behavior.

The turmoil of World War II and the fundamental question of man-made versus natural elements lead to the delay in confirming the discovery and naming of element 85 until 1947.

Friedrich Adolph Paneth (1887-1958), an Austrian radiochemist and member of the International Union of Pure and Applied Chemistry (IUPAC), helped establish a new order in chemical nomenclature. On January 4, 1947, Paneth published an editorial on the process of naming new elements in Nature (Paneth 1947), in which he described a procedure for deciding the names of elements in situations where several names were proposed. Friedrich Paneth would later chair the IUPAC committee responsible for recognizing the new elements.

Paneth insisted that the discovery of element 85 be credited to the group of researchers from Berkeley, respectively with the name astatine, from the Greek word astatos for „unstable” (D. R. Corson, Mackenzie, and Segrè 1947), a name proposed after their discovery was accepted (Dale R. Corson 2003).

Paneth stated that „former claims were open to grave objections and were experimentally disproved by very careful work by the Vienna physicists ” (Thornton and Burdette 2010). Thornton and Bourdette state that Paneth’s denial of Karlik and Bernert’s contributions may have been due to ” his knowledge that Karlik strongly disliked German war policies ” (Thornton and Burdette 2010). Paneth influenced all element discovery disputes, although some of those initially rejected by him are credited today as the true discoverers, such as Georges Urbain and von Welsbach for the discovery of element 71. Currently, Urbain’s proposed name of „lutetium” is the adopted one (Thornton and Burdette 2010).

It should be noted that Paneth initially made no comment about the studies of Hulubei and Cauchois (Appleton 2021). Paneth stated that what he called „earlier claims,” without naming any specific researchers, were disproved by the work of Karlik and Bernert.

”This is a rather crucial statement because it served to discredit the work of Hulubei and Cauchois, even though Karlik and Bernert had not actually addressed these claims whereas Paneth’s statement implied that they had” (Scerri 2013).

Hulubei’s discovery of astatine was downplayed by some researchers who stated that his means of detecting it were too weak by current standards to allow correct identification, and he could not perform chemical tests on the element (Thornton and Burdette 2010), being also implicated in an earlier false claim regarding the discovery of element 87 (francium) (Scerri 2013, 7). Thus, Karlik, according to Paneth, would have thought that the sample size of Hulubei and Cauchois was too small and that there was interference from other elements, even though the work of Hulubei and Cauchois had not been rejected. Paneth later noted that the work of Hulubei and Cauchois did not have sufficient means to characterize the discovery of element 85, and only received experimental confirmation through Valadares in Italy (Kostecka 2020).

Hulubei countered Paneth’s editorial by attributing Paneth’s omission to wartime communication difficulties. He denied that Karlik and Bernert dismissed his research on element 85, „contrary to what one would think after reading the expose of Mr. Paneth.” (Scerri 2013). To his 1946 presentation he added a short appendix before publication in which he attributed Paneth’s omission of his X-ray work on element 85 to difficulties caused by the war, a diplomatic statement, since both Karlik and the Berkeley researchers cited Hulubei and Cauchois (Hulubei 1947).

„Hulubei and Cauchois appeared to have been slighted unjustly since Karlik had not conducted any X-ray studies, but Paneth’s phrasing and lack of citations made it appear that Hulubei and Cauchois’ work was definitively erroneous”. (Thornton and Burdette 2010).

Although Karlik later suggested that the work of Hulubei and Cauchois was insufficient, „criticism seems to be based on the detection limits of traditional cathode ray tube X-ray spectrometry, not the technique pioneered by Rutherford and Wooster”. (Rutherford and Wooster 1925) In 1949, at the Amsterdam meeting, the International Chemical Union sanctioned the name astatine for element 85 (Thornton and Burdette 2010).

According to Thornton and Burdette’s analysis, there is no doubt that three teams of researchers can claim to have discovered element 85. They state that

”Unlike other flawed studies with X-ray spectroscopy, Hulubei and Cauchois indisputably had astatine in their samples. The only uncertainty is whether their instrument was sensitive enough to distinguish the spectral lines of element 85… Additionally, they were able to see clearly the L_α7 line of polonium, which has approximately a 500-fold lower transition probability than the observed astatine L_α1 and L_β1 lines, suggesting that the astatine lines would be visible to Hulubei and Cauchois.” (Thornton and Burdette 2010) (Scerri 2013, 7)

Scerri states that Horia Hulubei may very well have been considered the discoverer of astatine, „as it was later called by the physicists who synthesized the element artificially. It is these physicists who are generally accorded with the discovery of the element” (Scerri 2013, 7).

Conclusion

The discovery of astatine by Horia Hulubei and Yvette Cauchois was a remarkable achievement that expanded our understanding of the periodic table and the fundamental elements that make up our universe. Their collaborative efforts and innovative use of X-ray spectroscopy led to the confirmation of astatine as a new element, shedding light on its properties and behavior despite its extreme rarity and radioactivity. Horia Hulubei’s work in this discovery, along with his contributions to physics, continues to be remembered and celebrated in the scientific community.

Corson, MacKenzie and Segre were able to synthesize astatine in 1940 and perform chemical tests on the element, something Hulubei and Cauchois could not claim. Hulubei recognized this deficiency in his work, which probably explains his lack of significant protests after 1947. But unlike other flawed studies, Hulubei and Cauchois indisputably had astatine in their samples.

”Throughout history, the only constant for being credited with element discovery has been the ability to convince your scientific peers of your success. In some eras, Hulubei’s and Cauchois’ work might have been accepted, but at the time they reported their data their methods were not accepted widely. Convincing scientific peers of an experiment’s validity is often easier with an influential scientist as an advocate.” (Thornton and Burdette 2010)

Unfortunately, the laureate Jean Baptiste Perrin, the supporter of the discovery of element 85 by Horia Hulubei, had died in 1942.

Papers on element 85 published in languages other than English, and the wrong translation into English by reviewers of some of these articles, led to poor dissemination of the respective research (Nefedov et al. 1968). In most English-language sources, Corson, Segre, and MacKenzie are credited solely as the discoverers of astatine. German texts also mention Karlik, and French and Eastern European authors often acknowledge the contributions of Hulubei and Cauchois to the discovery of element 85 (Thornton and Burdette 2010).

The story of astatine showed that nationalistic biases strongly influenced the credit for the discovery and confirmation of this element of the periodic table.

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