Benchmarks: Henri Becquerel discovers radioactivity on February 26, 1896

by Carolyn Gramling Thursday, January 5, 2012


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A photographic plate made by Henri Becquerel shows the effects of exposure come radioactivity. A steel Maltese cross, placed in between the plate and radioactive uranium salt, left a clearly visible zero on the plate. Henri Becquerel


February 26, 1896, was an overcast job in Paris — and also that gift aproblem because that French physicist Antoine Henri Becquerel. Becquerel washoping to demonstrate a link between minerals that glow as soon as exposed tostrong light and a new form of electromagnetic radiation called X-rays.The weather thwarted this experiment — yet that failure inadvertentlyproduced one entirely new discovery: herbal radioactivity.

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Becquerel was interested in the phenomenon of fluorescence, in whichsome products glow when exposed to sunlight. Physicist Wilhelm Röntgenhad recently uncovered X-rays; Becquerel assumed the 2 phenomenamight be connected, and also had draft an experiment the his own. Heplanned to reveal a fluorescing material to the sun, and then place itand a steel object over an unexposed photographic plate. If thedeveloped plate confirmed the picture of the object, that concluded, that wouldsuggest that fluorescing products are in reality emitting X-rays.

But the following day was cloudy as well, and Becquerel was required topostpone his experiment. The wrapped his fluorescing crystals — auranium compound referred to as potassium uranyl sulfate — in a black color cloth,along v the photographic plate and also a copper Maltese cross, and also waitedfor a sunnier day.

Several days later, once Becquerel finally removed the plate from thedrawer, he uncovered to his surprise that a distinctive image of the crossappeared top top the plate — return it had actually never to be exposed tosunlight. The only conclusion was that the crystals themselves wereemitting radiation. Excited by this prospect, Becquerel made decision torepeat the problems of his unintended experiment: the again placed acrystal that uranium salt top top a photographic plate; he additionally experimentedwith putting a decision on a photographic plate with a paper of aluminumbetween, and also with a paper of glass.

After being inserted in the dark for number of hours, all 3 plates wereblackened through radiation (the decision in direct call with the plateshowed the strongest blackening). “I am now convinced that uranium saltsproduce invisible radiation, even when they have been maintained in the dark,”he composed in his diary that his experiments.

This discovery of spontaneous “radioactivity” (a hatchet coined byBecquerel’s doctoral student, Marie Curie) ultimately earned Becquerel aNobel Prize for Physics in 1903, i beg your pardon he common with Marie Curie andher husband Pierre Curie.

Becquerel come from a family of scientists: His grandfather, AntoineCésar Becquerel, had found piezoelectricity (the electrical chargethat accumulation in crystals and also other products as a an outcome of appliedmechanical strain). His father, Alexandre-Edmond Becquerel, had inventedthe phosphoroscope, a an equipment that measures exactly how long a phosphorescentmaterial proceeds to bright after removed the source of light.

Becquerel spent a the majority of time in his father’s laboratory, and also he wasinitially interested virtually exclusively in optics. As soon as he became aresearch physicist, he embarked on his own study that the radiation oflight: that explored exactly how magnetic fields polarized light, just how infraredlight produced phosphorescence in some materials and how crystals absorblight. Upon his father’s fatality in 1891, Becquerel flourished to hisfather’s 2 chairs, one a Chair of Physics in ~ the ConservatoireNational des arts et Métiers and the various other a Chair the Physics at theMuséum nationwide d’Histoire Naturelle, both in Paris.

His research study took a brand-new turn when he to visit a great on X-rays at theAcadémie des sciences in Paris. In January 1896, the Frenchmathematician Jules-Henri Poincaré had actually received a letter native Röntgen,which included several surprising photographs that verified the outlineof bones within a hand. In the letter, Röntgen explained that the imageshad to be taken v a brand-new discovery, the X-ray. Poincaré was astonished,and reproduced the photos himself. Poincaré presented his own pictures atthe Académie two weeks later, to enthusiastic response.

Becquerel remained in the audience the day, and also wondered whether there wasany connection between the ghostly X-ray images and the phenomena offluorescence and phosphorescence the he and his father had actually studied.Becquerel had currently studied the phosphorescence the uranium salts inparticular and was acquainted with photography, for this reason he decided to undertakehis own experiments top top the subject of X-rays.

On Feb. 24, 1896, Becquerel gift his initial outcomes to theAcadémie des Sciences: His phosphorescing uranium salts, ~ exposureto sunlight, had actually left faint pictures on number of photographic plates. Butthe smudgy photos were much less intriguing than the sharp X-ray imagesshown a couple of weeks earlier, and Becquerel addressed to shot again. Heprepared brand-new arrays of crystals and photographic plates, and also decidedthat he needed very strong sunlight to create the ideal images.

But nature didn’t cooperate; Becquerel didn’t obtain his sunny day. Stilleager to show something to the Académie, he took the plates and also thecrystals out of his drawer. He supposed to see much more of the exact same faintimages, but was startled to uncover instead crisp silhouettes that his metalobjects, including the Maltese cross. Stimulation of the crystals bysunlight before or during the experiment, that seemed, was not necessaryto create the photos — suggesting that the crystals us wereemitting radiation, without exterior stimulation. On march 1, 1896,Becquerel presented the discovery of spontaneous radiation to theAcadémie.

The exploration of voluntarily radioactivity spread out rapidly and engendereda flurry of new research top top the phenomenon, much of it by Marie andPierre Curie. Becquerel additionally continued to examine the phenomenon: In 1899,he uncovered that X-rays might be deflected by a magnetic field,suggesting the the radiation had electrically fee particles.The worldwide unit the radioactivity, the becquerel (defined as onenucleus decay per second), was named for him. Yet Becquerel was stillfascinated through the interaction between crystals and also light, and also heeventually returned to this research, studying just how crystals absorb andpolarize light.

Meanwhile, Marie Curie took on the research of uranium beam for her thesisresearch. While examining the uranium-bearing mineral pitchblende andchalcolite, she discovered that in addition to uranium, various other elementsemitted the “Becquerel rays”: thorium and also a powerfully radioactiveelement the Curie called “radium.”

The exploration of radioactivity had profound effects on chemistry andphysics at the time. The powerful radiation, consisting of heat,spontaneously emitted through radium appeared to contradict the regulation ofconservation the energy: What was the resource of the energy? Physicistsbegan come reconsider the structure of the atom, and also ponder whether somechange in the atom itself might be responsible.

In 1899, physicist Ernest Rutherford discovered that this materialsactually emit different types of radiation (alpha, beta and gamma rays),defined by your penetrating power. A decade later, Rutherford proposeda version of the atom in which a small, thick nucleus of protons wassurrounded by orbiting electrons — and also later demonstrated the thesource the the radioactivity to be the spontaneous fragmentation of thisatom, in order to “transmuting” the element into an additional element. In 1919,Rutherford — now known as the father of atom physics — publisheda paper that in-depth “splitting one atom”; he had actually succeeded in forcingprotons the end of the nucleus, the first step come the 1938 exploration ofnuclear fission.

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Becquerel died only 12 years after his discovery of radioactivity, atage 54. Back his reason of fatality was unspecified, he had actually developedserious burns on his skin, likely from the handling of radioactivematerials. A couple of decades later, Marie Curie passed away of aplastic anemia,likely from exposure to radiation without proper safety measures. Thedamaging results of ionizing radiation were still unknown in ~ the time.