Benchmarks: Henri Becquerel discovers radiation on February 26, 1896

by Carolyn Gramling Thursday, January 5, 2012


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A photographic plate made by Henri Becquerel shows the impacts of exposure to radioactivity. A metal Maltese cross, placed in between the plate and also radioactive uranium salt, left a plainly visible zero on the plate. Henri Becquerel


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

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Becquerel was interested in the phenomenon of fluorescence, in whichsome materials glow when exposed come sunlight. Physicist Wilhelm Röntgenhad recently found X-rays; Becquerel believed the two phenomenamight it is in connected, and also had draft an experiment of his own. Heplanned to reveal a fluorescing product to the sun, and then location itand a steel object over an unexposed photographic plate. If thedeveloped plate confirmed the photo of the object, the concluded, that wouldsuggest the fluorescing materials are actually emitting X-rays.

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

Several days later, once Becquerel finally removed the plate native thedrawer, he found to his surprise that a unique image that the crossappeared top top the bowl — return it had actually never to be exposed tosunlight. The just conclusion was that the crystals us wereemitting radiation. Excited through this prospect, Becquerel made decision torepeat the conditions of his unintended experiment: that again put acrystal that uranium salt on a photographic plate; he additionally experimentedwith placing a crystal on a photographic plate v a paper of aluminumbetween, and with a paper of glass.

After being put in the dark for number of hours, all 3 plates wereblackened through radiation (the crystal in direct contact with the plateshowed the the strongest blackening). “I to be now encouraged that uranium saltsproduce invisible radiation, even when they have been maintained in the dark,”he composed in his diary the his experiments.

This discovery of voluntarily “radioactivity” (a hatchet coined byBecquerel’s doctoral student, Marie Curie) eventually earned Becquerel aNobel Prize for Physics in 1903, which he shared with Marie Curie andher husband Pierre Curie.

Becquerel came from a household of scientists: His grandfather, AntoineCésar Becquerel, had found piezoelectricity (the electrical chargethat accumulates in crystals and also other materials as a result of appliedmechanical strain). His father, Alexandre-Edmond Becquerel, had actually inventedthe phosphoroscope, a machine that measures how long a phosphorescentmaterial proceeds to light after removed the source of light.

Becquerel spent a the majority of time in his father’s laboratory, and he wasinitially interested practically exclusively in optics. As soon as he ended up being aresearch physicist, he carried out his own study of the radiation oflight: he explored how magnetic areas polarized light, just how infraredlight created phosphorescence in some materials and also how crystals absorblight. Top top his father’s fatality in 1891, Becquerel succeeded to hisfather’s 2 chairs, one a Chair of Physics at the ConservatoireNational des arts et Métiers and the various other a Chair that Physics in ~ theMuséum nationwide d’Histoire Naturelle, both in Paris.

His study took a brand-new turn once he attend a lecture on X-rays in ~ theAcadémie des sciences in Paris. In January 1896, the Frenchmathematician Jules-Henri Poincaré had actually received a letter from Röntgen,which consisted of several surprising photographs that showed the outlineof bones in ~ 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 images himself. Poincaré presented his own photos atthe Académie two weeks later, to enthusiastic response.

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

On Feb. 24, 1896, Becquerel gift his initial outcomes to theAcadémie des Sciences: His phosphorescing uranium salts, after ~ exposureto sunlight, had left faint images on number of photographic plates. Butthe smudgy pictures were far less intriguing than the sharp X-ray imagesshown a few weeks earlier, and also Becquerel fixed to try again. Heprepared brand-new arrays of crystals and photographic plates, and decidedthat he necessary very strong sunlight to produce the best images.

But nature didn’t cooperate; Becquerel didn’t acquire his sunny day. Stilleager to show something to the Académie, he took the plates and also thecrystals the end of his drawer. He meant to see an ext of the same faintimages, yet was startled to discover instead crisp silhouettes the his metalobjects, consisting of the Maltese cross. Stimulation that the crystals bysunlight prior to or during the experiment, that seemed, was no necessaryto develop the pictures — saying that the crystals themselves wereemitting radiation, without external stimulation. On march 1, 1896,Becquerel gift the exploration of spontaneous radiation to theAcadémie.

The discovery of voluntarily radioactivity spread rapidly and also engendereda flurry of new research ~ above the phenomenon, lot of that by Marie andPierre Curie. Becquerel additionally continued to study the phenomenon: In 1899,he found that X-rays can be deflected by a magnetic field,suggesting the the radiation contained electrically fee particles.The worldwide unit that radioactivity, the becquerel (defined together onenucleus degeneration per second), was named for him. Yet Becquerel was stillfascinated by the interaction in between crystals and light, and also heeventually went back to this research, studying how crystals absorb andpolarize light.

Meanwhile, Marie Curie took on the examine of uranium light ray for she thesisresearch. While researching the uranium-bearing mineral pitchblende andchalcolite, she uncovered that in addition to uranium, various other elementsemitted the “Becquerel rays”: thorium and a powerfully radioactiveelement that Curie referred to as “radium.”

The discovery of radioactivity had actually profound results on chemistry andphysics in ~ the time. The an effective radiation, consisting of heat,spontaneously emitted by radium appeared to contradict the law ofconservation the energy: What was the resource of that energy? Physicistsbegan come reconsider the structure of the atom, and also ponder whether somechange in the atom itself could be responsible.

In 1899, physicist Ernest Rutherford found that these materialsactually emit different species of radiation (alpha, beta and also gamma rays),defined by your penetrating power. A decade later, Rutherford proposeda model of the atom in i m sorry a small, thick nucleus of proton wassurrounded by orbiting electron — and later demonstrated the thesource of the radioactivity was the spontaneous disintegration of thisatom, thereby “transmuting” the facet into another element. In 1919,Rutherford — now recognized as the father of nuclear physics — publisheda document that comprehensive “splitting one atom”; he had actually succeeded in forcingprotons out of the nucleus, the an initial step come the 1938 exploration ofnuclear fission.

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Becquerel passed away only 12 years after his discovery of radioactivity, atage 54. Back his cause of death 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 indigenous exposure come radiation without ideal safety measures. Thedamaging impacts of ionizing radiation were still unknown at the time.