Henri Becquerel

How to use this course

Henri Becquerel is remembered for one discovery: in February and March 1896 he found that uranium emits a penetrating radiation all by itself, with no light, heat or electricity to excite it. The discovery did not come from nowhere. Becquerel had spent twenty years on light and matter: the rotation of polarized light in a magnetic field, the glow of phosphorescent substances, the absorption of light by crystals. His family had studied uranium salts for two generations. When X-rays were announced, he was the person in Paris best placed to ask whether glowing bodies might emit them too.

This course follows the physics in the order he did it. Lessons 04 to 06 treat his optical work, which explains how he came to the uranium question. Lessons 07 to 09 treat the discovery itself and the two results that followed: that one part of the radiation consists of fast electrons, and that uranium regenerates an activity that can be chemically removed from it.

Each technical lesson opens with a paragraph for the general reader, then gives precise statements, derivations and exercises with folded solutions. Numerical exercises use SI units; where Becquerel reported values in the units of his day, the conversion is given.

Timeline

A family of physicists, 1852–1872

Antoine Henri Becquerel was born in Paris on 15 December 1852, in what was then the 12th arrondissement, the district of the Jardin des Plantes. The garden housed the Muséum national d’histoire naturelle, and the Muséum’s chair of physics was a family post. His grandfather, Antoine César Becquerel (1788–1878), was a pioneer of electrochemistry and a Fellow of the Royal Society. His father, Alexandre Edmond Becquerel (1820–1891), was a professor of applied physics.

Edmond Becquerel is known today for the photovoltaic effect, which he observed in 1839 as a teenager working in his father’s laboratory: light falling on an electrode in an electrolyte changed the current. His life’s main subject was phosphorescence, the glow that some substances keep after the light that excites them is removed. He built the phosphoroscope, a device of rotating discs that could measure afterglows lasting a small fraction of a second, and he studied the luminescence of uranium salts in detail. Uranium compounds, which glow a vivid green under ultraviolet light, were a fixture of the family laboratory.

Henri grew up among these instruments. He attended the Lycée Louis-le-Grand, one of the main preparatory schools for the competitive examinations of the grandes écoles, and in 1872 he entered the École polytechnique.

Polytechnique and the Corps des Ponts, 1872–1878

The École polytechnique trained the engineers of the French state in a demanding course of mathematics, mechanics and physics. Becquerel left it in 1874 for the École des ponts et chaussées, the school of the state corps of civil engineers responsible for roads, bridges and waterways. He qualified as an engineer in 1877. He kept his position in the corps for the rest of his life and rose to chief engineer in 1894. The engineering post gave him a secure salary, while the Muséum gave him a laboratory.

He married Lucie Jamin, daughter of Jules Jamin, the Sorbonne physicist known for his interferometer and his work on the optics of metals. Their son Jean was born in 1878, and Lucie died the same year. Jean Becquerel (1878–1953) became the fourth physicist of the family.

Becquerel’s first research, begun in 1875 while he was still an engineering student, was on the rotation of the plane of polarization of light by a magnetic field, the effect Michael Faraday had found in 1845. It is the subject of Lesson 04.

The Muséum, the thesis and the Academy, 1878–1895

In 1878 Becquerel became an assistant at the Muséum. His grandfather died that year, and his father now led the physics laboratory. Through the 1880s Henri worked across the family subjects: magneto-optics, terrestrial magnetism, phosphorescence, and the infrared spectrum, which he explored with phosphorescent screens (Lesson 05). From 1886 he studied how crystals absorb light depending on its polarization, the subject of his doctoral thesis, Recherches sur l’absorption de la lumière dans les cristaux, defended at the University of Paris in 1888 (Lesson 06).

The Académie des sciences elected him a member in 1889, a year after his doctorate, at the age of 36. He married Louise Lorieux in 1890. When Edmond Becquerel died in 1891, Henri succeeded him, and in 1892 he became professor of applied physics at the Muséum. In 1895 he was appointed professor of physics at the École polytechnique, succeeding Alfred Potier.

By the end of 1895 Becquerel was a respected, established physicist of 43, known for careful work on the optics of matter. Nothing in his record suggested that his name would one day mark a unit of the International System. The change came from a meeting of the Academy in January 1896.

Magnetic rotation of polarized light

For the general reader. Light is a wave whose electric field oscillates in a definite direction, its polarization. In 1845 Faraday found that a magnet can turn this direction as light passes through glass. Becquerel spent his first years of research measuring the effect in many substances, and in 1897 he gave a formula linking it to how strongly a material splits white light into colours.

Circular birefringence

A linearly polarized wave is the sum of two circularly polarized waves of opposite handedness:

If the two circular components travel with refractive indices \(n_+\) and \(n_-\), after a distance \(d\) they have acquired a relative phase \(\Delta\phi=(\omega d/c)(n_+-n_-)\). Recombined, they again give linear polarization, turned through half the relative phase:

A magnetic field produces exactly this difference. Model the medium as electrons (charge \(-e\), mass \(m\)) bound harmonically with natural frequency \(\omega_0\), and let \(\mathbf B=B\hat{\mathbf z}\). For a circular drive \(\mathbf E=E_0(\hat{\mathbf x}\pm i\hat{\mathbf y})e^{-i\omega t}\), the electron moves on a circle, and the magnetic force \(-e\,\dot{\mathbf r}\times\mathbf B\) is radial, adding to or subtracting from the restoring force. With the cyclotron frequency \(\omega_c=eB/m\), the equation of motion

gives a resonance shifted to \(\omega_0^2\mp\omega\omega_c\) for the two senses, so that

Becquerel’s relation

To first order in \(\omega_c\), the denominator \(\omega_0^2-\omega^2\mp\omega\omega_c\) equals \(\omega_0^2-(\omega\pm\omega_c/2)^2\) up to second-order terms. So each circular component sees the zero-field index at a slightly shifted frequency, \(n_\pm(\omega)\approx n(\omega\pm\omega_c/2)\). This is the content of Larmor’s theorem: a magnetic field acts on bound electrons like a rotation of the frame at angular velocity \(\omega_c/2\). Then \(n_+-n_-\approx\omega_c\,dn/d\omega\), and since \(\omega\,dn/d\omega=-\lambda\,dn/d\lambda\),

Becquerel proposed this relation in 1897, the year after Pieter Zeeman showed that a magnetic field splits spectral lines, an effect that the same electron model explains. The formula says that strongly dispersive media, such as the dense lead glass Faraday used, rotate most. Real substances usually need an empirical correction factor, because their dispersion comes from several kinds of oscillators, not all of them electrons, and because spin and quantum effects modify the simple picture. For diamagnetic liquids and glasses the formula gets the order of magnitude and the wavelength dependence right.

Phosphorescence and the infrared

For the general reader. Some substances glow after being lit, and the glow is always a different colour from the light that caused it. The Becquerels made a family science of this. Henri even used glowing screens as detectors for the infrared, light too red for the eye to see. The same habit of thought, that one kind of radiation can call forth another, led him to uranium in 1896.

Stokes’s rule

George Gabriel Stokes stated the rule from experiments on quinine and uranium glass. In modern terms an absorbed photon of energy \(h\nu_{\text{abs}}\) raises the molecule to an excited state; part of the energy is lost to vibrations before emission, so \(h\nu_{\text{em}}<h\nu_{\text{abs}}\). The difference is the Stokes shift. Uranium salts absorb in the violet and ultraviolet and emit green light in a series of bands.

Decay of luminescence

If excited centres return to the ground state independently, each with probability \(k\,dt\) in time \(dt\), the number \(N\) of excited centres obeys \(dN/dt=-kN\), and the emitted intensity \(I\propto N\) decays as

Many phosphors depart from this law. When emission requires an electron and a hole, separated by the excitation, to meet again, the rate is proportional to the product of their concentrations, and for equal concentrations \(dn/dt=-\alpha n^2\). This gives the hyperbolic decay \(n(t)=n_0/(1+\alpha n_0 t)\), which falls slowly at long times. Long-lasting phosphorescence in zinc and calcium sulphides is of this type. The luminescence of uranium salts, by contrast, dies away in a small fraction of a second, which Edmond Becquerel measured with his phosphoroscope.

The phosphoroscope

Two discs mounted on one axle carry openings that alternate: when the sample is lit through the front disc, the rear disc blocks the observer, and vice versa. The observer sees the sample only after a delay \(t=\theta/\Omega\), where \(\theta\) is the angle the discs turn between the end of illumination and the start of viewing and \(\Omega\) the angular speed. Spinning faster shortens the delay. If the glow is visible at delay \(t\) but not at larger delays, its lifetime is of order \(t\).

Seeing the infrared with phosphorescence

A phosphorescent screen, charged by exposure to light, responds to infrared radiation: the infrared first brightens the glow and then quenches it, releasing stored energy faster. In 1883 Henri Becquerel projected the spectra of metal vapours onto such screens. Where infrared lines fell, the screen behaved differently from its surroundings, and the pattern recorded line positions beyond the red end of the visible spectrum. The method extended spectroscopy into the near infrared before photographic plates sensitive there were available.

Absorption of light in crystals

For the general reader. Many crystals change colour when you turn them, because they absorb light differently depending on the direction in which its electric field vibrates. Becquerel’s doctoral thesis of 1888 studied this effect in crystals whose absorption spectra contain sharp dark bands, and used it to probe how light interacts with the oriented structure of a solid.

Derivation. Each thin slice \(dx\) removes a fixed fraction \(\alpha\,dx\) of the light crossing it: \(dI=-\alpha I\,dx\). Integrating gives the law. Becquerel used the logarithm of the ratio of intensities, read through a spectroscope across each absorption band, as his measure of absorption.

Anisotropic crystals

In a uniaxial crystal, light travelling perpendicular to the optic axis splits into two linearly polarized waves: the extraordinary wave, with \(\mathbf E\) parallel to the axis, and the ordinary wave, with \(\mathbf E\) perpendicular to it. Each has its own absorption coefficient, \(\alpha_e\) and \(\alpha_o\). The crystal is dichroic (in general, pleochroic) when these differ.

The two components decay independently because they are eigenmodes of the crystal. Becquerel examined crystals containing rare-earth elements, whose absorption spectra show narrow bands, and recorded how each band changed as the polarization was turned with respect to the crystal axes. In modern terms each band is a transition of a localized ion, and its strength depends on the orientation of the transition dipole relative to the field. The same physics is used in Polaroid sheet and in polarized-light microscopy of minerals.

February 1896: rays from uranium

For the general reader. In January 1896 the world learned that a German physicist, Wilhelm Röntgen, had found rays that pass through flesh and black paper. Röntgen’s rays came from the glowing spot where cathode rays hit glass. Becquerel wondered whether every glowing substance gives off such rays. He tested a uranium salt, and found a radiation that kept coming whether or not the salt glowed.

The hypothesis

At the Academy meeting of 20 January 1896, Henri Poincaré showed the first X-ray photographs sent by Röntgen. Röntgen’s tubes emitted X-rays from the region of the glass made fluorescent by cathode rays. Poincaré suggested that fluorescence and X-ray emission might be connected. Becquerel, who had a stock of uranium salts prepared years earlier in the family laboratory, set out to test the idea.

The experiments

In his note of 24 February 1896 Becquerel described wrapping a photographic plate in two sheets of thick black paper, placing crystals of potassium uranyl sulphate on the paper, and exposing the whole to the sun for several hours. The developed plate showed the silhouette of the crystals. A coin or a cut-out metal screen placed under the crystals appeared on the plate. A thin glass plate between the crystals and the paper did not prevent the effect, which ruled out vapours released by the warm salt.

On 26 and 27 February the sky over Paris was overcast. Becquerel put the prepared plates, with the uranium salt in place, in a dark drawer. On 1 March he developed them anyway, expecting faint images. The images were strong. On 2 March he told the Academy that the radiation did not need sunlight. Over the following weeks he established:

By May he had concluded that the radiation was a property of the element uranium itself. He published seven notes on the subject in 1896. He also believed for a time that the rays could be reflected and refracted like light. This was an error, corrected later. The rays did not seem to fit the existing categories, and interest faded in 1897 until Marie Curie and Gerhard Schmidt independently found the same activity in thorium in 1898.

The electroscope as a measuring instrument

A gold-leaf electroscope is a capacitor of capacitance \(C\) charged to potential \(V\). Radiation that creates ion pairs in the surrounding air lets the charge leak away. If the ionization current collected is \(I\), the potential falls at the rate

The rate of fall of the leaf is thus a direct measure of the ionizing power of the sample. Pierre and Marie Curie made this quantitative with an ionization chamber and the piezoelectric quartz electrometer of Pierre and Jacques Curie, and Rutherford used the same principle to separate the radiation into alpha and beta components.

Beta rays are electrons

For the general reader. In 1897 J. J. Thomson showed that cathode rays are streams of tiny negatively charged particles, later called electrons. Two years later Becquerel put radium in a magnetic field and found that part of its radiation bends just as cathode rays do. By 1900 he had measured the bending in both magnetic and electric fields and concluded that radioactive substances emit electrons at a large fraction of the speed of light.

Magnetic deflection

Derivation. The force \(q\mathbf v\times\mathbf B\) is perpendicular to \(\mathbf v\), so it does no work: \(|\mathbf v|\) and \(\gamma\) are constant. Then \(d\mathbf p/dt=\gamma m\,d\mathbf v/dt=q\mathbf v\times\mathbf B\) is uniform circular motion with centripetal acceleration \(v^2/r=|q|vB/(\gamma m)\), which gives the result.

Late in 1899 Becquerel placed radium preparations in a field and found that the rays striking a photographic plate were spread out: particles with different speeds followed circles of different radii and landed at different places, forming a continuous band. This was the first beta-ray spectrum. The sense of curvature showed negative charge.

Electric deflection and the ratio \(e/m\)

A particle of speed \(v\) crossing a region of length \(L\) with transverse electric field \(E\) acquires a transverse displacement, to first order and non-relativistically,

A magnetic measurement gives \(mv/q\) and an electric measurement gives \(mv^2/q\). Their ratio gives \(v\), and then either one gives \(q/m\). Using this method of Thomson’s, Becquerel reported in March 1900 that the rays from radium carry negative charge, move at about \(1.6\times10^{10}\) cm/s, and have a ratio of charge to mass of about \(10^{7}\) electromagnetic units per gram, that is, \(10^{11}\) C/kg. That is the order of magnitude Thomson had found for cathode rays; the modern value for the electron is \(1.76\times10^{11}\) C/kg.

Velocity dependence

At \(v=1.6\times10^8\) m/s, \(\beta=v/c=0.53\) and \(\gamma=1.18\). The deflections actually measure \(e/(\gamma m)\), which is lower than \(e/m\) by that factor. Walter Kaufmann, measuring radium beta rays over a range of speeds in 1901–1903, found exactly such a decrease of \(e/m\) with speed, the first experimental evidence for the velocity dependence of inertia that special relativity explains.

Uranium X, decay and the becquerel

For the general reader. In 1900 William Crookes found that he could chemically separate from uranium a small fraction, which he called uranium X, that carried most of the photographic activity. Becquerel then noticed that the uranium slowly recovered its activity while the separated fraction lost its own. The observation was a key clue: radioactivity is one element turning into another, at a rate fixed by nature.

The observations

Crookes precipitated a fraction from a uranium solution and found that the purified uranium no longer darkened photographic plates strongly, while the small precipitate did. Becquerel repeated the separation with his own samples and in 1901 reported that over the following months the uranium regained its activity and the separated material lost its activity. Ernest Rutherford and Frederick Soddy found the same behaviour with thorium and “thorium X” and in 1902–03 explained it as spontaneous transmutation. The photographic plate responds mainly to beta rays. Uranium-238 itself emits only alpha particles; the penetrating beta rays come from its short-lived descendants, thorium-234 (the historical uranium X\(_1\)) and protactinium-234.

Derivation. The daughter population obeys \(dN_D/dt=A_P-\lambda_D N_D\). With \(N_D(0)=0\), the solution is \(N_D=(A_P/\lambda_D)(1-e^{-\lambda_D t})\), and \(A_D=\lambda_D N_D\). The separated daughter receives no new supply, so it simply decays. The two curves are mirror images, the pattern Becquerel and then Rutherford and Soddy saw.

Units

The SI unit of activity, the becquerel (Bq), is one decay per second; it was adopted by the General Conference on Weights and Measures in 1975. The older unit, the curie (Ci), was defined from the activity of one gram of radium-226: \(1\ \mathrm{Ci}=3.7\times10^{10}\) Bq.

Where does the energy come from?

For the general reader. Becquerel’s uranium went on emitting rays for months without any sign of weakening and without being fed light or heat. That looked like a perpetual source of energy, in conflict with one of the firmest laws of physics. The answer, found between 1903 and 1905, is that each atom releases a fixed store of energy once, when it changes into another atom, and that this store is about a million times larger than any chemical energy.

The puzzle

The early measurements could detect no decline in the activity of uranium, and Becquerel’s 1896 notes stress its persistence. In his Nobel lecture of 1903 he discussed where the energy might come from. In the same year Pierre Curie and Albert Laborde measured directly that radium salts stay warmer than their surroundings, releasing roughly 100 calories per gram of radium per hour. Rutherford and Soddy’s transmutation theory supplied the answer: the energy is drawn from the atom itself, and the supply runs down, only extremely slowly for uranium.

For alpha decay of uranium-238 to thorium-234, \(Q\approx4.27\) MeV. Chemical reactions release a few eV per molecule. A gram of uranium therefore contains, in principle, about a million times more energy than a gram of coal, but releases it over billions of years. The same arithmetic, applied to the uranium and thorium in rocks, gave Rutherford the heat source missing from Kelvin’s cooling calculation of the age of the Earth.

Radioactivity and the Curies, 1896–1903

Becquerel’s uranium rays attracted less attention in 1896 than X-rays, which could photograph bones. In late 1897 Marie Curie, a young Polish physicist working in Paris with her husband Pierre Curie, chose the rays as the subject of her doctoral thesis. Measuring ionization currents with Pierre’s electrometer, she found that the activity of uranium compounds depends only on the amount of uranium they contain, that thorium is also active, and that the uranium mineral pitchblende is more active than its uranium content explains. In 1898 she and Pierre announced polonium and radium. She called the phenomenon radioactivity.

Becquerel followed and supported the work. He presented notes to the Academy, obtained radium samples from the Curies for his own experiments, and turned to the magnetic deflection of radium rays (Lesson 08). In 1901 he carried a tube of radium in his waistcoat pocket for some hours and later found a burn on his skin beneath it. He and Pierre Curie reported the physiological effects of the rays that year, an early step toward radiotherapy.

The Royal Society awarded him the Rumford Medal in 1900. In 1903 the Nobel Prize in Physics was divided: one half to Becquerel for his discovery of spontaneous radioactivity, the other to Pierre and Marie Curie for their joint research on the radiation phenomena he had discovered. Becquerel gave his Nobel lecture, On radioactivity, a new property of matter, on 11 December 1903.

Last years, death and legacy, 1903–1908

Becquerel continued to work on radioactivity and on the optics of crystals. He was by now a senior figure of French science. He belonged to the Accademia dei Lincei, the Royal Prussian Academy of Sciences in Berlin and the American Philosophical Society, among others. In 1908 the Académie des sciences elected him one of its two permanent secretaries, succeeding the chemist Marcellin Berthelot, and the Royal Society elected him a foreign member.

He died on 25 August 1908 at Le Croisic in Brittany, at the Lorieux family property, aged 55. He had suffered burns on his skin, probably from handling radioactive materials, though the effect of radiation on his early death cannot be established.

Legacy

• Physics: the discovery of radioactivity opened nuclear physics. It led within fifteen years to transmutation, the nuclear atom and the first estimates of the age of the Earth from radioactive decay.
• The unit: the becquerel, one decay per second, has been the SI unit of activity since 1975.
• The family: his son Jean Becquerel continued the dynasty at the Muséum, working on the magneto-optics of crystals at low temperature. Four generations of Becquerels held the Muséum’s physics chair.
• Honours: craters on the Moon and on Mars, the uranium mineral becquerelite, and the minor planet 6914 Becquerel bear his name, as does the Laboratoire national Henri Becquerel, France’s metrology laboratory for ionizing radiation.

Connections

People linked to Henri Becquerel across this collection. A name without a link has no page here yet. See Henri Becquerel in the network of all 337 lives →

Teachers and advisors

• Charles Friedel 1888

Influenced by

• George Gabriel Stokes
• Henri Poincaré 1896
• J. J. Thomson 1899-1900
• Michael Faraday
• Wilhelm Röntgen 1896

Influenced

• Ernest Rutherford
• Marie Curie

Colleagues and collaborators

• Marie Curie 1897-1908
• Pierre Curie 1898-1906
• William Crookes 1900-1901

Family

• Antoine César Becquerel
• Edmond Becquerel
• Jean Becquerel
• Jules Jamin

Also mentioned in this life

• Niels Henrik Abel
• William Thomson, Lord Kelvin
• Johann Heinrich Lambert
• Hendrik Antoon Lorentz
• Étienne-Louis Malus
• Count Rumford
• Frederick Soddy

Fields

• physics 13 lives
• radioactivity 10 lives
• optics 38 lives

On CoursesHub

• Nuclear Physics
• Optics & Photonics

Sources and further reading

Biography

1. Henri Becquerel, Wikipedia (English), and Henri Becquerel, Wikipédia (French).
2. Nobel Foundation, Henri Becquerel: Biographical.
3. H. Becquerel, On radioactivity, a new property of matter, Nobel lecture, 11 December 1903.
4. EBSCO Research Starters, Antoine-Henri Becquerel.
5. L. Badash, Radioactivity before the Curies, American Journal of Physics 33 (1965).
6. A. Romer (ed.), The Discovery of Radioactivity and Transmutation, Dover, 1964 (translations of the original papers).

Physics

1. H. Becquerel, Sur les radiations émises par phosphorescence, Comptes rendus 122 (1896), 420. English translation, Classic Chemistry, Le Moyne College.
2. H. Becquerel, Sur les radiations invisibles émises par les corps phosphorescents, Comptes rendus 122 (1896), 501.
3. H. Becquerel, Sur le rayonnement des corps radio-actifs, Comptes rendus 129 (1899), 1205.
4. H. Becquerel, Recherches sur l’absorption de la lumière dans les cristaux, thesis, Paris, 1888.
5. Faraday effect, Wikipedia, with the history of the Verdet constant and Becquerel’s 1897 formula.
6. C. V. Raman, Centenary of the Faraday effect, Current Science 14 (1945), 281–283.
7. E. Rutherford and F. Soddy, The cause and nature of radioactivity, Philosophical Magazine 4 (1902).
8. BIPM, The International System of Units (derived unit becquerel).