Experimental foundations of subatomic physics
A. Andronic
X-rays, radioactivity, the electron
• Prelude
• X-rays
• Radioactivity
• The electron
Experimental knowledge at end of XIXth century
A. Andronic
the gravitational field
Galileo’s falling bodies
Newton’s Principia, 1687; Halley’s comet predictions, 1705
enough (so to say:) to get the Rosetta probe (controlled from Darmstadt:) to travel 10 years and 7 billion km
to meet (at 10 km distance) a comet’s nucleus 4 km long and land on it ...for the first time ever on a comet
1: 03.04, 10: 08.14, 11: 11.14, 12: 12.15
Comet 67P, 19
http://en.wikipedia.org/wiki/Rosetta_(spacecraft)
Sep. 2014
Experimental knowledge at end of XIXth century
A. Andronic
the electro-magnetic field
Faraday, Maxwell, Hertz; radio: Marconi, Braun, Nobel Prize 1909
glows in rarefied gases (first observed by Faraday, 1838) under high voltages
(Rühmkorff coil) in Geissler / Crookes tubes (10−6 atm): cathode rays
main line of research (Hittorf, Goldstein; Lenard, Nobel Prize, 1905)
http://en.wikipedia.org/wiki/Crookes_tube
Experimental knowledge at end of XIXth century
A. Andronic
• Chemical elements (classification, Mendeleev, 1869)
caloric capacities, photoelectric effect (discovered by Hertz)
absorption and emission lines (Fraunhofer; Kirchhoff, Bunsen; Balmer)
• Law of energy conservation (Joule, Mayer, Helmholtz)
Planck’s PhD Thesis:
“The assumption of absolutely indivisible components of matter contradicts the principle of conservation of energy”
momentum was found to be conserved too
Experimental knowledge at end of XIXth century
A. Andronic
...and then there was light
Isaac Newton (1642-1726) imagined it as corpuscles,
Christiaan Huygens (1629-1695) as waves
reflexion, refraction, diffraction, dispersion
Thomas Young (1773-1829), Augustin-Jean Fresnel (1788-1827),
Josef von Fraunhofer (1787-1826), Gustav Kirchhoff (1824-1887)
What is just about to come
A. Andronic
...was anticipated by Newton in Opticks, 1680:
“The Attractions of Gravity, Magnetism and Electricity, reach to very sensible
distances, and so have been observed by vulgar Eyes, and there may be others
which reach to so small distances as hitherto escape Observation.”
(quoted by A. Pais, Inward Bound, 1986)
we will go to distances about 1 billion times smaller than what was known at
end of XIXth century
Discovery of X-rays
A. Andronic
Wilhelm Conrad Roentgen
1895, Würzburg, Germany
Nobel Prize 1901
studied cathode rays, which were
known to have short range in air,
and found penetrating rays, which he
called X rays, with properties:
- no reflexion or refraction
- no deflection in magnetic field
- originated at the tube’s spot where
cathode rays hit
...all done in a few weeks
(then no more research on X rays)
Impact of X-rays
A. Andronic
• Diffraction of X-rays (Max von Laue, 1912; Nobel Prize 1914)
• Analysis of crystal structure (W.H. Bragg, W.L. Bragg; Nobel Prize 1915)
• X-rays emitted after irradiation are unique for every element (Charles Barkla,
Edimburgh, UK, Nobel Prize 1917; discovered polarization of X-rays)
• precision X-ray spectroscopy (Karl Manne Georg Siegbahn, Uppsala, Nobel Prize
1924)
• X-ray imaging of astrophysical objects (Ricardo Giacconi, Nobel Prize 2002)
• Usage in medicine (already in WWI)
Discovery of radioactivity
A. Andronic
Henri Becquerel, 1896, Paris, France; Nobel Prize 1903
was studying phosphorescence, exposing potassium uranyl disulfate to sunlight
the effect was that photographic plates
were blackened even covered in black
paper
...during some cloudy days the photographic plates stayed in a drawer,
developing them showed a big surprise
penetrating rays like those of Roentgen, but without a Crookes tube!
“sans cause excitatrice”, as Becquerel demonstrated that phosphorescence
did not matter
Radioactivity follow-up
A. Andronic
During 1986 Becquerel demonstrated that the new “rays”
- come from the substance
- did not depend on type of chemical uranium compound
- effect did not decrease in time (after several years)
- effect was weaker for larger distances
- discharged charged bodies (electroscopes), like X-rays
also a mistake: Becquerel found reflexion and refraction
wondered about where is the energy coming from
The unit of radioactivity: Becquerel = 1 disintegration per second
Radioactivity follow-up
A. Andronic
Marie and Pierre Curie (1898, Paris) discovered:
- thorium is radioactive (independently by Gerhard C. Schmidt, Erlangen)
- 2 new elements: polonium, radium - with stronger radioactivity than uranium
(not by chance, but by observing that pitchblende and chalcite are more
radioactive than uranium and concluding that another element may be at play)
jump-started the discipline of radiochemistry
- radioactivity proportional to quantity of uranium
- radioactivity is an atomic property
introduced the term “radioactivity”
Nobel Prize, 1903; Nobel Prize Chemistry 1911, M. Curie
M. Curie died (1934) with health effects of radiation
(worked without any protection, handled considerable amount of radioactivity)
effects were known already in 1900 (Walkhoff, Giesel, Becquerel, Curie)
Radioactivity follow-up
A. Andronic
Ernest Rutherford (1898-1908, Cambridge, Montreal; & co.) discovered:
- reflection and refraction are not present (soon Becquerel concurred)
- two components of radioactivity of uranium:
one with a low penetration power, α, and another highly-penetrating, β
- identified α with He (He had been identified on earth only in 1895)
- the radioactive decay law
- element transmutation by radioactivity
Nobel Prize Chemistry: Rutherford, 1908; Frederick Soddy, 1921
Paul Villard (1900, Paris) discovered a 3rd type or radioactivity (of radium): γ
Rutherford and Andrade demonstrated its similarity to X-rays in 1914
Impact of radioactivity
A. Andronic
• fundamental laws of nature were discovered based on radioactivity
• essential for chains of element production (in stars)
• source of inner earth warmth (47 TW at surface, 1/2 radiogenic ...173000 TW
solar)
produced by 40K, 238,235U, 232Th
• radioactive carbon dating
• radioactive isotopes are used in medical investigations and treatment
Discovery of the electron
A. Andronic
J.B. Perrin, 1895: cathode rays are negatively-charged particles, magnetic
deflection (Nobel Prize, 1922)
P. Zeeman, 1896: light multiplets in magnetic field; e/m <0
(Nobel Prize 1902 with H.A. Lorentz)
Wiechert, Kaufmann (1897) were close to a measurement of e/m (assumed v)
J.J. Thomson
1897, Cambridge, England
Nobel Prize 1906
cancellation of electric and magnetic
deflection of cathode rays gives velocity; electric deflection, gives e/m
...770 times larger than for hydrogen
same for particles produced by ultraviolet light (1899)
e separately in cloud chamber (1899)
Discovery of the electron
A. Andronic
F
qE
qV
a=
=
=
m
m
mh
v = v0 + at;
d
t=
vx
qV d
vy =
= vx tan θ
mh vx
Impact of electron
A. Andronic
• first elementary particle
• Kaufmann demonstrates that electron and β rays (radium) are identical
• R.A. Millikan, 1910: measures e with precision (Nobel Prize, 1923)
now: e = 1.602176565(35) × 10−19 C, precision 22 ppb (2.2 × 10−8)
• J. Stark, 1913: electric splitting of spectral lines
• Electronics ...invention of the transistor: William Bradford Shockley, John
Bardeen and Walter Houser Brattain, 1947, USA (Nobel Prize 1956)
(now 1 mil. transistors per cm2)
• High-precision (10−5) electron spectroscopy to investigate vibrational energies
of molecules emitting X-rays, Kai M. Siegbahn (Nobel Prize 1981)
• Electron microscopy, Ernst Ruska (Nobel Prize 1986)
50 pm resolution, compared to 200 nm for light
Summary of radioactivity
A. Andronic
http://www.cyberphysics.co.uk/topics/radioact/Radio/deflection_fields.htm
Electric field: same charges repel, opposite charges attract
Magnetic field: left (right) hand rule for negative (positive) charges
(thumb is velocity, index to B field orientation, particle deflected out of hand)