The Physics
Opus in profectus

Radioactive Decay

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unstable isotopes

Quote that must be paraphrased

The turning point in the battle between theoretical physicists and empirical geologists and biologists occurred in 1896. In the course of an experiment designed to study x-rays discovered the previous year by Wilhelm Röntgen, Henri Becquerel stored some uranium-covered plates in a desk drawer next to photographic plates wrapped in dark paper. Because it was cloudy in Paris for a couple of days, Becquerel was not able to "energize'' his photographic plates by exposing them to sunlight as he had intended. On developing the photographic plates, he found to his surprise strong images of his uranium crystals. He had discovered natural radioactivity, due to nuclear transformations of uranium. The significance of Becquerel's discovery became apparent in 1903, when Pierre Curie and his young assistant, Albert Laborde, announced that radium salts constantly release heat. The most extraordinary aspect of this new discovery was that radium radiated heat without cooling down to the temperature of its surroundings. The radiation from radium revealed a previously unknown source of energy. William Wilson and George Darwin almost immediately proposed that radioactivity might be the source of the sun's radiated energy.

Historical quote

These experiments show that the uranium radiation is complex, and that there are present at least two distinct types of radiation — one that is very readily absorbed, which will be termed for convenience the α radiation, and the other of a more penetrative character, which will be termed the β radiation.

Paths of α, β, and γ radiation in a magnetic field. Alpha particles deflect upward in this field obeying the right hand rule of a positively charged particle. Beta particles deflect the opposite way indicating negative charge. Gamma particles are unaffected by the field and so must carry no charge. In addition, the radius of curvature of the α particles is larger than that of the β particles. This shows that the alphas are more massive than the betas.

alpha decay

Alpha particles cannot penetrate a piece of paper or even the thin layer of dead skin that coats us all. They will quickly find and join with two electrons to become an atom of helium before they can do much harm. Alpha particles are most dangerous, however, when inhaled. The inside of our lungs are moist and sticky and not as well coated with expendable cells as our exteriors are. Were a bit of alpha emitting debris to find its way into our lungs, chances are pretty good that it would stick there long enough to emit an energetic and massive nuclear projectile into our tissues, ionizing and dissociating molecules along the way. Such activities are one source of lung cancer. Workers who handle plutonium (a significant alpha emitter) are well aware of this hazard and take great care to keep it outside of their bodies at all times.

beta decay

Also called beta minus decay.

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The existence of neutrinos was first proposed by Wolfgang Pauli in a 1930 letter to his physics colleagues as a "desperate way out" of the apparent non-conservation of energy in certain radioactive decays (called beta decays) in which electrons were emitted. According to Pauli's hypothesis, which he put forward very hesitantly, neutrinos are elusive particles which escape with the missing energy in beta decays. The mathematical theory of beta decay was formulated by Enrico Fermi in 1934 in a paper which was rejected by the journal Nature because "it contained speculations too remote from reality to be of interest to the reader".

The name neutrino is a play on words in Italian. The Italian word for neutron (neutrone) is what linguists call an augmentative; that is, it indicates bigness or intensity. Some other augmentative words in Italian that readers might recognize are…

Change he suffix "-one" to "-ino" and you have a diminutive; that is, something small, cute, and innocuous. A small, cute, innocuous neutrone would be a neutrino in Italian and so it is in English now too. Some other diminutive words in Italian that readers might recognize are…

È un neutrone? (Is it a neutron?) No, è un neutrino. (No, it's a neutrino.)

The name “neutrino” (a funny and grammatically incorrect contraction of “little neutron” in Italian: neutronino) entered the international terminology through Fermi, who started to use it sometime between the conference in Paris in July 1932 [148)and the Solvay Conference in October 1933(159] where Pauliused it[281).The word came out in ahumorous conversation at the Istituto di Via Panisperna. Fermi, Amaldi and a few others were present and Fermi was explaining Pauli’s hypothesis abouthis “light neutron”. For distinguishing this particle from the Chadwick neutron Amaldi jokingly used this funny name, — says Occhialini, who recalls of having shortly later told around this little story in Cambridge.

electron capture


electron capture


reverse beta decay

Not really a form of decay.

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from the Nobel website…
The neutrino was postulated in 1930 by Pauli to explain the continuous energy spectrum of electrons emitted in nuclear β decay. Fermi's theory for weak interactions was developed during the 1930's. At the time, neutrino interaction cross-sections were considered too small for neutrino detection. However, the large neutrino fluxes that later became available with nuclear reactors opened the field of neutrino physics. The experimental confirmation of the neutrino came in an experiment in 1955 by Cowan and Reines. Pontecorvo and Alvarez (Pontecorvo 1946, Alvarez 1949) suggested reactor experiments using the reaction ν + 37Cl → 37Ar + e to detect the neutrino. At the time there was no clear distinction between neutrino and antineutrino. The reaction often goes by the name the Davis-Pontecorvo reaction. The threshold energy is 0.81 MeV and the Argon isotope decays via electron capture with a half-life of 35 days.

Gravity was long believed to be the energy source of the sun. By 1920 it was known that the sun was mainly composed of helium and hydrogen and Eddington proposed that nuclear fusion was the energy source. However, it took until 1938 before there was a complete theory of the nuclear reactions within the sun (Bethe and Chritchfield 1938, Bethe 1939). It was a challenge to prove experimentally that nuclear burning was the energy source of the sun.

Raymond Davis Jr did his first reactor-based chlorine neutrino experiment at Brookhaven in the early 1950s. He used a tank filled with 3,900 liters of CCl4 as a target. Helium was bubbled through the tank to remove the few argon atoms. The radioactive argon could then be removed from the helium by passing the gas through a charcoal trap with liquid nitrogen (-196 ℃) which adsorbs the argon quantitatively and allow helium to pass.

Neutrinos are very abundant in the Universe. Indeed, the ratio between neutrinos and nucleons (protons or neutrons) in the Universe is about 109. On the Earth, the dominant source of neutrinos is our sun. Every second more than 10 billion (1010) neutrinos pass through every cm2, the majority with low energy (< 0.4 MeV). Only 0.01% of the solar neutrinos have an energy larger than 5 MeV.

A new and 100 times larger experiment was proposed based on Davis' radiochemical method and on Bahcall's calculated rate of 40 ± 20  SNU (1 SNU = 1 Solar Neutrino Unit, 1 capture per second and per 1,036 target atoms) (Davis 1964, Bahcall 1964). Davis experiment was funded and installed in the Homestake Gold Mine, Lead, South Dakota (depth 1,500 m). The tank contained 615 tonnes of C2Cl4, an agent normally used for dry cleaning. It was ready to start data taking in 1967. The extraction of argon by helium was done approximately once every two months to match the half-life of 37Ar.

The first results from Davis came in 1968, based on 150 days of data taking (Davis et al. 1968). An upper limit of the solar neutrino flux of 3 SNU was given, much lower than the then calculated rate of 20 SNU (Davis et al. 1968). In this 1968 paper is discussed different possibilities to improve the sensitivity and in particular how to reduce the background. The experimental challenge, that Davis was so successful in meeting, was to extract an average of only 17 argon atoms among the 2 x 1030 chlorine atoms in the tank every second month.

Davis experiment was running almost continuously from 1970 until 1994. The final results were published in 1998 (fig. 2, Cleveland et al. 1998). During this time it is estimated that a total of 2,200 argon atoms were produced in the tank. Of these 1997 were extracted and 875 counted in the proportional counter. Of the latter, 776 are estimated to be produced by solar neutrinos and 109 by background processes. The production in the tank was 0.48 ± 0.03 (stat.) ± 0.03 (syst.) argon atoms per day, corresponding to 2.56 ± 0.16 (stat.) ± 0.16 (syst.) SNU. Davis was a true pioneer and his successful mastering of the extraction of a few atoms out of 1030 gave birth to a new field of neutrino physics.

gamma decay

1899: Ernest Rutherford discovers that uranium radiation is composed of positively charged alpha particles and negatively charged beta particles

1900: Paul Villard discovers gamma-rays while studying uranium decay

neutron emission


proton emission


decay chains


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Uranium decay series