Homestake Experiment
Encyclopedia
The Homestake experiment (sometimes referred to as the Davis experiment) was an experiment headed by astrophysicists Raymond Davis, Jr.
Raymond Davis Jr.
Raymond Davis, Jr. was an American chemist, physicist, and Nobel Prize in Physics laureate.-Early life and education:...

 and John N. Bahcall
John N. Bahcall
John Norris Bahcall was an American astrophysicist, best known for his contributions to the solar neutrino problem, the development of the Hubble Space Telescope and for his leadership and development of the Institute for Advanced Study in Princeton.-Early and family life:Bahcall was born in...

 in the late 1960s. Its purpose was to collect and count neutrino
Neutrino
A neutrino is an electrically neutral, weakly interacting elementary subatomic particle with a half-integer spin, chirality and a disputed but small non-zero mass. It is able to pass through ordinary matter almost unaffected...

s emitted by nuclear fusion
Nuclear fusion
Nuclear fusion is the process by which two or more atomic nuclei join together, or "fuse", to form a single heavier nucleus. This is usually accompanied by the release or absorption of large quantities of energy...

 taking place in the Sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...

. Bahcall did the theoretical calculations and Davis designed the experiment. After Bahcall calculated the rate at which the detector should capture neutrinos, Davis's experiment turned up only one third of this figure. The experiment was the first to successfully detect and count solar neutrinos, and the discrepancy in results essentially created the solar neutrino problem
Solar neutrino problem
The solar neutrino problem was a major discrepancy between measurements of the numbers of neutrinos flowing through the Earth and theoretical models of the solar interior, lasting from the mid-1960s to about 2002...

. The experiment operated continuously from 1970 until 1994. The University of Pennsylvania
University of Pennsylvania
The University of Pennsylvania is a private, Ivy League university located in Philadelphia, Pennsylvania, United States. Penn is the fourth-oldest institution of higher education in the United States,Penn is the fourth-oldest using the founding dates claimed by each institution...

 took it over in 1984. The discrepancy between the predicted and measured rates of neutrino detection was later found to be due to neutrino "flavor" oscillations
Neutrino oscillation
Neutrino oscillation is a quantum mechanical phenomenon predicted by Bruno Pontecorvowhereby a neutrino created with a specific lepton flavor can later be measured to have a different flavor. The probability of measuring a particular flavor for a neutrino varies periodically as it propagates...

.

Methodology

The experiment took place in the Homestake Gold Mine
Homestake Mine (South Dakota)
The Homestake Mine was a deep underground gold mine located in Lead, South Dakota. Until it closed in 2002 it was the largest and deepest gold mine in North America, producing more than 40 million ounces of gold. The Homestake Mine is famous in scientific circles for being the site at which the...

 in Lead, South Dakota
Lead, South Dakota
Lead is a city in Lawrence County, South Dakota, United States. The population was 3,124 at the 2010 census. Lead is located in western South Dakota, in the Black Hills near the Wyoming state line.-History:...

. Davis placed 1,478 meter (4,850 feet) underground a 380 cubic meter (100,000 gallon) tank of perchloroethylene, a common dry-cleaning fluid. A big target deep underground was needed to account for the very small probability of a successful neutrino capture, and to prevent interference from other forms of solar radiation. Perchloroethylene was chosen because it is rich in chlorine. Upon collision with a neutrino, a chlorine
Chlorine
Chlorine is the chemical element with atomic number 17 and symbol Cl. It is the second lightest halogen, found in the periodic table in group 17. The element forms diatomic molecules under standard conditions, called dichlorine...

 atom transforms into a radioactive isotope of argon
Argon
Argon is a chemical element represented by the symbol Ar. Argon has atomic number 18 and is the third element in group 18 of the periodic table . Argon is the third most common gas in the Earth's atmosphere, at 0.93%, making it more common than carbon dioxide...

, which can then be extracted and counted. Every few weeks, Davis bubbled helium
Helium
Helium is the chemical element with atomic number 2 and an atomic weight of 4.002602, which is represented by the symbol He. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas that heads the noble gas group in the periodic table...

 through the tank to collect the argon that had formed, and was able to determine how many neutrinos had been captured.

Conclusions

Davis's figures were consistently very close to one-third of Bahcall's calculations. The first response from the scientific community was that either Bahcall or Davis had made a mistake. Bahcall's calculations were checked repeatedly, with no errors found. Davis scrutinized his own experiment and insisted there was nothing wrong with it. The Homestake experiment was followed by other experiments with the same purpose, such as Kamiokande in Japan
Japan
Japan is an island nation in East Asia. Located in the Pacific Ocean, it lies to the east of the Sea of Japan, China, North Korea, South Korea and Russia, stretching from the Sea of Okhotsk in the north to the East China Sea and Taiwan in the south...

, SAGE in the former Soviet Union
Soviet Union
The Soviet Union , officially the Union of Soviet Socialist Republics , was a constitutionally socialist state that existed in Eurasia between 1922 and 1991....

, GALLEX
GALLEX
GALLEX or Gallium Experiment was a radiochemical neutrino detection experiment that ran between 1991 and 1997 at the Laboratori Nazionali del Gran Sasso...

 in Italy
Italy
Italy , officially the Italian Republic languages]] under the European Charter for Regional or Minority Languages. In each of these, Italy's official name is as follows:;;;;;;;;), is a unitary parliamentary republic in South-Central Europe. To the north it borders France, Switzerland, Austria and...

, Super Kamiokande, also in Japan, and SNO (Sudbury Neutrino Observatory)
Sudbury Neutrino Observatory
The Sudbury Neutrino Observatory is a neutrino observatory located 6,800 feet underground in Vale Inco's Creighton Mine in Sudbury, Ontario, Canada. The detector was designed to detect solar neutrinos through their interactions with a large tank of heavy water. The detector turned on in May 1999,...

 in Ontario, Canada. SNO was the first detector able to detect neutrino oscillation, solving the solar neutrino problem. The results of the experiment, published in 2001, revealed that of the three "flavors" between which neutrinos are able to oscillate, Davis' detector was sensitive to only one. After it had been proven that his experiment was sound, Davis shared the 2002 Nobel Prize in Physics
Nobel Prize in Physics
The Nobel Prize in Physics is awarded once a year by the Royal Swedish Academy of Sciences. It is one of the five Nobel Prizes established by the will of Alfred Nobel in 1895 and awarded since 1901; the others are the Nobel Prize in Chemistry, Nobel Prize in Literature, Nobel Peace Prize, and...

. Among those sharing the prize was Masatoshi Koshiba
Masatoshi Koshiba
is a Japanese physicist. He jointly won the Nobel Prize in Physics in 2002.He graduated from the University of Tokyo in 1951 and received a Ph.D. in physics at the University of Rochester, New York, in 1955...

 of Japan, who worked on the Kamiokande and the Super Kamiokande.

See also

  • Raymond Davis Jr.
    Raymond Davis Jr.
    Raymond Davis, Jr. was an American chemist, physicist, and Nobel Prize in Physics laureate.-Early life and education:...

  • Solar neutrino problem
    Solar neutrino problem
    The solar neutrino problem was a major discrepancy between measurements of the numbers of neutrinos flowing through the Earth and theoretical models of the solar interior, lasting from the mid-1960s to about 2002...

  • Cowan–Reines neutrino experiment (a contemporary experiment by Reines and Cowan which discovered the antineutrino)
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