Sunshine

What makes the Sun shine? B.N. Dwivedi Department of Physics, Indian Institute of Technology (Banaras Hindu University) Varanasi - 221005.

Basic overview of the Sun’s structure: Three major interior zones: core, radiative zone & convective zone Flare, sunspots & photosphere, chromosphere, corona, coronal holes & prominence are all clipped from actual images of the Sun obtained by the SOHO spacecraft (Credit: ESA/NASA)

Sun‟s visible surface

Granulation pattern on the photosphere

Courtesy: The Swedish 1-m Solar Telescope

“May the Almighty God illuminate our intellect to lead us along the righteous path.” The mantra is also a prayer to the “giver of light and life”: the Sun (savitur).

Why does the Sun shine? Fascinating account Involving → - geology - biology - evolution - meteors - E = mc2 - quantum mechanics - particle physics - two Nobel prizes. These questions challenged scientists since 19th century.

Physicists debated geologists & biologists over the age of the Sun.

The best physicists of 19th century → wrong answer • Unrelated Discoveries in Fundamental Physics → Theory of Nuclear-Energy Generation in stars •

• 1896: First hint of correct answer → Discovery of Radioactivity • 1905: Glimpse of solution in sight → Special Theory of Relativity • 1920 → Measurement of nuclear masses of Hydrogen & Helium 1928: Quantum Mechanical explanation→How protons come closer? •

Sun’s Interior → Controlled Thermonuclear Bomb on a giant scale

• 1968: Raymond Davis Jr → Detection of Solar Neutrinos (Nobel Prize in 2002) Confirmation → Sunshine by nuclear burning

Let‟s have a glimpse of the Sunshine (i) L = 3.85 x 1026 W = 3.85 x 1026 J/sec. An incandescent Lamp → 60 to 100 W

⇒ Sun more Luminous → 1024 light bulbs (ii) Imagine an ice cube in the Sun → ~ 40 min to melt (say)

That ice could be anywhere in a sphere (area = 4πr2) with (say RE) = 150,000,000 km around the Sun … Put all those ice cubes out there at same time → a shell 1 cm thick & surrounding the Sun would all melt in ~ 40 min. 1 calorie (= 4.2 J) → 1 gm water by 10C [(80 x 4.2) 4π (1.5 x 1013)2] / (40 x 60) ≈ 4 x 1026 W

Fundamental Role of the Sunshine

“The Sun‟s rays are the ultimate source of almost every motion which takes place on the surface of the Earth. By its heat are produced all winds,….By their vivifying action vegetables are elaborated from inorganic matter, and become, in their turn, the support of animals and of man, and the sources of those great deposits of dynamical efficiency which are laid up for human use in our coal strata.” --- 19th century astronomer John Herschel in his 1833 Treatise on Astronomy.

Among the many exotic astronomical objects in the universe, the proximity of the Sun provides the principal source of new physics, ranging from neutrino physics to the mysteries of the magnetohydrodynamic activity and to the geological evolution of terrestrial climate.

CORONA

SOLAR INTERIOR

Nuclear reactions burn every second about 700 million tons of Hydrogen into Helium. In the Sun‟s core the particles are packed so tightly, and the temperature is so hot, that individual atoms ram into each other, forming heavier helium atoms and releasing energy.

The Sun’s Age •

What is the fuel that makes the Sunshine?

•

What is the source of the Sun’s enormous supply of energy?

•

How old is the Sun?

•

How does the Sun shine? These questions puzzled scientists for a long time Total amount of energy radiated by the Sun since its birth ~ solar luminosity (3.85  1026 W) x age of Sun Older the Sun → Greater amount of solar energy radiated

19th Century Physicists → Gravitation could fuel the Sun 1854: Helmholtz → Origin of the Sun’s energy --Gravitational Contraction of a large mass →→ Lord Kelvin estimated age of the Sun & by implication Earth 1859: Charles Darwin

~ 30 million years

(First edition of The Origin of The Species by Natural Selection)

estimated age of Earth ~ 300 million years from geological activity, based on the erosion of the Weald in south England, cf., http://en.wikipedia.org/wiki/File:KentGeologyWealdenDomeSimple.svg

[time (300 x 106 years) enough for evolution to occur]

While physicists focused on the origin of the radiated energy, biologists and geologists considered the effects of the Sun’s radiation.

Problem → No fuel source known? Kelvin in agreement with Helmholtz → The Sun’s Luminosity caused by conversion of gravitational energy into heat Meteoric Impact? → Initially favoured by Lord Kelvin 1854: Kelvin suggested → Sun's heat might be produced continually by impact of meteors falling on its surface Kelvin later modified his hypothesis, based on astronomical evidence → Primary source of Sun’s energy due to Gravitational energy of primordial meteors from which it was formed.

With great Authority & Eloquence Lord Kelvin Declared: Darwin’s estimate of the age of the Earth was wrong & And also Darwin was wrong about the time available for natural selection to operate

Age of the Sun: 4.6 x 109 years

Helmholtz (1854) gravitational contraction

Charles Darwin (1859) > 0.3 x 109 years

Lord Kelvin (1862) ~ 0.03 x 109 years

Different estimates for the age of the Sun: von Helmholtz, Charles Darwin & Lord Kelvin.

Lord Kelvin → Second Law of Thermodynamics Absolute Temperature Scale → Kelvin Scale in his Honour. Second Law → Heat Flows from a Hotter to a Cooler body

⇒ Sun & Earth will cool unless an External Energy Source What was Wrong with Kelvin‟s Analysis? Analogy → How long your Laptop operating? ~ Few Hours Battery can supply Power Flaw in Analysis → Assumption that your Computer is powered only by a Battery But if powered from an Electric Outlet? So, Kelvin‟s Flaw was his Assumption → Only Gravitational Energy Powers the Sun

⇒ Different estimates of Sun’s age in 19th century → Answer depended on who did one ask … Prominent Physicists → ~ 30 million years. Geologists and Biologists → ~ 300 million years.

19th century physicists simply did not know about the conversion of nuclear mass into energy → They calculated the Sun’s age which was too short Exercise I: How long can gravity fuel a star?

Who was right? • Prominent Physicists → Few tens of million years • Geologists/Biologists → Several hundreds million years • Thus the age of the Sun, and the origin of the solar energy, were important questions not only for physics & astronomy, but also for geology & biology. • TODAY we know → Radioactive Dating of Meteorites shows that the Sun is 4.6 billion years old.

A Glimpse of Solution • Turning point → Henri Becquerel (1896) → Natural Radioactivity. • Pierre Curie & Albert Laborde (1903) → Radium salt constantly release heat → Radium radiated heat without cooling down to its surroundings’ temperature. • William Wilson & George Darwin immediately proposed → Radioactivity as the source of the Sun’s energy. • Possibility of Nuclear Energy. • This development freed theorists from gravitational energy. • But the Sun does not contain a lot of radioactive material. • Also, rate at which radioactivity delivers energy --- does not depend on the stellar temperature.

• Something else required to release Nuclear Energy

Discovery of Radioactivity 1896: Henri Becquerel → Experiment designed to study X-rays (discovered in 1895 by Wihelm Röntgen), stored some uranium-covered plates in a desk drawer atop photographic plates wrapped in dark paper. Developing photographic plates → found strong images of the uranium crystals. Discovery→ natural radioactivity due to nuclear transformations of uranium atoms

Problem → Requires massive Unstable elements → Inconsistent with Sun’s density & composition

Fission Heavy unstable nuclei split apart ⇒ Release of Energy

Radioactivity Discovered

1903: Pierre Curie & his assistant Albert Laborde → Radium salts constantly release heat Extraordinary → Radium gave off heat without cooling down the surrounding temperature → Previously Unknown Source of Energy William Wilson & George Darwin → Radioactivity might be source of Sun‟s radiated energy

1905 Albert Einstein: E = mc2 → Special Theory of Relativity (Extension of law of energy conservation of Helmholtz and Mayer to include conversion of mass into energy)

Fusion → Burning of Hydrogen → Sun is mostly Hydrogen → Fusion will form Helium → Lots of energy released 1920 → Key experimental element in the puzzle discovered by Francis Aston →

Four Hydrogen nuclei heavier than a Helium nucleus

Francis Aston

He H

H

H

H

H

Francis Aston (1920)

Four Hydrogen nuclei heavier than a Helium nucleus (Adapted from http://nobelprize.org/nobel_prizes/physics/articles/fusion/index.html) H

Sir Arthur Eddington (1920) Aston’s measurement of mass difference → Sun could shine by converting Hydrogen atoms into Helium • Burning of H into He would release ~ 0.7% of Hydrogen mass equivalent of energy & allow Sun to shine ~ 100 billion years “If, indeed, the sub-atomic energy in the stars is being freely used to maintain their great furnaces, it seems to bring a little nearer to fulfillment of our dream of controlling this latent power for the well-being of the human race or for its suicide.” --- Sir Arthur Eddington

“We do not argue with the critic who urges that the stars are not hot enough for this process; we tell him to go find a hotter place.” ---Sir Arthur Eddington

Albert Einstein and Sir Arthur Eddington

Radioactivity & Quantum Physics Next important step → How stars produce energy from nuclear burning? → Application of quantum mechanics → Explanation of nuclear radioactivity

But like charges repel → Protons will repel each other as they both have +ive charge Is Sun hot enough so that they slam into each other and get close enough for fusion to occur? Eddington (1925)→ Sun not collapsing → hydrostatic equilibrium (pressure in = pressure out) (For a gas: pressure = density x temperature) Working from surface down → He estimated temperature of Sun’s core at 10 x 106 K Not enough ! cf., Exercise II: Electrical repulsion

• Classical Physics → Probability of two +ively charged particles getting very close together ≈ zero

• But, something that cannot happen in classical physics, can happen in the real world.

George Gamow (1928)

• 1928: George Gamow (Russian-American Physicist): • Quantum Mechanical formula --- A non-zero probability (Gamow Factor)

Quantum again →

Since matter has wave-like behaviour → we can get ‘quantum-mechanical tunnelling’

Even though the protons don’t have enough energy to fuse, they can sort of leak into each other and fuse some of the time. (http://hyperphysics.phy-astr.gsu.edu/Hbase/quantum/barr.html)

p-p cycle 1938: Hans Bethe → Basic nuclear processes → Hydrogen fused into helium in stellar interiors

(i) pp chain → builds helium out of hydrogen → Dominant energy source in Sun-like stars & in less massive stars (ii) CNO cycle → Most important in stars →

more massive than Sun Hans Bethe received the 1967 Nobel Prize in Physics → “for his contributions to the theory of nuclear reactions, especially his discoveries concerning the energy production in stars”

Bethe's work → answer known in principle → ⇒ Sun produces energy by burning hydrogen Sun’s interior → a sort of controlled thermonuclear bomb on a giant scale

Bethe --- Basic theory: 1939.

Fowler --- Experiments: 1950s.

4p  4He + 2e+ + 2e + ~ 25 MeV.

Is this how the Sun shines?

Many important details added to Bethe’s theory • • • • •

AGW Cameron WA Fowler F Hoyle EE Salpeter M Schwarzschild etc.

Sun produces energy by burning hydrogen

The Proton-Proton (p-p) Chain Hydrogen burning → a sequence of nuclear fusion reactions → proton-proton chain → three main steps: 1st step → Two protons collide to produce a deuterium (D), a positron (e+) and an 1H + 1H  D + e+ +  . electron neutrino (e ), i.e., e 2nd step → A proton collides with deuterium to produce a nuclear isotope of helium (3He ) and a gamma-ray, i.e., D + 1H  3He + . 3rd step: Two 3He collide to produce a normal helium (4He) with the release of 3He + 3He  4He + 1H + 1H. two protons, i.e., First two reactions occur twice for each occurrence of 3rd reaction → Net Result :

4 1H  4He + 2e+ + 2e + 2

 Four protons fuse to make one helium nucleus

Nuclear reaction sequences Sun Particle energies in the core at few keV level → but typical Coulomb barriers for light elements ~ MeV

Fusion in main sequence stars → quantum mechanical tunnelling (wave functions of interacting particles penetrate each other)

Two nuclear reaction sequences in Sun Proton-proton (pp) chain – 99% energy Carbon-Nitrogen (CN) cycle – 1% energy Net Result 4p  4He + 2e+ + 2e plus energy. cf., Exercise III

Photon Transport in the Sun’s Interior • Time for a photon to travel from Sun’s core to its surface → Estimated from a random walk formula • Photons continually stopped by scattering off free electrons in the Sun’s Interior → Thomson scattering

• Random walk process:

(cf., Exercise IV)

End point

Starting point 34

CONFIRMATION • Impossible? → NO • Seeing into Sun’s interior with Neutrinos → Unique sub-atomic particles No electrical charge & interact weakly with matter Travel with speed of light

⇒ There‟ s a way of "seeing" into Sun‟s interior → Neutrinos As a doctor uses X-rays or ultra-sound to diagnose what happens inside our bodies, Measurement of neutrinos → Look inside the Sun

A cross section view of the Sun: Neutrinos enable us to look deep inside the Sun, into the Sun‟s core where nuclear burning occurs.

A “desperate remedy” The neutrino was thought to be a neutral, massless particle

“I have done a terrible thing. I have postulated a particle that cannot be detected.” - W. Pauli (1931)

W. Pauli Nobel Prize in 1945

Neutrinos are hard to detect. How hard? (cf, Exercise V) → probability of hitting something in Earth ~ 1/1012 → 100 billions pass through our thumbnail every second → Could travel through 100 light years of iron without hitting anything •

First 1964 Raymond Davis, Jr (Nobel Prize 2002) & John Bahcall: 100,000 gallons of cleaning fluid (perchloroethylene) Occasional collision with chlorine → Argon → Detect the Argon → Expect only a few atoms/week Davis performed the expt. & announced his results in 1968 →

Measured fewer electron-type solar neutrinos than Bahcall predicted

Neutrinos → unique sub-atomic particles, no electrical charge & interact weakly with matter. Travel with speed of light & come in three types:

electron neutrinos (e), muon neutrinos () & tau neutrinos (). They are also produced on Earth in Particle Accelerators, in Nuclear Reactors, and in Natural Radioactivity. Process → Sun-like stars generate energy

Neutrinos → so elusive that we don’t notice hundred billion solar neutrinos passing through our thumbnail every second

37Cl Detector (cf., Exercise VI)

e 

37

Cl  e 

37

Ar

(Homestake, SD)

Homestake Gold Mine Neutrino Detector in South Dakota. Tank contains 400 m3 perchloroethylene & 1.5 km deep underground. Solar neutrinos are captured by chlorine atoms in the perchloroethylene and changed into argon atoms. Ar is removed from tank and measured, indicating the rate of solar neutrinos. See: Raymond Davis on catwalk above tank. (Credit: Brookhaven National Laboratory)

Other Experiments Include: Chemical detection in large Ga targets, either aqueous (GALLEX & later GNO) [Itallian] or solid (SAGE) [Russian]. Ge decays by electron capture with a half-life of 11 days → resulting electrons detected as for 37Cl experiment Neutrino scattering by electrons orbiting atoms in water molecules in highly purified water (KAMIOKANDE and SUPER KAMIOKANDE) [Japanese] The electron generates Čerenkov radiation → detected by arrays of photo-multipliers

SUPER-KAMIOKANDE also detects muon neutrinos Čerenkov radiation generated by electrons with v > c / n (n refractive index) Radiation is emitted in a cone with half-angle cos  = 1 / n , where  = v / c. KAMIOKANDE thus determines the direction of neutrinos

SUPER-KAMIOKANDE Neutrino Detector (Japan) → one km underground in a Japanese zinc mine. Huge stainless-steel vessel [40 m (tall) × 40 m (wide)] filled with 50 million liters of highly purified water. Walls lined with 13,000 light sensors (photo-multiplier tubes) → Detects Čerenkov radiation from recoiling electrons struck by incoming neutrinos. Unlike other neutrino detectors, it can determine the neutrino direction.

Neutrino Sun: Constructed in an unused mine in Japan, an ambitious large-scale experiment designed to detect and study neutrinos → Super-Kamiokande or "Super-K“ → Exposure of 500 days needed to produce this Neutrino Sun (The Size of the Emission ~ 30 degrees, Spatial Resolution Poor) From the archives of the Astronomy Picture of the Day

Measured and Predicted Neutrino Fluxes Observed neutrino rate is 1.9—3.6 times ‹ predicted

Three different neutrino states or flavours → e ,  ,  associated with electron, muon, & tau particle in weak interaction decays Most experiments detected electron neutrinos MSW effect: Mikheyev & Smirnov (1985) & Wolfenstein (1978) → Neutrinos change state in presence of other matter if they have a tiny rest mass

Emitted electron neutrinos → “mixed” into muon or tau particle neutrinos modified as they leave the Sun

⇒ electron neutrinos would only be 1/3 of the total number of neutrinos Solar Neutrino Problem → resolved by mixing of the neutrino flavours Most detectors → electron neutrinos → ~ 1/3 of the total neutrinos SUPER-KAMIOKANDE detects  (muon neutrinos) → same discrepancy

John Bahcall

Raymond Davis Jr.

John Bahcall & Raymond Davis Jr.

Many different possibilities examined by physicists, chemists and astronomers Both experiments & theoretical calculations → correct

Two ideas widely discussed: Either model of the Sun wrong,

Or, something happens to neutrinos on their way to Earth. Each detector: No. of neutrinos observed → somewhat lower than standard theory

Scientists rejoiced detection of neutrinos but worried about fewer neutrinos than predicted

Bahcall → Solar Neutrino Problem (SNP)

Science is unpredictable and fun --- John Bahcall

What was wrong?

• • • •

Understanding of the sunshine? Bahcall’s calculations? Experiments? Something happened to Neutrinos after their creation in the Sun? • Many possibilities analyzed • Two ideas → Solar Model & Neutrinos

Situation changed dramatically in 2001 • “The Sudbury Neutrino Observatory (SNO) found that the Solution lies not with the Sun, but with Neutrinos, which change as they travel from the core of the Sun to the Earth.” • What do all these experiments’ results mean? • Solar Neutrinos detected in 5 experiments → source of the Sun’s energy → fusion of hydrogen nuclei in Sun’s interior.

Normal Astronomical Observations (using Light rather than Neutrinos) and Theoretical Calculations of Solar Characteristics → much more Precise Internal Structure of the Sun → observations of Solar Vibrations ⇒ Standard Solar Model predicts Temperature at Sun‟s core to be consistent with observations to an accuracy of better than 0.1%. But then what can explain the disagreement by a factor of 2 to 3 between the measured and the predicted neutrinos? We do not know what causes the multiple personality disorder of solar neutrinos. Answer to this question may provide clue to physics beyond the current Standard Model of elementary particles & their interactions.

Does the identity change occur while neutrinos travel from Sun to Earth → Vladimir Gribov & Bruno Pontecorvo (1969)? Or, does matter induce solar neutrinos to flip out? Experiments underway in Canada, Italy, Japan, Russia & the US → Attempting to pin down the exact cause of solar neutrino oscillations by measuring their masses and how they transform from one type into another. Non-zero neutrino masses → A clue to a still undiscovered realm of physical theory „… the discrepancy between theory and the first solar neutrino experiment could be due to an inadequacy in the textbook description of particle physics, rather than in the solar standard model.‟ Pontecorvo & Gribov (1969)

19th century debate between Physicists, Geologists & Biologists settled • But just we thought we had NATURE figured out, experiments showed → fewer neutrinos detected than predicted • We have now learnt → solar neutrinos tell us about nature of neutrinos

• Nature has written its Mystery in an International Language that can be read by curious people of all nations • AND what MYSTERIES of NATURE will be unlocked by the new solar neutrino experiment? WHO KNOWS! “Science is unpredictable and fun.” ---John Bahcall

Thank You

Exercise I: How long can gravity fuel a star? A star is held together by gravitation. If this were the only force acting, it would rapidly contract and the time to lose all of its gravitational potential energy (PE) is given by: GM2/ R L ~

3 x 107 million years: Kelvin-Helmholtz timescale.

If a star contracts slowly, the shrinkage produces gravitational potential energy (P.E.) which can be converted into heat.

P.E.() = G(Ms2/Rs), where G = 6.67 x 10-11 Nm2 kg-2 . The change, , in gravitational P.E. created by a decrease in radius Rs is:  = G(Ms2/Rs) Rs = 0.55 x 103 (Ms/M)2 (R/Rs)2 Rs Joule (where M = 1.989 x 1030 kg, and R = 6.955 x 108 m). ---contd.---

If the energy change provides an absolute luminosity Ls in a time interval t, Then Ls = /t And the rate of change in radius is Rs/t = 7.05 x 10-7(Ls/L) (M/Ms)2 (Rs/R)2 m/sec. Sun’s luminosity

L = 3.85 x 1026 W

1 year = 3.1557 x 107 sec.,

 contraction of only 22.3 m/year, will power the Sun at its present rate. If the source of the Sun’s present luminosity were gravitational P.E., then the current radius would shrink to zero in a total gravitational lifetime of

s = Rs/(Rs/t) = /Rs = 3.1 x 107 (Ms/M)2 (R/Rs) (L/Ls) year

 If the Sun shines by converting gravitational P.E. into heat, it would disappear in only about 31 million years.

Exercise II: Electrical repulsion A proton moving close to another proton : repelled by a Coulomb force. A proton moving fast enough might get close to another proton to overcome the electrical repulsion. Determine the velocity, fast enough for one proton to move into the another: Equate Kinetic Energy (K.E.) = ½ mp v2 = e2/4πε0 r = Electrical Potential Energy (P.E.): v = (2e2/4πε0 mp r)1/2 = 1.66 x 107 m/sec (Assuming separation between two protons ~ size of an atomic nucleus: r = 1o-15 m) Determine the mean speed of a proton inside the Sun by equating: Kinetic Energy (K.E.) = ½ mp v2 = 3/2 kT = Thermal Energy. Thermal velocity (v) = (3kT/mp)1/2 = 157 T1/2 m/sec. For T = 15 MK, thermal speed is = 6.1 x 105 m/sec which is much below the speed (i.e., 1.66 x 107 m/sec) to overcome the electrical repulsion between two protons. So, classical physicists: Proton Fusion cannot occur in the Sun. Sir Arthur Eddington was certain that sub-atomic energy fueled the stars, stating in 1926:

Famous Quote: “We do not argue with the critic who urges that the stars are not hot enough for this process; we tell him to go find a hotter place.”

Exercise III Every completion of the p-p chain results in the release of energy as a consequence of E = mc2. The mass difference between four hydrogen atoms and one helium atom is: 4  1.00813 – 4.00386 = 0.02866 atomic mass unit (amu). Since one atomic mass unit is 1.66  10-27 kg, this mass difference is 4.8  10-29 kg. The energy liberated from this mass difference will be: E = mc2 = (4.8  10-29 kg) x (3x108 m)2 = 4.3  10-12 J.

A small amount of this energy (~ 0.1  10-12 J) is taken away by the neutrinos, leaving 4.2  10-12 J which makes the Sunshine.

Exercise IV: Photon Transport in the Sun’s Interior The time for a photon to travel from the solar core to the solar photosphere can be estimated from a random walk formula. The photons are continually stopped by scattering off free electrons in the Sun’s Interior – Thomson scattering. Random walk process: Mean

free path for photon is given by

λ = 1 / (σT ) where σT = Thomson scattering cross section = 6.65 × 10-29 m2, and is the mean electron number density ~ mean proton number density ~ M / (V mp): gas is fully ionized = 2 × 1030 / ( 1.4 × 1027 × 1.67 × 10-27) = 8.6 × 1029 m-3. So

λ = 0.018 m.

The total number of random walk steps for a photon to get to the Sun’s surface is: (R / λ)2 = (7 × 108 /0.018)2 = 1.5 × 1021. The time taken for a photon to travel a distance (λ) = λ / c = 6 × 10-11 s. So the time for a typical photon to reach the Sun’s surface is ttravel = (R/λ)2 × (λ/c) = 1.5 × 1021 × 6 × 10-11 s = 9 × 1010 s = 3000 years.

Exercise V Every time the proton-proton chain creates one helium nucleus, it releases an energy E = (m) c2 = 0.007 (4mp) c2 = 4.2 x 10-12 J, where m is the mass difference between the helium nucleus, & four protons. The Sun’s luminosity L = 3.85 x 1026 W. The total number of helium-producing proton-proton chains required to fuel the Sun’s energy/sec is L/E. Two neutrinos are emitted every time one helium nucleus is made, & Therefore, the Sun emits 2L/E = 2 x 1038 neutrinos/sec. No. of neutrinos passing through Earth/sec = (RE/AU)2 (2L/E) = 4 x 1029. (Note: RE = 6.378 x 106 m , and 1 AU = 1.496 x 1011 m)

Number of neutrinos passing through every square meter at the Earth can be calculated by dividing it by πRE2 to get

3 x 1015 neutrinos/m2.

Exercise VI: The Homestake Neutrino Detector First solar neutrino experiment was set up in 1967 by Raymond Davis Jr & collaborators. The experiment is 1.5 km deep in the Homestake gold mine in South Dakota (to shield it from cosmic rays). Its operation is based on the reaction:

e + 37Cl  37Ar + e-

The neutrino energy threshold is 0.814 MeV. From the standard model most (77%) detectable neutrinos for this reaction are from reaction: 8B

 8Be* + e+ + e in the p—p chain, i.e.  decay of 8B to 8Be & ~ 13% are from reaction: 7Be + e-  7Li + e .

The detector consists of a large (400 m3 ) tank of the cleaning fluid perchloroethylene (C2Cl4) containing 2.2 × 1030 Cl atoms. ---contd.---

About 1 solar neutrino-induced reaction is expected per day. A typical “run” lasts 80 days, after which He gas is bubbled through the tank to pick up 37Ar atoms. A measured volume of 37Ar gas is then placed in a proportional counter (similar to a Geiger counter). 37Ar decays by electron capture and emits 2.82 keV electrons that are detected by the proportional counter. Counting is carried out over 8 months to estimate the background : 37Ar decays in ~ 3 months. New unit introduced by Bahcall → Solar Neutrino Unit (SNU) to specify flux 1 SNU  1 neutrino capture/sec in 1036 target atoms 5.35 SNU  1 37Ar atom per day

Observed 37Ar production rate = 2.55 ± 0.25 SNU Prediction rate from standard model = 9.3 ± 1.3 SNU Observed rate = 3.6 times less than predicted This is (or was) the solar neutrino problem