Diagram showing the Sun's components. The core is where nuclear fusion takes place, creating solar neutrinos.

A solar neutrino is a neutrino originating from nuclear fusion in the Sun's core, and is the most common type of neutrino passing through any source observed on Earth at any particular moment. Neutrinos are elementary particles with extremely small rest mass and a neutral electric charge. They only interact with matter via the weak interaction and gravity, making their detection very difficult. This has led to the now-resolved solar neutrino problem. Much is now known about solar neutrinos, but the research in this field is ongoing.

History and background

Diagram of the Homestake experiment set-up

Neutrino observatories

The Sudbury Neutrino Observatory

Main article: Sudbury Neutrino Observatory

The Sudbury Neutrino Observatory (SNO), a 2,100 m (6,900 ft) underground observatory in Sudbury, Canada, is the other site where neutrino oscillation research was taking place in the late 1990s and early 2000s. The results from experiments at this observatory along with those at Super-Kamiokande are what helped solve the solar neutrino problem.

The SNO is also a heavy-water Cherenkov detector and designed to work the same way as the Super-Kamiokande. The Neutrinos when reacted with heavy water produce the blue Cherenkov light, signaling the detection of neutrinos to researchers and observers.^[8]

Borexino detector exterior

Formation process

Solar neutrinos are produced in the core of the Sun through various nuclear fusion reactions, each of which occurs at a particular rate and leads to its own spectrum of neutrino energies. Details of the more prominent of these reactions are described below.

Solar neutrinos (proton–proton chain) in the standard solar model

The main contribution comes from the proton–proton chain. The reaction is:

${\displaystyle {\text{p}}+{\text{p}}\to {\text{d}}+{\text{e}}^{+}+\operatorname {\nu } _{\text{e}}}$

or in words:

two protons ${\displaystyle \to }$ deuteron + positron + electron neutrino.

Of all Solar neutrinos, approximately 91% are produced from this reaction.^[12] As shown in the figure titled "Solar neutrinos (proton–proton chain) in the standard solar model", the deuteron will fuse with another proton to create a ³He nucleus and a gamma ray. This reaction can be seen as:

${\displaystyle {\text{d}}+{\text{p}}\to {^{3}}{\text{He}}+\operatorname {\gamma } }$

The isotope ⁴He can be produced by using the ³He in the previous reaction which is seen below.

${\displaystyle {^{3}}{\text{He}}+{^{3}}{\text{He}}\to {^{4}}{\text{He}}+2\,{\text{p}}}$

Solar Neutrino Generation

With both helium-3 and helium-4 now in the environment, one of each weight of helium nucleus can fuse to produce beryllium:

${\displaystyle {^{3}}{\text{He}}+{^{4}}{\text{He}}\to {^{7}}{\text{Be}}+\operatorname {\gamma } }$

Beryllium-7 can follow two different paths from this stage: It could capture an electron and produce the more stable lithium-7 nucleus and an electron neutrino, or alternatively, it could capture one of the abundant protons, which would create boron-8. The first reaction via lithium-7 is:

${\displaystyle {^{7}}{\text{Be}}+{\text{e}}^{-}\to {^{7}}{\text{Li}}+\operatorname {\nu } _{\text{e}}}$

This lithium-yielding reaction produces approximately 7% of the solar neutrinos.^[12] The resulting lithium-7 later combines with a proton to produce two nuclei of helium-4. The alternative reaction is proton capture, that produces boron-8, which then beta⁺ decays into beryllium-8 as shown below:

${\displaystyle {^{7}}{\text{Be}}+{\text{p}}\to {^{8}}{\text{B}}+\operatorname {\gamma } }$

${\displaystyle {^{8}}{\text{B}}\to {^{8}}{\text{Be}}{^{*}}+{\text{e}}^{+}+\operatorname {\nu } _{\text{e}}}$

This alternative boron-yielding reaction produces about 0.02% of the solar neutrinos; although so few that they would conventionally be neglected, these rare solar neutrinos stand out because of their higher average energies. The asterisk (*) on the beryllium-8 nucleus indicates that it is in an excited, unstable state. The excited beryllium-8 nucleus then splits into two helium-4 nuclei:^[13]

${\displaystyle {^{8}}{\text{Be}}{^{*}}\to {^{4}}{\text{He}}+{^{4}}{\text{He}}}$

Observed data

The greatest number of solar neutrinos are direct products of the proton–proton reaction (tall, dark blue curve on the left). They have a low energy – only reaching up to 400 keV. There are several other significant production mechanisms, with energies up to 18 MeV.^[14]

The highest flux of solar neutrinos come directly from the proton–proton interaction, and have a low energy, up to 400 keV. There are also several other significant production mechanisms, with energies up to 18 MeV.^[14] From the Earth, the amount of neutrino flux at Earth is around 7·10¹⁰ particles·cm⁻²·s ⁻¹.^[15] The number of neutrinos can be predicted with great confidence by the standard solar model, but the number of neutrinos detected on Earth versus the number of neutrinos predicted are different by a factor of a third, which is the solar neutrino problem.

Solar models additionally predict the location within the Sun's core where solar neutrinos should originate, depending on the nuclear fusion reaction which leads to their production. Future neutrino detectors will be able to detect the incoming direction of these neutrinos with enough precision to measure this effect.^[16]

Theoretical curves of survival probability of solar neutrinos that arrive on day (orange, continuous) or on night (purple, dashed), as a function of the energy of the neutrinos. Also shown the four values of the energy of the neutrinos at which measurements have been performed, corresponding to four different branches of the proton–proton chain.

The energy spectrum of solar neutrinos is also predicted by solar models.^[17] It is essential to know this energy spectrum because different neutrino detection experiments are sensitive to different neutrino energy ranges. The Homestake experiment used chlorine and was most sensitive to solar neutrinos produced by the decay of the beryllium isotope ⁷Be. The Sudbury Neutrino Observatory is most sensitive to solar neutrinos produced by ⁸B. The detectors that use gallium are most sensitive to the solar neutrinos produced by the proton–proton chain reaction process, however they were not able to observe this contribution separately. The observation of the neutrinos from the basic reaction of this chain, proton–proton fusion in deuterium, was achieved for the first time by Borexino in 2014. In 2012 the same collaboration reported detecting low-energy neutrinos for the proton–electron–proton (pep reaction) that produces 1 in 400 deuterium nuclei in the Sun.^[18][19] The detector contained 100 metric tons of liquid and saw on average 3 events each day (due to ¹¹C production) from this relatively uncommon thermonuclear reaction. In 2014, Borexino reported a successful direct detection of neutrinos from the pp-reaction at a rate of 144±33/day, consistent with the predicted rate of 131±2/day that was expected based on the standard solar model prediction that the pp-reaction generates 99% of the Sun's luminosity and their analysis of the detector's efficiency.^[20][21] And in 2020, Borexino reported the first detection of CNO cycle neutrinos from deep within the solar core.^[22]

Note that Borexino measured neutrinos of several energies; in this manner they have demonstrated experimentally, for the first time, the pattern of solar neutrino oscillations predicted by the theory. Neutrinos can trigger nuclear reactions. By looking at ancient ores of various ages that have been exposed to solar neutrinos over geologic time, it may be possible to interrogate the luminosity of the Sun over time,^[23] which, according to the standard solar model, has changed over the eons as the (presently) inert byproduct helium has accumulated in its core.

Key contributing astrophysicists

Wolfgang Pauli was the first to suggest the idea of a particle such as the neutrino existing in our universe in 1930. He believed such a particle to be completely massless.^[24] This was the belief amongst the astrophysics community until the solar neutrino problem was solved.

Frederick Reines, from the University of California at Irvine, and George A. Cowan were the first astrophysicists to detect neutrinos in 1956. They won a Nobel Prize in Physics for their work in 1995.^[25]

Raymond Davis Jr receives the Medal of Science from President George W. Bush.

Raymond Davis and John Bahcall are the pioneers of solar neutrino studies. While Bahcall never won a Nobel Prize, Davis along with Masatoshi Koshiba won the Nobel Prize in Physics in 2002 after the solar neutrino problem was solved for their contributions in helping solve the problem.

Pontecorvo, known as the first astrophysicist to suggest the idea neutrinos have some mass and can oscillate, never received a Nobel Prize for his contributions due to his passing in 1993.

Arthur B. McDonald, a Canadian physicist, was a key contributor in building the Sudbury Neutrino Observatory (SNO) in the mid 1980s and later became the director of the SNO and leader of the team that solved the solar neutrino problem.^[24] McDonald, along with Japanese physicist Kajita Takaaki both received a Nobel Prize for their work discovering the oscillation of neutrinos in 2015.^[24]

Current research and findings

The critical issue of the solar neutrino problem, that many astrophysicists interested in solar neutrinos studied and attempted to solve in late 1900s and early 2000s, is solved. In the 21st century, even without a main problem to solve, there is still unique and novel research ongoing in this field of astrophysics.

See also

• Neutrino detector
• Neutral particle oscillation
• Solar neutrino unit
• Stellar nucleosynthesis
• Supernova neutrinos
• Diffuse supernova neutrino background (DSNB)

References

Further reading

• Haxton, W.C.; Hamish Robertson, R.G.; Serenelli, Aldo M. (18 August 2013). "Solar Neutrinos: Status and Prospects". Annual Review of Astronomy and Astrophysics. 51 (1): 21–61. arXiv:1208.5723. Bibcode:2013ARA&A..51...21H. CiteSeerX 10.1.1.755.6940. doi:10.1146/annurev-astro-081811-125539. S2CID 119255372.