(Redirected from Subatomic astronomy)
This is an image obtained from muon radiography of Japan's Asama volcano. Credit: H T M Tanaka.

Subatomic astronomy is an observational astronomy using one or more subatomic particles or radiation.

A variety of subatomic particle astronomies have already been developed. These are highlighted below.

Potential particle astronomies are examined for their likelihood of becoming a successful astronomy.

## Theoretical subatomic astronomy

The bare nuclei of atoms may qualify as a form of subatomic astronomy.

Def. "particles that are constituents of the atom, or are smaller than an atom; such as proton, neutron, electron, etc"[1] or "any length or mass that is smaller in scale than a the diameter of a hydrogen atom"[1]

are called subatomics, or subatomic, respectively.

As a bare uranium nucleus is smaller than a hydrogen atom in diameter, but much larger in mass, it qualifies as one of the subatomics. Here, subatomic is used to mean smaller than the diameter of a hydrogen atom.

A neutron star is one nucleus surrounded by an electron cloud. But, it is much larger than a hydrogen atom in diameter.

## Galactic cosmic rays

Cosmic Ray Intensity (blue) and Sunspot Number (green) is shown from 1951 to 2006 Credit: University of New Hampshire.{{fairuse}}
Space weather conditions are associated with solar activity. Credit: Daniel Wilkinson.{{free media}}
Galactic cosmic rays (GCR) are displayed from 1951 to 2006. Credit: Jbo166.

The "effect of time-variations in galactic cosmic rays on the rate of production of neutrons in the atmosphere [was studied using] a series of balloon and airplane observations of the [fast neutron] flux and spectrum of 1-10 MeV neutrons, in flights at high geomagnetic latitude, during [quiet times as well as during Forbush decreases, which are rapid decreases in the observed galactic cosmic rays following a coronal mass ejection (CME), and solar particle events for] the period of increasing solar modulation, 1965-1969. It also included latitude surveys in 1964-1965 and in 1968."[2]

In the image on the right for Forbush decreases, data include GOES-15 X-rays, energetic particles, and magnetometer. Cosmic Rays from the Moscow station show a Forbush Decrease.

The graph on the right shows an inverse correlation between sunspot numbers (solar activity) and neutron production from galactic cosmic rays.

Notation: let the symbol Z stand for atomic number.

let the symbol PeV stand for 1015 electron volts.

"The most dominant group is the iron group (Z = 25 − 27), at energies around 70 PeV more than 50% of the all-particle flux consists of these elements."[3]

In the graph on the right, the black line is cosmic-ray data and the red line is temperature. Ulysses data is included.

## Ultra-high energy cosmic rays

The Oh-My-God particle was observed on the evening of 15 October 1991 over Dugway Proving Ground, Utah. Its observation was a shock to astrophysicists, who estimated its energy to be approximately 3×1020
eV
[4](50 joules)—in other words, a subatomic particle with kinetic energy equal to that of a baseball (142 g or 5 oz) traveling at 100 km/h (60 mph).

It was most probably a proton with a speed very close to the speed of light (approximately 0.9999999999999999999999951c), so close that in a year-long race between light and the cosmic ray, the ray would fall behind only 46 nanometers (5 x 10-24 light-years), or 0.15 femtoseconds (1.5 x 10-16 s).[5]

“The energy spectrum of cosmic rays extends to ~1020 eV (and smoothly to 1019).”[6]

## Cosmic rays

The flux of cosmic-ray particles is a function of their energy. Credit: Sven Lafebre, after Swordy.[7]

At right is an image indicating the range of cosmic-ray energies. The flux for the lowest energies (yellow zone) is mainly attributed to solar cosmic rays, intermediate energies (blue) to galactic cosmic rays, and highest energies (purple) to extragalactic cosmic rays.[7]

About 89% of cosmic rays are simple protons or hydrogen nuclei, 10% are helium nuclei or alpha particles, and 1% are the nuclei of heavier elements. Solitary electrons constitute much of the remaining 1%.

In cosmic-ray astronomy, cosmic rays are not charge balanced; that is, positive ions heavily outnumber electrons.

There is "a correlation between the arrival directions of cosmic rays with energy above 6 x 1019 electron volts and the positions of active galactic nuclei (AGN) lying within ~75 megaparsecs."[8]

## Anomalous cosmic rays

A mechanism is suggested for anomalous cosmic rays (ACRs) of the acceleration of pick-up ions at the solar wind termination shock. Credit: Eric R. Christian.{{fairuse}}

"While interstellar plasma is kept outside the heliosphere by an interplanetary magnetic field, the interstellar neutral gas flows through the solar system like an interstellar wind, at a speed of 25 km/sec. When closer to the Sun, these atoms undergo the loss of one electron in photo-ionization or by charge exchange. Photo-ionization is when an electron is knocked off by a solar ultra-violet photon, and charge exchange involves giving up an electron to an ionized solar wind atom. Once these particles are charged, the Sun's magnetic field picks them up and carries them outward to the solar wind termination shock. They are called pickup ions during this part of their trip."[9]

"The ions repeatedly collide with the termination shock, gaining energy in the process. This continues until they escape from the shock and diffuse toward the inner heliosphere. Those that are accelerated are then known as anomalous cosmic rays."[9]

"ACRs [may] represent a sample of the very local interstellar medium. They are not thought to have experienced such violent processes as GCRs, and they have a lower speed and energy. ACRs include large quantities of helium, oxygen, neon, and other elements with high ionization potentials, that is, they require a great deal of energy to ionize, or form ions. ACRs are a tool for studying the movement of energetic particles within the solar system, for learning the general properties of the heliosphere, and for studying the nature of interstellar material itself."[9]

## Solar energetic particles

Mean Fe charge states as a function of energy for the same event (in red) with overall mean charge state and test result for null-hypothesis (i.e. random distribution around mean). Credit: Zhangbo Guo, Eberhard Moebius, and Mark Popecki.{{fairuse}}
Charge state of Fe is a function of energy for an impulsive event in September 2000 in comparison with that for a CME-related event in June 1999 and the charge state of adjacent solar wind. Credit: Berndt Klecker and Eberhard Moebius.{{fairuse}}

"Earlier observations with ACE/SEPICA, SAMPEX/LICA, and SOHO/STOF have shown that highly ionized Fe in solar energetic particle (SEP) events (mean QFe > 14) is usually coupled with an increase of the mean charge state with energy in the range from 0.01 to 1 MeV/amu [...]. At the lowest energies the mean charge state of Fe is typically found to be well below QFe = 14. Recently, this has been demonstrated for all impulsive SEP events that were observed with SEPICA (DiFabio et al., ApJ, Nov 2008), indicating that the greater degree of ionization at higher energies was established by electron stripping in the low corona (e.g. Kartavykh et al., ApJ, 671, 947, 2007). However, observations of solar wind charge states have shown a widespread presence of QFe ≥ 16, associated with a hot plasma environment in solar wind from active regions and in interplanetary [Coronal Mass Ejections] CMEs (e.g. Lepri et al., JGR, 106, 29231, 2001; ACE News #52)."[10]

"Mean Fe charge states [in the figure on the right are] a function of energy for the same event (in red) with overall mean charge state and test result for null-hypothesis (i.e. random distribution around mean). Shown for comparison is an impulsive [Solar Energetic Particle] SEP event from June 2000 (in blue)."[10]

"Impulsive solar energetic particle events are well known for their dramatic over-abundances in 3He and heavy ions. ACE observations have extended these composition peculiarities to overabundances in the heavy isotopes of Ne and Mg."[11]

"The first charge-state measurements of impulsive events, averaged over all such events observed during one year with ISEE ULEZEQ, suggested that impulsive events feature rather high charge states with Q ≈ 20 for Fe and all elements up to Mg essentially fully stripped. These high charge states appeared to be well separated from the group of large, CME-related events with Q ≈ 14 for Fe."[11]

"With ACE SEPICA we have found that solar energetic particle events generally show a wide variety of mean charge states for Fe ranging from Q ≈ 10 continuously up to Q ≈ 20. Also, element abundance ratios appear to correlate with the ionic charge states (see ACE News #33). These two results seemed to present a puzzle, as the highest overabundances of heavy ions were observed for events with essentially fully-ionized ions up to Mg, which would not lend itself to an M/Q-based explanation for the observed fractionation. Therefore, it was suggested that fractionation and acceleration occur among lower charge state ions, with the final high charge states attained through stripping. This idea appears to be corroborated now by the observation of a very strong energy dependence of the iron charge states from 0.2 to 0.5 MeV/nuc with ACE SEPICA, a pattern that is even more pronounced when extended to ~0.01 MeV/nuc with the SOHO CELIAS STOF instrument."[11]

The "charge state of Fe [in the second figure down on the right is] a function of energy for an impulsive event in September 2000 in comparison with that for a CME-related event in June 1999 and the charge state of adjacent solar wind. Whereas the CME-related event shows Q ≈ 10 over the entire energy range, commensurate with that of the solar wind, in the impulsive event the charge state increases from Q ≈ 12 at low energies up to Q ≈ 17 at 0.5 MeV/nuc. This observation suggests that the original source material which is accelerated in these events has a much lower temperature than previously thought and is only partially ionized, thereby lending itself to M/Q fractionation. The sharp increase of the charge state with energy can be explained by electron stripping that increases with energy. This also implies that the acceleration in impulsive events occurs in the lower corona."[11]

## Ultra-heavy element nuclei

Absolute flux Φ0Z of cosmic–ray elements at E0 = 1 TeV/nucleus is plotted versus nuclear charge. Credit: Jörg R. Hörandel.{{fairuse}}

"The iron group and the ultra–heavy elements are more pronounced in cosmic rays as compared to the solar system. Especially the r–process elements beyond xenon (Z=54) are enhanced, partly due to spallation products of the platinum and lead nuclei (Z=78, 82). For the latter direct measurements at low energies around 1 GeV/n yield about a factor two more abundance as compared to the solar system and a factor of four for the actinides thorium and uranium (Z=90, 92) [66]. This has been attributed to the hypothesis that cosmic rays are accelerated out of supernova ejecta–enriched matter [67]."[12]

## Heavier element nuclei

The distribution of galactic cosmic-ray (GCR) particles is shown in atomic number (charge) and energy. Credit: W. Schimmerling, J. W. Wilson, F. Cucinotta, and M-H Y. Kim.{{fairuse}}

"These charged particles are hydrogen nuclei (protons), helium nuclei (α particles), and the nuclei of heavier elements such as iron and nickel."[13]

"Primary cosmic radiation mainly consists of the nuclei of atoms which have lost their electrons due to their extremely high velocity; these charged particles are hydrogen nuclei (protons), helium nuclei (alpha particles) and the nuclei of heavier elements such as iron and nickel; there are also some electrons (1%) and positrons (1‰)."[13]

"The relative abundances of GCR particles (9) are shown in [the figure on the right] (a), and typical energy spectra (10), are shown in [...] (b). The GCR particles of interest for radiation protection of crews engaged in space exploration range from protons (nuclei of hydrogen) to nuclei of iron; the abundances of heavier elements are orders of magnitude lower."[14]

Heavier element nuclei consist primarily of Li, Be, B, C, N, O, F, Ne, Na, Mg, Al, Si, P, S, Cl, Ar, K, Ca, Sc, Ti, V, Cr, Mn, Fe, Co and Ni.

"The two groups of elements Li, Be, B and Sc, Ti, V, Cr, Mn are many orders of magnitude more abundant in the cosmic radiation than in solar system material."[15]

## Irons

"Iron from outside the solar system has sprinkled down on Antarctica in recent years. Measurements of half a ton of snow turned up interstellar iron deposited within the last two decades [...]. That iron comes from the explosions of massive stars, or supernovas."[16]

"Within the snow, [...] 10 atoms of iron-60, a radioactive variety, or isotope, of iron with a total of 60 protons and neutrons in its nucleus [were isolated]. Previous studies have found iron-60, an isotope spewed from supernovas, in ocean sediments and on the moon [...]. But those depositions were a few million years old, and are thought to be the result of ancient nearby explosions blasting waves of debris through space."[16]

The "snow [was] transported [...] — still frozen thanks to careful packing and shipping — back to [the] lab. [It was] melted, filtered and evaporated [...], and [...] a technique called accelerator mass spectrometry [was used] on the remnants to identify iron-60."[16]

High-energy "particles called cosmic rays can create the isotope when they slam into dust in the solar system. To eliminate that explanation, [...] the amount of iron-60 in the snow [was compared] with another isotope produced by cosmic rays, manganese-53. The ratio of iron-60 to manganese-53 found was much higher than expected if both isotopes were produced by cosmic rays. The iron-60 might also have been the result of past nuclear weapons tests, but similar logic ruled out that option."[16]

"By the measurement of the cosmogenically produced radionuclide 53Mn, an atomic ratio of 60Fe/53Mn = 0.017 was found, significantly above cosmogenic production."[17]

"This gives us a clear indication that this stuff comes from outside of the solar system."[18]

"The solar system resides within a low-density pocket of gas, known as the local bubble. It’s thought that exploding supernovas created shock waves that blasted out that bubble."[16]

"The detection of recently deposited iron-60 suggests that the Local Interstellar Cloud may also have been sculpted by supernovas."[18]

"This is actually quite a profound thing. It’s telling us about the recent history of our whole neighborhood in the galaxy and about the lives and deaths of massive stars."[19]

## Aluminums

"The dominant reactions for making 26Al by [cosmic-ray] proton and α bombardment of refractory rocks in impulsive flares are 27Al(p, pn)26Al (β=0.92), 26Mg(p, n)26Al (β=1.0), 24Mg(α, pn)26Al (β=2.5 and yCR = 0.1), 28Si(p, 2pn)26Al (β=0.10), and 28Si(α, αpn)26Al (β=0.41)."[20]

## Neon nuclei

The ACE-CRIS measurements of the ratios 22Ne/20Ne and 21Ne/20Ne are plotted as a function of energy. Credit: W.R. Binns, M.E. Wiedenbeck, M. Arnould, A.C. Cummings, J.S. George, S. Goriely, M.H. Israel, R.A. Leske, R.A. Mewaldt, G. Meynet, L. M. Scott, E.C. Stone, and T.T. von Rosenvinge.{{fairuse}}

On the right, "the ACE-CRIS measurements of the ratios 22Ne/20Ne and 21Ne/20Ne are plotted as a function of energy. Abundances measured by other experiments (Wiedenbeck & Greiner 1981 [ISEE-3]; Lukasiak et al. 1994 [Voyager]; Connell & Simpson 1997 [Ulysses]; DuVernois et al. 1996 [CRRES]) are plotted as open symbols and the energy intervals for their measurements are shown as horizontal bars at the bottom of the figure."[21]

## Oxygen nuclei

Oxygen fluences were observed by the Advanced Composition Explorer (ACE). Credit: Richard Mewaldt, Caltech.

The fluences of oxygens in the galactic cosmic rays (GCRs) are plotted on the graph at right using data from the Cosmic Ray Isotope Spectrometer (CRIS) aboard the Advanced Composition Explorer (ACE). The fluences of solar 'cosmic rays' add to the GCRs at lower energy.

## Nitrogen nuclei

"For cosmic rays the low abundance ”valleys” in the solar system composition around Z=4, 21, 46, and 70 are not present. This is usually believed to be the result of spallation of heavier nuclei during their propagation through the galaxy. Hydrogen, helium, and the CNO–group are suppressed in cosmic rays. This has been explained by the high first ionization potential of these atoms [63] or by the high volatility of these elements which do not condense on interstellar grains [64]. Which property is the right descriptor of cosmic–ray abundances has proved elusive, however, the volatility seems to become the more accepted solution [65]."[12]

## Carbon nuclei

These "are nevertheless present in the cosmic radiation as spallation products of the abundant nuclei of carbon and oxygen (Li,Be,B) and of iron (Sc,Ti,V,Cr,Mn)."[15]

## Boron nuclei

Absolute boron and carbon fluxes multiplied by E2.7 as measured by PAMELA. Credit: O. Adriani, G. C. Barbarino, G. A. Bazilevskaya, R. Bellotti, M. Boezio, E. A. Bogomolov, M. Bongi, V. Bonvicini, S. Bottai, A. Bruno, F. Cafagna, D. Campana, R. Carbone, P. Carlson, M. Casolino, G. Castellini, I. A. Danilchenko, C. De Donato1, C. De Santis, N. De Simone, V. Di Felice, V. Formato, A. M. Galper, A. V. Karelin, S. V. Koldashov, S. Koldobskiy, S. Y. Krutkov, A. N. Kvashnin, A. Leonov, V. Malakhov, L. Marcelli, M. Martucci, A. G. Mayorov, W. Menn, M. Mergé, V. V. Mikhailov, E. Mocchiutti, A. Monaco, N. Mori, R. Munini, G. Osteria, F. Palma, B. Panico, P. Papini, M. Pearce, P. Picozza, C. Pizzolotto, M. Ricci, S. B. Ricciarini, L. Rossetto, R. Sarkar, V. Scotti, M. Simon, R. Sparvoli, P. Spillantini, Y. I. Stozhkov, A. Vacchi, E. Vannuccini, G. I. Vasilyev, S. A. Voronov, Y. T. Yurkin, G. Zampa, N. Zampa, and V. G. Zverev.{{fairuse}}

"In cosmic rays, both the isotopes 10B and 11B are present in comparable quantities."[22]

In the figure on the right are absolute boron and carbon fluxes multiplied by E2.7 as measured by PAMELA, together with results from other experiments (AMS02 Oliva et al. (2013), CREAM Ahn et al. (2008), TRACER Obermeier et al. (2011), ATIC-2 Panov et al. (2007), HEAO Engelmann et al. (1990), AMS01 Aguilar et al. (2010), CRN Swordy et al. (1990)) and a theoretical calculation based on GALPROP, as functions of kinetic energy per nucleon.

## Berylliums

The "presence in ... cosmic radiation [is] of a much greater proportion of "secondary" nuclei, such as lithium, beryllium and boron, than is found generally in the universe."[15]

## Lithium nuclei

The "evidence for the overwhelming majority of the Li-atoms in photospheres has its origin not only in nuclear synthesis near the stellar centers, but also by active processes in stellar atmospheres. [...] the lithium [resonance] line [is] near 478 keV."[23]

"Approximately 90% of lithium atoms originate from α - α reactions for the typical spectra of an accelerated particles on the Sun [...] During impulsive flares, interaction between the accelerated particles and the ambient medium occurs mainly at low altitudes, i.e., close to the footprints of loops."[23]

## Alpha particles

About 89% of cosmic rays are simple protons or hydrogen nuclei, 10% are helium nuclei or alpha particles, and 1% are the nuclei of heavier elements. Solitary electrons constitute much of the remaining 1%.

"Natural alpha decay takes place in heavy nuclei [...]. Each alpha decay leads to ΔΛ = 4, ΔZ = 2. Since this tends to move nuclei off the line of beta stability to the neutron-rich side, beta (-minus) decays are found in conjunction with alpha decays. There are thus four series (or chains) of alpha decays into which the natural alpha decays can be fitted; these correspond to Λ = 4n, 4n + 1, 4n + 2 and 4n + 3, where n is an integer.

4n
Thorium series: 232
Th
208
Pb
4n+1
Neptunium series: 237
Np
209
Bi
4n+2
Uranium series: 238
U
206
Pb
4n+3
Actinium series: 235
U
207
Pb
"[24]

"A decay starting with the heaviest nucleus in a series can continue down to the lightest, with a sequence of alpha and beta decays following roughly the line of stability."[24]

"It follows from conservation of energy and momentum that the kinetic energy of the alpha and the residual nucleus has a fixed value. Alpha particles are thus emitted with a sharply peaked spectrum."[24]

## Helions

An idealized image of protium shows the relative sizes of the proton and the atom. Credit: Bensaccount.

Def. a "nucleus of a helium-3 atom"[25] is called a helion.

Def. the "lightest and most common isotope of hydrogen, having a single proton and no neutrons- 1
1
H
"[26] is called protium.

Def. an "isotope of hydrogen formed of one proton and one neutron in each atom - 2
1
H
"[27] is called deuterium.

"Heavy water is “heavy” because it contains deuterium."[27]

"There were about 80 deuteriums for every million protiums, and virtually no tritium."[27]

Def. a "radioactive isotope of the element hydrogen, (symbol T or 3
1
H
), having one proton and two neutrons"[28] is called tritium.

Def. a "highly unstable, synthetic isotope of the element hydrogen, 4
1
H
, having one proton and three neutrons"[29] is called quadrium.

1
1
H
(p,β+ν)2
1
H

${\displaystyle \mathrm {_{1}^{1}H} +\mathrm {_{1}^{1}H} \rightarrow \mathrm {_{1}^{2}D} +e^{+}+\nu _{e}+\gamma (0.42MeV).}$

At 10-million-kelvin, hydrogen fuses to form helium in the proton-proton chain reaction:[30]

41
1
H
→ 22
1
H
+ 2e+ + 2νe (4.0 MeV + 1.0 MeV)
21
1
H
+ 22
1
H
→ 23
2
He
+ 2γ (5.5 MeV)
23
2
He
4
2
He
+ 21
1
H
(12.9 MeV)

These reactions result in the overall reaction:

41
1
H
4
2
He
+ 2e+ + 2γ + 2νe (26.7 MeV)

where e+ is a positron, γ is a gamma ray photon, νe is a neutrino, and H and He are isotopes of hydrogen and helium, respectively. The energy released by this reaction is in millions of electron volts, which is actually only a tiny amount of energy.

"The light elements deuterium, lithium, beryllium, and boron pose a special problem for any theory of the origin of the elements which proposes that all the elements are built up from hydrogen in the stars. ... The difficulty arises because the lifetimes of these elements against proton capture, at the temperatures and pressures at which most stellar matter exists, are short compared to the stable lifetimes of stars. These elements then cannot be produced in stellar interiors unless they are transported rapidly to the surface, and if they are produced at the surface, non-equilibrium processes must be involved. Further, they can exist in significant quantities at the surface only in the absence of rapid mixing to the interior."[31]

## Tritons

Energetic deuterons and tritons have been detected in solar flares.[32]

## Deuterons

"The flux [of deuterons in cosmic rays at a geomagnetic latitude of 7.6°N] is found to be 4 ± 1.3 M-2 sec-1 sterad-1".[33]

## Secondary cosmic rays

Def. cosmic rays that are created when primary cosmic rays interact with interstellar matter are called secondary cosmic rays.

Carbon and oxygen nuclei collide with interstellar matter to form lithium, beryllium and boron in a process termed cosmic ray spallation. Spallation is also responsible for the abundances of scandium, titanium, vanadium, and manganese ions in cosmic rays produced by collisions of iron and nickel nuclei with interstellar matter.

Observations of the lunar shadowing of galactic cosmic rays (GCRs) has demonstrated that there does not appear to be an antiproton component of the galactic cosmic rays, but the antiprotons detected are instead produced by the GCR interaction with interstellar hydrogen gas.[34]

For an interstellar medium "composed of 90% H and 10% He, [with a density of 0.3 atoms cm-3] and using the most recently measured cross sections (Webber, 1989; Ferrando et al., 1988b), the escape length has been found equal to 34βR-0.6 g cm-2 for rigidities R above 4.4 GV, and 14β g cm-2 below. ... where R and β are the interstellar values of the rigidity and the ratio of the velocity of the particle to the velocity of light."[35]

## Canal rays

In 1886, Eugen Goldstein discovered canal rays (also known as anode rays) and showed that they were positively charged particles (ions) produced from gases. However, since particles from different gases had different values of charge-to-mass ratio (e/m), they could not be identified with a single particle, unlike the negative electrons discovered by J. J. Thomson.

"Full multiple scattering theory must take account of the angular dependence of hadron-nucleon scattering, which affects the degree of screening."[15]

## Baryons

In "dense nuclear matter, such as neutron stars [it] has recently been discovered that kaon condensation in nuclear matter at a density of a few times normal nuclear matter may significantly reduce the upper mass limit of neutron stars [...] This clearly has an impact on astronomical observations. By exploiting the electron fermi level, we are able to predict kaon production at reasonable baryon number densities [...] Experimental detection of [dibaryons, hyperons] is a subtle matter [...] there is strong theoretical evidence that such states [as the dibaryon] do exist in nature. [...] the lightest dibaryon [...] is energetically stable against strong decay to [ΛΛ baryons] by 88 MeV. [The H dibaryon] is bound by 250 MeV."[36]

## Neutrons

The neutron probe is in the hole on the Moon. Credit: NASA.

Neutron astronomy deals with the study of astronomical neutron sources (such as stars, planets, comets, nebulae, star clusters and galaxies and phenomena that originate outside the Earth's atmosphere, such as cosmic rays.

"Due to nγ collisions of the ultrarelativistic neutrons with the submillimeter-IR photons, the neutrons with Lorentz factors Γ > Γesc [...] should degrade in the region r ≤ rmx responsible for the low-frequency radiation of [active galactic nuclei] AGN."[37]

The neutron is a subatomic hadron particle which has the symbol n or n0
, no net electric charge and a mass slightly larger than that of a proton.

Outside the nucleus, free neutrons are unstable and have a mean lifetime of 885.7±0.8 s (about 14 minutes, 46 seconds); therefore the half-life for this process (which differs from the mean lifetime by a factor of ln(2) = 0.693) is 613.9±0.8 s (about 10 minutes, 11 seconds).[38] Free neutrons decay by emission of an electron and an electron antineutrino to become a proton, a process known as beta decay:[39]

n0
=> p+
+ e
+ ν
e

Because free neutrons are unstable, they can be obtained only from nuclear disintegrations, nuclear reactions, and high-energy reactions (such as in cosmic radiation showers or accelerator collisions).

The neutron has a negatively charged exterior, a positively charged middle, and a negative core.[40]

## Antiprotons

The antiproton (p, pronounced p-baer) is the antiparticle of the proton. Antiprotons are stable, but they are typically short-lived since any collision with a proton will cause both particles to be annihilated in a burst of energy.

Antiprotons have been detected in cosmic rays for over 25 years, first by balloon-borne experiments and more recently by satellite-based detectors. The standard picture for their presence in cosmic rays is that they are produced in collisions of cosmic ray protons with nuclei in the interstellar medium, via the reaction, where A represents a nucleus:

p +  A → p + p + p + A

The secondary antiprotons (p) then propagate through the galaxy, confined by the galactic magnetic fields. Their energy spectrum is modified by collisions with other atoms in the interstellar medium. The antiproton cosmic ray energy spectrum is now measured reliably and is consistent with this standard picture of antiproton production by cosmic ray collisions.[41]

## Protons

The diagram shows one of the Van Allen Probes with various components and subsystems labeled. Credit: JHU/APL.

Proton astronomy per se often consists of directly or indirectly detecting the protons and deconvoluting a spatial, temporal, and spectral distribution.

"Proton astronomy should be possible; it may also provide indirect information on inter-galactic magnetic fields."[42]

"The Relativistic Proton Spectrometer (RPS) [measures] inner radiation belt protons with energies from 50 MeV-2 GeV. Such protons are known to pose a number of hazards to humans and spacecraft, including total ionizing dose, displacement damage, single event effects, and nuclear activation. The objectives of the investigation are to: (1) support the development of a new AP9/AE9 standard radiation model for spacecraft design; (2) to develop and test the model for RBSP data in general and RPS specifically; and, (3) to provide standardized worst-case specifications for dose rate, internal and deep dielectric chargins, and surface charging."[43]

The proton is a subatomic particle with the symbol p or p+
and a positive electric charge of 1 elementary charge. One or more protons are present in the nucleus of each atom, along with neutrons. The number of protons in each atom is its atomic number.

Nucleon spin structure describes the partonic structure of proton intrinsic angular momentum (spin). The key question is how the nucleon's spin, whose magnitude is 1/2ħ, is carried by its [suggested] constituent partons (quarks and gluons). In the late 1980s, the European Muon Collaboration (EMC) conducted experiments that suggested the spin carried by quarks is not sufficient to account for the total spin of [protons]. This finding astonished particle physicists at that time, and the problem of where the missing spin lies is sometimes referred to as the "proton spin crisis".

Experimental research on these topics has been continued by the Spin Muon Collaboration (SMC) and the COMPASS experiment at CERN, experiments E154 and E155 at [SLAC National Accelerator Laboratory] SLAC, HERMES at DESY, experiments at [Thomas Jefferson National Accelerator Facility] JLab and RHIC, and others. Global analysis of data from all major experiments confirmed the original EMC discovery and showed that the quark spin [may] contribute about 30% to the total spin of the nucleon.

New measurements performed by European scientists reveal that the radius of the proton is 4 percent smaller than previously estimated.[44]

## Mesons

Mesons are hadronic subatomic particles, bound together by the strong interaction. Because mesons are composed of sub-particles, they have a physical size, with a radius roughly one femtometre, which is about ​23 the size of a proton or neutron.

Charged mesons decay (sometimes through intermediate particles) to form electrons and neutrinos. Uncharged mesons may decay to photons.

Mesons are not produced by radioactive decay, but appear in nature only as short-lived products of very high-energy interactions in matter. In cosmic ray interactions, for example, such particles are ordinary protons and neutrons. Mesons are also frequently produced artificially in high-energy particle accelerators that collide protons, anti-protons, or other particles.

Mesons are subject to both the weak and strong interactions. Mesons with net electric charge also participate in the electromagnetic interaction.

While no meson is stable, those of lower mass are nonetheless more stable than the most massive mesons, and are easier to observe and study in particle accelerators or in cosmic ray experiments. They are also typically less massive than baryons, meaning that they are more easily produced in experiments, and thus exhibit certain higher energy phenomena more readily than baryons composed of the same quarks would.

Potential mesons to be detected astronomically include: π, ρ, η, η′, φ, ω, J/ψ, ϒ, θ, K, B, D, and T.

## B mesons

"The K0-K0 bar, D0-D0 bar, and B0-B0 bar oscillations are extremely sensitive to the K0 and K0 bar energy at rest. The energy is determined by the values mc2 with the related mass as well as the energy of the gravitational interaction. Assuming the CPT theorem for the inertial masses and estimating the gravitational potential through the dominant contribution of the gravitational potential of our Galaxy center, we obtain from the experimental data on the K0-K0 bar oscillations the following constraint: |(mg/mi)K0 - (mg/mi)K0 bar| ≤ 8·10-13, CL=90%. This estimation is model dependent and in particular it depends on a way we estimate the gravitational potential. Examining the K0-K0 bar, B0-B0 bar, and D0-D0 bar oscillations provides us also with weaker, but model independent constraints, which in particular rule out the very possibility of antigravity for antimatter."[45]

## Upsilon mesons

A plot of the invariant mass of muon pairs, the peak at about 9.5 GeV is due to the contribution of the Upsilon meson. Credit: Leon Lederman and the E288 collaboration, Fermilab.

The plot on the right shows a peak at about 9.5 GeV due to the Upsilon meson.

## Psions

J/Ψ production is graphed. Credit: Fermilab.

On the right is a graph of the production of psions at Fermilab.

## Omega mesons

Omega meson production:[46]

1. ${\displaystyle p+d\rightarrow He^{3}+\omega ,}$
2. ${\displaystyle {\bar {p}}+p\rightarrow \omega +\eta +\pi _{0},}$
3. ${\displaystyle \pi ^{-}+p\rightarrow \omega +n,}$
4. ${\displaystyle p+{\bar {p}}\rightarrow \mathrm {K} ^{+}+\mathrm {K} ^{-}+\omega ,}$
5. ${\displaystyle p+{\bar {p}}\rightarrow \mathrm {K} 1+\mathrm {K} 1+\omega ,}$

## Phi mesons

The phi meson ${\displaystyle \Phi ^{0}}$(1020) has a mass of 1019.445 MeV. It decays per[47]

1. ${\displaystyle \Phi ^{0}\rightarrow \mathrm {K} ^{+}+\mathrm {K} ^{-}or}$
2. ${\displaystyle \Phi ^{0}\rightarrow \mathrm {K} _{S}^{0}+\mathrm {K} _{L}^{0}.}$

## Rho mesons

Rho mesons occur in three states: ρ+, ρ-, and ρ0.[47] The rest masses are apparently the same at 775.4±0.4 and 775.49±0.34.[47] Decay products are π± + π0 or π+ + π-, respectively.[47]

## Eta mesons

Eta mesons (547.863 ± 0.018 MeV) have the decay schemes:[46]

1. η : ${\displaystyle \eta \rightarrow \gamma +\gamma ,}$
2. η : ${\displaystyle \eta \rightarrow \pi ^{0}+\pi ^{0}+\pi ^{0},or}$
3. η : ${\displaystyle \eta \rightarrow \pi ^{+}+\pi ^{0}+\pi ^{-},}$

Eta prime mesons (957.78 ± 0.06 MeV) have the decay schemes:[46]

1. η' : ${\displaystyle \eta ^{'}\rightarrow \pi ^{+}+\pi ^{-}+\eta or}$
2. η' : ${\displaystyle \eta ^{'}\rightarrow \pi ^{0}+\pi ^{0}+\gamma ,}$

The charmed eta meson ηC(1S) has a rest mass of 2983.6 ± 0.7 MeV.[46]

## D mesons

${\displaystyle D_{S}\rightarrow \tau +{\bar {\nu }}_{\tau }\rightarrow \nu _{\tau }+{\bar {\nu }}_{\tau }.}$[48]

## Kaons

"The muons created through decays of secondary pions and kaons are fully polarized, which results in electron/positron decay asymmetry, which in turn causes a difference in their production spectra."[49]

The "highest energy neutrinos from GRBs mainly come from kaons."[50]

## Pions

"The Gamma-Ray Spectrometer (GRS) on [Solar Maximum Mission] SMM has detected [...] at least two of the flares have spectral properties >40 MeV that require gamma rays from the decay of neutral pions. [Pion] production can occur early in the impulsive phase as defined by hard X-rays near 100 keV."[51]

"Neutral current single π0 production induced by neutrinos with a mean energy of 1.3GeV is measured at a 1000 ton water Cherenkov detector as a near detector of the K2K long baseline neutrino experiment."[52]

"The single π0 production rate by atmospheric neutrinos could be usable to distinguish between the νµ ↔ ντ and νµ ↔ νs oscillation hypotheses. The NC rate is attenuated in the case of transitions of νµ’s into sterile neutrinos, while it does not change in the νµ ↔ ντ scenario."[52]

Based on interactions between cosmic rays and the photons of the cosmic microwave background radiation (CMB), cosmic rays with energies over the threshold energy of 5x1019 eV interact with cosmic microwave background photons ${\displaystyle \gamma _{\rm {CMB}}}$ to produce pions via the ${\displaystyle \Delta }$ resonance,

${\displaystyle \gamma _{\rm {CMB}}+p\rightarrow \Delta ^{+}\rightarrow p+\pi ^{0},}$

or

${\displaystyle \gamma _{\rm {CMB}}+p\rightarrow \Delta ^{+}\rightarrow n+\pi ^{+}.}$

Pions produced in this manner proceed to decay in the standard pion channels—ultimately to photons for neutral pions, and photons, positrons, and various neutrinos for positive pions. Neutrons decay also to similar products, so that ultimately the energy of any cosmic ray proton is drained off by production of high energy photons plus (in some cases) high energy electron/positron pairs and neutrino pairs.

The pion production process begins at a higher energy than ordinary electron-positron pair production (lepton production) from protons impacting the CMB, which starts at cosmic ray proton energies of only about 1017eV. However, pion production events drain 20% of the energy of a cosmic ray proton as compared with only 0.1% of its energy for electron positron pair production. This factor of 200 is from two sources: the pion has only about ~130 times the mass of the leptons, but the extra energy appears as different kinetic energies of the pion or leptons, and results in relatively more kinetic energy transferred to a heavier product pion, in order to conserve momentum. The much larger total energy losses from pion production result in the pion production process becoming the limiting one to high energy cosmic ray travel, rather than the lower-energy light-lepton production process.

## Tauons

"For ultrahigh energies the neutrino spectrum at the detector is influenced by neutrino-nucleon interactions and tauon decays during the passage through the interior of the earth."[53]

## Muons

The Moon's cosmic ray shadow, as seen in secondary muons generated by cosmic rays in the atmosphere, and detected 700 meters below ground, at the Soudan II detector. Credit: Deglr6328.

"TeV muons from γ ray primaries ... are rare because they are only produced by higher energy γ rays whose flux is suppressed by the decreasing flux at the source and by absorption on interstellar light."[54]

"[T]here is a window of opportunity for muon astronomy with the AMANDA, Lake Baikal, and MILAGRO detectors."[54]

"The muons created through decays of secondary pions and kaons are fully polarized, which results in electron/positron decay asymmetry, which in turn causes a difference in their production spectra."[49]

## Neutrinos

The diagram contains the reactions in the proton-proton chain including neutrino production. Credit: Dorottya Szam.

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. [55]

## Beta particles

This graph is a chart of the nuclides for carbon to fluorine. Decay modes:

Credit: original: National Nuclear Data Center, stitched: Neokortex, cropped: Limulus.

Beta particles are high-energy, high-speed electrons or positrons emitted by certain types of radioactive nuclei such as potassium-40. The beta particles emitted are a form of ionizing radiation also known as beta rays. The production of beta particles is termed beta decay. They are designated by the Greek letter beta (β).

At right is a graph or block diagram that shows the boundaries for nuclear particle stability. The boundaries are conceptualized as drip lines. The nuclear landscape is understood by plotting boxes, each of which represents a unique nuclear species, on a graph with the number of neutrons increasing on the abscissa and number of protons increasing along the ordinate, which is commonly referred to as the table of nuclides, being to nuclear physics what the more commonly known periodic table of the elements is to chemistry. However, an arbitrary combination of protons and neutrons does not necessarily yield a stable nucleus, and ultimately when continuing to add more of the same type of nucleons to a given nucleus, the newly formed nucleus will essentially undergo immediate decay where a nucleon of the same isospin quantum number (proton or neutron) is emitted; colloquially the nucleon has 'leaked' or 'dripped' out of the target nucleus, hence giving rise to the term "drip line". The nucleons drip out of such unstable nuclei for the same reason that water drips from a leaking faucet: the droplet, or nucleon in this case, sees a lower potential which is great enough to overcome surface tension in the case of water droplets, and the strong nuclear force in the case of proton emission or alpha decay. As nucleons are quantized, then only integer values are plotted on the table of isotopes, indicating that the drip line is not linear but instead looks like a step function up close.

Beta particles (electrons) are more penetrating than alpha particles, but still can be absorbed by a few millimeters of aluminum. However, in cases where high energy beta particles are emitted shielding must be accomplished with low density materials, e.g. plastic, wood, water or acrylic glass (Plexiglas, Lucite). This is to reduce generation of Bremsstrahlung X-rays. In the case of beta+ radiation (positrons), the gamma radiation from the electron-positron annihilation reaction poses additional concern.

As an example, "[t]he power into the Crab Nebula is apparently supplied by an outflow [wind] of ~1038 erg/s from the pulsar"[56] where there are "electrons (and positrons) in such a wind"[56]. These beta particles coming out of the pulsar are moving very close to light speed.

"[M]odels in which γ-rays are absorbed in collisions with X-rays producing nonthermal electron-positron pairs, which in turn radiate further X-rays [have been developed]."[57]

"[T]he reprocessing of radiation by e+ e- pairs could be a sufficiently robust mechanism to yield the canonical spectrum, independent of the details of the particle acceleration mechanism and the parameters of the source, such as the X- and γ-ray luminosity, L, and the size, R."[57]

"[T]he hard X-ray spectrum of a growing number of [active galactic nuclei] AGN [in] the 1-30 keV X-ray emission has four distinct components":[57]

1. "an incident power law spectrum with a spectral index αix ≃ 0.9,"[57]
2. "an emission line at the energy ~6.4 keV (interpreted as a fluorescent iron K-line),"[57]
3. "an absorption edge at 7-8 keV (interpreted as an iron K-edge), and"[57]
4. "a broad excess of emission with respect to the underlying power law at energies ≳ 10 keV (interpreted as Compton reflection from cold [T < 106 K, optically thick] material)." [57]

## Electrons

Beam of electrons are moving in a circle in a magnetic field (cyclotron motion). Lighting is caused by excitation of atoms of gas in a bulb. Credit: Marcin Białek.

Although electron astronomy is usually not recognized as a formal branch of astronomy, the measurement of electron fluxes helps to understand a variety of natural phenomena.

""[E]lectron astronomy" has an interesting future".[58]

"Electron beams can be generated by thermionic emission, field emission or the anodic arc method. The generated electron beam is accelerated to a high kinetic energy and focused towards the [target]. When the accelerating voltage is between 20 kV – 25 kV and the beam current is a few amperes, 85% of the kinetic energy of the electrons is converted into thermal energy as the beam bombards the surface of the [target]. The surface temperature of the [target] increases resulting in the formation of a liquid melt. Although some of incident electron energy is lost in the excitation of X-rays and secondary emission, the [target] material evaporates under vacuum."[59]

The "emission phenomena observed in active galactic nuclei [includes] the production of compact radio sources separating at superluminal speeds".[60]

Outbursts "of cosmic ray electrons from the Galactic Center [may] penetrate the Galaxy relatively undamped and [may be able] to have a major impact on the Solar System through their ability to vaporize and inject cometary material into the interplanetary environment. [One] such 'superwave', passing through the Solar System toward the end of the Last Ice Age, [may have been] responsible for producing major changes in the Earth's climate and for indirectly precipitating the terminal Pleistocene extinction episode. The high concentration of 10Be, NO3-, Ir and Ni observed in Late Wisconsin polar ice are consistent with this scenario."[60]

## Delta rays

A delta ray is characterized by very fast electrons produced in quantity by alpha particles or other fast energetic charged particles knocking orbiting electrons out of atoms. Collectively, these electrons are defined as delta radiation when they have sufficient energy to ionize further atoms through subsequent interactions on their own.

"The conventional procedure of delta-ray counting to measure charge (Powell, Fowler, and Perkins 1959), which was limited to resolution sigmaz = 1-2 because of uncertainties of the criterion of delta-ray ranges, has been significantly improved by the application of delta-ray range distribution measurements for 16O and 32S data of 200 GeV per nucleon (Takahashi 1988; Parnell et al. 1989)."[61] Here, the delta-ray tracks in emulsion chambers have been used for "[d]irect measurements of cosmic-ray nuclei above 1 TeV/nucleon ... in a series of balloon-borne experiments".[61]

## Epsilon rays

Epsilon radiation is tertiary radiation caused by secondary radiation (e.g., delta radiation). Epsilon rays are a form of particle radiation and are composed of electrons. The term is very rarely used today.

## Positrons

Positron astronomy results have been obtained using the INTEGRAL spectrometer SPI shown. Credit: Medialab, ESA.

"Positron astronomy is 30 years old but remains in its infancy."[62]

In 2009, the Fermi Gamma Ray Telescope in Earth orbit observed an intense burst of gamma rays corresponding to positron annihilations coming out of a storm formation. Scientists wouldn't have been surprised to see a few positrons accompanying any intense gamma ray burst, but the lightning flash detected by Fermi appeared to have produced about 100 trillion positrons. This has been reported by media in January 2011, it is an effect, never considered to happen before.[63]

"The positrons can annihilate in flight before being slowed to thermal energies, annihilate directly with electrons when both are at thermal energies, or form positronium at thermal energies (or at greater than thermal energies if positronium formation occurs via charge exchange with neutrals)."[64]

"Positrons entering a gaseous medium at [0.6 to 4.5 MeV] are quickly slowed by ionizing collisions with neutral atoms and by long-range Coulomb interactions with any ionized component."[64]

## Nuclear transmutations

This graph shows positron emissions, among others, from nuclear transmutation. Credit: .

If the proton and neutron are part of an atomic nucleus, these decay processes transmute one chemical element into another. For example:

${\displaystyle _{Z}^{A}N\rightarrow ~_{Z-1}^{~~~A}N'+e^{+}+\nu _{e},}$

where A = 22, Z = 11, N = Na, Z-1 = 10, and N' = Ne.

Beta decay does not change the number of nucleons, A, in the nucleus but changes only its charge, Z. Thus the set of all nuclides with the same A can be introduced; these isobaric nuclides may turn into each other via beta decay. Among them, several nuclides (at least one) are beta stable, because they present local minima of the mass excess: if such a nucleus has (A, Z) numbers, the neighbour nuclei (A, Z−1) and (A, Z+1) have higher mass excess and can beta decay into (A, Z), but not vice versa. For all odd mass numbers A the global minimum is also the unique local minimum. For even A, there are up to three different beta-stable isobars experimentally known. There are about 355 known beta-decay stable nuclides total.

In β+
decay, or "positron emission", the weak interaction converts a nucleus into its next-lower neighbor on the periodic table while emitting an positron (e+
) and an electron neutrino (ν
e
):

${\displaystyle _{Z}^{A}N\rightarrow ~_{Z-1}^{~~~A}N'+e^{+}+\nu _{e}.}$

β+
decay cannot occur in an isolated proton because it requires energy due to the mass of the neutron being greater than the mass of the proton. β+
decay can only happen inside nuclei when the value of the binding energy of the mother nucleus is less than that of the daughter nucleus. The difference between these energies goes into the reaction of converting a proton into a neutron, a positron and a neutrino and into the kinetic energy of these particles.

Positron emission' or beta plus decay (β+ decay) is a type of beta decay in which a proton is converted, via the weak force, to a neutron, releasing a positron and a neutrino.

Isotopes which undergo this decay and thereby emit positrons include carbon-11, potassium-40, nitrogen-13, oxygen-15, fluorine-18, and iodine-121. As an example, the following equation describes the beta plus decay of carbon-11 to boron-11, emitting a positron and a neutrino:

${\displaystyle _{6}^{11}C\rightarrow ~_{5}^{11}B+e^{+}+\nu _{e}+\gamma {(0.96MeV)}.}$

## Positroniums

An electron and positron orbit around their common centre of mass. This is a bound quantum state known as positronium. Credit: Manticorp.

Def. an exotic atom consisting of a positron and an electron, but having no nucleus or an onium consisting of a positron (anti-electron) and an electron, as a particle–anti-particle bound pair is called positronium.

Being unstable, the two particles annihilate each other to produce two gamma ray photons after an average lifetime of 125 ps or three gamma ray photons after 142 ns in vacuum, depending on the relative spin states of the positron and electron.

The singlet state with antiparallel spins ([spin quantum number] S = 0, Ms = 0) is known as para-positronium (p-Ps) and denoted 1
S
0
. It has a mean lifetime of 125 picoseconds and decays preferentially into two gamma quanta with energy of 511 keV each (in the center of mass frame). Detection of these photons allows for the reconstruction of the vertex of the decay. Para-positronium can decay into any even number of photons (2, 4, 6, ...), but the probability quickly decreases as the number increases: the branching ratio for decay into 4 photons is 1.439(2)×106
.[65]

${\displaystyle t_{0}={\frac {2\hbar }{m_{e}c^{2}\alpha ^{5}}}=1.244\times 10^{-10}\;{\text{s}}}$

The triplet state with parallel spins (S = 1, Ms = −1, 0, 1) is known as ortho-positronium (o-Ps) and denoted 3S1. The triplet state in vacuum has a mean lifetime of 142.05±0.02 ns[66] and the leading mode of decay is three gamma quanta. Other modes of decay are negligible; for instance, the five photons mode has branching ratio of ~1.0×106
.[67]

${\displaystyle t_{1}={\frac {{\frac {1}{2}}9h}{2m_{e}c^{2}\alpha ^{6}(\pi ^{2}-9)}}=1.386\times 10^{-7}\;{\text{s}}}$

## Annihilations

Naturally occurring electron-positron annihilation is a result of beta plus decay. Credit: .
A Germanium detector spectrum shows the annihilation radiation peak (under the arrow). Note the width of the peak compared to the other gamma rays visible in the spectrum. Credit: Hidesert.

The positron or antielectron is the antiparticle or the antimatter counterpart of the electron. The positron has an electric charge of +1e, a spin of ½, and has the same mass as an electron. When a low-energy positron collides with a low-energy electron, annihilation occurs, resulting in the production of two or more gamma ray photons.

Def. the process of a particle and its corresponding antiparticle combining to produce energy is called annihilation.

The figure at right shows a positron (e+) emitted from an atomic nucleus together with a neutrino (v). Subsequently, the positron moves randomly through the surrounding matter where it hits several different electrons (e-) until it finally loses enough energy that it interacts with a single electron. This process is called an "annihilation" and results in two diametrically emitted photons with a typical energy of 511 keV each. Under normal circumstances the photons are not emitted exactly diametrically (180 degrees). This is due to the remaining energy of the positron having conservation of momentum.

Electron–positron annihilation occurs when an electron (e
) and a positron (e+
, the electron's antiparticle) collide. The result of the collision is the annihilation of the electron and positron, and the creation of gamma ray photons or, at higher energies, other particles:

e
+ e+
→ γ + γ

The process [does] satisfy a number of conservation laws, including:

As with any two charged objects, electrons and positrons may also interact with each other without annihilating, in general by elastic scattering.

The creation of only one photon can occur for tightly bound atomic electrons.[68] In the most common case, two photons are created, each with energy equal to the rest energy of the electron or positron (511 keV).[69] It is also common for three to be created, since in some angular momentum states, this is necessary to conserve C parity.[70] Any larger number of photons [can be created], but the probability becomes lower with each additional photon. When either the electron or positron, or both, have appreciable kinetic energies, other heavier particles can also be produced (such as D mesons), since there is enough kinetic energy in the relative velocities to provide the rest energies of those particles. Photons and other light particles may be produced, but they will emerge with higher energies.

At energies near and beyond the mass of the carriers of the weak force, the W and Z bosons, the strength of the weak force becomes comparable with electromagnetism.[70] It becomes much easier to produce particles such as neutrinos that interact only weakly.

The heaviest particle pairs yet produced by electron–positron annihilation are W+
W
pairs. The heaviest single particle is the Z boson.

Annihilation radiation is not monoenergetic, unlike gamma rays produced by radioactive decay. The production mechanism of annihilation radiation introduces Doppler broadening.[71] The annihilation peak produced in a gamma spectrum by annihilation radiation therefore has a higher full width at half maximum (FWHM) than other gamma rays in [the] spectrum. The difference is more apparent with high resolution detectors, such as Germanium detectors, than with low resolution detectors such as Sodium iodide. Because of their well-defined energy (511 keV) and characteristic, Doppler-broadened shape, annihilation radiation can often be useful in defining the energy calibration of a gamma ray spectrum.

## Pair production

The reverse reaction, electron–positron creation, is a form of pair production governed by two-photon physics.

Two-photon physics, also called gamma-gamma physics, [studies] the interactions between two photons. If the energy in the center of mass system of the two photons is large enough, matter can be created.[72]

γ → e
+ e+

In nuclear physics, [the above reaction] occurs when a high-energy photon interacts with a nucleus. The photon must have enough energy [> 2*511 keV, or 1.022 MeV] to create an electron plus a positron. Without a nucleus to absorb momentum, a photon decaying into electron-positron pair (or other pairs for that matter such as a muon and anti-muon or a tau and anti-tau can never conserve energy and momentum simultaneously.[73]

These interactions were first observed in Patrick Blackett's counter-controlled cloud chamber. In 2008 the Titan laser aimed at a 1-millimeter-thick gold target was used to generate positron–electron pairs in large numbers.[74] "The LLNL scientists created the positrons by shooting the lab's high-powered Titan laser onto a one-millimeter-thick piece of gold."[74]

## Submillimeters

"Radio observations at 210 GHz taken by the Bernese Multibeam Radiometer for KOSMA (BEMRAK) [of] high-energy particle acceleration during the energetic solar flare of 2003 October 28 [...] at submillimeter wavelengths [reveal] a gradual, long-lasting (>30 minutes) component with large apparent source sizes (~60"). Its spectrum below ~200 GHz is consistent with synchrotron emission from flare-accelerated electrons producing hard X-ray and γ-ray bremsstrahlung assuming a magnetic field strength of ≥200 G in the radio source and a confinement time of the radio-emitting electrons in the source of less than 30 s. [... There is a] close correlation in time and space of radio emission with the production of pions".[75]

## Solar neutrinos

This "neutrino image" of the Sun is produced by using the Super-Kamiokande to detect the neutrinos from nuclear fusion coming from the Sun. Credit: R. Svoboda and K. Gordan (LSU).

Neutrinos are hard to detect. The Super-Kamiokande, or "Super-K" is a large-scale experiment constructed in an unused mine in Japan to detect and study neutrinos. The image at right required 500 days worth of data to produce the "neutrino image" of the Sun. The image is centered on the Sun's position. It covers a 90° x 90° octant of the sky (in right ascension and declination). The higher the brightness of the color, the larger is the neutrino flux.

"The detection of solar neutrinos demonstrates that fusion energy is the basic source of energy received from the sun."[76]

In detecting solar neutrinos, it became clear that the number detected was half or a third than that predicted by models of the solar interior. The problem was solved by revising the properties of neutrinos and understanding the limits of the detection mechanisms - only one third of the forms of neutrinos coming in was being detected and all neutrinos oscillate between the three forms.

The first experiment to detect the effects of neutrino oscillation was Ray Davis's Homestake Experiment in the late 1960s, in which he observed a deficit in the flux of solar neutrinos with respect to the prediction of the Standard Solar Model, using a chlorine-based detector. This gave rise to the Solar neutrino problem. Many subsequent radiochemical and water Cherenkov detectors confirmed the deficit, but neutrino oscillation was not conclusively identified as the source of the deficit until the Sudbury Neutrino Observatory provided clear evidence of neutrino flavor change in 2001. Solar neutrinos have energies below 20 MeV and travel an astronomical unit between the source in the Sun and detector on the Earth. At energies above 5 MeV, solar neutrino oscillation actually takes place in the Sun through a resonance known as the Mikheyev–Smirnov–Wolfenstein effect (MSW) effect, a different process from the vacuum oscillation.

Most neutrinos passing through the Earth emanate from the Sun. About 65 billion (6.5 x 1010) solar neutrinos per second pass through every square centimeter perpendicular to the direction of the Sun in the region of the Earth.[77]

The Mikheyev Smirnov Wolfenstein (MSW) effect is important at the very large electron densities of the Sun where electron neutrinos are produced. The high-energy neutrinos seen, for example, in the Sudbury Neutrino Observatory (SNO) and in Super-Kamiokande, are produced mainly as the higher mass eigenstate in matter ν2m, and remain as such as the density of solar material changes. (When neutrinos go through the MSW resonance the neutrinos have the maximal probability to change their nature, but it happens that this probability is negligibly small—this is sometimes called propagation in the adiabatic regime). Thus, the neutrinos of high energy leaving the sun are in a vacuum propagation eigenstate, ν2, that has a reduced overlap with the electron neutrino νe = ν1 cosθ + ν2 sinθ seen by charged current reactions in the detectors.

For high-energy solar neutrinos the MSW effect is important, and leads to the expectation that Pee = sin²θ, where θ = 34° is the solar mixing angle. This was dramatically confirmed in the Sudbury Neutrino Observatory (SNO), which has resolved the solar neutrino problem. SNO measured the flux of Solar electron neutrinos to be ~34% of the total neutrino flux (the electron neutrino flux measured via the charged current reaction, and the total flux via the neutral current reaction). The SNO results agree well with the expectations.

For the low-energy solar neutrinos, on the other hand, the matter effect is negligible, and the formalism of oscillations in vacuum is valid. The size of the source (i.e. the Solar core) is significantly larger than the oscillation length, therefore, averaging over the oscillation factor, one obtains Pee = 1 − sin²2θ / 2. For the same value of the solar mixing angle (θ = 34°) this corresponds to a survival probability of Pee ≈ 60%. This is consistent with the experimental observations of low energy Solar neutrinos by the Homestake experiment (the first experiment to reveal the solar neutrino problem), followed by GALLEX, the Gallium Neutrino Observatory (GNO), and Soviet–American Gallium Experiment (SAGE) (collectively, gallium radiochemical experiments), and, more recently, the Borexino experiment. These experiments provided further evidence of the MSW effect.

The transition between the low energy regime (the MSW effect is negligible) and the high energy regime (the oscillation probability is determind by matter effects) lies in the region of about 2 MeV for the Solar neutrinos.

## Interstellars

As of December 5, 2011, "Voyager 1 is about ... 18 billion kilometers ... from the [S]un [but] the direction of the magnetic field lines has not changed, indicating Voyager is still within the heliosphere ... the outward speed of the solar wind had diminished to zero in April 2010 ... inward pressure from interstellar space is compacting [the magnetic field] ... Voyager has detected a 100-fold increase in the intensity of high-energy electrons from elsewhere in the galaxy diffusing into our solar system from outside ... [while] the [solar] wind even blows back at us."[78]

"In the first 18 months of operations, AMS-02 [image under Cherenkov detectors] recorded 6.8 million positron (an antimatter particle with the mass of an electron but a positive charge) and electron events produced from cosmic ray collisions with the interstellar medium in the energy range between 0.5 giga-electron volt (GeV) and 350 GeV. These events were used to determine the positron fraction, the ratio of positrons to the total number of electrons and positrons. Below 10 GeV, the positron fraction decreased with increasing energy, as expected. However, the positron fraction increased steadily from 10 GeV to 250 GeV. This increase, seen previously though less precisely by instruments such as the Payload for Matter/antimatter Exploration and Light-nuclei Astrophysics (PAMELA) and the Fermi Gamma-ray Space Telescope, conflicts with the predicted decrease of the positron fraction and indicates the existence of a currently unidentified source of positrons, such as pulsars or the annihilation of dark matter particles. Furthermore, researchers observed an unexpected decrease in slope from 20 GeV to 250 GeV. The measured positron to electron ratio is isotropic, the same in all directions."[79]

## Hypotheses

1. Each extant subatomic particle may help to understand a radiation source.

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