# Neutron astronomy

The neutron probe is in the hole on the Moon. Credit: NASA.
 Completion status: About halfway there. You may help to clarify and expand it.

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.

 Development status: this resource is experimental in nature.

It is concerned with the evolution, physics, chemistry, meteorology, and motion of astronomical objects, as well as the physical cosmology (the formation and development) of the universe.

 Educational level: this is a secondary education resource.

Because of the short half-life of neutrons outside of the nucleus, neutron astronomy is often restricted to nearby objects such as the Earth, the Moon, Mars, and the Sun.

 Educational level: this is a tertiary (university) resource.
 Educational level: this is a research resource.
 Resource type: this resource is an article.
 Resource type: this resource contains a lecture or lecture notes.
 Subject classification: this is an astronomy resource.
 Subject classification: this is an astrophysics resource.

# Notation

Notation: let the symbol Def. indicate that a definition is following.

Notation: let the symbols between [ and ] be replacement for that portion of a quoted text.

Notation: let the symbol ... indicate unneeded portion of a quoted text.

Sometimes these are combined as [...] to indicate that text has been replaced by ....

# Universals

To help with definitions, their meanings and intents, there is the learning resource theory of definition.

Def. evidence that demonstrates that a concept is possible is called proof of concept.

The proof-of-concept structure consists of

1. background,
2. procedures,
3. findings, and
4. interpretation.[1]

The findings demonstrate a statistically systematic change from the status quo or the control group.

# Control group

A control group for neutron astronomy may include a laboratory neutron detector or monitor operating at levels expected in or from the interplanetary medium.

# Astronomy

Around EeV (1018 eV) energies there may be associated ultra high energy neutrons “observed in anisotropic clustering ... because of the relativistic neutrons boosted lifetime.”[2] “[A]t En = 1020 eV, [these neutrons] are flying a Mpc, with their directional arrival (or late decayed proton arrival) ... more on-line toward the source.”[2] Although “neutron (and anti-neutron) life-lengths (while being marginal or meaningless at tens of Mpcs, the growth of their half-lives with energy may naturally explain an associated, showering neutrino halo.”[2]

The principal component of radiation through great thicknesses of shielding (such as concrete or regolith) consists of neutrons in the very high energy range (above 50 MeV) associated with a 20 GeV synchrotron.[3]

# Neutrons

“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."[4]

“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).[5] Free neutrons decay by emission of an electron and an electron antineutrino to become a proton, a process known as beta decay:[6][4]

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).

“[T]he neutron has a negatively charged exterior, a positively charged middle, and a negative core.[7]”.[4]

# Planetary science

"The nuclear processes that produce cosmogenic 36Cl in rocks are spallation, neutron capture, and muon capture. The first two processes dominate production on the land surface; muon production in Ca and K becomes more important with increasing depth (Rama and Honda, 1961)."[8]

"The decay of radioactive U and Th also give rise to the production of 36Cl, via neutron capture (Bentley et al., 1986)."[8]

"The production rate of cosmogenic 36Cl in bedrock and regolith exposed at Earth's surface is dependent on its calcium, potassium, and chloride content and can be expressed by the equation

$P = \psi_{Ca}(C_{Ca}) + \psi_K(C_K) + \psi_n(\sigma_{35}N_{35}/\Sigma \sigma_iN_i),$

where $\psi_K$ and $\psi_{Ca}$ are the total production rates (including production due to slow negative muons) of 36Cl due to potassium and calcium, respectively; $C_K$ and $C_{Ca}$ are the elemental concentrations of potassium and calcium, respectively; and $\psi_n$ is the thermal neutron capture rate, which is dependent on the fraction of neutrons stopped by 35Cl $(\sigma_{35} N_{35}/\Sigma \sigma_i N_i),$ as determined by the effective cross sections of 35Cl$(\sigma_{35})$ and all other absorbing elements $(\Sigma \sigma)$ and their respective abundances $(N_{35}$ and $N_i)$."[8]

# Colors

"The neutron detection temperature, also called the neutron energy, indicates a free neutron's kinetic energy, usually given in electron volts. The term temperature is used, since hot, thermal and cold neutrons are moderated in a medium with a certain temperature. The neutron energy distribution is then adopted to the Maxwellian distribution known for thermal motion. Qualitatively, the higher the temperature, the higher the kinetic energy is of the free neutron. Kinetic energy, speed and wavelength of the neutron are related through the De Broglie relation."[9]

"Moderated and other, non-thermal neutron energy distributions or ranges are ...

• Fast neutrons [with] kinetic energies greater than 1 eV, 0.1 MeV or approximately 1 MeV, depending on the definition.
• Slow neutrons ... a kinetic energy less than or equal to 0.4 eV.
• Epithermal neutrons ... an energy from 1 eV to 10 keV.
• Hot neutrons ... an energy of about 0.2 eV.
• Thermal neutrons ... an energy of about 0.025 eV.[10] This is the most probable energy, while the average energy is 0.038 eV.
• Cold neutrons ... an energy from 5 × 10−5 eV to 0.025 eV.
• Very cold neutrons ... an energy from 3 × 10−7 eV to 5 × 10−5 eV.
• Ultra cold neutrons ... an energy less than 3 × 10−7 eV.
• Continuum region neutrons ... an energy from 0.01 MeV to 25 MeV.
• Resonance region neutrons ... an energy from 1 eV to 0.01 MeV.
• Low energy region neutrons ... an energy less than 1 eV."[9]

# Minerals

The geological situation in Gabon leading to natural nuclear fission reactors is described
1. Nuclear reactor zones
2. Sandstone
3. Uranium ore layer
4. Granite. Credit: .

"A natural nuclear fission reactor is a uranium [mineral] deposit where self-sustaining nuclear chain reactions have occurred. This can be examined by analysis of isotope ratios. The existence of this phenomenon was discovered in 1972 at Oklo in Gabon, Africa [...] Oklo is the only known location for this in the world and consists of 16 sites at which self-sustaining nuclear fission reactions took place approximately 1.7 billion years ago, and ran for a few hundred thousand years, averaging 100 kW of thermal power during that time.[11][12]"[13]

"Gadolinium as a metal or salt has exceptionally high absorption of neutrons and therefore is used for shielding in neutron radiography and in nuclear reactors."[14]

"The report by Hoffman et al. (1971) of 8.2 x 107 y 244Pu in terrestrial bastnesite is supported by some unpublished evidence at Argonne National Laboratory for 244Pu in terrestrial gadolinite (Metta et al., 1971)."[15]

# Sources

Neutron emitters to left of lower dashed line
Z → 0 1 2
n ↓ n H He 3 4
0 1H Li Be 5 6
1 1n 2H 3He 4Li 5Be B C 7
2 2n 3H 4He 5Li 6Be 7B 8C N 8
3 4H 5He 6Li 7Be 8B 9C 10N O 9
4 4n 5H 6He 7Li 8Be 9B 10C 11N 12O F 10
5 6H 7He 8Li 9Be 10B 11C 12N 13O 14F Ne 11
6 7H 8He 9Li 10Be 11B 12C 13N 14O 15F 16Ne Na 12
7 9He 10Li 11Be 12B 13C 14N 15O 16F 17Ne 18Na Mg 13
8 10He 11Li 12Be 13B 14C 15N 16O 17F 18Ne 19Na 20Mg Al 14
9 12Li 13Be 14B 15C 16N 17O 18F 19Ne 20Na 21Mg 22Al Si
10 14Be 15B 16C 17N 18O 19F 20Ne 21Na 22Mg 23Al 24Si
11 16B 17C 18N 19O 20F 21Ne 22Na 23Mg 24Al 25Si
12 18C 19N 20O 21F 22Ne 23Na 24Mg 25Al 26Si
13 20N 21O 22F 23Ne 24Na 25Mg
26Al
27Si
14 22O 23F 24Ne 25Na 26Mg 27Al 28Si

“Neutrons are produced when alpha particles impinge upon any of several low atomic weight isotopes including isotopes of lithium, beryllium, carbon and oxygen."[16]

Gamma radiation with an energy exceeding the neutron binding energy of a nucleus can eject a neutron. Two examples and their decay products:”[16]

9Be + >1.7 Mev photon → 1 neutron + 2 4He
2H (deuterium) + >2.26 MeV photon → 1 neutron + 1H

“Traditional particle accelerators with hydrogen (H), deuterium (D), or tritium (T) ion sources may be used to produce neutrons using targets of deuterium, tritium, lithium, beryllium, and other low-Z materials. Typically these accelerators operate with voltages in the > 1 MeV range”.[16]

“Neutrons (so-called photoneutrons) are produced when photons above the nuclear binding energy of a substance are incident on that substance, causing it to undergo giant dipole resonance after which it either emits a neutron ([photodisintegration) or undergoes fission (photofission). The number of neutrons released by each fission event is dependent on the substance. Typically photons begin to produce neutrons on interaction with normal matter at energies of about 7 to 40 MeV, which means that megavoltage photon radiotherapy facilities may produce neutron radiation as well, and require special shielding for it. In addition, electrons of energy over about 50 MeV may induce giant dipole resonance in nuclides by a mechanism which is the inverse of internal conversion, and thus produce neutrons by a mechanism similar to that of photoneutrons.[17]".[16]

“A spallation source is a high-flux source in which protons that have been accelerated to high energies hit a target material, prompting the emission of neutrons.”[16]

Nuclear fusion, the combining of the heavy isotopes of hydrogen, also has the potential to produce large quantities of neutrons.”[16]

"Neutron emission is a type of radioactive decay of atoms containing excess neutrons, in which a neutron is simply ejected from the nucleus. Two examples of isotopes which emit neutrons are beryllium-13 (mean life 2.7x10-21 sec) and helium-5 (7x10-22 sec)."[18]

"Neutron emission usually happens from nuclei that are in an excited state, such as the excited O-17* produced from the beta decay of N-17. The neutron emission process itself is controlled by the nuclear force and therefore is extremely fast, sometimes referred to as "nearly instantaneous." The ejection of the neutron may be as a product of the movement of many nucleons, but it is ultimately mediated by the repulsive action of the nuclear force that exists at extremely short-range distances between nucleons. The life time of an ejected neutron inside the nucleus before it is emitted is usually comparable to the flight time of a typical neutron before it leaves the small nuclear "potential well," or about 10-23 seconds.[19] A synonym for such neutron emission is "prompt neutron" production, of the type that is best known to occur simultaneously with induced nuclear fission. Many heavy isotopes, most notably californium-252, also emit prompt neutrons among the products of a similar spontaneous radioactive decay process, spontaneous fission."[18]

"Most neutron emission outside prompt neutron production associated with fission (either induced or spontaneous), is from neutron-heavy isotopes produced as fission products. These neutrons are sometimes emitted with a delay, giving them the term delayed neutrons, but the actual delay in their production is a delay waiting for the beta decay of fission products to produce the excited-state nuclear precursors that immediately undergo prompt neutron emission. Thus, the delay in neutron emission is not from the neutron-production process, but rather its precursor beta decay which is controlled by the weak force, and thus requires a far longer time. The beta decay half lives for the precursors to delayed neutron-emitter radioisotopes, are typically fractions of a second to tens of seconds."[18]

# Objects

"[G]alactic (X-ray pulsars, binary systems - black hole candidates) and extragalactic (blasars) objects [emit] gamma-rays [in periodical processes] and high-energy neutral radiation (gamma-rays, neutrons) [is emitted from] solar flares."[20]

# Solar flares

"This graph shows the neutrons detected by a neutron detector at the University of Oulu in Finland from May 16 through May 18, 2012. The peak on May 17 represents an increase in the number of neutrons detected, a phenomenon dubbed a ground level enhancement or GLE. This was the first GLE since December of 2006. Credit: University of Oulu/NASA's Integrated Space Weather Analysis System"[21].

Fairly large fluxes of neutrons have been observed during solar flares such as that of November 12, 1960, with a flux of 30-70 neutrons per cm-2 s-1.[22]

"On May 17, 2012 an M-class flare exploded from the sun. The eruption also shot out a burst of solar particles traveling at nearly the speed of light that reached Earth about 20 minutes after the light from the flare. An M-class flare is considered a "moderate" flare, at least ten times less powerful than the largest X-class flares, but the particles sent out on May 17 were so fast and energetic that when they collided with atoms in Earth's atmosphere, they caused a shower of particles to cascade down toward Earth's surface. The shower created what's called a ground level enhancement (GLE)."[21]

"[O]n Saturday, May 5, ... a large sunspot rotated into view on the left side of the sun. ... [J]ust before [Active Region 1476] disappeared over the right side of the sun, it ... erupted with an M-class flare."[21]

# Mercury

This is an image of the neutron spectrometer aboard the MESSENGER spacecraft in orbit around Mercury. Credit: NASA/JHU/APL.

The neutron spectrometer on the MESSENGER spacecraft "[d]etermines the hydrogen mineral composition to a depth of 40 cm by detecting low-energy neutrons that result from the collision of cosmic rays and the minerals.[23][24]"[25]

"During large solar flares, the region near Mercury may be strongly illuminated with solar neutrons."[26]

# Imaging Compton Telescope

The Imaging Compton Telescope (COMPTEL) utilizes the Compton Effect and two layers of gamma-ray detectors. Credit: NASA.

"In addition to observing gamma rays from a solar flare, COMPTEL is also capable of detecting solar neutrons. Neutron interactions within the instrument occur when an incident solar neutron elastically scatters off a hydrogen nucleus in the liquid scintillator of an upper D1 module. The scattered neutron may then interact and deposit all or a portion of its energy in one of the lower D2 modules, providing the internal trigger signal necessary for a double scatter event. The energy of the scattered neutron is deduced from its time of flight from the upper to lower detector, which is summed with the energy measured for the recoil proton in the upper D1 module to obtain the energy of the incident solar neutron. The computed scatter angle of the neutron, as with gamma rays, yields an event circle on the sky, which can be further constrained since the true source of the detected neutrons is assumed to be the Sun."[27]

"In practice, neutron observations are conducted in the following manner: within two minutes of an initial burst trigger, BATSE sends a second signal to COMPTEL indicating that the burst originates from the general direction of the Sun. COMPTEL can then automatically be commanded to enter an alternate event selection mode to measure solar neutrons for a period of 90 minutes (or approximately one orbit of the spacecraft). This is achieved by shifting the acceptance window for the time of flight of particles from the upper to the lower detector to allow for the slower-moving neutrons, compared to the speed-of-light gamma rays. Gamma ray events continue to be accumulated simultaneously with the neutrons; the two types of particles are later distinguished by their respective time-of-flight and pulse-shape signatures."[27]

# International Space Station

This image shows a Bonner Ball Neutron Detector which is housed inside the small plastic ball when the top is put back on. Credit: NASA.

"Bonner Ball Neutron Detector (BBND) [shown at right with its cap off] measures neutron radiation (low-energy, uncharged particles) which can deeply penetrate the body and damage blood forming organs. Neutron radiation is estimated to be 20 percent of the total radiation on the International Space Station (ISS). This study characterizes the neutron radiation environment to develop safety measures to protect future ISS crews."[28]

Six BBND detectors were distributed around the International Space Station (ISS) to allow data collection at selected points.

"The six BBND detectors provided data indicating how much radiation was absorbed at various times, allowing a model of real-time exposure to be calculated, as opposed to earlier models of passive neutron detectors which were only capable of providing a total amount of radiation received over a span of time. Neutron radiation information obtained from the Bonner Ball Neutron Detector (BBND) can be used to develop safety measures to protect crewmembers during both long-duration missions on the ISS and during interplanetary exploration."[28]

"The Bonner Ball Neutron Detector (BBND) developed by Japan Aerospace and Exploration Agency (JAXA) was used inside the International Space Station (ISS) to measure the neutron energy spectrum. It consisted of several neutron moderators enabling the device to discriminate neutron energies up to 15 MeV (15 mega electron volts). This BBND characterized the neutron radiation on ISS during Expeditions 2 and 3."[28]

The "BBND ... determined that galactic cosmic rays were the major cause of secondary neutrons measured inside ISS. The neutron energy spectrum was measured from March 23, 2001 through November 14, 2001 in the U.S. Laboratory Module of the ISS. The time frame enabled neutron measurements to be made during a time of increased solar activity (solar maximum) as well as observe the results of a solar flare on November 4, 2001."[28]

"BBND results show the overall neutron environment at the ISS orbital altitude is influenced by highly energetic galactic cosmic rays, except in the South Atlantic Anomaly (SAA) region where protons trapped in the Earth's magnetic field cause a more severe neutron environment. However, the number of particles measured per second per square cm per MeV obtained by BBND is consistently lower than that of the precursor investigations. The average dose-equivalent rate observed through the investigation was 3.9 micro Sv/hour or about 10 times the rate of radiological exposure to the average US citizen. In general, radiation damage to the human body is indicated by the amount of energy deposited in living tissue, modified by the type of radiation causing the damage; this is measured in units of Sieverts (Sv). The background radiation dose received by an average person in the United States is approximately 3.5 milliSv/year. Conversely, an exposure of 1 Sv can result in radiation poisoning and a dose of five Sv will result in death in 50 percent of exposed individuals. The average dose-equivalent rate observed through the BBND investigation is 3.9 micro Sv/hour, or about ten times the average US surface rate. The highest rate, 96 microSv/hour was observed in the SAA region."[28]

"The November 4, 2001 solar flare and the associated geomagnetic activity caused the most severe radiation environment inside the ISS during the BBND experiment. The increase of neutron dose-equivalent due to those events was evaluated to be 0.19mSv, which is less than 1 percent of the measured neutron dose-equivalent measured over the entire 8-month period."[28]

# Oriented Scintillation Spectrometer Experiment

The Oriented Scintillation Spectrometer Experiment (OSSE) consists of four NaI scintillation detectors, sensitive to energies from 50 keV to 10 MeV. Credit: NASA GSFC.

"The Compton Gamma Ray Observatory was the second of NASA's Great Observatories. Compton, at 17 tons, was the heaviest astrophysical payload ever flown at the time of its launch on April 5, 1991 aboard the space shuttle Atlantis. Compton was safely deorbited and re-entered the Earth's atmosphere on June 4, 2000."[29]

"The Oriented Scintillation Spectrometer Experiment (OSSE) will conduct a broad range of observations in the 0.05-250 MeV energy range. Major emphasis is placed on scientific objectives in the 0.1-10.0 MeV region with a limited capability above 10 MeV, primarily for observations of solar gamma-rays and neutrons and observations of high-energy emission from pulsars."[27]

"Pulse-shape discrimination in the highest range is also used to separate neutron and gamma-ray energy losses in the NaI portion of the phoswich by utilizing the differing time characteristics of the secondaries produced by these interactions."[27]

"Solar Flare Neutrons Sensitivity: 5 x 10-3 n cm-2 s-1".[27]

# SEDA-AP

The Space Environment Data Acquisition equipment-Attached Payload (SEDA-AP) aboard the Kibo (International Space Station module) "measures neutrons, plasma, heavy ions, and high-energy light particles in ISS orbit."[30]

# Skylab

"The student experiments performed on Skylab 3 included ... neutron analysis."[31]

# Moon

The image shows the hydrogen concentrations on the Moon detected by the Lunar Prospector. Credit: NASA.

At right is the result of an all Moon survey by the Lunar Prospector using an onboard neutron spectrometer (NS). Cosmic rays impacting the lunar surface generate neutrons which in turn loose much of their energy in collisions with hydrogen atoms trapped within the Moon's surface.[32] Some of these thermal neutrons collide with the helium atoms within the NS to yield an energy signature which is detected and counted.[32] The NS aboard the Lunar Prospector has a surface resolution of 150 km.[32]

At the top of this page is an image showing a neutron detector put into a pre-dug hole on the surface of the Moon by Eugene Cernan of the Apollo 17 lunar surface crew. Also, in the image is a boulder. "Now, this (boulder) ought to shield that thing (the neutron probe) from the doggone (RTG)"[33] "[T]he neutron probe consists of targets containing either boron or uranium-235 which, upon capturing neutrons, emit alpha particles or fission fragments which are then captured by plastic or mica detectors. The instrument consists of an outer tube containing the detectors and a central core containing the targets. Because the targets and detectors do not cover the whole surfaces of the core and tube, respectively, the core can be twisted so that the target/detector pairs are either next to each other or 180 degrees apart. In the latter case, very few alpha particles or fission fragments are captured by the detectors and, therefore, the instrument is "off".[34] "The neutron probe is a self-contained unit and, among other things, has no cable connecting it to electronics on the surface, a cable that would prevent the probe from falling out of reach to the bottom of the core hole. At the end of the third EVA, Jack will return to the ALSEP site and retrieve the probe so that he and Gene can bring it back to Earth for analysis."[34]

# Mars

This image contains polar maps of thermal and epithermal neutrons as detected by the Mars Odyssey spacecraft in orbit around Mars. The images are from July 22, 2009. Credit: NASA/JPL-Caltech.

Several neutron detectors and spectrometers have been and are currently being used to measure surface properties associated with neutron emission. The Dynamic Albedo of Neutrons (DAN) spectrometer is aboard the Curiosity rover.

"The Dynamic Albedo of Neutrons (DAN) is an active/passive neutron spectrometer that measures the abundance and depth distribution of H- and OH-bearing materials (e.g., adsorbed water, hydrated minerals) in a shallow layer (~1 m) of Mars' subsurface along the path of the MSL rover. In active mode, DAN measures the time decay curve (the "dynamic albedo") of the neutron flux from the subsurface induced by its pulsing 14 MeV neutron source."[35] "The science objectives of the DAN instrument are as follows: 1) Detect and provide a quantitative estimation of the hydrogen in the subsurface throughout the surface mission; 2) Investigate the upper <0.5 m of the subsurface and determine the possible layering structure of hydrogen-bearing materials in the subsurface; 3) Track the variability of hydrogen content in the upper soil layer (~1 m) during the mission by periodic analysis; and 4) Track the variability of neutron radiation background (neutrons with energy < 100 keV) during the mission by periodic analysis."[35]

Both the neutron spectrometer, from Los Alamos National Laboratories in New Mexico, and the High Energy Neutron Detector (HEND), from the the Russian Aviation and Space Agency, are operating aboard the Odyssey spacecraft in orbit around Mars since 2001.

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