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.

Educational level: this is a secondary education resource.

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 tertiary (university) 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 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.

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.

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

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."

“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). Free neutrons decay by emission of an electron and an electron antineutrino to become a proton, a process known as beta decay:”

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.”.

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."

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

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”.

“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.".

“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.”

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

"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)."

"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. 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."

"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."

Temperatures

"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."

"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. 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."

Ultra high energy neutron 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.” “[A]t E_n = 10²⁰ eV, [these neutrons] are flying a Mpc, with their directional arrival (or late decayed proton arrival) ... more on-line toward the source.” 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.”

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.

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".

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.

"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)."

"[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."

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."

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

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."

"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."

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."

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."

"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."

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."

"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."

"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."

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."

"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."

"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."

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

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."

Skylab

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

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. Some of these thermal neutrons collide with the helium atoms within the NS to yield an energy signature which is detected and counted. The NS aboard the Lunar Prospector has a surface resolution of 150 km.

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)" "[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". "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."

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." "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."

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.

See also

• Cosmic-ray astronomy
• Proton astronomy
• Principles of Radiation Astronomy
• Radiation

Further reading

• Coleman FJ, Thomas DC, Saxon G (1971). "An experiment to determine shielding requirements for a multi-GeV electron synchrotron ring". Daresbury Nuclear Physics: 581-600. 

External links

• African Journals Online
• Bing Advanced search
• GenomeNet KEGG database
• Google Books
• Google scholar Advanced Scholar Search
• Home - Gene - NCBI
• International Astronomical Union
• JSTOR
• Lycos search
• NASA/IPAC Extragalactic Database - NED
• NASA's National Space Science Data Center
• NCBI All Databases Search
• NCBI Site Search
• Office of Scientific & Technical Information
• PsycNET
• PubChem Public Chemical Database
• Questia - The Online Library of Books and Journals
• SAGE journals online
• The SAO/NASA Astrophysics Data System
• Scirus for scientific information only advanced search
• SDSS Quick Look tool: SkyServer
• SIMBAD Astronomical Database
• Spacecraft Query at NASA.
• SpringerLink
• Taylor & Francis Online
• Universal coordinate converter
• WikiDoc The Living Textbook of Medicine
• Wiley Online Library Advanced Search
• Yahoo Advanced Web Search

This is a research project at http://en.wikiversity.org