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Neutron monitor

Neutron monitor

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A neutron monitor is a ground-based detector
Particle detector
In experimental and applied particle physics, nuclear physics, and nuclear engineering, a particle detector, also known as a radiation detector, is a device used to detect, track, and/or identify high-energy particles, such as those produced by nuclear decay, cosmic radiation, or reactions in a...

 designed to measure the number of high-energy charged particle
Subatomic particle
In physics or chemistry, subatomic particles are the smaller particles composing nucleons and atoms. There are two types of subatomic particles: elementary particles, which are not made of other particles, and composite particles...

s striking the Earth's atmosphere
Earth's atmosphere
The atmosphere of Earth is a layer of gases surrounding the planet Earth that is retained by Earth's gravity. The atmosphere protects life on Earth by absorbing ultraviolet solar radiation, warming the surface through heat retention , and reducing temperature extremes between day and night...

 from outer space
Outer space
Outer space is the void that exists between celestial bodies, including the Earth. It is not completely empty, but consists of a hard vacuum containing a low density of particles: predominantly a plasma of hydrogen and helium, as well as electromagnetic radiation, magnetic fields, and neutrinos....

. For historical reasons the incoming particles are called "cosmic ray
Cosmic ray
Cosmic rays are energetic charged subatomic particles, originating from outer space. They may produce secondary particles that penetrate the Earth's atmosphere and surface. The term ray is historical as cosmic rays were thought to be electromagnetic radiation...

s", but in fact they are particles, predominantly protons and Helium
Helium
Helium is the chemical element with atomic number 2 and an atomic weight of 4.002602, which is represented by the symbol He. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas that heads the noble gas group in the periodic table...

 nuclei. Most of the time, a neutron monitor records galactic cosmic ray
Galactic cosmic ray
Galactic cosmic rays are cosmic rays that have their origin inside our Galaxy. GCRs are high-energy charged particles, and are usually protons, electrons, and fully ionized nuclei of light elements...

s and their variation with the 11-year sunspot cycle and 22-year magnetic cycle. Occasionally the Sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...

 emits cosmic rays of sufficient energy and intensity to raise radiation levels on Earth's surface to the degree that they are readily detected by neutron monitors. They are termed "Ground Level Enhancements" (GLE).

The neutron monitor was invented by University of Chicago
University of Chicago
The University of Chicago is a private research university in Chicago, Illinois, USA. It was founded by the American Baptist Education Society with a donation from oil magnate and philanthropist John D. Rockefeller and incorporated in 1890...

 Professor John A. Simpson
John Alexander Simpson
John Alexander Simpson worked as an experimental nuclear, and cosmic ray physicist who was deeply committed to educating the public and political leaders about science and its implications. The year he died, his instruments in space had been sending data back for nearly 40 years...

 in 1948. The "18-tube" NM64 monitor, which today is the international standard, is a large instrument weighing about 36 tons.

Atmospheric cascades


When a high-energy particle from outer space ("primary" cosmic ray) encounters Earth, its first interaction is usually with an air molecule at an altitude of 30 km or so. This encounter causes the air molecule to split into smaller pieces, each having high energy. The smaller pieces are called "secondary" cosmic rays, and they in turn hit other air molecules resulting in more secondary cosmic rays. The process continues and is termed an "atmospheric cascade". If the primary cosmic ray that started the cascade has energy over 500 MeV, some of its secondary byproducts (including neutron
Neutron
The neutron is a subatomic hadron particle which has the symbol or , no net electric charge and a mass slightly larger than that of a proton. With the exception of hydrogen, nuclei of atoms consist of protons and neutrons, which are therefore collectively referred to as nucleons. The number of...

s) will reach ground level where they can be detected by neutron monitors.

Measurement strategy


Since they were invented by Prof. Simpson in 1948 there have been various types of neutron monitors. Notable are the "IGY-type" monitors deployed around the world during the 1957 International Geophysical Year
International Geophysical Year
The International Geophysical Year was an international scientific project that lasted from July 1, 1957, to December 31, 1958. It marked the end of a long period during the Cold War when scientific interchange between East and West was seriously interrupted...

 (IGY) and the much larger "NM64" monitors (also known as "supermonitors"). All neutron monitors however employ the same measurement strategy that exploits the dramatic difference in the way high and low energy neutrons interact with different nuclei. (There is almost no interaction between neutron
Neutron
The neutron is a subatomic hadron particle which has the symbol or , no net electric charge and a mass slightly larger than that of a proton. With the exception of hydrogen, nuclei of atoms consist of protons and neutrons, which are therefore collectively referred to as nucleons. The number of...

s and electron
Electron
The electron is a subatomic particle with a negative elementary electric charge. It has no known components or substructure; in other words, it is generally thought to be an elementary particle. An electron has a mass that is approximately 1/1836 that of the proton...

s.) High energy neutrons interact rarely but when they do they are able to disrupt nuclei, particularly heavy nuclei, producing many low energy neutrons in the process. Low energy neutrons have a much higher probability of interacting with nuclei, but these interactions are typically elastic
Elastic collision
An elastic collision is an encounter between two bodies in which the total kinetic energy of the two bodies after the encounter is equal to their total kinetic energy before the encounter...

 (like billiard ball collisions) that transfer energy but do not change the structure of the nucleus. The exceptions to this are a few specific nuclei (most notably 10B and 3He
Helium-3
Helium-3 is a light, non-radioactive isotope of helium with two protons and one neutron. It is rare on Earth, and is sought for use in nuclear fusion research...

) that quickly absorb extremely low energy neutrons, then disintegrate releasing very energetic charged particles. With this behavior of neutron interactions in mind, Professor Simpson ingeniously selected the four main components of a neutron monitor:
  1. Reflector. An outer shell of proton-rich material – paraffin
    Paraffin
    In chemistry, paraffin is a term that can be used synonymously with "alkane", indicating hydrocarbons with the general formula CnH2n+2. Paraffin wax refers to a mixture of alkanes that falls within the 20 ≤ n ≤ 40 range; they are found in the solid state at room temperature and begin to enter the...

     in the early neutron monitors, polyethylene
    Polyethylene
    Polyethylene or polythene is the most widely used plastic, with an annual production of approximately 80 million metric tons...

     in the more modern ones. Low energy neutrons cannot penetrate this material, but are not absorbed by it. Thus environmental, non-cosmic ray induced neutrons are kept out of the monitor and low energy neutrons generated in the lead are kept in. This material is largely transparent to the cosmic ray induced cascade neutrons.
  2. Producer. The producer is lead
    Lead
    Lead is a main-group element in the carbon group with the symbol Pb and atomic number 82. Lead is a soft, malleable poor metal. It is also counted as one of the heavy metals. Metallic lead has a bluish-white color after being freshly cut, but it soon tarnishes to a dull grayish color when exposed...

    , and by weight it is the major component of a neutron monitor. Fast neutrons that get through the reflector interact with the lead to produce, on average about 10 much lower energy neutrons. This both amplifies the cosmic signal and produces neutrons that cannot easily escape the reflector.
  3. Moderator. The moderator, also a proton rich material like the reflector, slows down the neutrons now confined within the reflector, which makes them more likely to be detected.
  4. Proportional Counter. This is the heart of a neutron monitor. After very slow neutrons are generated by the reflector, producer, moderator, and so forth, they encounter a nucleus in the proportional counter
    Proportional counter
    A proportional counter is a measurement device to count particles of ionizing radiation and measure their energy.A proportional counter is a type of gaseous ionization detector. Its operation is similar to that of a Geiger-Müller counter, but uses a lower operating voltage. An inert gas is used to...

     and cause it to disintegrate. This nuclear reaction
    Nuclear reaction
    In nuclear physics and nuclear chemistry, a nuclear reaction is semantically considered to be the process in which two nuclei, or else a nucleus of an atom and a subatomic particle from outside the atom, collide to produce products different from the initial particles...

     produces energetic charged particles that ionize gas in the proportional counter, producing an electrical signal. In the early Simpson monitors, the active component in the gas was 10B, which produced a signal via the reaction (n + 10B → α + 7Li). Recent proportional counters use the reaction (n + 3He → 3H + p) which yields 764 keV.

What it measures


Neutron monitors measure by proxy the intensity of cosmic rays striking the Earth, and its variation with time. These variations occur on many different time scales (and are still a subject of research). The three listed below are examples:

Solar cycles


In a process termed “solar modulation” the Sun and solar wind
Solar wind
The solar wind is a stream of charged particles ejected from the upper atmosphere of the Sun. It mostly consists of electrons and protons with energies usually between 1.5 and 10 keV. The stream of particles varies in temperature and speed over time...

 alter the intensity and energy spectrum of Galactic cosmic rays that enter the solar system
Solar System
The Solar System consists of the Sun and the astronomical objects gravitationally bound in orbit around it, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago. The vast majority of the system's mass is in the Sun...

. When the Sun is active, fewer Galactic cosmic rays reach Earth than during times when the Sun is quiet. For this reason, Galactic cosmic rays follow an 11-year cycle like the Sun, but in the opposite direction: High solar activity corresponds to low cosmic rays, and vice versa.

Forbush decreases


Occasionally the Sun expels an enormous quantity of mass and energy in a "Coronal Mass Ejection
Coronal mass ejection
A coronal mass ejection is a massive burst of solar wind, other light isotope plasma, and magnetic fields rising above the solar corona or being released into space....

" (CME). As this matter moves through the solar system, it suppresses the intensity of Galactic cosmic rays. The suppression was first reported by Scott Forbush
Scott Forbush
Scott Ellsworth Forbush was an American astronomer, physicist and geophysicist who is recognized as having laid the observational foundations for many of the central features of solar-interplanetary-terrestrial physics, which at the time was an under-developed field of study...

 and hence is termed a "Forbush decrease
Forbush decrease
A Forbush decrease is a rapid decrease in the observed galactic cosmic ray intensity following a coronal mass ejection . It occurs due to the magnetic field of the plasma solar wind sweeping some of the galactic cosmic rays away from Earth. The term Forbush decrease was named after the American...

".

Ground level enhancements


Approximately 10-15 times per decade, the Sun emits particles of sufficient energy and intensity to raise radiation levels on Earth's surface. The largest of these events, termed a "Ground Level Enhancement", (GLE) was observed on February 23, 1956.

Neutron monitor arrays


In the early days of neutron monitoring, discoveries could be made with a monitor at a single location. However, the scientific yield of neutron monitors is greatly enhanced when data from numerous monitors are analyzed in concert. Modern applications frequently employ extensive arrays of monitors. In effect the observing instrument is not any isolated instrument, but rather the array.
Networking neutron monitors yields new information in several areas, among them:
  1. Anisotropy: Neutron monitor stations at different locations around the globe view different directions in space. By combining data from these stations, the anisotropy of cosmic rays can be determined.
  2. Energy Spectrum: Earth’s magnetic field repels cosmic rays more strongly in equatorial regions than in polar regions. By comparing data from stations located at different latitudes, the energy spectrum can be determined.
  3. Relativistic Solar Neutrons: These are very rare events recorded by stations near Earth’s equator that face the Sun. The information they provide is unique because neutrally charged particles (like neutrons) travel through space unaffected by magnetic fields in space. A relativistic solar neutron event was first reported for a 1982 event.