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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 B, which produced a signal via the reaction (n + B → α + Li). Recent proportional counters use the reaction (n + He → H
281:
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.
95:
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
260:
Approximately 10-15 times per decade, the Sun emits particles of sufficient energy and intensity to raise radiation levels on Earth's surface. The official list of GLEs is kept by the
International GLE database. The largest of these events, termed a "ground level enhancement" (GLE) was observed on
148:
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
301:
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
156:, 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.
112:(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
96:"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
120:.) 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
205:. 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.
136:) 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:
180:
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:
297:
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
294:
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.
261:
February 23, 1956. The most recent GLE, (#72) occurred on
September 10, 2017, as a result of an X-class flare and was measured on the surface of both the Earth (by Neutron Monitors) and Mars (by the
162:
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
638:
67:
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 "
108:
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
159:
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.
236:" (CME). As this matter moves through the solar system, it suppresses the intensity of Galactic cosmic rays. The suppression was first reported by
213:
The main advantage of the neutron monitor is its long-term stability making them suitable for studied of cosmic-ray variability through decades.
128:
collisions) that transfer energy but do not change the structure of the nucleus. The exceptions to this are a few specific nuclei (most notably
286:(Real-time Neutron Monitor DataBase) gives access to the largest network of stations worldwide (more than 50 stations) through its interface
519:
Meyer, P.; Parker, E. N.; Simpson, J. A. (1956). "Solar cosmic rays of
February, 1956 and their propagation through interplanetary space".
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Usoskin, I. (2017). "Heliospheric modulation of cosmic rays during the neutron monitor era: Calibration using PAMELA data for 2006-2010".
609:
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Moraal, H.; Belov, A.; Clem, J. M. (2000). "Design and coordination of multi-station international neutron monitor networks".
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in 1948. The "18-tube" NM64 monitor, which today is the international standard, is a large instrument weighing about 36 tons.
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The most stable long-running neutron monitors are: Oulu, Inuvik, Moscow, Kerguelen, Apatity and Newark neutron monitors.
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Simpson, J. A. (2000). "The cosmic ray nucleonic component: The invention and scientific uses of the neutron monitor".
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An overview of the space environment shows the relationship between the sunspot cycle and galactic cosmic rays.
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262:
450:
Forbush, S. E. (1937). "On the effects in cosmic-ray intensity observed during the recent magnetic storm".
48:
696:
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Chupp, E. L.; et al. (1987). "Solar neutron emissivity during the large flare on 1982 June 3".
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Mavromichalaki, H. (2010). "Establishing and Using the Real-Time
Neutron Monitor Database (NMDB)".
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In effect the observing instrument is not any isolated instrument, but rather the array.
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290:. Networking neutron monitors yields new information in several areas, among them:
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alter the intensity and energy spectrum of
Galactic cosmic rays that enter the
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Klein, K. L. (2010). "WWW.NMDB.EU: The real-time
Neutron Monitor database".
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Occasionally the Sun expels an enormous quantity of mass and energy in a "
100:) will reach ground level where they can be detected by neutron monitors.
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Steigies, C. (2009). "NMDB: towards a global neutron monitor database".
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Cosmic ray variability recorded by Oulu neutron monitor since 1964
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36:. For historical reasons the incoming particles are called "
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designed to measure the number of high-energy charged
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Reflector. An outer shell of proton-rich material –
197:In a process termed “solar modulation” the Sun and
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557:"A Rare Type of Solar Storm Spotted by Satellite"
40:", but in fact they are particles, predominantly
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569:
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51:. Most of the time, a neutron monitor records
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302:event was first reported for a 1982 event.
256:Ground Level Enhancement — September 1989.
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610:American Geophysical Union, Fall Meeting
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55:and their variation with the 11-year
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74:The neutron monitor was invented by
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166:and cause it to disintegrate. This
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492:National Geophysical Data Center
370:National Geophysical Data Center
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639:38th COSPAR Scientific Assembly
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144:in the early neutron monitors,
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488:"Extreme Space Weather Events"
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366:"Extreme Space Weather Events"
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110:International Geophysical Year
1:
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263:Radiation Assessment Detector
506:"International GLE Database"
7:
385:J. Geophys. Res. Space Phys
10:
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171:+ p) which yields 764 keV.
152:Producer. The producer is
697:The Astrophysical Journal
474:10.1103/PhysRev.51.1108.3
248:Ground level enhancements
149:induced cascade neutrons.
69:ground level enhancements
594:10.1023/A:1026504814360
543:10.1103/PhysRev.104.768
344:10.1023/A:1026567706183
267:Mars Science Laboratory
240:and hence is termed a "
738:Cosmic-ray experiments
277:Neutron monitor arrays
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668:ASP Conference Series
573:Space Science Reviews
323:Space Science Reviews
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234:Coronal Mass Ejection
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76:University of Chicago
415:10.1002/2016JA023819
164:proportional counter
104:Measurement strategy
91:Atmospheric cascades
53:galactic cosmic rays
710:1987ApJ...318..913C
681:2010ASPC..424...75M
652:2010cosp...38.1685K
623:2009AGUFMSH51B1280S
586:2000SSRv...93..285M
535:1956PhRv..104..768M
466:1937PhRv...51.1108F
407:2017JGRA..122.3875U
336:2000SSRv...93...11S
209:Long-term stability
63:. Occasionally the
437:"Oulu NM database"
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30:Earth's atmosphere
20:is a ground-based
460:(12): 1108–1109.
228:Forbush decreases
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242:Forbush decrease
176:What it measures
168:nuclear reaction
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80:John A. Simpson
18:neutron monitor
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718:10.1086/165423
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617:: SH51B–1280.
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529:(3): 768–783.
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330:(1/2): 11–32.
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57:sunspot cycle
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559:. June 2012.
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203:Solar System
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185:Solar cycles
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146:polyethylene
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86:How it works
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59:and 22-year
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704:: 913–925.
298:determined.
38:cosmic rays
34:outer space
398:1705.07197
307:References
199:solar wind
78:Professor
352:117949880
118:electrons
71:" (GLE).
26:particles
732:Category
423:55768360
142:paraffin
114:neutrons
98:neutrons
22:detector
706:Bibcode
677:Bibcode
648:Bibcode
619:Bibcode
582:Bibcode
531:Bibcode
462:Bibcode
403:Bibcode
332:Bibcode
265:on the
122:elastic
42:protons
675:: 75.
421:
350:
124:(like
49:nuclei
46:Helium
646:: 3.
419:S2CID
393:arXiv
348:S2CID
32:from
615:2009
288:NEST
284:NMDB
154:lead
132:and
116:and
44:and
714:doi
702:318
673:424
590:doi
539:doi
527:104
470:doi
411:doi
389:122
340:doi
273:).
269:'s
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65:Sun
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130:B
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