Neutron Monitor

Neutron Monitor (NM)

Mode of operation for a neutron monitor

The measuring process involves the following steps

  1. The direction of the high energy protons to be measured is refracted inside the magnetosphere. Under a certain magnetic rigidity (momentum per charge and magnetic field strength), the refraction is so strong that the particles don’t reach the atmosphere, but disappear into space again. The rigidity limit depends on the geomagnetic latitude and is about 2.3 GV/c for the monitor in Kiel. The rigidity limit is the highest at the geomagnetic equator.
    Most of the protons that reach Kiel’s monitor deviate due to magnetic refraction on average about 80 degrees to the east longitude from their angle of incidence. Because the geographic longitude of Kiel is 10 degrees, the asymptotic incidence longitude is about 90 degrees. The asymptotic incidence latitude is about 0 degrees. This directional dependence of the neutron monitor can be used in general to reveal the possible presence of anisotropy in cosmic radiation.
    High energy neutrons, like those that are shot out from the sun during certain solar events, are of course not subject to this refraction.
  2. The high energy nuclei develop three secondary components in the atmosphere:
    • The soft components consisting of electromagnetic cascades of electrons and gammaquanten
    • The hard muon components
    • The nucleon components in which on average the same number of protons and neutrons are released through nuclear collision
  3. Calculating the intensity in space from measurements on earth has to take into account the effects of the atmosphere. This is the easiest to do when the neutron monitor only focuses on the nucleon components, because they are the ones that are only affected by the penetrated air mass and not from the variable expansion of the atmosphere that depends on temperature conditions. Both of the other components are shielded out by a 5 cm thick lead jacket that surrounds the counting tube of the monitor. However, incoming nucleons, which typically reach the earth with 200-300 MeV, create, among other things, an average of eight 2 MeV-neutrons through nuclear collision when they enter the lead. These neutrons are registered as the measurement for the primary intensity and give the system the name of “neutron monitor”. The secondary protons that are also created in this process have such a short range due to their small energy (in the MeV range) that they practically never make it out of the lead.
  4. The neutrons are changed into thermal energy in a second inner counting tube covering made out of polyethylene. They are finally registered in BF3 counting gas inside the counting tube as alpha particles by the reaction B(n,alpha)Li.

Literature

C. J. Hatton, H. Carmichael
Experimental Investigation of the NM-64 Neutron Monitor.
Canadian Journal of Physics, Volume 42, 1964.

L. I. Dorman
Cosmic Rays - Variations and Space Explorations.
North Holland Publishing Company, 1974.

Technical Data and International Cooperation

IQSY-Type with 18 counting tubes in 3 sections, counting tube length of 2.20 m, total counting surface approximately 21 m2, average counting rate app. 10,000 pulses per min. registered continuously since January 1958 (type IGY until August 1964, after type IQSY)

A network of neutron monitors are in place around the globe that have different asymptotic lines of site due to their different geographical locations which are described by latitude and longitude. However, only the stations close to the poles stick out of the equator level with their asymptotic lines of site. Presently, it is being attempted – especially by the Izmiran work group in Moscow – to coordinate the measurements from different monitors in order that the intensity of the high energy cosmic radiation can be given as a function of time and angle of incidence

There is a close cooperation at the moment with the World Data Canter in Nagoya/Japan and in Boulder/USA. The data from the neutron monitor in Kiel can also be found there.

Results from Kiel’s neutron monitor’s measurements

The measurement results of Kiel’s neutron monitor are available at the World Data Center A in the USA as daily averages starting from 1958. The data can also be found here in numerical form as monthly and daily averages.
The graphic representation of the monthly averages shows that the neutron monitor counting rates over several sun-cycles stay relatively constant. The value fluctuates rarely more than 10 percent. It can be seen in a time-spread representation that there is a systematic variation that runs inversely, but noticeably proportional to the number of sun-spots on the sun. In the sun-spot cycle 21 (maximum number of sun-spots around the year 1980), the variation is delayed about a year against the number of sun-spots, while a delay can hardly be seen in the two neighboring cycles. This shows that the magnetic interference in the interplanetary medium that increases with the number of sun-spots and is carried by the solar wind at about 0.01 AU/h hinders the oncoming flow of cosmic radiation up to a distance of about 100 AU. The variable delay of the lowering of the cosmic radiation from cycle to cycle can depend on the alternating polarity of the solar magnetic field. The statistical quality of the neutron monitor data allows a chronological resolution down to five minute average rates, for which the random variations stay under one percent. These data show short-term events, especially Forbush declines of the counting rates.

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