Neutron Monitor

In the late 1940s, the neutron monitor was developed by John A. Simpson (University of Chicago) to investigate the hadronic component of cosmic particles. Only ten years later, a worldwide network of about 50 neutron monitors was established. The detectors take continuously data to register cosmic weather events like sun eruptions.

A major contribution of the neutron monitors was the discovery that the intensity of cosmic particles originating from outside our solar system decreases with increasing solar activity (11-year cycle). The increase of the solar wind (particles emitted by the Sun such as electrons, protons and neutrons) leads to the shielding of those cosmic particles of galactic origin. Neutron monitors are also suitable for the registration of solar flares. During these solar flares, large quantities of particles are emitted in an explosion, which can affect and even destroy electronic systems in satellites and on Earth.

The neutron monitor is specialized in measuring the nucleon component (protons and neutrons) of cosmic rays on the Earth's surface. Primary cosmic particles consist essentially of protons and light nuclei. When they hit the Earth's atmosphere, three different components of secondary particles are produced when they interact with the air molecules:

• muons and neutrinos,
• electrons and gamma,
• nucleons

The North-West University of Potchefstroom in South Africa has built mini neutron monitors which are compared to the standard neutron monitors light and transportable. One of them was installed at the “Polarstern” research vessel, a second at the Neumayer III station in Antarctica (see photo).

The schematic figure shows the shell structure of the cylindrical neutron monitor. The outer layer (1) is a moderator made of polyethylene to reduce the energy of high-energy neutrons and to absorb low-energy neutrons from the background radiation. The next layer (2) consists of 5 cm lead. Here the number of neutrons is considerably increased when the nucleons collide with the lead atoms. A nucleon produces an average of eight neutrons in the lead layer, which hit the moderator layer (3) with an energy of a few MeV. Most of the protons will be absorbed by the lead. The neutrons are further slowed down in the moderator and finally hit the proportional counter tube (4) inside the cylinder. Depending on the gas filling of the counter tube (¹⁰BF₃ at the neutron monitor at Neumayer Station or ³He at Polarstern) the thermal neutrons can be detected by the following reactions:

n + ¹⁰BF₃ → α + ⁷Li         or         n + ³He → ³H + p

The alpha particles or protons produce an electrical signal in the counter tube, which is registered in the electronic box (5) and stored together with the time information, atmospheric pressure and temperature. Since the other two components do not produce any reactions, the neutron monitor can only detect nucleons.