Neutron Monitors

Despite their decades of tradition, ground based neutron monitors (NMs) remain the state-of-the-art instrumentation for measuring cosmic rays, and they play a key role as a research tool in the field of space physics, solar-terrestrial relations, and space weather applications. They are sensitive to cosmic rays penetrating the Earth's atmosphere with energies from about 0.5-20 GeV, i.e. in an energy range that cannot be measured with detectors in space in the same simple, inexpensive, and statistically accurate way. Two types of standardized detectors (IGY and NM64) are in operation in a worldwide network that presently consists of about 50 stations.

NM64 neutron monitor with three counter tubes (right, wood casing of reflector and counter tubes are visible) and rack (left) with counter electronics, high-voltage power supplies, and barometer.

Components of a neutron monitor

Design of a neutron monitor

There are two types of standardized neutron monitors. The IGY neutron monitor was designed by Simpson (1958) in the early fifties of last century. It was the standard detector to study the time variations of the primary cosmic ray intensity at GeV-energies near Earth during the International Geophysical Year (IGY) 1957/1958. About ten years later Carmichael (1964) designed the larger NM64 neutron monitor with an increased counting rate. The NM64 was the standard ground-based cosmic ray detector for the International Quiet Sun Year (IQSY) of 1964.

Neutron monitors consist of special gas-filled proportional counters surrounded by a moderator, a lead producer, and a reflector. The incident nucleon component (protons and neutrons) of the secondary cosmic ray flux causes nuclear reactions in the lead, and evaporation as well as low-energy neutrons are produced. These MeV-neutrons are slowed down to thermal energies by the moderator, and in e.g. the NM64 about 6% of the MeV-neutrons are finally detected by the proportional counter tubes. The fact that finally neutrons are detected gives this cosmic ray detector its name: neutron monitor.

Schematic view of an IGY neutron monitor. The incident nucleon, here a proton, interacts with the lead. In the illustrated case three evaporation neutrons are produced in this nuclear reaction. In a random walk the neutrons travel in the different materials of the NM. Two neutrons are stopped in the reflector (absorbed neutron) and one evaporation neutron enters the moderator where it is slowed down, and finally it is detected in the counter tube.

Gas-filled counter tube

The counter tubes in a neutron monitor detect mainly thermal neutrons, i.e. a kinetic energy of about 0.025 eV. The counter gas is usually boron trifluoride (BF3), enriched to 96% of the 10B isotope, with a pressure of 0.27 bar in the NM64.

The detection of the thermal neutrons in the counter tube occurs by their interactions with a 10B nucleus in the exothermic reaction: $$ ^{10}\mathrm{B}_5 \quad + \quad \mathrm{n} \quad \rightarrow \quad ^{7}\mathrm{Li}_3 \quad + \quad ^4\mathrm{He}_2 $$

The reaction products are detected by their ionization of the counter gas. The counter tube is operated as a proportional counter with an operating voltage of about -2800 V (NM64).

Since 1990 counter tubes filled with 3He gas instead of BF3 have been also used in neutron monitors. The 3He counters have a simpler design. The exothermic reaction of the neutrons with the 3He is: $$ ^{3}\mathrm{He}_2 \quad + \quad \mathrm{n} \quad \rightarrow \quad ^{3}\mathrm{H}_1 \quad + \quad \mathrm{proton} $$

A further advantage of 3He as counter gas is the fact that the counter tube can be operated at a much higher gas pressure and with a voltage of less than 1500 V. At higher pressure in the counter gas a greater detection efficiency per unit volume can be achieved.

Although counters based on the above-mentioned reactions (1) and (2) are most efficient for detecting thermal neutrons due to the 1/v dependence of the cross-section (v: speed), faster neutrons can be detected by surrounding the counter tubes with moderator materials that contain hydrogen, such as paraffin wax or polyethylene.

Moderator

If the incoming neutrons are too fast, they will have a small chance to react according to (1) and (2) with the counter gas and to be detected. In order to enhance the probability of detection, the neutrons have to be slowed down. The function of the moderator is to decrease the energies of the neutrons and to bring them as close to thermal energies (~ 1/40 eV) as possible.

This is done by making the neutrons collide with other nuclei. The exchange of kinetic energy works the better, the closer the mass of the nucleus is to the neutron itself - this is an elementary law of mechanics. Materials with low atomic mass A, which are usually materials containing hydrogen, such as paraffin wax in the IGY neutron monitor, water and polyethylene in the NM64, are used as moderator material.

Lead producer

Surrounding the moderator is a lead producer. The function of the lead in the neutron monitor is twofold:

1. Evaporation neutrons and low-energy neutrons are produced in nuclear reactions of the incident energetic nucleons with the lead. The evaporation neutrons produced have an energy distribution that shows a maximum at about 2 MeV and that reaches energies up to about 15 MeV.
2. The average number of evaporation neutrons per incident nucleon that makes a nuclear interaction in the lead is ~15, and thus the lead increases the overall detection probability.

Lead is chosen as producer, because an element with a high atomic mass, A, provides a large nucleus target for producing evaporation neutrons. In addition, lead has a relatively low absorption cross-section for thermal neutrons.

Open NM64 neutron monitor without counter tubes. Visible are the lead rings and the polyethylene reflector.

The following specifications apply to the NM64 neutron monitor type. The probability for a cosmic ray neutron or proton that hits the neutron monitor to interact with a nucleus of the lead target is ~50%. The average number of evaporation neutrons produced per nuclear reaction in the lead is ~15 and the detection probability for evaporation neutrons by the counter tube is ~6%. With these parameters, the average count rate of a high latitude sea level NM64 neutron monitor with 6 BF3 counter tubes is ~70 cts/second and ~50 cts/second for an equatorial sea level neutron monitor.

Reflector

The assembly of counter tubes, moderator, and lead is enclosed by polyethylene in the NM64 and by paraffin in the IGY. This assembly moderates and reflects the evaporation neutrons produced in the lead into the counter tube. The reflector also shields and absorbs low energy neutrons that are produced in the surrounding materials outside the neutron monitor. This prevents that changes of material in the environment of the detector (e.g. snow accumulation on the detector housing) have a major change in the neutron monitor count rate.

Characteristics of neutron monitors

Further reading

J.A. Simpson, Cosmic Radiation Neutron Intensity Monitor, Annals of the Int. Geophysical Year IV, Part VII, Pergamon Press, London, p. 351, 1958

H. Carmichael, IQSY Instruction Manual, vol. 7, Deep River, Canada, 1964

C.J. Hatton, The Neutron Monitor, in J., G. Wilson and S.A. Wouthuysen (eds.), Progress in Elementary Particle and Cosmic-ray Physics, vol. 10, chapter 1, North Holland Publishing Co., Amsterdam, 1971

P.H. Stoker, L.I. Dorman, and J.M. Clem, Neutron Monitor Design Improvements, Space Science Review, vol. 93, pp. 361-380, 2000

J.M. Clem and L.I. Dorman, Neutron Monitor Response Functions, Space Science Review, vol. 93, pp. 335-359, 2000