Chapter 3 Space Weather and operational needs for NMs
Chapter 4 Other science applications of NM data
Chapter 5 Current status and outlook
Chapter 6 Recommendations and priorities
Chapter 7 Citation Listing
Chapter 8 References
The National Science Foundation, the University of New Hampshire and the Bartol Research Institute sponsored a workshop entitled Neutron Monitor Community Workshop on October 24-25, 2015 in Honolulu, Hawaii. The workshop was organized with generous help from the University of Hawaii at Manoa. Its objective was to review and evaluate the US-supported neutron monitor network, and articulate a community consensus on the network utility and value with recommended priorities and actions. This paper is the principal product of that workshop, integrating contributions of the many participants.
John Bieber University of Delaware
Doug Biesecker NOAA
Veronica Bindi UH of Hawaii Manoa
Mirko Boezio Istituto Nazionale di Fisica Nucleare
The purpose of the neutron monitor is to detect, deep within the atmosphere, variations of intensity in the interplanetary cosmic ray spectrum. Interactions of the primary cosmic rays with the atmosphere produce, among other things, a lower energy secondary nucleonic component consisting of nucleons, in particular neutrons that are not slowed by ionization loss, These secondaries fall in the energy range of a few hundred MeV up to about one GeV. Because of the falling energy spectrum of the primary cosmic rays, the neutron monitors are most sensitive to the low energy (1-20 GeV) portion of the spectrum. These nucleons in turn produce further nuclear interactions, either in the atmosphere, or in lead target material surrounding the monitor. The interaction rate may be measured most conveniently and reliably by detecting the reaction product neutrons rather than by detecting the charged fragments directly.
Excerpt from Simpson, J.A., "Cosmic-Radiation Neutron Intensity Monitor", in Annals of the IGY, 1955
Neutron monitors were identified decades ago as useful instruments for studying the near-Earth environment. Since their wide deployment in the International Geophysical Year 1957-1958, they have been used to (1) measure the energetic particle emission from the Sun during periods of intense solar activity, (2) study the dynamics of the near and far heliosphere as sensed through solar modulation over the course of several solar cycles and (3) study the dynamics of what are now known as Coronal Mass Ejections through the transient modulation of galactic cosmic rays. Since then other disciplines have found uses for neutron monitor data that go beyond near-Earth science.
The initial chapter addresses the history and workings of the instrument and its modes of deployment. The remainder of the paper is organized into discussions of the utility and importance of neutron monitors for science, service and space weather. The paper finishes with a discussion of recommended actions and priorities to support these diverse needs.
A neutron monitor workshop held in October 2015 reviewed the status, utility, science and future of the monitor network. The participants formulated recommended actions to preserve and enhance these aspects of the network. Also in October 2015, two documents from the Executive Branch were published in response to the country’s needs from the risks posed by Space Weather events, the National Space Weather Strategy and the National Space Weather Action Plan. The documents were prepared by the Space Weather Operations, Research, and Mitigation (SWORM) Task Force and published by the National Science and Technology Council. Co-chairs of the task force included the National Oceanic and Atmospheric Administration, Department of Homeland Security and the Office of Science and Technology Policy. The role of neutron monitors in the multi-agency plan is part of the multipronged approach to identify and characterize space weather events and mitigate their negative effects on society and governmental functions.
A neutron monitor is a ground-based instrument that continuously records the rate of high-energy particles (E>500MeV) impacting the Earth’s atmosphere. For historical reasons these particles, mostly protons and helium nuclei, are called "cosmic rays." Because the intensity of cosmic rays hitting Earth is not uniform, it is important to place neutron monitors at multiple locations to form a complete picture of cosmic rays in space. The advent of the neutron monitor came after the discovery by Simpson  that the intensity of the nucleonic component in cosmic ray air showers is several times more sensitive to changes in low energy regime (0.5-3.0GeV) of primary cosmic rays than that of the electromagnetic and muon components, as measured by the ionization chambers. In 1952 the first neutron monitor stations were established at Chicago, Climax, Huancayo, Peru and Sacramento Peak, and by the 1957 IGY, sixty stations were established world-wide. Some these early stations are still in operation today. The reliability and basic simplicity of neutron monitors provide a means for studying the longer-term variations, while the sensitivity to low energy primary cosmic rays and high count rates make possible the measurement of short term intensity changes. As the result of the long term reliability and on-line availability of neutron monitor data, users outside the traditional circles of Space Sciences have discovered innovative ways to utilize these data for new and unforeseen investigations, radiation monitoring or real-time background. Over the past decades neutron monitor data have become an operational resource in many different areas.
Unfortunately, from the point of view of basic science, service to the community and as part of the national space weather strategy, the current state of the US neutron monitor network is not good. The network now consists of aging neutron monitors, many of which have been neglected due to a lack of funding. The network is not functioning as it once did or envisioned during the International Geophysical Year in 1957-1958. Several key measurement sites have been de-commissioned or switched off. A perceived lack of interest by the US makes foreign sites consequently vulnerable to cuts and closings. There is minimal coordination between supporting groups with each neutron monitor station fighting for existence. New and inexpensive technology cannot make its way into the field to modernize and greatly improve the performance of those stations now operating. Recommendations emerged from the workshop that if implemented would bring the network into a condition to conduct the best science and support operational needs. In order of priority, they are:
Fully restore the scientific functionality and update the existing US network. This will constitute a major step in restoring the global network by restoring coverage provided by the US stations and by making a statement to international funding agencies that the global network is important,
Establish a desired network concept that would fulfill the needs of the science and operational communities. This would involve identifying new key strategic sites to complete the global coverage,
Improve station and data uniformity and accessibility,
Train a new generation of scientists and expand educational outreach,
Given the unavailability of standard detectors, design and deploy a new generation of neutron monitors in the form of inexpensive kits to be widely deployed as part of global strategy for the network, and
Modernize or install new timing electronics to study rapid phenomena, not anticipated in 1957.
History and Heritage
The history of the neutron monitor starts with the discovery of "high altitude radiation" identified with a balloon experiment conducted by Victor Hess in 1912 (Hess, 1912). Soon afterwards temporal and spatial variations in the radiation intensity were observed with ionization chambers and charged particle telescopes. The analysis of these variations led to a better understanding of the nature of primary cosmic-rays. Travelling by sea from Indonesia to Europe in 1927, Jacob Clay observed variation of cosmic ray intensity with latitude indicating primary cosmic rays are deflected by the Earth’s magnetic field and consequently must be electrically charged particles. In 1933 Bruno Rossi observed that the number of particles coming from the west is greater than from the east, indicating most primaries are positive charged (the East-West effect). During the same experiment, while testing his detectors, Rossi also discovered the existence of particle showers produced by interactions of cosmic rays in the atmosphere, a phenomenon studied by Pierre Auger in 1937, whose name became associated with the discovery.
Routine monitoring of the cosmic radiation was initiated in January 1932 with the operation of an ionization chamber at Hafelekar, Austria (Shea and Smart, 2000). The first network of cosmic ray detectors was sponsored by the Carnegie Institution and consisted of four shielded ionization chambers of the same design at Christchurch, New Zealand (April, 1936), Huancayo, Peru (June 1936), Cheltenham, USA (March 1937) and Godhavn, Greenland (October, 1938). The Carnegie Network of ionization chambers was developed primarily to detect the change in cosmic radiation as a function of geomagnetic activity as evidenced by some of Forbush's early papers (Forbush, 1937, 1938). By the early 1940s there were a number of ionization chambers in operation both at stationary locations and on shipboard.
Figure 1. Anatomy of a cosmic ray air shower.
During the decades from 1930s to 1940s, a wide variety of experiments confirmed that primary cosmic rays entering the top of the atmosphere produced nuclear interactions deep in the atmosphere. An incoming primary cosmic ray with sufficient energy will interact with an air nuclei, initiating a cascade of secondary interactions that ultimately produce a particle shower. A significant fraction of these interactions occur at the nuclear level, which can be thought of as a two stage process (inelastic collision and nuclear de-excitation). In the first stage (inelastic collision), the primary particle reacts with nucleons inside the nucleus creating an intranuclear cascade of high-energy (>20 MeV) protons, neutrons, and pions within the nucleus. Some of these energetic hadrons escape as secondary particles while others deposit their energy in the nucleus leaving it in an excited state. The escaping secondary particles may collide with another air nuclei and produce successively spallation products and excited nuclei. In the second stage (nuclear de-excitation), evaporation takes place when the excited nucleus relaxes by emitting low-energy (< 20 MeV) neutrons, protons, alpha particles, etc., with the majority of the particles being neutrons. Evaporation neutrons interacting with nuclei are typically elastic that transfer energy without changing the structure of the nucleus. After multiple interactions, the evaporation neutron will lose energy and eventually thermalize, and/or be absorbed or undergo decay.
In 1946, Simpson initiated the first investigation on the dependence of nucleonic intensity with incident primary cosmic-ray energy using the geomagnetic cut-off effect. These observations took place at 30,000ft on a B29 aircraft. The nucleonic flux in the atmosphere was inferred by measurements of the evaporation energy neutron intensity (Simpson, 1949). This required a detector that was insensitive to neutron energies lower than the evaporation energy, as thermal neutrons are highly dependently on local conditions. An additional requirement was the exclusion of the muon and electron components at all energies. To achieved these requirements, Simpson designed a detector utilizing a 10BF3 gas proportional counters encapsulated in paraffin. 10B has a neutron capture cross-section inversely proportional to the neutron and responds to neutrons by the exothermic reaction 10B(n,) 7Li with a Q value of 2.3MeV. With properly chosen gas density, the pulse-height threshold could eliminate the muon and electromagnetic component from neutron absorptions. Paraffin serves to reduce the energy of neutrons, thus increasing the probability of an absorption inside the proportional counter while also providing a shielding barrier against unwanted thermal and epithermal energy neutrons.
Figure 2. The lower curve represents the latitude dependence for ionization chamber. The upper curve represents the evaporation neutron intensity. Both curves are normalized to 1 at zero magnetic latitude. The data for the curves were obtained in June, 1948. (Need Permission?)
Utilizing this new neutron detector along with Geiger-Muller counters on an air-borne latitude measurements in 1946–1947, Simpson  discovered the latitude variation of the intensity of evaporation neutrons in the atmosphere is several times larger than that of the ionizing and hard components. It immediately became apparent from this discovery that variations in the low energy cosmic-ray proton intensity down to 1–5 GV could be investigated continuously for the first time. This new detector opened a new window to the primary cosmic-ray energy spectrum.
Figure 3. IGY Neutron Monitor
The observed variation in the nucleonic component intensity at high latitudes from survey to survey motivated the need for a ground base system to continuously monitor the evaporation neutrons. It was during this time Simpson (2000) considered the effect of atmospheric production and local production of evaporation of neutrons, estimating that the multiplicity per interaction of neutron production in materials with high atomic mass, A, increased as A0.7. The anticipation that the count rate could be greatly enhanced led to the concept of a cosmic-ray neutron monitoring system based on measuring the local production of evaporation neutrons in a high atomic mass target such as lead. The fragmentation of a lead nucleus by an incident high-energy secondary nucleon would yield a multiplicity of evaporation neutrons, which would then become thermalized in the surrounding paraffin wax and be detected with 10BF3 proportional counters embedded in the ‘pile’.
The development of a leaded neutron monitor was undertaken in 1949, leading to a basic ‘standard’ pile design with an account of its dependence only on barometric pressure (Simpson, 1953; Simpson et al., 1953b). Drawing of IGY is shown in Figure 3. Furthermore, this design could be extended in size to multiply the counting rate from the pile. The 12 counter configuration became the standard neutron monitor design for Chicago and Climax, Colorado in 1949; and with the influence of Scott Forbush, later it was the design adopted for the International Geophysical Year (1957–1958) at more than sixty (60) sites world-wide (Simpson, 1958). The detector was renamed the IGY neutron monitor.
The IGY neutron monitor opened a new window to cosmic ray intensities and produced an understanding of six basic phenomena:
(1) Variation of cosmic ray intensity with geomagnetic latitude (Simpson, 1948).
(2) Variation of cosmic radiation intensity in response to the 11-year solar activity cycle and the 22-year solar magnetic cycle (Meyer and Simpson, 1955) that was originally indicated in ion chamber data variations from the time period 1937–1952 by Forbush (1954).
(3) Correlation of the 27-day modulation of cosmic ray intensity with the Carrington rotation of the Sun (Simpson, Babcock and Babcock, 1955).
(4) Anisotropy of the cosmic rays exhibited as diurnal variations (Simpson 1953).
(5) The origin of the rapid decrease in cosmic ray intensity (Forbush Decrease) lies outside of the Earth’s geomagnetic field (Simpson 1953).
(6) Based on the ground level event observation that the sudden and short burst of relativistic nucleons of a solar flare event (23 February 1956) could only slowly escape to the interstellar medium through a continuous barrier region beyond the orbit of Earth, implying a dynamical heliosphere (Bieber et al., 2000, Meyer et al., 1956; Simpson, 1985; Parker, 1956).
Following these discoveries, Parker (1963) developed a quantitative theory for coronal expansion of the solar wind into interplanetary space. This was the start of our current understanding of the heliosphere and the cosmic ray modulation.
Figure 4.Cross-sectional view of an NM64 Neutron Monitor
In 1964 a new neutron monitor design was developed (H. Carmichael, 1964, Fowler 1962, Hatton 1971) to greatly increase the neutron counting rates. The new design was then called the super-monitor and now simply the NM64. Such an increase (to over an order of magnitude larger than the IGY monitor) involved designing a neutron monitor to cover a greater area than that of the IGY monitor. This was achieved with a much larger (BP28) BF3 proportional counters developed at Chalk River by Fowler (1962). Another design development was the use of polyethylene as a moderator and reflector which provided a more stable mechanical structure. Although the lead producer and moderator were chosen to be similar to that of the IGY, the reflector thickness was reduced from 11 inches to 3 inches based on an optimization study by Hatton and Carmichael (1964) with the objective to maximize neutrons counts above evaporation energies while minimizing the contribution from externally produced evaporation neutrons. Most importantly the NM64 design has achieved a globally accepted standardization (Hatton 1971). Today this same design is still recognized as the “Neutron Monitor” standard.