Executive Summary Chapter 1 Introduction History, heritage and operation

THE USE OF THE EARTH AS A MAGNETIC SPECTROMETER Dr. Alessandro Bruno - INFN and University of Bari, Italy

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Dr. Alessandro Bruno - INFN and University of Bari, Italy

Fig.1. PAMELA’s pitch angle distribution (GCR background subtracted) in three rigidity ranges (top). Also shown is the world-wide neutron monitor pitch angle distribution (bottom) averaged between 0158 and 0220 UT.

The PAMELA space experiment is providing first direct observations of SEPs with energies from about 80 MeV to several GeV in near-Earth orbit, bridging the low energy measurements by in-situ spacecrafts and the GLE data by the worldwide network of NMs. Its unique observational capabilities include not only the possibility of measuring the flux energetic spectrum and composition, but also its angular distribution, thus investigating possible anisotropies associated to SEP events [1]. Cosmic Ray cutoff rigidities and asymptotic arrival directions are commonly evaluated by simulations accounting for the effect of the geomagnetic field on the particle transport. Using spacecraft ephemeris data (position, orientation, time), and the particle rigidity and direction provided by the PAMELA tracking system, trajectories of all detected protons are reconstructed by means of a tracing program based on numerical integration methods, and implementing the IGRF-11 and the TS07D [2] models for the description of internal and external geomagnetic sources, respectively. Solar wind and IMF parameters are obtained from the high-resolution Omniweb database. Each trajectory is back propagated from the measurement location with no constraint limiting the total path-length or tracing time, and the corresponding asymptotic arrival direction is evaluated with respect to the IMF direction. Since the PAMELA aperture is 20 deg, the observable pitch-angle range is quite small (a few deg) except in regions close to the geomagnetic cutoff (discarded from the analysis). However, because it is a moving platform, it sweeps through pitch angle space allowing one to construct a pitch angle distribution of the SEPs. Consequently, a quite large pitch-angle range is covered during the whole polar pass. Fig.1 reports PAMELA's vertical asymptotic directions of view (0.39-2.5 GV) during the first polar pass (0158 - 0220 UT) that registered the May 17, 2012 event [3], for different values of particle rigidity (color code). The spacecraft position is indicated by the grey curve. The contour curves represent values of constant pitch angle with respect to the IMF direction, denoted with crosses. In this case the IMF direction is almost perpendicular to the sunward direction. As PAMELA is moving (eastward) and changing its orientation along the orbit, observed asymptotic directions rapidly vary performing a (clockwise) loop over the region above Brazil. PAMELA data can be combined with data from NMs and other space-based detectors, in order to model the directional distribution of solar events, estimating the omnidirectional density and weighted anisotropy.

[1] A. Bruno et al. (2015), Proc. 34th Intl. Cosmic Ray Conf., PoS(ICRC2015)085.

[2] N. A. Tsyganenko & M. I. Sitnov ( 2007), J. Geophys. Res., 112, A06225.
[3] O. Adrian et al. (2015), ApJ 801 L3.

Dr. Alessandro Bruno - INFN and University of Bari, Italy

Fig1: Time variations in the intensity (1.57 - 5.70 GV) of protons, He nuclei, electrons and

positrons, during the Forbush Decrease event associated with the 13 Dec 2006 CME [5].

The PAMELA space experiment is providing first direct observations of Solar Energetic Particles (SEPs) with energies from about 80 MeV to several GeV in near-Earth orbit, bridging the low energy measurements by other spacecrafts and the GLE data by the worldwide network of neutron monitors. Its unique observational capabilities include the possibility of measuring the flux angular distribution and thus investigating possible anisotropies associated to SEP events. The analysis is supported by an accurate back-tracing simulation based on a realistic description of the Earth's magnetosphere, which is exploited to estimate the SEP energy spectra as a function of the asymptotic direction of arrival with respect to the IMF. Fig.1 reports the results for the May 17, 2012 event [1]. Proton fluxes are averaged over the first PAMELA’s polar pass (0158-0220 UT) which registered the event. Two populations with very different pitch angle distributions can be noted: a low-energy component (<1 GV) confined to pitch angles <90 deg and exhibiting significant scattering or redistribution; and a high-energy component (1-2 GV) that is beamed with pitch angles <30 deg and relatively unaffected by dispersive transport effects, consistent with neutron monitor observations. The presence of these simultaneous populations can be explained by postulating a local scattering/redistribution in the Earth's magnetosheath. The quasi-perpendicular orientation of the IMF may be a key factor in the anisotropy effect observed in the particle intensities because entry into the magnetosphere on the flank significantly increases the diffusive volume compared to the nominal 45 deg of the Archimedes spiral. This is the first time that we observe distinct effects of the magnetosphere in the transport of SEPs. This type of analysis is only possible with the unique capability offered by the PAMELA instrument.

[1] O. Adrian et al. (2015), ApJ 801 L3.


Dr. Mirko Boezio - INFN and University of Trieste, Italy

The PAMELA [1] and the AMS-02 [2] space experiments represent the state-of-the-art of the investigation of the charged Cosmic-Ray (CR) radiation in the near-Earth environment. The former was launched into a semi-polar (70 deg inclination) and elliptical (350–610 km altitude) orbit on June 2006 onboard of the Resurs-DK1 Russian satellite; the latter was installed in May 2011 on the ISS. Both the instruments are composed by several subdetectors, with the core constituted by a magnetic spectrometer, providing accurate particle identification and rigidity measurement. While the mission temporal coverage and geometric factor are limited in comparison to ground-based detectors, PAMELA and AMS-02 are able to directly measure the spectral shape and the composition of CR fluxes. The high-precision data collected at low energies are significantly improving our understanding of the solar modulation effects on CRs, allowing the investigation of the long- and short-term CR variations between solar cycles 23 and 24 [3]. In particular, PAMELA measured the temporal evolution of different CR species (p, He, e-, e+), founding evidence of particle charge-sign dependent modulation effects. In addition, PAMELA is providing comprehensive observations of SEP events during the solar cycles 23 and 24, including energetic spectra and pitch angle distributions in a wide interval (>80 MeV), bridging the low energy data by in-situ spacecrafts and the GLE data by the worldwide network of neutron monitors. Major PAMELA’s results include the first direct evidence of magnetosheath effects on SEPs [4]. Similar results are being achieved by AM-02 experiment at relatively higher energies, due to the higher geomagnetic cutoff related to the ISS orbit. Space- and ground-based measurements can be combined with data from NMs in order to model the directional distribution of solar events, estimating the omnidirectional density and weighted anisotropy. Finally, PAMELA and AMS-02 are performing detailed observations of geomagnetic storms and Forbush Decrease (FD) effects induced by CME events. Complementing the integrated fluxes measured by NMs, PAMELA and AMS-02 provide information on the dependency of FD effects on particle composition and energy. As an example, Fig.1 reports the variations in the intensity of the different CR species (1.57 - 5.70 GV), during the FD event associated with the 13 December 2006 CME [5].

[1] O. Adriani et al. (2014), Physics Reports, 544, 4, 323–370.
[2] M. Aguilar et al. (2013), Phys. Rev. Lett., 110, 141102.
[3] M. Boezio et al. (2015), Proc. 34rd Intl. Cosmic Ray Conf., PoS(ICRC2015)037.
[4] O. Adrian et al. (2015), ApJ 801 L3.
[5] M. Mergé et al. (2013), Proc. 33rd Intl. Cosmic Ray Conf., 1215.

Spectral Information from Neutron Monitor Multiplicity Measurement

A new generation of electronics recently developed by Bartol Research Institute, also records the time between pulses. The average number of evaporated neutrons produced in an inelastic interaction is energy dependent and can be roughly described as a power law, consequently spectral information can be in principle extracted this timing data. An example of this new system as a resource utility was demonstrated during a GLE. Over a 6-minute time period on January 20, 2005, the neutron monitor rate at the sea level station of McMurdo, Antarctica increased by a factor of 30, while the rate at the high-altitude (2820 m) station of South Pole increased by a factor of 56. For a number of years Bartol Research Institute and University of Tasmania have conducted an annual latitude survey with a portable monitor aboard a U.S. Coast Guard icebreaker. At the time of the January 20, 2005 GLE, this instrument was in McMurdo Sound and therefore recorded essentially the same primary flux as the stationary monitor at McMurdo. Unlike the stationary monitor, however, the survey instrument was equipped with the new electronics to record the distribution of elapsed times δt between successive counts in a single detector tube.

Figure 6 displays δt distributions for an hour preceding the GLE (i.e., for a pure Galactic spectrum) and for an hour that includes the peak of the GLE. Only the first 12 δt values in each second are accumulated in the distribution. Hence the majority of δt values are discarded during the high count rate interval of the GLE peak, but the values that are recorded should represent an unbiased sample of neutron multiplicities.

During quiet periods the Galactic δt distribution has two populations. For time intervals above ~2 ms, the distribution is a flat, smooth exponential representing single uncorrelated counts with the slope corresponding to the count rate. For time intervals less than ~2 ms, there is an additional sharp spike representing multiplicity events, in which multiple evaporation neutrons are counted from a single incident particle. As shown in Figure 5, the Galactic distribution (black curve) clearly reveals these features.
The GLE distribution (red curve), however, displays a number of differences. First, the number of uncorrelated counts has increased relative to the multiplicity spike, because the solar particles have a lower energy on average than Galactic cosmic rays. Second, the GLE distribution displays a different slope in the uncorrelated region, owing to the higher count rate. Third, the GLE distribution deviates from a pure uncorrelated region, owing to the higher count rate. Third, the GLE distribution deviates from a pure exponential below 50 ms reflecting rapidly changing count rates. Work is ongoing to model these differences, thereby gaining new information on the energy spectrum of GLEs.
Chapter 3

Space Weather

In contrast to the first major deployment of neutron monitors, space weather concerns now are a major consideration in the number and placement of stations. During the IGY, the objective of the neutron monitor network was research and the advancement of our knowledge of the Earth’s environment. Space Weather, however, is a practical concern, that is, understanding, predicting and mitigating effects of transient space events on society has tangible, financial and security factors. One of the phenomena that drives this interest is ionizing radiation, coming from galactic cosmic rays and solar energetic particles. Intense high-energy events, as manifested in ground level enhancements, affect communications at high latitudes and pose radiation hazards for personnel and avionics at aircraft altitudes and orbiting platforms. A network of ground-based network of neutron monitors offers a stable, isotropic and uniform system for the detection and registration of energetic particle events, immune to the operational hiccups that can plague a space-based network in a time of need. Additionally, we would be able to continue building the long term cosmic-ray space climate data base that now extends from the mid 1950s to the present.

The importance of this was articulated in the National Space Weather Action Plan of October 2015, where it states:

1.2…Changes in the near-Earth radiation environment can affect satellite operations, astronauts in space, commercial space activities, and the radiation environment on aircraft at relevant latitudes or altitudes. Understanding the diverse effects of increased radiation is challenging, but the ionizing radiation benchmarks will help address these effects…

5.3.8 DOC, DOD, and NSF, in collaboration with academia, the private sector, and international partners, will develop options to sustain or enhance the worldwide ground-based neutron-monitoring network to include real-time reporting of ground-level events to operational space-weather-forecasting centers.

Deliverable: Complete plan to ensure a sufficient number of neutron detectors are deployed, worldwide, to adequately characterize the radiation environment and support a real-time alert and warning system.

Historically, the global network of monitors was established by international scientific collaboration toward the common goal of the IGY. What and where a country could support a monitor were the main factors in how many and where stations were placed, with scientific considerations secondary—a reasonable strategy for a never-attempted exercise. However, with the potential global impact of a major space weather event, a more judicious plan for the number and location of stations is warranted. The stations should be numerous enough to cover a full range of cutoff rigidities and asymptotic directions so as to be sensitive to beamed SEPs over a wide spectral range. High latitude stations would have the lowest threshold, of course, but by themselves would not provide the spectral information necessary to assess the event’s potential radiological impact.

Medium term action: Construct a plan that has as its main priority a global network of monitors that covers the full range of geomagnetic rigidities over a mesh of asymptotic directions with some minimal angular separation determined through the analysis of archival GLEs.

Long term action: Solicit international collaboration to deploy and support these stations with potential assistance by the US.

Medium term action: Given that traditional IGY or NM-64 BF3 tubes are no longer manufactured, design around commercially available BF3 tubes, an inexpensive neutron monitor kit with a yield function as close as possible to that of the IGY or NM-64 designs. The magnitude of the yield function may be less than the traditional monitors, but should possess a similar spectral response. Keeping it small and inexpensive would facilitate wide deployment.

Implementing this plan could be accomplished as budgets permit, working toward the ultimate goal of a systematic global array of monitors with a minimum number of blind spots. The implementation of the plan must include electronic networking of the instruments, so that real-time data are immediately available to concerned parties. How these data are used can be left to the particular stakeholders and affected agencies and offices.

Linking neutron monitors together in a realtime network enables a number of space weather forecasting and “nowcasting” applications (Kuwabara et al. 2006a; Mavromichalaki et al. 2011). Backtesting studies have shown that automated detection of a Ground Level Enhancement (GLE) onset can provide ~10-30 minutes earlier warning of a major radiation storm than the earliest proton alert issued by the Space Weather Prediction Center (Kuwabara et al. 2006b; Souvatzoglou et al. 2009). Three such GLE alert systems are currently in operation by the University of Delaware (http://www.bartol.udel.edu/~takao/ neutronm/glealarm/), IZMIRAN (http://cr0.izmiran.ru/GLE-AlertAndProfilesPrognosing/), and the National & Kapodistrian University of Athens (http://cosray.phys.uoa.gr/index.php/ glealertplus). Existing systems could be substantially improved if realtime communications could be implemented at additional remote stations (e.g., the U.S. station at South Pole and the Russian station at Tixie Bay).

Numerous other space weather applications of neutron monitors have been proposed. For instance, the possibility of near-term forecasting the energy spectrum of solar energetic particle events has been demonstrated in principle (Oh et al. 2012), as has the possibility of nowcasting radiation levels on Earth’s surface during GLE (Mavromichalaki et al. 2011; see their Fig. 6). Additional proposed applications include the use of “loss cone” anisotropies to warn of approaching ICME (Leerungnavarat et al. 2003 and references therein), and prediction of the north-south orientation of the IMF (i.e., “BZ”) using transport theory to link cosmic ray fluctuations with IMF fluctuations (Bieber et al. 2013)

Chapter 4

Other Science Applications of NM Data

In addition to space weather forecasting, data from neutron monitors are also used in support of other science investigations, beyond studies of cosmic rays and heliospheric physics. One use of neutron monitor data is to determine the flux of neutrons produced by cosmic rays for a broad range of practical applications, including detecting nuclear threats for homeland and national security, calculating the radiation dose to airplane crews and passengers, understanding the rate of single-event upsets (soft errors) in microelectronic devices, measuring soil and snow moisture content, and calculating the production rate of cosmogenic radionuclides used for atmospheric tracers and nuclear treaty verification. In all these applications, neutrons and other secondary particles produced in the atmosphere and surface materials by galactic cosmic rays (and occasionally by solar particles) are either the source of the effect or an important background. Starting from the cosmic-ray local interstellar spectrum or interplanetary spectrum together with a model of the geomagnetic field, the flux and energy spectrum of cosmic-ray-produced neutrons can be calculated using Monte Carlo computer codes to model radiation transport through the atmosphere as a function of altitude/air pressure, geomagnetic cutoff rigidity, and local materials, but the effect of solar modulation on the neutron flux near the Earth’s surface at a given time must come from neutron monitor data. Without ongoing data from stable neutron monitors the cosmogenic neutron flux cannot be determined accurately, and all the practical calculations and measurements that depend on knowing the neutron flux will be severely impaired.

The Neutron Monitor Community Workshop that gave rise to this paper included four presentations on practical applications related to the terrestrial and atmospheric cosmogenic neutron flux determined from neutron monitor data by scientists from the Department of Homeland Security (DHS), the Federal Aviation Administration (FAA), IBM Corporation, and (by proxy) the University of Arizona. Below are articles summarizing those presentations. In addition, the editors received a brief summary from the Byrd Polar Research Center of Ohio State University describing their use of neutron measurements and neutron monitor data to determine snow mass on the surface of ice sheets.
Uses of Neutron Data for Homeland and National Security1

Paul Goldhagen

U.S. Department of Homeland Security National Urban Security Technology Laboratory

The detonation of a terrorist nuclear device in the U.S. is one of the worst things that could happen to our country, and the Departments of Homeland Security, Energy, and Defense fund programs to improve our ability to detect any such device, or the fissile nuclear materials that could be used to make one, before it could reach its target. One way to detect nuclear threats is by detecting the penetrating radiation they give off. Uranium can be detected by the gamma rays it emits, and plutonium emits both gamma rays and neutrons. Gamma-ray detection is more widely employed, but it suffers from frequent innocent nuisance alarms from nuclear medicine patients and medical and industrial radioactive sources. There are no medical patients or medical sources that emit neutrons, and fewer industrial neutron sources, so there are far fewer nuisance alarms for neutron detection, and a neutron detection alarm is a serious indicator that plutonium may be present. Neutrons are also harder to shield than gamma rays because the required shielding is bulkier. Neutron detection is also used in active interrogation, where pulsed high-energy x rays from an accelerator are used to induce fission in fissile material that might be hidden in a container. So detecting neutrons emitted by nuclear threats is an important addition to gamma detection for homeland security. Neutron detection is also used for verification of nuclear treaties and could be used to make standoff measurements of the operating power of foreign nuclear reactors where direct access is denied and for other national security applications.

The sensitivity of detection measurements with an acceptable false alarm rate is limited by the background count rate and by its variation. In addition to coping with statistical fluctuations, detection systems should not alarm just because the background is higher in one situation than in another. The only significant natural background for neutron radiation comes from cosmic rays. So accurate determination of the flux (and energy-angle distribution) of the cosmogenic neutrons at a given place and time is a significant capability for homeland and national security. This can only be done with the aid of neutron monitor data.

The background count rate in deployed detection systems is continually measured, but we need to understand the background in advance in order to:

  • Design new, better detection systems with improved signal/background ratio

  • Compare developmental detection systems tested at different times in different places

  • Optimize alarm threshold settings

  • Prepare search teams for what changes in background rate to expect

  • Deal with rapidly varying position-dependent background

  • Mobile standoff detection in cities – varying shielding by buildings

  • Searching ships.

While there are no free neutrons in the primary galactic cosmic rays (GCR), cosmic rays liberate neutrons when they strike the nuclei of atoms in the atmosphere and cause cosmic-ray air showers. The resulting cosmogenic neutron rate depends on altitude (atmospheric depth), location in the geomagnetic field, solar magnetic activity (solar modulation), and the materials nearby. While determining the effects of these complex dependencies is difficult, with one exception they can be predicted from calculations. The exception is the solar modulation. The only practical way to accurately determine the cosmogenic neutron flux on the surface of the Earth at a given time is to use data from neutron monitors.

The Los Alamos National Laboratory (LANL), in collaboration with the University of Delaware and the National Urban Security Technology Laboratory (NUSTL), has calculated the flux and energy distribution of GCR secondary particles, including neutrons, at the surface of the Earth and in the atmosphere on a global grid of locations as a function of date (McKinney et al. 2012; McGrath et al. 2014; McGrath and McKinney 2014). The work was done with support from the Department of Homeland Security Domestic Nuclear Detection Office. The calculations are similar to those done by several groups to determine the radiation dose to airplane crews (see article below by Kyle Copeland) and neutron monitor yield functions, but employ a unique feature of the MCNP6 Monte Carlo radiation transport code (Pelowitz et al. 2014): the ability to transport charged particles curving through a magnetic field while multiple scattering in air. In this case, it allowed the simulated protons in the air shower to bend in the geomagnetic field and on average spread away from vertical incidence. This feature was crucial for accurate transport through the full depth of the atmosphere to sea level. The cosmic source, calculational methods, and results are available within MCNP6, so scientists and engineers in the homeland security community can use them to make further calculations for all sorts of situations – for example, to calculate the production of cosmogenic radionuclides useful for nuclear treaty verification or to calculate the neutron background spectrum and count rate in a given detector in a building or on soil with a given water content or at each layer of containers on a container ship of a particular size.

The count rate in a neutron detector is given by the integral over neutron incident energy and angle of the response of the detector as a function of energy and angle times the flux and energy-angle distribution of the neutrons. So to determine the background count rate, it is not enough to calculate the cosmogenic neutron total flux; we need the energy and angular distribution, too. The MCNP6 calculations provide these, and there are measurements of the energy distribution to test the accuracy of the calculations.

NUSTL, in collaboration with a series of other laboratories, has made measurements of the flux and energy spectrum of cosmic-ray-produced neutrons on the ground at various locations and elevations (Gordon et al. 2004) and aboard airplanes (Goldhagen, 2000; Goldhagen et al. 2004) and ships, including container ships. The measurements were made using two extended-range Bonner sphere neutron spectrometers (Goldhagen 2011) and covered a wide range of altitudes/elevations and geomagnetic locations. These measurements are being used to benchmark the LANL calculations.

The cosmogenic neutron spectrum covers an extremely wide range of energies—from less than 0.01 eV to more than 10 GeV. In order to show the details of a neutron spectrum covering such a wide range of energy, E, it is useful to plot the energy on a logarithmic scale and the fluence rate, or flux, d/dt or , per ln(E) rather than as d/dE. In this so-called lethargy representation, the characteristic 1/E dependence of d/dE is flattened out because d/d(ln(E) E d/dE, and a linear scale can be used on the vertical axis. In the lethargy representation, the flux in each energy region is proportional to the area under the curve, allowing the viewer to see at a glance what energy regions have more or less flux. Figure ?.1 shows three cosmogenic neutron spectra, one measured on land and two on different container ships, plotted both ways.

Figure ?.1. Three measured cosmogenic neutron spectra plotted in two representations.

Figure ?.2 shows the cosmogenic neutron spectrum measured inside a 40-foot shipping container in a building in Livermore, CA, (green curve) and the spectrum calculated by LANL using MNP6 together with a detailed model of the building and container (black curve). The measured spectrum is the same one shown as the green curve in Figure ?.1. The agreement between the calculation and the measurement is excellent. The spectra show three broad peaks: around 100 MeV from knock-on spallation collisions, around 1 MeV from nuclear evaporation that follows spallation, and around 0.025 eV from thermal neutrons. Between the evaporation peak and the thermal peak there is a plateau where scattering neutrons are slowing slow down and d/dE is proportional to1/E. These four features are typical of terrestrial cosmic-ray neutron energy spectra. Cosmogenic spectra in the atmosphere away from the ground do not have a thermal peak because the nitrogen in air absorbs neutrons that slow to thermal energies. Neutron spectra from fission do not have the high-energy peak.

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