Neutron Monitors How Sausage is Made

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Neutron MonitorsHow Sausage is Made

• LWS Coordinated Data Analysis Workshop on Ground
  Level Enhancement (GLE) Events
• Palo Alto
• January 6, 2008
• Paul Evenson

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Reference Document

• NEUTRON MONITORS IN THE 21ST CENTURY ?A report
  presented to the NRC Committee on Solar and Space
  Physicson the justification for neutron monitors
  and the status (as of April 2008)of the
  University of Delaware neutron monitor network.
• http//neutronm.bartol.udel.edu/cssp_report.pdf
• Update Goose Bay has been closed and NSF support
  for McMurdo has been continued (Thanks!!)

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WHAT IS A NEUTRON MONITOR ?

• A large instrument, weighing 32 tons (standard
  18-tube NM64)
• Detects secondary neutrons generated by collision
  of primary cosmic rays with air molecules
• Detection Method
• Older type proportional counter filled with
  BF3 n 10B ? a 7Li
• Modern type counter filled with 3He
  n 3He ? p 3H

Neutron Monitor in Nain, Labrador Construction
completed November 2000
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Neutron Monitor Principle

• An incoming hadron interacts with a nucleus of
  lead to produce several low energy neutrons.
• These neutrons thermalize in polyethylene or
  other material containing a lot of hydrogen.
• Thermal neutrons cause fission reaction in a 10B
  (7Li 4He) or 3He (3H p) gas proportional
  counter.
• The large amount of energy released in the
  fission process dominates that of all penetrating
  charged particles. There is essentially no
  background.

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Simulated Interaction In a Neutron Monitor
http//www.bartol.udel.edu/clem/nm/display/intro.
html
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BP-28 Neutron Detector

• Stainless steel cylinder 1.90 m long and 15 cm
  diameter
• Filled to 20 cmHg with 96 enriched 10BF3
• 0.2 mm diameter anode wire
• Capacitance is about 20 pF

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BP-28 Neutron Detector
n

• 10Boron has a huge cross section for neutron
  absorption
• It then splits into 7Li and 4He (an alpha
  particle)
• Unlike 235U you cannot make a bomb out of it
  because no secondary neutrons are produced

He
Li
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BP-28 Neutron Detector

• About 94 of the reactions leave the 7Li in an
  excited state, releasing 2.30 MeV of kinetic
  energy
• About 6 go directly to the ground state,
  releasing 2.78 MeV
• This produces about 2x105 free electrons
• Most of the kinetic energy appears in the alpha
  particle
• The particles can hit the wall of the detector
  and not deposit all of their energy

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BP-28 Neutron Detector

• The cathode is maintained at about negative 2.8
  kilovolts
• The anode is near ground
• Electrons drift toward the anode in the resulting
  electric field
• Using VQ/C this would produce a signal of about
  1.5 millivolts

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BP-28 Neutron Detector

• Actually the signal is amplified somewhat in the
  strong electric field very near the wire
• Electrons ionize the gas, producing a cascade
• For appropriate potentials, the amplification is
  proportional
• The gain is approximately 20 for the BP-28 design

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Multi-GeV Particles Are Rare

• Even in a moderately large event like that of 13
  December 2006 the SEP spectrum falls below the
  background at a few GeV
• Detectors must be large to measure the increase
  with statistical precision

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Neutron Monitors are Large

• Calculated yield functions for protons (left) and
  helium (right) incident at the top of the
  atmosphere at South Pole. Black lines, top to
  bottom IceTop thresholds of 1, 5, 10, 20, and
  100 PE. Red lines, top to bottom 3NM64, 3NM64m1.
  Green Line 12XB

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Monitors are Characterized by Yield Functions

• By definition, the convolution of a yield
  function and the top of atmosphere particle
  spectrum gives the counting rate of a monitor
• Apart from geometry factors of order unity this
  is an effective area

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Response Functions

• The product of the yield function and the
  particle spectrum is termed the response
  function
• For steep spectra, such as cosmic and solar, this
  function has a maximum that can be used to
  estimate the particle energy observed by the
  detector
• The response function is also directly measurable
  using a latitude survey

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Latitude Survey Calibration of a Cosmic Ray
Detector

• By observing the change in signal distribution as
  a function of geomagnetic cutoff, response
  functions can be directly measured

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The 20 January 2005 Event

• Expanded view of the 20 January event seen on the
  latitude survey
• The structure on day 384 results from an
  instrument problem, and will be a data gap in the
  final analysis

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Barometer Correction

• All neutron monitor data must be corrected for
  variations in barometric pressure
• The two panels above show the latitude survey
  data before and after a very simple barometer
  correction

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Barometer Correction

• Overall barometer correction is about 1 per mmHg
• A major advantage of neutron monitors is that the
  correction depends almost entirely on the total
  air mass (measured by a barometer) and not on the
  atmospheric density profile
• Actual barometer coefficients are latitude and
  altitude dependent and are individual to station
• Corrections also depend on the spectrum at the
  top of the atmosphere
• For study of long term modulation, a time
  independent correction is typically used
• For solar events one must use a two pressure
  method, correcting the background and the
  increase separately

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Measuring a Response Function

• The response function is just the derivative of
  the counting rate with respect to geomagnetic
  cutoff
• A fit to the (phenomenological) Dorman Function
  is often used to smooth the data

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Monitor Networks

• One monitor alone is of marginal use
• Monitors looking in different directions are
  needed to measure anisotropy
• Monitors with different yield functions are
  needed to measure spectra
• Disentangling spectrum and anisotropy is a major
  problem

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Obtaining Different Yield Functions

• The most common way of obtaining different
  (effective) yield functions is to locate monitors
  at different cutoffs
• Monitors at different altitudes can also be used
  in this way
• Particularly at high altitude, Neutron Moderated
  Detectors (unleaded bare counters) have
  significantly different yield functions from the
  NM64
• Water (Ice) Cherenkov detectors such as Milagro
  and IceTop now are entering the picture

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ENERGY SPECTRUM POLAR BARE METHOD

• South Pole station had both a standard neutron
  monitor (NM64) and a monitor lacking the usual
  lead shielding (Bare). The Polar Bare responds to
  lower particle energy on average. Comparison of
  the Bare to NM64 ratio provides information on
  the particle spectrum.
• This event displays a beautiful dispersive onset
  (lower panel), as the faster particles arrive
  first.
• Later, the rigidity spectrum softens to P 5
  (where P is rigidity), which is fairly typical
  for GLE.

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Cutoff Calculation

• Cutoffs are calculated numerically and are done
  backwards
• Negatively charged particles of known rigidity
  (momentum per unit charge) are launched and
  then followed to see if they escape

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Calculation for Newark, Delaware

• Color Code
• Red Forbidden
• Green Allowed ( touch magnetopause)
• Yellow Allowed (enter magnetotail)
• Blue Undetermined
• Some are simple, some are not
• Most of the time the undetermined trajectories
  are actually forbidden
• Sometimes they suddenly become allowed. We have
  seen this in our balloon flights but are still
  not sure just what is going on

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Some Important Concepts

• Conjugate Point
• Penumbral Bands
• Close relation of some of the clearly forbidden
  and class of undefined trajectories

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Asymptotic Direction

• Here the calculation is restricted to a more
  realistic rigidity range, 1-10 GV.
• Below 1 GV the atmosphere absorbs all trace of
  the particle.
• Above 10 GV there are few particles because the
  spectrum is steep.
• Note that particles come from specific
  directions, the so-called asymptotic directions.
• The asymptotic direction is a function of
  rigidity

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High Latitude Neutron Monitors

• At higher latitude the complicated trajectories
  all lie below the atmospheric cutoff.
• Therefore the asymptotic direction is better
  defined, particularly for solar particles with
  their steep energy spectra.
• Also, the energy response of all such monitors is
  nearly identical.
• So a matched set of monitors such as Spaceship
  Earth allows precise study of angular
  distributions.

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The Name Spaceship Earth

• Follows from the use of the atmosphere and
  magnetic field of the earth to form one giant
  instrument similar in function to lower energy
  detectors on spacecraft.
• Symbolizes the international cooperation
  necessary to build and operate this instrument.
• Reminds us that we are stewards of this planet
  and have a responsibility to use our science for
  societal benefit wherever possible.

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• Spaceship Earth
• Spaceship Earth is a network of neutron
  monitors strategically deployed to provide
  precise, real-time, 3-dimensional measurements of
  the cosmic ray angular distribution
• Eleven neutron monitors on four continents
• Multi-national participation U.S.A., Russia,
  Australia, Canada
• Nine stations view equatorial region at 40-degree
  intervals
• Thule and McMurdo provide crucial 3-dimensional
  perspective

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Earth as a Giant Scientific Instrument

• Circles denote station geographical locations.
• Average asymptotic direction (squares) and range
  (lines) are separated from station geographical
  locations.

STATION CODES IN Inuvik, Canada FS Fort Smith,
Canada PE Peawanuck, Canada NA Nain, Canada
MA Mawson, Antarctica AP Apatity, Russia NO
Norilsk, Russia TB Tixie Bay, Russia CS Cape
Schmidt, Russia TH Thule, Greenland MC
McMurdo, Antarctica
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Directional Response of Spaceship Earth

• Counting rates recorded by neutron monitors
  viewing in different directions are shown in
  different colors
• You can easily see the beam of solar particles
  undergoing scattering and becoming isotropic

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New Developments (I)

• Tunable Yield Functions (Water/Ice Cherenkov
  detectors like IceTop and HAWC) will give
  improved spectra from specific viewing directions
• This will help resolve the ambiguity inherent in
  global fits

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Adding IceTop to the Mix

• Continuous determination of a precise spectral
  index from IceTop illustrates the difficulty of
  obtaining a simultaneous spectral and anisotropy
  solution from the full network
• All information on anisotropy comes from the
  monitor network
• HAWC will extend the independent spectrum
  determination to higher energy

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Continuously Tunable Yield Functions

• With improvements in the data collection system
  now in progress IceTop will be able to look for
  curvature and cutoffs in the spectrum
• HAWC will do the same thing at higher energy

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New Developments (II)

• In addition to being tunable, Cherenkov yield
  functions have a different shape from the neutron
  monitor yield functions. Left IceTop Right
  Milagro (Morgan et al. 2007 -- Merida ICRC)

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Element Composition and Spectrum are Separated
with Combined IceTop and Neutron Monitor Analysis

• Simulated loci of constant value of the indicated
  ratio, varying spectral index (horizontal) and
  helium fraction (vertical). Statistical errors
  (/- one sigma) are shown by thickened lines.
• 20 January 2005 spectrum and galactic composition
  are assumed
• IceTop alone does not resolve composition and
  spectrum
• Adding the information from a Neutron Moderated
  Detector and a standard NM64 neutron monitor
  differentiates composition and spectrum

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Composition at High Energy Lopate (ICRC 2001)

• It would be really interesting to be able to
  measure composition at high energy!