Docstoc

Evenson

Document Sample
Evenson Powered By Docstoc
					                              Neutron Monitors
                            How Sausage is Made


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


                             Paul Evenson
Paul Evenson January 2009
                                                  1
                            Reference Document

• NEUTRON MONITORS IN THE 21ST CENTURY ?
  A report presented to the NRC Committee on Solar
  and Space Physics
  on 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!!)



Paul Evenson January 2009
                                                          2
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 → α + 7Li
  Neutron Monitor in Nain, Labrador       – Modern type – counter
Construction completed November 2000        filled with 3He:
                                            n + 3He → p + 3H
 Paul Evenson January 2009
                                                              3
              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.
Paul Evenson January 2009
                                                                    4
Simulated Interaction In a Neutron Monitor
   http://www.bartol.udel.edu/~clem/nm/display/intro.html




Paul Evenson January 2009
                                                            5
                  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


Paul Evenson January 2009
                                                         6
                    BP-28 Neutron Detector
n



                                        •    10Boronhas a huge
                                          cross section for
                            He            neutron absorption
       Li                               • 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

Paul Evenson January 2009
                                                                7
                            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
Paul Evenson January 2009
                                                                        8
                            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 V=Q/C this
                                         would produce a
                                         signal of about 1.5
                                         millivolts
Paul Evenson January 2009
                                                                    9
                            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



Paul Evenson January 2009
                                                                       10
                        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


Paul Evenson January 2009
                                                                 11
                            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
Paul Evenson January 2009
                                                                   12
                            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

Paul Evenson January 2009
                                                                   13
                            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
Paul Evenson January 2009
                                                                 14
               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
Paul Evenson January 2009                   measured          15
                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
Paul Evenson January 2009
                                                       16
                                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

    Paul Evenson January 2009
                                                                              17
                            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
Paul Evenson January 2009
                                                                    18
                            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

Paul Evenson January 2009
                                                                19
                            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
Paul Evenson January 2009
                                               20
                            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
Paul Evenson January 2009
                                                                        21
   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.
Paul Evenson January 2009
                                                      22
                            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

Paul Evenson January 2009
                                                              23
                       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

Paul Evenson January 2009
                                                                         24
                            Some Important Concepts

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




Paul Evenson January 2009
                                                              25
                            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
Paul Evenson January 2009
                                                                     26
                            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.

Paul Evenson January 2009
                                                    27
                       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.


Paul Evenson January 2009
                                                           28
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

    Paul Evenson January 2009
                                      29
    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




 Paul Evenson January 2009
                                                                   30
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


Paul Evenson January 2009
                                                    31
                            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”


Paul Evenson January 2009
                                                   32
                            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
Paul Evenson January 2009
                                                                        33
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



  Paul Evenson January 2009
                                                      34
                            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)
Paul Evenson January 2009
                                                                    35
                       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

Paul Evenson January 2009
                                                                             36
                        Composition at High Energy:
                           Lopate (ICRC 2001)




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

Paul Evenson January 2009
                                                              37

				
DOCUMENT INFO
Shared By:
Categories:
Stats:
views:46
posted:3/13/2010
language:English
pages:37