# Presentation: 769971

## Neutron MonitorsHow Sausage is Made  

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

## Reference Document

- _[NEUTRON MONITORS IN THE 21ST CENTURY ?](http://neutronm.bartol.udel.edu/cssp_report.pdf)_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](http://neutronm.bartol.udel.edu/cssp_report.pdf)_
- Update: Goose Bay has been closed and NSF support for McMurdo has been continued (Thanks!!)

## 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
- Modern type – counter filled with 3He:                         n + 3He → p + 3H

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

## 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.

## Simulated Interaction In a Neutron Monitor

- _[http://www.bartol.udel.edu/~clem/nm/display/intro.html](http://www.bartol.udel.edu/~clem/nm/display/intro.html)_*** ***

## 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

## BP-28 Neutron Detector

- 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

***n***

***He***

***Li***

## 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

## 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

## 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

## 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

## 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

## 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

## 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

## 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

## 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

## 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

## 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

## 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

## 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

## 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

## 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.**

## 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

## 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

## Some Important Concepts

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

## 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

## 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.

## 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.

## 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

***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

## 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**

## 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

## 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”

## 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

## 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

## 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)

## 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

## Composition at High Energy: Lopate (ICRC 2001)