Neutron Monitor Detection Efficiency
2004 Annual CRONUS Collaboration Meeting
John Clem
University of Delaware
During the late 1940s John Simpson discovered that the latitude dependence of
nucleonic intensity in the atmosphere is many times that of the ionizing component.
The realization of this discovery gave him the inspiration to invent the cosmic ray
neutron monitor that could better exploit the geomagnetic field as a magnetic
spectrometer.
By 1951 Simpson had set up five monitoring stations (the first neutron monitor
network) from the city of Chicago to the magnetic equator in Peru, allowing him to use
the Earth's own magnetic field as an analyzer.
In 1954 and 1955 a sea-level survey by Rose et al., verified experimentally that the
neutron monitor responded to lower energy primary cosmic rays than a muon monitor.
By 1957-1958 (International Geophysical Year) the neutron monitor network expanded
world wide to 50 stations.
In 1964 the IGY network was supplemented with a second generation of Neutron
Monitors which had a much higher count rate called the NM64 while the original design
is called the IGY monitor.
Substantial contribution to space science was made by this early data. One of
these is the discovery of solar modulation which is the influence the Sun exerts
upon the intensity of galactic cosmic rays. When the sun is active, we get fewer
cosmic rays here. As solar activity rises (top panel), the count rate recorded by
a neutron monitor in McMurdo Antarctica decreases.
But how does a Neutron Monitor work ?? What does it detect ??
The process begins in space with primary galactic cosmic rays
entering the atmosphere after passing through the heliosphere
(solar modulation) and geomagnetic field (rigidity cutoff).
Most of these primaries are energetic enough to produce
a nuclear or high energy interaction initiating a cascade
of particles through the atmosphere.
As the ensemble of cascades
develop the particle density and
the particle type distribution varies
with atmospheric depth as shown
in the Figure.
The passage of each particle type
through atmosphere is
determined by different interaction
channels. The dominate
interaction depends on particle
type, energy and material.
Above energies of 100MeV muons are the dominate species at
sea-level, however when considering all energies neutrons
dominate in numbers.
The question remains, how does the Neutron Monitor
respond to this particles ??
In a neutron monitor, neutron sensitive proportional tubes, surrounded by
moderator material and a lead target, detect thermal neutrons produced
locally from interacting incident particles.
Even though neutrons do not leave an ion trail in the proportional tube, the
absorption of a neutron by a nucleus is usually followed by the emission of
charge particles which can be detected.
A proportional tube filled with either 10BF3 or 3He gas respond to neutrons
by the exothermic reaction 10B(n,α)7Li or 3He(n,p)3H.
The reaction cross-sections
for both nuclei is inversely
proportional to the neutron
speed, having a thermal
endpoint (0.025eV)
of roughly 3840 barns and
5330 barns respectively, as
shown in the Figure.
Surrounding each counter is a Moderator which serves to reduce the energy of
neutrons, thus increasing the probability of an absorption inside the counter while also
providing a reflecting medium for low energy neutrons.
Due to conservation of momentum, the neutron energy loss per elastic collision
increases with decreasing atomic mass A, therefore materials with a high concentration
of hydrogen are most effective.
The moderator material used in NMs are either Low Density Polyethelene (NM64) or
Paraffin (IGY).
The Lead Producer, which surrounds the moderator, provides a thick large-nucleus
target for incident particles . A large nucleus such as lead is preferred as the neutron
production rate per unit mass of a material is roughly proportional to A0.7.
Inelastic interactions can separated into 2 stages:
1) Knock on phase: High energy nucleons, fragments, mesons, typically escape
detection, but sometimes produce additional inelastic interactions
2) De-excitation phase: Wounded target nucleus ejects neutrons spectrum peaked
near 1.0~MeV (evaporation neutrons). Source of counts in the proportional tubes.
Low neutron capture cross-sections are also a required characteristic of the produce
Surrounding the lead is an outer moderator, usually referred to as to the
Reflector, which serves to contain low energy neutrons produced in interactions
within the lead as well as rejecting unwanted low energy neutrons (external
evaporation neutrons) produced in the local surroundings from entering into the
detector.
Same material as used in the moderator tubes.
Comparison between IGY
and NM64 Neutron Monitors
To determine the neutron monitor
detection response a simulation was
performed using a transport package
entitled FLUKA combined with
programs written by the author to
simulate the proportional tube and
electronics response
to energy deposition in the gas.
The standard dimensions and
composition of materials of a IGY and a
NM-64 were used as input to the
geometry.
4 meter diameter parallel beam of
mono-energetic particles at a fixed
angle fully illuminates the
neutron monitor as shown in the
illustration, and is repeated for
different incident beam angles, initial
energy and particle species.
Event1
Event2
1GeV Neutron simulated event examples
The resulting detection efficiency of a NM-64 for 6 different particle species including neutrons,
protons, positive and negative pions and muons for vertical incident direction.
It quite clear from this figure that the detector's response is optimized to measure the hadronic
component as muons above 1 GeV is more 3 orders of magnitude less than the hadrons.
In this energy region, muon generated counts is from neutron production in photo-nuclear
interactions and from electromagnetic showers producing multiple ionization tracks in the counter.
Energies below 1GeV stopping negative charge muons (or pions) are captured by a lead nucleus
into a mesic orbit and absorbed by a nucleus which de-excites of the nucleus through the emission
of neutrons which is reflected in the rise in detection efficiency with decreasing energy.
A comparison of the calculated detection efficiency for a NM-64 and IGY with data (Shibata,
et al. 1999) and a previous calculation (Hatton 1971). The solid circular symbols are
laboratory measurements with a neutron beam on a NM-64. The dashed lines represent a
calculation by Hatton (1971) while the solid lines are the results of this work. Black
represents the NM-64 response to neutrons and protons while red represents the IGY
response to neutrons and protons. Within the energy range of the measurement, the data is
in fair agreement with both models which is coincidentally the only region of agreement of
the two calculations as well as the region of peak response when folded with sea-level
particle spectra.
Left: Particle Fluxes measured at Sea-Level as published by Allkofer and
Grieder 1984. Right: The fluxes weighted by the NM64 detection
efficiency (as displayed in previous slides). As shown the detection
efficiency biases the energy spectrum of these particles. The detectable
neutron spectrum is harder than the actual spectrum while the detectable
muon spectrum is much softer with break at 1GeV. The suppression of
protons at low energy is mainly due to ionization energy loss while the
low energy suppression of neutrons is due to change in inelastic crosssections, reduction in evaporation neutrons with lower energy (n-Pb) and
the energy lost due to elastic collisions in the reflector.
Yield function spectra of NM-64 counts at sea level from
vertical incident primary protons is shown (top line).
The contribution from different secondary particle species
are separated into different curves. As shown the dominating
contribution is from neutrons (black line), however protons
and negative muons provide a significant contribution above
5GV.
High latitude and altitude balloon and
spacecraft measurements of the rigidity
spectra of primary cosmic ray protons (upper
spectra) and Helium ions (lower spectra)
above of the Earth’s atmosphere (points) and
global fit to all the spectra (curves).
The count rates from a NM is the integral of response function.
Observations and models of two NM latitude surveys
NM Response Function is the
product of the Primary cosmic
ray spectrum and total NM yield
function.
Relative contribution of different secondary particle
species to NM for a Sea-Level Latitude Survey
We have recently augmented the electronics for our neutron
monitor latitude survey so as to record the elapsed time (dT)
between detected neutrons in each proportional tube
Left; Distribution of the elapsed time dT (modulo 142 ms) between measured single
tube counts for 14.2~GV cutoff rigidity (lower curve and data) and 0.01~GV cutoff
rigidity (upper curve and data). The histograms represent the observed data. The
curves represent simulated data. Right: The same data on a compressed scale
illustrating the transition from correlated to uncorrelated counts.
The integral of dT distributions shown in
previous slide as a function of the integral
lower limit. The integral is the neutron
monitor count rate when the dead time is
the integral lower limit.
The average number of counts in a 6
tube NM-64 calculated for different
dead times.