Cosmic-ray neutron probe: non-invasive measurement of soil water content
Marek Zreda, Darin Desilets, Ty Ferré
Department of Hydrology and Water Resources, University of Arizona, Tucson, Arizona, USA
([email protected]; [email protected]; [email protected])
1 MeV source
collision
with hydrogen
6 eV
60
k
0.01
eV
-1050
30
y-ccordinate (cm)
MeV
-1045
0.2 eV
eV
-1040
20
10
-1055
0
-1060
20
-10
10
0
-10
x-ccordin
ate (cm)
-20
-30
-20
-40
2. COSMIC RAYS ON EARTH
Supported by: NSF grant EAR-01-26241 (Hydrology)
HV
+ 24 V
Neutrons are detected using proportional
counters (photo above). Detectors are
placed on or above the ground (photo to
the right), which results in the
FOOTPRINT of tens of meters. However,
detecors can be placed below the ground
for smaller measurement volume.
Explanation:
Thermal
8
7
6
5
10
Epithermal
8
6
4
2
25
Fast
20
granite
basalt
quartz
limestone
15
Thermal
Left: variations of neutron counting rates
(fluxes) with soil water content for four
representative parent rocks.
2
1
Right: Sensitivity of the neutron counting
rate to soil water content calculated as
derivatives of the functions in the left
panels.
0
15
Observations:
Epithermal
10
(1) Thermal neutrons have lowest
sensitivity to water; epithermal have
highest.
5
0
(2) Thermal neutrons are sensitive to rock
chemistry; epithermal and fast are not.
12
9
(3) Sensitivity is highest at low water
contents.
Fast
6
(4) Fast neutrons have the best
combination of sensitivity and absolute
counting rate for soil water content
measurement.
3
10
0.3
0.2
N
n
e
p
p
Cosmic-ray cascade
0.0
0.4
0.1
0.2
0.3
0.4
Volumetric moisture content
6. FOOTPRINT
1700
Dry
Wet
1600
-200
200
1500
N
n
-100
0
100
200
-200
-100
0
100
200
200
nucleonic
cascade
n
n
n
e
100
300
N
e
250
N
air
ground
ground
0.3
p
n
e
0.2
Volumetric moisture content
p
y-coordinate (m)
e
0.1
0.1
Secondary cascade in the atmosphere
P
0.0
Water content
0.4
Epithermal neutron
count rate (cph)
N
100
75%
75%
50%
50%
0
0
-100
-100
-200
-200
200
n
p
N, P - high-energy neutron or proton
n, p - evaporation neutron or proton
p - pion
m - muon
e - electron
g - gamma ray
- nuclear disintegration
Primary cosmic rays, mostly protons,
enter the earth (top), collide with
atmospheric nitrogen and oxygen
nuclei, and create cascades of protons,
neutrons and other secondary particles
(photo and diagram on the left). Many
energetic secondary particles
penetrate the atmosphere and interact with terrestrial nuclei. These interactions
generate neutrons at the earth’s surface. These secondary neutrons are the
source of fast neutrons that are used in our cosmic-ray probe.
0
10
20
30
40
50
60
70
Days since 14 January 2003
Field measurements of soil moisture content using time domain reflectometry
(TDR) and thermal and epithermal neutron fluxes using neutron detectors
revealed a clear relationship between soil moisture and neutron intensity. The
first decrease in neutron fluxes is due to a rain event (day 30); the second (day
42) and third (day 60) are due to snow melt following two snow storms. The
magnitude of the change of neutron intensity is consistent with theoretical
predictions (see SENSITIVITY), indicating that our knowledge of neutron
response to variable soil water content is adequate.
z-coordinate (m)
The cosmic-ray neutron probe can be used jointly with
other techniques, thus bridging the gap between smallscale point measurements and satellite imaging. In this
fashion, it can be used to calibrate remote sensing
instruments.
(1) None: for thermal neutrons
(2) Cd foil: for epithermal neutrons
(3) Polyethylene+Cd: for fast neutrons
pulse counter
0.5
p
(5) It measures soil water at an intermediate scale of tens
of meters.
waterproof electronics
(preamp and discriminator)
9
0
p
(1) It can be non-invasive and non-contact, allowing for
broad coverage of undisturbed soil conditions;
(4) It can be deployed below or above the ground surface;
He-3 proportional counter
Shielding:
Trajectory of a 5.1 GV proton (p)
as it enters the earth's magnetic field
Our method has five useful characteristics:
(3) It can be automated (computerized) easily;
+5 V
4. FIELD EXAMPLE
primary cosmic ray
(2) It does not contain a radioactive source, so it can be
transported easily and left in the field unattended;
3
Thermal neutron
count rate (cph)
Coupling neutron detectors with cadmium and
polyethylene shielding (see INSTRUMENTATION)
changes the energy sensitivity of the instrument, and
thereby also the sensitivity to water content. Results from
a series of laboratory and field experiments show that our
cosmic-ray probe is capable of measuring changes of
water content resulting from irrigation or infiltration after a
storm event (see EXAMPLE). Our numerical
SENSITIVITY experiments indicate that 1% change in
volumetric water content corresponds to between <1%
and 17% change in neutron flux, depending on neutron
energy and soil water content. Neutron intensities are
sensitive to water in the upper 10-100 cm of soil and over
a footprint of 10-100 m in diameter (see FOOTPRINT).
Both the depth and the footprint decrease with the
increasing soil water content.
Cosmic-ray fast neutrons are
produced in the ground when
cosmic-ray particles collide
with nuclei. These neutrons
are moderated (slowed)
through elastic collisions, lose
energy, and eventually
become thermalized. The
energy loss is inversely
proportional to the atomic
weight of nuclei. Because the
efficiency with which
hydrogen thermalizes
neutrons is very high, the
number of low-energy
neutrons depends strongly on
the hydrogen content of the
soil, and thus on its water
content.
5. SENSITIVITY
Sensitivity (% of neutron flux change per % of soil water content change)
-1035
3. INSTRUMENTATION
Neutron flux (= counting rate) (arbitrary units, the same for the three energies)
1. BASIS OF THE METHOD
0.8
We are developing a novel technique for soil water content
determination that operates on the horizontal scale of
dekameters. The method is based on the same principle
that underlies conventional neutron probes: thermalization
of neutrons by hydrogen atoms (see BASIS OF THE
METHOD). The standard neutron probe works by emitting
fast neutrons from a source in the instrument, and
measuring the flux of neutrons that are scattered back.
Our new probe uses a similar detector, but has cosmic-ray
neutrons as a source (see COSMIC RAYS ON EARTH).
te (cm)
z-ccordina
DESCRIPTION
0.0
0.0
-0.5
-0.5
-1.0
-1.0
-1.5
-1.5
-2.0
-200
-100
0
x-coordinate (m)
100
200
-200
-100
0
x-coordinate (m)
100
-2.0
200
Measurement area (footprint)
and thickness for dry and wet
silica sand (100% SiO2),
calculated using (Monte Carlo
N-Particle (MCNP) code for
epithermal neutrons. Both the
footprint and thickness
decrease with increasing soil
water content.