measuring snow pack thickness using cosmic rays juliana araujo march 11, 2004
TRANSCRIPT
Measuring Snow Pack Thickness Using
Cosmic Rays
Juliana Araujo
March 11, 2004
Outline
Introduction Some definitions Previous attempts to measure snow water
equivalent (SWE) Thermal & Epithermal Neutrons Conclusion
Introduction
Cosmic rays neutrons have been a topic of studies for many years. They are useful in quantifying production of isotopes, such as 36Cl.
Safe alternative to rays from highly radioactive 60Co, commonly used in snow gauge
Introduction
Measuring snow pack thickness and SWE is of great importance for river forecasting and water resources planning.
Automation for remote areas: snow pillows radio nuclear devices,
Attenuation of rays in snow monitoring of attenuation of natural isotopes in snow profiling snow density through back scattering X-rays
Topics of DiscussionPrevious Attempts
Bissell 1974 Kodama 1975 Kodama 1980
Experiment
Basic Theory
Equipment
Similarities
Unanswered questions
Thermal & Epithermal Neutrons
Purpose
Goals
Some Preliminary results
New Technique
Some Definitions Thermal, Epithermal, Fast, and High-energy neutrons
Thermal Neutrons: Practically, the Cd-cutoff range Neutrons with an energy <0.6 eV
Epithermal Neutrons: Those between the thermal range and 1eV
Fast Neutrons: Those that are produced in the atmosphere, due to secondary
cascade, through ‘evaporation-like’ process from nuclear interactions of nuclear active particles with higher energies.
Some say, 1eV-100Kev, while others define as <10MeV. The energy spectrum peaks at 1 MeV for fast neutrons.
High Energy neutrons: Those produced from primary cosmic rays, E > 10MeV
More Definitions
“evaporation” & “ground albedo” neutrons These are slow neutrons produced in the soil, and
escape back into the atmosphere, where it is absorbed by the 14N(n,p)14C reaction (Hendrick & Edge, 1966)
‘Neutrons that are created into the soil and are backscattered from soil to air’ (Kodama 1980)
Function of soil moisture content due to diffusion and absorption of “albedo” neutrons.
Bissell, 1974
Deep Snow Measurements Highly penetrating cosmic radiation Counts are produced by NaI(TI) scintillator,
rays are >3MeV High-energy, to ensure that what they detect
is entirely produced by comic radiation The detectors function primarily by photons
generated by cosmic interaction with nuclei in air, water, soil, and in the system.
Bissell, 1974
Lake Mead, Nevada test dampening
effect in the flux due to water at various depths
buried one detector in soil and other, suspended above snow
Counts/min >3 MeV, in 10cmX10cm scintillator as function of water depthCounts/min >3 MeV, in 10cmX10cm scintillator as function of water depthCounts/min >3 MeV, in 10cmX10cm scintillator as function of water depth
Bissell, 1974
Bissell, 1974
Underground detector counts >3MeVflux as
attenuated by snow cover
<3MeV -flux from radioisotopes in soil, and soil moisture near detector
Suspended detector >3MeV fluxes
“unattenuated” by snow <3MeVnatural
terrestrial -radiation attenuated by snow
serves as a control from barometric pressures, seasonal and solar variations
Kodama, 1975 Preliminary Test to
investigate absorption effects of neutrons in water.
Type A and WS detector with counting rates ~170-300n/hr in Tokyo
T can be accurate to a % of the depth, with one measurement per day
Water absorptions of neutrons in Tokyo, compared with 60Co measurements of -rays.
Kod
ama,
1975
Kodama, 1975 Mt. Norikura (2,770m) One detector was
placed inside a snow-free building
The other placed on the ground
The difference in counts from the two detectors with use of empirical curvewater equivalent of snow pile
Experimental error based on counting rates to measuring time and snow depth
Kodama, 1975
Time profile of neutron counting rates.
a) barometric pressure
b) indoor
c) outdoor
d) water depth
1974 November December
Date
wat
er e
quiv
. (d
epth
, cm
)ne
utro
ns c
ount
s/hr
Snow fall
Kodama,1980
Winter season of 1977-1978 Estimated to be effective for deep snow, >1m Only have statistical errors due to n-counting,
and change in moisture content in soil Goal:
how cosmic-gauge is useful on continuous observations of SWE.
Kodama, 1980Experiment:
Takada, 13m a.s.l. Hirosaki (302m) Oritate, 1330m Ohtawa, 1440m
Instrumentation: moderated BF3 counter, 2 cm polyethylene constant response in 1ev-1MeV range two sensors, WS, and HP
After corrections for barometric pressures they used the following to convert the counts to water equivalents:
(1) Nw =Noexp(1-0.753(1-exp(-0.77w))); w<30cm
(2) Nw = N30exp(1-0.00578(w-30)); w>30cm
(3) w1=13ln(0.753/(0.753-ln(No/ Nw))), cm
(4) w2=173ln(N30/Nw) if w1>30cm
Kodama, 1980
Kodama, 1980 Correlation
between barometric pressure and cosmic ray neutron flux, under snow
Kodama, 1980
Good correlation between cosmic-ray gauge, and snow sampler, except near the snow cover maximum due to discordance or field discrepancies
Atmospheric pressure effects, varies with barometric pressure
The greater the snow cover depth, the harder the energy spectrum
Primary Cosmic ray modulation daily variations affects the apparent swe of snow pack
Statistical Fluctuations
Conclusions from the past
All three experiments are used for high energy neutrons.
Whether they use -ray or neutrons, they measure these effects under snow shielding
Related to the attenuation of neutrons in the snow, and moisture in soil.
They do not look at lower energies.
As in the case of Kodama 1980, the method was successful for long term measurements, and can be used for deep snow packs
The new technique
Uses thermal and epithermal neutrons, as means to quantify moisture in the soil, and possibly applicable to snow pack thickness
Unique, because there has been no previous work of this nature, with thermal neutrons
My definition: Thermal neutrons: 0-0.6eV Epithermal neutrons: 0.6-100KeV
normalized thermal neutron flux
0.0 0.4 0.8 1.2 1.6 2.0
de
pth
in
co
ncre
te (
g c
m-2)
0
50
100
150
200
uncovered concrete block, Los Alamos experimentblock covered with ~ 19 cm of water,Los Alamos experimentMCNP calculated flux, uncoveredMCNP calculated flux, 20 cm of water
The new techniqueComparison of depth profiles for measured and calculated thermal neutron fluxes.
Des
ilets
—P
erso
nal C
omm
unic
atio
n
Dep
th in
con
cret
e (g
/cm
2 )
The new Technique
Scalable—volume could be corrected by adjusting the height of the instrument
* F
. M P
hilli
ps e
t al.,
200
0.
Fig.1: Comparison of epithermal and thermal
neutron fluxes in a concrete block at Los Alamos
National Laboratory
The new technique
*Edge, R.D. 1958
Fig. 8. Approximate neutron density near a water surface
Speculation: Snow pack depresses
neutron flux Along with ponding, it
could significantly skew the count rates of thermal neutrons.
Fig. 3a. Epithermal neutron flux as a function of water content (%)
* F. M Phillips et al., 2000.
Fig. 3b. Thermal neutron flux as a function of water content. The thermal flux data of Hendrick and Edge 1966 are shown for comparison.
What This Means Made-up Basalt
varying water content, 3%, 20%, 40% varying amount of water on top of saturated
soil, 5cm, 10cm, and 20cm of water. Results are a good indication that we are
on the right path Although, some of it still is ambiguous
Real Life Simulation, MCnp
Different energies above ground
Neutron flux n/cm^2/yr0 2e-8 4e-8 6e-8 8e-8 1e-7
Dep
th D
en
sity
g/c
m^2
-200
-150
-100
-50
0
50
100
<0.025 eV vs depth density 0.025--0.6 eV vs depth density
Different energies above and below ground
Neutron flux n/cm^2/yr
0.0 2.0e-7 4.0e-7 6.0e-7 8.0e-7 1.0e-6 1.2e-6 1.4e-6
De
pth
De
nsi
ty g
/cm
^2
-200
-150
-100
-50
0
50
100
<0.025 eV vs depth density 0.025--0.06 eV vs depth density 0.06--1 eV vs depth density 1 eV--1 KeV vs depth density 1 KeV--1 MeV vs depth density totals vs depth density
For a typical Montana soil, 20% water content
Thermal & Epithermal above and below ground
Different energies above ground
Neutron flux n/cm^2/yr
0 5e-8 1e-7 2e-7 2e-7 3e-7 3e-7 4e-7 4e-7
Dep
th D
ensi
ty g
/cm
^2
-200
-150
-100
-50
0
50
100
0.6--1 eV vs depth density 1 eV--1 KeV vs depth density
Real Life Simulation, MCnpDifferent energies above and below ground
Neutron flux n/cm^2/yr
0.0 2.0e-7 4.0e-7 6.0e-7 8.0e-7 1.0e-6 1.2e-6 1.4e-6
Dep
th D
ensi
ty g
/cm
^2
-200
-150
-100
-50
0
50
100
<0.025 eV vs depth density 0.025--0.06 eV vs depth density 0.06--1 eV vs depth density 1 eV--1 KeV vs depth density 1 KeV--1 MeV vs depth density totals vs depth density
Thermal & Epithermal above and below ground
20 cm of water
Different energies above ground
Neutron flux n/cm^2/yr0 2e-8 4e-8 6e-8 8e-8 1e-7
Dep
th D
ensi
ty g
/cm
^2
-200
-150
-100
-50
0
50
100
<0.025 eV vs depth density 0.025--0.6 eV vs depth density
Different energies above ground
Neutron flux n/cm^2/yr
0 1e-7 2e-7 3e-7 4e-7
De
pth
De
nsi
ty g
/cm
^2
-200
-150
-100
-50
0
50
100
0.06--1 eV vs depth density 1 eV--1 KeV vs depth density
3% WC Basalt
Flux n/cm^2/yr
0 2e-7 4e-7 6e-7 8e-7
de
pth
, g
/cm
^2
-200
-150
-100
-50
0
50
100
40% WC Basalt
Flux n/cm^2/yr
0 2e-7 4e-7 6e-7 8e-7
dept
h, g
/cm
^2
-200
-150
-100
-50
0
50
100
Fake Basalt, variable WC
20% WC Basalt
Flux n/cm^2/yr0 2e-7 4e-7 6e-7 8e-7
dept
h, g
/cm
^2
-200
-150
-100
-50
0
50
100
ThermalEpithermal
total
Thermal
Epithermal
total
Thermal
Epithermal
total
Air/soil boundary effect
Fig. 4 Slow cosmic-ray neutron density below water.
*Edge, R.D. 1958
20 cm of water
Flux n/cm^2/yr
0 2e-7 4e-7 6e-7 8e-7
de
pth
, g
/cm
^2
-200
-150
-100
-50
0
50
100
Thermal Epithermal Total
5cm of water
Flux n/cm^2/yr
0 2e-7 4e-7 6e-7 8e-7
de
pth
, g
/cm
^2
-200
-150
-100
-50
0
50
100
Thermal Epithermaltotal
Variable Water Depth
10cm of water
Flux n/cm^2/yr0 2e-7 4e-7 6e-7 8e-7
de
pth
, g
/cm
^2
-200
-150
-100
-50
0
50
100
ThermalEpithermal total
Next Steps Analysis of current results, what do
they mean in terms of estimating snow pack thickness?
Correlation to snow water equivalent? Boundary affects between
air/water/soil. Function of thickness of water in between?
References:Avdyushin, S.I., V.V. Abelentsev, E.V. Kolomeets, V.V. Oskomov, R.G.-E. Pfeffer, K.O. Syundikova, and S.D. Fridman, 1988. Estimating snow moisture reserve and soil humidity from cosmic rays. Izvestiya Akademii Nauk SSSR, Seriya Fizicheskaya 52, 2454-2456.
Bissell, V.C., and Z.G. Burson, 1974. Deep snow measurements suggested using cosmic radiation. Water Resources Research 10, 1243-1244.
Kodama, M., S. Kawasaki, and M. Wada, 1975. A cosmic-ray snow gauge. International Journal of Applied Radiation and Isotopes 26, 774-775.
Kodama, M.,1980. Continuous monitoring of snow water equivalent using cosmic ray neutrons. Cold Regions Science and Technology, 3: 295-303.
Above groundThermal Neutrons
Flux n/cm^2/yr1e-7 2e-7 3e-7 4e-7 5e-7
dept
h, g
/cm
^2
-200
-150
-100
-50
0
50
100
3%
20%
40%
Thermal Neutrons
Flux n/cm^2/yr1e-7 2e-7 2e-7 3e-7 3e-7 4e-7 4e-7
dept
h, g
/cm
^2
-200
-150
-100
-50
0
Epithermal Neutrons
Flux n/cm^2/yr
2e-7 4e-7 6e-7 8e-7
dept
h, g
/cm
^2
-200
-150
-100
-50
0
50
100
Thermal & Epithermal
Epithermal Neutrons
Flux n/cm^2/yr
1e-7 2e-7 3e-7 4e-7
dept
h, g
/cm
^2
-200
-150
-100
-50
0
3% 20%
40%
3%
20%
40%