Probe background, technology, calibration, MCNP

DARIN DESILETSHydroinnova

Darin Desilets 2

The instrument: CRS-1000

80 cm

Bare tube (secondary sensor)

Moderated tube (primary sensor)

Charge controller and30 Ah battery

Data logger

Pulse module

Connection to Iridium or cell phone

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The instrument: CRS-1000/B

Pulse module

Bare or moderated tube inside

130 cm

Charge controller and30 Ah battery

Data logger

Connection to Iridium or cell phoneantenna

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Comparison

Tradeoffs

size/weight

long-term performance

power consumption

sensor cost

expandability

protection from moisture

Sensitivity to soil moisture

CRS1000 CRS1000/B

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Same sensitivity?

0 10 20 30 40 50 60 70350

400

450

500

550

600

650

700

750

CRS1000 CRS1000/B

Days

Neutroncounting

rate(counts per

hour)

majorrainevent

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Neutron scattering theory

ss (barns) x (-)

H 20.0 1.00C 4.8 0.16O 4.2 0.12Si 2.0 0.07

xsPower Moderating

2s

3-

ss

cm

cm nuclei

s

s

N

N

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Neutron scattering theory

a

s

xE < 0.1 eV

E > 10 eV x

sa

1Moderatedtube

Bare tube

as

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Energy sensitivity for moderated and bare detectors

-10 -8 -6 -4 -2 0 20.00.51.01.52.02.53.03.54.04.55.0

bare moderated

Logarithm of neutron energy (MeV)

Logarithmof counting

rate(relative)

a

s

x

sx

1

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Why use bare counter?o Built-in pressure/solar activity correctiono Provides long-term check on stabilityo Separate snow from soil moisture (?)o Basic research

– Canopy snow and water– Biomass

o Added issues:– Presence of local absorbers (no universal cal!)– Some very local responses– Greater depth penetration?

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Bare counter enhancements?

2009

02-Jan 04-Jan 06-Jan 08-Jan 10-Jan 12-Jan

SW

E (c

m)

12

14

16

18

20

Neu

tron

coun

ts p

er 1

5 m

inut

es

1300

1400

1500

1600

1700

1800

Snow Pillow

Cosmic-ray probe

Kerplop!

BareModerated

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Improved uncertainty for soil moisture determinations

o Limiting case 1: bare tube has same sensitivity to soil moisture as moderated tube.Add counting rates, reduce sstat by ~40%

o Limiting case 2: bare tube has low sensitivity to soil moistureUse bare to reduce ssyst

s2total=s2

stat + s2syst

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Modeling neutrons with MCNP

o Monte Carlo Neutral-Particleo The original Monte Carlo applicationo Remains the “gold standard” for neutron

transporto Flexible source specification, geometries,

tallieso Overkill? Maybe so

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Summary

o Current configuration is optimized for COSMOS—but other configurations are possible.

o Bare counter: not critical but has certain advantages when used with mod tube.

o MCNP code: an essential tool for basic research with the cosmic-ray probe.