Far-field Monitoring

Far-field Monitoring

of

Rogue Nuclear Activity

with

an Array of Antineutrino Detectors

Neutrino Sciences 2005University of Hawaii, Manoa

December 14-16, 2005

Eugene H. GuillianUniversity of Hawaii, Manoa

Neutrino Sciences 2005University of Hawaii, Manoa

December 14-16, 2005

Eugene H. GuillianUniversity of Hawaii, Manoa

Rogue Nuclear Activity

Fission ReactorFission Bomb

Detonation

PurposeProduce Weapons-Grade Plutonium (93.8% 239Pu)

Make sure a given bomb design works (i.e. it explodes,

not just fizzling)

Expected Size < ≈100 MWth ≈ 1 kiloton TNT

Typical commercial reactor ≈ 2500 MWth

Little Boy ≈ 15 kilotonFat Man ≈ 23 kiloton

Hard to Detect!

Small!

Far-Field Monitoring

Uncooperative Regime

Access within ~100 km not allowed

Far Away

Small event rate (1 / distance2)

• Large Detector• Shielding from cosmic rays• Detector must be affordable

These factors strongly constrain the detector &

array specifications

Antineutrino Detector Module Specification

Far-field monitoring• Distance > ≈ 100 km

Detector must be on the order of 106 m3

100 m

100

m

100 m

Water is the only economically feasible target

• H2O loaded with 0.2% GdCl3

• C.f. GADZOOKS! (Super-K with GdCl3) J. F. Beacom & M. R, Vagins, Phys. Rev. Lett. 93, 171101 (2004)

Detection Mechanism

€

ν e + p → n + e+Inverse Beta Decay

Delayed Event

≈ 20µs

n + Gd Gd + cascade

Evis ≈ 3~8 MeV

Prompt Event

Cherenkov radiation

€

Ee + ≈ Eν −1.3 MeV

Neutrino Energy Spectrum

GADZOOKS! Threshold• Eν > 3.8 MeV

KamLAND Threshold• Eν > 3.4 MeV

GADZOOKS! Efficiency58% of entire spectrum (Eν > 1.8 MeV)

82% of KamLAND efficiency

A Very Basic Look at a Megaton Detector Module

100 m

100

m

100 m

Photo-Sensor Requirement≈ 120,000 units (10 Super-Kamiokande)

Gadolinium2000 metric tons

Water Purification200 Super-Kamiokande’s capacity

~$120 Million @ $1000 per unit

~$10 Million @ $3 / kg

Cost?

The cost of just one module looks to be easily about $500 Million!

Is a Megaton Module Outlandish?

The linear dimensions are not that much larger than those of Super-Kamiokande

Challenges• Deep-Ocean environment

• Remote operations• Mega-structure engineering

Cosmic Ray Background

• Like bullets!• Occasionally they destroy atomic nuclei

Unstable nuclei

Sometimes indistinguishable from antineutrinos!

Shielding from Cosmic Rays

Super-Kamiokande• Shielded by 1000 m of rock

(equivalent to 2700 m of water)• Mitsui Mining Co. property

Super-Kamoikande (and similar experiments) would have cost too much if shielding had to be erected from scratch!

For the megaton module array, we assume that cost of shielding on land is prohibitive.

Ocean & Lake = Affordable Shielding

Array ConfigurationsGlobal Monitoring Regime Regional Monitoring Regime

Want sensitivity to anywhere on Earth Want sensitivity to a well-defined region

Can’t optimize module positioning

Module positions can be optimized because of prior knowledge of likely

locations

Larger Modules Required• 10 Megatons• 1 year exposure

Smaller Modules Will Do• 1 Megatons• 1 year exposure

Global Array 15º 5º Array

Total of 1596 modules

Global Array 2Equidistant Array


Minimum nearest-neighbor distance ≈

600 km

Global Array 3Coast-hugging Array


Minimum nearest-neighbor distance ≈

100 km

Modules removed from coast line by ≈

100 km

Regional ArrayNorth Korea

€

log10 S / S + B

Choose locations based on sensitivity map

(red dots are candidate module positions)

• 250 MWth fission reactor deep inside of

North Korea• Background from commercial nuclear

reactors

Rogue Activity Detection Strategy

(1) Assume that no rogue activity is taking place

(2) If this assumption is incorrect AND if the rogue activity is sufficiently large, there would be a discrepancy between observation & expectation

(3) Use a statistical technique (minimum log-likelihood) to estimate the position & power of the rogue activity

Illustration of the Detection Strategy

If no rogue activity takes place, module 1, 2, & 3 detects B1,

B2, and B3 events

With rogue activity, module 1, 2, and 3 sees an extra S1, S2,

and S3 events

The size of the excess goes as:

Power / Distance2

Seeing the Rogue Activity Above Random Fluctuations

ObservedNumber

ofEvents

Backgroundonly

ObservedNumber

ofEvents

Small Signal + Background

RandomStatistical

Fluctuation

Large Signal + Background

B = # background events

S = # signal events

Signal Strength

€

B = statistical uncertainty

Signal Strength

€

B

SS

€

B

Map of Signal Strength

Rogue Activity

2000 MWth

Equidistant Detector Array Configuration10 Megaton per module

1 year exposure

Detectors with Signal Strength > 3

Remarks on Rogue Activity Detection

Rogue Activity:1. Has sufficiently large power2. Is sufficiently close to detector modules

Cluster of nearby detector modules with significant excess

Pin-PointingRogue activity location

given roughly by the position of the cluster

Cutting on Signal Strength

• Tight cut low background noise, but

loss of signal • Loose cut more

signal, but more background noise

Measuring PowerUse log-likelihood to obtain the most likely

power

P99: Benchmark for Array Performance

Log-likelihood Function• A statistical tool used for hypothesis testing

Hypothesis• No rogue activity is taking place

Information Used in Log-likelihood Function

• Expected number of background events in each detector (from commercial nuclear reactors)• Observed number of events in each detector

• The log-likelihood value is not defined a priori because of random fluctuations in the measurement• Its distribution, however, is defined a priori

99% of measurements give log-likelihood above the

alarm threshold1% of measurements sets

off false alarm

1% False Positive

Rogue Reactor Exists

Hypothesis Incorrect

Log-likelihood function is biased to lower values

Rogue Reactor Power is Weak

Large overlap between observed vs. expected distributions

Can’t reliably detect rogue activity

Unacceptably Large Frequency of False Negatives

Definition of P99

P99 = Rogue reactor power which gives 1% chance of false negative

Global Array Performance

• For each array configuration, make a map of P99

• Procedure for making map:1. Vary the rogue reactor position

2. At each location, determine P99

P99 Map: 5º 5ºMWth

P99 Map: EquidistantScaled to 1596 Modules MWth

P99 Map: Coast-huggingScaled to 1596 Modules MWth

5º 5º

Equidistant

Coast-Hugging

P99 Summary

In Water < 100 MWth

W/in several 100 km of

coast

Several 100 MWth

Deep in continent

Up to 2000 MWth

Regional Monitoring

Example:• A rogue reactor in North Korea

Signal

Background

Signal StrengthAbout the Plots

Signal

• Rogue power = 250 MWth

• Detector mass = 1 Megaton

• Exposure = 1 year

Background

• Commercial nuclear reactors

• 1 Megaton

• 1 year

€

log10 S

€

log10 B

€

log10 S / S + B

Detector Locations

€

log10 S / S + B

23 candidate locations based on map of sensitivity

Performance of Various Array Configurations

Consider configurations with 2, 3, and 4 detector modules

For each configuration, determine:• P99

• Estimated area that contains rogue reactor

Two Modules

95% Confidence

99% Confidence

P99 = 250 MWth

Two Modules

95% Confidence

99% Confidence

P99 = 120 MWth

Three Modules

95% Confidence

99% Confidence

P99 = 626 MWth

Four Modules

95% Confidence

99% Confidence

P99 = 336 MWth

Four Modules

95% Confidence

99% Confidence

P99 = 502 MWth

What if a Georeactor Exists?The Georeactor Hypothesis:

• Unorthodox, but surprising things can happen….• If it does exist, its power is likely to be 1~10 TWth

Total commercial nuclear activity ≈ 1 TWth

If a terawatt-level georeactor does exist, the background level for rogue activity

monitoring increases significantly!

log10 BackgroundNo Georeactor

log10 Background3 TWth Georeactor

Ratio3 TWth / No Georeactor

Squeezing More Information from the Data

Fission Bomb Monitoring

ConclusionGlobal Monitoring

≈ 1000 10 Megaton modules

10 Gigaton-year

P99 Water < ≈ 100 MWth

Several 100 km from

coast

Several 100 MWth

Deep in continent

< ≈ 2000 MWth

Regional Monitoring

Several Megaton-year

P99 100 ~ several 100 MWth

Location(95%

Confidence Level)

Best Worst

< ~ 100 kmBand stretching over

several 100 km

• One module costs several hundred million dollars multiply this by number of modules in array

• A terawatt-level georeactor increases the background level by a factor of several in most locations around the world• A regional monitoring regime seems not-too-outlandish

Appendix

Antineutrino Detection Rate for H2O + GdCl3 Detector

€

3040 Events ⋅T

1 year

⎛

⎝ ⎜

⎞

⎠ ⎟⋅

V

106 m3

⎛

⎝ ⎜

⎞

⎠ ⎟⋅

100 km

D

⎛

⎝ ⎜

⎞

⎠ ⎟2

⋅P

100 MWth

⎛

⎝ ⎜

⎞

⎠ ⎟

€

2.25 events ⋅V

106 m3

⎛

⎝ ⎜

⎞

⎠ ⎟⋅

100 km

D

⎛

⎝ ⎜

⎞

⎠ ⎟2

⋅Y

1 kiloton

⎛

⎝ ⎜

⎞

⎠ ⎟

Reactor• Assume 100% detection efficiency for Eν > 1.8 MeV

Fission Bomb• Assume 100% detection efficiency for Eν > 1.8 MeV• Integrated over 10 sec. burst time

Antineutrino Detection Rate for H2O + GdCl3 Detectors

€

832 Events ⋅T

1 day

⎛

⎝ ⎜

⎞

⎠ ⎟⋅

V

109 m3

⎛

⎝ ⎜

⎞

⎠ ⎟⋅

1000 km

D

⎛

⎝ ⎜

⎞

⎠ ⎟2

⋅P

1 GWth

⎛

⎝ ⎜

⎞

⎠ ⎟

€

22.5 events ⋅V

109 m3

⎛

⎝ ⎜

⎞

⎠ ⎟⋅

1000 km

D

⎛

⎝ ⎜

⎞

⎠ ⎟2

⋅Y

1 kiloton

⎛

⎝ ⎜

⎞

⎠ ⎟

Reactor• Assume 100% detection efficiency for Eν > 1.8 MeV

Fission Bomb• Assume 100% detection efficiency for Eν > 1.8 MeV• Integrated over 10 sec. burst time

Background Processes

Antineutrinos from sources other than the rogue reactor

Non-antineutrino background mimicking antineutrino events

• Commercial nuclear reactors• Geo-neutrinos• Georeactor (possibly)

• Cosmic rays• Radioactivity in the detector

• Require Eν > 3.4 MeV

• Place detector at > 3 km depth under water

• Fiducial volume cut + radon free environment

Antineutrino Detection with a H2O + GdCl3 Detector

Inverse beta decay on target hydrogen nuclei

νe + p n + e+ Prompt Event

Delayed Event

Eν > 1.8 MeVEe ≈ Eν – 1.3 MeV

Detector Threshold: Ee > 2.5 MeV

Eν > 3.8 MeV

Physics Threshold:≈ 20 µs

n + Gd Gd*

Ecascade ≈ 3~8 MeV

Gd + cascade

90% neutron captured by Gd @ 0.2% concentration

Commercial Nuclear Reactors

• 433 reactors• Total thermal power ≈ 1 TW

The effect of commercial nuclear reactors on the

detection sensitivity for a rogue nuclear reactor

Assume that a rogue reactor with P = 250 MWth is operating just north of Hawaii

Top:

Middle:

Bottom:

log10 S

log10 B

€

log10 S / S + B

# events from rogue reactor

# events from commercial

reactors

3.5

7.0

1.5

• Detector target mass = 10 megatons• 1 year exposure• Detectors allowed only in oceans & large lakes• 100% detection efficiency

Possible Detector Locations

23 Locations based on S/sqrt(S+B)

log10(S)

log10(B)

€

log10 S / S + B

Map of S, B, and S/sqrt(S+B) for 1 megaton target exposed

for 1 year

If a Geo-Reactor Exists…

• If it does exist, its power is expected to be 1 ~ 10 TWth, 3 TWth being the most favored value.

• The total power from all commercial reactors world-wide ≈ 1 TWth

In most locations around the world, antineutrinos from a georeactor would outnumber those from commercial reactors

€

2.25 ×104 Events T

1 year

⎛

⎝ ⎜

⎞

⎠ ⎟⋅

M

1 Megaton

⎛

⎝ ⎜

⎞

⎠ ⎟ 3 TWth Georeactor

Far-field Monitoring

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