Can One Build A 

Global Neutron 

Surface Spectrum 

Map?

ROBERT VALDILLEZ

DR. HANS PIETER MUMM
Outline

 Background

 Analysis of Previously Gathered Data

 Analytical Function to Predict Neutron Spectra

 Data Collection

 Next Steps

 Conclusion
Outline

 Background

 Analysis of Previously Gathered Data

 Analytical Function to Predict Neutron Spectra

 Data Collection

 Next Steps

 Conclusion
What is a surface neutron spectrum 

map and why is it useful?

 A tool to determine the neutron background 

anywhere, anytime

 Usefulness

Searches for illicit special nuclear material

Radiation Monitoring

Fundamental Nuclear Physics Research
Neutron Spectrum

 Neutron energy Energy range

 0.0–0.025 eV Cold neutrons

 0.025 eV Thermal neutrons

 0.025–1eV Epithermal Neutrons

 1–10 eV Slow neutrons

 10–300 eV Resonance neutrons

 300 eV–1 MeV Intermediate neutrons

 1–20 MeV Fast neutrons

 > 20 MeV Ultrafast neutrons
Neutron Spectrum

 Neutron energy Energy range

 0.0–0.025 eV Cold neutrons

 0.025 eV Thermal neutrons

 0.025–1eV Epithermal Neutrons

 1–10 eV Slow neutrons

 10–300 eV Resonance neutrons

 300 eV–1 MeV Intermediate neutrons

 1–20 MeV Fast neutrons

 > 20 MeV Ultrafast neutrons
Neutron Flux

 Number of neutrons passing through a given 

area in a specified amount of time.

 Usually measured in neutrons per cm2 per 

second.
Sources of Neutrons 

 Cosmic Rays

 Galactic Cosmic Rays

Supernovae

Quasars

Gamma-ray bursts

 Solar Energetic Particles

 Terrestrial

 Natural Radioactivity of Spontaneously Fissionable 

Elements in the Earth’s Crust

 NIST
How cosmic rays produce neutrons
What affects neutron flux?

 Dominant Effects

Barometric Pressure

Changes in Cosmic Ray Flux

Soil Moisture Content

 Other Effects

The Ship-Effect

Column Water Vapor

Humidity
Outline

 Background

 Analysis of Previously Gathered Data

 Analytical Function to Predict Neutron Spectra

 Data Collection

 Next Steps

 Conclusion
University of Delaware

Bartol Research Institute 

Neutron Monitor Program

 Neutron Flux data from four sites around the world

 Newark, Delaware

 McMurdo, Antarctica

 South Pole, Antarctica

 Thule, Greenland

 Uncorrected Data

 Corrected Data

 Pressure Data
One Year of Data from Single Site
Pressure Effect About 25%
Solar Effect About 6%
All Other Effects About 1.5%
Outline

 Background

 Analysis of Previously Gathered Data

 Analytical Function to Predict Neutron Spectra

 Data Collection

 Next Steps

 Conclusion
Input Variables 

 Solar Modulation Potential (Sunspot Number)

 Influences energy spectrum of primary cosmic rays

 Vertical Cut-off Rigidity

A measure of the shielding provided by the 

earth’s magnetic field

 Atmospheric Depth (Barometric Pressure)
Spectrum at Two Pressures 

Recorded During One Weekend[1]
Outline

 Background

 Analysis of Previously Gathered Data

 Analytical Function to Predict Neutron Spectra

 Data Collection

 Next Steps

 Conclusion
FaNS-1 – Schematic
FaNS-1 – Output Signal
FaNS-1 – Picture
Fixing the Detector

PMTs were coming 

loose from the 

scintillator blocks

Silicone adhesive 

used to bond 

them together

Tape used to 

make them light 

tight
Data Logger
Outline

 Background

 Analysis of Previously Gathered Data

 Analytical Function to Predict Neutron Spectra

 Data Collection

 Next Steps

 Conclusion
What are the next steps in the 

project?

 Take long term data with FaNS-1 and the data 

logger

 Take data a different locations and during 

different weather conditions

 Compare these data with the function output

 Collect sources of global real-time data for 

pressure, sunspot number, and other effects

 Create a web tool that will predict the spectrum 

at any location and time
Outline

 Background

 Analysis of Previously Gathered Data

 Analytical Function to Predict Neutron Spectra

 Data Collection

 Next Steps

 Conclusion
Conclusions

 Based on the Bartol data it is feasible to create a 

neutron surface spectrum map with fairly high 

precision

 Previous work with an analytical function can 

predict neutron flux throughout the spectrum

 Neutron flux lends well to a web tool and/or an 

app that anyone can use that would like to 

know the background neutron spectrum at their 

location in near real-time
Acknowledgements

 Dr. Hans Pieter Mumm

 Dr. Paul Huffman

 NIST SURF Program

 NCNR

 NSF

 DoE
References

 1. Sato, T., & Niita, K. (2006). Analytical functions to predict cosmicray neutron spectra in the atmosphere. Radiation research, 166(3), 

544-555.
Questions?