Monte-Carlo Exploration of Focused 

Neutron Guide Geometries

Samantha Isaac

Mentor: Leland Harriger
NIST Center for Neutron Research

https://commons.wikimedia.org/wiki/File:NCNR_in_2010_from_the_air.jpg

Thermal Cold
New Cold Source

• A new cold source 

will be replacing 

the current cold 

source

• This produces 

most of the cold 

neutrons used at 

the NCNR
Replacing NG5 and SPINS
Software used

• Mcstas

• http://www.mcstas.org/

• Guide_bot_distribution

• Courtesy of Mads Bertelsen

• iFit

• NCNR Rocks Cluster
1st Generation Guide: NG5

• Neutron guides contain coatings that line the inner 

walls that allow the neutrons to bounce down the 

guide

• NG5 is a 41 meter long straight rectangular guide

• Coated in Ni58
NG5 Baseline

2.3 meV: 1.5e8 Flux , 30% brilliance transfer

17 meV: 1.6e8 Flux , 6% brilliance transfer

2
2nd Generation Guides

• Fairly recent

• Supermirror coatings

• Ballistic elliptical shape
Ballistic Ellipse

• Use a ballistic elliptical geometry 

• Each neutron should ideally only bounce once 

down the guide
Ballistic Ellipse

2.3 meV: 5e8 Flux , 80% brilliance transfer

17 meV: 11e8 Flux , 45% brilliance transfer
Improvements to 2nd Generation 

Guides

• Things to look forward to:

• Perfect sample masking

• A focused beam

• No fast neutrons
Sample Masking

• With larger beam 

intensities, we can 

look at smaller 

sample sizes

• Previously sample 

sizes were ok with 

current masking 

techniques

• Mask can only be 

placed at the edge of 

sample 

environment, 

allowing beam to 

widen again by the 

time it reaches the 

sample
• Place a slit at 

monochromator 

focal point & 

focus beam via 

Selene Tip

• Can include a slit 

that provides 

exact selection of 

divergence

• Breaks line of 

sight so fast 

neutrons do not 

hit sample

Selene Tip
Selene Tip

• Over 1 order of magnitude 

reduction in flux at the 

sample position for a 5mm 

sample

• First attempt (ever) at 

optimizing this type of 

instrument configuration

-3 -2 -1 0 1 2 3 -3

-2

-1

0

1

2

3

Selene Tip

Integrated Intensity: 2.0e6

X divergence [deg]

Y divergence [deg]

0

1

2

3

4

5

x 104

-3 -2 -1 0 1 2 3 -3

-2

-1

0

1

2

3

No Selene Tip

Integrated Intensity: 4.7e7

X divergence [deg]

Y divergence [deg]

0

0.5

1

1.5

2

2.5

3

x 105
Double Ellipse with Kink

• Considered on 

the BIFROST 

spectrometer 

at ESS.

• Breaks line of 

sight
Double Ellipse with Kink

• 20cm x 30cm Sample

• Increase in neutron 

flux (~1.2x more 

neutrons)

• ~8.5x more neutrons 

than NG5

-3 -2 -1 0 1 2 3 -3

-2

-1

0

1

2

3

Double Ellipse w/ Kink

Integrated Intensity: 2.6e10

X divergence [deg]

Y divergence [deg]

0

0.5

1

1.5

2

2.5

3

x 106

-3 -2 -1 0 1 2 3 -3

-2

-1

0

1

2

3

Single Ellipse

Integrated Intensity: 2.3e10

X divergence [deg]

Y divergence [deg]

0

0.5

1

1.5

2

x 106
Double Ellipse with Kink

• All fast neutrons 

are eliminated at 

the sample 

position

-3 -2 -1 0 1 2 3 -3

-2

-1

0

1

2

3

Single Ellipse

Integrated Intensity: 5.1e7

X divergence [deg]

Y divergence [deg]

0

0.5

1

1.5

2

2.5

x 106

-3 -2 -1 0 1 2 3 -3

-2

-1

0

1

2

3

Double Ellipse w/ Kink

Integrated Intensity: 0

X divergence [deg]

Y divergence [deg]

0

0.2

0.4

0.6

0.8

1
Future Work

• Perform a careful study of energy resolution & the 

beam profile at the sample position of the double 

ellipse with a kink

• Inhomogeneous phase space typical for a bended guide

• Put in an order for guide parts ASAP since ESS will 

be buying up all of the guides very soon!
Acknowledgments

•Leland Harriger

•Mads Bertelson

•Jeff Lynn

•Julie Borchers, Joe Dura, and all 

SURF Directors

•NSF and CHRNS
Monte Carlo

• Many systems cannot be perfectly modeled

• Monte Carlo randomly samples a distribution 

function to determine the probability of a neutron 

reflecting or transmitting
Momentum Transfer

• The momentum 

and collision 

angle determine 

the momentum 

transfer (Q)