1. Overall Efficiency of Neutron Sources
F. X. Gallmeier
Proton Efficiency Workshop
February 29, 2016

2. Layout Spallation neutron source concepts Proton Sources Targets
Moderators
Neutron Optics
Instrument Backend
Neutron Accounting

3. Spallation Neutron Sources
Quantity
Continuous
Long-pulse
Short-pulse
Facilities
PSI-SINQ
ESS
SNS,JPARC,CSNS, ISIS, Lujan
target
Watercooled, lead-filled zircalloy tubes
Helium cooled, rotating tungsten wheel
Liquid mercury, water-cooled tungsten plate
moderators
D2O, D2, water scatterer
Para-H2, H2O
(Para-)H2, H2O, lq. methane
reflector
Large volume D2O
Water-cooled Be
Light/heavy water cooled Be
power
1.3 MW
5 MW
1.4/1/0.2/0.16/0.1 MW
proton energy
590 MeV
2.5 GeV
1.0/3.0/1.6/0.8/0.8 GeV
pulse length
continuous
2.6 ms
< 1 μs
Repetition rate -
14 Hz
60/25/25/50/20

4. SINQ – Continuous Source
High-intensity thermal flux buildup in D2O reflector similar to modern research reactors (ILL, FRM2)
Maximizes neutron leakage from target
Very low thermal neutron absorption in target (Pb, Zr, D2O, Al)
Extremely low neutron absorption in reflector
Maximizes neutron storage time in reflector

5. ESS (in construction) Medium compact target/moderator configuration
Rotating helium-cooled tungsten target
Butterfly moderator configuration of lq. H2 and ambient H2O for bispectral neutron extraction
Reduced-size moderator height boosts neutron brightness
First long-pulse facility dedicated for neutron scattering
Protons
Neutrons H2 H2
H2O
H2O

6. SNS Target-Reflector-Moderator Assembly
Target Module with jumpers
Be Inner Reflector Plug
Target Inflatable seal
4 Moderators
Core Vessel water cooled shielding
Protons
Core Vessel Multi-channel flange
Hg Target

7. Neutron Output Characteristics requires different strategies for Neutron Instrumentation
Short-pulse coupled
Short-pulse decoupled
Long-Pulse
Continuous
Short-pulse coupled
Short-pulse decoupled
Long-Pulse
Continuous

8. Neutron Utilization Quantity continuous Long-pulse Short-pulse
Facilities
PSI-SINQ
ESS
JPARC, SNS, ISIS
Energy-selection
Crystal monochromators, bandpass filters, velocity selectors, spin-echo
Choppers, crystal monochromators, spin-echo, requires more chopping than for short-pulse
Choppers, crystal monochromators, spin-echo
Spectrometers
Triple-axis spectrometers
Chopper spectrometers
Reflectometers
Incident-beam energy selector
Broad-band beam utilization
Diffractometers ?
Chopper-based E selection
Full-pulse utilization
Instrument concepts can be very creative

9. Complexity of Neutron Scattering Instruments
Detector
Proton Beam
Target
36 meters distance from moderator to sample
Sketch of CNCS at SNS

10. About the Proton Sources

11. How are the Protons Beam being produced
Linac to intermediate energy + synchrotron to final energy (JPARC, CSNS, ISIS) - short-pulse beams
Linac to final energy (ESS, SNS, Lujan) long-pulse beams
Linac to final energy + compressor ring (Lujan, SNS), short-pulse beams
Cyclotron, continuous beam
Source pulsing is best imposed by the accelerator

12. The Accumulator Ring Compresses the long beam from the linear accelerator
Front-End: Produce a 1-msec long, chopped, low-energy H- beam
LINAC: Accelerate the beam to ~90% speed of light
Accumulator Ring: Compress by a factor of 1000.
Deliver beam to Target
Chopper system makes gaps
945 ns
Current
1ms
mini-pulse
Current
macropulse

13. ISIS using a Rapid Cycling Synchrotron
H- Ion Source 35 keV
H- RFQ 665 keV
H- Linac 70 MeV
Proton synchrotron 800 MeV Hz
Repeated Use of Relatively Low RF Acceleration Voltage.
Max ~160 kV per turn, (163 m), but over turns
H- stripping at injection into ring
Pulse length 250 μs

14. Cyclotron

15. Motivation to skip Proton Beam Compression
Compression ring is an expensive investment
Synchrotron is power limited?
Long-pulse mode is well suited for small-angle scattering, reflectometers and imaging
Long-pulse mode can for spectrometers be overcome to some degree by utilizing more choppers and mode complex beam optics
Long-pulse mode buys more flexibility

16. The Target Aspects:

17. High Neutron Production needs high-Z Target Material
Neutron Yield:
E<1.5 GeV
Material with high atomic number
Proton energies of GeV

18. Compact Neutron Production Zone for feeding of moderators in pulsed sources
High-density target materials
Compact proton beam footprint
Minimizing in-beam material “dilution” by cooling media

19. Target Choices
Water-cooled heavy metal targets: tantalum-clad tungsten, lead-filled tubes
Flowing liquid mercury or lead eutectics
Rotating tungsten target
Uranium target (ISIS, IPNS)

20. But we live in a world of constraints!
Proton beam footprint reduction has its limits:
Beam-induced heat has to be safely removed
Beam-induced material damage causes lifetime of beam windows to scale with 1/Area*P-Energy
Cycling stresses due to temperature gradients lead to material fatigue
Pulse-induced shock waves in liquid target arrangements lead to dynamic stresses and cavitation induced corrosion
And materials are not perfect: welds, inclusions

21. Moderators to produce thermal and cold neutron beams:

22. Moderator Properties Material #coll 1MeV-1eV Σs (1/cm) Σa(1/cm) Σs/ΣaH2 14
1.2
0.015 80 D2 20
0.2
100000
H2O 15
1.47
0.019 71
D2O pure 23
0.29
5700
D2O(99.8%)
2500 Be 65
0.75
0.001
143 C 86
0.38
0.0003
192
Neutrons are produced with high energy E> 0.1 MeV
Neutrons used in neutron scattering are thermal and cold E<100meV

23. Choices of Moderator Materials at High-power pulsed Sources
ortho
para
Hydrogenous materials for fast neutron slowing down and moderation
Liquids for heat extraction
Radiation resistant
Para-hydrogen moderator T=20 K
Ambient water as pre-moderator and ambient temperature moderator

24. Choices for reflector: Beryllium
Medium-quality moderator
High scattering cross section
Low neutron absorption
Well-behaving structural material
Good conductor
Neutron booster due to (n,2n) reactions
Alternative: lead

25. Coupled/Decoupled Moderators in pulsed facilities
Pre-moderator
Decoupler
Poison plate
Decoupled cold H2 moderator: wrapped with neutron absorber (decoupler) and central absorber (poison plate) plate to reduce the residence time of neurons in moderator and to emit sharp pulses.
Coupled cold H2 moderator: enclosed with pre-moderator (H2O) to scatter neutrons leaving the moderator into the reflector back into the moderator.

26. High-brightness Hydrogen Moderator
By reducing the moderator viewed area and subsequently the moderator size gains of brightness of 2-3 can be obtained.
PMB
PMR
VIEWH
PMT

27. Cold Source Brightness a Metric of Efficiency?

28. Neutron Optics tune the neutron beams to the needs of the Instruments:

29. Neutron Guides: Most frequent Optics Component at Neutron Sources
Low-loss transport of thermal and cold neutrons to sample position
Shape beam size and beam divergence, but phase-space density cannot be improved
Guides consist of mirrors made of glass substrates coated with multi- layer metal coatings with different coherent scattering lengths
Neutron momentum component perpendiclar to mirror survace

30. Neutron optics improvements
Recent developments
High m-value mirrors (m ≥ 7 today)
Metal guide substrates become feasible
True curved guides – elliptical, parabolic
Guides may not be the answer – curved single mirrors (HFIR IMAGINE: 15 x 18 mm2  2x3mm2 with uniform divergence. Solution: 2 bandpass mirrors + 2 elliptical focusing mirrors)
Strategy
Match new technologies to instrument requirements
Test devices with collaborators (e.g. Wolter optics, K-B mirrors, novel combinations of guide shapes, adaptive optics)
CG1 measured and simulated guide exit
Compact SANS employing Wolter optic tested at HFIR
Structure in CG1 is real and largely expected based on simulation model. Specifics of installation and alignment, however, make prediction of detailed performance extremely difficult. - one of the reasons for in situ alignment and adjustment needed – (guide is 11 mm wide by 15 cm tall).

31. Curtesy: Phil Bentley/ESS

32. Neutron Scattering and Detection
Single scatter requirement in samples make most of the incident neutrons end up in beamstop
Large detector banks desired to detect as many of the scattered neutrons as possible
A detected neutron is the only neutron that counts
Sequoia detector Banks

33. Neutron Economy at SNS 1.4 MW SNS produces: 2 ×1017 n/s
Thermal neutrons at beamline start: 2×1012 n/s
Neutrons at sample position (white): 2×1011 n/s
Neutrons at sample (chopped): 2×1010 n/s
Neutrons scattered: 2×108 n/s
Neutrons counted: 5×107 n/s
Neutron counted/Neutrons produced: 3×10-10

34. Costs Analysis SNS construction cost: $ 1400M Yearly SNS budget: $170M
SNS expected life: yrs
Yearly counted neutrons: 9×1014 n/yr
Neutrons per dollar: 4×106 n/$

35. Ways to Improve #neutrons/$
Beam power increase
Target/Moderator/Reflector optimization
Neutron optics optimization
Detector coverage
Background reduction
New instrument concepts

36. Conclusion Showed that many factors play into efficiency
Ultimately it is the science output (increase of knowledge base – often quantified in number of publications) that tells if a facility is being used efficiently
Support staff
Sample environments
Data analysis assistance
Neutron scattering schools …
At SNS we will get the chance to build a second target station and associated 20 additional neutron instruments; a chance to consider more efficient ways of making use of the resources being provided

37. Backup slides

38. ORNL Neutron Science Performance Expectations
Run two targets or less
Run 4500 hours scheduled operations hours per year
Perform target station at 1.4 MW power
Run 7 cycles of HFIR
Invest $10M into instrument upgrades per year out of operations funds
Produce 400+ science papers per year
Inherent expectations:
Operate accident free
Loss of working hours due to injuries not tolerated
Meet or better exceed expectations

39. Para-H2 coupled cylindrical mod.
Compact Target for STS
Choice of target material: tantalum-clad tungsten (high atomic mass, high density, suppressed fission)
Compact proton beam footprint: 30 cm2 (FTS: 140 cm2)
Intermediate energy proton beam:1.3 GeV proton will result in neutron yield of about 33
Edge-cooled rotating tungsten 4cm high
Bright compact neutron source: 10 cm x 10 cm high luminosity
Vertical cut along
the proton
beam direction
Proton Beam
Coupled para-H2
box mod.
Para-H2 coupled cylindrical mod.
Decoupled para-H2
and H2O mod.

40. STS small sample cold spectrometer multiplies gains across elements
Source
1.5
Beam Divergence 2
Detector Coverage 3
280x Gain over CNCS (1.4MW)
High Brightness Moderator
2.6
Repetition Rate Multiplication 12
Source gain is peak brightness 467 kW STS 10/10 cm2 moderator compared to 2MW FTS coupled moderator.
Curtesy: K. Herwig/ORNL

41. Crossing new thresholds in instrument capabilities requires total optimization
> x100 Performance Gains
Source Parameters
Repetition Rate
Target
Moderators
Beam Transport
Small Moderators to Small Samples
Polarization
Sample
Size
Beam Divergence
Sample Environment
Detectors
Spatial Resolution
Count Rates
Area
$$$
Data Acquisition/Data Reduction
Integrated
Live Results
Visualization
Data Analysis
Computational Sciences
High Performance Computing
Theory
Science mission dictates the neutron phase space that is required at the sample position.
Instruments will likely be more narrow in what they can do – but do that much better in order to obtain these types of gains (e.g. smaller samples)
Curtesy: K. Herwig/ORNL