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

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

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

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

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

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

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

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

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

About the Proton Sources

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

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

ISIS using a Rapid Cycling Synchrotron H- Ion Source 35 keV H- RFQ 665 keV H- Linac 70 MeV Proton synchrotron 800 MeV 50 Hz Repeated Use of Relatively Low RF Acceleration Voltage. Max ~160 kV per turn, (163 m), but over 12000 turns H- stripping at injection into ring Pulse length 250 μs

Cyclotron

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

The Target Aspects:

High Neutron Production needs high-Z Target Material Neutron Yield: E<1.5 GeV Material with high atomic number Proton energies of 0.8-3 GeV

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

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)

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

Moderators to produce thermal and cold neutron beams:

Moderator Properties Material #coll 1MeV-1eV Σs (1/cm) Σa(1/cm) Σs/Σa H2 14 1.2 0.015 80 D2 20 0.2 0.00002 100000 H2O 15 1.47 0.019 71 D2O pure 23 0.29 0.00003 5700 D2O(99.8%) 0.00017 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

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

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

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.

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

Cold Source Brightness a Metric of Efficiency?

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

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

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).

Curtesy: Phil Bentley/ESS

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

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

Costs Analysis SNS construction cost: $ 1400M Yearly SNS budget: $170M SNS expected life: 40 yrs Yearly counted neutrons: 9×1014 n/yr Neutrons per dollar: 4×106 n/$

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

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

Backup slides

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

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.

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

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