Roger Pynn

Indiana University&

Spallation Neutron Source

Neutron Production

What Do We Need to Do a Basic Neutron Scattering Experiment?

• A source of neutrons• A method to prescribe the wavevector of the neutrons incident on the sample• (An interesting sample)• A method to determine the wavevector of the scattered neutrons

– Not needed for elastic scattering• A neutron detector

Incident neutrons of wavevector k0 Scattered neutrons of wavevector, k1

k0

k1Q

Detector

Remember: wavevector, k, & wavelength, λ, are related by: k = mnv/(h/2π) = 2π/λ

Sample

Commercially-Available Neutron Sources

• 252Cf (Californium)– ~2.6 year half life; mainly alpha decay; fission produces neutrons– 1 mg emits 2.3x109 n/s with average energy 2.1 MeV– Up to 1011 n/s from a single source

• Sealed-tube – Accelerated deuterons 80 -225 keV; Deuterium target– Average neutron energy is ~ 2.5 MeV– ~ 4x1011 n/s

• Small (2.5 MeV) proton linac– Li target; used for BNCT– Up to 1013 n/s

Neutron Scattering Requires Intense Sources of Neutrons

• Neutrons for scattering experiments can be produced either by nuclear fission in a reactor or by spallation when high-energy (~1 GeV) protons strike a heavy metal target (W, Ta, or U).– In general, reactors produce continuous neutron beams and spallation sources produce

beams that are pulsed between 20 Hz and 60 Hz– The energy spectra of neutrons produced by reactors and spallation sources are different,

with spallation sources producing more high-energy neutrons– Neutron spectra for scattering experiments are tailored by moderators – solids or liquids

maintained at a particular temperature – although neutrons are not in thermal equilibrium with moderators at a short-pulse spallation sources

• Both reactors and spallation sources are expensive to build and require sophisticated operation.

– SNS at ORNL will cost about $1.5B to construct & ~$140M per year to operate

• Either type of source can provide neutrons for 30-50 neutron spectrometers– Small science at large facilities

Nuclear Fission & Spallation are the Methods of Choice to Produce Neutrons for Scattering

Nuclear Fission

Spallation

Artist’s view of spallation

Nuclear Fission

• Prompt neutrons “evaporate” from excited nuclei with energy ~ 2 MeV. About 2.5 neutrons produced/fission.– About 0.5% of neutrons are delayed by a few secs (decay chain).

• ~1 neutron from each fission is “useful”– 1 required to sustain reaction; ~0.5 lost to absorption

• Each fission event produces ~ 190 MeV (neutron & fragment KE, γ, β, ν)

Energy distribution of prompt fission neutrons

J. M. CarpenterNeutron Production & Moderationand the Characterization of Sources

http://www.neutron.anl.gov/NeutronProduction.pdf

Spallation• Complex series of reactions when a high-energy (~1 GeV) particle hits a

heavy nucleus– Incident particles collide with & excite nuclei which give off further particles that collide

with nuclei…..– Excited nuclei reduce their energy by giving off (mostly) neutrons and (eventually) e+

(because they have too many protons) and γ• ~20 neutrons produced per incident proton

– Most with (evaporation) energies ~2 MeV but some up to initial particle energy• ~60% of initial proton energy deposited in target (as heat)

Y(E,A) = 0.1(EGeV – 0.12)(A + 20)

Thick target

Evaporation + angle-dependent tail

The Energy Cost of Various Neutron Sources

• For high-power sources the driving issue is heat removal => use spallation for high power sources– ~ 190 MeV per neutron for fission– ~ 25 MeV per neutron for spallation with protons (threshold at Ep~ 120 MeV)– ~ 1500 MeV per neutron for (n,p) on Be using 13 MeV protons– ~ 3000 MeV per neutron for electrons

• Driving issue for low-intensity sources is cost (electric power, regulatory, manpower etc)– Cost has to be kept “low” (i.e. construction ~$10-20M)– Cost/benefit is still the metric– Spallation and fission cost too much (absent a “killer app” money maker)– Use Be (p,n) or electrons on Ta

Neutrons From Reactors and Spallation Sources Must Be Moderated Before Being Used for Scattering Experiments

• Reactor spectra are Maxwellian• Intensity and peak-width ~ 1/(E)1/2 at

high neutron energies at spallationsources

• Cold sources are usually liquid hydrogen (though deuterium is alsoused at reactors & methane is some-times used at spallation sources)

• Hot source at ILL (only one in theworld) is graphite, radiation heated.

Moderators at Reactors• Thermal moderators are ~1m3 of H20, D20 or Be

– Neutrons lose energy with each collision (~50% in H20; ~40% in D20)– Thermalization takes ~16 collisions in H20; ~29 in D20 and ~ 69 in Be

• Cold moderators are either liquid H2 or D2 @ ~20K– LD2 moderators are usually ~ 10 liter spheres– LH2 moderators are thin, spherical shells 1-3 cm thick– H2 is more effective moderator than D2 but absorbs neutrons more than D2

• Hot moderators (only at ILL) are made of graphite @ ~2000K– Self-heated by radiation from the reactor

Cavity cold-neutron moderator

The Institut Laue Langevin Reactor

1. Safety rod2. Neutron guide pool3. Reflector tank4. Double neutron guide5. Vertcial cold source6. Reactor core7. Horiz cold source8. Control rod

Reactor pool; light water

Spent fuel pool

Fuel element:top view

Cross section ofreactor

The ILL Reactor (contd)

• ~ 1018 n/s; 55 MW of thermal power• Flux at beam tubes ~ 1015 n/cm2/s• 1 hot and 2 cold sources; many beam tubes & guides

700

520

450

VCS

HCS1

HS

D2O

Thermal and Cold Neutron Guides Ready for Installation at the ILL

Neutron Sources Provide Neutrons for Many Spectrometers: Schematic Plan of the ILL Facility

A ~ 30 X 20 m2 Hall at the ILL Houses About 30 Spectrometers. Neutrons Are Provided Through Guide Tubes

The ESRF* & ILL* With Grenoble & the Belledonne Mountains

*ESRF = European Synchrotron Radiation Facility; ILL = Institut Laue-Langevin

The National Institute of Standards and Technology (NIST) Reactor Is a 20 MW Research Reactor With a Peak Thermal Flux of 4x1014 N/cm2/sec. It Is Equipped With a Liquid-hydrogen Moderator That Provides Neutrons for

Seven Neutron Guides

Cold Neutron Source at NCNR was Optimized by Sophisticated Simulation

The original (top) and new (bottom)designs for the liquid hydrogen coldsource cryostat. Changes include elliptical shape, size, H2 pumped out.Gain was 1.5 – 2 times

For Neutron Scattering, Pulsed SpallationSources have Recently Overtaken Reactors

• At reactors we use a narrow wavelength band of neutrons all the time

• At pulsed sources we can often use a broad wavelength band from each pulse and do not have to “throw neutrons away”

Figure from Skold & Price

Complex Codes are Used to Calculate Spallation Processes and Neutron Moderation

Viewgraph courtesy of Phil Ferguson,SNS

Spallation Product Production Rates for 1-GeV Protons on a W Target

Viewgraph courtesy of Phil Ferguson,SNS

Neutron Production Rate per Unit Beam Power as a Function of Proton Beam Energy: it Saturates

(50-cm-diam × 200-cm-long targets bombarded on axis by ~1-GeV protons)

0.0

0.5

1.0

1.5

2.0

2.5

0 500 1000 1500 2000 2500

W targetPb target

LOW

-EN

ER

GY

(< 2

0 M

eV) N

EU

TRO

N P

RO

DU

CTI

ON

RA

TE

(×10

17 n

/s/M

W)

PROTON ENERGY (MeV)

P

Viewgraph courtesy of Phil Ferguson,SNS

Peak Power Density vs. Depth into Target:Highest Power Deposition at the Front of the Target

0.0

0.5

1.0

1.5

2.0

2.5

3.0

0 5 10 15 20 25 30

800 MeV1200 MeV1600 MeV2000 MeV2400 MeV

PE

AK

DE

PO

SIT

ED

PO

WE

R D

EN

SIT

Y (M

W/li

ter)

DEPTH INTO TARGET (cm)

P

(Bare solid W target, proton beam peak power density of 52 kW/cm2)

Viewgraph courtesy of Phil Ferguson,SNS

Neutronic Performance of Lead and Tungsten Targets

0

4

8

12

16

20

24

28

32

0 50 100 150 200

Total ProductionTotal LeakageCylindrical-Surface LeakageFront-Surface LeakageBack-Surface Leakage

NE

UTR

ON

S (<

20 M

eV) P

ER

INC

IDE

NT

PR

OTO

N

TARGET DIAMETER (cm)

55-cm-long Natural Lead

(Stopping-length targets bombarded on axis by 1-GeV protons)

P

0

4

8

12

16

20

24

28

32

0 50 100 150 200

Total ProductionTotal LeakageCylindrical-Surface LeakageFront-Surface LeakageBack-Surface Leakage

NE

UTR

ON

S (<

20 M

eV) P

ER

INC

IDE

NT

PR

OTO

N

TARGET DIAMETER(cm)

30-cm-long Natural Tungsten

Viewgraph courtesy of Phil Ferguson,SNS

Be

protons

NiFe

Cd liner/decoupler

WReflectors

TMRS – Target-Moderator-Reflector-Shield Assembly

>1 MeV>10 keV>1 keV>1eV

Moderators at Spallation Sources• Old idea – keep pulses short for TOF & maximize intensity

– Moderator contains highest possible hydrogen density (~50% energy loss per collision and high cross section)

– Moderator as close as possible to target– Reflector (often Be) around moderators– Decoupler (e.g. Cd) between reflector and moderator– Poison (e.g. Cd, Gd) to limit effective depth of moderator

• Most Short-Pulse Spallation Sources (SPSS) designed this way– Decoupled, poisoned moderators, mostly water

• More recently, recognition of the usefulness of coupled H2moderators– First installed at the Lujan Center in Los Alamos– Basis for LENS

Pulse Shapes for Decoupled Moderators

• Pulse shapes can be fit by Ikeda-Carpenter function: see J. M. Carpenter, Neutron Production & Moderation and the Characterization of Sources http://www.neutron.anl.gov/NeutronProduction.pdf

• Note – Sharp rising edge – time constant depends weakly on coupling– Long tail – time constant increases dramatically with coupling– Peak height increases with coupling (up to ~ factor of 2)

Emission time distribution for ~60 meV neutrons emitted from a poisoned, room-temperature polyethylene moderator at the IPNS

Los Alamos Neutron Science Center

800-MeV Linear Accelerator

LANSCEVisitor’s Center

Manuel Lujan Jr.Neutron Scattering

Center

WeaponsNeutron Research

Proton StorageRing

ProtonRadiography

Neutron Production at LANSCE• Linac produces 20 H- (a proton + 2 electrons) pulses per second

– 800 MeV, ~800 μsec long pulses, average current ~100 μA• Each pulse consists of repetitions of 270 nsec on, 90 nsec off• Pulses are injected into a Proton Storage Ring with a period of 360 nsec

– Thin carbon foil strips electrons to convert H- to H+ (I.e. a proton)– ~3 x 1013 protons/pulse ejected onto neutron production target

H Source+IPF

PSR

123

4

5

6

7

8 9 1011a

1213

14

1516

Area C

Area B

Area A

H Source–DT Linac SC Linac

Line D

11b

Lujan

Blue90 R

60 R30 R

15 R00

15 L30 L

90 L

WNR

Room

Components of a Spallation SourceA half-mile long proton linac…

….a proton accumulation ring….

…and a neutron production target

The Spallation Neutron Source

Producing Neutrons at SNS --Target/Moderators

Core Vessel water-cooled

shielding

Core Vessel multi-channel

flange

Outer Reflector

Plug

Target inflatable

seal

Target Module with jumpers

Proton Beam

The SNS During Construction

Hot cell manipulators

Target area

Beam shutters

A Comparison of Neutron Flux Calculations for the ESS SPSS (50 Hz, 5 MW) & LPSS (16 Hz, 5W) With Measured Neutron Fluxes at the ILL

0 1 2 3 4

1010

1011

1012

1013

1014

1015

1016

1017

ILL hot source ILL thermal source ILL cold source

average flux poisoned m. decoupled m. coupled m.

and long pulse

Flux

[n/c

m2 /s

/str/

Å]

Wavelength [Å]

0 1 2 3 4

1010

1011

1012

1013

1014

1015

1016

1017

ILL hot source ILL thermal source ILL cold source

peak flux poisoned m. decoupled m. coupled m. long pulse

Flux

[n/c

m2 /s

/str/

Å]

Wavelength [Å]

0 500 1000 1500 2000 2500 30000.0

5.0x1013

1.0x1014

1.5x1014

2.0x1014

λ = 4 Å poisoned moderator decoupled moderator coupled moderator long pulse

Inst

anta

neou

s flu

x [n

/cm

2 /s/s

tr/Å]

Time [μs]

Pulsed sources only make sense ifone can make effective use of theflux in each pulse, rather than theaverage neutron flux

Simultaneously Using Neutrons With Many Different Wavelengths Enhances the Efficiency of Neutron Scattering Experiments

Potential Performance Gain relative to use of a Single Wavelengthis the Number of Different Wavelength Slices used

ki

ki

kf

kf

Δλres = Δλbw

Δλres α ΔT

Δλbw α T

λ

λ

I

I

I

λ

NeutronSource Spectrum

A Comparison of Reactors & Short Pulse Spallation Sources

Neutron spectrum is MaxwellianNeutron spectrum is “slowing down”spectrum – preserves short pulses

Energy deposited per useful neutron is ~ 180 MeV

Energy deposited per useful neutron is ~20 MeV

Neutron polarization has been easierSingle pulse experiments possible

Pulse rate for TOF can be optimized independently for different

spectrometers

Low background between pulses => good signal to noise

Large flux of cold neutrons => very good for measuring large objects and

slow dynamics

Copious “hot” neutrons=> very good for measurements at large Q and E

Resolution can be more easily tailored to experimental requirements

Constant, small δλ/λ at large neutron energy => excellent resolution

especially at large Q and E

ReactorShort Pulse Spallation Source