Roger Pynn
Indiana University
&
Spallation Neutron Source
Neutron Production
What Do We Need to Do a Basic Neutron Scattering Experiment?
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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
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A neutron detector
Detector
Incident neutrons of wavevector k0
Scattered neutrons of
wavevector, k1
Sample
k0
Q
k1
Remember: wavevector, k, &
wavelength, λ, are related by:
k = mnv/(h/2π) = 2π/λ
Commercially-Available Neutron Sources
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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
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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
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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
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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
Artist’s view of spallation
Spallation
Nuclear Fission
Nuclear Fission
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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
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Each fission event produces ~ 190 MeV (neutron & fragment KE, γ, β, ν)
J. M. Carpenter
Neutron Production & Moderation
and the Characterization of Sources
http://www.neutron.anl.gov/NeutronProduction.pdf
Energy distribution of prompt fission neutrons
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
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~60% of initial proton energy deposited in target (as heat)
Thick target
Y(E,A) = 0.1(EGeV – 0.12)(A + 20)
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
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~ 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)
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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
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Reactor spectra are Maxwellian
Intensity and peak-width ~ 1/(E)1/2 at
high neutron energies at spallation
sources
Cold sources are usually liquid
hydrogen (though deuterium is also
used at reactors & methane is sometimes used at spallation sources)
Hot source at ILL (only one in the
world) is graphite, radiation heated.
Moderators at Reactors
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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
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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
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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.
2.
3.
4.
5.
6.
7.
Safety rod
Neutron guide pool
Reflector tank
Double neutron guide
Vertcial cold source
Reactor core
Horiz cold source
8.
Control rod
Reactor pool; light water
Cross section of
reactor
Fuel element:
top view
Spent fuel pool
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
HS
HCS1
D2O
52
450
0
70 0
VCS
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 cold
source cryostat. Changes include
elliptical shape, size, H2 pumped out.
Gain was 1.5 – 2 times
For Neutron Scattering, Pulsed Spallation
Sources 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
2.5
P
1.5
17
n/s/MW)
2.0
( ×10
LOW-ENERGY (< 20 MeV) NEUTRON PRODUCTION RATE
(50-cm-diam × 200-cm-long targets bombarded on axis by ~1-GeV protons)
1.0
W target
Pb target
0.5
0.0
0
500
1000
1500
PROTON ENERGY (MeV)
2000
2500
Viewgraph courtesy of Phil Ferguson,SNS
Peak Power Density vs. Depth into Target:
Highest Power Deposition at the Front of the Target
2
PEAK DEPOSITED POWER DENSITY (MW/liter)
(Bare solid W target, proton beam peak power density of 52 kW/cm )
3.0
800 MeV
1200 MeV
1600 MeV
2000 MeV
2400 MeV
2.5
2.0
1.5
P
1.0
0.5
0.0
0
5
10
15
20
25
30
DEPTH INTO TARGET (cm)
Viewgraph courtesy of Phil Ferguson,SNS
Neutronic Performance of Lead and Tungsten Targets
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NEUTRONS (<20 MeV) PER INCIDENT PROTON
NEUTRONS (<20 MeV) PER INCIDENT PROTON
(Stopping-length targets bombarded on axis by 1-GeV protons)
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Total Production
Total Leakage
Cylindrical-Surface Leakage
Front-Surface Leakage
Back-Surface Leakage
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20
16
12
8
4
0
0
50
100
150
TARGET DIAMETER (cm)
200
55-cm-long Natural Lead
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28
24
20
Total Production
Total Leakage
Cylindrical-Surface Leakage
Front-Surface Leakage
Back-Surface Leakage
16
12
8
4
0
0
50
100
150
TARGET DIAMETER(cm)
200
30-cm-long Natural Tungsten
P
Viewgraph courtesy of Phil Ferguson,SNS
protons
moderator
poison
Cd liner/
decoupler
Reflectors
W
Be
Fe
Ni
TMRS – Target-Moderator-Reflector-Shield Assembly
>1 MeV
>10 keV
>1 keV
>1eV
<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 H2
moderators
– First installed at the Lujan Center in Los Alamos
– Basis for LENS
Pulse Shapes for Decoupled Moderators
Emission time distribution for ~60 meV neutrons
emitted from a poisoned, room-temperature polyethylene
moderator at the IPNS
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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
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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)
Los Alamos Neutron Science Center
Manuel Lujan Jr.
Neutron Scattering
Center
Proton
Radiography
800-MeV Linear
Accelerator
LANSCE
Visitor’s Center
Proton Storage
Ring
Weapons
Neutron Research
Neutron Production at LANSCE
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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
Area C
Area B
+H
Source
IPF
Area A
DT Linac
– H Source
SC Linac
Line D
PSR
8
9 10
7
6
11a
11b
12
13
5
14
4
3
90 R
Blue
Room
60 R
30 R
15 R
00
15 L
30 L
WNR
90 L
15
2
1
16
Lujan
Components of a Spallation Source
A half-mile long proton linac…
…and a neutron production target
….a proton accumulation ring….
The Spallation Neutron Source
Producing Neutrons at SNS --Target/Moderators
Target Module with jumpers
Outer
Reflector
Plug
Target
inflatable
seal
Core Vessel
watercooled
shielding
Core Vessel
multi-channel
flange
Proton
Beam
The SNS During Construction
Beam shutters
Target area
Hot cell manipulators
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
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15
10
14
10
13
10
12
10
ILL hot source
ILL thermal source
ILL cold source
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10
10
10
0
1
2
3
Wavelength [Å]
2.0x10
2
Instantaneous flux [n/cm /s/str/Å]
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17
10
16
10
15
10
14
10
13
10
12
10
11
10
10
average flux
poisoned m.
decoupled m.
coupled m.
and long pulse
2
2
Flux [n/cm /s/str/Å]
10
Flux [n/cm /s/str/Å]
peak flux
poisoned m.
decoupled m.
coupled m.
long pulse
17
10
4
ILL hot source
ILL thermal source
ILL cold source
0
1
2
3
Wavelength [Å]
14
1.5x10
14
1.0x10
14
5.0x10
13
Pulsed sources only make sense if
one can make effective use of the
flux in each pulse, rather than the
average neutron flux
λ=4Å
poisoned moderator
decoupled moderator
coupled moderator
long pulse
0.0
0
500
1000
1500
Time [μs]
2000
2500
3000
4
Simultaneously Using Neutrons With Many Different Wavelengths
Enhances the Efficiency of Neutron Scattering Experiments
Neutron
Source Spectrum
I
ki
Δλres = Δλbw
kf
I
λ
Δλbw α T
ki
kf
I
Δλres α ΔT
λ
λ
Potential Performance Gain relative to use of a Single Wavelength
is the Number of Different Wavelength Slices used
A Comparison of Reactors & Short Pulse
Spallation Sources
Short Pulse Spallation Source
Reactor
Energy deposited per useful neutron is
~20 MeV
Energy deposited per useful neutron is
~ 180 MeV
Neutron spectrum is “slowing down”
spectrum – preserves short pulses
Neutron spectrum is Maxwellian
Constant, small δλ/λ at large neutron
energy => excellent resolution
especially at large Q and E
Resolution can be more easily tailored
to experimental requirements
Copious “hot” neutrons=> very good
for measurements at large Q and E
Large flux of cold neutrons => very
good for measuring large objects and
slow dynamics
Low background between pulses =>
good signal to noise
Pulse rate for TOF can be optimized
independently for different
spectrometers
Single pulse experiments possible
Neutron polarization has been easier