neutron sources - pdfs.semanticscholar.org
TRANSCRIPT
September 6, 2013
Mitg
lied
der H
elm
holtz
-Gem
eins
chaf
t
Neutron sources Jörg Voigt
September 6, 2013 Folie 2
1. How do we get free neutrons?
2. How do we make free neutron useful?
3. How do we bring the neutrons to the experiment?
4. How do we detect neutrons?
Contents:
September 6, 2013 Folie 4
What users want: Good data !!!
What users have: small samples with weak effects
€
Idet = εprεsecεdet ⋅σ s ⋅Vs ⋅ I0
High flux, good resolution, low background !!!
Small cross section for neutron scattering
Sample
Analyzer
Detector
Monochromator Primary
spectrometer
Secondary spectrometer
Beam from a neutron source
Efficiencies of primary, secondary spectrometers, detector system - subjects of dedicated lectures
September 6, 2013 Folie 5
Neutron source
Nuclear installation emitting neutrons
Neutron spectrometer
Neutron transport system
Spectrum transformer
Neutron
spectrometer
September 6, 2013 Folie 6
How to obtain free neutrons?
6
Fission nuclear reactions
are used in modern continuous neutron sources.
Spallation nuclear reactions
are used in modern pulsed neutron source
September 6, 2013 Folie 7
Nuclear fission reaction
The first free neutrons (Chadwick,1932):
~ 100 n/cm2 s!
September 6, 2013 Folie 8
The breakthrough in 40ies: nuclear fission reactors
~ 107 n/cm2 s!
CP-1 reactor in USA
235U + neutron → fission fragments + 2.52 neutrons + 180 MeV
September 6, 2013 Folie 9
2000
FRM II
9
CP-1 reactor in USA
High-flux reactor at the ILL
From 1942 to 2009
September 6, 2013 Folie 10
Nuclear fission reaction
→ capture of a slow neutron → deformation of nucleus → splitting into two fragments, simultaneously
releasing 2 or 3 (on average 2.5) “prompt” neutrons with energies 1.29 MeV
235U + neutron → fission fragments + 2.52 neutrons + 180 MeV
Chain reaction
September 6, 2013 Folie 11
Nuclear fission reaction 235U + neutron → fission fragments + 2.52 neutrons + 180 MeV.
The critical mass Mc.
⇒ the number of neutrons will increase exponentially ⇒ the reaction will become uncontrollable very quickly ⇒ a huge energy release (an explosion: A-bomb)
⇒ the number of neutrons will decrease over time ⇒ it is impossible to sustain a chain reaction:
So, this neutron producing reaction is unstable.
How to obtain a stable neutron flux?
If the mass of fissile material M>Mc:
If the mass of fissile material M<Mc:
September 6, 2013 Folie 12
Prompt neutrons
Delayed neutrons
Delayed neutrons
Nuclear fission reactors: delayed neutrons
M < Mc
Considering prompt neutrons in a reactor
Delayed neutrons keep reactor burning.
September 6, 2013 Folie 13
How to control a fission reactor? Remove thermal neutrons
September 6, 2013 Folie 14
Spallation reaction
The de Broglie wavelength of the proton nucldmEh <<= 22λ
A high-energy proton hits a nucleous:
September 6, 2013 Folie 15
Spallation reaction
Proton pulse determines neutron pulse
b The spallation process takes 10-15 s.
This pulse can be made:
• rather long, about 5 ms (the long pulse spallation source (LPSS)) • or rather short, about 1 µs (the short pulse spallation source (SPSS))
September 6, 2013 Folie 16
Spallation neutron source
For the long pulse spallation source - LPSS (τ pulse = 5 ms): the protons go to the target directly
H+
Linac H –ion source
Carbon seeve Target
n
n
n
RF structures
Ekin ~ 1 GeV
Carbon sieve
Liquid metal target (Bi, Pb or Hg) plays a role of the reactor core
Target
September 6, 2013 Folie 17
Spallation neutron source
For the short pulse spallation source - SPSS (τpulse =1 µs): accumulate protons in short bunch
H+
Linac H –ion source
Carbon seeve Target
n
n
n
RF structures
Compressor ring
September 6, 2013 Folie 18
Comparison of neutron-producing reactions (neutron yields and deposited heat)
Reaction Energy/event Yield (neutron/event) Deposited heat (MeV/neutron)
(T,d) fusion ~1 neutron/fusion 3
235U fission ~1 neutron/fission 180
Pb spallation 1 GeV ~ 20 neutron/proton 23 238U spallation 1 GeV ~ 40 neutron/proton 50
The heat deposition results in the cooling problem ⇒ the real limiting factor for all kinds of neutron sources!
• fusion is the most attractive process → a technique of a far future • spallation is more attractive than fission
September 6, 2013 Folie 19
Thermal moderators:The time average flux is defined for 50 Hz (5 MW total power) operation for the short pulsemoderators and 16.667 Hz operation (also 5 MW total power) for the long pulse.
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]
Pulsed sources Reaction Energy/event Yield (neutron/event) Deposited heat
235U fission ~1 neutron/fission 180 MeV/neutron Pb spallation 1 GeV ~20 neutron/proton 23 MeV/neutron 238U spallation 1 GeV ~40 neutron/proton 50 MeV/neutron
5 MW spallation source
September 6, 2013 Folie 20
4. Peak, per pulse and time average fluxesThermal moderators:
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 1 2 3 4
1010
1011
1012
1013
1014
1015
1016
1017
ILL hot source ILL thermal source ILL cold source
flux per pulse poisoned m. decoupled m. coupled m. long pulse
Flux
[n/c
m2 /s
/str/
Å]
Wavelength [Å]
Pulsed sources Reaction Energy/event Yield (neutron/event) Deposited heat
235U fission ~1 neutron/fission 180 MeV/neutron Pb spallation 1 GeV ~20 neutron/proton 23 MeV/neutron 238U spallation 1 GeV ~40 neutron/proton 50 MeV/neutron
5 MW spallation source
September 6, 2013 Folie 22
Nuclear installation emitting neutrons
Neutron spectrometer
Spectrum transformer
22
Neutron source
Ultra cold E <0.5 µeV λ > 400 Å Very cold E=0.5µeV-0.05 meV λ = (40-400) Å Cold E=(0.05-5) meV λ = (4-40) Å Thermal E=(5-100) meV λ = (0.9-4) Å Hot E=100 meV -1eV λ = (0.3-0.9) Å
Desired neutron spectrum
E > 1 MeV
Source neutron spectrum
€
λ(Å) =hmv
=3956v(m /s)
=81.8
E(meV )
September 6, 2013 Folie 23
ΔE A( )
A20 40
0
0.2
0.4
0.6
ΔE
A
How to cool neutrons?
To bring them into a cold body – moderator
€
ΔE =2AA +1( )2
⇒ multiple elastic collisions with the light atoms of the moderator (like billiard balls)
⇒ Energy loss per collision
till E = EM = kTM ≈ 25 meV (TM = 300 K ) The thermalization process takes ~10 µs
H2, D2 – the best choices
September 6, 2013 Folie 24
Nuclear reactor
4
2
1
5
6
3
5 m reactor core
heavy water moderator of high-energy fission neutrons (TM =300 K)
Maximum of the thermal neutron flux density is at r = 10-15 cm
core
Φt
Radius r0
Full thermalization
Absorption ~ vn-1
September 6, 2013 Folie 25
4
2
1
5
6
3
FRM-2 reactor in Garching, Germany
5 m
Neutron beam tubes • the entrance should be placed exactly at r0 • no direct view of the core (background of fast
neutrons and γ-rays from the core)
reactor core
heavy water moderator
r0= 10-15 cm
Light water tank (biological shielding)
concrete Φt
Radius
20 MW (8 kg 235U) D2O H2O
⇒ Tangential beam tubes
September 6, 2013 Folie 26
Ultra cold E <0.5 µeV λ > 400 Å Very cold E=0.5µeV-0.05 meV λ = (40-400) Å Cold E=(0.05-10) meV λ = (3-40) Å Thermal E=(10-100) meV λ = (0.9-3) Å Hot E=100 meV -1eV λ = (0.3-0.9) Å
1.2
0.4
0.8
1.6
0 2 4 6 8 10 λ, Å
Φ(E
)
Interatomic distances in solids ~ 5 Å
Lattice energies ~ 10-100 meV
Thermal neutron spectrum (T=300K)
( )⎭⎬⎫
⎩⎨⎧−=Φ
MMkTE
TkEE exp2
33π
September 6, 2013 Folie 27
Cold neutrons: Larger distance or lower energies
Hot neutrons: Small lattice spacing or higher energies
⇒ heating or cooling of the moderator
( )⎭⎬⎫
⎩⎨⎧−=Φ
MMkTE
TkEE exp2
33π
Maxwellian distribution Cold E=(0.5-10) meV λ = (3-40) Å Thermal E=(10-100) meV λ = (0.9-3) Å Hot E=100 meV -1eV λ = (0.3-0.9) Å
1.2
0.4
0.8
1.6
0 2 4 6 8 10 λ, Å
Φ(E
)
hot neutrons T=1000 K
thermal neutrons T=300 K
cold neutrons T=50 K
September 6, 2013 Folie 28
1.2
0.4
0.8
1.6
0 2 4 6 8 10 λ, Å
Φ(E
)
hot source
thermal neutrons
cold source
• The hot source: Graphite block, T = 2400 K • The cold sources: Liquid H2 or D2, T = 20 K.
Up to 20 times gain in the corresponding
neutron flux!
4
2
1
5
6
3
Hot and cold sources
September 6, 2013 Folie 29
Nuclear installation emitting neutrons
Neutron spectrometer
Neutron transport system
Spectrum transformer
Neutron source
Neutron transport system
September 6, 2013 Folie 30
Neutron beam transport
1°
24 Rcore
out πΦ
=Φ
4
2
1
5
6
3
5 m
R = 2.5 m ⇒ The neutron flux available at the output is drastically reduced by about 6 orders of magnitude in comparison to the core flux.
Flux at the output of a neutron beam tube:
September 6, 2013 Folie 31
Neutron beam transport
1°
24 Rcore
out πΦ
=Φ
4
2
1
5
6
3
5 m
The neutron flux available at the input of a neutron spectrometer is reduced by about 8 orders of magnitude in comparison to the core flux!
( )24 LRcore
spectr+
Φ=Φ
πL
At an instrument : L+R = 20 m ⇒
September 6, 2013 Folie 32
Solution: Neutron guides
Similar to light guides – Total external reflection for θ < θc:
Because the intensity at the neutron guide output is proportional to θc2 ,
neutron guides provide an order of magnitude flux increase as compared to a beam tube.
Light guide:
Glass, n>1
Air, n=1
Glass, n>1
Air, n=1
Neutron guide:
πρ
λθ cc
b2=
Air, n=1
Glass, n>1 cθ
Neutrons Light: Air, n=1
cθ θ
θc≈45° θc≈ 0.1 deg/Å Glass, n>1
September 6, 2013 Folie 33
Neutron beam transport:
More space
outΦ
4
2
1
5
6
3
L = XX m
€
Φspectr =Φoutθc2 =10−4Φout
θc≈ 0.1 deg/Å: for λ=5Å θc is about 0.01 rad
September 6, 2013 Folie 34
Neutron beam transport:
Avoid direct view
4
2
1
5
6
3
• Bent neutron guides - no direct line-of-sight to the reactor core, drastically reduced γ- and neutron background
September 6, 2013 Folie 35
Neutron beam transport:
Choose collimation or size
outΦ
4
2
1
5
6
3
• Parabolic or elliptic neutron guides -focusing of neutrons on a sample - smaller samples with the same intensity
September 6, 2013 Folie 36
Neutron detection
Neutron absorption
• Gd, 10B, 3He ...
Detection of light or charge
• Gas counter • Szintillation
detector • Distinguish Γ
Localization in space and time
• 1mm< Δ s<3cm • Δt ≈ 1 µs • Dead time
September 6, 2013 Folie 37
Conclusions
ü The flux at the neutron scattering instrument becomes an ultimate parameter that defines the quality of the experiment.
ü High continuous flux from reactor <-> high peak flux from spallation source
ü Neutron properties tailored by moderators (hot, thermal, cold) ü Neutron guides:
ü Reduced losses during neutron transport ü Bending and focusing of neutron beams. ü Reduced background & higher intensity
ü Today and tomorrow neutron sources are providing extremely high
neutron fluxes thus opening exiting opportunities for neutron scattering