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September 6, 2013 Mitglied der Helmholtz-Gemeinschaft Neutron sources Jörg Voigt

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Page 1: Neutron sources - pdfs.semanticscholar.org

September 6, 2013

Mitg

lied

der H

elm

holtz

-Gem

eins

chaf

t

Neutron sources Jörg Voigt

Page 2: Neutron sources - pdfs.semanticscholar.org

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:

Page 3: Neutron sources - pdfs.semanticscholar.org

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

Page 4: Neutron sources - pdfs.semanticscholar.org

September 6, 2013 Folie 5

Neutron source

Nuclear installation emitting neutrons

Neutron spectrometer

Neutron transport system

Spectrum transformer

Neutron

spectrometer

Page 5: Neutron sources - pdfs.semanticscholar.org

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

Page 6: Neutron sources - pdfs.semanticscholar.org

September 6, 2013 Folie 7

Nuclear fission reaction

The first free neutrons (Chadwick,1932):

~ 100 n/cm2 s!

Page 7: Neutron sources - pdfs.semanticscholar.org

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

Page 8: Neutron sources - pdfs.semanticscholar.org

September 6, 2013 Folie 9

2000

FRM II

9

CP-1 reactor in USA

High-flux reactor at the ILL

From 1942 to 2009

Page 9: Neutron sources - pdfs.semanticscholar.org

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

Page 10: Neutron sources - pdfs.semanticscholar.org

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:

Page 11: Neutron sources - pdfs.semanticscholar.org

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.

Page 12: Neutron sources - pdfs.semanticscholar.org

September 6, 2013 Folie 13

How to control a fission reactor? Remove thermal neutrons

Page 13: Neutron sources - pdfs.semanticscholar.org

September 6, 2013 Folie 14

Spallation reaction

The de Broglie wavelength of the proton nucldmEh <<= 22λ

A high-energy proton hits a nucleous:

Page 14: Neutron sources - pdfs.semanticscholar.org

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))

Page 15: Neutron sources - pdfs.semanticscholar.org

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

Page 16: Neutron sources - pdfs.semanticscholar.org

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

Page 17: Neutron sources - pdfs.semanticscholar.org

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

Page 18: Neutron sources - pdfs.semanticscholar.org

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

Page 19: Neutron sources - pdfs.semanticscholar.org

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

Page 20: Neutron sources - pdfs.semanticscholar.org

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 )

Page 21: Neutron sources - pdfs.semanticscholar.org

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

Page 22: Neutron sources - pdfs.semanticscholar.org

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

Page 23: Neutron sources - pdfs.semanticscholar.org

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

Page 24: Neutron sources - pdfs.semanticscholar.org

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

Page 25: Neutron sources - pdfs.semanticscholar.org

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

Page 26: Neutron sources - pdfs.semanticscholar.org

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

Page 27: Neutron sources - pdfs.semanticscholar.org

September 6, 2013 Folie 29

Nuclear installation emitting neutrons

Neutron spectrometer

Neutron transport system

Spectrum transformer

Neutron source

Neutron transport system

Page 28: Neutron sources - pdfs.semanticscholar.org

September 6, 2013 Folie 30

Neutron beam transport

24 Rcore

out πΦ

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:

Page 29: Neutron sources - pdfs.semanticscholar.org

September 6, 2013 Folie 31

Neutron beam transport

24 Rcore

out πΦ

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 ⇒

Page 30: Neutron sources - pdfs.semanticscholar.org

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

Page 31: Neutron sources - pdfs.semanticscholar.org

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

Page 32: Neutron sources - pdfs.semanticscholar.org

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

Page 33: Neutron sources - pdfs.semanticscholar.org

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

Page 34: Neutron sources - pdfs.semanticscholar.org

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

Page 35: Neutron sources - pdfs.semanticscholar.org

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