Introduction to particle accelerators and their applications 
- Parte III: Production, detection and application of neutrons

Introduction to particle accelerators and their applications 


- Parte III:

Production, detection and application of neutrons

Gabriele Chiodini

Istituto Nazionale di Fisica Nucleare

Sezione di Lecce

PhD lessons in Physics for Università del Salento

2015-16 (20 hours, 4 CFD)

Overview: Production

Neutron production:

• Radioactive sources

• Photo-production

• Spontaneous fission

• Nuclear reactor

• Commercial accelerators

• Spallation sources

• Neutron guides

Radioactive sources

Alpha radioactive source on Beryllium target

Large spread in energy and emission angle:

- many energy decay levels

- slowing down in material target

- C-12 excitations

Rate=10E7 n/s per 1 Ci of Ra-226 Source activity:

1 Curie (Ci) = 3.7E10 Becquerel

1 Becquerel=1 disintegration/s

Commercial radioactive sources

Neutron photo-production

• Almost mono-energetic neutron for mono-energetic photon.

• Na-24 emits γ of 2.76 MeV, which is higher than neutron bond energy.

• Last neutron bonding energy for Be-9 is only 1.66 MeV.

• The yield is good 2E6 n/s for 1 Ci of Na-24 , but short average life time (15h) .

• Another source of γ is (antimony) Sb-124 ( 60 d of average life time)

with energy slightly higher than the binding energy of the neutron, which

is emitted with a low energy, just 24 KeV.

Spontaneous fission

• Iper-uranium isotopes, such as Cf-252, subject to spontaneous fission constitute excellent neutron sources.

• The neutrons are produced directly in the process at a rate of about 4 neutrons per fission.

• The fission rate of Cf-252 is 3% and alpha decay rate 97 % .

• The yield is of 2.3x10E12 n/s /g or 4.3x10E9 n/s for Ci .

• Average energy of the neutrons is 1-3 MeV , typical of the fission processes.

• The cost is high cost because it is an artificial transuranic

product and the average half-life is just 2.65 y.

Accelerator production

With the reaction p+3H -> 3He+n - 0.735 MeV can get mono energetic neutrons 3He do not have excited states.

By choosing the energy of the incident particle and the neutron emission angle, you can get monoenergetic neutron from few keV to 20 MeV.excited states.

Dependence of the neutron

energy from projectile energy

for three different values of the

reaction angle iin the nuclear

reaction 3H(d,n)4He

Accelerator based commercial neutron source

Nuclear reactors

Nuclear reactions ia a fission reactors

The fission reaction involves heavy nuclei such as U-235, U-238 , U-233 , Pu-239 etc.

The compound nuclei C* is a state extremely excited which decays into a wide variety of ways, including the fission .

n n

A two stages process

I) Nucleus excitation

II) Nucleus evaporation

Fission cross-section induced by neutrons

The fission in nuclear reactors is induced by neutrons with kinetic energies > MeV for even-even

nuclei (more stable) and for kinetic energy < eV for the even-odd nuclei (less stable).

Neutron spectrum and flux from reactor

• Prompt neutron as produced in a fission event: evaporation of a the excited core.

• The distribution reflects the distribution of the energies of neutrons inside the nucleus.

Thermal and cold neutron

To obtain thermal neutron the prompt neutron are slowed down and transported out of the reactor.

• Liquid H2 or D2 at 20K moderate to cold n (25 K)

• H2O,D2O or Be moderate to thermal n (300 K)

• Graphite moderate to hot n (2000

K)

Spallation source

• Energetic particles such as a proton of E>1GeV on heavy nucleus produces a reaction called "spallation".

• For example 1 GeV proton bunches fired on high A targets, such as tungsten or mercury, produce on average 30 to 40 neutrons for each incident proton

• The pulsed neutrons are slowed down and brought to experimental areas through special beam guides.

• The neutron intensity is about 50-100 times more than neutrons from reactors.

• Each pulse contains neutrons with different energy range

• synchrotrons: short pulses (us)

• linacs: long pulses (ms)

• cyclotrons: continuos beam

Why spallation source?

European Spallation Source (ESS)

Lund is in Copenaghen

Construction cost: 1843 Meuro

In-kind cost: 750 Meuro

Operation cost: 140 Meuro

Decommissioning cost: 177 Meuro

ESS components

LEBTF=Low Energy Beam Transfer Line RFQ=Radio Frequency Quadrupole

DTL=Drift Tube Linac

Medium and High beta SC

elliptical cavities

Neutron Yield

Fraser (about 1965) has found experimentally a linear relationship

between yield and plate thickness in function of the energy of the

protons accidents .

Neutron spectra

Neutrons of energy around 1 MeV have almost a isotropic peak.

!

For energy above 10

M e V t h e d i r e c t

i n t e r a c t i o n o f t h e

neutrons dominates

f a v o u r i n g f o r w a r d

distribution.

Neutron absorpion

• A beam of neutrons are attenuated in a material due to nuclear reactions

• For fast neutron the most probable nuclear reactions are (n,p), (n,α) or (n,2p)

• For slow neutrons or thermal neutron the most probable nuclear reaction is the nuclear capture (n,γ)

• Outside the area of the resonances (around 1 eV), the cross section decreases with the increase of the speed (1/ v): slower neutrons are more absorbed then fast

neutrons

The intensity decreases exponentially with the material thickness, where:

• σt = total cross section=σs + σa= scattering + absorption cross section

• N = atomic density

• x = material thickness of material

neutron absorbed before scattering

!

neutron scattered many time before being absorbed

=

Neutron guides

• The neutron flux as a function of the distance decreases as 1/r ^2

• Neutron guides are needed to avoid a such fast decrease using the principle of total reflection

(similarly to fibre light).

• Neutron guides can be several meters long

• Total reflection occurs when the angle of incidence is

less than the critical angle

Neutron reflection

Neutron reflectivity

Neutron monochromator

Supermirror

!

To improve the reflectivity Bragg reflection from supermirrors with different layers of

variable thickness for scattering different total length: alternate layers with positive

( Ni ) and negative ( Ti) scattering length.

Overview: Detection

Neutron detection:

• How to detect particles or radiation

• How to detect neutrons

• Detector type

• Neutron converter layers

• 2D neutron detectors

• Application overview

Particle or radiation detection

What means to detect a radiation quanta or a particle? It means to produce a detectable electronic signal.

1. A low noise electronic channel has an equivalent input noise of about 1000 e-. To distinguish a real count from count noise the signal to noise ratio must be better 6 sigma. The detector must produces at least 5,000 electrons

2. Leakage current << 1 uA in order to have a shot noise from sensor less than 1000 e-

+ - + - + - + - + -

+ - + -

Gas filled volume

Ar NTP ~100 e-ions/cm + - + - + -

Solid state detector.

8000e-hole/100um

Leakage current = 0 but you need avalanche

multiplication in gas to detect the signal. Good signal but need inverse biased p- n junction to reduce leakage current.

In either case an electric field is need to separate the opposite

Neutron detection

• What does it mean to “detect” a neutron?

• Can’t directly “detect” neutrons because no primary ionizazion (no coulomb scattering with atomic electrons)

• Need to use nuclear reactions to “convert” neutrons into charged particles (secondary ionization)

!

Then we can use one of the many types of charged particle detectors

– Gas proportional counters

– Scintillation detectors

– Semiconductor detectors

Nuclear Reactions for 
 Neutron Detectors

• ^{n + 3 He → 3 H + 1} H + 0.764 MeV

• ^{n + 6 Li → 4 He + 3} H + 4.79 MeV

• ^{n + 10 B → 7 Li* + 4 He→ 7 Li + 4} He + 0.48 MeV γ +2.3 MeV (93%)
 → ⁷ Li + ⁴ He +2.8 MeV ( 7%)

• ^{n + 155} Gd → Gd* → γ-ray spectrum → conversion electron spectrum

• ^{n + 157} Gd → Gd* → γ-ray spectrum → conversion electron spectrum

• ^{n + 235} U → fission fragments + ~160 MeV

• ^{n + 239} Pu → fission fragments + ~160 MeV

Gas Detectors for Neutrons

• Proportional Mode!

– if voltage is high enough, electron collisions ionize gas atoms producing even more electrons

- gas amplification

- gas gains of up to a few thousand are possible

n +

³

He →

³

H +

¹

H + 0 76 . MeV

σ λ

= 5333

18 . barns

~25,000 ions and electrons produced per neutron

World Helium-3

crisis

Semiconductor Detectors for Neutrons

• ~1,500,000 holes and electrons produced per neutron

– Standard semiconductors do not contain enough neutron-absorbing nuclei to give reasonable neutron detection efficiency

– put neutron absorber on surface of semiconductor

• Neutron absorber coated layer must be thin (a few microns) for charged particles to reach detector !

n +

⁶

Li →

⁴

He +

³

H + 4 79 . MeV

barns 8

.

940 λ 1

σ =

Scintillation detectors

A scintillator is a transparent material that exhibits scintillation:


the property of luminescence when excited by ionizing radiation. ! The scintillation material can be a crystal, a plastic or a liquid.

scintillator + photodetector ~ particle or radiation detector A photodetector (like PMT) converts one or more photons in a detectable electric pulse (>10,000e-).

Ionization charge not detected by electric drift but by photon detections

Other photodetectors

SiPm (Silicon Photo Multiplier):

• Matrix of avalanche diodes with quench resistors

• Avalanche multiplication triggered by VI photons

• Signal proportional to the fired avalanche diodes, than increases linearly with the number of incident photons.

• PMT and SiPm are single photon devices

• SiPm is cheaper, more robust and can work in B

Some Common Scintillators for Neutron Detectors

Li glass (Ce) 1.75×10

²²

0.45 % 395 nm ~7,000

LiI (Eu) 1.83×10

²²

2.8 % 470 ~51,000

ZnS (Ag) - LiF 1.18×10

²²

9.2 % 450 ~160,000

Material

Density of

6

Li atoms (cm

^-3)

!

Scintillation efficiency

Photon wavelength

(nm)

Photons per neutron

Scintillating glass fibre for neutrons

•

Fast neutrons are thermalized by hydrogen-rich moderator

•

Thermal neutron capture by 6Li ·

•

Alpha particle and Triton are produced

•

Triton particle excites Ce3+

•

Ce3+ fluoresces

•

Light guided to PMTs

Crossed-Fiber Neutron Scintillation Detector

• Size: 25-cm x 25-cm!

• Thickness: 2-mm!

• Number of fibers: 48 for each axis!

• Multi-anode photomultiplier tube:

Phillips XP1704!

• Coincidence tube: Hamamastu 1924!

• Resolution: < 5-mm!

• Shaping time: 300 nsec!

• Count rate capability: ~ 1 MHz!

Neutron camera

• 2-D neutron imaging with CCD

• Composit material= scintillator + high xsec to neutron (Li, Gd, …)

• Spatial resolution about 10-20 um (as secondary ion range)

Neutron energy measurements

Thermal neutrons

Bragg law total reflection Crystals: Cu,Be,C (pirolitic graphite),Ge and Si

Fast neutrons

Recoil technique

Pulse height distribution peak

Neutron scattering with light nuclei

such as H,d,T,He-3,He-4

Summary: neutron applications

• Neutron as a probe for material

• Neutron vs X rays

• Imaging vs scattering

• Radiography and tomography

• Scattering experiments:

• nuclear and magnetic diffraction

• spectroscopy

• small angle scattering

• ^diffusion

Neutron interactions

• Nuclear interactions (electrons ignored contrary to X ray)

• Deeply penetrating

• Dipole-dipole magnetic interaction

with unpaired electrons of atomic shell

• Some order such as nuclear

Unravel structure, dynamics and magnetic properties of

Neutrons vs Xray

Neutrons

absorber

Isotopic contrast for neutrons

Imaging vs scattering

(U)SANS=(Ultra)SmallAngleNeutronScattering

Radiography

A radiography is a 2D plot given by the ratio between the transmitted intensity and the intensity of a collimated and monoenergetic source.

T(x,y)=I(x,y)/I ₀

Neutron radiography uses the

transmission difference of: For a clear radiography

Tomography

A tomography is a algorithmic 3D reconstruction of an object

from a set radiography made at different angles

X-ray and n tomograpy

Visible light

Motion imaging

Fuel cell radiography with neutrons

Neutron imaging

Particle scattering

Measure number S(Q,ε) of neutrons scattered as function of Q and ω.

Van Hove in 1954 proved that S(Q,ε) is the Fourier Transform of the probability to find two atoms at a distance x at the time t.

For elastic scattering:

• energy transfer is zero

• ^|Ki|=|Kf|

Momentum transfer

Energy transfer

Wave scattering

Elastic and anelastic scattering

• Where the atoms are

• Equilibrium positions (crystal structure)

• What the atoms do

• Collective motion (atomic diffusion, lattice vibration or phonos, molecular modes)

Diffraction 
 (|Ki|=|Kf|)

Spectroscopy 


(|Ki|=|Kf|)

Nucleus and lattice

Scattering length

Nucleus position

b _R depends on the nucleus and on the spin SINGLE NUCLEUS

LATTICE: MANY NUCLEI (PHASE MATTER)

Differential

xsec Total xsec

For a generic Q the phase is randomly

changing between 0 and 360 degrees

and the xsec is small

Bragg peak

If Q=K (reciprocal lattice K=1/a a=lattice period) 
 the phase is 0 and the sum is large (Bragg peak).

!

The sum is a Dirac delta function in the reciprocal lattice.

Number of

cells Volume of

the cells Reciprocal lattice

For different nuclei

Same nuclei

Coherent and incoherent scattering

Isotopes and spin are distributed randomly and the incoherent term doesn’t depend on Q

Average on Isotopes and spin

Coherent and incoherent scattering

Coherent scattering

• The neutron wave interacts coherently with all atoms (same phase) and the neutron scattered waves interfere

• Strong dependence on scattering angle

• Material structure information

Incoherent scattering

• The neutron wave interacts with single atoms (random phase) and the neutron scattered waves add incoherently

• Smooth dependence on scattering angle

• Atomic diffusion and lattice vibration information

Contrast matching

between H and D used

to study polymers and

biology structure

For small angles (small momentum transfer) the intensity of the

scattered wave is easy to calculate because the wave change slowly

with respect to the atomic distance and is given by the Fourier

transform of the scattering length b(r) along the sample thickness.

SANS applications

• Study large objects: from 1 to 10,000 nm

• Thick samples can be used (few mm)

• Light elements sensitivity (H, C, N)

• Isotope composition sensitivity (H,D)

• 10-120 degrees scattering related to crystal properties

•

Neutron magnetic scattering

• The magnetic moment of the neutron can interact with the magnetic moment of unpaired electron of an atoms.

• The magnetic interaction is similar in strength to the nuclear interaction

• The neutron magnetic scattering is sensitive to the material magnetization and magnetic domain size and position.

Neutron magnetic moment

about 960 smaller than

electron magnetic moment

Neutron magnetic diffraction

! • The neutron magnetic diffraction in not isotropic because only the material magnetization orthogonal to Q is contributing.

• Applying an external magnetic field is possible to separate magnetic scattering from nuclear scattering:

• For external B parallel to Q only nuclear scattering because electron spin because alined with B

• For external B orthogonal to Q nuclear and magnetic scattering both present.

• Antiferromagnetic material exhibits magnetic bragg peaks

at half the nuclear Bragg peaks because the magnetic

periodicity of the lattice is half the lattice periodicity

HighT SC due to quantized magnetic vortex

Diffusion with neutrons

Diffusion (proportionality between flux and gradient) can be studied with neutrons using Quasi-elastic neutron scattering looking at the incoherent scattering function.

Flick’s law gives rise to exponential relaxation: Lorentzian

half-width Γ=DQ ²