Transcript Chapter 3. Basic Instrumentation for Nuclear Technology

Chapter 3. Basic Instrumentation for
Nuclear Technology
1. Accelerators
2. Detectors
3. Reactors
Outline of experiment:
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get particles (e.g. protons, …)
accelerate them
throw them against each other
observe and record what happens
analyse and interpret the data
1.Accelerators
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History-Why
Particle Sources
Acceleration stage
Space charge
Diagnostics
Application
Nature’s Particle Accelerators
Examples from the nature – electrostatic discharge, αand β-decays, cosmic rays.
• Naturally occurring radioactive sources:
– Up to 5 MeV Alpha’s (helium nuclei)
– Up to 3 MeV Beta particles (electrons)
• Natural sources are difficult and limited:
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Chemical processing: purity, messy, and expensive
Low intensity
Poor geometry
Uncontrolled energies, usually very broad
“Start the ball rolling…”
1927: Lord Rutherford requested a “copious supply” of projectiles
more energetic than natural alpha and beta particles. At the opening
of the resulting High Tension Laboratory, Rutherford went on to
reiterate the goal:
What we require is an apparatus to give us a potential
of the order of 10 million volts which can be safely
accommodated in a reasonably sized room and operated by
a few kilowatts of power. We require too an exhausted tube
capable of withstanding this voltage… I see no reason why
such a requirement cannot be made practical.
Why study...
• The construction, design and operation of
particle accelerators uses knowledge from
different branches of physics:
electromagnetism, high frequency electronics,
solid states physics, optics, vacuum technology,
cryogenics, ...
• Learning about particle accelerator is a good
opportunity to learn about many different
physical phenomenon.
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Why
They have wide ranging applications well beyond
physics: health, life science, materials and even
archaeology!
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Early accelerators
1870: Discovery of the cathode rays by William Crookes
- Charged rays
- Propagation from the Cathode to the anode
A Crookes tube in which the Cathode
rays are deflected by a magnetic field.
Images source: Wikipedia
1896: J.J. Thomson shows that the cathode rays are made
of “particles” and measure the charge/mass ratio.
These particles are called “electrons”
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Bremsstrahlung
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A charged particle emits radiation when it is accelerated.
An electron that Coulomb scatters on a heavy nucleus will change
direction => acceleration
Bremsstrahlung, braking radiation, is the name of the radiation
emitted when a charged particle scatters on a heavy nucleus.
• When a charged beam hits an object,
X-rays are emitted. This is used to produce Xrays in hospitals but it is also a source of
hazardous radiations in accelerators.
• Bremsstrahlung is similar to synchrotron
radiation.
Image source:
http://www.ndt-ed.org/EducationResources/
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Improved resolution
• In quantum mechanics the wavelength of an object is
related to its energy by
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The reach better resolutions, the energy of the probe
must be increased.
The energy of the electrons in Cathodic ray tubes is
limited by the electrostatic generators available.
In the 1930s several generators where invented to
produce high electric fields.
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vacuum
Ion
source
acceleration
steering
analyzer
1.Accelerators
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History-Why
Particle Sources
Acceleration stage
Space charge
Diagnostics
Application
Particle sources
• How particles are first produced?
• How to extract particles with the right
properties?
• What are the limitations of the sources?
• The quality of the source is very important. If
the particles emitted by the source do not have
the right properties, it will be very difficult
and/or expensive to rectify it later.
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Beams of nanoamperes to hundreds of amperes
Very thin to very broad beams (μm2 to m2)
Negative to highly charged state
e to protein molecule
Emission of electron:
Thermionic effect
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When a metal is heated more electrons
can populate high energy levels.
Above a certain threshold they
electrons can break their bound and be
emitted:
This is thermionic emission.
(image source: wikipedia)
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Work function
• To escape from the metal the electrons must reach an
energy greater than the edge of the potential well.
• The energy that must be gained above the Fermi energy is
called the “work function” of the metal.
• The work function is a property
specific to a given metal. It can
be affected by many parameters
(eg: doping, crystaline state,
surface roughness,...)
• Example values:
(image source: wikipedia)
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(image source:
http://cnx.org/content/m13458/lates
t/
Summary: electrons in solids
• At low temperature all electrons are in the lowest possible energy
level, below the Fermi level.
• As the temperature increase some electrons will go above the Fermi
level.
• But only those with an energy above the Fermi level greater than
the work function are “free”.
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Thermionic emission
• The Richardson-Dushman equation gives the electronic
current density J (A/m-2) emitted by a material as a
function of the temperature:
With A the Richardson constant:
(image source:
Masao Kuriki, ILC school)
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Thermionic cathode material
• Two parameters are important when considering a
thermionic cathode material:
– W=Work function (as low as possible)
– Te=Operation Temperature (preferably high)
• Cesium has a low work function (W~2eV) but a low
operation temperature (Te=320K)
=> not good for high current
• Metals: Ta (4.1eV, 2680K), W(4.5eV, 2860K)
• BaO has good properties (1eV; 1000K) but can oxidize by
exposure to air => sinter of BaO+W
BaO provided slowly to the surface.
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field enhanced thermionic emission
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Under a very intense electric
field some electrons will be
able to tunnel across the
potential barrier and become
free.
This is known as field effect
emission.
(image source: answers.com)
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Electric field bias
• Once the electrons are free they may
fall back on the cathode.
• To avoid this an electric field needs to
be applied.
• If a negative potential is applied to the
cathode the electrons will be attracted
away from the cathode after being
emitted.
• However this field affects the work
function.
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Photon-enhanced thermionic
emission
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A photon incident on a piece of metal
can transfer its energy to an electron
If the photon transfers enough energy
the electron can be emitted.
By using powerful lasers the
photoelectric effect can be used to
produce electron beams.
This is known as the photo-electric
emission.
(image source: wikipedia)
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Photo-electric emission (2)
• A UV photon at 200nm carries an energy of about 6 eV,
this is enough to “jump” over the work function of most
metals.
• As seen in electromagnetism, electromagnetic waves
(photons) can penetrate inside a metal.
• The photo-electric
emission may thus
take place away from
the surface.
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(image source: Dowell et al., Photoinjectors lectures)
The 3 steps of
photo-electric emission
Photo-electric emission takes place in 3 steps:
1) Absorption of a photon by an electron inside the metal. The
energy transferred is proportional to the photon energy.
2) Transport of the photo to the physical surface of the metal.
The electron may loose energy by
scattering during this process.
3) Electron emission (if
the remaining energy is
above the work function;
including Schottky effect)
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Quantum efficiency (QE)
• For photo-electric emission, it is useful to define the
“quantum efficiency”:
Number of photo electrons
QE=
Number of photons
• Typical QE for a photo-cathode is only a few percent or
less!
• The quantum efficiency will decrease during the life of the
cathode: it may get damaged or contaminated.
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Thermionic emittance (1)
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Velocity distribution of
thermionic electrons:
− mv 2x
2kB T
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1 dn v x
m
=
vx e
ne dv x
kBT
The higher the temperature, the
wider the transverse energy
(momentum) spread.
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300K => 0.049eV spread
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2500K => 0.41eV spread
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The transverse momentum (image source: Dowell et al., Photoinjectors lectures)
spread determines the beam
divergence.
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Ion (and proton) sources
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An electric discharge creates a plasma in
which positively and negatively charged
ions are present (as well as neutrals).
If such plasma experiences an intense
electric field ions will separate in opposite
directions.
This is a rather crude and inefficient (but
very simple) way of producing any sort of
ions.
In a Penning ion source a magnetic field
is used to increase the probability the free
electron ionize extra neutrals.
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(images source: CERN)
Ion source SINCS
Source of Negative Ions by Cesium Sputtering - SNICS II
Principle of Operation
Focused Ion Beam
liquid metal ion source (LMIS),
Electrospray ionisation
Tube lens
ESI needle
4kV
Octapole
Lenses
Skimmer
1 mbar
Heated
Capillary
(~180°C)
Rotary
pump
Acceleration tube
10-3 mbar
Turbo
pump
10-5 mbar
10-6 mbar
Turbo
pump
Fused silica
capillary
Charge Residue Model electrospray droplets undergo
evaporation and fission cycles, eventually leading
progeny droplets that contain on average one analyte ion
or less. The gas-phase ions form after the remaining
solvent molecules evaporate, leaving the analyte with the
charges that the droplet carried.
1.Accelerators
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History-Why
Particle Sources
Acceleration stage
Space charge
Diagnostics
Application
Acceleration stage
Lorentz Force
F  qE  v  B 
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Only works on charged particles
Electric Fields for Acceleration
Magnetic Fields for Steering
Magnetic fields act perpendicular to the direction of
motion.
• For a relativistic particle, the force from a 1 Tessla
magnetic field corresponds to an Electric field of 300
MV/m
types of accelerators:
electrostatic (DC) accelerators
Cockcroft-Walton accelerator (protons up to 2 MeV)
Van de Graaff accelerator (protons up to 10 MeV)
Tandem Van de Graaff accelerator (protons up to 20 MeV)
resonance accelerators
cyclotron (protons up to 25 MeV)
linear accelerators: electron linac: 100 MeV to 50 GeV
proton linac: up to 70 MeV
synchronous accelerators
synchrocyclotron (protons up to 750 MeV)
proton synchrotron (protons up to 900 GeV)
electron synchrotron (electrons from 50 MeV to 90 GeV)
Induction: Induction linac, betatron
electrostatic accelerators:
generate high voltage between two electrodes
⇒ charged particles move in electric field,
energy gain = charge times voltage drop;
Cockcroft-Walton and Van de Graaff
accelerators differ in method to achieve high
voltage.
Cockcroft-Walton
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High voltage source using rectifier units
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Voltage multiplier ladder (made of
diodes and capacitors) allows reaching
up to ~1 MeV (sparking).
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First nuclear transmutation reaction
achieved in 1932: p + 7Li → 2·4He
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CW was widely used as injector until the
invention of RFQ
Fermilab 750 kV C-W
preaccelerator
Van de Graaff
Voltage buildup by mechanical
transport of charge using a
conveyor belt. up to ~20 MV
The charged particles are extracted
from an ion source housed inside the
high-voltage terminal and accelerated
down an evacuated tube to ground
potential.
Tandem Van de Graaff
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Negative ions accelerated
towards a positive HV
terminal, then stripped of
electrons and accelerated
again away from it,
doubling the energy.
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Negative ion source
required!
The Million Volt Barrier
Summary of Problems in getting HV ~ 1929
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Voltage Generators
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Insulators – 750 kV max holding !
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Power
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Safety in using HV
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Funding
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Imagination
RF Accelerators
Radiofrequency oscillating voltage
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High voltage gaps are very difficult to maintain
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Solution: Make the particles pass through the
voltage gap many times!
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First proposed by G. Ising in 1925
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First realization by R. Wiederöe in 1928 to
produce 50 kV potassium ions
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Many different types
RF LINAC – basic idea
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Particles accelerated between the cavities
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Cavity length increases to match the increasing speed of
the particles
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EM radiation power P = ωrfCVrf2 –
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the drift tube placed in a cavity so that the EM energy is stored.
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Resonant frequency of the cavity tuned to that of the accelerating
field
RF LINAC – phase focusing
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E. M. McMillan – V. Veksler 1945
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The field is synchronized so that the slower
particles get more acceleration
1.Accelerators
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History-Why
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Particle Sources
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Acceleration stage
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Space charge
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Diagnostics
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Application
1.Accelerators
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History-Why
Particle Sources
Acceleration stage
Space charge
Diagnostics
Application
What do you want to know about
the beam?
• Intensity (charge) (I,Q)
• Position (x,y,z)
• Size/shape (transverse
and longitudinal)
• Emittance (transverse and
longitudinal)
• Energy
• Particle losses
Properties of a charged beam
• Almost all accelerators accelerate
charged particles which interact with matter.
• That's almost all what you need to use to build
diagnostics (together with some clever tricks).
Faraday cup (1)
• Let's send the beam
on a piece of copper.
• What information
can be measured
after the beam has
hit the copper?
Faraday cup (2)
• Two properties can be
measured:
– Beam total energy
– Beam total charge
• By inserting an ammeter
between the copper and
the ground it is possible to
measure the total charge
of the beam.
Image source: Pelletron.com
• At high energy Faraday
cups can be large: More
than 1m at Diamond.
Beam current monitor
• Remember: as the charge
travelling in the beam
pipe is constant the
current induced on the
walls (of the beam pipe)
will be independent of the
beam position.
• By inserting a ceramic gap
and an ammeter the total
charge travelling in a
beam pipe can be
measured.
Beam current monitor
vs Faraday cup
• Both devices have pros
and cons.
• A Faraday cup destroys
the beam but it gives a
very accurate charge
measurements
• A Beam current monitor
does not affect the beam
but must be calibrated.
• Both tend to be used at
different locations.
Screen (1)
• If a thin screen is inserted in
the path of the particles,
they will deposit energy in
the screen.
• If this screen contains
elements that emit light
when energy is deposited
then the screen will emit
light.
• Example of such elements;
Phosphorus, Gadolinium,
Cesium,...
Screen (2)
• It is not possible to stay in
the accelerator while the
beam is on so the screen
must be monitored by a
camera.
• To avoid damaging the
camera the screen is at
45 degrees.
• On this screen you can see
both the position of the
beam and its shape.
• Note the snow on the
Wire-scanner
• By inserting a thin wire in
the beam trajectory
(instead of a full screen) it
is possible to sample parts
of the beam.
• By moving the wire in the
transverse direction one
can get a profile of the
beam.
• It is possible to use wire
diameters of just a few
micrometres.
Longitudinal properties
• It is not possible to
directly image the
longitudinal profile of a
bunch.
• By giving longitudinal
impulsion to the beam it
is possible to make it
rotate and observe its
longitudinal profile.
Beam losses
• It is important to monitor
the beam losses directly:
• Small beam losses may
not be detected by other
systems
• Beam losses are a source
of radiation and activation
• Most beam losses indicate
that there is a problem
somewhere.
Limitation of these monitors
• Monitors in which the
matter interacts are prone
to damage.
• With high energy high
intensity colliders such
damages are more likely
to occur.
• To the left: hole punched
by a 30 GeV beam into a
scintillating screen.
Laser-wire
• To mitigate the problem
of broken wires in wirescanners it is possible to
replace the wire by a
laser.
• This technique called
“laser-wire” also allow to
reach better resolutions.
• High power lasers (or long
integration times) are
needed.
Synchrotron radiation
• Synchrotron
radiation carries
information about
the beam which
emitted it.
• It is commonly used
to study the beam
shape.
Energy measurements
• To measure (or
select) the energy of
the particles a
bending magnet is
often the best
solution.
Diagnostics overview
Interaction
with matter
Charge
Charge
Faraday cup
Beam current
monitor
Position
Screen
BPM
Size or shape
(transv.)
Screen or wire- Synchrotron
scanner/LW
rad. OTR/ODR
Size or shape
(longit)
RF cavity +
screen
Bending
magnet
Energy
Losses
Radiation
detectors
Scintillator
Summary
• There are two ways of
measuring the properties
of a beam:
– By forcing it to interact
with matter
– By looking at the EM
radiation emitted.
• How to build the best
diagnostic is then a matter
of imagination!
1.Accelerators
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History-Why
Particle Sources
Acceleration stage
Space charge
Diagnostics
Application
Several accelerator based methods can
be used to date old artefacts.
Hospitals use accelerators everyday to
treat some forms of Cancer.
The data storage capacity of electronic
devices has been improved.
The structure of molecules, including
drugs, can be studied with intense
sources of X-rays.
Material hardness can be studied with
neutrons
Intense flux of neutrons can burn
unwanted nuclear materials
Dating old artefacts
The Shroud of Turin
The shroud of Turin is a piece of cloth
which was first mentioned in the
middle age.
On it the face of a man can be seen.
Some claim that it is the shroud that
was used after the Christ's crucifixion.
In the 1980s 3 AMS laboratory
independently dated the sample they
were provided to 1260-1390.
Therapy
Comparison of the physical
dose distribution (upper
diagram) and the survival rate
of cells (lower diagram) as a
function of penetration depth for
ion and photon beams. The
enhanced energy deposition at
the end of the particle range
and the corresponding dramatic
decrease of cell survival show
that heavy ion beams are
excellent tools for the treatment
of deep seated tumours.
therapy
Sub-micron micromachining
interactions
Masked processes
(electromagnetic)
•Light
•X-rays
Direct write processes
•Electrons
•Low energy heavy ions
(eg gallium)
•High energy light ions
(protons)
Proton Beam Micro-machining
Examples of structures in PMGI and PMMA
(2 MeV proton beam)
Structures produced in a 12 mm
thick PMGI resist layer.
Map of Singapore (60 mm high) Cogs (60 mm high) produced
produced in bulk PMMA.
in bulk PMMA
Pharmaceutical drugs
To be efficient a drug need to target the
correct molecule.
This can only be achieved by studying
the diffraction of intense on the
molecule.
What type of machine (gun,
accelerator, ...) is best suited to deliver
an intense stable beam of X-rays?
Channeltron detector
Ion bunch
+
[d(A)2+H]
Magnet
Na oven
1m
Counts
Accelerator with
electrospray ion source
0
20
40
60
80
Time (ms)
Fig.2.3 Schematic drawing of electrostatic storage ring (ELISA).
Irradiation of dinucleotide (二核苷酸） cations [d(A)2+H]+ in
ELISA. The initial count rate after injection is due to the
decay of ‘‘hot’’ ions, but after ca. 20 ms the signal is
dominated by collisional decay. Laser excitation after 60 ms
of storage time causes a large increase in the count rate.
The depletion of the ion beam is reflected in a lower rate of
collisional decay after laser excitation than before.
Microchannel
plate detector
HIRFL-CSR是重离子物理及相关学科研究的
兰
州
重
离
子
加
速
器
综合性实验平台
回旋2
回旋1
核物理
核天体物理
电子冷却
核物理
粒子物理
●
HIRFL
ECR源
电子冷却
●
●
原子物理
应用物理
●
治癌
具有全离子，宽能区的特点
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实现重离子束深层癌症临床治疗研究
回旋
加速器
109个离子
治癌
同步加速器
均匀慢引出
2008-2011，利用物理实验的间隙
成功治疗了45例患者
肝癌、肺癌、胰腺癌
脑胶质瘤、恶性脑膜瘤
头颈部肿瘤、骨和软组织肉瘤
直肠癌、前列腺癌、卵巢癌等
实现笔形束
点扫描
达到适形治疗
主要创新：采用回旋与同步组合的
独特治癌专用机器模式，打破了国
外相关行业对我国的专利禁锢。
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Producer
Wuxi EL PONT Accelerator Research Institute 无锡爱邦
http://www.elpont.net/abfs/EN/
HV High Voltage Engineering Europa B.V.
http://www.highvolteng.com/
National Electrostatics Corporation (NEC)
http://www.pelletron.com/index.html
Kobelco
http://www.kobelco.co.jp/english/machinery/products/
function/hrbs/index.html
IBA
http://www.iba-industrial.com/e-beam-accelerators
Jobs and graduate studies
Accelerators do not operate on their own.
A team is needed to manage the accelerator operations.
All accelerators facilities have a wide-range of staff at all
levels.
There are also many jobs connected to the usage of
accelerators.
New machines bring new challenges and there are many
opportunities for graduate studies in Accelerator science.
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•
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Timothy Koeth
Physics, Oxford University
www-w2k.gsi.de/charms/Talks/CHARMS/
Greg LeBlanc
盛丽娜
刘波