Download Lecture 3.

Topics
1. Elements of Nuclear Physics
2. History of Nuclear Physics; beginning of nuclear
energetics
3. Detecting nuclear radiations, radiation detectors
4. A nuclear power plant
5. Types of nuclear reactors based on usage of the
reactors
6. Generations of nuclear power plants
7. Nuclear reactors in Hungary
8. Mini or small nuclear power plants
9. Reactor safety, radiation safety
10. Nuclear accidents, abnormal operations
11. Nuclear power plants and environmental protection
12. Fusion, fusion devices, fusion power plants
13. Natural nuclear reactors or nuclear reactors in nature
Lecture #3.
Detecting nuclear
radiations, radiation
detectors
Chapter 3.
Lecture 3.
About detectors
o Definition (detectors):
o
o
o
o
Detectors help to observe and detect or sometimes help to specify
some specialties of particles taking part in introductions in atomic
physics. These particles are generating during nuclear processes.
Simple detectors:
• Counters
• Its task is to indicate presence of a particle in a given place in a
well defined time moment.
Little complicated detectors:
• It is able to specify some parameters of the particles (for example:
identification, charge, mass, kinetic energy, pulse or moment
measurement)
Physical background of detection:
Particle (or its radiation) interacts with the material of the detector (In
most cases the type of the interaction is electromagnetic).
Neutral particles cannot be detected directly. Neutral particles can
be detected by charged particles generated by neutral particles.
(Secondary particles.)
Lecture #3.
Types of Detectors
1. Gas-filled counters
2. Scintillation counters
3. Semiconductor detectors
4. Cherenkov-radiation and counters
5. Particle trace detectors (Visual
detectors)
6. Neutrino detectors
Lecture #3.
1. Gas-filled counters
1. Ionization chamber:
o Definition (Ionization chamber):
This is the most simple particle detector, which is a gas insulated
plane capacitor.
o Schematics:
o Operation:
If particles propagate through the plane electrodes of a gasinsulated capacitor, then the working gas can be ionized. If the
gas is ionized, then ions (particles with charges) will be
generated. Due to the voltage fells on the electrodes the ions will
propagate to the electrodes. In other words: directional flow of
charged particles will be presented. It means: electric current will
flow. The current can be measured by meters. This detector is
well used for high intensity particle detection.
Lecture #3.
1. Gas-filled counters
2. Proportional counter:
o What does it do?:
Number of particles generated by ionizing radiation and energies of these
particles can be counted by proportional counters.
o Operation:
The detector is filled by natural gas.
Free electrons moving by high electric field can excite the gas molecules.
o If the electric field is big enough, then the electrons can absorb energy during
two elastic collisions. The energy is equal to the ionization potential of the gas.
During the coming collision every electron can ionize an atom. After this
process the original electron and a new (just generated from ionization)
electron will propagate. These two electrons can ionize one-one atom (in
other words two atoms), and after the process another plus one-one electron
will be generated. Etc…
o Avalanche of electrons can be generated this way. The strength of the
avalanche and the measured electric signal are proportional to the number of
the primer electrons. After this effect the name is: proportional counter. (There
is a gas which brakes the avalanche to grow up to infinity strength.)
o Comments:
• Size: several centimeters – meter; distance of the planes is several
centimeters.
• Gas fill: argon; Braking gas: methane (P-10)
• U  2000 V
• Application field: measuring neutron flux
Lecture #3.
1. Gas-filled counters
2. Proportional counter:
o Schematics:
E=
𝑈
𝑟1
𝑟 ∙ 𝑙𝑛
𝑟2
r1 : radius of the cathode
r2 : radius of the anode
r : distance from axis,
where electric field
is measured
U : voltage
E : electric field
Source: Wikipedia
Lecture #3.
1. Gas-filled counters
3. Proportional chamber:
o Operation:
Numerous proportional counters are installed in a flat container.
Numerous wires are outspreaded parallel to each other in the flat
container. These are the anode wires. Above and below the
theoretical planes of the wires solid metal planes are installed
(see the figure below). These metal planes are the cathode
planes. The gas fill is joint, but every single wires can work as
independent proportional counters.
o Schematics:
Lecture #3.
1. Gas-filled counters
3. Proportional chamber:
o Structure of the electric field
inside the proportional
chamber:
o Comments:
• Application fields: mainly used for detecting energetic radiations
or particles
• Size: from dm2 - to m2
• Density of wires: 1-2 wire(s) / 3mm
• 102 – 105 wires are installed in every meters
• Expensive detectors!
Lecture #3.
1. Gas-filled counters
4. Drift chamber:
o Definition (drift chamber):
Drift chamber is a proportional chamber, where the distances
between the wires fell to the range of 10 cm. Drift chamber is a
special proportional chamber.
o Operation:
There is proportionality between the position of the generated ion
column and the time interval needs for collecting ions by the
anode wire. This proportionality can be used for precise
describing of the position of the particle trace.
o By time measurement  position of the penetrating particle can
be characterized  coordinate detector
Electric field must be kept homogeneous. Path – flying time
connection (proportionality) is linear.
Lecture #3.
1. Gas-filled counters
5. Geiger – Müller-counter:
o Operation:
If the power of a proportional chamber is increased above the
normal operating power, a threshold can be reached, where the
gas amplification will grow up to infinite value. In this case, the
discharge generated by one electron is going to be selfsupported. If the discharge will be extinguish externally, a new
type of counter can be designed: penetration of the
propagating particle elicits the electron pulse (the effect), but
the strength of the effect will be independent of the energy of
the particle.  Trigger counter.
o It is possible to find a mixture of gas fill to get a trigger counter. It
means that the counter goes back to starting position after one
discharge cycle.  Trigger counter.
Gas mixtures possible:
• Argon – argon vapor  for lower intensity
• Argon – halogen gas (bromine vapor)
Lecture #3.
1. Gas-filled counters
5. A Geiger – Müller-counter:
o Scheme:
Lecture #3.
Source: Wikipedia
1. Gas-filled counter
6. Spark chamber:
o Operation:
Spark chambers consist of a stack of metal plates placed in a sealed
box filled with a gas such as helium, neon or a mixture of the two.
When a charged particle ray travels through the box, it ionizes the
gas between the plates. Ordinarily this ionization would remain
invisible. However, if a high enough voltage can be applied between
each adjacent pair of plates before that ionization disappears, then
sparks can be made to form along the trajectory taken by the ray in
effect becomes visible as a line of sparks. In order to control when this
voltage is applied, a separate detector (often containing a pair
of scintillators placed above and below the box) is needed. When
this trigger senses that a ray has just passed, it fires a fast switch to
connect the high voltage to the plates. The high voltage cannot be
connected to the plates permanently, as this would lead to arc
formation and continuous discharging.
o Comments:
• At the beginning there is a 100 V voltage to clear the field from
the ions stay there for longer time.
• Working voltage is several kV.
• Pictures can be taken by photo machine  coordinate detector
operation.
Lecture #3.
Types of Detectors
1. Gas-filled counters
2. Scintillation counters
3. Semiconductor detectors
4. Cherenkov-radiation and counters
5. Particle trace detectors (Visual
detectors)
6. Neutrino detectors
Lecture #3.
2. Scintillation counters
o Definition (Scintillation counter):
A scintillation counter is an instrument for detecting and
measuring ionizing radiation by using the excitation effect of incident
radiation on a scintillator material, and detecting the resultant light
pulses.
o Structure of the device:
a. Scintillator material, where the energy of the radiation is
transformed to energy of light.
b. Photomultiplier, the device which transforms light pulses to
electric signals.
c. Amplification of electric pulses, analyzing of electric pulses,
register by electronics.
Lecture #3.
2. Scintillation counters
1. Scintillator:
o Scintillating materials:
• Inorganic crystals (ZnS – zink-sulfid, NaI – sodium-iodid, CsI, LiI)
• Organic single crystal (anthracene, naphthalene, stilbene)
• Scintillating liquids(toluol)
• scintillator (polimerized liquid scintillator)
o Radiations (α-, β-, γ-radiation) transformed to light pulses.
Lecture #3.
2. Scintillation counters
2. Photomultiplier:
o Definition (photomultiplier):
This is an electron tube which holds two parts: photocathode
(it is sensitive for light) + amplifier part
o Photocathode: It transforms the light pulses to electron current
o Amplifier: It amplifies the electron current to stronger values.
Lecture #3.
2. Scintillation counters
3. An important application: detection of gamma radiation
o
o
Lecture #3.
NaI - Sodium-iodide as scintillating screen
Photoeffect, Compton-scattering and pair-generation – some of
them is needed to generate the effect. (Light pulses in the
crystal)
Types of Detectors
1. Gas-filled counters
2. Scintillation counters
3. Semiconductor detectors
4. Cherenkov-radiation and counters
5. Particle trace detectors (Visual
detectors)
6. Neutrino detectors
Lecture #3.
3. Semiconductor detectors
1. Parameters:
o
o
o
o
o
2.
From the beginning of 1960´s
Small size
Good energy distribution capability
It is nicely used for radiating surfaces with small radiation
Silicon, Germanium are mostly well known materials
Operation + definition (semiconductor detectors):
o
3.
We can say that, semiconductor detectors are special ionization
chambers, where the ionization occurs in solid, semiconductor
material. (Instead of gas the ionization works is solid material.)
Comparison with gas-filled ionization chamber:
o
o
o
o
o
Lecture #3.
Solid state is denser material than gas  solid state needs weaker
radiation to start the ionization
Taking same irradiation, in solids more charged particles can be
generated, than in gas ionization chamber.
In case of solid semiconductor the measurement is more stable,
errors are lower (lower error bars during the measurements)
In case of semiconductors the energy resolution is better than in
gases
Specific conductivity is better for semiconductors
3. Semiconductor detectors
4. Main application fields:
o
o
o
o
Lecture #3.
Measurement on heavy charged particles
Detecting electrons
Detecting gamma-radiations
Observing neutrons
Types of Detectors
1. Gas-filled counters
2. Scintillation counters
3. Semiconductor detectors
4. Cherenkov-radiation and counters
5. Particle trace detectors (Visual
detectors)
6. Neutrino detectors
Lecture #3.
4. Cherenkov-effect
1. Definition (Cherenkov radiation):
If a particle is moving in a medium and its speed is higher
than the speed of the light for the same medium, then the
particle emits a radiation in a cone-like structure. This emitted
radiation use to call as Cherenkov-effect or Cherenkov
radiation after famous Russian Nobel Prize laureate physicist
scientist Pavel Alekszejevics Cserenkov Cherenkov.
Cherenkov
(1904 - 1990)
Lecture #3.
Very typical blue light shows presence of
Cherenkov radiation
Geometry of Cherenkov radiation
Direction of the radiation
𝑣
𝟏
𝒄,
𝐜𝐨𝐬 𝝋 =
=
𝒏𝜷 𝒗
Direction of the radiation
𝑣
𝛽=
𝑐
Lecture #3.
n: index of refraction
of the medium
𝜑: angle of cone,
cone angle
4. Cherenkov-counters
1. Cherenkov-counter:
It is an application for detection of high speed charged
particles. Cherenkov-effect can be used for detectors.
o
2.
Schematics:
photomultiplier
Radiator
Cylindrical mirror
slit
Medium of the radiator is gas.
Lecture #3.
Types of Detectors
1. Gas-filled counters
2. Scintillation counters
3. Semiconductor detectors
4. Cherenkov-radiation and counters
5. Particle trace detectors (Visual
detectors)
6. Neutrino detectors
Lecture #3.
5. Particle trace detectors
Detectors
Counters
It shows information of the
presence of a particle
- in a given time minute
- in a given place.
Lecture #3.
Particle trace detectors
Particle leaves mark or trace along with
their path during their movement.
This trace can be visually projected or saved.
Operation of particle trace detector is
based on showing the trace of the moving
particle.
5. Particle trace detectors
1. Cloud chamber – Wilson-type cloud chamber:
o
The oldest trace detector ever built! 1912.
o
When a charged particle (for example, an alpha or beta
particle) interacts with the mixture, the fluid is ionized. The
resulting ions act as condensation nuclei, around which a mist
will form (because the mixture is on the point of condensation).
The high energies of alpha and beta particles mean that a trail is
left, due to many ions being produced along the path of the
charged particle. These tracks have distinctive shapes (for
example, an alpha particle's track is broad and shows more
evidence of deflection by collisions, while an electron's is thinner
and straight). When any uniform magnetic field is applied across
the cloud chamber, positively and negatively charged particles
will curve in opposite directions, according to the Lorentz force
law with two particles of opposite charge.
Lecture #3.
5. Particle trace detectors
• Three steps of the operation:
1.
2.
3.
4.
Expansion of the gas fill
Trace generation
Taking photos on the particle traces
Preparation of the chamber for the next experiment
Source: Paks Nuclear Power Plant
Lecture #3.
5. Particle trace detectors
Lecture #3.
5. Particle trace detectors
2. Bubble chamber:
o Cloud chambers work on the same principles as bubble chambers,
but are based on supersaturated vapor rather than superheated
liquid.
o While bubble chambers were extensively used in the past, they have
now mostly been supplanted by wire chambers and spark chambers.
Historically, notable bubble chambers include the Big European
Bubble Chamber (BEBC) and Gargamelle.
Lecture #3.
Source: Wikipedia
5. Particle trace detectors
3.
Solid state trace detectors:
o
o
When a heavy (charged) particle travels through solid insulators, single
crystals, glass-like materials or organic polymers, permanent changes
occur. These permanent changes are sub-microscopic, but they can be
observed under microscopes.
Main features of these type of detectors:
•
They are threshold detectors, it means that they can register
energies above a given energy threshold or limit. (Energy threshold
value is material specific.)
o
𝑑𝐸
They gives possibility to measure energy, and/or determine
,
𝑑𝑥
identify particles, diagnose direction and depth of penetration
•
They have good geometrical resolution (5 – 20 nm)
•
They are not sensitive for extreme environmental impacts
•
It is relatively easy and fast to prepare them, to develop them, and
to see them
Main application fields:
•
Investigation of nuclear fission
•
Investigation of reaction caused by alpha particles
•
Identification of heavy particles during high energy reaction
•
Investigation of primary components of cosmic radiation
•
Lecture #3.
5. Particle trace detectors
3. Figure of particle trace on solid state detector:
Lecture #3.
Types of Detectors
1. Gas-filled counters
2. Scintillation counters
3. Semiconductor detectors
4. Cherenkov-radiation and counters
5. Particle trace detectors (Visual
detectors)
6. Neutrino detectors
Lecture #3.
6. Neutrino detectors
1. Countering neutrino detectors:
o
o
In these devices liquid scintillation detectors alternate with drift
chambers which are used as coordinate detectors.
Seconder particles are detected. Seconder particles can be
generated from neutrino interactions.
Lecture #3.
Cherenkov circles in neutrino scattering