Tools for Nuclear & Particle Physics
Experimental Background
Basic Structure of Experimentation
Ion Source
AcceleratorBeam
Target
Detectors
Accelerators
Van de Graaff generator (~1935) By transporting charges, it makes a DC field to
accelerate an ion source.
The voltage used is about 20-30 keV, and it provides 10 MeV potential.
It had become obsolete in nuclear & particle field although the technological applications are still common.
Note: Tandem Van de Graaff can utilize twice the maximum voltage.
Accelerators continued
Linear Accelerators [Linacs] (~1955) These are used mainly for electrons.
The idea is to utilize radio frequency to accelerate electrons through a number of connected gaps.
It needs less energy to get close to speed of light.
It can obtain up to 100 MeV.
Accelerators continued
Cyclotron (~1940) By using a magnetic field, a particle is tracked in a
circular orbit. An alternating electric field accelerates the particle
at each gap. It can gain up to 500 MeV. Nowadays, it is used for medical physics, and
other applications.
Accelerators continued
Synchrotron (~1955) Particles are accelerated in a circle of constant
diameter. The main idea is to use bending magnets and
gaps to accelerate particles. The particles must be “pre-accelerated” because
of a large difference of magnetic field at the end. It can gain up to 100 GeV.
Accelerators continued
Colliders (~1975) Colliders make two accelerated particles collide
each other.
It can gain the TeV order of energy.
Collision and Total Energy
The laboratory frame (The target is at rest.) plab
b = 0, Elabb = mbc2
The center-of-momentum frame (The center-of-momentum is fixed.) pCM
a + pCMb = 0
plaba b
plab
ma mb
Center of momentum
paCM bpCM
ma mb
Collision and Total Energy (cont.) The total energy obtained by the collision
2422 2 cmEcmmE blabbaCMtotal
22 cmEE blabCMtotal
When the energy of incident particle increases, it willbe approximated as
Note: The derivation will be presented in the lecture.
Passage of Radiation Through Matter The idea is to find out the input and output relation
of particle beams through a slab of matter Two basic interactions
Many small interactions It describes the input and output energies in a statistical
manner.
“All-or-nothing” interactions It describes how many particles going out from a slab of
matter. xNxN exp0
Particle-Dependent Properties Heavy charged particles
The energy loss depends upon not only the length, but the density.
There occurs an ionization minimum.
The range of a particle gives the specific range and energy lost. (Bragg peak)
Particle-Dependent Properties (cont.) Photons
There are mainly three processes. Photoelectric effect
At law energies, it is dominant.
Compton effect At intermediate, it is dominant.
Pair production At an energy of 2mec2, it becomes possible, and then it will
be completely dominant.
Particle-Dependent Properties (cont.) Electrons
The high-energy electrons get energy loss by radiation.
Because of the radiation energy loss, there is the separation of the region, critical energy.
Ionization region (E<Ec)
Radiation region (E>Ec)
ZEc MeV600
Detectors
The main purposes
To identify particles
To measure positions
To measure time differences
Detectors (cont.)
Scintillation counters This utilizes the fact that charged particles
traversing solids excite the electrons and emit light in such materials.
The light will be collected and amplified by photomultipliers.
The time response is very fast (200 pico second). A pair of scintillation counters can measure the
time of flight and velocity, but only for (v<<c).
Detectors (cont.)
Scintillation counters For the problems, the scintillation counter is not so
efficient, and the result is always statistical.
Detectors (cont.)
Semiconductor detectors This utilizes the fact that charged particles
traversing solid excite the electrons in semiconductor.
Measurement of position is accurate (500 m or less).
The problem is radiation damage (because of harsh conditions).
Detectors (cont.)
Bubble chambers This utilizes the fact that the highly heated
transparent liquid gives the path of incident particles in the chamber.
This is a supplemental detector for counters.
Detectors (cont.)
Spark chambers This utilizes the fact that the ions remained, after
particles’ passing through, can be sparked by voltage.
This is selective detector unlike a babble chamber.
This can distinguish between electrons and muons.
Other Detectors
Wire chambers Very good time resolution and position accuracy
Time projection chambers Giving very good spatial (three dimensional)
resolution Spectrometer
Measuring mass and momentum of a particle using magnetic fields
Counters and its Statistics
What is the probability of finding a specific value? If the total number of detected particles is small, it
follows Poisson distribution.
If the total number of detected particles is large, it follows Gaussian distribution.
Note: The detailed discussion will be given in the lecture and lab.
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