Tools for Nuclear & Particle
Physics
Experimental Background
Basic Structure of Experimentation
Ion
Source
Accelerator
Beam
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.)


ma
plabb = 0, Elabb = mbc2
The center-ofmomentum frame (The
mb
plab
a
ma
mb
pCM
a
p CM
b
center-of-momentum is fixed.)

pCMa + pCMb = 0
plab
b
Center of
momentum
Collision and Total Energy (cont.)

The total energy obtained by the collision
CM
total
E

m
2
a

 m c  2Elabmbc
2
b
4
2
When the energy of incident particle increases, it will
be approximated as
CM
total
E
 2Elabmbc
2
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.
N x  N 0exp  x
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


Compton effect


At law energies, it is dominant.
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.
Ec  600MeV Z

Ionization region (E<Ec)

Radiation region (E>Ec)
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


Time projection chambers


Very good time resolution and position accuracy
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.