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.