Tools for Nuclear & Particle Physics Experimental Background

Basic Structure of Experimentation

Accelerators Van de Graaff generator (~1935)  By transporting charges, it makes a DC field to accelerate an ion source.  The voltage used is about 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.)  p lab b = 0, E lab b = m b c 2 The center-of- momentum frame (The center-of-momentum is fixed.)  p CM a + p CM b = 0

Collision and Total Energy (cont.) The total energy obtained by the collision When the energy of incident particle increases, it will be 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.

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 2m e c 2, 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<E c ) Radiation region (E>E c )

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.