Tools for Nuclear & Particle Physics

1. Tools for Nuclear & Particle Physics Experimental Background

2. Basic Structure of Experimentation

3. 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.

4. 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.

5. 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.

6. 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.

7. Accelerators continued • Colliders (~1975) • Colliders make two accelerated particles collide each other. • It can gain the TeV order of energy.

8. Collision and Total Energy • The laboratory frame (The target is at rest.) • plabb= 0, Elabb = mbc2 • The center-of-momentum frame (The center-of-momentum is fixed.) • pCMa + pCMb = 0

9. 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.

10. 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.

11. 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)

12. 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.

13. 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)

14. Detectors • The main purposes • To identify particles • To measure positions • To measure time differences

15. 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).

16. Detectors (cont.) • Scintillation counters • For the problems, the scintillation counter is not so efficient, and the result is always statistical.

17. 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).

18. 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.

19. 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.

20. 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

21. 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.