Particle Detectors and Quantum Physics (1) Stefan Westerhoff Columbia University NYSPT Summer Institute 2002

Size: px

Start display at page:

Download "Particle Detectors and Quantum Physics (1) Stefan Westerhoff Columbia University NYSPT Summer Institute 2002"

Emil Bates
5 years ago
Views:

1 Particle Detectors and Quantum Physics (1) Stefan Westerhoff Columbia University NYSPT Summer Institute 2002

2 Lab Classes related to this lecture: Photoelectric Effect (this morning) Oscilloscopes (this afternoon) Photomultipliers (tomorrow)

3 Physics is an experimental science.

4 Quantum Mechanics Most particle detectors (for example the detectors we will work with in the next weeks) are quantum mechanical devices you can not understand them without a basic knowledge of quantum physics! Quantum mechanics: the laws of physics that apply on very small (atomic) scales the basic message is that physical quantities like energy, momentum, and charge come in discrete amounts (or quanta). Sort of like money

5 Quantum Numbers A quantum number you already know: A particle has a charge Q, and this charge is always a multiple of the electron charge q electron = e = C An object can have a charge 1e, 2e, 3e,, but never 0.5 e Charge is quantized!

6 Does the Electrician Care? Current is charge per time. 1 Ampere passing a point along a wire means that a charge of 1 Coulomb has gone by in 1 second I = t Q, 1A = 1C s = e s In normal life, you need not take into account that charge is quantized only when you are down to a few electrons, the quantization becomes crucial

7 Do We Care? We want to detect single particles, so the quantization is important! Charge is a quantum number carried by a particle (it can be zero, of course ) What else characterizes a particle? mass spin (angular momentum of a particle)

8 What Do We Want to Detect? Particles in air showers electrons, protons, nuclei muons µ neutrons photons γ Detecting a particle means identifying it one must be able to tell different particles apart! What are the fundamental differences between these particles?

9 Charge Electrons, protons, muons, are charged! Neutrons and photons are neutral. Particles differ in their charge quantum number What does that mean in terms of quantum mechanics? The charge quantum number determines whether the particle participates in an interaction connected to the quantum number. Charged particles interact with electric and magnetic fields.

10 Charged Particles and Electromagnetic Fields

11 Electric Fields a charged particles experiences a force in an electric field which is proportional to its charge F = q E the electric field E is defined as the force per unit charge E = F q

12 Electric Fields Field lines indicate the direction of that force Particle in electric field can be deflected Particle in electric field can be accelerated Particle accelerators TV tubes,

13 Parallel Plate Capacitor Two parallel conducting plates of area A and distance d Plates have equal but opposite charges +Q (anode) -Q (cathode) Particle in the field between the plates experiences force and can be accelerated (potential energy is converted into kinetic energy)

14 Electronvolt ev Convenient unit in particle physics kinetic energy acquired by an electron in passing through a potential difference of 1 Volt in vacuum 1eV = J

15 How to Create Free Electrons Electron gun (TV tube, oscilloscope) A heated metal plate emits electrons which are accelerated towards a positively charged metal plate with a hole Electron beam is created by the electrons which pass through the hole because of inertia

16 Magnetic Fields A particle in a magnetic experiences a force which is proportional to its charge and its velocity!!! F = q v B sinθ Lorentz Force Law F v, F B ( right hand rule ) particle in a magnetic field can be deflected magnetic fields can not increase a particle s energy!!!

17 Magnetic Fields Since the force is perpendicular to the motion, the particle is forced on a circular path m v r 2 = q v B r = m v q B Larmor radius is inversely proportional to the magnetic field strength and the charge

18 Electrons in Magnetic Fields Electron beam in a chamber containing gas at low pressure Uniform magnetic field Does the magnetic field point into the page or out of the page? Out of the page remember electron charge is negative!

19 Radioactivity Marie Curie, 1910 One can separate the three sorts of radiation using a magnetic field α and β particles have different and opposite charges γ rays are not charged

20 Cathode Rays crossed magnetic and electric fields

with electric fields are the basic element of the oscilloscope An oscilloscope

21 Applications Thomson discovered the electron with a cathode ray tube with crossed fields by measuring the ratio e/m of the (unknown) beam particle Tubes with electric fields are the basic element of the oscilloscope An oscilloscope makes voltages visible as deflections of an electron beam on a screen J.J. Thomson

22 Electromagnetic Fields Charged particles can be identified by their interaction with electromagnetic fields Bubble chamber photograph, LBNL 1950 Electron/positron pair (same mass, opposite charge)

23 Particle Detection To detect particles, we need them to interact!! (for example with electromagnetic fields) Problems: How do we tell apart particles that do not interact with electromagnetic fields? Example: photons (light)

24 Positron or Electron? Discovery of the positron (Andersen, 1931) Magnetic field B is perpendicular to the paper plane, particles pass through a lead plane Positive charge going up, or negative charge going down? Particle loses energy in the lead (absorber)!

25 What Is Light?

26 Light as a Wave n 1865/1888: Electromagnetic Theory n n Maxwell, Faraday: laws of electromagnetic field Maxwell, Hertz: light is an electromagnetic wave, and basically consists of electric and magnetic fields that are disconnected from their source and travel through space at the speed of light

27 Wavelength and Frequency Frequency f = number of wavelengths passing per unit time

28 The Electromagnetic Spectrum λ = c f

29 Evidence for the Wave Theory Optical Phenomena Refraction Dispersion

30 Evidence for the Wave Theory Interference

31 An Interesting Discovery Hertz 1886 While trying to prove the wave nature of light (which he did!) Hertz was the first to discover (1886) a strange effect: sparks bridge the gap between two charged spheres more easily if they are exposed to light Effect was further studied by Lenard and Thomson and finally explained in 1905 by Einstein (based on work by Planck) this is the beginning of quantum mechanics! (in fact, this is Einstein s Nobel Prize)

32 Photoelectric Effect If a beam of light is directed onto a metal surface (emitter plate), the Ammeter measures a current, the photoelectric current Electrons are kicked out of the emitter plate by the incoming light!

33 Lenard s Experiment (1902) Even if the collector plate is negative with respect to the emitter, there still measure a current some electrons have enough energy to overcome the potential difference If the potential is too high, the current stops (stopping potential)

34 Kinetic Energy of the Electrons If the potential difference is too large, the emitted electrons won t reach the collector plate This is a way to measure the maximum kinetic energy of the ejected electrons q V = 1 2 m v 2 max How does the maximum energy depend on the frequency (color) and the intensity of the light?

35 Classical Explanation (wrong!) Incoming light is an oscillating electromagnetic wave Atoms in the cathode contain electrons which are shaken loose by the oscillating fields of the electromagnetic waves If the amplitude of the wave the light (its intensity) increases, we expect the electron to get a more energetic kick and have a higher kinetic energy when leaving the cathode This is not what happens: for a given frequency f, intense and feeble light beams give the same maximum kick

36 Dependence on Frequency The stopping potential depends on the frequency of the light The photoelectric effect does not occur if the frequency of the light is below some critical cutoff frequency which depends on the cathode material

37 Classical Physics Fails If light is a wave, we expect that no matter how the low the frequency is, electrons can always be ejected if only the light is bright enough. This is not what happens The photoelectric effect can t be explained if we insist that light is a wave. Well, what else can it be?

Planck s Quantum Hypothesis Depending on their temperature, objects emit light (electric stove, wires in toasters, sun, ) Electrons in the material oscillate and produce

38 Planck s Quantum Hypothesis Depending on their temperature, objects emit light (electric stove, wires in toasters, sun, ) Electrons in the material oscillate and produce electromagnetic waves In 1900, Max Planck tried to explain this radiation, but found that the only way to understand the experiments was to assume that these oscillations were quantized

The Birth of Modern Physics Quantum means fixed amount Planck assumed that the electrons do not oscillate at any possible frequency, but only at some fixed frequencies think of the difference between

39 The Birth of Modern Physics Quantum means fixed amount Planck assumed that the electrons do not oscillate at any possible frequency, but only at some fixed frequencies think of the difference between a piano and a violin: on a piano, frequencies are quantized Planck thought this was a nice way to explain the data, but nothing deeper Einstein picked the idea up in 1905 to explain the photoelectric effect

40 Einstein s Explanation (1905) Photoelectric effect can be explained if we assume that light comes in small bundles of energy, so-called photons (light is a particle!) When a photon hits the surface of a metal, its energy is absorbed by an electron in the metal, and if this energy is greater than the surface potential energy barrier, the electron is knocked out of the metal The minimum energy necessary to liberate the electron depends on the metal and is called work function Φ it depends on the metal.

41 Photons The electron energy is proportional to the frequency The energy of a light quantum or photon is E = h f h is Planck s constant: h = J s (you can get h from the slope of the previous plot!)

42 Light Is Quantized When the electron is emitted, it gets the entire energy of the photon so its kinetic energy is 1 2 mv 2 = hf Φ Only if the photon energy is greater than the work function of the target material can the electron escape h f > Φ

43 Photon Energy The least energy a light wave of frequency f can have is the energy of a single photon, hf If the wave has more energy, the total energy must be an integer multiple of hf E = h f = ( J s) f Examples: red light at 650 nm. blue light at 475 nm E E red green = 1.9 ev = 2.6 ev

44 Photons From a Light Bulb How many photons does a 100 W light bulb emit per second? Assume λ = 500 nm 100 W = 100 J/s, so 100 J in 1 second Roughly 5% of the energy of a light bulb goes into light, the rest goes into heat. So we have 5 J/s. E n = = h f = 5 J 2.5eV 2.5 ev 19 10

45 Compton Effect (1923) Experiment in support of the photon theory: A single photon strikes an electron in some material The scattered photon has less energy (or longer wavelength) Compton effect can be explained by using the laws of conservation of energy and momentum just as if the photons were particles like billiard balls

46 Wave or Particle? Ok, so light is a particle. But what about optics?? How do you explain dispersion and refraction? Ok, ok, so light is a wave. But what about the photoelectric effect? Bottom line: we don t know. Light is a wave and/or a particle ( wave-particle duality ). This is one of the cornerstones of quantum mechanics.

47 Principle of Complementarity To understand any given experiment, one must assume that light is either a wave or a particle, but not both. The wave and the particle aspects complement one another. (Niels Bohr) Photoelectric effect: light is photons Diffraction: light is a wave

48 Solvay Conference 1927

49 Intermission

50 Detecting Light

51 A Light Detector The human eye: sensitive to light between 400 nm and 700 nm ( visible light ) Sensitivity is not uniform over the wavelength range this is typical for detectors of all types they only see a small range of the spectrum

52 Photomultiplier Tubes Photomultipliers are electron tubes which transform light into an electric current How? Use photoelectric effect Photosensitive cathode if photon hits the cathode, an electron is emitted and directed towards the anode Signal current can be picked up at the anode Problem: one electron is a current of I = Q t = t 19 C Way too small!!

53 Dynodes How can we go from one electron to a measurable current? using an electric field, we direct and accelerate the first electron towards another metal plate (dynode) where it kicks out some secondary electrons, which we direct and accelerate towards another dynode where they kick out electrons Electron cascade is finally collected at the anode Typical gains: 10 7 for 10 to 12 dynodes ( stages ) Typical voltage: 1000 V (don t worry, not yours!)

54 Schematic Diagram Basic Components: Photocathode: transforms light to a primary electron via photoelectric effect Dynodes: increase the number of particles ( secondary electrons ) Anode: measure the secondary electrons

55 Photomultipliers The components are housed in an evacuated glass tube, so a PMT looks pretty much like the electron tube from Thomson s lab

56 Examples Depending on the application, photomultipliers come in all shapes and sizes Photonis

57 SuperKamiokande Underground neutrino detector in Japan 11,146 photomultipliers observe 50,000 tons of water

58 The Photocathode The photosensitive material is deposited as a thin layer on the inside of the PMT window Different cathode materials are sensitive to light of different wavelength Like your eye, photomultipliers are sensitive in a limited wavelength only depending on the cathode material Certain minimum frequency is required (of course) E = h f Φ

59 Quantum Efficiency Even above the threshold, the probability for conversion of a photon to an electron is not 100% Important number: Quantum Efficiency η(λ) = number of photoelect rons released number of incident photons on cathode Typical quantum efficiencies are 25-30%, so you see only about a quarter or a third of the incoming photons

60 Quantum Efficiency Quantum efficiency as a function of wavelength for some materials If you build a detector with photomultipliers, pick the right cathode material Most tubes today have semiconductors (30% quantum efficiency) rather than metals (0.1%)

61 Pulse Shape The signal at the anode is a charge pulse (current) with a total charge proportional to the number of electrons emitted by the cathode Typical anode current pulse of a fast-response PMT vertical scale 20 ma/div. horizontal scale 2 ns/div.

62 Light, Yes, But Not Too Much! Photomultipliers are extremely light sensitive (down to the one photon level) They should not be exposed to ambient light while under high voltage, since the large current can destroy the tube When used in our experiment, the photomultipliers should be shielded from ambient light with black cloth Question: Photomultipliers don t work well in strong magnetic fields why not?

63 New Developments Flat photomultipliers with large area and small pixellation for better resolution (where did the photon hit the tube surface?) Burle 85001

64 Micro Channel Plates Dynodes are replaced by array of microscopic channels (typically 5-10µm diameter) Inner surface of channels acts as dynode and releases secondary electrons

65 How to Make It Flat photon Faceplate Photocathode Dual MCP Photoelectron V ~ 200V V ~ 2000V Gain ~ 10 6 V ~ 200V Anode

66 Summary If we want to apply physics on very small (atomic) scales, we need the laws of quantum mechanics The important message of quantum mechanics is that charges, energy, and momentum come in discrete amounts (quanta) In certain experiments (photoelectric effect, Compton effect) light behaves as if it was made up of particles of energy E = h f (photons) However, light can also behave as a wave (diffraction, refraction, )

67 Summary The deflection of charged particles in electromagnetic fields can be used to identify the particles Photomultipliers are light detectors which use the photoelectric effect to convert incoming photons to an anode current

68 Literature W.R. Leo, Techniques for Nuclear and Particle Physics Experiments, Springer (1987) D.C. Giancoli, Physics, Vol. 2, 5 th Edition, Prentice Hall (1998) D. Halliday, R. Resnick, J. Walker, Fundamentals of Physics, Vol. 2, 6 th Edition, J. Wiley (2001) Photomultiplier Tubes, Principles and Applications, Philips Photonics

Particle Detectors and Quantum Physics (2) Stefan Westerhoff Columbia University NYSPT Summer Institute 2002

Particle Detectors and Quantum Physics (2) Stefan Westerhoff Columbia University NYSPT Summer Institute 2002 More Quantum Physics We know now how to detect light (or photons) One possibility to detect