The Picture by ~1932 Electrons were discovered ~1900 by J. J. Thomson Protons being confined in a nucleus was put forth ~1905 Neutrons discovered 1932 by James Chadwick Quantum theory of radiation had become “widely accepted”, although even Einstein had his doubts HW8 due Wed.

Quick recap on radiation from atoms Energetic gamma rays come from excited nuclei (Co60, for example) These photons emerge from the nucleus of the atom !!! They are generally in the gamma ray region of the EM spectrum ------------------------------------------------------------------------------ Ordinary atoms also radiate photons when their atomic electrons “fall” from a higher energy state to a lower one. (The configuration where all the electrons are in their lowest energy state is referred to as the ground state) The transitions of atomic electrons from a high energy state to a lower energy state produces radiation (light)! The radiation which emerges when electrons make these transitions (ie., quantum transitions) is generally in the visible or X-ray region. Let’s continue on this issue of transitions in atoms…

Bohr Atom & Radiation Before After Allowed Orbits Radiated photon n = 1 2 3 4 5 n = Electron in lowest “allowed” energy level (n=1) Electron in excited state (n=5) Before Electrons circle the nucleus due to the Electric force 1 2 3 4 5 Electron falls to the lowest energy level After Radiated photon Note: There are many more energy levels beyond n=5, they are omitted for simplicity

Atomic Radiation It is now “known” that when an electron is in an “excited state”, it spontaneously decays to a lower-energy stable state. E5 > E4 > E3 > E2 > E1 The difference in energy, DE, is given by: DE = E5 – E1 = hn = Ephoton h = Planck’s constant = 6.6x10-34 [J s] = frequency of light [hz] The energy of the light is DIRECTLY PROPORTIONAL to the frequency, n. Recall that the frequency, n, is related to the wavelength by: c = n l (n = c / l) So, higher frequency  higher energy  lower wavelength This is why UV radiation browns your skin but visible light does not ! One example could be: Before n = 1 n = 2 n = 3 n = 4 n = 5 Energy Electron in excited state (higher PE) E5 E4 E2 E3 E1 After n = 1 n = 2 n = 3 n = 4 n = 5 Energy Electron in lowest state (lower PE) E5 E4 E2 E3 E1

Hydrogen atom energy “levels” Quantum physics provides the tools to compute the values of E1, E2, E3, etc…The results are: En = -13.6 / n2 1 2 3 4 5 Energy Level Energy En (eV) 1 -13.6 2 -3.4 3 -1.51 4 -0.85 5 -0.54 These results DO DEPEND ON THE TYPE OF ATOM OR MOLECULE So, the difference in energy between the 3rd and 1st quantum state is: Ediff = E3 – E1 = -1.51 – (-13.6) = 12.09 (eV) When this 3 1 atomic transition occurs, this energy is released in the form of electromagnetic energy.

Example 4 In the preceding example, what is the frequency, wavelength of the emitted photon, and in what part of the EM spectrum is it in? E = 12.1 [eV]. First convert this to [J]. Since E = hn  n = E/h, so: n = E/h = 1.94x10-18 [J] / 6.6x10-34 [J s] = 2.9x1015 [1/s] = 2.9x1015 [hz] l = c/n = (3x108 [m/s]) / (2.9x1015 [1/s]) = 1.02x10-7 [m] = 102 [nm] This corresponds to low energy X-rays !

Some Other Quantum Transitions Initial State Final State Energy diff. [eV] Energy diff. [J] Wavelength [nm] Region 2 1 10.2 1.6x10-18 121 X-ray 3 12.1 1.9x10-18 102 4 12.8 2.0x10-18 97 1.89 3.0x10-19 654 Red 2.55 4.1x10-19 485 Aqua 5 2.86 4.6x10-19 432 Violet

This completed the picture, or did it… Electrons were discovered ~1900 by J. J. Thomson Protons being confined in a nucleus was put forth ~1905 Neutrons discovered 1932 by James Chadwick Quantum theory of radiation had become “widely accepted”, although even Einstein had his doubts Radiation is produced when atomic electrons fall from a state of high energy  low energy. Yields photons in the visible/ X-ray region. A nucleus can also be excited, and when it “de-excites” it also gives off radiation  Typically gamma rays !

Cosmic Rays Cosmic Rays are energetic particles that impinge on our atmosphere (could be from sun or other faraway places in the Cosmos) They come from all directions. When these high energy particles strike atoms/molecules in our atmosphere, they produce a spray of particles. Many “exotic” particles can be created. As long as they are not so massive as to violate energy conservation they can be created. Some of these particles are unstable and “decay” quickly into other stable particles. Any of these exotic particles which live long enough to reach the surface of the earth can be detected !

Discoveries in Cosmic Rays 1932 : Discovery of the antiparticle of the electron, the positron. Confirmed the existence and prediction that anti-matter does exist!!! 1937 : Discovery of the muon. It’s very much like a “heavy electron”. 1947 : Discovery of the pion. We’ll touch on these today… and some other things…

Positron Discovery in Cosmic Rays (1932) A “Cloud Chamber” is capable of detecting charged particles as they pass through it. The chamber is surrounded by a magnet. The magnet bends positively charged particles in one direction, and negatively charged particles in the other direction. By examining the curvature above and below the lead plate, we can deduce: (a) the particle is traveling upward in this photograph. (b) it’s charge is positive Using other information about how far it traveled, it can be deduced it’s not a proton. Cloud Chamber Photograph Lead plate Larger curvature of particle above plate means it’s moving slower (lost energy as it passed through) It’s a particle who’s mass is same as electron but has positive charge  POSITRON ! Positron

Significance of Positron Discovery The positron discovery was the first evidence for ANTIMATTER. That is, the positron has essentially all the same properties as an electron, except, it’s charge is positive ! Carl Anderson award Nobel prize for the discovery of the positron Carl Anderson 1905-1991 If an electron and a positron collide, they ANNIHILATE and form pure energy (EM Radiation). This conversion of matter into energy is a common event in the life of physicists that studies these little rascals…

Example: Matter  Energy E=5 [MeV] E=5 [MeV] E=5 [MeV] %*&* e+ e- e+ e- e+ e- e- e+ An electron and positron, each with energy 5 [MeV] collide, and annihilate into pure energy in the form of 2 photons. Each photon carries away ½ of the total energy available.

Example follow-up You will find that this corresponds to gamma rays ! In the preceding example, what are the wavelengths of the photons which emerge from this interaction, and from what part of the spectrum are they? Since E=hc / l, We can get wavelength using: l = hc/ E First we need to convert the 5 [MeV] to the equivalent number of [J] First note that: 5 [MeV] = 5x106 [eV] = 1 l = hc/ E = (6.6x10-34)(3x108) / 8.0x10-13 = 2.5x10-13 [m] You will find that this corresponds to gamma rays ! Very energetic photons !!!

Discovery of the muon The muon was discovered in 1937 by J. C. Street and E. C. Stevenson in a cloud chamber. Again, the source is cosmic rays produced in the atmosphere. The muon behaves identally to an electron, except: It is about 200 times as massive It’s unstable, and decays in about 2x10-6 [s] = 2 [ms] (m  e + n + n) More on these n guys later ! Note that many muons are able to reach the earth from the upper atmosphere because of time dilation ! Because of their large speed, we observe that their “clocks” run slow  they can live longer !!!

Discovery of the Pion m p Cecil Powell and colleagues at Bristol University used alternate types of detection devices to see charged tracks (called “emulsions”) in the upper atmosphere. In 1947, they annouced the discovery of a particle called the p-meson or pion (p) for short. Cecil Powell 1903-1969 1950 Nobel Prize winner Muon (m) comes to rest here, and then decays: m  e + n + n Two more neutrinos are also produced but also escape undetected. Pion (p) comes to rest here, and then decays: p  m + n + n Two neutrinos are also produces but escape undetected. m e p

The Plethora of Particles Because one has no control over cosmic rays (energy, types of particles, location, etc), scientists focused their efforts on accelerating particles in the lab and smashing them together. Generically people refer to them as “particle accelerators”. (We’ll come back to the particle accelerators later…) Circa 1950, these particle accelerators began to uncover many new particles. Most of these particles are unstable and decay very quickly, and hence had not been seen in cosmic rays. Notice the discovery of the proton’s antiparticle, the antiproton, in 1955 ! Yes, more antimatter !