PHYS-3301 Lecture 5. CHAPTER 3 The Experimental Basis of Quantum. 3.8: Compton Effect. 3.8: Compton Effect. Sep. 11, 2018

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1 CHAPTER 3 The Experimetal Basis of Quatum PHYS-3301 Lecture 5 Sep. 11, Discovery of the X Ray ad the Electro 3.2 Determiatio of Electro Charge 3.3 Lie Spectra 3.4 Quatizatio 3.5 Blackbody Radiatio (Plak; 1900; 1918*) 3.6 Photoelectric Effect (Eistei; 1905; 1921*) 3.7 X-Ray Productio (Rötge;1895; 1901*) 3.8 Compto Effect (Compto; 1927; 1927*) 3.9 Pair Productio ad Aihilatio (Aderso; 1932; 1936*) Homework [1] Chapter 3 :: 4, 8, 11, 19, 20, 23, 35, 39, 42, 47, 51, 54 (due: 9/18) 3.8: Compto Effect 3.8: Compto Effect Whe a photo eters matter, it is likely to iteract with oe of the atomic electros. The photo is scattered from oly oe electro, rather tha from all the electros i the material, ad the laws of coservatio of eergy ad mometum apply as i ay elastic collisio betwee two particles. The mometum of a particle movig at the speed of light is The electro eergy ca be writte as This yields the chage i wavelegth of the scattered photo which is kow as the Compto effect:

2 The Compto effect (Arthur Compto 1927) Mometum & Eergy whe a photo strike a free electro Is this true? Compto provided the 1 st experimetal evidece!! mometum eergy Hypothesis: Experimet? Before Collisio: A photo approaches a electro at rest Mometum & Eergy whe a photo strike a free electro Mometum & Eergy whe a photo strike a free electro After Collisio: The electro scatters at speed u, agle f. A photo of wavelegth l scatters at agle q Eergy ad Mometum Coservatio

3 X-ray detector The Compto Effect We ow discuss a third! X-ray tube target (light atoms, e.g. graphite) The photoelectric effect ad the Compto effect are two importat ways i which EM radiatio iteracts as a particle with matter. e- Photos carry mometum like particles ad scatter idividually with other particles ideed, the wavelegth shift is idepedet of the target material ad the iitial photo wavelegth.

4 3.9: Pair Productio ad Aihilatio Particle-Atiparticle Pair Creatio If a photo ca create a electro, it must also create a positive charge to balace charge coservatio. I 1932, C. D. Aderso observed a positively charged electro (e + ) i cosmic radiatio. This particle, called a positro, had bee predicted to exist several years earlier by P. A. M. Dirac. A photo s eergy ca be coverted etirely ito a electro ad a positro i a process called pair productio. Bubble Chamber Q: Calculate the eergy ad wavelegth of the least-eergetic photo capable of producig a electro-positro pair. [Hit] Photo E goes to the massive particles as iteral eergy + KE. The least eergetic oe must still create the particles but would leave them o KE.

5 Pair Productio i Empty Space Coservatio of eergy for pair productio i empty space is Cosiderig mometum coservatio yields This eergy exchage has the maximum value Recall that the total eergy for a particle ca be writte as However this yields a cotradictio: ad hece the coversio of eergy i empty space is a impossible situatio. Pair Productio i Matter Sice the relatios ad cotradict each other, a photo ca ot produce a electro ad a positro i empty space. I the presece of matter, the ucleus absorbs some eergy ad mometum. Pair Aihilatio A positro passig through matter will likely aihilate with a electro. A positro is draw to a electro by their mutual electric attractio, ad the electro ad positro the form a atomlike cofiguratio called positroium. Pair aihilatio i empty space will produce two photos to coserve mometum. Aihilatio ear a ucleus ca result i a sigle photo. Coservatio of eergy: Coservatio of mometum: The photo eergy required for pair productio i the presece of matter is The two photos will be almost idetical, so that The two photos from positroium aihilatio will move i opposite directios with a eergy:

6 Electromagetic Waves behavig like Particles (Chapter 2) PHOTONS Black Body Radiatio The Photoelectric Effect The Productio of X-Rays The Compto Effect PHOTONS E = hf PHOTONS p = hf/c = h/! Particle-Atiparticle Pair Productio CHAPTER 4 Structure of the Atom 4.1 The Atomic Models of Thomso ad Rutherford 4.2 Rutherford Scatterig 4.3 The Classic Atomic Model 4.4 The Bohr Model of the Hydroge Atom 4.5 Successes ad Failures of the Bohr Model 4.6 Characteristic X-Ray Spectra ad Atomic Number 4.7 Atomic Excitatio by Electros Bohr s differet; he s a football [U.S. soccer] player! Erest Rutherford, givig a ucharacteristic complimet to a theorist-niels Bohr i this case. 4.1 The Atomic Models of Thomso ad Rutherford Pieces of evidece that scietists had i 1900 to idicate that the atom was ot a fudametal uit: 1) There seemed to be too may kids of atoms, each belogig to a distict chemical elemet. 2) Atoms ad electromagetic pheomea were itimately related. 3) The problem of valece. Certai elemets combie with some elemets but ot with others, a characteristic that hited at a iteral atomic structure. 4) The discoveries of radioactivity, of x rays, ad of the electro

7 Thomso s Atomic Model Thomso s plum-puddig model of the atom had the positive charges spread uiformly throughout a sphere the size of the atom with, the ewly discovered egative electros embedded i the uiform backgroud. I Thomso s view, whe the atom was heated, the electros could vibrate about their equilibrium positios, thus producig electromagetic radiatio.