RUTHERFORD SCATTERING

1. RUTHERFORD SCATTERING Edan Bainglass Jose Chavez Kennedy Izuagbe

2. After his discovery of alpha particle emissions from a radioactive isotope of Radon gas, for which he was awarded the 1898 Nobel prize in Chemistry, Rutherford spent a considerable amount of time and effort into their investigation • As a student of J.J. Thomson at Cambridge University, Rutherford was familiar with the Plum Pudding model and used it as the standard for his investigations How it all started…

3. Shortly after discovering the electron in 1897, Prof. J.J. Thomson proposed the “Plum Pudding” model • “We suppose that the atom consists of a number of corpuscles moving about in a sphere of uniform positive electrification” – J.J. Thomson The“Plum pudding” model

4. Rutherford initially failed in counting individual alpha particles • Upon moving to the University of Manchester, he teamed up with Dr. Hans Geiger, and with the use of Geiger’s apparatus, they began recording individual scintillations of alpha particles The initial experiments

5. The assumption, based on the Plum Pudding model, was that the alpha particles will mostly pass right through the atom, with minimal deflection • Geiger originally calculated the most probable deviation of alpha particles to be within 2° • Geiger suggested that the experiments should be given to a young student named Ernest Marsden • Marsden returned to Geiger shortly after conducting the experiment with strange results – some of the alpha particles had been deflected at a “considerable” angle • Rutherford instructed Geiger and Marsden to perform further investigation into the matter Strange results

6. Geiger’s Scintillation Method • Radium was used as a powerful, continuous source of alpha particles • A low powered microscope was used to observe scintillations on a ZnS screen • Different reflecting materials were used • Alpha particles were reflected unto the ZnS screen regardless of the angle of incidence • About 1 in 8000 particles were reflected at angles greater than 90° Geiger and Marsden's 1909 Experiment

7. Rutherford was shocked!!! • “It was almost as incredible as if you fired a 15-inch shell at a piece of tissue paper and it came back and hit you.“ – E. Rutherford • In order to explain such events, Rutherford proposed a new atomic model – one with a massive central charge confined to a very small volume surrounded by an opposite and equal charge uniformly distributed across the remainder of the atom • Rutherford later dubbed the central charge “the nucleus” Rutherford's Theory – The nucleus

8. Rutherford’s model explained both small and large deflections • As the ratio of the impact parameter p to the instantaneous distance from the nucleus b decreases, the deflection angle φincreases – stronger coulomb force () • Rutherford suggested that such large deflection angles most likely occurred due to a single scattering event b φ Rutherford's Theory (Cont.)

9. y = number of scattered particles Q = number of incident particles n = atoms per unit volume t = target thickness Z1 = atomic number of alpha particle Z2 = atomic number of target nucleus K = kinetic energy of incident particle Φ = scattering angle • Rutherford showed that the number of alpha particles scattered per unit area into the detector at scattering angle φ is given by or • Rutherford noted that the number of scattered particles is proportional to • The inverse square of the kinetic energy of the incident particle • The inverse 4thpower of the sine of half the deflection angle • The square of the atomic number of the nucleus • The thickness of the target (for thin foils) Rutherford's Theory (Cont.)

10. Geiger and Marsden went on to prove their professor’s theory. They tackled his four main conclusions by investigating the change of the number of scattered particles with: • Variation of angle • Variation of thickness • Variation of atomic weight • Variation of velocity Experimental Proof (1913)

11. The equipment

12. Geiger and Marsden found Rutherford’s theory to be correct • “It may be mentioned in anticipation that all the results of our investigation are in good agreement with the theoretical deductions of Prof. Rutherford, and afford strong evidence of the correctness of the underlying assumption that an atom contains a strong charge at the center of dimensions, small compared with the diameter of the atom” – Geiger and Marsden (1913) • They concluded that it would be possible to calculate the probability of an alpha particle being scattered through any angle under any specified conditions in good agreement

13. Rutherford’s theory and subsequent experiments provided a stepping stone for future research into the structure of the atom • A few examples of such research: • Bohr’s stationary quantized energy states (1912) describing the electron structure of the atom (later adjusted by quantum theory) • Rutherford’s discovery of the proton (1920) • The discovery of the neutron by James Chadwick (1932) • The discovery of quarks (1968, 1974, 1977, 1995) Where did we go from there?

14. Rutherford’s work was invaluable to science and allowed us to have a clearer picture of the inner workings of our world • It is interesting to note that similar to the strange results of his team, Rutherford turned out to be quite the anomaly, as his greatest achievements – the theory of the nucleus, the discovery of the proton – all came after he had already been awarded the Nobel prize - a first! • The methods used by Rutherford and his team are still used today to further investigate the atomic world Conclusions

15. 1. E. Rutherford, F.R.S.*, The Scattering of  and β Particles by Matter and the Structure of the Atom, Philosophical Magazine. Series 6 vol. 21, p. 669-688 (1911). • 2. H. Geiger and E. Marsden, On a Diffuse Reflection of the Particles. (1909). • 3. H. Geiger and E. Marsden, Assistants Paper. Philosophical Magazine 25. p 605-623 (1913). • 4. J.J. Thomson, On the Structure of the Atom: an Investigation of the Stability and Periods of Oscillation of a number of Corpuscles arranged at equal intervals around the Circumference of a Circle; with Application of the Results to the Theory of Atomic Structure. Philosophical Magazine. Series 6, Volume 7, Number 39. p. 237-265. (1904). references