LEO SZILARD National Physics Competition |
| FINAL 1999 Selecting Competition |
| 1. Hofsadter used high energy electrons to measure the distribution of charges in a nucleus. At least what energy the electrons he had to have for this purpose? (National Leo Szilárd Competition 2000, selecting term). (5 scores) Data: The mass of electron is m = 9.11ˇ10-31 kg. Let us assume that the diameter of nucleus is 10-14 m. 2. In 1923 George de Hevesy determined the (upper limit of the) age of a rock based on the ratio of 238U and 206Pb atoms in it, which was 2:3. How old he considered the rock could have been? (5 scores) Data: the half-life period of 238U is 4,51.109 years. 3. Which one among the following two-body systems has the lowest energy: proton-proton, proton-neutron or neutron-neutron? (5 scores) Data: The mass of a proton is 1,6726.10-27 kg, the mass of a neutron is 1,6749.10-27 kg, let us consider the effective range of nuclear attractive forces is 10-15 m. 4. Most of the noble gases left the Earth when it was hot. However, one percent of the atmosphere is 40Ar. George de Hevesy explained that it was generated during the natural decay of 40K, which by absorption of electrons in 11 % decays into 40Ar. Based on this approach please give an estimate for the age of the Earth. (5 point) Data: Half-life of 40K is 1.26.109 years. The proportion of occurrence of 40Ar/40K is 0.9. 5. There is a room, which is not ventilated and its volume is 40 m3. The Rn does not escape the room. The radioactivity of the air in the room is 600 Bq/m3. How much Rn enters the room through the floor in one hour? (5 scores) 6. If in a nucleus there are too many protons, this condition is abolished by a proton-neutron transformation. It can happen through electron absorption or a positron emission. What do you think: is the number of electron absorber or positron emitter isotopes higher? (5 scores) 7. The binding energy of 14N is 16.19 pJ, and of 14C is 16.37 pJ. Which nucleus decays into the other, and why? (5 scores) 8. A reactor operates on natural uranium and uses graphite as neutron moderator. H2O, D2O (heavy water), He (gas), CO2 (gas), or liquid metal (pl. Na, Bi, Pb) can be used for cooling. Compare the behaviour of the reactor in case of overheating for the cases of these different cooling agents. Which cooling agent(s) make(s) the reactor unstable and which ones are neutral from the point of view of stability. (5 scores) 9. Where will be the radon concentration higher in wintertime: in a cool or in a heated room of a one-floor building? (5 scores) 10. Why are the fission products of uranium radioactive? (5 scores) |
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