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Transcript
Gilbert Kirss Foster Chapter 2 Big Bang Theory Big Bang Theory β’ Theoretical physicists believe that the universe began 13.7 billion years ago with an enormous release of energy β’ All of the mass of the universe was compressed into one infinitely dense mass, which exploded and expanded β’ Within milliseconds, energy converted to matter, which formed subatomic particles. © 2014 W. W. Norton Co., Inc. Big Bang Theory β’ Within 4 minutes, the universe had expanded and cooled to 109 K. Neutrons and protons collided and fused to form deuterium, 2 1π» β’ When all of the free neutrons were consumed, deuterium atoms underwent fusion to form Helium: 2 1π» © 2014 W. W. Norton Co., Inc. + 21π» β 42π»π Big Bang Theory β’ Most of the matter in the universe today is still hydrogen and helium, which supports the theory, but how did other elements form? β’ As the universe cooled further, the particles slowed, allowing electrons to combine with the nuclei to produce neutral atoms © 2014 W. W. Norton Co., Inc. Big Bang Theory β’ Large masses of hydrogen and helium formed the first stars. Energy released from the fusion fuels the stars. β’ Inside stars, 4He would fuse to create 12C, which fused with other 4He to produce 16O, which fused with other 4He to produce 20Ne, and so onβ¦β¦ until 56Fe forms β’ Once a star produces iron, the star is near death, because the formation of iron does not release energyβ¦it consumes it. Thus, a star with an iron core has run out of fuel. © 2014 W. W. Norton Co., Inc. Big Bang Theory β’ The mass of a stable nucleus is always less than the sum of the separate masses of the subatomic particles. For example, consider 4He 2 neutrons + 2 protons = 6.69510 x 10-27 kg Actual mass of 4He nucleus = 6.64466 x 10-27 kg Difference = 0.05044 x 10-27 kg β’ This difference is called the mass defect, Ξm. This βmissingβ mass is converted to energy according to E =Ξmc2. Some of this excess energy is used to hold the nucleus together, so it is called the binding energy, or mass energy. β’ Takes significant energy to counteract the repulsion of protons in the nucleus © 2014 W. W. Norton Co., Inc. Big Bang Theory © 2014 W. W. Norton Co., Inc. Big Bang Theory β’ A collapsing star is hot enough to disintegrate nuclei and produce free neutrons. Trans-iron elements can form by successive neutron capture. Ex: formation of Co 56 26πΉπ + 3 10π β 59 26πΉπ Ξ² β 59 27πΆπ + β10π β’ The eventual explosion of the star leads to the dispersion of the elements across the universe. All matter in the solar system is star debris. © 2014 W. W. Norton Co., Inc. Artificial Nuclides β’ Following the gold foil experiment, scientist began bombarding elements with alpha particles to artificially create isotopes. The first radionuclide not found in nature was 30P, created in 1933 27 13π΄π + 42π»π β 30 15π + 10π β’ Trans-uranium elements are obtained by a combination of neutron capture and Ξ± particle capture. β’ For elements 93<Z<103, neutron bombardment is easier than Ξ± bombardment because neutrons donβt repel, so less energy is required β’ For elements with Z>103, bombardment of 249 98πΆπ with other nuclei leads to formation of radionuclides up to Z=118. © 2014 W. W. Norton Co., Inc. Artificial Nuclides 238 92π + 10π β 239 94π π + 2 β10π 249 98πΆπ + 126πΆ β 257 104π π + 4 10π 249 98πΆπ 18 8π 263 106ππ 4 10π © 2014 W. W. Norton Co., Inc. + β + Trans-uranium neutron capture Trans-californium nuclei fusion 10
