Today. Clicker Question. Announcements: Today: SETI loose ends Atoms and Nuclei. Clicker question. Clicker Question

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1 Announcements: Today HW#9 is due Friday April 16 at 8:00 am Exam #3 Thurs. April 29 Final Exam will be on CAPA Today: SETI loose ends Atoms and Nuclei If the Sun suddenly became a black hole, what would happen to the Earth s orbit? A). The Earth would start a spiral into the Sun B). The Earth would fly off out of the solar system C). Depending of the mass of the Sun, the Earth s orbit would approximately double or be approximately half of what it is now D). The Earth would join all the other plants at the same radius from the black hole E). Nothing ISP209s10 Lecture ISP209s10 Lecture If the Sun suddenly became a black hole, what would happen to the Earth s orbit? A). The Earth would start a spiral into the Sun B). The Earth would fly off out of the solar system C). Depending of the mass of the Sun, the Earth s orbit would approximately double or be approximately half of what it is now D). The Earth would join all the other plants at the same radius from the black hole E). Nothing Clicker question True or False. In principle, it is only possible for extremely large Masses to form a black hole. A) True B) False ISP209s10 Lecture ISP209s10 Lecture 23-4-

2 Clicker question True or False. In principle, it is only possible for extremely large Masses to form a black hole. A) True B) False Given any mass M, if you squish it down to a sufficiently small radius ( Schwartzchild radius ), the escape velocity becomes equal to c, which signals that a black hole has formed. ISP209s10 Lecture Drake Equation # civilizations in Milky Way = N* f p n e f l f i f c f L N number of stars in the Milky Way (200 billion) F p fraction of stars with planets (0.2 to 0.4) n e number of planets where life may exist per star (1 to 5) f l fraction where life begins (0-1) f i fraction of life that develops intelligence (0-1) f c fraction of intelligent life that develops communication (0-1) f L fraction of a star s life when intelligent life survives (0-1) The red factors are complete guesses, so the final answer can be billions or zero! Some critics say this makes the Drake Equation meaningless. Others say it still helps identify key factors. ISP209s10 Lecture Drake Equation # civilizations in Milky Way = N* f p n e f l f i f c f L N number of stars in the Milky Way (200 billion) F p fraction of stars with planets (0.5) n e number of planets where life may exist per star (2) f l fraction where life begins (1) f i fraction of life that develops intelligence (0.2) f c fraction of intelligent life that develops communication (1) f L fraction of a star s life when intelligent life survives (3x10-7 ) Drake s guesses are shown in blue. His final answer is ~ 10,000 ISP209s10 Lecture Fermi s Question: Where is Everybody? Nobel-winning physicist who was famous for back of the envelope estimates. Much like Drake, he estimated there should be many intelligent civilizations in our universe. Why then, haven t we made contact? Fermi came up with 3 possible answers: 1) Interstellar travel is impossible 2) Interstellar travel is possible, but not worth the effort 3) Civilizations don t survive long enough to develop interstellar travel ISP209s10 Lecture 23-8-

3 Search for Extra Terrestrial Intelligence (SETI) We have been broadcasting EM signals (radio/tv) into space unintentionally for ~ 100 years Recently (1970 s), we have sent intentional signals to a star cluster 27,000 light-years away We also have been listening for radio signals from intelligent civilizations (SETI institute) About 10% of stars within 40,000 light-years have been searched with no luck. They re either using too weak signals, or a different frequency range, or not transmitting EM signals, or maybe they re not there! ISP209s10 Lecture Want to help SETI make 1st contact? go to and download the software you find there. Your computer will search for extraterrestrial signals by crunching data from SETI s telescopes when you are not using it. Watch Film => ISP209s10 Lecture From Large to Small Atomic Nuclei Atoms Atomic Nucleus A Proton Nucleus at center of atom discovered by the backscattering of!-particles (which are much heavier than electrons) off gold atoms (1911) Contradicted the previous plum pudding model Electrons embedded in a positive charge background like raisins in pudding Made of nuclei and electrons. Size: 10-9 m Made of neutrons and protons. Size: m ISP209s10 Lecture Made of quarks. -Atomic # (aka Proton Number) Z determines the element Size: Neutron # N m -Mass # (aka atomic mass A = N + Z) A neutron has ddu quarks. ISP209s10 Lecture

4 The Nucleus The nucleus contains more than 99.95% of an atom s mass, but only 1E-15 of its volume! The nucleons are held together by the strong force, even though the protons repel each other very strongly. Mass Number Atomic Number (or Proton Number) Element Neutron Number ISP209s10 Lecture The Chart of the Nuclides: A Map of All Known and Possible Nuclei Atomic Nuclei and Elements The number of protons determine the element (oxygen). The number of neutrons+protons determines the atomic mass (16 as shown). This nucleus is 16 O. ISP209s10 Lecture Radioactivity A radioactive nucleus is unstable: Alpha Decay 238 Pu " 234 U + 4 He Beta Decay Gamma Decay 152 Dy* " 152 Dy +! Fission 238 Pu heated by its own radioactivity. 248 Cm " 144 Ce Sr n (spontaneous) 1 n U " 141 Ba + 92 Kr n (induced) ISP209s10 Lecture Why are some nuclei unstable? Two fundamental tendencies in nature: Move toward higher entropy Move toward lower potential energy For most nuclear processes the change in entropy is ~0. Therefore, energy dominates. Total Energy per Mass Unit 6 Li 56 Fe Mass Number 238 U ISP209s10 Lecture

5 Alpha Decay as a Quantum Tunneling Event The alpha particle is like a ball that is contained in a box (the nucleus). QM tells us the wave function of the ball in non-zero outside The box. There is a finite probability you find the ball outside the box as if it tunneled thru the wall. ISP209s10 Lecture Half-Life and Radioactive Decay Law Radioactive decay is a quantum phenomena. Can t say when one given nucleus will decay. Only averages. Find a exponential decay law Amount at time t Original Amount N = N 0 e -" t Decay constant (different for each Type of nucleus) ISP209s10 Lecture Half-Life and Radioactive Decay Law Radioactive decay is a quantum phenomena. Can t say when one given nucleus will decay. Only averages. A more useful form: ( t 1/2 = ln(2)/# ~.693/#) ISP209s10 Lecture Sample Problem Suppose we find a sample of material that has 43.5% of the expected original amount. The half-life of the material is 23.0 days. What is the best estimate for the age of the sample? ( t t 1 / 2 ) ( t 23.0 d) N & 1 # N & 1 # = $! ' = = $! N0 % 2 " N0 % 2 " - ln = ln +,! t = ' % ( 1/ 2) & t $ " 23.0 d # ( 23.0 d)( ln 0.435) ln ( 1/ 2) * ' t $ ( = % ln(1/ 2) 23.0 " ) & d # = 27.6 d Math tip: ln(x y ) = yln(x) ISP209s10 Lecture

6 Radioisotope Dating 14 C is an isotope of carbon that is produced continually by the interaction of cosmic rays with 14 N in the atmosphere: 1 n + 14 N " 14 C + 1 H An organism establishes an equilibrium with 14 C by breathing in 14 CO 2. After death, no new 14 C is absorbed, so it decays away with a half-life of 5730 years. ISP209s10 Lecture Examples of Radioactive Dating 14 C (half-life = 5730 y) is used to date archeological objects. Normal living material has a certain amount of 14 C, which is produced in the atmosphere. 40 K (half-life = 1.25 By) is used to date rocks. It decays 10% of the time to 40 Ar which is not naturally found in most rocks. 238 U (half-life = 4.5 By) is used to date the Earth, the Sun, and other stars. This is one of the ways we estimate that the Earth is 4.5 By old. ISP209s10 Lecture Suppose that fresh tuna contains a certain amount of an isotope that has a half-life of 10 years. Once canned the radioactive isotope decays and is not replaced. If we find a tuna can that has half the expected amount of the nucleus, how old is it? A). We can not tell B). 1 year C). 10 years D). 20 years E).! year Suppose that fresh tuna contains a certain amount of an isotope that has a half-life of 10 years. Once canned the radioactive isotope decays and is not replaced. If we find a tuna can that has half the expected amount of the nucleus, how old is it? A). We can not tell B). 1 year C). 10 years t 1/2 = time for 1/2 of the nuclei to decay D). 20 years E).! year ISP209s10 Lecture ISP209s10 Lecture

7 After three half-lives, what fraction of a radioactive material is left? A). 1/2 B). (1/2) (1/2) (1/2) = 1/8 C). (1/2) (1/2) = 1/4 D). (1/2) (1/2) (1/2) (1/2) (1/2) (1/2) = 1/64 E). 2 3 = 8 After three half-lives, what fraction of a radioactive material is left? A). 1/2 B). (1/2) (1/2) (1/2) = 1/8 C). (1/2) (1/2) = 1/4 D). (1/2) (1/2) (1/2) (1/2) (1/2) (1/2) = 1/64 E). 2 3 = 8 3 half-lives means the Amount has been cut By 1/2 three times. ISP209s10 Lecture ISP209s10 Lecture The equation for fraction remaining is A & 1 # f = = $! B % 2 " Which letter in the above equation is the half-life? A). A B). B C). C C ISP209s10 Lecture The equation for fraction remaining is A & 1 # f = = $! B % 2 " Which letter in the above equation is the half-life? A). A B). B C). C C ISP209s10 Lecture

8 DATA: Take the half-life of 14-C to be 6000 years. If we find a sample of old bone that has (1/16) the normal amount of 14-C found in living bone, how old is the bone? A) years B). 12,000 years C). 18,000 years D). 24,000 years E). (1/6000) 4 years ISP209s10 Lecture DATA: Take the half-life of 14-C to be 6000 years. If we find a sample of old bone that has (1/16) the normal amount of 14-C found in living bone, how old is the bone? A) years B). 12,000 years C). 18,000 years D). 24,000 years E). (1/6000) 4 years You could have used the equation: Equivalently, you could notice that 1/16 = 1/2 x 1/2 x 1/2 x 1/2 => 4 half-lives have elapsed. 4 x 6000 = 24,000 years ISP209s10 Lecture Applications: Nuclear Power A nuclear reactor is a fancy way of generating heat to produce steam. The rest works like a traditional power plant. All nuclear plants produce fissile material as part of their operation. This had led to concerns about proliferation of this material, which could be used for a dirty bomb or possibly to make a nuclear weapon. Nuclear plants produce radioactive wastes which must be stored safely, possibly for tens of thousands of years. Fission Chain Reaction This process repeats continuously. Nuclear Reactor: Stays at criticality. Nuclear Weapon: Reaction goes supercritical. ISP209s10 Lecture ISP209s10 Lecture Nuclear plants do not produce greenhouse gases.

9 Example of use Nuclear Medicine Radiolabelled compounds can be injected into the body for diagnostic purposes. Similarly, compounds can be attached to tumors to kill them (but leaves them in place). X-rays, CT scans, PET scans. Average Dose per Year We are constantly exposed to small amounts of radiation. As long as the damage is not too great our natural repair mechanisms fix the damage: Source Dose (mrem/year) Radon 200 Medical/Dental X-Rays 40 Internal Sources 40 Other Natural Sources 60 Consumer Products ~11 Roger Ressmeyer/Corbis ISP209s10 Lecture ISP209s10 Lecture Average Dose per Year (continued) Source Dose (mrem/year) Nuclear Medicine ~15 Fallout from weapons testing ~2 Nuclear Power ~0.4 Coal Power ~1.2 Total (U.S. Average) ~360 Smoking one pack per day Smoking two packs per day ISP209s10 Lecture RISK Average Loss of Life Expectancy (days) Being male 2800 Being unmarried 2000 Smoking cigarettes (1 pack/day) 1600 Coal Mining 1100 Cancer lb overweight 900; 15 lb overweight 450 Driving a car (risk of accidents) 200 Alcohol 130 Homicide 90 Drowning 40 Diet drinks, one per day for life 2 Power generated by all nuclear 1.5 (Union Concerned Scientists) Hurricanes and tornadoes 1 ISP209s10 Lecture Airline crashes 1