Physics 4 – April 17, 2018 P3 Challenge – What is the half-life of a nuclide if a 30.0 mg reduces to 28.5 mg after 2.00 weeks? (Mass weighed at the same time of day…. 9 am Monday routine maintenance routine in a radiation lab.)

Objectives/Agenda/Assignment 7.2 Nuclear reactions Assignment: p294 #16-24 Agenda: Binding energy and nuclear stability Radiation Units Measuring Radiation Nuclear reactions (fission/fusion)

Binding Energy and nuclear stability The energy released in a decay come from the energy required to bind a nucleus together. It is the energy of the mass defect that occurs when a nucleus forms. A nucleus has a mass that is less than the sum of its parts by  = the mass defect The binding energy is given by E = c2 . (the famous equation) Graph of Binding Energy / Mass number Ni-62 is most stable

Units of Radiation SI Metric unit for counting radioactivity is the Curie (Symbol: Ci) 1 Curie = 3.70 x 1010 d/s (disintegrations per second) Radiation is also measured in terms of its effects on tissue. The rem is the unit of radiation dosage equivalent to a given amount of tissue damage in a human. Takes into account the different effects on tissue (alpha most destructive) The mrem (millirem) is by far the most commonly used radiation unit.

Radiation Dosage Typical public dosage per year: 500 mrem (about 350 mrem from background environmental sources – cosmic rays, the sun, Radon in the atmosphere, isotopes in the ground, isotopes in brick/stone building materials + about 150 mrem from medical testing or plane trips or TV/Computer screens or power plants) Limit for general public: 2000 mrem Limit for radiation workers: 5000 mrem Health effects: none below 25000 mrem

Measuring Radioactivity Geiger Counter: An ionization counter detects radioactive emissions as they ionize a gas Ionization produces free electrons and gaseous cations, which are attracted to electrodes and produce an electric current.

Measuring Radioactivity Scintillation Counter: detects radioactive emissions by their ability to excite atoms and cause them to emit light. Radioactive particles strike a light- emitting substance, which emits photons. The photons strike a cathode and produce an electric current.

Measuring Radioactivity Thermoluminescent dosimeter (TLD) badge. Radiation workers are required to wear this kind of badge to monitor the number of mrems they are receiving. When I was a radiation worker at Cornell, our badges served as our door key. If the badge were ever to go over the 5000 mrem limit, the door to the facility would simply not allow entry.

Induced Transmutation We can induce nuclear reactions by bombarding nuclei with neutrons, protons, alpha particles or other particles including small atoms like oxygen. These nuclear reactions follow the same rules as nuclear decay processes. Ex: Alumunim-27 reacts with alpha particles to produce a neutron and another element.

Nuclear Fission In 1939, Lise Meitner and Otto Hahn discovered that when Uranium 235 is bombarded with neutrons a nuclear reaction happens that releases a large amount of energy. They isolated two daughter atoms and when comparing the mass defect of the proposed reaction, they could explain the amount of energy that they got from the reaction. Because of the split into two smaller atoms, this was called a fission reaction. In 1944, Hahn, but not Meitner, received the Nobel prize in Chemistry for the discovery of nuclear fission. Meitner, as a Jew, did the bulk of the work and calculations while living in exile from Hitler in Sweden but was denied credit.

Chain reaction Because 3 neutrons are produced from the reaction, the products of one reaction can start the reaction of 3 new fissionable atoms. Because the natural abundance of U-235 is 0.7%, there is a limited chance that a neutron released will hit another fissionable atom. The mass required for a nuclear reaction to be sustained by the chain reaction is the critical mass. Samples with a mass greater than the critical mass are used to make weapons. Uncontrolled fission reactions are bombs. Controlled fission reactions are nuclear power plants.

Nuclear Energy Reactors Nuclear Energy Reactors operate near the critical mass and use control rods containing neutron “sponges” to slow the reaction as needed. Parts: Fuel Rods enriched to 3 % U-235 in a uranium oxide ore. Control rods of B or Cd absorb neutrons (They integrate into the fuel rods and can be lowered for more contact slowing the reaction or raised for less contact speeding the reaction. This is usually monitored constantly by a computer.) Moderator of Deuterium in heavy water D2O; slows down neutrons

Nuclear Energy Reactors Parts: Reactor Parts shielded in concrete containment structure. Why contain? Provide sufficient shielding in the event of a run away nuclear reaction (nuclear meltdown) Isolate nuclear reaction from earthquakes and other environmental disasters (E.g. Fukushima Tsunami) Protect nuclear reaction from explosions elsewhere in the power plant (Not present on Chernobyl reactor – steam explosion released radiation)

Nuclear Energy Reactors Parts: Heat from the reactor Sent to Electric generator via steam turbine (same generator system used by coal plants) Sent to a nearby body of water to keep the reactor cool The typical nuclear towers are cooling towers that release excess steam heat into the atmosphere.

Nuclear Plant Locations The spot in the center of Missouri is Calloway. 1st built during WWII By 2018, 99 in US 440 worldwide in 30 countries

Nuclear Waste Nuclear waste has two characteristics: volume and radioactivity. A sample may be small, but highly radioactive, or it may be large but relatively low radioactive. Low level radioactive waste is placed in sealed containers and buried 20 feet deep in concrete lined trenches. The low volume but highly radioactive spent fuel rods from nuclear power plants cannot be disposed this way. They are currently stored on site and there is no permanent long term waste disposal facility exists for this highly radioactive waste. Yucca Mt, NV Proposed 1987, cancelled 2009 (under discussion) Carlsbad, NM opened 1999 (under discussion)

Nuclear Fusion In addition to Fission reactions, nuclear fusion also releases energy. Note: All possible nuclear reactions go from less stable to more stable states with the release of some amount of energy. True for both natural decays and induced transmutations, including both fission and fusion reactions. Fusion is the nuclear process the sun uses to create energy. (3-10x more than fission) The H-bomb is an application of an uncontrolled Fusion reaction. We have not yet learned how to maintain a controlled fusion reaction. Why? Because Fusion requires a very high temperature to get the reaction to ignite.

Nuclear Fusion In the sun, the process is three steps: Two protons produce a deuterium, and a gamma ray A third proton collides with the deuterium to form helium-3 and another gamma ray Two of the helium-3 nuclei collide to create a helium-4 nucleus and two extra protons. The net reaction is A Holy grail of nuclear research is to achieve this kind of fusion reaction at or near room temperature. So called “cold fusion”. (Cold here means something less than 104 Kelvin. Even fusion at 100oC would be considered cold.) Fusion also doesn’t use fuel rods so it doesn’t create problematic nuclear waste from the spent fuel rods.

Nuclear reactions Fission: When a large atom is reacted with a neutron, it may form a nuclide that splits into two daughter nuclei. Ex: Fusion: When two small atoms collide to form a larger nuclide. Nuclides larger than Ni-62 may do fission. Smaller may do fusion. The energy of nuclear reactions is also related to it’s mass changes.

Exit slip and homework p294 #16-24 Exit Slip – Nitrogen-14 has a mass of 14.003074 u. What is the binding energy of a Nitrogen-14 nucleus in J? (Atomic number= 7) What’s due? (homework for a homework check next class) p294 #16-24 What’s next? (What to read to prepare for the next class) Read 7.1, 7.2 and 7.3