oct01

PHYSICS/ HONORS 3375: 

PHYSICS/ HONORS 3375 Sep. 19, 2007 Curie and Nuclear Radiation

Maria Sklowdowka Curie: 

Maria Sklowdowka Curie Born in Warsaw Arrived in Paris in 1891 for university studies at the Sorbonne. Received degree in physics in 1893, another in mathematics in 1894, and a teacher’s diploma in 1896 1895: married Pierre Curie: who had already discovered “Piezoelectric Effect” and was to submit his Ph.D. thesis on magnetism (“Curie’s Law”: M=CB/T) the same year. Maria Sklowdowka in 1891 before departing for Paris Pierre Curie

Radiation: 

Radiation In 1897 Mme. Curie started her Ph.D. thesis research on a systematic investigation of “radiation” discovered by Röntgen and Becquerel. Becquerel’s discovery of ionizing radiation from uranium was not met with excitement. He reported it at l’Académie des sciences on a routine Monday meeting where his colleagues listened politely and then moved on to the next item on the agenda Wilhelm Röntgen Henri Becquerel Mme. Curie had at her disposal the piezoelectric electrometer, invented by spouse Pierre and his brother Jacques, for the measurement of very weak currents

Surprising Results: 

Surprising Results Very early in her work, Mme. Curie discovered that thorium gives off the same radiation as uranium She also observed that the amount of radiation depended only on the amount of U or Th atoms present, independent of the chemical compound!!!. Pierre abandoned his own research and joined her in radiation research She went on to look at ores with U and Th. In pitchblende, they found evidence of much more radioactive components. Source: Lecture by Nanny Fröman to the Royal Academy of Sciences in Stockholm, Sweden, February 28, 1996

Discovery!: 

Discovery! They soon isolated what appear to be two previously unknown elements One is a metal chemically similar to bismuth: they named it polonium in honor of her homeland The second was an alkali metal with properties almost identical to barium: named radium. The Curies were a true partnership: evidence by the intertwined entries in their lab notebook. These discoveries were submitted as Mme. Curie’s Ph.D. thesis in 1903.

Nobel Prize and Honor!: 

Nobel Prize and Honor! In the same year (1903) in which Mme. Curie presented her Ph.D. thesis, the Curies were jointly awarded ½ the Nobel Prize in Physics. Mm.e Curie went on to win the Nobel Prize in Chemistry in 1911. She was the first woman to win a Nobel Prize. In 1995 the French government honored the Curies by disinterring their bodies and reburying them at the Panthéon in Paris (near the Sorbonne).

Nuclear Isotopes: 

Nuclear Isotopes Mendeleev and Dalton introduced to us the idea of “chemical” classification of basic (indivisible…or so they thought) components of substances by Elements Elements are differentiated by the charge of the nucleus (number of protons in it) and the equal number of electrons that orbit them But you cannot just put protons together they are guaranteed to be unstable (why? Answer: electrical repulsion) In order to make nuclei stable you need to add neutrons (to dilute the charge) It turns out that you usually need to add the ~same number of neutrons as protons for “light” elements, but you need more neutrons as you get to heavier elements

Nuclear Isotopes: 

Nuclear Isotopes Notation: Z=number of protons (atomic number) N=number of neutrons A=Z+N (mass number, or loosely: atomic/isotopic weight) Written as: AZX Where X = conventional element symbol Example: lead-207 and uranium-235 20782Pb 23592U

Nuclear Radiation: 

Nuclear Radiation Nuclei are made of protons (p) and neutrons (n) Mass of proton : mp = 1.6726E-27 kg (mpc2= 938.272 013 MeV) Mass of neutron: mn = 1.6749E-27 kg (mnc2= 939.565 346 MeV) Mass of electron: me = 0.00091E-27 kg (mec2= 0.510 999 MeV) mp + me = 1.6735E-27 kg < mn !!!!!!! So neutrons can decay via “Beta Decay” (electrons were known as “beta rays” n p + e (conserves charge) + “anti-electron neutrino” + excess energy (in the form of motion) In the decay process, energy DE = Dmc2 is released So Dmc2  (0.0014E-27kg) x (2.99E8m/s)2  1.25E-13 J 1.25E-13 J may seem small, but it is mostly shared between the electron and the anti-neutrino, with a small fraction going to the “recoil proton”. Comparison: an electron passing through a 1.5V AA cell battery gains 2.4E-19 J (~1/1,000,000 that of the beta decay)

Uranium Alpha Decay: 

Uranium Alpha Decay Uranium-238: 23892U: mU-238 = 238.0507826 u Helium 4: 42He: m He-4 = 4.0026033 u Thorium-234: 23490Th: mTh-234 = 234.043601 u mTh-234 + m He-4 = 238.0462043 u < mU-238 Fission decay of U-238: 23892U  23490Th + 42He Called an “alpha decay”, where the He-4 nucleus was referred to as an “alpha particle”, a name that persist to this day Here Dm  0.0045783 u x (0.001 kg / 6.02E23 u )  7.605E-30 kg And Dmc2  6.80E-13 J (again: ~ 1,000,000 times the energy scale of the electron through a ~1V battery) Alpha decays are a form of (spontaneous) nuclear fission

Fission: 

Fission What we conventionally call “fission” is a little different: Example of U-235 (not 238) 23592U + n (10n)  23692U* (* denotes excited state) The capture process is small unless the neutron is slow (i.e. of “thermal energy” (i.e. ~5E-21J in kinetic energy) 1/7 times the excited U-236 emits a photon (“gamma radiation”) and then alpha-decays to Th-232 6/7 times: the excited U-236 nucleus fragments See http://hyperphysics.phy-astr.gsu.edu/hbase/hframe.html Fragments range from A~80 to 160, for example 23692U*  14456Ba + 8936Kr + 310n + 177 MeV (2.8310-11 J) The important thing is to note the 3 neutrons coming from the reaction (and carrying most of the energy released)