Transcript Lecture
by Dr. Adriana Banu JMU Department of Physics and Astronomy January 28, Meet the Scientist: Saturday Morning Physics at JMU’12 A few fundamentals… What chemists like… 92 building blocks (chemical elements) Periodic Table of Elements 1896 Mendeleyev o all matter (solids, liquids and gases) is composed of atoms o chemical element = matter that is composed of only one type of atom o atom = the smallest particle of any element that still retains the characteristics of the element not to scale Atom = nucleus + electrons -e (10-10 m) +Ze Nucleus = protons + neutrons (10-14 m) Electron (discovered in 1897 by J. J. Thomson) Neutron (discovered in 1932 by J. Chadwick) 1906 1935 Stable and Unstable Atoms How do atomic particles interact inside the atom? o there are forces within the atom that account for protons (positive charges) and electrons (negative charges): like charges repel and unlike charges attract each other q1 F21 F12 q2 Attractive force between q1 and q2. r F21 q1 q2 Repulsive force between q1 and q2. r F12 Stable and Unstable Atoms What makes the protons stay together in an atom? o the strong nuclear force which opposes and overcomes the electrical repulsion force between the protons The energy associated with the strong force is called the binding energy. Does the nucleus of an atom ever lose particles? o stable(unstable) nucleus: the binding energy is(not) great enough to hold the nucleus together. Unstable atoms will lose neutrons or protons as they attempt to become stable. They are called radioactive atoms. What is a radioactive decay? o spontaneous breakdown of an atomic nucleus resulting in the release of nuclear radiation What is nuclear radiation? o energy and matter released during radioactive decay 3 types observed: , and What nuclear physicists like… ~ 3000 currently known nuclides ~ 270 stables only ! ~ 7000 expected to exist Color Key: Stable + emission - emission particle emission Spontaneous fission Z N A X Z N A chemical element (X) is uniquely identified by the atomic number Z ! Nuclides that have the same Z but different N are called isotopes ! Mass number: A = N + Z What is radioactivity? o emission of nuclear radiation due to a change in the nucleus Transmutation occurs when a radioactive element attempts to become stabilized and transforms into a new element. He-4 (2 protons + 2 neutrons) Radium-226 (88 protons + 138 neutrons) Radon-222 (86 protons + 136 neutrons) The Beginnings… It was 1895 in Europe… the discovery of X-rays 1869 – first observation of “cathode rays” Wilhelm Conrad Röntgen 1901 – Nobel Prize for Physics (first ever) Modern “Alchemy”: radioactivity 1896 Becquerel discovered radioactivity The Nobel Prize in Physics 1903 A. H. Becquerel Pierre Curie Uranium mineral Marie Curie 1898 – Marie Curie discovered two new radioactive elements: Polonium & Radium The Nobel Prize in Chemistry 1911 Curies’ work on radioactivity… Marie Curie studied the “uranium rays” through their electrical effects using the quartz piezoelectroscope, an unusually sensitive electroscope invented by Pierre Curie himself (see picture below) Classical electroscope used to detect electrical effects in the air (briefly)Marie Curie’s legacy o coined the word “radioactivity” to describe each substance’s power to give off invisible ionizing rays/radiations o radioactivity is an atomic property o discovery of new radioactive elements- Polonium and Radium o isolated Radium and determined its atomic weight o suggested that Becquerel rays might be pieces of matter o the first scientist honored with 2 Nobel prizes (physics & chemistry) o first female professor at the prestigious University of Paris (Sorbonne) o founded a private Radium Institute (1914) to conduct research in chemistry, physics and medicine (the beginning of radiotherapy) Rutherford joins the quest Meet the “uranium rays”: Ernest Rutherford The Nobel Prize in Chemistry 1908 “for his investigations into the disintegration of the elements and the chemistry of radioactive substances” Rutherford – father of nuclear physics Discovery of the nucleus! fluorescent ffl screen u non-deflected particles gold foil deflected particles radioactive source “Father of nuclear physics” in his own words: “It was as though you had fired a fifteen inch shell at a piece of tissue paper and it had bounced back and hit you.” Where does the energy come from? He-4 (2 protons + 2 neutrons) Radium-226 (88 protons + 138 neutrons) Radon-222 (86 protons + 136 neutrons) 1 kg of radium would be converted into 0.999977 kg of radon and alpha particles. The loss in mass is only 0.000023 kg = 23 mg! Energy = mc2 = mass x (speed of light)2 = 0.000023 x (3 x 108)2 = 2.07 x 1012 joules. Equivalent to the energy from over 400 tonnes of TNT!!! 1 kg Ra (nuclear) 4*105 kg TNT (chemical) 238Pu Vanishing radioactivity o no matter how much radioactive material is present initially, half of it will disappear after a time interval known as the “half-life”, T1/2 o after two half-lives, only a quarter of the original material remains… o radioactivity is a random process that follows the laws of probability Exponential decay law: N N 0e 0.693 T1/ 2 (decay probability) t Radioactive half-life Half-lives for various radioisotopes can range from a few microseconds (10-6 s) to billons of years: Radioisotope Half-life Polonium-215 0.0018 seconds Radon-222 38 seconds Sodium-24 15 hours Iodine-131 8.07 days Cobalt-60 5.26 years Caesium-137 30.17 years Radium-226 1600 years Carbon-14 5730 years Uranium-234 0.25 million years Potassium-40 1.3 billion years Uranium-238 4.5 billion years ~ 99% ~ 0.7% ~ 0.006% How to measure the decay of radioactive isotopes? Basic unit of measure the radioactivity: curie (C) 1 C = 37 x 109 decays/second Exposure Amount of radiation striking an object: Röntgen (R) Dose Amount of energy absorbed by an object exposed to radiation: gray (Gy) (in USA: 1 rad = 0.01 Gy) Dose equivalent Amount of biological damage caused by radiation: Sievert (Sv) (in USA: 1 rem = 0.01 Sv = 10 mSv) Human body is naturally radioactive This is the story of a mysterious science which entered history at the onset of major transitions in science, and itself triggered these changes… From its modest beginnings as a minor phenomenon, radioactivity quickly developed into a major research field… The radioactive decays of naturally occurring minerals containing uranium and thorium are in large part responsible for the birth of the study of nuclear physics… A remarkable episode in modern science, radioactivity has the dual attractions of a fascinating history and dramatic consequences for humanity… What did we learn? Got questions?