Structure of the nucleus L 37 Modern Physics [3] The diameter of the nucleus is about 10 million times • Nuclear physics smaller that the overall – what’s inside the nucleus and what diameter of the atom. holds it together 0.00000000000001 m =10-15 m – what is radioactivity – carbon dating neutrons • Nuclear energy protons + – nuclear fission – nuclear fusion – nuclear reactors 0.00000001 m – nuclear weapons = 10-8 m
The atom and the nucleus The structure of the nucleus • the attractive force between the positive • terminology protons and the negative electrons is what – atomic number Z – the number of protons in holds the atom together the nucleus, this is equal to the number of • the neutrons and protons have about the electrons in the atom, since atoms are same mass, and are each about 2000 electrically neutral. The atomic number is what times more massive than the electrons distinguishes one atom from another • Î the nucleus accounts for about 99.9% of – N = the number of neutrons in the nucleus, the total mass of the atom atoms with the same Z but different N’s are • the neutrons have no charge Î what role called isotopes do they play????
– Atomic mass number A = Z + N = the number of protons + neutrons, A determines the examples mass of the nucleus • Hydrogen1H 1 proton, 0 neutrons • symbol for a nucleus whose chemical 1 2 symbol is X • Deuterium 1H 1 proton, 1 neutron Number of protons and neutrons 3 • Tritium1H 1 proton, 2 neutrons A • Carbon 12 13 6 protons, 6 or 7 neutrons 66C, C Z X • Uranium-235 235 has 235 – 92 = 143 neutrons 92U
Number of this is “enriched” uranium, natural uranium U-238 protons contains only 0.7% of this fissionable isotope.
1 What holds the nucleus together? the nuclear force Î The nuclear glue • in addition to the repulsive electric force • The nucleus contains between the protons, the protons and positively charged protons all neutrons also exert an attractive nuclear in a very small volume and all + + force on each other. repelling each other + • However the nuclear force of the protons • so what keeps the nucleus isn’t enough to hold the nucleus together, together? but the neutrons add more “nuclear glue” • Î the nuclear force (glue) without adding the repulsive electric force. • this is where the neutrons play a role • stable nuclei have as many neutrons as protons or more neutrons than protons
What is radioactivity? Natural radioactivity • in some nuclei, there is a delicate balance • some nuclei are naturally radioactive and of electric repulsion and nuclear attraction give off either alpha rays (He nucleus), forces. bets rays (electrons) or gamma rays (high energy photons) randomly • sometimes the nuclei are just on the verge • the particles are classified in terms their of falling apart and need to release some ability to penetrate matter, gammas are excess energy Î an unstable nucleus the most penetrating and alphas the least • an unstable nucleus can disintegrate penetrating. Gammas can go right through spontaneously by spitting out certain kinds several inches of lead! of particles or very high energy photons • how do we detect these particles – using a called gamma rays (γ’s)Å radioactivity Geiger counter
Geiger Counters Gamma Knife Radiosurgery
Îgas filled metal cylinder with a wire down the center
Îthe ray ionizes the gas, and the resulting electrons are collected by the center + wire, the result is a pulse Gamma rays (from Cobalt -60) are focused (BLIP) of current to a point in the brain to kill tumors
2 Half-Life of radioactive nuclei START Half Life • the decay of radioactive nuclei is a random T1/2 process. If you have a sample of many unstable nuclei, you cannot predict when any one of them will disintegrate
• if you start with No radioactive nuclei now, then the HALF LIFE T1/2 is defined as the time for half of the nuclei present to disintegrate.
Nuclear reactions
y = 6050.9 * e^(-0.27681x) R= 0.99961 222 218 7000 •86 Rn decays to 84 Po by emitting an 0.0000 6129.0 4 0.50000 5268.0 alpha particle ( 2 He ) with a half life of 3.8 6000 1.0000 4583.0 137 1.5000 3987.0 days. Ba 2.0000 3503.0 5000 2.5000 2953.0 • If we started with 20,000 atoms of Rn-222, 3.0000 2584.0 4000 3.5000 2320.0 then in 3.8 days we would have 10,000 4.0000 2024.0 4.5000 1753.0 atoms of Rn-222 and 10,000 atoms of Po- 3000 2.5, 3000 218 2000 • In 7.6 days we would have 5000 atoms of
1000 Rn-222, in 11.4 days, 2500 Rn-222’s, etc
0 012345 time (min)
Smoke detectors use radioactivity Dating a Fossil Smoke detectors have • As soon as a living organism dies, it stops taking a radioactive alpha in new carbon. The ratio of carbon-12 to carbon- emitting source. The 14 at the moment of death is the same as every alpha particles ionize other living thing, but the carbon-14 decays and is the air in the detector not replaced. The carbon-14 decays with its half- creating a current. life of 5,700 years, while the amount of carbon-12 If smoke particles enter remains constant in the sample. By looking at the the detector they can ratio of carbon-12 to carbon-14 in the sample and interfere with the comparing it to the ratio in a living organism, it is current causing it to possible to determine the age of a formerly living drop, which sets off the Americium 241 thing fairly precisely. alarm.
3 Natural Radioactivity Nuclear activation • Radon gas 222 • some nuclei that are stable can be 86Rn activated (made unstable) by exposing occurs in soil and can leak into basements. It them to neutrons. can attach to dust particles and be stable nucleus inhaled. • cosmic rays – energetic particles from the cosmos enter the Activated nuclei are atmosphere and decay neutron embedded in tumors as a cancer treatment
210 84 Po killed Alexander Litvenko • Polonium 210 is a highly toxic substance that decays by emitting an 5.3 MeV α 210 4 206 particle in the reaction 84Po→+ 2 He 82 Pb • the half-life of Po-210 is 140 days • Po-210 is made by bombarding Bismuth with neutrons in a nuclear reactor: 209 1 210 83Bin+→ 0 83 Bi • Bi -210 decays via beta decay to Po-210
210 210 BiPobeta decay11 n→+ p e− 83→ 84 01 beta decay
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