Structure of the nucleus L 36 Modern Physics [3] The diameter of the nucleus is about 10 million times • 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 • Nuclear energy protons + – – – 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 • 1H 1 proton, 1 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 • -235 235 has 235 – 92 = 143 neutrons 92U

Number of this is “enriched” 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

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 Smoke detectors use radioactivity 222 Rn 218 Po Smoke detectors have •86 decays to 84 by emitting an a radioactive alpha alpha particle ( 4 He ) with a half life of 3.8 2 emitting source. The days. alpha particles ionize • If we started with 20,000 atoms of Rn-222, the air in the detector then in 3.8 days we would have 10,000 creating a current. atoms of Rn-222 and 10,000 atoms of Po- If smoke particles enter 218 the detector they can • In 7.6 days we would have 5000 atoms of interfere with the Rn-222, in 11.4 days, 2500 Rn-222’s, etc current causing it to drop, which sets off the Americium 241 alarm.

Dating a Fossil Natural Radioactivity

• As soon as a living organism dies, it stops taking • Radon gas 222Rn in new carbon. The ratio of carbon-12 to carbon- 86 14 at the moment of death is the same as every occurs in soil and can other living thing, but the carbon-14 decays and is leak into basements. It not replaced. The carbon-14 decays with its half- can attach to dust life of 5,700 years, while the amount of carbon-12 particles and be remains constant in the sample. By looking at the inhaled. ratio of carbon-12 to carbon-14 in the sample and • cosmic rays – energetic comparing it to the ratio in a living organism, it is particles from the possible to determine the age of a formerly living cosmos enter the atmosphere and decay thing fairly precisely.

3 210Po Nuclear activation 84 killed Alexander Litvenko • Polonium 210 is a highly toxic substance • some nuclei that are stable can be that decays by emitting an 5.3 MeV α activated (made unstable) by exposing particle in the reaction 210Po→+ 4 He 206 Pb them to neutrons. 84 2 82 • the half-life of Po-210 is 140 days stable nucleus • Po-210 is made by bombarding Bismuth with neutrons in a : 209 1 210 83Bin+ 0→ 83 Bi • Bi -210 decays via beta decay to Po-210 Activated nuclei are neutron 210 210 embedded in tumors BiPobeta decay11 n→+ p e− 83→ 84 01 as a cancer treatment beta decay

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