Alpha, Beta, and Gamma Decay Nuclear Reactions Alpha, Beta, and Gamma Decay

The Atom The atom consists of two parts: 1. The nucleus which contains: protons neutrons 2. Orbiting electrons.

X A Z Mass number = number of protons + number of neutrons Element symbol Z Atomic number = number of protons

X A Z A = number of protons + number of neutrons Z = number of protons A – Z = number of neutrons Number of neutrons = Mass Number – Atomic Number

U U 235 92 238 92 There are many types of uranium: A Z Number of protons Number of neutrons A Z Number of protons Number of neutrons

U U 235 92 238 92 There are many types of uranium: A 235 Z 92 Number of protons Number of neutrons 143 A 238 Z 92 Number of protons Number of neutrons 146 Isotopes of any particular element contain the same number of protons, but different numbers of neutrons.

Most of the isotopes which occur naturally are stable. A few naturally occurring isotopes and all of the man-made isotopes are unstable. Unstable isotopes can become stable by releasing different types of particles. This process is called radioactive decay and the elements which undergo this process are called radioisotopes/radionuclides.

Radioactive Decay Radioactive decay results in the emission of either: an alpha particle (a), a beta particle (b), or a gamma ray(g).

A. Types of Radiation 2+ 1- 1+ Alpha particle () Beta particle (-) helium nucleus paper 2+ Beta particle (-) electron 1- lead Positron (+) positron 1+ Gamma () high-energy photon concrete

Alpha Decay An alpha particle is identical to that of a helium nucleus. It contains two protons and two neutrons.

X Y + He Alpha Decay A Z A - 4 Z - 2 4 2 unstable atom alpha particle more stable atom

Alpha Decay Rn 222 86 He 4 2 Ra 226 88

Alpha Decay X A Z Y A - 4 Z - 2 + He 4 2 Ra 226 88 Rn 222 86 + He 4 2

Alpha Decay Rn 222 86 + Y A Z He 4 2 Rn 222 86 He 4 2 + Po 218 84

Alpha Decay X A Z + Th 230 90 He 4 2 He 4 2 U 234 92 + Th 230 90

Alpha Decay Th 230 90 + Y A Z He 4 2 He 4 2 + Ra 226 88 Th 230 90

Alpha Decay X A Z + Pb 214 82 He 4 2 He 4 2 + Pb 214 82 Po 218 84

Beta Decay A beta particle is a fast moving electron which is emitted from the nucleus of an atom undergoing radioactive decay. Beta decay occurs when a neutron changes into a proton and an electron.

Beta Decay As a result of beta decay, the nucleus has one less neutron, but one extra proton. The atomic number, Z, increases by 1 and the mass number, A, stays the same.

Beta Decay b -1 At 218 85 Po 218 84

Beta Decay X A Z Y Z + 1 + b -1 Po 218 84 At 85 + b -1

Beta Decay Th 234 90 Y A Z + b -1 Th 234 90 Pa 91 + b -1

Beta Decay X A Z Pb 210 82 + b -1 Tl 210 81 Pb 82 + b -1

Beta Decay Bi 210 83 Y A Z + b -1 Bi 210 83 Po 84 + b -1

Beta Decay X A Z Bi 214 83 + b -1 Pb 214 82 Bi 83 + b -1

Gamma Decay Gamma rays are not charged particles like a and b particles. Gamma rays are electromagnetic radiation with high frequency. When atoms decay by emitting a or b particles to form a new atom, the nuclei of the new atom formed may still have too much energy to be completely stable. This excess energy is emitted as gamma rays (gamma ray photons have energies of ~ 1 x 10-12 J).

Nuclear Decay Beta Emission electron Positron Emission positron

Nuclear Decay Electron Capture electron Gamma Emission Usually follows other types of decay. Transmutation One element becomes another.

Nuclear Decay Why nuclides decay… need stable ratio of neutrons to protons DECAY SERIES TRANSPARENCY

Half-life Half-Life time half is the required for of a radioisotope’s nuclei to decay into its products. For any radioisotope, # of ½ lives % Remaining 100% 1 50% 2 25% 3 12.5% 4 6.25% 5 3.125% 6 1.5625%

Half-life Half-life (t½) Time required for half the atoms of a radioactive nuclide to decay. Shorter half-life = less stable. C. Johannesson

Half-Life For example, suppose you have 10.0 grams of strontium – 90, which has a half life of 29 years. How much will be remaining after x number of years?   You can use a table: # of ½ lives Time (Years) Amount Remaining (g) 10 1 29 5 2 58 2.5 3 87 1.25 4 116 0.625

mf = mi x (0.5)n Half-Life initial mass Final mass # of half-lives Or an equation! initial mass mf = mi x (0.5)n Final mass # of half-lives

Half-life t½ = 5.0 s mf = mi (½)n mi = 25 g mf = (25 g)(0.5)12 mf = ? Fluorine-21 has a half-life of 5.0 seconds. If you start with 25 g of fluorine-21, how many grams would remain after 60.0 s? GIVEN: t½ = 5.0 s mi = 25 g mf = ? total time = 60.0 s n = 60.0s ÷ 5.0s =12 WORK: mf = mi (½)n mf = (25 g)(0.5)12 mf = 0.0061 g C. Johannesson

Half-Life Example 1: If gallium – 68 has a half-life of 68.3 minutes, how much of a 160.0 mg sample is left after 1 half life? ________ 2 half lives? __________ 3 half lives? __________

Half-Life Example 2: Cobalt – 60, with a half-life of 5 years, is used in cancer radiation treatments. If a hospital purchases a supply of 30.0 g, how much would be left after 15 years? ______________

Half-Life Example 3: Iron-59 is used in medicine to diagnose blood circulation disorders. The half-life of iron-59 is 44.5 days. How much of a 2.000 mg sample will remain after 133.5 days? ______________

Half-Life Example 4: The half-life of polonium-218 is 3.0 minutes. If you start with 20.0 g, how long will it take before only 1.25 g remains? ______________

Half-Life Example 5: A sample initially contains 150.0 mg of radon-222. After 11.4 days, the sample contains 18.75 mg of radon-222. Calculate the half-life.

Nuclear Fission and Fusion

Nuclear power Power can be obtained two ways. Fission Splitting atoms Get energy if the nucleus is big. The smaller ones are more stable. What we do in nuclear reactors. Fusion Joining atoms Get energy if the nuclei are small. The larger one is more stable. This is how the sun works.

NUCLEAR FISSION A reaction in which an atomic nucleus of a radioactive element splits by bombardment from an external source, with simultaneous release of large amounts of energy, used for electric power generation

Nuclear Fission Fission is the splitting of atoms These are usually very large, so that they are not as stable Fission chain has three general steps: 1. Initiation. Reaction of a single atom starts the chain (e.g., 235U + neutron) 2. Propagation. 236U fission releases neutrons that initiate other fissions 3. Termination.

Nuclear Fission A very heavy nucleus splits into more stable nuclei of intermediate mass. The mass of the products is less than the mass of the reactants. Missing mass is converted to energy

Fission of 238U

Neutron induced in U235 Fission is Exothermic The sum of the masses of the resulting nuclei is less than the original mass (about 0.1% less) The “missing mass” is converted to energy according to E=mc2

Neutrons may: 1 - Cause another fission by colliding with a U235 nucleus 2 - Be absorbed in other material 3 - Lost in the system If sufficient neutrons are present, we may achieve a chain reaction Creates two smaller nuclides and free neutrons The free neutrons potentially collide with nearby U235 nuclei May cause the nuclide to split as well Each split (fission) is accompanied by a large quantity of E-N-E-R-G-Y

Uranium Isotopes Naturally occurring Uranium contains two major isotopes Uranium-238 (99.3%) Uranium-235 (0.7%) As it turns out the only isotope of Uranium that undergoes fission is Uranium-235

235U Fission 23592U + 10n  23692U* and 10-14 seconds later... 23692U*  9236Kr + 14156Ba + 3 10n + ENERGY 50 possible sets of fission products (sum of atomic numbers = 92) 3 neutrons released for ONE 23592U each neutron can split another 23592U CHAIN REACTION POSSIBLE If amount of 23592U is sufficient (CRITICAL MASS) then the number of neutrons generated is high enough to result in a nuclear explosion )

Where does all this energy come from?

E = mc2 E = Energy (joules) m = mass (kg) c = speed of light = 3 x 108 m/s

Nuclear Fission & POWER Currently about 103 nuclear power plants in the U.S. and about 435 worldwide. 17% of the world’s energy comes from nuclear.

Fusion Light-mass nuclei combine to form a heavier, more stable nucleus. More energetic than fission reactions Source of energy for the H-bomb Origin of the elements

Nuclear Fusion

FUSION 411H  42He + 2 ? + + energy

Stars energy is produced through fusion reactions Fusion occurs until Fe is produced because less energy is released than required to fuse Fe nuclei = _____ ____ ____ Star burns out

The most destructive force on the planet H-bombs 1000s of times more powerful than A-bombs

Cold Fusion: Efforts are being made to start and sustain a fusion reaction at lower temperatures, in other words with a lower amount of input energy