Nuclear Decay Reading Assignment: pp. 965-980. Particles can be identified based on how they interact with a magnetic field: –Alpha particles will curve.

Nuclear DecayNuclear Decay

Reading Assignment:Reading Assignment:

pp. 965-980pp. 965-980

Particles can be Particles can be identified based identified based on how they on how they interact with a interact with a magnetic field:magnetic field:– Alpha particles Alpha particles

will curve slightlywill curve slightly– Beta particles will Beta particles will

be deflected be deflected significantly, and significantly, and in the opposite in the opposite direction from direction from alphaalpha

– Gamma rays—no Gamma rays—no charge, so no charge, so no deflection at alldeflection at all

Nuclear StabilityNuclear Stability

Strong Nuclear ForceStrong Nuclear Force depends on depends on maintaining a delicate balance maintaining a delicate balance between the number of protons and between the number of protons and the number of neutrons in a nucleus.the number of neutrons in a nucleus.

The higher the atomic number, the The higher the atomic number, the larger the neutron:proton ratio must larger the neutron:proton ratio must be in order to remain stablebe in order to remain stable

Difference of +/- 1 neutron can result Difference of +/- 1 neutron can result in an unstable nucleusin an unstable nucleus

Natural Radioactive DecayNatural Radioactive Decay

For all decay, two properties must For all decay, two properties must remain constant: remain constant: total chargetotal charge and and total masstotal mass beforebefore the decay to the decay to afterafter the decaythe decay

Daughter Product Daughter Product the atom that the atom that is created as a result of nuclear is created as a result of nuclear decaydecay

Parent NucleiParent Nuclei the unstable atom the unstable atom that is undergoing nuclear decaythat is undergoing nuclear decay

Think about this:Think about this:– If an atom releases an alpha particle, If an atom releases an alpha particle,

what will happen to its atomic number?what will happen to its atomic number?

A. A. Increase by 2Increase by 2

B.B. Decrease by 2 Decrease by 2

C.C. Increase by 4 Increase by 4

D.D. Decrease by 4 Decrease by 4

– What will happen to its atomic mass?What will happen to its atomic mass?

Types of DecayTypes of Decay

Alpha Decay (Alpha Decay (emissionemission))– Alpha particle is emitted from the Alpha particle is emitted from the

nucleusnucleus– Atomic number decreases by 2Atomic number decreases by 2

For example: Uranium-232 decays For example: Uranium-232 decays by alpha emission. What is its by alpha emission. What is its daughter product?daughter product?

232 492 2

AZU He X 232 4 228

92 2 90U He Th

Types of DecayTypes of Decay

Alpha Decay (Alpha Decay (emissionemission))

Types of DecayTypes of Decay

Beta DecayBeta Decay– Beta minus Beta minus an electron is emitted an electron is emitted

from the nucleusfrom the nucleusAtomic number increases by 1Atomic number increases by 1

Types of DecayTypes of Decay

Beta Decay—example Beta Decay—example – Sulfur-35 emits Sulfur-35 emits -- particles when it particles when it

decays radioactively. What is its decays radioactively. What is its daughter product?daughter product?

35 016 1

AZS e X

35 0 3516 1 17S e Cl

Types of DecayTypes of Decay

Beta DecayBeta Decay– Beta plus Beta plus a positron (positive a positron (positive

electron) is emitted from the nucleuselectron) is emitted from the nucleusAtomic number decreases by 1Atomic number decreases by 1Not as common as beta-minusNot as common as beta-minus

Types of DecayTypes of Decay

Beta Decay—example Beta Decay—example – Carbon-11 emits Carbon-11 emits ++ particles when it particles when it

decays radioactively. What is its decays radioactively. What is its daughter product?daughter product?

11 06 1

AZC e X 11 0 11

6 1 7C e B

Types of DecayTypes of Decay Electron CaptureElectron Capture

– Essentially the same result as positron Essentially the same result as positron emissionemission

– An electron from the lowest energy level An electron from the lowest energy level is “captured” by the nucleus, turning a is “captured” by the nucleus, turning a proton into a neutronproton into a neutron

Types of DecayTypes of Decay

Gamma DecayGamma Decay– Occurs when there is an unstable Occurs when there is an unstable

amount of internal energy in the nucleusamount of internal energy in the nucleus– Energy is released, returning the Energy is released, returning the

nucleus to a more stable state, in the nucleus to a more stable state, in the form of a gamma ray (photon energy)form of a gamma ray (photon energy)

61 6128 28Ni Ni

Products of sequential nuclear decaysProducts of sequential nuclear decays

When an atom undergoes alpha or beta When an atom undergoes alpha or beta decay, it often has a daughter product that decay, it often has a daughter product that is also unstable…and therefore will decay. is also unstable…and therefore will decay. (anything with Z > 83 are naturally (anything with Z > 83 are naturally radioactive)radioactive)

A series of decays occur, starting with the A series of decays occur, starting with the parent nuclei, and result in a series of parent nuclei, and result in a series of different radioactive particle emissions different radioactive particle emissions until the original parent nuclei has finally until the original parent nuclei has finally decayed into a stable atom (i.e. Lead)decayed into a stable atom (i.e. Lead)

Decay Series Example:Decay Series Example: Uranium-238 Uranium-238 alpha decay into Thorium- alpha decay into Thorium-

234234 Thorium-234 Thorium-234 beta(-) into Palladium 234 beta(-) into Palladium 234 Palladium 234 Palladium 234 beta (-) into Uranium-234 beta (-) into Uranium-234 Uranium-234 Uranium-234 goes through 4 separate goes through 4 separate

alpha decays into Thorium-230, then alpha decays into Thorium-230, then Radium-226, then Radon-222, and then Radium-226, then Radon-222, and then Polonium-218Polonium-218

Polonium-218 Polonium-218 several possible several possible combinations of alpha and beta decays combinations of alpha and beta decays until the final product, Lead-206until the final product, Lead-206

Half-LifeHalf-Life

Radioactive decay is a Radioactive decay is a random random processprocess!!– It is impossible to predict exactly when a It is impossible to predict exactly when a

specific nucleus will decayspecific nucleus will decay For a given isotope, however, there is For a given isotope, however, there is

a 50% chance that a nucleus will a 50% chance that a nucleus will decay during a particular time perioddecay during a particular time period

Half-LifeHalf-Life = the time it takes for 50% = the time it takes for 50% of the remaining unstable nuclei to of the remaining unstable nuclei to decaydecay

Determining Half-LifeDetermining Half-Life

Decay Curve options: Decay Curve options: Number of parent nuclei vs. timeNumber of parent nuclei vs. timeRate of decay (decays per second) vs. timeRate of decay (decays per second) vs. time

The half life is the amount of time (x-The half life is the amount of time (x-axis) that passes by before the axis) that passes by before the number of parent nuclei OR the rate number of parent nuclei OR the rate of decay has decreased by 50%of decay has decreased by 50%

Half-LifeHalf-Life Decay Curve for a Radioactive Sample Decay Curve for a Radioactive Sample

with twith t1/21/2 = 2 days= 2 days

Determining Half-LifeDetermining Half-Life

Simple Calculation:Simple Calculation:

N = number (mass) of parent nuclei N = number (mass) of parent nuclei remainingremaining

NN00 = original number (mass) of parent nuclei = original number (mass) of parent nuclein = # of half-lives that have passed byn = # of half-lives that have passed by

012n

N N