THE PERIODIC TABLE

OF ELEMENTSAtomic number = # of protons in nucleus

# of nucleons = # of protons + # of neutrons

The number of neutrons can vary slightly for a given element (isotopes)

Atomic weight is equal to average number of nucleons in nucleus

Radioactivity: Birth of a new scienceRadioactivity: Birth of a new science

Milestones (important events) leading to establishment of nuclear Milestones (important events) leading to establishment of nuclear science as a subjectscience as a subject

Discovery of XDiscovery of X--Rays by W.C. RoentgenRays by W.C. Roentgen

Discovery of Radioactivity by H. BecquerelDiscovery of Radioactivity by H. Becquerel

Discovery of Polonium and Radium by Marie and Pierre CuriesDiscovery of Polonium and Radium by Marie and Pierre Curies

Discovery of electron by J.J. ThompsonDiscovery of electron by J.J. Thompson

Classification of radioactive emissions by E. RutherfordClassification of radioactive emissions by E. Rutherford

Discovery of atomic nucleus by E. RutherfordDiscovery of atomic nucleus by E. Rutherford

Enunciation of RutherfordEnunciation of Rutherford--Soddy displacement lawSoddy displacement law

Discovery of neutron by J. ChadwickDiscovery of neutron by J. Chadwick

Discovery of artificial radioactivity by Irene and J. CuriesDiscovery of artificial radioactivity by Irene and J. Curies

Discovery of nuclear fission by O. Hahn and StrassmannDiscovery of nuclear fission by O. Hahn and Strassmann

Atomic StructureAtomic Structure

Inner

electron

shell

Proton

Nucleus

Outer

electron

shell

Proton

Neutron

Relative scale model of an atom and the

solar system

Do you perceive a gold ring to contain a larger fraction of solid matter

than the solar system?

On this scale, the nearest star would be a little over 10,000 miles

away

Nuclear notationNuclear notation

• Z = atomic number or proton number, is the number of protons in the nucleus.

• N = neutron number, is the number of neutrons in the nucleus.

• A = Z + N = mass number, is the number of • A = Z + N = mass number, is the number of nucleons in the nucleus.

• In general, the notation is Z X N

• For example, 6 C6 has atomic mass 12.000

A

12

RadioactivityRadioactivity

• Questions

– How and why do nuclei decay?

– How do we use nuclear decay to tell time?

– What is the evidence for presence of now extinct radionuclides in the early solar system?

– How much do you really need to know about – How much do you really need to know about secular equilibrium and the U-series?

• Tools

– First-order ordinary differential equations

Enrico Fermi

(1901-1954)-------------------------------

One fermi (f) = 10-15 m

r = 1.2 A1/3 (in f)

-------------------------Helium: A = 4

r = 1.2 (4)1/3

= 1.9 f

-------------------------Uranium: A = 238

r = 1.2 (238)1/3r = 1.2 (238)1/3

= 7.4 f

Protons which would otherwise strongly repel at close distances are held in place by an extremely strong, but extremely short rangeforce called the strong

force. Other names for the strong force are strong nuclear force, or nuclear force.

Beyond about one fermi

the strong force declinesextremely rapidly.

As more protons areadded to the nucleus, more neutrons are needed to bind theprotons together, butthe larger the nucleusbecomes, the farther

STRONG FORCE

Protons and neutrons in the nucleus are collectively referred to asnucleons.

nuclear force.

The strong force between two protons is about the same as the strong force between two neutrons, or a proton and a neutron.

becomes, the farther apart are the protonsand the less effectiveis the strong force

Isotopes: Nuclides with same atomic number but different atomic

weight (or different neutron number)

All the nuclides belong to the same element

1H1, 1H

2 (D), 1H3 (T) 6C

12, 6C13

19K39, 19K

40, 19K41

92U234, 92U

235,92U238

Isobars: Nuclides with same atomic weight but different atomic

number (Nuclides belong to different elements)

18Ar40, 19K40, 20Ca40

Isotones: Nuclides with the same number of neutrons.

5B12, 6C

13 both have 7 Neutrons

Mirror nuclei: Nuclides with neutron and proton number

interchanged

7N15 and 8O

15

In general, the mass defect is calculated by summing the mass of

protons, neutrons, and electrons in an atom, and subtracting the

atom’s actual atomic mass. The general formula is:

Md = Zmp + Nmn - Ma

Where Z is the atomic number, N is the number of neutrons in the

atom, and Ma is the actual measured mass of the atom. Placing Md

into Einstein's equation for relating mass and energy gives the into Einstein's equation for relating mass and energy gives the

energy release from forming the atom from its constituent

particles:

E = Mdc2

Electric force is longer range than the strong force.

Eventually separation becomes too great for the strong force to compensate for the repulsive forces.

Nuclei spontaneously disintegrate for proton numbers larger than 83.

The release of light and or particles which accompanies the disintegration is called radiation, first discovered by Henri Becquerel in 1896.

Fundamental law of radioactive decayFundamental law of radioactive decay

• Each nucleus has a fixed probability of decaying per unit time. Nothing affects this probability (e.g., temperature, pressure, bonding environment, etc.)

[exception: very high pressure promotes electron capture slightly]

• This is equivalent to saying that averaged over a large enough number of atoms the number of decays per unit time is proportional to the number of atoms present.present.

• Therefore in a closed system:dN

dt= −λN (Equation 3.1)

– N = number of parent nuclei at time t

– λ = decay constant = probability of decay per unit time (units: s–1)

• To get time history of number of parent nuclei, integrate 3.1: N t( ) = Noe

−λt (3.2)

– No = initial number of parent nuclei at time t = 0.

DefinitionsDefinitions

• The mean life τ of a parent nuclide is given by the number present divided by the removal rate (recall this later when we talk about residence time):

τ =N

λN=

1

λ

– This is also the “e-folding” time of the decay:

N(τ ) = N e−λτ = N e

−1 =No

(3.3)

• The half life t1/2 of a nucleus is the time after which half the parent remains:

N(τ ) = Noe−λτ = Noe

−1 =No

e

N(t1/2) =No

2= Noe

−λt1/2 ⇒ λt1/2 = ln2 ⇒ t1/2 =ln 2

λ≈

.693

λ

• The activity is decays per unit time, denoted by parentheses: N( ) = λN (3.4)

Decay of parentDecay of parent

0 τ 2τ 3τ 4τ 5τtime

λNo

λNo2

t1/2

λNoe

0 τ 2τ 3τ 4τ 5τtime

t1/2

0

-1

-2

-3

-4

-5

slope = -1

Activity

ln(λ

N)–

ln(λ

No)

Some dating schemes only consider measurement of parent nuclei Some dating schemes only consider measurement of parent nuclei

because initial abundance is somehow known.

• 14C-14N: cosmic rays create a roughly constant atmospheric 14C inventory, so that living matter has a roughly constant 14C/C ratio while it exchanges CO2 with the environment through photosynthesis or diet. After death this 14C decays with half life 5730 years. Hence even through the daughter 14N is not retained or measured, age is calculated using:

t =1

λ14

ln(14

C) / C

(14

C) / C[ ]o

Modes of decayModes of decay

• A nucleus will be radioactive if by decaying it can lower the overall mass, leading to larger (negative) nuclear binding energy– Yet another manifestation of the 2nd Law of thermodynamics

• Nuclei can spontaneously transform to lower mass nuclei by one of five processes

– α-decay

– β-decay– β-decay

– positron emission

– electron capture

– spontaneous fission

• Each process transforms a radioactive parent nucleus into one or more daughter nuclei.

αα--decaydecay

Emission of an α-particle or 4He nucleus (2 neutrons, 2 protons)#

pro

to

ns

s23

237

238

238 U

234 Th

92

91

90

144 145 146

# p

roto

ns

# neutrons s23

237

238

α-decay The parent decreases its mass number by 4, atomic number by 2.

Example: 238U -> 234Th + 4He

Mass-energy budget:238U 238.0508 amu234Th –234.0436

# n

ucleons

234

235

236

7

# neutrons

# n

ucleons

234

235

236

7

234Th –234.04364He –4.00260

mass defect 0.0046 amu= 0.0046 x 930.5 = 4.5 MeV

This is the preferred decay mode of nuclei heavier than This is the preferred decay mode of nuclei heavier than 209209Bi with a proton/neutron ratio along the valley of Bi with a proton/neutron ratio along the valley of stabilitystability

4 42 2

A AZ Z

X Y He−−

→ +

X is called the X is called the parent nucleus and parent nucleus and Y is called the Y is called the daughter nucleusdaughter nucleus

ββ--decaydecay

Emission of an electron (and an antineutrino) during conversion of a neutron into a proton

The mass number does not change, the atomic number increases by 1.

Example: 87Rb -> 87Sr + e– + ν

Mass-energy budget:87Rb 86.909186 amu87Sr –86.908882

# p

roto

ns

cleo

ns

86

87

88

87 Rb

87 Sr38

37

49 50

# p

roto

ns

# neutrons

cleo

ns

86

87

88

β-decay

Sr –86.908882

mass defect 0.0003 amu= 0.0003 x 931 = 0.28 MeV

This is the preferred decay mode of nuclei with excess neutrons compared to the valley of stability

# nucl

e86

# nucl

e86

The emission of the electron is from the nucleusThe emission of the electron is from the nucleusThe nucleus contains protons and neutronsThe nucleus contains protons and neutronsThe process occurs when a neutron is The process occurs when a neutron is transformed into a proton and an electrontransformed into a proton and an electronEnergy must be conservedEnergy must be conserved

Beta Decay Beta Decay

• Symbolically

– ν is the symbol for the neutrino

ν++→

ν++→

+−

−+

eYX

eYX

A1Z

AZ

A1Z

AZ

– ν is the symbol for the neutrino

– is the symbol for the antineutrino

• To summarize, in beta decay, the following pairs of particles are emitted– An electron and an antineutrino

– A positron and a neutrino

ν

ββ++--decay and electron capturedecay and electron capture

Emission of a positron (and a neutrino) or capture of an inner-shell electron during conversion of a proton into

a neutronThe mass number does not change, the atomic number decreases by 1.

Examples: 40K -> 40Ar + e+ + ν50V+ e– -> 50Ti + ν + γ

# p

roto

ns

cleo

ns

3940

41

40 Ar

40 K19

18

21 22

# p

roto

ns

# neutrons

cleo

ns

3940

41

Electron Capture

V+ e -> Ti + ν + γ

In positron emission, most energy is liberated by remote matter-antimatter annihilation. In electron capture, a gamma ray carries off the excess energy.

These are the preferred decay modes of nuclei with excess protons compared to the valley of stability

# n

ucle39

# n

ucle39

Gamma DecayGamma Decay• Gamma rays are given off when an excited nucleus

“falls” to a lower energy state– Similar to the process of electron “jumps” to lower energy

states and giving off photons

• The excited nuclear states result from “jumps” made by a proton or neutron

• The excited nuclear states may be the result of violent collision or more likely of an alpha or beta emission

• Example of a decay sequence– The first decay is a beta emission

– The second step is a gamma emissionγ+→

ν++→ −

C*C

e*CB

126

126

126

125

Spontaneous FissionSpontaneous Fission

Certain very heavy nuclei, particular those with even mass numbers (e.g., 238U and 244Pu) can spontaneously fission.

Odd-mass heavy nuclei typically only fission in response to neutron capture (e.g., 235U, 239Pu)

There is no fixed daughter product but rather a statistical distribution of fission products with two peaks (most fissions are asymmetric).

Because of the curvature of the valley of

1

10

235 U+n

Because of the curvature of the valley of stability, most fission daughters have excess neutrons and tend to be radioactive (β-decays).

You can see why some of the isotopes people worry about in nuclear fallout are 91Sr and 137Cs.

Recoil of daughter products leave fission tracksof damage in crystals about 10 µm long, which only heal above ~300°C and are therefore useful for low-temperature thermochronometry.

Fis

sio

n Y

ield

(%

)

80 100 120 140 160 1800.0001

0.001

0.01

0.1

Atomic Mass (amu)

Fis

sio

n Y

ield

(%

)

Natural RadioactivityNatural Radioactivity

•• Classification of nucleiClassification of nuclei

–– Unstable nuclei found in natureUnstable nuclei found in nature

•• Give rise to Give rise to natural radioactivitynatural radioactivity

–– Nuclei produced in the laboratory through nuclear Nuclei produced in the laboratory through nuclear reactionsreactions

•• Exhibit Exhibit artificial radioactivityartificial radioactivity•• Exhibit Exhibit artificial radioactivityartificial radioactivity

•• Three series of natural radioactivity existThree series of natural radioactivity exist

–– UraniumUranium--235 (4n + 3 series) 235 (4n + 3 series) ends at Pbends at Pb--207207

–– UraniumUranium--238 (4n + 2 series)238 (4n + 2 series) ends at Pbends at Pb--206206

–– ThoriumThorium--232 (4n series)232 (4n series) ends at Pbends at Pb--208208

4n + 1 series starting from Neptunium4n + 1 series starting from Neptunium--237 is extinct237 is extinctends at Biends at Bi--209209

Uses of RadioactivityUses of Radioactivity•• Carbon DatingCarbon Dating

–– Beta decay of Beta decay of 1414C is used to date organic samplesC is used to date organic samples–– The ratio of The ratio of 1414C to C to 1212C is usedC is used

•• Smoke detectorsSmoke detectors–– Ionization type smoke detectors use a radioactive Ionization type smoke detectors use a radioactive

source to ionize the air in a chambersource to ionize the air in a chamber–– A voltage and current are maintained A voltage and current are maintained –– A voltage and current are maintained A voltage and current are maintained –– When smoke enters the chamber, the current is When smoke enters the chamber, the current is

decreased and the alarm soundsdecreased and the alarm sounds•• Radon pollutionRadon pollution

–– Radon is an inert, gaseous element associated Radon is an inert, gaseous element associated with the decay of radiumwith the decay of radium

–– It is present in uranium mines and in certain types It is present in uranium mines and in certain types of rocks, bricks, etc that may be used in home of rocks, bricks, etc that may be used in home buildingbuilding

–– May also come from the ground itselfMay also come from the ground itself

Nuclear ReactionsNuclear Reactions

• Structure of nuclei can be changed by bombarding them with energetic particles

– The changes are called nuclear reactions

• As with nuclear decays, the atomic • As with nuclear decays, the atomic numbers and mass numbers must balance on both sides of the equation

Which of the following are possible Which of the following are possible reactions?reactions?

(a) and (b). Reactions (a) and (b) both (a) and (b). Reactions (a) and (b) both conserve total charge and total mass number conserve total charge and total mass number

as required. Reaction (c) violates as required. Reaction (c) violates conservation of mass number with the sum conservation of mass number with the sum

of the mass numbers being 240 before of the mass numbers being 240 before reaction and being only 223 after reaction.reaction and being only 223 after reaction.

Determine the product of the reaction Determine the product of the reaction

What is the What is the Q Q value of the reaction?value of the reaction?

Given:

reaction

In order to balance the reaction, the total amount of nucleons (sum of A-numbers) must be the same on both sides. Same for the Z-number.

7 4 1 10+ = + ⇒ =X X

3 2 0 5+ = + ⇒ =Y Y

Number of nucleons (A):

Number of protons (Z):

7 43 2

?XY

Li He n+ → +

Find:

Q = ? The Q-value is then

( ) ( )7 4 10

2 2 2.79= ∆ = + − − = −nLi He B

Q m c m m m m c MeV

3 2 0 5+ = + ⇒ =Y YNumber of protons (Z):

Thus, it is B, i.e. 7 4 10 1

3 2 5 0+ → +Li He B n

Processes of Nuclear EnergyProcesses of Nuclear Energy

• Fission

– A nucleus of large mass number splits into

two smaller nuclei

• Fusion• Fusion

– Two light nuclei fuse to form a heavier

nucleus

• Large amounts of energy are released in either case

Nuclear FissionNuclear Fission• A heavy nucleus splits into two smaller nuclei

• The total mass of the products is less than the original mass of the heavy nucleus

• First observed in 1939 by Otto Hahn and Fritz Strassman following basic studies by Fermi

• Lisa Meitner and Otto Frisch soon explained what had happenedhappened

• Fission of 235U by a slow (low energy) neutron

– 236U* is an intermediate, short-lived state

– X and Y are called fission fragments

• Many combinations of X and Y satisfy the requirements of conservation of energy and charge

neutronsYX*UUn 23692

23592

10 ++→→+

Sequence of Events in FissionSequence of Events in Fission

• The 235U nucleus captures a thermal (slow-moving) neutron

• This capture results in the formation of 236U*, and the excess energy of this nucleus causes it to undergo violent oscillations

• The 236U* nucleus becomes highly elongated, and the force of repulsion between the protons tends to increase the distortion

• The nucleus splits into two fragments, emitting several neutrons in the process

Natural (radioactive) decay (fission)

Neutron-induced fission

• Many heavy elements (eg.

Uranium) decay (slowly) into

lighter elements (natural decay)

• However, this fission can also be

induced by an incoming neutron.

• Fission reaction release a lot of

energy.

• Fission often creates new

neutrons!!

Fission and

chain reaction

Fission releases neutrons …

… these neutrons cause new fission

reactions in surrounding Uranium …

… creating more neutrons …

… chain reaction

Energy in a Fission ProcessEnergy in a Fission Process

• Binding energy for heavy nuclei is about 7.2 MeV per nucleon

• Binding energy for intermediate nuclei is about 8.1 MeV per nucleon

• Therefore, the fission fragments have less mass than the nucleons in the original nuclei

• This decrease in mass per nucleon appears as released • This decrease in mass per nucleon appears as released energy in the fission event

• An estimate of the energy released

– Assume a total of 236 nucleons

– Releases about 0.9 MeV per nucleon

• 8.1 MeV – 7.2 MeV

– Total energy released is about 212 Mev

• This is very large compared to the amount of energy released in chemical processes

Chain ReactionChain Reaction• Neutrons are emitted when 235U undergoes fission

• These neutrons are then available to trigger fission in other nuclei

• This process is called a chain reaction

–If uncontrolled, a violent explosion can violent explosion can occur

–The principle behind the nuclear bomb, where 1 g of U can release energy equal to about 20000 tons of TNT

Carbon dating is a variety of radioactive Carbon dating is a variety of radioactive

dating which is applicable only to matter dating which is applicable only to matter

which was once living and presumed to be in which was once living and presumed to be in

equilibrium with the atmosphere, taking in equilibrium with the atmosphere, taking in

carbon dioxide from the air for photosynthesis.carbon dioxide from the air for photosynthesis.

Cosmic ray protons blast nuclei in the upper Cosmic ray protons blast nuclei in the upper

atmosphere, producing neutrons which in turn atmosphere, producing neutrons which in turn

bombard nitrogen, the major constituent of bombard nitrogen, the major constituent of

the atmosphere . This neutron bombardment the atmosphere . This neutron bombardment

produces the radioactive isotope carbonproduces the radioactive isotope carbon--14. 14. produces the radioactive isotope carbonproduces the radioactive isotope carbon--14. 14.

The radioactive carbonThe radioactive carbon--14 combines with 14 combines with

oxygen to form carbon dioxide and is oxygen to form carbon dioxide and is

incorporated into the cycle of living thingsincorporated into the cycle of living things..

TheThe carboncarbon--1414 formsforms atat aa raterate whichwhich appearsappears toto bebe constant,constant, soso thatthat byby

measuringmeasuring thethe radioactiveradioactive emissionsemissions fromfrom onceonce--livingliving mattermatter andand

comparingcomparing itsits activityactivity withwith thethe equilibriumequilibrium levellevel ofof livingliving things,things, aa

measurementmeasurement ofof thethe timetime elapsedelapsed cancan bebe mademade..

Radioactive DatingRadioactive Dating

RadioactiveRadioactivehalfhalf--lifelife ofof aa givengiven radioisotoperadioisotope isis notnot affectedaffected

byby temperature,temperature, physicalphysical oror chemicalchemical state,state, oror anyany otherother

influenceinfluence ofof thethe environmentenvironment outsideoutside thethe nucleusnucleus..

RadioactiveRadioactive samplessamples continuecontinue toto decaydecay atat aa predictablepredictable raterate..

ThisThis makesmakes severalseveral typestypes ofof radioactiveradioactive datingdating feasiblefeasible..ThisThis makesmakes severalseveral typestypes ofof radioactiveradioactive datingdating feasiblefeasible..

ThereThere areare twotwo mainmain uncertaintiesuncertainties inin thethe datingdating processprocess::

1.1. WhatWhat waswas thethe amountamount ofof thethe daughterdaughter elementelement whenwhen

thethe rocksrocks werewere formed?formed?

2.2. HaveHave anyany ofof thethe parentparent oror daughterdaughter atomsatoms beenbeen addedadded

oror removedremoved duringduring thethe process?process?

Balancing Nuclear Decay Equations

92U238 --------> 90Th234 + 2He4

-----------------------------------------Subscripts are "proton numbers"Superscripts are "nucleon numbers"

Proton and nucleon counts mustbe the same:92 = 90 + 2238 = 234 + 4

Distribution of Energy in Alpha Emission ∆m = 0.0046 u

E = 0.0046 x 931= 4.3 MeV

-----------------------Which particlehas the greaterkinetic energy?

Energy Distribution in Radioactive Decay

Ratio of kinetic energies:

Conservation of momentum:Mv = mV (2)

Rearranging, we getV/v = M/m (3)

KEm / KEM: (1/2 mV2) / (1/2 Mv2)

= (m/M)(V2/v2)

= (m/M)(V/v)2 (1)

Substitute (3) into (1):

Ratio = (m/M)(M/m)2 (4)= M/m

Smaller mass gets more energy

Smoke Detector

Alpha particles emitted from source ionize the air and provide the charge necessary to conduct current through the air.the air.

Charges stick to the heavy smoke particles and the current drops, causing the alarm to buzz.

Wavelength of a Gamma Ray

What is the wavelength of a 1 MeV gamma ray?

Using the 1234 rule:

λ = 1234 eV-nm / E= 1234 eV-nm / 1 x 106 eV= 1.23 x 10-6 nm= 1.23 x 10-15 m= 1.23 x 10-15 m= 1.23 fermi

This gamma radiation is extraordinarily harmfulto humans and other living things since its wavelength is comparable to the diameter ofa nucleon; transmutations are likely whensuch radiation reaches nuclei.

Measuring the Age of Organic Matter

A German tourist inthe Italian Alpsdiscovered thediscovered theremains of the "Iceman" in the iceof a glacier in 1991

Calculating the Iceman's Age

The current activity per gram of carbon is 0.23 Bq per gram. Iceman's carbon showed

0.121, or about half what itwould be if the Iceman were

alive.alive.

Since the half-life of carbon-14 is about 5700 years, the

Iceman's remains are about 5700 years old.

The Shroud of Turin

Since the1354 AD, a yellowing piece of linen14-ft long has been stored in Turin, Italy.

It bears the image of a person who seems to be wearing a crown of be wearing a crown of thorns.

Could the Shroud of Turin have been the burial cloth of a person who died two thousand years ago?

Dating of the Shroud of Turin

At the time of the public exhibition of the shroud in 1354, a bishop declared it to be fraud. Most religious bodies take a

neutral stance on the shroud's authenticity.

In 1988, three laboratories were given four pieces of fabric; three were control pieces similar in appearance, and one pieces similar in appearance, and one was a piece from the shroud. The labs all agreed that the shroud was 608-

728 years old, which means that it came into existence sometime between1260 and 1380 AD, a time span which includes the year the shroud was first shown to the public.

In 1934, Irene and Frederic Joliot-Curie discover the

artificial radioactivity, making a great step toward the use

and the control of radioactivity. For this discovery, they

received the Nobel price of chemistry in 1935.

They were the first to show that mankind could build under

control some news radioactive nuclei. By shooting an

aluminium sheet with alpha particles (helium nuclei), they

were able to make radioactive phosphorus, a new isotope of

the stable phosphorus that was never observed in nature.

They demonstrated it by chemically isolating the phosphorus

produced before it becomes silicium by its radioactivity. The

creation an unnatural radioactive element is what we call the

creation of artificial radioactivity.

PositronsIn 1930 Paul Dirac calculated the existence of electrons with positive charges. These "anti-electrons" would be

expected to have the same mass as the electron, but opposite electric charge. In 1932 Carl Anderson was examining

tracks produced by cosmic rays in a cloud chamber. One particle made a track like an electron, but the curvature of

its path in the magnetic field showed that it was positively charged. He named this positive electron a positron. We

know that the particle Anderson detected was the anti-electron predicted by Dirac. An electron and positron

annihilate one another producing two gamma rays (β- + β+® γ + γ).

Irene Curie-Joliot (1897-1956), the daughter of Marie & Pierre, and her husband Frédéric Joliot prepared

phosphorus-30 by bombarding aluminum with alpha particles..

Phosphorus-30 does not occur in nature and is radioactive. This was the first artificial radioactive substance ever Phosphorus-30 does not occur in nature and is radioactive. This was the first artificial radioactive substance ever

prepared. Aside from the three natural types of radioactivity (α,β,γ), artificially made nuclei can undergo:

Both positron emission and electron capture tend to occur for radioactive isotopes that need to convert a proton into

a neutron. The Curie-Joliots were awarded the Nobel Prize in Chemistry in 1935 for discovering artificial

radioactivity.

Chemical ReactionChemical Reaction Nuclear reactionNuclear reaction

Atoms are rearranged by Atoms are rearranged by the breaking and formation the breaking and formation of chemical bondsof chemical bonds

Elements (or isotopes of Elements (or isotopes of the same elements) are the same elements) are converted from one to converted from one to anotheranother

Only electrons in atomic Only electrons in atomic orbitals are involved in the orbitals are involved in the breaking and forming of breaking and forming of

Protons, neutrons, Protons, neutrons, electrons and other electrons and other elementary particles may elementary particles may breaking and forming of breaking and forming of

bondsbondselementary particles may elementary particles may be involvedbe involved

Absorption or release of Absorption or release of small amounts of energysmall amounts of energy

Absorption or release of Absorption or release of tremendous amounts of tremendous amounts of energyenergy

Rates of reactions are Rates of reactions are affected by temperature, affected by temperature, pressure, concentration pressure, concentration and catalystsand catalysts

Rates of reactions are NOT Rates of reactions are NOT affected by temperature, affected by temperature, pressure, concentration pressure, concentration and catalystsand catalysts

Producing Radioactive Isotopes:

TRANSMUTATION is the process of changing one element

into another.

A stable atom can be bombarded with fast-moving a particles,

protons, or neutrons.

A radioactive isotope is called a RADIOISOTOPE.A radioactive isotope is called a RADIOISOTOPE.

Half-Life:

The HALF-LIFE of a radioisotope is the amount of time it

takes for half of the sample to decay.

A DECAY CURVE is a graph of the decay of a radioisotope

(amount vs. time).

Some radioisotopes have long half-lives. For other

radioisotopes, the half-life can be short.

The image cannot be displayed. Your computer may not have enough memory to open the image, or the image may have been corrupted. Restart your computer, and then open the file again. If the red x still appears, you may have to delete the image and then insert it again.

Penetrating power of different forms of radiation:

Radioactivity

Chemical reactions

CH4 + 2O2 � CO2 + 2H2O + some energy

One molecule or element reacts with another one.

�Get a rearrangement (different combination) of elements.

No new elements are created (C, H, O before and C, H, O after)

– a nuclear reaction

As an example, when uranium 238 emits an alpha particle, it loses 2 protons and 2 neutrons.

238 234 4− − > +U Th He

– Nuclear reactions must balance just like any other chemical reaction, but we must also be aware of balancing protons and neutrons

238 234 4

92 90 2− − > +U Th He

Nuclear ReactionsNuclear ReactionsNuclear reactions occur when a nucleus is struck by a Nuclear reactions occur when a nucleus is struck by a

particle or other nucleus.particle or other nucleus.

1 4 1 47 6

n N C p+ → +

4 1 4 1 7 172 8 1

H e N O H+ → +72 8 1

••The second reaction was observed by Rutherford and is the The second reaction was observed by Rutherford and is the

first nuclear reaction observed.first nuclear reaction observed.

••It should be noted that in the first reaction, the neutron can It should be noted that in the first reaction, the neutron can

enter the nucleus with very little energy but the enter the nucleus with very little energy but the 44He is repelled He is repelled

by the nucleus and thus has to overcome the Coulomb barrier by the nucleus and thus has to overcome the Coulomb barrier

in order to come close enough to cause a nuclear reaction.in order to come close enough to cause a nuclear reaction.

ParameterParameter Chemical ReactionChemical Reaction Nuclear ReactionNuclear Reaction

Reaction H + H → H2 H + H → 2H (D)

Mechanism Interaction of electrons

Interaction of nuclei

Species Do not change New species form

Energy change

∆H = 104.2 kCal/mol

1.73 x 10-22 kCal.atom

(4.5 eV/atom)

Q = 33.47 x 106

kCal/mol

5.56 x 10-17 kCal/atom

(1.452 MeV/atom)

Conservation of mass and energy

Maintained Maintained

Radioactivity in Nature

Our world is radioactive and has been since it was created

Over 60 radionuclides (radioactive isotopes) can be found in nature.

Radionuclides are found in air, water, food and soil

Radionuclides are even found in our body

Everyday we ingest and inhale radionuclides

In addition to radionuclides found in nature

We have

Cosmogenic radionuclides: formed as a result of cosmic ray interactions

Man-made radionuclides

Number of radionuclides > 2000

Number of elements: 111

Natural Radioactivity in soil

How much natural radioactivity is found in a volume of soil that is 2.6 sq KM, 30 cm deep (total volume = 7.894 x 105 m3)

Every day, we ingest/inhale nuclides in our air we breath, in the food we eat and the water we drink. Radioactivity is common in the rocks and soil that makes up our planet, in the water and oceans, and even in our building materials and homes. It is just everywhere. There is no where on Earth that you materials and homes. It is just everywhere. There is no where on Earth that you can get away from Natural Radioactivity.

Radioactive elements are often called radioactive isotopes or radionuclides. There are over 1,500 different radioactive nuclides

Food 40K (pCi/kg) 226Ra (pCi/kg)

Banana 3,520 1

Carrot 3,400 0.6 - 2

White potatoes 3,400 1 – 2.5

Natural Radioactivity in Food

Beer 390 ----

Red meat 3,000 0.5

Drinking water ----- 0 – 0.17

Handbook of radiation measurement and protection

MaterialMaterial Uranium (Uranium (µg/g)µg/g) Thorium (Thorium (µg/g)µg/g) Potassium (Potassium (µg/g)µg/g)

GraniteGranite 4.74.7 22 44

SandstoneSandstone 0.450.45 1.71.7 1.41.4

CementCement 3.43.4 5.15.1 0.80.8

Limestone Limestone concreteconcrete

2.32.3 2.12.1 0.30.3

Sandstone Sandstone concreteconcrete

0.80.8 2.12.1 1.31.3

Radionuclides in building materialsRadionuclides in building materials

concreteconcrete

Dry Dry wallboardwallboard

11 33 0.30.3

Byproduct Byproduct gypsumgypsum

13.713.7 16.116.1 0.020.02

Natural Natural gypsumgypsum

1.11.1 1.81.8 0.50.5

WoodWood -- -- 11.311.3

Clay brickClay brick 8.28.2 10.810.8 2.32.3

Nuclide Total mass of nuclide in the body

Total activity

Daily intake

Uranium 90 µg 1.1 Bq 1.9 µg

Thorium 30 µg 0.11 Bq 3 µg

Some radionuclides in human bodySome radionuclides in human body

Thorium 30 µg 0.11 Bq 3 µg

Potassium-40

17 mg 4.4 kBq 0.39 mg

Radium 31 pg 1.1 Bq 2.3 pg

Carbon-14 95 µg 15 kBq 1.8 µg

Tritium 0.06 pg 23 Bq 0.003 pg