Nuclear Physics

Chapter 29

Fossils

How are scientists able to determine the age of fossils?– The “iceman”

Nuclear Physics

The year 1896 marked the birth of nuclear physics– Henri Becquerel accidentally discovered

natural radioactivity in uranium compounds.• Researchers tried to identify

the radiation from atomic nuclei.

1 Thorite ------->

Rutherford’s Discovery

Rutherford showed that there were three types of nuclear radiation.

Alpha ()– Helium nuclei– Least penetrating– Positively charged

Beta ()– Electrons or positrons– More penetrating than alpha particles– Negatively or positively charged

Gamma ()– High-energy photons– Most penetrating– No charge

Rutherford’s Experiment

Rutherford bombarded gold foil with alpha particles and disproved the Thompson model of the atom..

The Strong Nuclear Force

Rutherford and his students discovered the nuclear strong force in 1911– It explained why the protons don’t go flying off

from the nucleus.

Milestones in Nuclear Physics

Nuclear reactions were observed in 1930.The neutron was discovered in 1932.Artificial radioactivity was produced in 1933.Nuclear fission was discovered in 1938.Nuclear fission was first controlled in 1942.

Atomic Nuclei

All nuclei are composed of two types of particles.– The atomic number, Z, equals the number of protons

in the nucleus.– The neutron number, N, equals the number of

neutrons n the nucleus.– The mass number, A, equals the number of

nucleons. (A = Z + N)• A nucleon can be either a proton or a neutron

Nuclear Symbols

The symbol we use to represent nuclei is

Sometimes Z is not shown when the chemical symbol is obvious.

€

ZA X

Isotopes

Isotopes have the same number of protons but different numbers of neutrons.– There are four isotopes of carbon.

C116 C12

6 C136 C14

6

has a 98.9 % natural abundance.C126

Hydrogen Isotopes

Hydrogen has three isotopes.– Protium– Deuterium– Tritium

( )H11

( )H21

( )H31

Symbols for Other Particles

– alpha– beta or– gamma– neutron

( )He42

€

-10β( )

€

00γ( )

€

+10β( )

( )n10

Artificial Isotopes

Artificial isotopes do not occur naturally and are produced in the laboratory.

17-1, 17-2

Charge and Mass

The proton and electron have charges that are equal in magnitude but opposite in sign.The neutron has no charge.The masses of the proton and neutron are nearly equal. (See Table 29.1) on pg. 914.The masses of selected isotopes are in Appendix B on pg. A.14.

Unified Mass Units

Unified mass units (u) are based upon the carbon-12 atom which has a mass of exactly 12 u.– 1 u = 1.660540 x 10-27 kg– The proton and neutron each have a mass of

approximately 1 u.– The mass of the electron is much less.

Particle Masses

Proton mass = 1.007276 u

Neutron mass = 1.008665 u

Electron mass = 0.000549 u

Mass Energy Conversion

The energy equivalent of one atomic mass unit is 931.494 MeV.

€

1 u = 931.494 MeV

Rutherford’s Experiment

Rutherford found that an alpha particle on a head-on collision with a nucleus will stop instantaneously at a distance d from the nucleus because of Coulomb repulsion.

29.1

A neutron has the best chance of causing a nuclear reaction because it has no charge.

Nuclear Radii

The average radius of most atomic nuclei can be found by using

ro = 1.2 x 10-15 mA is the mass number

285

3

1

oArr =

Nuclear Density

All nuclei have nearly the same density.

Nuclear Stability

Why don’t the protons in the nucleus fly apart because of the repulsive Coulomb forces?

The Strong Nuclear Force

The strong nuclear force is an attractive force between all nuclear particles.– The strong nuclear force dominates the

Coulomb force over short distances.

Nuclear Stability

There are over 260 stable nuclei.– Hundreds more are unstable

Light nuclei are stable when N = Z.Heavier nuclei are stable when N > Z.Elements with more than 83 protons are always unstable (radioactive).

29.3

Nuclear Mass

The total mass of the nucleus is always less than the sum of the masses of its nucleons.

Nuclear Energy

The total energy of the bound nucleus is always less than the combined energy of the separated nucleons.– The difference is called the binding energy of the

nucleus.

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Splitting a Nucleus

In order to break apart a nucleus, energy must be added to the system.

Binding Energy

Nuclei with an atomic mass near 60 are the most stable.– The average binding energy per nucleon is

about 8 MeV / nucleon.

29.4, 291

Natural Radioactivity

Natural radioactivity was accidentally discovered by Becquerel in 1896.– It was later named “radioactivity” by Marie

Curie.

Radioactive Elements

Marie and Pierre Curie discovered radium and polonium after years of separating the radioactive elements from tons of pitchblende, a radioactive ore.– Experiments indicated that the radiation was

the result of nuclear decay.– Marie Curie died of leukemia in 1934.

The Weak Nuclear Force

The weak nuclear force is responsible for radioactivity.

Anti Matter

What is a positron?– It is an antiparticle with the mass of an

electron and the charge of a proton.• Symbol

€

−10e

Identifying Nuclear Particles

A magnetic field can be used to identify the particles involved in nuclear radiation.

29.5, 292

Rutherford’s Mousetrap

Rutherford’s mousetrap experimentally proved that alpha particles were composed of helium nuclei.

T-41

The Decay Constant

The decay constant () determines the rate at which isotopes decay

• A large value for indicates a rapid rate of decay.

Activity

The activity (R) is defined as the number of decays per second– 1 Bq = 1 decay / second– 1 Ci = 3.7 x 1010 decays / second = 3.7 x 1010 Bq

• This is the approximate activity of 1 gram of radium.• The mCi and the Ci are most commonly used.

Remaining Nuclei

The number of nuclei (N) remaining after a given amount of time can be found by using:

€

N = Noe−λ t

Decay Rate

The decay rate (Ro) can be found by using:

€

Ro = λNo

Activity

The activity (R) after a given amount of time can be found by using:

€

R = Roe−λ t

Half Life

Half–life– The time it takes or half of a given number of

radioactive nuclei to decay is given by:

6, 294, 29.6

⎟⎟⎠

⎞⎜⎜⎝

⎛

2

1T

€

T1

2

=ln 2

λ=

0.693

λ

Questions

1, 12Pg. 933

The Decay Processes

Alpha decay– The parent nucleus emits a helium nucleus.

• The remaining nucleus is called the daughter.• The daughter nucleus has two less protons and two less

neutrons.

– ExamplesHe Th U 4

2234

9023892 +→

He Rn Ra 42

22286

22688 +→

Transmutation

Transmutation is the spontaneous decay of of one element into another.– In the decay process, excess mass is

converted into energy of other forms, mostly into the KE of the nuclei.

39-1

Artificial Transmutation

Rutherford accomplished the first artificial transmutation.– He bombarded nitrogen-14 with alpha

particles and produced Oxygen-17.

Beta decay– The parent nucleus emits an electron or a positron.

How are these particles produced?• The daughter nucleus has the same number of

nucleons as the parent nucleus.• The atomic number increases by 1 or decreases by 1.

– Examples

32-1

e C N 01

126

127 ++→e N C 0

1-147

146 +→

The Decay Chain

Radioisotopes undergo a series of decays which eventually result in the formation of a stable isotope of lead.– Radioisotopes may decay by either alpha or beta

emission.

Various half-lives are involved.

295, 74

The neutrino– No charge– Little if any mass– A spin of 1/2– Very little interaction with matter

There is also an antineutrino.

Gamma decay– The parent nucleus emits a gamma ray

photon.• This results when the nucleus is in an excited

state (*) after a collision or previous decay.• The atomic number and the mass number do not

change.– Example

C C 126

*126 +→

Summary of the Three Decay Processes

Penetrating Ability

Which type of particle would have the best chance of reaching the nucleus?

A gamma ray has the best chance of reaching the nucleus because it has no charge.

Applications Involving Gamma Rays

Baggage scanners at airports

294

Natural Radioactivity

There are two groups of radioactive nuclei– Natural

• Found in nature• Nature continuously resupplies us with

radioactive isotopes with short half-lives– Artificial

• Produced in the laboratory

Radon Hazzard

Radon is produced by the decay of radium in the soil.– Which type of decay is

responsible?

Health concerns?

13

The Carbon 14 Cycle

Carbon Dating

Carbon dating depends upon the beta decay of carbon-14

• Carbon-14 is produced when cosmic rays bombard nitrogen-14 in the atmosphere.

• The ratio of carbon-14 to carbon-12 in the atmosphere remains constant.

– All living things have the same ratio.– When organisms die, the ratio begins to change because of the

beta decay of carbon-14.

The Shroud of Turin

The Shroud of Turin is a centuries old linen cloth that bears the image of a crucified man. A man that millions believe to be Jesus of Nazareth. Is it really the cloth that wrapped his crucified body, or is it simply a medieval forgery, a hoax perpetrated by some clever artist?

Positive Image of the Shroud

Negative Image of the Shroud

Biological Damage

Radiation damage in matter– The amount of damage depends upon the type of

radiation and upon the absorbing material.• Cancer or death may result

– In cells, ionization is the primarycause of damage.

• Ions and free radicals may beformed.

Cancer Treatment

Radiation can be used to kill cancer cells.– Cancer cells are more vulnerable to damage

from radiation than are normal cells.

293

Cyberknife

Biological Damage

Somatic damage– Radiation damage to any cells but the

reproductive organs

Genetic damage– Damage to the reproductive cells

• Birth defects may result

Natural Radiation

Low level radiation from natural sources– Cosmic rays– Air– Soil– Food– Water– Building materials

Biological Units of Radiation

Roentgen (R)– The amount of radiation that deposits 8.6 x

10-3 J of energy into 1 kg of air

Rad– The amount of radiation that deposits 10-2 J of

energy into 1 kg of absorbing material

RBE

Relative Biological Effectiveness is the number of rad of x-radiation or gamma radiation that will produce the same biological damage as 1 rad of the radiation being used. (Table 29.3) on pg. 930.

rem

A rem (roentgen equivalent man) is the product of the dose in rad and the RBE factor.

€

rem = rad x RBE

Radiation Exposure Standards

Government limits– 0.5 rem/year for the general population

• Roentgen Equivalent Man – 5.0 rem/year for those in occupations

involving higher levels of exposure– Higher limits are allowed for certain parts of

the body• Hands and forearms

Lethal Dose

400 to 500 rem results in a 50% mortality rate.– This is called lethal dose 50 (LD 50).

Radiation Exposure

Ingestion is the most dangerous form of exposure.– Strontium-90 is present in the radioactive fallout from

above ground nuclear testing.– Behaves like calcium chemically

– Iodine-131(also in radioactive fallout) • Affects the thyroid gland

The Chernobyl Disaster

Radioactive isotopes were released into the environment.

Chernobyl (after the fire)

Applications

Involving

Radiation

Sterilization of food by radiation exposure kills:– Bacteria– Worms– Insects and their eggs

Exposing food to radiation is very controversial.

Sterilization in medicine reduces the chance of infection.– Surgical equipment– Grafts

• Bone• Cartilage• Skin

– Chromium-53 is used as a radioactive tracer to locate hemorrhages.

Cholangiogram, radithor capsules

Radioactive tracers in medicine must have a short half-life!– Examples

• Iodine-131– Evaluating the

performanceof the thyroid

Radioactive tracers in agriculture are used to evaluate the effectiveness of fertilizers

• Nitrogen use is tracked

Smoke detectors use an isotope of americiumto ionize air molecules– Smoke particles interfere with

this ionization.

Radioactive tracers in industryare used to check for pistonring wear.

Computerized Axial Tomography (CAT Scan) images have greater clarity and detail than a normal x-ray picture.– X-rays enter the body from different directions and the

results are evaluated by a computer.• A brain scan can be made in about 2 seconds.• A full body scan requires about 6 seconds

– X-rays do present health risks to the patient.

CAT scan

CAT Scan Images

Magnetic Resonance Imaging

Very strong magnetic fields are used to affect proton spin for ~ 30 min.Changes can be detectedand used to form a highlydetailed image.MRI is much less damaging to cells.

MRI Image

Magnetic ResonanceImaging (MRI) of thespine

PET Scans

A PET scan involves injecting a very small dose of a radioactive chemical, called a radiotracer, into the vein of your arm. The tracer travels through the body and is absorbed by the organs and tissues being studied.

299

Next, you will be asked to lie down on a flat examination table that is moved into the center of a PET scanner—a doughnut-like shaped machine. This machine detects and records the energy given off by the tracer substance and, with the aid of a computer, this energy is converted into three-dimensional pictures.

A physician can then look at cross-sectional images of the body organ from any angle in order to detect any functional problems.

Particle Counters

The Geiger counter– Radiation ionizes a confined gas which

has a large potential difference applied.

292

The semiconductor diode detector– Uses a reverse biased p-n junction

The scintillation counter– Counts flashes of light produced by

impacts

Particle Track Detectors

Photographic emulsion– Uses photographic film

Cloud chamber– Uses a super cooled gas– Particles form vapor tracks

Bubble chamber– Uses liquid hydrogen or

liquid helium.– Particles form tracks in

the liquid.

Your Annual Exposure

Calculate your own yearly exposure.

Questions

2 - 4, 6 - 9, 11Pg. 933