Atomic Nucleus and Radioactivity

Atomic Nucleus and Radioactivity

Eleanor Roosevelt High SchoolChin-Sung Lin

Lesson 28

Atomic Nucleus and Radioactivity

X-Rays and Radioactivity

• In 1895, a German physicist, W. C. Roentgen was working with a cathode ray tube in his lab and discovered X-rays produced by a beam of electrons striking the glass surface of a gas-discharge tube


• Roentgen found that X-rays could pass through solid materials, could ionize the air, showed no refraction in glass, and were undeflected by magnetic fields


• X-rays are high-frequency electromagnetic waves, usually emitted by the de-excitation of the innermost orbital electrons of atoms


• An energetic beam of electrons striking a solid surface excites the innermost electrons and produces higher-frequency photons of X-radiation.


• X-ray photons have high energy and can penetrate many layers of atoms before being absorbed or scattered

• X-rays do this when they pass through your soft tissue to produce an image of the bones inside your body


Radioactivity

• Radioactivity is the process of nuclear decay (radioactive decay)

• Nothing new in the environment; it’s been going on since time zero

• It warms Earth’s interior, is in the air we breathe, and is present in all rocks (some in trace amounts)

• It is natural

The radioactive decay of nature’s elements occurs in the

A. soil we walk on.B. air we breathe. C. interior of Earth.D. All of the above.


CHECK YOUR NEIGHBOR

A. soil we walk on.B. air we breathe.C. interior of Earth.D. All of the above.


CHECK YOUR ANSWER

The radioactive decay of nature’s elements occurs in the

Alpha, Beta, and Gamma Rays


Alpha, Beta, and Gamma Rays•Radioactive elements emit three distinct types of radiation:

— alpha: positively charged (helium nuclei)

— beta: negatively charged (electrons)

— gamma: no charge(electromagnetic radiation)

•Relative penetration


CHECK YOUR NEIGHBOR

The origins of radioactivity go back to

A. military activities in the mid-20th century.B. the Industrial Revolution two centuries ago. C. the beginning of human error. D. before humans emerged on Earth.

A. military activities in the mid-20th century.B. the Industrial Revolution two centuries ago.C. the beginning of human error.D. before humans emerged on Earth.


CHECK YOUR ANSWER

The origins of radioactivity go back to

Alpha, Beta, and Gamma Rays•Alpha decay

Alpha, Beta, and Gamma Rays•Alpha decay

Alpha, Beta, and Gamma Rays•Beta decay




Alpha, Beta, and Gamma Rays•Gamma decay

Alpha, Beta, and Gamma Rays•Gamma decay

Any atom that emits an alpha particle or beta particle

A. becomes an atom of a different element, always.B. may become an atom of a different element. C. becomes a different isotope of the same element. D. increases its mass.


CHECK YOUR NEIGHBOR

Any atom that emits an alpha particle or beta particle

A. becomes an atom of a different element, always.B. may become an atom of a different element. C. becomes a different isotope of the same element. D. increases its mass.


CHECK YOUR ANSWER

A. AlphaB. BetaC. Gamma


CHECK YOUR NEIGHBOR

Which of these is actually a high-speed electron?

A. AlphaB. BetaC. Gamma

Which of these is actually a high-speed electron?


CHECK YOUR ANSWER

Radioactive Isotopes

• The number of protons in an atomic nucleus determine the number of electrons surrounding the nucleus in a neutral atom

• When there is a different number of electrons than the nuclear protons, the atom is charged and is called an ion

• When there is a same number of protons (atomic numbers) but different number of neutrons (mass numbers), they are called isotopes

• When an isotope is radioactive, it is called radioactive isotope


• Three common isotopes of Hydrogen:


• Tritium is a radioactive isotope:


• Tritium is a radioactive isotope:

Exercise: Radioactive Isotopes

The nucleus of beryllium-8 (Be-8) undergoes a special kind of radioactive decay: it split into two equal halves. What nuclei are the products of this decay? What is the form of this decay?

Exercise: Radioactive Isotopes

The nucleus of beryllium-8 (Be-8) undergoes a special kind of radioactive decay: it split into two equal halves. What nuclei are the products of this decay? What is the form of this decay?

8Be –> 2 4He

alpha decay24


• The electric force of repulsion between the protons in a heavy nucleus acts over a greater distance than the attractive forces among the neutrons and protons in the nucleus

• Each proton is repelled by every other proton in the nucleus, but is attracted only by the nucleons closest to it

• In a large nucleus, electric repulsion can exceed nuclear attraction

• This instability makes all the heaviest atoms radioactive

The strong force is more effective with smaller nuclei.particles occupying the nucleus is nucleonsMass=2000 times the mass of electrons

The Atomic Nucleus and the Strong Force

• The strong force holds nucleons together

• Nuclear force holds the nucleus together called the strong interaction which is an attractive force and is a short range force

• Electrical interaction is a long range force


• A lone neutron is radioactive and spontaneously transforms to a proton and an electron

• A neutron needs protons around to keep this from happening


Alpha emission


A. atom that holds electrons in orbit.B. nucleus that holds nucleons together.C. Both A and B.D. Neither A nor B.

The Atomic Nucleus and the Strong ForceCHECK YOUR NEIGHBOR

The strong force is a force in the

A. atom that holds electrons in orbit.B. nucleus that holds nucleons together.C. Both A and B.D. Neither A nor B.

The Atomic Nucleus and the Strong ForceCHECK YOUR ANSWER

The strong force is a force in the

A. short-range force.B. long-range force.C. unstable force.D. neutralizing force.

The Atomic Nucleus and the Strong ForceCHECK YOUR NEIGHBOR

In the nucleus of an atom, the strong force is a relatively

A. short-range force.B. long-range force.C. unstable force.D. neutralizing force.

The Atomic Nucleus and the Strong ForceCHECK YOUR ANSWER

In the nucleus of an atom, the strong force is a relatively

Radioactive Half-Life

The rate of decay for a radioactive isotope is measured in terms of a characteristic time, the half-life, the time for half of an original quantity of an element to decay

A. 1 minuteB. 2 minutesC. 5 minutesD. 9 minutes

Radioactive Half-LifeCHECK YOUR NEIGHBOR

Suppose the number of neutrons in a reactor that is starting up doubles each minute, reaching 1 billion neutrons in 10 minutes. When did the number of neutrons reach half a billion?

Suppose the number of neutrons in a reactor that is starting up doubles each minute, reaching 1 billion neutrons in 10 minutes. When did the number of neutrons reach half a billion?

A. 1 minuteB. 2 minutesC. 5 minutesD. 9 minutes

Explanation:This question would be appropriate with Appendix D, Exponential Growth and Doubling Time. Can you see that working backward, each minute has half the number of neutrons?

Radioactive Half-LifeCHECK YOUR ANSWER

A. zero.B. one-quarter. C. Half.D. the same.

Radiation Half-LifeCHECK YOUR NEIGHBOR

A certain isotope has a half-life of 10 years. This means the amount of that isotope remaining at the end of 10 years will be

A certain isotope has a half-life of 10 years. This means the amount of that isotope remaining at the end of 10 years will be

A. zero.B. one-quarter.C. Half.D. the same.

Radiation Half-LifeCHECK YOUR ANSWER

Exercise: Radioactive Half-Life

If a sample of radioactive isotope has a half-life of 1 year, how much of the original sample will be left at the end of the second year?


If a sample of radioactive isotope has a half-life of 1 year, how much of the original sample will be left at the end of the second year?

(1/2)2 = 1/4


If Fukushima Daiichi nuclear power plant in Japan releases a certain amount of radioactive iodine (I-131), how many percent of the original material will be left at the end of the 50 days? (The half-life of I-131 is 8.02 day)


If Fukushima Daiichi nuclear power plant in Japan releases a certain amount of radioactive iodine (I-131), how many percent of the original material will be left at the end of the 50 days? (The half-life of I-131 is 8.02 day)

(1/2) (50 d/8.02 d) = 1.33%


The half-life of Th-227 is 18.2 day. How many days are required for 0.70 of a given sample to decay?


The half-life of Th-227 is 18.2 day. How many days are required for 0.70 of a given sample to decay?

(1/2) (t/18.2 days) = (1.0 – 0.7)

t = 31.61 days

Radioactive Half-Life

Radioactive decay series:

• Uranium-238 to lead-206 through a series of alpha and beta decays

• In 4.5 billion years, half the uranium presently in Earth will be lead

Alpha, Beta, and Gamma Rays• Irradiation is the process by which an item is exposed to

radiation

• Food irradiation kills microbes, but doesn’t make the food radioactive

A. Alpha B. Beta C. Gamma D. All are different forms of helium.


CHECK YOUR NEIGHBOR

Which of these is the nucleus of the helium atom?

A. Alpha B. Beta C. Gamma D. All are different forms of helium

Explanation: Contrary to the failures of alchemists of old to change elements from one to another, this was going on all around them—unnoticed


CHECK YOUR ANSWERS

Which of these is the nucleus of the helium atom?

• Transmutation: The changing from one chemical element to another

• With alpha or beta particle, a different element is formed. This is transmutation, which occurs in natural events and is also initiated artificially in the laboratory

• Uranium naturally transmutes to Thorium when an alpha particle is emitted

Transmutation of the Elements

• Uranium naturally transmutes to Thorium when an alpha particle is emitted

Transmutation of the Elements

Transmutation of the Elements• Natural transmutation

• Thorium naturally transmutes to protactinium when a beta particle is emitted.

• An electron is e– Superscript 0 indicates electron’s mass is insignificant compared

with nucleons– Subscript -1 is the electric charge of the electron

Radioactive Half-LifeU-238 Radioactive decay series

A. reduces by 2.B. reduces by 4.C. increases by 2.D. increases by 4.

Transmutation of the ElementsCHECK YOUR NEIGHBOR

When an element ejects an alpha particle, the atomic number of the resulting element

When an element ejects an alpha particle, the atomic number of the resulting element

A. reduces by 2.B. reduces by 4. C. increases by 2.D. increases by 4.

Explanation:An alpha particle (a helium nucleus) has atomic number 2. Ejection of an alpha particle means a loss of 2 protons, so the atomic number of the element is lowered by 2

Transmutation of the ElementsCHECK YOUR ANSWER

A. reduces by 1.B. increases by 1. C. reduces by 2.D. increases by 2.


When an element ejects an alpha particle and a beta particle, the atomic number of that element

When an element ejects an alpha particle and a beta particle, the atomic number of that element

A. reduces by 1.B. increases by 1.C. reduces by 2.D. increases by 2.

Explanation:Alpha emission reduces atomic number by 2, and beta emission increases atomic number by 1, so net result is 1.


Exercise: Transmutation of the Elements

Write the nuclear reactions from Uranium-238 to lead-206 through a series of alpha and beta decays

Transmutation of the ElementArtificial transmutation

• An alpha particle fired at and impacting on a nitrogen atom, which transmutes to oxygen and hydrogen

Atoms can transmute into completely different atoms in

A. nature.B. laboratories. C. Both A and B.D. Neither A nor B.


A. nature.B. laboratories. C. Both A and B.D. Neither A nor B.

Explanation:Atomic transmutation occurs in nature, in laboratories, and as far as we know, throughout the cosmos


Atoms can transmute into completely different atoms in

A. 0.B. -1. C. -2.D. None of the above.


An element emits 1 beta particle, and its product then emits 1 alpha particle. The atomic number of the resulting element is changed by

An element emits 1 beta particle, and its product then emits 1 alpha particle. The atomic number of the resulting element is changed by

A. 0.B. -1. C. -2.D. None of the above.

Explanation:Beta emission increases atomic number by 1, then alpha emission decreases atomic number by 2, so the net change is –1.


• Geiger counter detects incoming radiation by a short pulse of current triggered when radiation ionizes a gas in the tube

Radiation Detectors

• Scintillation counter indicates incoming radiation by flashes of light produced when charged particles or gamma rays pass through the counter

Radiation Detectors

Radiation DetectorsCloud chamber:• Charged particles moving

through supersaturated vapor leave trails

• When the chamber is in a strong electric or magnetic field, bending of the tracks provides information about the charge, mass, and momentum of the particles

Radiation DetectorsBubble chamber:

• Liquid hydrogen is heated under pressure in a glass and stainless steel chamber to a point just short of boiling

• If the pressure in the chamber is suddenly released at the moment an ion-producing particle enters, a thin trail of bubbles is left along the particle’s path

Radiation Units• Different units of measure are used depending on what

aspect of radiation is being measured

– The amount of radiation being emitted

– The radiation dose absorbed by a person

– The biological risk of exposure to radiation

Radiation UnitsAspect Conventional SI

The amount of radiation being emitted

Curie (Ci) Becquerel (Bq)

The radiation dose absorbed by a person

Rad (rad) Gray (Gy)

The biological risk of exposure to radiation

Rem (rem) Sievert (Sv)

Radiation Units – Ci and Bq• The amount of radiation being emitted, by a radioactive

material is measured using the conventional unit curie (Ci), or the SI unit becquerel (Bq)

• of radioactive atoms in a radioactive material over a period of time

• 1 Bq = 1 disintegration per second

• 1 Ci = 37 X 109 Bq

• Ci or Bq may be used to refer to the amount of radioactive materials released into the environment

Radiation Units – rad and Gy• The radiation dose absorbed by a person (that is, the

amount of energy deposited per unit of weight of human tissue by radiation) is measured using the conventional unit rad or the SI unit gray (Gy)

• The rad, which stands for radiation absorbed dose, was the conventional unit of measurement

• 1 Gy = 100 rad

Radiation Units – rem and Sv• The biological risk of exposure to radiation is measured using

the conventional unit rem or the SI unit sievert (Sv)

• Scientists have assigned a number, Quality Factor (Q), to each type of ionizing radiation (alpha and beta particles, gamma rays, and x-rays) depending on that type's ability to transfer energy to the cells of the body

• To estimate a person's biological risk in rems

rem = rad X Q

• 1 Sv = 100 rem

Environmental Radiation• Units of radiationParticle Radiation Quality Factor Health

effectalpha 1 rad 10

= 10 remsbeta 10 rad 1

= 10 rems

• Doses of radiation– Lethal doses of radiation begin at 500 rems

Environmental Radiation

• Source received annually

Natural origin Typical dose (mrem)

Cosmic radiation 26

Ground 33

Air (Radon-222)198

Human tissues (K-40; Ra-226) 35


• Radon, a common environmental hazard• Most radiation from natural background• About 1/5 from nonnatural sources


• Doses of radiationHuman origin Typical

dose (mrem)Medical procedures

Diagnostic X-rays 40

Nuclear diagnostics 15

TV tubes, other consumer products 11Weapons-test fallout

1Commercial fossil-fuel power plants < 1Commercial nuclear power plants << 1


Radioactive tracers• Radioactive isotopes used to trace such pathways are

called tracers

Radiometric Dating• Earth’s atmosphere is continuously bombarded by

cosmic rays, which causes many atoms in the upper atmosphere to transmute

Radiometric Dating• A nitrogen that captures a neutron and becomes an

isotope of carbon by emitting a proton:

Radiometric Dating• Because living plants take

in carbon dioxide, any C-14 lost by decay is immediately replenished with fresh C-14 from the atmosphere

• Dead plants continue emitting C-14 without replenishment

Radiometric Dating• Carbon-14 is a beta emitter and decays back to nitrogen

• The half-life of Carbon-14 is about 5730 years

Radiometric Dating

Relative amounts of C-12 to C-14 enable dating of organic materials

The half-life of carbon-14 is about 5730 years, which means that the present amount in your bones will reduce to zero

A. when you die.B. in about 5730 years. C. in about twice 5730 years.D. None of the above.

Radiometric DatingCHECK YOUR NEIGHBOR

The half-life of carbon-14 is about 5730 years, which means that the present amount in your bones will reduce to zero

A. when you die.B. in about 5730 years. C. in about twice 5730 years.D. None of the above.

Explanation:In theory, the amount never reaches zero. In eons to come, trace amounts of the carbon-14 in your bones, even if completely dissolved, will still exist.

Radiometric DatingCHECK YOUR ANSWER

Q & A

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Atomic Nucleus and Radioactivity