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2-Dec-10

Chapter 33X-Rays & RadioactivityNuclear Physics - Part I

Lecture 38

Quiz 5: Monday Dec. 6 (Chaps. 29-32)

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X-Rays and Radioactivity• Roentgen discovered X-rays

produced by a beam of electrons striking the glass surface of a gas-discharge tube.

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

cathode

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X-Rays and Radioactivity

• X-rays are high-frequency electromagnetic waves (high frequency photons).

• An energetic beam of electrons striking atoms of a solid excites the innermost atomic electrons and produces X-rays.

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X-Rays and Radioactivity• 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.

Transmission X-Ray image.

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X-Ray Back-Scatter Images• Look at X-Rays that “bounce back” from an

object instead of transmitted X-Rays. (Less radiation exposure.)

• Used in new airport scanners.

Back-Scatter X-Ray image.

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Radioactivity• Radioactivity is the process of nuclear decay

(radioactive decay). Atomic nuclei emit particles or photons as they change state.

• 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.

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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.

X-Rays and RadioactivityCHECK YOURSELF

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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.

X-Rays and RadioactivityCHECK YOUR ANSWER

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Alpha, Beta, and Gamma RaysRadioactive elements emit three distinct types of radiation:• α —alpha particles: helium nuclei (2 protons

& 2 neutrons); positively charged • β — beta particles: electrons; negatively

charged• γ —gamma photons (very high frequency

electromagnetic radiation); no charge

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Alpha, Beta, and Gamma Rays

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Alpha, Beta, and Gamma RaysRelative penetrations

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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.

Alpha, Beta, and Gamma RaysCHECK YOURSELF

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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.

Alpha, Beta, and Gamma RaysCHECK YOUR ANSWER

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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.

Alpha, Beta, and Gamma RaysCHECK YOURSELF

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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.

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

Alpha, Beta, and Gamma RaysCHECK YOUR ANSWER

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Alpha, Beta, and Gamma RaysFood irradiation kills microbes.• Doesn’t make the food radioactive.• There is no diarrhea with astronauts in space

(their food is first irradiated).

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Which of these is the nucleus of the helium atom?

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

Alpha, Beta, and Gamma RaysCHECK YOURSELF

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Which of these is the nucleus of the helium atom?

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

Alpha, Beta, and Gamma RaysCHECK YOUR ANSWER

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Which of these is actually a high-speed electron?

A. AlphaB. BetaC. GammaD. All are high speed.

Alpha, Beta, and Gamma RaysCHECK YOURSELF

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Which of these is actually a high-speed electron?

A. AlphaB. BetaC. GammaD. All are high speed.

Explanation: Choice D may be true, but doesn’t directly answer the question.

Alpha, Beta, and Gamma RaysCHECK YOUR ANSWER

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Environmental Radiation

• Most radiation from natural background• About 1/5 from non-natural sources

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Environmental Radiation

Test kit for Radon, a common environmental hazard

Radon gas is emitted by uranium deposits underground.Since it is a heavy gas, it collects in basements.

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Radiation Dosage and EffectsRadiation dose measured in rads; rad = 0.01 J/kgBiological effect measured in rem; depends on

type of radiation. rem = RBE*rad

Biological equivalence factors (RBE) of radiationParticle Radiation Dosage RBE Factor Health effectalpha 1 rad × 20 = 20 remsbeta 1 rad × 1 = 1 remGamma 1 rad x 1 = 1 remX-Ray 1 rad x 1 = 1 remneutron 1 rad x 5 = 5 rem• Doses of radiation

– 100 mrem/yr is legal limit for man-made non-medical exposure– Lethal doses of radiation begin at 500 rems (over a short time)

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Environmental RadiationSource Typical annual dose (mrem)Natural originCosmic radiation - San Francisco 50

- Denver 200- Pocos de Caldos, Brazil 7000

Ground 33Air (Radon-222) 198Human tissues (K-40; Ra-226) 35

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Environmental RadiationSource Typical dose (mrem)Human originMedical procedures

Diagnostic X-rays 50-2000CAT Scan 25,000Nuclear diagnostics 15Radiation cancer therapy 200,000 (local)

Cross-country jet flight 3Weapons-test fallout 1Commercial fossil-fuel power plants <1Airport back-scatter X-ray 0.009Commercial nuclear power plants <<1

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Environmental RadiationRadioactive tracers• Radioactive isotopes used to map out flow

pathways in plants or animals are called tracers.

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The Atomic Nucleus and the Strong Nuclear Force

The strong nuclear force holds nucleons together.It is a very short range force (10-15 m distance).

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The strong force is more effective with smaller nuclei.

The Atomic Nucleus and the Strong Force

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The Atomic Nucleus and the Strong 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.

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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 YOURSELF

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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

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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 YOURSELF

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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

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Radioactive Half-LifeThe 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.

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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.

Radioactive Half-LifeCHECK YOURSELF

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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.

Radioactive Half-LifeCHECK YOUR ANSWER

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A sample of radioactive material initially shows 80 decays per minute. Six hours later, it shows 10 decays per minute. What is the half-life of the material?

A. 1 hourB. 2 hoursC. 4 hoursD. 6 hours

Radioactive Half-LifeA challenge…

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A sample of radioactive material initially shows 80 decays per minute. Six hours later, it shows 10 decays per minute. What is the half-life of the material?

A. 1 hourB. 2 hoursC. 4 hoursD. 6 hours

After 6 hours, there is only 1/8 of the original amount remaining, so the material has gone through 3 half-lives. 6 hours/3 = 2 hours

Radioactive Half-LifeCheck Your Answer

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Key Points of Lecture 38Key Points of Lecture 38

Before Monday Dec. 6, read Hewitt Chap. 32.

Homework #25 due by 11:00 PM Friday Dec. 3

Homework #26 due by 11:00 PM Sunday Dec. 5

X-Rays and RadioactivityAlpha, Beta and Gamma RaysEnvironmental RadiationRadiation Units The Atomic Nucleus and the Strong ForceRadioactive Half-Life