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Chapter 9 Nuclear Radiation

9.3 Radiation Measurement9.4 Half-Life of a Radioisotope9.5 Medical Applications Using

Radioactivity

Copyright © 2005 by Pearson Education, Inc.Publishing as Benjamin Cummings

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Radiation Measurement

A Geiger counter • detects beta and gamma radiation.• uses ions produced by radiation to create an

electrical current.

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Detecting Radiation

A Geiger-Müller counter

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Geiger-Müller counter with radioactive antique orange plate

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Radiation Units

Units of radiation include• Curie

- measures activity as the number of atoms that decay in one second.

• rad (radiation absorbed dose) - measures the radiation absorbed by the

tissues of the body.• rem (radiation equivalent)

- measures the biological damage caused by different types of radiation.

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Units of Radiation Measurement

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Exposure to Radiation

Exposure to radiation occurs from

• naturally occurring radioisotopes.

• medical and dental procedures.

• air travel, radon, and smoking cigarettes.

TABLE 9.6

Copyright © 2005 by Pearson Education, Inc.Publishing as Benjamin Cummings

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Half-Life

The half-life of a radioisotope is the time for the radiation level to decrease (decay) to one-half of the original value.

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Decay Curve

A decay curve shows the decay of radioactive atoms and the remaining radioactive sample.

Copyright © 2005 by Pearson Education, Inc.Publishing as Benjamin Cummings

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Half-Lives of Some Radioisotopes

Radioisotopes• that are naturally occurring tend to have long half-lives.• used in nuclear medicine have short half-lives. Half-Lives of Some Radioisotopes

Radioisotope Half-life 14C 5730 yr

40K 1.3 x 109 yr226Ra 1600 yr238U 4.5 x 109 yr

51Cr 28 days131I 8 days59Fe 46 days

99mTc 6.0 hr

Naturally Occurring

Medical Uses

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In one half-life, 40 mg of a radioisotope decays to 20 mg. After two half-lives, 10 mg of radioisotope remain.

40 mg x 1 x 1 = 10 mg 2 2

1 half-life 2 half-lives

Initial40 mg

20 mg10 mg

Half-Life Calculations

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The half life of 123I is 13 hr. How much of a 64 mg sample of 123I is left after 26 hours?

1) 32 mg2) 16 mg3) 8 mg

Learning Check

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Solution

Half-life

13 hr 13 hr

2) 16 mg

STEP 1 Given 64 g; 26 hr; 13 hr/half-life

STEP 2 Plan 26 hours Number of half-lives

STEP 3 Equalities 1 half-life = 13 hr

STEP 4 Set Up Problem

Number of half-lives = 26 hr x 1 half-life = 2 half-lives

13 hr

64 mg 32 mg 16 mg

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Radioactive Decay (cont)

a. Radioactive Dating1. The radioactive decay of carbon-14 can

be used to estimate the age of organic materials.

6C126C13

6C14

Types of Carbon Isotopes

Mass number = # protons + # neutrons

The process of Carbon-14 Dating

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The Shroud of Turin

Credit: The Image Works

Reputed as the burial cloth of Jesus Christ. C-14 dating by 3 independent labs report the Cloth originated during the Medieval times, between A.D. 1260-1390.

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Mummified remains found frozen in the Italian Alps

Credit: Landov

At least 5000 years oldBy carbon-14 dating

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Medical Applications

Radioisotopes with short half-lives are used in nuclear medicine because

• they have the same chemistry in the body as the nonradioactive atoms.

• in the organs of the body, they give off radiation that exposes a photographic plate (scan) giving an image of an organ. Thyroid scan

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Some Radioisotopes Used in Nuclear Medicine

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Learning Check

Which of the following radioisotopes are most likely to be used in nuclear medicine?

1) 40K half-life 1.3 x 109 years2) 42K half-life 12 hours3) 131I half-life 8 days

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Solution

Which of the following radioisotopes are most likely to be used in nuclear medicine?

Radioisotopes with short half-lives are used in nuclear medicine.

2) 42K half-life 12 hours3) 131I half-life 8 days