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Amount of radiation

Cobalt-60 ( t 1/2 = 5.26 y) is used in cancer therapy since the γ rays it emits can be focused in small areas where the cancer is located. A 5.00-g sample of Co-60 is available for cancer treatment.

(a) What is its activity in Bq?

(b) What is its activity in Ci?

Solution

The activity is given by:

Activity = λ N = ( ln 2 t 1 / 2 ) N = ( ln 2 5.26 y ) × 5.00 g = 0.659 g y of Co−60 that decay

And to convert this to decays per second:

0.659 g y × 1 y 365 d × 1 d 24 h × 1 h 3600 s × 1 mol 59.9 g × 6.02 × 10 23 atoms 1 mol × 1 decay 1 atom = 2.10 × 10 14 decay s

(a) Since 1 Bq = 1 decay s , the activity in Becquerel (Bq) is:

2.10 × 10 14 decay s × ( 1 Bq 1 decay s ) = 2.10 × 10 14 Bq

(b) Since 1 Ci = 3.7 × 10 11 decay s , the activity in curie (Ci) is:

2.10 × 10 14 decay s × ( 1 Ci 3.7 × 10 11 decay s ) = 5.7 × 10 2 Ci

Check your learning

Tritium is a radioactive isotope of hydrogen ( t 1/2 = 12.32 y) that has several uses, including self-powered lighting, in which electrons emitted in tritium radioactive decay cause phosphorus to glow. Its nucleus contains one proton and two neutrons, and the atomic mass of tritium is 3.016 amu. What is the activity of a sample containing 1.00mg of tritium (a) in Bq and (b) in Ci?

Answer:

(a) 3.56 × 10 11 Bq; (b) 0.962 Ci

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Effects of long-term radiation exposure on the human body

The effects of radiation depend on the type, energy, and location of the radiation source, and the length of exposure. As shown in [link] , the average person is exposed to background radiation, including cosmic rays from the sun and radon from uranium in the ground (see the Chemistry in Everyday Life feature on Radon Exposure); radiation from medical exposure, including CAT scans, radioisotope tests, X-rays, and so on; and small amounts of radiation from other human activities, such as airplane flights (which are bombarded by increased numbers of cosmic rays in the upper atmosphere), radioactivity from consumer products, and a variety of radionuclides that enter our bodies when we breathe (for example, carbon-14) or through the food chain (for example, potassium-40, strontium-90, and iodine-131).

A bar graph titled “Radiation Doses and Regulatory Limits, open parenthesis, in Millirems, close parenthesis” is shown. The y-axis is labeled “Doses in Millirems” and has values from 0 to 5000 with a break between 1000 and 5000 to indicate a different scale to the top of the graph. The y-axis is labeled corresponding to each bar. The first bar, measured to 5000 on the y-axis, is drawn in red and is labeled “Annual Nuclear Worker Doses Limit, open parenthesis, N R C, close parenthesis.” The second bar, measured to 1000 on the y-axis, is drawn in blue and is labeled “Whole Body C T” while the third bar, measured to 620 on the y-axis, is drawn in blue and is labeled “Average U period S period Annual Dose.” The fourth bar, measured to 310 on the y-axis, is drawn in blue and is labeled “U period S period Natural Background Dose” while the fifth bar, measured to 100 on the y-axis and drawn in red reads “Annual Public Dose Limit, open parenthesis, N R C, close parenthesis.” The sixth bar, measured to 40 on the y-axis, is drawn in blue and is labeled “From Your Body” while the seventh bar, measured to 30 on the y-axis and drawn in blue reads “Cosmic rays.” The eighth bar, measured to 4 on the y-axis, is drawn in blue and is labeled “Safe Drinking Water Limit, open parenthesis, E P A, close parenthesis” while the ninth bar, measured to 2.5 on the y-axis and drawn in red reads “Trans Atlantic Flight.” A legend on the graph shows that red means “Dose Limit From N R C dash licensed activity” while blue means “Radiation Doses.”
The total annual radiation exposure for a person in the US is about 620 mrem. The various sources and their relative amounts are shown in this bar graph. (source: U.S. Nuclear Regulatory Commission)

A short-term, sudden dose of a large amount of radiation can cause a wide range of health effects, from changes in blood chemistry to death. Short-term exposure to tens of rems of radiation will likely cause very noticeable symptoms or illness; a dose of about 500 rems is estimated to have a 50% probability of causing the death of the victim within 30 days of exposure. Exposure to radioactive emissions has a cumulative effect on the body during a person’s lifetime, which is another reason why it is important to avoid any unnecessary exposure to radiation. Health effects of short-term exposure to radiation are shown in [link] .

Health Effects of Radiation Source: US Environmental Protection Agency
Exposure (rem) Health Effect Time to Onset (without treatment)
5–10 changes in blood chemistry
50 nausea hours
55 fatigue
70 vomiting
75 hair loss 2–3 weeks
90 diarrhea
100 hemorrhage
400 possible death within 2 months
1000 destruction of intestinal lining
internal bleeding
death 1–2 weeks
2000 damage to central nervous system
loss of consciousness; minutes
death hours to days

It is impossible to avoid some exposure to ionizing radiation. We are constantly exposed to background radiation from a variety of natural sources, including cosmic radiation, rocks, medical procedures, consumer products, and even our own atoms. We can minimize our exposure by blocking or shielding the radiation, moving farther from the source, and limiting the time of exposure.

Key concepts and summary

We are constantly exposed to radiation from a variety of naturally occurring and human-produced sources. This radiation can affect living organisms. Ionizing radiation is the most harmful because it can ionize molecules or break chemical bonds, which damages the molecule and causes malfunctions in cell processes. It can also create reactive hydroxyl radicals that damage biological molecules and disrupt physiological processes. Radiation can cause somatic or genetic damage, and is most harmful to rapidly reproducing cells. Types of radiation differ in their ability to penetrate material and damage tissue, with alpha particles the least penetrating but potentially most damaging and gamma rays the most penetrating.

Various devices, including Geiger counters, scintillators, and dosimeters, are used to detect and measure radiation, and monitor radiation exposure. We use several units to measure radiation: becquerels or curies for rates of radioactive decay; gray or rads for energy absorbed; and rems or sieverts for biological effects of radiation. Exposure to radiation can cause a wide range of health effects, from minor to severe, and including death. We can minimize the effects of radiation by shielding with dense materials such as lead, moving away from the source, and limiting time of exposure.

Key equations

  • rem = RBE × rad
  • Sv = RBE × Gy

Chemistry end of chapter exercises

If a hospital were storing radioisotopes, what is the minimum containment needed to protect against:

(a) cobalt-60 (a strong γ emitter used for irradiation)

(b) molybdenum-99 (a beta emitter used to produce technetium-99 for imaging)

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Based on what is known about Radon-222’s primary decay method, why is inhalation so dangerous?

Alpha particles can be stopped by very thin shielding but have much stronger ionizing potential than beta particles, X-rays, and γ-rays. When inhaled, there is no protective skin covering the cells of the lungs, making it possible to damage the DNA in those cells and cause cancer.

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Given specimens uranium-232 ( t 1/2 = 68.9 y) and uranium-233 ( t 1/2 = 159,200 y) of equal mass, which one would have greater activity and why?

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A scientist is studying a 2.234 g sample of thorium-229 ( t 1/2 = 7340 y) in a laboratory.

(a) What is its activity in Bq?

(b) What is its activity in Ci?

(a) 7.64 × 10 9 Bq; (b) 2.06 × 10 −2 Ci

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Given specimens neon-24 ( t 1/2 = 3.38 min) and bismuth-211 ( t 1/2 = 2.14 min) of equal mass, which one would have greater activity and why?

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Source:  OpenStax, Chemistry. OpenStax CNX. May 20, 2015 Download for free at http://legacy.cnx.org/content/col11760/1.9
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