20.6 Biological Effects of Radiation | Chemistry: Atoms First 2e

Learning Objectives

Explain how radiation damages cells and how penetration affects exposure
Distinguish radiation activity, absorbed dose, and biological effect
Calculate activity and convert dose units

Core Concepts & Principles

Radiation and Cell Damage

Nonionizing radiation, such as visible light and microwaves, cannot remove electrons; it can heat or excite molecules. Ionizing radiation—alpha and beta particles, gamma rays, X-rays, and sufficiently energetic ultraviolet—can remove electrons and break bonds.

Damage can be direct, through ionization of DNA or other biomolecules, or indirect, through water breakdown producing hydroxyl radicals, OH•. A free radical has an unpaired electron and reacts readily with DNA, proteins, and enzymes. Unrepaired damage can cause cell malfunction, cancer, or death. Rapidly dividing cells (bone marrow, digestive lining, hair follicles, embryos) are especially sensitive. Damage to body cells is somatic; damage to reproductive cells can affect offspring. Acute high doses can cause serious illness; repeated exposure can increase long-term cancer risk.

Penetration and Protection

Alpha: Stopped by paper or outer skin, but particularly harmful when a source is inhaled or swallowed.
Beta: Penetrates farther; plastic or thin metal can provide shielding.
Gamma: Highly penetrating; dense lead or thick concrete reduces exposure.
Neutrons: Need different shielding, including hydrogen-rich water to slow them and materials that absorb them.

Reduce exposure time, increase distance, and use appropriate shielding. Natural sources include cosmic rays, radioactive rocks, and radon, a gas from uranium decay that can accumulate indoors. Medical procedures and some consumer products add exposure.

Measuring Radiation

Geiger counter: Radiation ionizes gas in a tube, producing electrical signals.
Scintillation counter: Radiation excites a material that emits light; a sensor converts the flashes into signals.
Radiation dosimeter: Records personal exposure, using devices such as film badges or electronic detectors.

Quantity	Units and relationship
Activity: decays per second	1 Bq = 1 decay/s; 1 Ci = $3.7\times10^{10}$ Bq; 1 mCi = $10^{-3}$ Ci = $3.7\times10^7$ Bq
Absorbed dose: energy absorbed per mass	1 Gy = 1 J/kg; 1 rad = 0.01 Gy
Biological effect estimate: dose adjusted for radiation type	Sv = RBE × Gy; rem = RBE × rad; 100 rem = 1 Sv

Relative biological effectiveness (RBE) estimates damage relative to a reference radiation. The source gives approximate factors of 1 for beta/gamma, about 10 for alpha, and 2 or more for protons/neutrons; effectiveness depends on energy and conditions. Use the factor supplied in a problem. Gray is the SI absorbed-dose unit; rad is non-SI. The source glossary's rad and millicurie entries are clarified below.

Activity Is Not Dose

Activity measures a source's decay rate. Absorbed dose depends on how much emitted energy reaches and is absorbed by tissue; biological effect also depends on radiation type.

Problem-Solving Routines & Methods

Activity from Isotope Mass and Half-Life

N=\frac{m}{M}N_A,\qquad \text{activity}=\lambda N=\frac{\ln 2}{t_{1/2}}N

Variables & Constants

m

=radioactive isotope mass;

M

=its molar mass;

N

=its nuclei count;

N_A

=Avogadro constant

Calculating Activity or Biological Dose

1
For activity, convert isotope mass to nuclei using molar mass and Avogadro's constant.
2
Convert half-life to seconds, then multiply N by ln(2)/half-life to obtain Bq. Divide Bq by 3.7 × 10¹⁰ to obtain Ci.
3
For biological dose, multiply absorbed dose by the supplied RBE: Gy to Sv, or rad to rem.

Pro-Tip: Keep activity (Bq/Ci), absorbed dose (Gy/rad), and biological dose (Sv/rem) separate.

Practice & Concept Checks

Concept Check

A gamma exposure deposits 0.25 Gy, with RBE = 1. What is the estimated biological dose?

Concept Check

Estimate the activity of 1.00 mg of tritium, with molar mass 3.016 g/mol and half-life 12.32 years. Use 365.25 days/year.

Key Terms & Vocabulary

becquerel (Bq)Radiation Units and Dose

SI unit for rate of radioactive decay; 1 Bq = 1 disintegration/s

curie (Ci)Radiation Units and Dose

larger unit for rate of radioactive decay frequently used in medicine; 1 Ci = 3.7

\times

10¹⁰ disintegrations/s

Geiger counterRadiation Detection

instrument that detects and measures radiation via the ionization produced in a Geiger-Müller tube

gray (Gy)Radiation Units and Dose

SI unit for measuring radiation dose; 1 Gy = 1 J absorbed/kg tissue

ionizing radiationRadiation Effects

radiation that can cause a molecule to lose an electron and form an ion

millicurie (mCi)Radiation Units and Dose

larger unit for rate of radioactive decay frequently used in medicine; 1 Ci = 3.7

\times

10¹⁰ disintegrations/s

Example: Source clarification: 1 mCi = 0.001 Ci = 3.7 × 10⁷ disintegrations/s; the source definition above gives the value for 1 Ci.

nonionizing radiationRadiation Effects

radiation that speeds up the movement of atoms and molecules; it is equivalent to heating a sample, but is not energetic enough to cause the ionization of molecules

radiation absorbed dose (rad)Radiation Units and Dose

SI unit for measuring radiation dose, frequently used in medical applications; 1 rad = 0.01 Gy

Example: Source clarification: rad is a non-SI absorbed-dose unit. Gray (Gy) is the SI unit; 1 rad = 0.01 Gy.

radiation dosimeterRadiation Detection

device that measures ionizing radiation and is used to determine personal radiation exposure

relative biological effectiveness (RBE)Radiation Units and Dose

measure of the relative damage done by radiation

roentgen equivalent man (rem)Radiation Units and Dose

unit for radiation damage, frequently used in medicine; 100 rem = 1 Sv

scintillation counterRadiation Detection

instrument that uses a scintillator—a material that emits light when excited by ionizing radiation—to detect and measure radiation

sievert (Sv)Radiation Units and Dose

SI unit measuring tissue damage caused by radiation; takes into account energy and biological effects of radiation