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Alpha particles have low penetration ability and are stopped by outer skin layers. However, when ingested, they directly contact internal tissues and directly ionize biomolecules within cells, causing highly damaging effects. This direct ionization in sensitive internal organs results in severe biological harm compared to external exposure.

Gamma radiation ionizes water molecules in living tissue, producing highly reactive hydroxyl radicals. These radicals then react with biological molecules like DNA, proteins, and enzymes, disrupting their structure and function. This indirect damage pathway is particularly dangerous because hydroxyl radicals can damage multiple types of biomolecules throughout the cell.

Absorbed dose measures energy deposited per unit mass, expressed in grays. Equivalent dose accounts for different radiation types' varying ionizing and penetration powers by multiplying absorbed dose by a radiation weighting factor, expressed in sieverts. This adjustment reflects that equal absorbed doses of different radiation types cause different biological damage.

Different body tissues have varying sensitivities to ionizing radiation. When radiation exposure is concentrated in one area or unevenly distributed, tissue weighting factors adjust equivalent doses to reflect each organ's sensitivity. Summing these weighted doses determines the overall effective dose to the body, providing a more accurate assessment of total biological risk.

A Geiger-Müller counter contains a gas-filled tube with electrodes at high voltage. Ionizing radiation ionizes gas molecules, creating a cascade of ionizations that produces current between the electrodes. This current is collected, amplified, and displayed as counts per minute or disintegrations per second, allowing measurement of radiation activity.

Background radiation comes from cosmic rays from the sun, radon from uranium in the ground, medical procedures like CAT scans and X-rays, airplane flights with increased cosmic ray exposure, consumer products, and radionuclides entering the body through breathing or food chains, such as carbon-14, potassium-40, and strontium-90.

An acute dose of 500 rems or 5 sieverts has an estimated 50% probability of causing death within 30 days of exposure. Short-term exposure to tens of rems causes very noticeable symptoms or illness. Radiation exposure also has cumulative effects over a person's lifetime, making it important to avoid unnecessary exposure.