Unit 10: Properties & Dangers of Radiation

Comparing the characteristics of different types of radiation and understanding their effects on matter and living tissue.

10.6 Penetrating Power of α, β, γ

Penetrating power describes the ability of radiation to pass through matter. The three main types of radiation have vastly different penetrating abilities.

  • Alpha (α) Particles: Have very low penetrating power. They are large and heavy (2 protons, 2 neutrons), so they collide frequently with air molecules and quickly lose their energy. They can be stopped by a few centimetres of air or a single sheet of paper.
  • Beta (β) Particles: Have medium penetrating power. Being much smaller and lighter than alpha particles, they travel further. They can pass through paper but are stopped by a thin sheet of aluminium (a few millimetres thick).
  • Gamma (γ) Rays: Have very high penetrating power. As they have no mass or charge, they interact much less with matter. They can pass through the human body easily and require several centimetres of lead or thick concrete to be significantly reduced.
Solved Examples:
  1. Which type of radiation is the most penetrating?
    Solution: Gamma rays.
  2. What material can be used to stop beta particles but not gamma rays?
    Solution: A thin sheet of aluminium.
  3. Why do alpha particles have such low penetrating power?
    Solution: Because they are relatively large, heavy, and have a +2 charge, causing them to interact strongly and frequently with atoms they encounter, quickly losing their energy.
  4. A radioactive source emits radiation that is stopped by a sheet of paper. What type of radiation is it?
    Solution: Alpha radiation.
  5. Arrange alpha, beta, and gamma radiation in order of increasing penetrating power.
    Solution: Alpha < Beta < Gamma.
  6. What is needed to provide effective shielding from a strong gamma source?
    Solution: Thick layers of a dense material, such as lead or concrete.
  7. Can beta particles pass through human skin?
    Solution: Yes, they can penetrate the skin and superficial tissues, but they are generally stopped before reaching internal organs.
  8. Why are gamma rays so penetrating?
    Solution: Because they are a form of electromagnetic radiation with no mass and no charge, they are less likely to interact with the atoms they pass through.
  9. A radioactive sample is placed in a box made of 5mm thick aluminium. Which type(s) of radiation could be detected outside the box?
    Solution: Gamma radiation. The alpha and beta particles would be stopped by the aluminium.
  10. How far can alpha particles typically travel in air?
    Solution: Only a few centimetres.

10.7 Ionising Power of α, β, γ

Ionising power is the ability of radiation to knock electrons out of the atoms or molecules it passes through, creating ions. This property is inversely related to penetrating power.

  • Alpha (α) Particles: Have very high ionising power. Their large size and +2 charge strongly attract electrons from nearby atoms, causing intense ionisation along a short path.
  • Beta (β) Particles: Have medium ionising power. They are smaller and faster than alpha particles, with a -1 charge, causing less intense ionisation over a longer path.
  • Gamma (γ) Rays: Have very low ionising power. Since they are uncharged, they are much less likely to interact with and remove electrons from atoms.
Solved Examples:
  1. Which type of radiation is the most ionising?
    Solution: Alpha particles.
  2. Why are alpha particles so strongly ionising?
    Solution: Their large mass and strong +2 charge exert a powerful electrostatic force on the electrons of atoms they pass, easily pulling them off.
  3. What is the relationship between penetrating power and ionising power?
    Solution: They are generally inversely proportional. Radiation with high ionising power (like alpha) loses its energy quickly and has low penetrating power. Radiation with low ionising power (like gamma) loses energy slowly and has high penetrating power.
  4. Arrange alpha, beta, and gamma radiation in order of decreasing ionising power.
    Solution: Alpha > Beta > Gamma.
  5. What does it mean for radiation to "ionise" an atom?
    Solution: It means the radiation has enough energy to knock one or more electrons out of the atom, turning the neutral atom into a positive ion.
  6. Why are gamma rays weakly ionising?
    Solution: They have no charge and are a form of energy, not a particle with mass, so they interact much less frequently with atomic electrons.
  7. A beta particle passes through a gas. What happens to some of the gas atoms?
    Solution: Some of the gas atoms will be ionised (lose an electron) by the passing beta particle.
  8. Which type of radiation would cause the most damage over a very short distance inside the body?
    Solution: Alpha radiation, due to its very high ionising power.
  9. How does a beta particle cause ionisation?
    Solution: Its negative charge repels atomic electrons, and if it passes close enough with enough energy, it can knock an electron out of its orbit.
  10. Which property of radiation is most directly responsible for its ability to damage living cells?
    Solution: Its ionising power.

10.8 Detection of Radiation (Geiger-Muller Tube)

Since radiation is invisible, special instruments are needed to detect it. The most common is the Geiger-Müller (GM) tube, often connected to a counter that produces an audible "click" or a digital reading for each radiation event detected.

How a GM Tube Works:
  1. The tube contains a low-pressure inert gas (like argon).
  2. A central wire (anode) is kept at a high positive voltage relative to the outer metal casing (cathode).
  3. When a single particle of ionising radiation (α or β) or a high-energy photon (γ) enters the tube, it collides with a gas atom and ionises it, knocking off an electron.
  4. The high voltage accelerates this electron, causing it to ionise more gas atoms, which in turn releases more electrons. This creates a cascade of electrons, known as an avalanche.
  5. This avalanche creates a short, detectable pulse of electric current that flows from the wire to the casing.
  6. The counter registers this pulse as a single "count" or "click".

A GM counter measures the rate of radiation, typically in counts per second or counts per minute.

Solved Examples:
  1. What is the most common device used to detect radiation?
    Solution: A Geiger-Müller (GM) tube and counter.
  2. What property of radiation allows it to be detected by a GM tube?
    Solution: Its ionising power.
  3. What is inside a GM tube?
    Solution: An inert gas at low pressure, a central positive electrode (anode), and an outer negative electrode (cathode).
  4. What does each "click" of a Geiger counter represent?
    Solution: The detection of a single particle or ray of radiation entering the tube.
  5. What is the "avalanche effect" in a GM tube?
    Solution: It is the cascade of ionisations where one initial ionisation event leads to a large number of electrons, creating a detectable electrical pulse.
  6. Can a standard GM tube distinguish between alpha, beta, and gamma radiation?
    Solution: No, it simply counts the total number of ionising events. Distinguishing between them requires using absorbers (like paper or aluminium).
  7. What is "background radiation"?
    Solution: The low level of ionising radiation that is always present in the environment from natural and man-made sources.
  8. A student points a GM tube at a source and gets a reading of 250 counts per minute. The background count is 30 counts per minute. What is the corrected count rate from the source?
    Solution: 250 - 30 = 220 counts per minute.
  9. Why does the GM tube need a high voltage?
    Solution: To accelerate the electrons produced by ionisation with enough energy to cause the electron avalanche.
  10. What is measured by a Geiger counter?
    Solution: The rate of radioactive decay or the activity of a source.

10.9 Dangers of Radiation Exposure

The danger from radiation comes from its ability to ionise atoms within living cells. This ionisation can damage or destroy important biological molecules, including DNA.

  • High doses of radiation can kill cells outright, leading to radiation sickness or burns (similar to sunburn).
  • Low doses may not kill cells but can damage DNA. This can lead to mutations that may cause the cell to become cancerous, often many years after the exposure.

The type of danger depends on the type of radiation and whether the source is outside (external) or inside (internal) the body.

  • External Hazard: Gamma rays are the most dangerous external source because they are highly penetrating and can reach internal organs. Beta particles can cause skin burns. Alpha particles are not an external hazard as they are stopped by the outer layer of dead skin cells.
  • Internal Hazard: If a radioactive source is inhaled or ingested, alpha particles become the most dangerous. Their high ionising power causes intense, localised damage to the surrounding tissues. Beta particles are also a significant internal hazard.
Solved Examples:
  1. What is the primary way radiation harms living cells?
    Solution: By ionising atoms within the cells, which can damage critical molecules like DNA.
  2. Which type of radiation poses the greatest danger from a source outside the body? Why?
    Solution: Gamma radiation, because it is highly penetrating and can reach vital internal organs.
  3. Which type of radiation is most dangerous if its source is inhaled or ingested? Why?
    Solution: Alpha radiation, because it is highly ionising and will cause a large amount of damage in a very localised area.
  4. What are two possible long-term effects of radiation exposure?
    Solution: Cancer and genetic mutations.
  5. Why is an alpha source safe to hold in your hand (with gloves) but extremely dangerous to swallow?
    Solution: The alpha particles are stopped by the outer layers of skin and the glove (low penetration). If swallowed, the source is in direct contact with living tissue, and the high ionising power of the alpha particles causes severe internal damage.
  6. What is a high dose of radiation likely to cause?
    Solution: Widespread cell death, leading to radiation sickness or burns.
  7. What is a potential risk of a low dose of radiation?
    Solution: It can cause mutations in DNA, which may lead to cancer later in life.
  8. Name a natural source of background radiation.
    Solution: Radon gas from the ground, cosmic rays from space, or radioactive elements in food and drink.
  9. How can workers who handle radioactive materials protect themselves?
    Solution: By minimising exposure time, maximising distance from the source, and using appropriate shielding (e.g., lead aprons).
  10. Is beta radiation considered an internal or external hazard?
    Solution: It can be both. It can penetrate skin to cause burns (external) and can cause significant damage if ingested (internal).

Knowledge Check (20 Questions)

Answer: Gamma.

Answer: Alpha.

Answer: A Geiger-Müller tube.

Answer: A sheet of paper or a few cm of air.

Answer: Increased risk of cancer due to DNA mutation.

Answer: Its ionising power.

Answer: Gamma.

Answer: Alpha.

Answer: Thick lead or concrete.

Answer: They are inversely related.

Answer: Background radiation.

Answer: The rate of radiation (counts per unit time).

Answer: No, they are stopped by the outer layer of dead skin cells.

Answer: It gets ionised.

Answer: A thin sheet of aluminium.

Answer: Because it can ionise and damage important biological molecules like DNA.

Answer: Beta.

Answer: A cascade of electrons created from a single ionising event.

Answer: Gamma.

Answer: Kill them.
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