Nuclear & Radioactivity — Set 4

Correct Answer: B. A sheet of paper

• **A sheet of paper** = Alpha particles have very low penetrating power due to their large charge (+2e) and mass; they are stopped by a single sheet of paper or by just a few centimetres of air. • **Range ≈ 3–7 cm in air** — Their intense ionisation causes rapid energy loss; even the outer dead layer of human skin blocks external alpha particles, though internal alpha sources are very hazardous. • 💡 Wrong-option analysis: A vacuum only: a vacuum contains no stopping material at all — alpha particles would travel much farther in vacuum than in any material; A steel wall: steel would certainly stop alpha particles, but it is gross overkill; a sheet of paper suffices and is the standard answer; A thick lead block: thick lead is needed for gamma radiation, not alpha; using it for alpha particles misunderstands the shielding requirements for each radiation type.

Correct Answer: C. A thin sheet of aluminum

• **A thin sheet of aluminum** = Beta particles (fast electrons) have intermediate penetrating power; they are effectively stopped by a few millimetres of aluminium or similar low-Z metal. • **Range ~1–10 mm in Al** — A thin aluminium plate (e.g. 3 mm) absorbs beta particles from most common radioisotopes; choosing lead over aluminium for beta shielding can actually increase bremsstrahlung X-ray production. • 💡 Wrong-option analysis: A thin plastic film only: a thin plastic film may stop very low-energy beta but is insufficient for high-energy beta emitters; A vacuum only: vacuum provides no shielding — beta particles travel even farther in vacuum than in air; A thin paper sheet: paper stops alpha particles but is inadequate for beta, which can penetrate several mm of aluminium.

Correct Answer: B. Thick lead or thick concrete

• **Thick lead or thick concrete** = Gamma rays require dense, thick shielding because they are penetrating electromagnetic photons; several centimetres of lead or metres of concrete are needed to reduce intensity to safe levels. • **HVL of lead for 1 MeV gamma ≈ 1.3 cm** — The half-value layer is the thickness that reduces intensity by half; nuclear reactors use up to 1.5 m of concrete or 15 cm of lead for gamma shielding. • 💡 Wrong-option analysis: A sheet of paper: paper stops alpha particles but does virtually nothing to attenuate gamma rays; A rubber band: rubber is a low-density organic material — it provides essentially no gamma shielding; A thin cloth: cloth is even less effective than paper for gamma — it offers negligible attenuation of high-energy photons.

Correct Answer: D. +2e

• **+2e** = An alpha particle is a helium-4 nucleus consisting of 2 protons and 2 neutrons; the two protons give it a charge of +2 elementary charges = +2e = +3.2 × 10⁻¹⁹ C. • **+2e = 3.2 × 10⁻¹⁹ C** — This positive charge causes alpha particles to deflect toward the negative plate in an electric field and explains their strong ionising effect on surrounding matter. • 💡 Wrong-option analysis: −2e: a charge of −2e would make it resemble a double-negative ion, but alpha particles are positively charged nuclei; −1e: −1e is the charge of a beta-minus particle (electron), not an alpha particle; 0: zero charge describes gamma photons, not alpha particles which are heavily charged.

Correct Answer: C. -1e

• **−1e** = A beta-minus particle is an electron emitted from the nucleus; it carries a charge of −1e = −1.6 × 10⁻¹⁹ C, the same as a free electron. • **−1.6 × 10⁻¹⁹ C** — In beta-minus decay, a neutron converts to a proton (atomic number Z increases by 1), with the electron and antineutrino being emitted; the small mass makes beta particles deflect strongly in magnetic fields. • 💡 Wrong-option analysis: +1e: +1e is the charge of a beta-plus particle (positron) or a proton — opposite to beta-minus; 0: zero charge describes gamma photons and neutrons, not beta particles which are clearly deflected by electric fields; +2e: +2e is the charge of an alpha particle (helium nucleus), not a beta-minus particle.

Correct Answer: D. Photons with no mass and no charge

• **Photons with no mass and no charge** = Gamma radiation consists of high-energy photons — packets of electromagnetic energy with zero rest mass and zero charge, travelling at the speed of light. • **E = hf, typically 0.1–10 MeV** — Gamma photons are emitted by excited nuclei transitioning to lower energy states; they have the shortest wavelengths in the electromagnetic spectrum (< 10 pm). • 💡 Wrong-option analysis: Electrons only: electrons describe beta-minus radiation; gamma photons are fundamentally different — they have no mass and no charge; Helium nuclei only: helium nuclei describe alpha particles, which are massive and carry +2 charge — very different from massless gamma photons; Neutrons only: free neutrons are massive, neutral particles emitted in fission reactions, not the same as gamma photons.

Correct Answer: C. Ionization of gas leading to an electric pulse

• **Ionization of gas leading to an electric pulse** = In a GM counter, incoming radiation ionises the filling gas (usually argon + halogen); the freed electrons avalanche toward the anode under a high voltage, producing a detectable electrical pulse. • **Operating voltage ~300–900 V** — Each radiation event produces a single pulse counted electronically; the GM counter is most efficient for beta and gamma, and less efficient for alpha due to the thin window needed. • 💡 Wrong-option analysis: Measuring mass change only: mass change occurs in nuclear reactions at the atomic level — no ordinary counter can measure such tiny mass changes directly; Producing nuclear fuel: a GM counter is a passive detection instrument; it produces no fuel and has no nuclear reaction; Measuring temperature change only: temperature change is used in bolometric detectors, not in the ion-current-based GM counter mechanism.

Correct Answer: B. Condensation of vapor on ions

• **Condensation of vapor on ions** = As a charged particle traverses a cloud chamber (supersaturated vapour of water or alcohol), it ionises the gas along its path; these ions act as condensation nuclei, forming a trail of tiny liquid droplets that becomes visible. • **Wilson cloud chamber (1911)** — C.T.R. Wilson invented this device; alpha tracks appear as thick, straight lines while beta tracks are thin and curved; the curvature in a magnetic field reveals charge and momentum. • 💡 Wrong-option analysis: Sound production by particles: nuclear particles do not produce audible sound in cloud chambers — that effect is used in acoustic detectors, not cloud chambers; Chemical burning of air: the vapour in a cloud chamber condenses by phase transition, not by combustion — no burning occurs; Magnetic levitation: magnetic levitation is used in Maglev trains and some bearing systems — it has no role in making particle tracks visible.

Correct Answer: D. Cobalt-60

• **Cobalt-60** = Cobalt-60 is a widely used gamma-emitting radioisotope in cancer radiotherapy; it emits two gamma photons of 1.17 MeV and 1.33 MeV, which penetrate tissue deeply enough to treat deep-seated tumours. • **T₁/₂ (Co-60) = 5.27 years** — A cobalt-60 teletherapy unit (the 'cobalt machine') was the standard before linear accelerators; sources must be replaced every few years as activity decreases. • 💡 Wrong-option analysis: Helium gas: helium is a stable noble gas and does not emit gamma rays; it is used in cryogenics and balloons; Copper: copper is a stable metal with no therapeutic gamma-emitting isotope in clinical use; Sodium chloride: NaCl is ordinary table salt — it is chemically stable and emits no significant gamma radiation.

Correct Answer: A. Technetium-99m

• **Technetium-99m** = Technetium-99m (Tc-99m) is the most widely used diagnostic radioisotope; it emits a 140 keV gamma ray ideal for gamma camera imaging and has a short half-life of ~6 hours, minimising patient dose. • **T₁/₂ = 6 hours, 140 keV gamma** — The 'm' stands for metastable; Tc-99m decays to Tc-99 by isomeric transition; it is produced from molybdenum-99 generators installed in hospitals. • 💡 Wrong-option analysis: Oxygen-16: oxygen-16 is the most abundant stable oxygen isotope — it is not radioactive and emits no radiation useful for imaging; Iron-56: iron-56 is the most stable of all nuclides with the highest binding energy per nucleon — it is not a radioactive tracer; Carbon-12: carbon-12 is the stable reference isotope for atomic mass — it is not radioactive.