Radioactivity — ICSE Class 10 Physics Important Questions

Answer: (c) a helium nucleus.

An alpha particle is a helium nucleus $_2^4\text{He}$_2^4He, consisting of 2 protons and 2 neutrons, and carries a charge of $+2e$+2e.

Answer: (c) gamma.

Gamma rays are high-energy electromagnetic waves with no charge and no mass, so they penetrate deeply and are stopped only by thick lead or concrete. Alpha is the least penetrating.

Answer: (b) mass number $A$A, atomic number $Z+1$Z+1.

In beta decay a neutron converts to a proton and an emitted electron: $_Z^A X\rightarrow{}_{Z+1}^{A}Y+{}_{-1}^{0}e$_Z^A X→_Z+1^AY+_-1^0e. The mass number is unchanged while the atomic number rises by one.

Answer: (a) the same.

Alpha decay lowers $Z$Z by 2; each beta decay raises $Z$Z by 1, so two beta decays raise $Z$Z by 2. Net change $=-2+2=0$=-2+2=0, so the atomic number is unchanged (the mass number, however, decreases by 4).

Answer: (a) Gamma rays are undeflected by electric and magnetic fields, and this is precisely because they are chargeless electromagnetic waves — only charged particles are deflected by such fields. R correctly explains A.

1. An alpha particle is a helium nucleus $_2^4\text{He}$_2^4He with charge $+2e$+2e; a beta particle is a fast-moving electron $_{-1}^{0}e$_-1^0e with charge $-1e$-1e.

2. An alpha particle is much heavier (mass $\approx4\text{ u}$4 u) and less penetrating but highly ionising; a beta particle is very light, more penetrating and less ionising than alpha.

Background radiation is the weak nuclear radiation that is always present in our surroundings, coming from natural radioactive sources, to which all living things are constantly exposed.

Source (any one): cosmic rays from outer space, radioactive elements in rocks and soil (such as uranium, thorium, radon gas), or radioactive materials in building materials and food.

(i) Nature and charge:

• Alpha ($\alpha$): helium nucleus $_2^4\text{He}$_2^4He, charge $+2$+2.
• Beta ($\beta$): fast electron $_{-1}^{0}e$_-1^0e, charge $-1$-1.
• Gamma ($\gamma$): electromagnetic wave, no charge, no mass.

(ii) Penetrating power:

• Alpha: least penetrating (stopped by paper).
• Beta: moderate (stopped by a few mm of aluminium).
• Gamma: most penetrating (needs thick lead/concrete).

(iii) In an electric field:

• Alpha (positive) is deflected towards the negative plate.
• Beta (negative) is deflected towards the positive plate, and more than alpha because it is much lighter.
• Gamma (no charge) is not deflected.

Conservation of mass number and atomic number fixes the missing particle.

(a) Mass number: $226-222=4$226-222=4; atomic number: $88-86=2$88-86=2. So the particle is $_2^4\text{He}$_2^4He, an alpha particle.
$_{88}^{226}\text{Ra}\rightarrow{}_{86}^{222}\text{Rn}+{}_2^4\text{He}.$_88^226Ra→_86^222Rn+_2^4He.

(b) Mass number: $14-14=0$14-14=0; atomic number: $7-6=+1$7-6=+1 for the daughter means the emitted particle has charge $-1$-1. So the particle is $_{-1}^{0}e$_-1^0e, a beta particle.
$_{6}^{14}\text{C}\rightarrow{}_{7}^{14}\text{N}+{}_{-1}^{0}e.$_6^14C→_7^14N+_-1^0e.

Radioactivity is a nuclear change because:

1. It originates in the nucleus of the atom (emission of $\alpha$, $\beta$ or $\gamma$), whereas chemical changes involve only the outer electrons.

2. It is spontaneous and unaffected by external conditions such as temperature, pressure or chemical combination, unlike chemical reactions whose rates depend on these factors.

3. It can change one element into another (transmutation) and releases enormously more energy than any chemical change of the same mass.

(Also, the atomic number and/or mass number of the atom changes, which never happens in a chemical reaction.)

(a) When the mixed beam passes through a magnetic field directed into the page, the three radiations behave differently according to their charge:

• The alpha beam ($+$+ charge, heavy) bends slightly to one side.
• The beta beam ($-$- charge, very light) bends strongly to the opposite side.
• The gamma beam (no charge) passes straight through, undeflected.

(b) The beta radiation is deflected the most. Although alpha carries a larger charge, the beta particle has an extremely small mass, so for a given magnetic force it experiences a much greater acceleration and curves far more than the heavy alpha particle. Gamma, being chargeless, is not deflected at all.

(a) Fission vs fusion:

• Nuclear fission is the splitting of a heavy nucleus into two lighter nuclei with the release of energy, e.g. $_{92}^{235}\text{U}$_92^235U splits when it captures a neutron. It is used in nuclear reactors and atom bombs.
• Nuclear fusion is the combining of two light nuclei into a heavier nucleus with the release of even greater energy, e.g. hydrogen nuclei fusing to form helium in the Sun. It requires extremely high temperature and pressure.

(b) Uses (any two):

1. Radioisotopes are used in medicine to diagnose and treat diseases (e.g. cobalt-60 for cancer, radio-iodine for the thyroid).
2. Used for sterilising surgical instruments, in carbon dating, and to detect leaks/flaws and generate electricity.

Hazards (any two):

1. Radiation damages living cells and tissues, causing radiation sickness and burns.
2. It can cause cancer, genetic mutations and, in large doses, death.

(c) Safety precautions (any two):

1. Handle sources with long tongs/remote handling tools, never with bare hands, and store them in thick lead containers.
2. Wear protective lead aprons and gloves, maintain a safe distance and limit exposure time, and use radiation-monitoring badges.

(i) Carbon-14 decays by emitting a beta particle (a fast electron, $_{-1}^{0}e$_-1^0e).

(ii) In beta decay the mass number stays the same and the atomic number increases by 1:
$_{6}^{14}\text{C}\rightarrow{}_{7}^{14}\text{N}+{}_{-1}^{0}e.$_6^14C→_7^14N+_-1^0e.

(iii) Any one: using cobalt-60 gamma rays to treat cancer (radiotherapy), radio-iodine to diagnose/treat thyroid disorders, or radioisotopes as tracers to study organ function.

(iv) Radioactive sources emit ionising radiation that can damage living cells, cause radiation sickness, cancer or genetic mutations. So they must be handled with tongs, stored in lead containers and used with proper shielding to limit exposure.