JEE Physics Nuclei MCQs Set 15

Answer: (c) Gamma rays

Explanation:
1. Deflection needs an electric charge on the radiation.
2. Alpha particles are positive and beta particles are negative, but gamma rays are uncharged electromagnetic waves, so they go straight.

Teacher's Note:
1. Neutral radiation is not affected by electric or magnetic fields.

Answer: (a) a helium nucleus (two protons and two neutrons)

Explanation:
1. An alpha particle contains 2 protons and 2 neutrons, so its mass number is 4 and its charge is +2e.
2. This is the nucleus of a helium atom, \( {}^{4}_{2}\text{He} \).

Teacher's Note:
1. Alpha particle means doubly ionised helium.

Answer: (d) \( \alpha > \beta > \gamma \)

Explanation:
1. Alpha particles are heavy and doubly charged, so they ionise a gas the most.
2. Beta particles ionise less, and gamma rays, being neutral photons, ionise the least.

Teacher's Note:
1. Ionising power and penetrating power run in opposite orders.

Answer: (d) \( \gamma > \beta > \alpha \)

Explanation:
1. The more strongly a radiation ionises, the sooner it loses its energy.
2. So alpha is stopped quickly (a sheet of paper), beta needs a thin metal sheet, and gamma needs thick lead.

Teacher's Note:
1. Penetration is highest for gamma rays.

Answer: (a) Z = 90, A = 234

Explanation:
1. An alpha particle carries away 2 protons and 2 neutrons: A decreases by 4 and Z by 2.
2. So A = 238 − 4 = 234 and Z = 92 − 2 = 90 (thorium-234).

Teacher's Note:
1. Alpha decay: A − 4, Z − 2.

Answer: (a) Z = 91, A = 234

Explanation:
1. Inside the nucleus a neutron changes into a proton, an electron and an antineutrino.
2. So the mass number stays 234 and Z rises by 1 to 91.

Teacher's Note:
1. Beta-minus decay: A unchanged, Z + 1.

Answer: (d) 8 alpha and 6 beta

Explanation:
1. Fall in mass number = 238 − 206 = 32, so number of alpha particles = 32 ÷ 4 = 8.
2. 8 alpha particles reduce Z by 16, giving 76, but the final Z is 82, so 6 beta particles (each raising Z by 1) are needed.

Teacher's Note:
1. Find alpha from the change in A, then beta from the change in Z.

Answer: (d) isotones

Explanation:
1. Na-23 has 23 − 11 = 12 neutrons and Mg-24 has 24 − 12 = 12 neutrons.
2. Nuclei with different Z and A but the same number of neutrons are isotones.

Teacher's Note:
1. Isotopes: same Z. Isobars: same A. Isotones: same N.

Answer: (c) neither A nor Z changes

Explanation:
1. A gamma ray is a photon with no mass and no charge.
2. The nucleus only drops from an excited state to a lower energy state, so A and Z stay the same.

Teacher's Note:
1. Gamma emission does not change the element.

Answer: (a) 186 MeV

Explanation:
1. E = Δm × 931 MeV per u = 0.20 × 931 MeV.
2. E ≈ 186 MeV.

Teacher's Note:
1. Use \( E = \Delta m\,c^2 \) with the 931 MeV per u conversion.

Answer: (c) \( 9\times10^{16} \) J

Explanation:
1. E = mc² = 1 × (3 × 10⁸)² J.
2. E = \( 9\times10^{16} \) J.

Teacher's Note:
1. Square the speed of light carefully.

Answer: (d) 3

Explanation:
1. Mass number balance: 235 + 1 = 144 + 89 + x, so x = 3.
2. Charge balance: 92 = 56 + 36 confirms the equation.

Teacher's Note:
1. Balance A and Z on both sides.

Answer: (b) absorbing some neutrons with a moderator and control rods such as graphite or heavy water

Explanation:
1. Each fission releases neutrons that cause more fissions.
2. If some of these neutrons are absorbed or slowed down, the reaction proceeds at a steady controlled rate.

Teacher's Note:
1. A reactor uses a controlled chain reaction; a bomb uses an uncontrolled one.

Answer: (d) the nuclei must overcome the electrostatic repulsion between them

Explanation:
1. Light nuclei are positively charged and repel each other strongly.
2. Very high temperature gives them enough kinetic energy to come close enough for the nuclear force to act.

Teacher's Note:
1. Coulomb repulsion is the barrier to fusion.

Answer: (d) the nucleus, when a neutron changes into a proton

Explanation:
1. No electrons exist inside the nucleus as such; in beta-minus decay a neutron converts into a proton, an electron and an antineutrino.
2. The electron is thrown out as the beta particle.

Teacher's Note:
1. Cathode rays come from orbital electrons; beta rays come from the nucleus.

Answer: (d) much larger

Explanation:
1. Fusion of light nuclei releases a larger fraction of the mass as energy per nucleon than fission of heavy nuclei.
2. So per unit mass, fusion gives much more energy.

Teacher's Note:
1. Per nucleon, fusion releases more energy than fission.

Answer: (b) \( {}^{14}_{6}\text{C} \)

Explanation:
1. Carbon-14 is radioactive; living things keep its fraction constant, and after death it decays.
2. Measuring the remaining fraction gives the age.

Teacher's Note:
1. Only carbon-14 is radioactive among these.

Answer: (b) Gamma rays from cobalt-60

Explanation:
1. Gamma rays are highly penetrating and kill germs and cancer cells.
2. Cobalt-60 is a common gamma source.

Teacher's Note:
1. Penetrating gamma rays reach deep tissue.

Answer: (a) they are much lighter (greater charge-to-mass ratio)

Explanation:
1. Deflection depends on the charge-to-mass ratio q/m for particles of similar speed.
2. A beta particle has the same size of charge as a proton but much smaller mass than an alpha particle, so it bends more (in the opposite direction).

Teacher's Note:
1. Lighter charged particles are deflected more.

Answer: (d) the same as the speed of light

Explanation:
1. Gamma rays are electromagnetic waves, like light and X-rays.
2. Every electromagnetic wave travels at \( 3\times10^8 \) m/s in vacuum.

Teacher's Note:
1. Gamma rays differ from light only in wavelength.