19 past paper MCQs on Spontaneous and Random Nuclear Decay, from the Nuclear Physics unit of MDCAT Physics. The year each question appeared is shown where known; tap “Show answer” for the correct option and a short solution.
1.It has been observed that thorium $^{234}_{90}\mathrm{Th}$ is transformed into protactinium $^{234}_{91}\mathrm{Pa}$ after the emission of a ______ particle. (MDCAT 2022)
Correct answer: (b) beta
The mass number is unchanged and the atomic number rises by one, which is beta-minus decay.
2.Which of the following has the maximum ionizing power? (UHS MDCAT 2017)
Correct answer: (a) $\alpha$
The alpha particle is the heaviest and carries two units of charge, so it ionizes most strongly and is stopped soonest.
3.Stability of radioactivity element is determined by: (NUMS MDCAT 2025)
Correct answer: (a) Decay constant
The decay constant is the probability of decay per second, so the smaller it is the more stable the nuclide.
4.The unit of decay constant is ; (UHS MDCAT 2024)
Correct answer: (c) s-1
The decay constant is a probability per second, so its unit is the reciprocal second.
5.An artificial radioactive element can be made by bombarding (UHS MDCAT 2024)
Correct answer: (c) High energy particles on stable elements
A stable nucleus struck by a fast particle is left in an unstable form that then decays.
6.Q.175 It has been observed that Thorium (_90^234)Th is transformed into Protactinium (_91^234)Pa after the emission of ______________ particle: (UHS MDCAT 2023)
Correct answer: (b) Beta
The mass number is unchanged and the atomic number rises by one, which is beta emission.
7.Q.176 The half-life of Strontium (Sr) is 8.70 hours. Its decay constant is: (UHS MDCAT 2023)
Correct answer: (c) 0.000022 / s
$\lambda=0.693/t_{1/2}=0.693/(8.70\times3600)$, about $2.2\times10^{-5}\,\mathrm{s^{-1}}$.
8.The decay rate of radioactive substance is: (UHS MDCAT 2023)
Correct answer: (c) Decreases exponentially with time
The activity is proportional to the number left, and that falls exponentially.
9.When gamma photon is entered in nucleus it_ _ : (PMC MDCAT 2022)
Correct answer: (b) excite the nucleus
A gamma photon absorbed by a nucleus lifts it to a higher energy state.
10.C-14 decays by emitting a beta particle. After decay new element becomes: (PMC MDCAT 2022)
Correct answer: (b) N-14
Beta emission leaves the mass number alone and raises the atomic number by one, giving nitrogen-14.
11.Which radiation will be deflected more by magnetic fleld? (PMC MDCAT 2022)
Correct answer: (b) β-radiation
A beta particle is far lighter than an alpha, so the same force bends it much more.
12.Three types of radioactive elements are emitted when unstable nuclei undergo radioactive decay. Which of the following is not one of them? (PMC Practice 2021)
Correct answer: (d) Delta
Radioactive decay gives alpha, beta and gamma radiation; there is no delta radiation.
13.When the nucleus of an unstable atom emits only gamma radiation, the nucleus must: (PMC Practice 2021)
Correct answer: (b) Lose energy
A gamma photon carries energy away and leaves the same nucleus in a lower state.
14.Which of the following is a radioactive substance? (PMC Practice 2021)
Correct answer: (d) All of the above options are correct
Uranium, radium and thorium are all radioactive.
15.In relation, which quantity is represented by λ? (UHS MDCAT 2019)
Correct answer: (a) Decay constant
$\lambda$ is the decay constant, the probability of decay per second.
16.Calculate the activity (decaying atom per unit time) of radioactive strontium – 90 having 6.7 x 1021 atoms at t = 0. Decay constant of strontium – 90 is 8.3 x 10-10 s-1 (UHS MDCAT 2018)
Correct answer: (c) 5.6 x 1012Bq
$A=\lambda N=8.3\times10^{-10}\times6.7\times10^{21}$, about $5.6\times10^{12}\,\mathrm{Bq}$.
17.Which of the following has maximum ionizing power? (UHS MDCAT 2017)
Correct answer: (a) α
The alpha particle is heavy and doubly charged, so it ionizes most strongly.
18.Thorium is transformed after the transmission of β-particle into: (UHS MDCAT 2016)
Correct answer: (b) Protactinium
Beta emission raises the atomic number by one, taking thorium to protactinium.
19.Emission of γ-rays from radioactive element results into: (UHS MDCAT 2016)
Correct answer: (c) Polonium
Gamma emission changes neither the mass number nor the atomic number, so the element is unchanged in kind.