Free NEET Physics multiple-choice questions on Radioactivity & Decay Laws. Attempt each question and reveal the answer with a full explanation.
The half-life of a radioactive substance is 20 s. The time taken for the sample to decay by 7/8 th of its initial value is: 60 s 40 s 80 s 20 s The probability that a radioactive nucleus will decay in a time interval equal to its half-life is: 50 % 100 % 37 % 63 % Which of the following is not emitted by a radioactive substance? Protons Electrons Helium nuclei Electromagnetic waves A radioactive sample has an initial activity of 10 Ci . If the half-life is 2 days, the activity after 6 days will be: 1.25 Ci 2.50 Ci 5.00 Ci 3.33 Ci Which of the following is the correct relation between decay constant and mean life ? = 1/ = 0.693/ = = e 2 / The half-life of a radioactive substance is 20 minutes. In how much time will the activity of the substance drop to 1 16 th of its initial value? 80 minutes 60 minutes 40 minutes 20 minutes Which of the following particles has the highest penetrating power? -rays -particles -particles Protons During a - decay, the particle emitted along with the electron is: Antineutrino Neutrino Positron Photon In the nuclear reaction 92 235 U + 0 1 n 56 144 Ba + 36 89 Kr + 3x , the particle x is: Neutron Proton Electron Alpha particle In a radioactive material, the fraction of the initial amount remaining undecayed after five half-lives is: 1/32 1/16 1/64 1/5 The phenomenon of radioactivity was discovered by: Henri Becquerel Marie Curie Ernest Rutherford J.J. Thomson A radioactive nucleus Z A X decays by emitting one -particle and two - particles. The daughter nucleus formed is an: Isotope of the parent nucleus Isobar of the parent nucleus Isotone of the parent nucleus Isomer of the parent nucleus The half-life of a radioactive isotope is 5 hours . What fraction of the original activity will remain after 15 hours ? 1/8 1/3 1/4 1/16 A radioactive sample has an activity of A 0 at t=0 . If its activity becomes A 0/e after time t 1 , then t 1 is equal to: The mean life The half-life Twice the half-life The decay constant When a nucleus Z A X undergoes + decay, the daughter nucleus is represented as: Z-1 A Y Z+1 A Y Z A-1 Y Z-2 A-4 Y A radioactive material has an initial mass of 16 g . After 120 days , the mass remaining is 1 g . The half-life of the material is: 30 days 40 days 60 days 15 days In a nuclear reactor, the purpose of control rods is to: Absorb excess neutrons to control the reaction rate. Slow down fast neutrons. Cool the reactor core. Transfer heat to the turbine. The variation of decay rate ( R = -dN/dt ) with the number of active nuclei ( N ) is correctly shown by which graph? A straight line passing through the origin A rectangular hyperbola A parabola A decreasing exponential curve A radioactive sample has an activity of 100 mCi . If its half-life is 5 days, what will be its activity after 20 days? 6.25 mCi 12.5 mCi 25 mCi 3.125 mCi Which of the following particles is used as a projectile to start a nuclear fission reaction in 235 U ? Thermal neutron Proton Alpha particle Fast electron For a radioactive sample, the count rate changes from 1600 counts/s to 100 counts/s in 80 minutes . The half-life of the sample is: 20 minutes 10 minutes 40 minutes 80 minutes The plot of the number of undecayed nuclei N versus time t for a radioactive sample is: Exponentially decaying curve Straight line with negative slope Parabola opening upwards Hyperbola A radioactive substance has an initial mass of N 0 . The amount of substance remaining undecayed after a time interval of 3 mean lives ( t = 3 ) is: N 0/e 3 N 0/3e 3N 0/e N 0/e 2 In a nuclear reaction 2 4 He + 7 14 N 8 17 O + Z A X , the nucleus X is: A proton A neutron An electron A positron In the nuclear reaction 13 27 Al + 2 4 He 15 30 P + X , the particle X is: A neutron A proton An electron A positron A radioactive sample has 10 6 active nuclei. If its half-life is 10 seconds , how many nuclei will remain undecayed after 30 seconds ? 1.25 10 5 2.5 10 5 5 10 5 0.33 10 6 A nucleus Z A X emits one - particle. The resulting nucleus is an: Isobar of X Isotope of X Isotone of X Isomer of X For a radioactive material, the half-life is 10 minutes. If initially there are 600 number of nuclei, the time taken (in minutes) for the disintegration of 450 nuclei is: 20 10 30 15 In a radioactive decay, a nucleus Z A X emits one particle and then one - particle. The final nucleus formed is: Z-1 A-4 Y Z-2 A-4 Y Z-3 A-4 Y Z A-4 Y The activity of a radioactive sample is 1.6 10 -3 Ci . If its half-life is 1 day, its activity after 4 days will be: 10 -4 Ci 10 -3 Ci 4 10 -4 Ci 0.8 10 -3 Ci A radioactive substance has a half-life of T . The time taken for the substance to decay by 75 % and 50 % are t 1 and t 2 respectively. The ratio t 1/t 2 is: 2 1.5 0.5 3 In a nuclear reactor, the material used to slow down the fast-moving neutrons without capturing them is: Moderator Control rods Coolant Shielding A radioactive nucleus Z A X undergoes spontaneous decay in the sequence Z A X Z-1 A Y Z-3 A-4 B Z-2 A-4 C , where Z is the atomic number of element X . The possible decay particles in the sequence are: +, , - -, , + , -, + +, -, The half-life of a radioactive sample is 10 days . The time required for 1/4 th of the sample to remain undecayed is: 20 days 30 days 40 days 5 days The half-life of a radioactive isotope X is 50 years. It decays to another element Y which is stable. The two elements X and Y were found to be in the ratio of 1:15 in a sample of a given rock. The age of the rock was estimated to be: 200 years 150 years 100 years 250 years A radioactive sample has a half-life of 10 days. The fraction of the sample that will decay in 30 days is: 7/8 1/8 1/4 3/4 In the process of + (positron) decay, which of the following particles is also emitted to conserve lepton number and energy? Neutrino Antineutrino Photon Neutron The mean life of a radioactive sample is . The time required for 75 % of the sample to decay is: 4 2 2 / 2 The probability of a radioactive nucleus not decaying during one mean life is: 1/e 1 - 1/e 2 1/2 For a radioactive substance, a plot of the logarithm of activity ( A ) versus time ( t ) results in a straight line. The magnitude of the slope of this line represents: The decay constant The half-life The mean life The initial activity In the thorium decay series, 90 232 Th disintegrates into 82 208 Pb . The number of and - particles emitted in the process are respectively: 6 , 4 - 4 , 1 - 6 , 0 - 5 , 2 - A radioactive isotope has a decay constant . If it is being produced in a reactor at a constant rate P , the number of nuclei N after a long time will be: P/ /P P P e - In a radioactive decay process, the activity R of a sample is found to reduce from 800 Bq to 100 Bq in 6 hours . The decay constant is: 2 2 hr -1 2 6 hr -1 2 hr -1 2 2 hr -1 A radioactive sample decays by two different processes with half-lives T 1 and T 2 respectively. The effective half-life T eff is given by: T 1 T 2 T 1 + T 2 T 1 + T 2 T 1 + T 2 2 T 1 T 2 A radioactive substance has a half-life of 60 minutes. After 3 hours, the fraction of the substance that has decayed is: 7/8 1/8 5/6 1/6 The half-life of a radioactive substance is T . The time taken for the activity to reduce to 3.125 % of its initial value is: 5T 4T 6T 8T A radioactive sample has N 0 active nuclei at t=0 . The number of nuclei that decay in the time interval between t=T 1/2 and t=2T 1/2 is: N 0/4 N 0/2 3N 0/4 N 0/8 The half-life of a radioactive substance is T . The probability of decay of a nucleus in time 2T is: 3/4 1/4 1/2 1 In - decay, the energy of the emitted electrons follows a continuous spectrum because of the simultaneous emission of: Antineutrinos Neutrinos Photons Positrons A radioactive sample has two types of nuclei A and B . At t=0 , the number of nuclei of A is twice that of B . The half-life of A is 2 hours and the half-life of B is 1 hour . After how many hours will the number of nuclei of A and B be equal? 2 hours 4 hours 1 hour log 2 hours The probability of a radioactive nucleus surviving (not decaying) after a time interval equal to two mean lives ( t = 2 ) is: 1/e 2 1/2e 1 - 1/e 2 1/e If the half-life of a radioactive substance is T , then the activity after time t = 3.5T is what fraction of the initial activity? 1/(8 2 ) 1/8 1/16 2 /8 The half-life of a radioactive nuclide is 100 hours. The fraction of original activity that will remain after 150 hours would be: 1 2 2 2 3 1 2 1 4 In a - decay, the energy spectrum of the emitted electrons is continuous because: Energy is shared between the electron and an antineutrino. The nucleus has a continuous set of energy levels. Electrons lose energy by collisions before exiting the atom. The mass of the electron is not constant. A radioactive sample has two types of nuclei A and B . Initially, there are N 0 nuclei of each. The decay constant of A is 3 and that of B is . After what time will the ratio of the number of nuclei of A to those of B be 1/e 2 ? 1/ 2/ 1/2 2 / If 10 grams of a radioactive material with a half-life of 5 days is kept for 20 days, the amount of undecayed material left is: 0.625 g 1.25 g 2.5 g 5.0 g A radioactive sample has a half-life of T . If the initial activity is A 0 , the activity after time t = 2.5T is: A 0 / (4 2 ) A 0 / 4 A 0 / 8 A 0 / (2 2 ) The activity of a radioactive sample is measured as N 0 counts per minute at t=0 and N 0/e counts per minute at t=10 minutes. The time (in minutes) at which the activity reduces to half of its initial value is: 10 ln 2 10 ln 2 10 log 10 2 5 ln 2 In a + decay, the mass number A and atomic number Z change as: A remains same, Z decreases by 1 A remains same, Z increases by 1 A decreases by 4, Z decreases by 2 A increases by 1, Z remains same A nucleus with Z=92 emits the following particles in succession: , -, -, , , . The atomic number of the resulting nucleus is: 84 82 86 88 A radioactive sample has two isotopes X and Y with half-lives of 3 hours and 6 hours respectively. Initially, the sample contains equal numbers of nuclei of both. The ratio of the number of nuclei of X to Y after 12 hours is: 1:4 1:2 4:1 1:8 Which of the following graphs correctly shows the variation of (N/N 0) with time t for a radioactive decay? A straight line with negative slope A straight line with positive slope An exponential decay curve A parabola opening upwards A radioactive substance has a mean life . What fraction of the substance decays in a time interval equal to one mean life? (e-1)/e 1/e 1/2 (e-1)/2 The half-life of a radioactive sample is T . The time taken for the activity of the sample to decrease to 10 % of its initial value is approximately: 3.32 T 2.30 T 4.60 T 1.44 T For a radioactive material, half-life is T . In time T/2 , the amount of substance decayed is: 1 - (1/ 2 ) of initial amount 1/ 2 of initial amount 1/4 of initial amount 1/2 of initial amount A radioactive substance X decays into another radioactive substance Y . At t=0 , only X is present. The decay constants are x and y respectively. The number of nuclei of Y will be maximum when: x N x = y N y x = y N x = N y x N y = y N x The half-life of a radioactive material is T . Its average life is: 1.44 T 0.693 T T 2 T Two radioactive materials X 1 and X 2 have decay constants 10 and respectively. If initially they have the same number of nuclei, then the ratio of the number of nuclei of X 1 to that of X 2 will be 1/e after a time: 1/(9 ) 1/(11 ) 11/(10 ) 1/(10 ) In an -decay of a stationary nucleus Z A X , if the total energy released is Q , the kinetic energy of the daughter nucleus is approximately: 4Q/A Q/A (A-4)Q/A 4Q/(A-4) A radioactive nucleus emits 3 and 2 + particles. The ratio of number of neutrons to number of protons in the daughter nucleus as compared to the parent nucleus 92 238 U will: Increase Decrease Remain same Initially increase then decrease If a radioactive nucleus undergoes decay, the kinetic energy of the particle is K . If Q is the total energy released and A is the mass number of the parent nucleus, then: K = Q A-4 A K = Q 4 A K = Q A A-4 K = Q A-4 4 In a nuclear reaction Z A X Z+1 A Y + e - + , the particle is called: Antineutrino Neutrino Positron Photon A substance has a decay constant of . Its mean life is: 1/ (2)/ (2)