**NEET Physics Chapter Wise Mock Test – Radioactivity**

**Question 1.**

The activity of a radioactive sample is measured as 9750 counts/min at t = 0 and as 975 counts/min at t = 5 min. The decay constant is approximately

(a) 0.922/min

(b) 0.691/min

(c) 0.461/min

(d) 0.230/min

**Question 2.**

Samples of two radioactive nuclides A and B are taken. X_{A} and X_{B} are the disintegration constants of A and B, respectively. In which of the following cases, the two samples can simultaneously have the same decay rate at any time?

(a) Initial rate of decay of A is twice the initial rate of decay of 6 and X_{A} = X_{B}

(b) Initial rate of decay of A is twice the initial rate of decay of 6 and X_{A} > X_{B}

(c) Initial rate of decay of 6 is twice the initial rate of decay of A and X_{A} > X_{B}

(d) Initial rate of decay of B is always same

**Question 3.**

If a radioactive substance decays for time interval equal to its mean life, then the fraction of the substance remaining undecayed, will be

(a) 1/e

(b) 1/e^{2}

(c) e^{2}

(d) e

**Question 4.**

The variation of decay rate of two radioactive samples A and B with time is shown in figure.

Which of the following statements is/are true?

(a) Decay constant of A is greater than that of B, hence A always decays faster than B

(b) Decay constant of B is greater than that of A but its decay rate is always smaller than that of A

(c) Decay constant of A is greater than that of B but it does not always decay faster than B

(d) Decay constant of B is same as that of A but still its decay rate becomes equal to that of A at a later instant

**Question 5.**

Two Cu^{64} nuclei touch each other. The electrostatic repulsive energy of the system will be

(a) 0.788 MeV

(b) 7.88 MeV

(c) 126.15 MeV

(d) 788 MeV

**Question 6.**

The graph of log(R/R_{0}) versus A (R=radius of a nucleus and A=mass number) is

(a) a circle

(b) an ellipse

(c) a parabola

(d) a straight line

**Question 7.**

Which stable nucleus has radius one third of that of Os^{189}?

(a) Be^{9}

(b) Li^{7}

(c) F^{19}

(d) None of these

**Question 8.**

If the nuclear radius of ^{27}Al is 3.6 fm, the approximate nuclear radius of ^{27}Cu in Fermi is

(a) 2.4

(b) 1.2

(c) 4.8

(d) 3.6

**Question 9.**

The half-life of a radioactive material is 3h. If the initial amount is 300g, then after 18h, it will remain

(a) 4.68 g

(b) 46.8 g

(c) 9.375 g

(d) 93.75 g

**Question 10.**

The half-life of a radioactive isotope X is 50 yr. 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

(a) 200 yr

(b) 250 yr

(c) 100 yr

(d) 150 yr

**Question 11.**

A mixture consists of two radioactive materials A_{1} and A_{2} with half-lives of 20s and 10s, respectively. Initially, the mixture has 40 g of A_{1} and 160 g of A_{2}. The amount of the two in the mixture will become equal after

(a) 60s

(b) 80s

(c) 20s

(d) 40s

**Question 12.**

The half-life of radioactive element is 600 yr. The fraction of sample that would remain after 3000 yr, is

(a) 1/2

(b) 1/16

(c) 1/8

(d) 1/32

**Question 13.**

If 10% of a radioactive substance decays in every 5 yr, then the percentage of the substance that will have decayed in 20 yr, will be

(a) 40%

(b) 50%

(c) 65.6%

(d) 34.4%

**Question 14.**

A radioactive sample has an initial activity of 50 dpm, 20 min later, the activity is 25 dpm. How many atoms of the radioactive nuclide were there originally?

(a) 20

(b) 80

(c) 1443

(d) 5441

**Question 15.**

The activity of a radioactive sample is measured as Ne counts per minute at t=0 and N_{0}/e counts per minute at t=5 min. The time (in minute) at which the activity reduces to half its value, is

(a) log_{e} 2/5

(b) 5/log_{e} 2

(c) 5 log_{10 }2

(d) 5log_{e }2

**Question 16.**

Half-life of a radioactive substance is 20 min. The time between 20% and 80% decay will be

(a) 20 min

(b) 40 min

(c) 80 min

(d) 60 min

**Question 17.**

**Question 18.**

If mass of proton = 1.008 amu and mass of neutron = 1.009 amu, then the binding energy per nucleon for _{4}Be^{9} (mass = 9.012 amu) will be

(a) 0.0672 MeV

(b) 0.672 MeV

(c) 6.72 MeV

(d) 67.2 MeV

**Question 19.**

The mass of _{18}Ar^{40} is 39.9480 amu. Its mass defect will be (given that, m_{p} = 1.0078 amu and m_{n}=1.0087 amu)

(a) 0.3694

(b) 0.3318

(c) 0.3480

(d) 0.3838

**Question 20.**

The binding energy of neutron in deuterium _{1}H^{2} will be (given that, m_{p}= 1.0078 amu, m_{n}= 1.0087 amu and m_{d} = 2.0141 amu)

(a) 2.2344 MeV

(b) 4.4688 MeV

(c) 1.1172 MeV

(d) 7.8 MeV

**Question 21.**

**Question 22.**

**Question 23.**

If the mass defect in a fusion process is 0.3%, then the energy released in the fusion of 1kg of material will be

(a) 2.7 x 10^{14} J

(b)2.7x 10^{-14} J

(c)2.7x 10^{14} erg

(d)2.7x 10^{14} eV

**Question 24.**

The binding energy of deuteron is 2.2 MeV and that of ^{4}_{2}He, is 28 MeV. If two deuterons are fused to form one ^{4}_{2}He, then the energy released is

(a) 25.8 MeV

(b) 23.6 MeV

(c) 19.2 MeV

(d) 30.2 MeV

**Question 25.**

Heavy stable nuclei have more neutrons than protons. This is because of the fact, that

(a) neutrons are heavier than protons

(b) electrostatic force between protons is repulsive

(c) neutrons decay into protons through β-decay

(d) nuclear forces between neutrons is weaker than that between protons

**Question 26.**

If 200 MeV energy is obtained per fission of _{92}U^{235}, then the number of fissions per second to produce 1kW power will be

(a) 1.25×10^{18}

(b) 3.2×10^{-8}

(c) 3.125×10^{13}

(d) 0.125×10^{13}

**Question 27.**

If all the atoms of 1kg of deuterium undergo nuclear fusion, then the amount of energy released will be

(a) 6×10^{27} cal

(b) 2×10^{7} kWh

(c) 56.9×10^{13} J

(d) 8×10^{23} MeV

**Question 28.**

On fission of one nucleus of U^{235}, the amount of energy obtained is 200 MeV. The power obtained in a reactor is 1000 kW. Number of nuclei fissioned per second in the reactor, is

(a) 9.4×10^{16} s^{-1}

(b) 2.3×10^{8} s^{-1}

(c) 3.125×10^{16} s^{-1}

(d) 4.2×10^{8} s^{-1}

**Question 29.**

The mass of uranium required per day to generate 1MW power from the fission of _{92}U^{235}, will be

(a) 1.05g

(b) 2.05g

(c) 3.05g

(d) 4.05g

**Question 30.**

10^{14} fissions per second are taking place in a nuclear reactor having efficiency 40%. The energy released per fission is 250 MeV. The power output of the reactor is

(a) 2000 W

(b) 4000 W

(c) 1600 W

(d) 3200 W

**Question 31.**

An electron of 1.02 MeV and a positron of 1.02 MeV collide and annihilate into energy producing two γ-photons. The energy of each γ-photon will be

(a) 1.02 MeV

(b) 2.04 MeV

(c) 0.51 MeV

(d) 1.53 MeV

**Question 32.**

A nuclear explosive is designed to deliver 1 MW power in the form of heat energy. If the explosion is designed with a nuclear fuel consisting of U^{235} to run a reactor at this power level for one year, then the amount of fuel needed is (Given energy per fission is 200 MeV)

(a) 1 kg

(b) 0.01 kg

(c) 3.84 kg

(d) 0.384 kg

**Question 33.**

When a nucleus in an atom undergoes a radioactive decay, the electronic energy levels of the atom

(a) do not change for any type of radioactivity

(b) change fora and β-radioactivity but not for γ-radioactivity

(c) change for α-radioactivity but not for others .

(d) change for β-radioactivity but not for others

**Question 34.**

**Question 35.**

**Question 36.**

A neutron causes fission in _{92}U^{235} producing _{40}Zr^{97} and Te^{134} and some neutrons. The atomic number of Te will be

(a) 50

(b) 51

(c) 52

(d) 53

**Question 37.**

A uranium reactor _{92}U^{235} takes 30 days to use up 2 kg of fuel, each fission gives 185 MeV of usable energy, then power output is

(a) 32.23 MW

(b) 22.28 MW

(c) 58.46 MW

(d) None of these

**Direction (Q.Nos. 38-42):** In each of the following questions, a statement of Assertion is given followed by a corresponding statement of Reason just below it. Of the statements mark the correct answer as

(a) If both Assertion and Reason are true and the Reason ‘ is the correct explanation of the Assertion

(b) If both Assertion and Reason are true but the Reason is not correct explanation of the Assertion

(c) If Assertion is true but Reason is false

(d) If both Assertion and Reason are false

**Question 38.**

**Assertion(A):** On α-decay, daughter nucleus shifts two places to the left from the parent nucleus.

**Reason(R):** An α-particle carries four units of mass.

**Question 39.**

**Assertion (A):** ^{90}Sr from the radioactive fall out from a nuclear bomb ends up in the bones of human beings through the milk consumed by them. It causes impairment of the production of red blood cells.

**Reason (R):** The energetic β-particles emitted in the decay of ^{90}Sr damage the bone marrow.

**Question 40.**

**Question 41.**

**Assertion (A):** the ratio of time taken for light emission from an atom to that for release of nuclear energy in fission is 1:100.

**Reason (R):** Time taken of the light emission from an atom is of the order of 10^{-8} s.

**Question 42.**

**Assertion (A):** The mass of β-particles when they are emitted, is higher than the mass of electron obtained by other means.

**Reason (R):** β-particle and electron, both are similar particles.

**Question 43.**

**Question 44.**

A radio isotope X with a half life 1.4 x 10^{9} years decays to Y which is stable. A sample of the rock from a cave was found to contain X and Y in the ratio 1 : 7. The age of the rock is

(a) 1.96 x 10^{9} years

(b) 3.92 x 10^{9} years

(c) 4.20 x 10^{9} years

(d) 8.40 x 10^{9} years

**Question 45.**

**Question 46.**

The half-life of a radioactive isotope X is 20 yr. It decays to another element Y which is stable. The two elements X and Y were found to be in the ratio 1 : 7 in a sample of a given rock. The age of the rock is estimated to be

(a) 40yr

(b) 60yr

(c) 80yr

(d) 100yr

**Question 47.**

A certain mass of hydrogen is changed to helium by the process of fusion. The mass defect in fusion reaction is 0.02866u. The energy liberated per u, is (Given, 1u=931 MeV)

(a) 2.67 MeV

(b) 26.7 MeV

(c) 6.675 MeV

(d) 13.35 MeV

**Question 48.**

If the half-life of any sample of radioactive substance is 4 days, then the fraction of sample will remain undecayed after 2 days, will be

(a) √2

(b) 1/√2

(c) (√2-1)/√2

(d) 1/2

**Question 49.**

Consider the nuclear reaction X^{200} —–> A^{110}+B^{80}. If the binding energy per nucleon for X, A and B are 7.4 MeV, 8.2 MeV and 8.1 MeV, respectively. Then, the energy released in the reaction is

(a) 70 MeV

(b) 200 MeV

(c) 190 MeV

(d) 10 MeV

**Question 50.**

Band spectrum is also called

(a) molecular spectrum

(b) atomic spectrum

(c) flash spectrum

(d) line absorption spectrum

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