A radioactive sample with a half life of 1 monthhas the label : ‘Activity = 2 micro curies on 1–8–1991. What would be its activity two monthsearlier ? 
In two half lives, the activity becomes one fourth.
Activity on 1–8–91 was 2 micro–curie
∴ Activity before two months, 4 × 2 micro-curie = 8 micro curie
The nucleus after two successive β – decay will give
Two successive β decay increase the charge number by 2. Therefore, (d) is correct.
A radioactive element has half life period 800 years. After 6400 years what amount will remain? 
No. of half lives,
An element A decays into element C by a two step process
From eqn. (ii), B has 2 units of charge more than C.
From eqn. (i), A loses 2 units of charge by emission of alpha particle. Hence, A and C are isotopes as their charge numbers are same.
Ultraviolet radiations of 6.2 eV falls on analuminium surface. K.E. of fastest electronemitted is (work function = 4.2 eV) 
K.E. of fastest electron = E – W0 = 6.2 – 4.2 = 2.0 eV
= 2 × 1.6 × 10–19 = 3.2 × 10– 19 J
Curie is a unit of 
Curie is a unit of radioactivity.
The average binding energy of a nucleon insidean atomic nucleus is about 
Average B.E./nucleon in nuclei is of the order of 8 MeV.
The nuclei 6C13 and 7N14 can be described as 
As 6C13 and 7N14 have same no. of neutrons (13 – 6 =7 for C and 14 – 7 =7 for N), so they are isotones.
Which of the following statements is true fornuclear forces? 
Nuclear forces are short range attractive forces which balance the repulsive forces between the protons inside the nucleus.
The ratio of the radii of the nuclei 13Al27 and 52Te125 is approximately 
The nucleus 126C absorbs an energetic neutronand emits a beta particle (β). The resultingnucleus is 
The constituents of atomic nuclei are believedto be 
Nucleus contains only neutrons and protons.
The half life of radium is 1600 years. The fractionof a sample of radium that would remain after6400 years 
An electron with (rest mass m0) moves with a speed of 0.8 c. Its mass when it moves with this speed is 
In the nucleus of 11Na23, the number of protons,neutrons and electrons are 
Z = 11 i.e., number of protons = 11, A = 23
∴ Number of neutrons = A – Z = 12
Number of electron = 0 (No electron in nucleus) Therefore 11, 12, 0.
If the nuclear force between two protons, two neutrons and between proton and neutron is denoted by Fpp, Fnn and Fpn respectively, then 
Nuclear force is the same between any two nucleons.
Solar energy is due to 
The energy equivalent of one atomic mass unitis 
1 a.m.u = 931 MeV
The mass of α-particle is 
α-particle = 2He4 . It contains 2p and 2n.
As some mass is converted into B.E., therefore, mass of α particle is slightly less than the sum of the masses of 2p and 2n.
The mass density of a nucleus varies with massnumber A as 
The nuclear radius r varies with mass number A according to the relation
r ∝ A1/3 or A ∝ r3
Further, mass ∝ A and volume ∝ r3
In a given reaction
Radioactive radiations are emitted in thesequence of 
Increase of charge number. by 1 indicates β emission. Decrease of mass number by 4 and charge number by 2 indicates α emission.
No change of mass number and charge number indicates γ-emission.
The binding energy per nucleon is maximum in case of
From B.E. curve, the curve reaches peak for
Energy released in the fission of a single nucleus is 200 MeV. The fission rate of a filled reactor operating at a power level of 5 W is 
= 1.56 × 1011 s–1
If the binding energy per nucleon in and nuclei are respectively 5.60 MeV and 7.06 MeV, then the energy of proton in the reaction 
Applying principle of energy conservation, Energy of proton = total B.E. of 2α– energy of Li7
= 8 × 7.06 – 7 × 5.6
= 56.48 – 39.2 = 17.28 MeV
Heavy water is used as a moderator in a nuclearreactor. The function of the moderator is
Moderator slows down the neutrons to thermal energies.
The mass number of He is 4 and that for sulphur is 32. The radius of sulphur nuclei is larger than that of helium by [1994, 1995]
The nature of ions knocked out from hotsurfaces is 
Ion is an atom or a group of atoms that has lost or gained electrons having negative or positive charge respectively. Thus, the ions knocked out from hot surfaces is electrons.
What is the respective number of α and β particles emitted in the following radioactive decay ? 
We know that
Therefore, in this process,
200 = 4n + 168 or n
Also, 90 = 2n – m + 80
or, m = 2n + 80 – 90 = (2 × 8 + 80 – 90) = 6.
Thus, respective number of α and β-particles will be 8 and 6.
The count rate of a Geiger Muller counter for theradiation of a radioactive material of half-life 30minutes decreases to 5 sec–1 after 2 hours. Theinitial count rate was 
Half-life = 30 minutes; Rate of decrease (N) = 5 per second and total time = 2 hours = 120 minutes. Relation for initial and final count rate
Therefore, N0 = 16 × N = 16 × 5 = 80 s–1.
A nucleus ruptures into two nuclear parts, whichhave their velocity ratio equal to 2:1. What willbe the ratio of their nuclear size (nuclear radius)? 
Applying law of conservation of momentum,
m1v1 = m2v2