Try not to be a person of success, but rather a person of virtue.

– Albert Einstein

Here are the two questions from nuclear physics which were included in KEAM (Medical) 2008 question paper:

(1) If the mass defect of _{8}O^{16} nucleus is 0.128 amu, then the binding energy per nucleon of oxygen is

(a) 8.2 MeV

(b) 7.45 MeV

(c) 7.3 MeV

(d) 7.1 MeV

(e) 8.15 MeV

One atomic mass unit (amu) is equivalent to 931 MeV. Therefore, the *total *binding energy of the _{8}O^{16} nucleus is 0.128×931 MeV.

Since there are 16 nucleons in the _{8}O^{16} nucleus, the binding energy *per nucleon* of oxygen is (0.128×931)/16 = **7.45 MeV**, very nearly.

(2) Two radioactive samples have decay constants 15*x* and 3*x*. If they have the same number of nuclei initially, the ratio of number of nuclei after a time 1/6*x* is

(a) 1/*e*

(b) e/2

(c) 1/*e*^{4}

(d) 2*e*/3

(e)** **1/*e*^{2}

The number *N *of nuclei at time *t* is given by** **

** ***N = N*_{0}*e*^{–}^{λ}^{t}* *where *N*_{0} is the initial number,* e *is the base of natural logarithms, and *λ* is the decay constant.

The required ratio is (*N*_{0}*e*^{–15}^{x}^{/}^{6x})/ (*N*_{0}*e*^{–3x/6x}) = *e*^{–2.5}/* e*^{–0.5} = *e*^{–2} = **1/ e^{2}**