161.

A radioactive substance containing N atoms emits n $\alpha$ - particles per second. Half-life of the substance is :

• $\frac{\mathrm{N}}{\mathrm{n}}$ sec

• $\frac{\mathrm{n}}{\mathrm{N}}$ sec

• $\frac{0.693\mathrm{n}}{\mathrm{N}}$ sec

• $\frac{0.693\mathrm{N}}{\mathrm{n}}$ sec

In a radioactive decay,

           ₉₂U²³⁸ _{$\stackrel{\mathrm{\alpha }}{\to }$  X  $\stackrel{\mathrm{\alpha }}{\to }$  Z}Y^A

The values of Z and A

• 92 , 236

• 88 , 230

• 90 , 130

• 88 , 234

In a sample of radioactive substance, what percentage decays in one mean life time?

• 69.3 %

• 63 %

• 50 %

• 36 %

Number of nuclei of a radioactive substance at time t = O are 2000 and 1800 at time t = 2s. Number of nuclei left after t = 6s is

• 1442

• 1554

• 1652

• 1458

₉₂U²³⁸ on absorbing a neutron goes over to ₉₂U²³⁹. This nucleus emits an electron to go over to neptunium which on further emitting an electron goes over to plutonium. The resulting plutonium can be expressed as

• ₉₄Pu²³⁹

• ₉₀Pu²¹⁹

• ₉₃Pu²⁴⁰

• ₉₂Pu²⁴⁰

The radius of nucleus of silver (atomic number = 47) is 3.4 x 10^-14 m. The electric potential on the surface of nucleus is (e = 1.6 x 10^-19 C)

• 1.99 x 10⁶ V

• 2.9 x 10⁶ V

• 4.99 × 10⁶ V

• 0.99 x 10⁶ V

Which one of the following is used as a moderator in nuclear reaction?

• Uranium

• Heavy water

• Cadmium

• Plutonium

The K_{$\mathrm{\alpha }$} line from molybdenum (atomic number = 42) has a wavelength of 0.7078 $\stackrel{\circ }{\mathrm{A}}$. The wavelength of K_{$\mathrm{\alpha }$} line of zinc (atomic number = 30) will be

• 1 $\stackrel{\circ }{\mathrm{A}}$

• 1.3872 $\stackrel{\circ }{\mathrm{A}}$

• 0.3541 $\stackrel{\circ }{\mathrm{A}}$

• 0.5 $\stackrel{\circ }{\mathrm{A}}$

Decay constant of radioactive element is 1.5 × 10^-9/s. Average age of element is

• 1.5 × 10⁹

• 4.62 × 10⁸

• 6.67 × 10⁸

• 10.35 × 10⁸

Half-life of radium is 1600 yr, then its 1/16th part will remain undecayed after

• 1200 yr

• 6400 yr

• 1800 yr

• 2200 yr