291.

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

Atomic number of an atom is Z and mass number is m, then number of neutrons in its nucleus is

• m − Z

• $\frac{\mathrm{m}}{\mathrm{Z}}$

• m + Z

• m × Z

Nucleus of an atom has

• e and n

• e and p

• p and n

• e, p and n

A nucleus undergoes β-decay. The mass number and  atomic number change as

• mass number and atomic number both increases

• mass number remain same and atomic number increases

• mass number increases and atomic number remain same

• mass number and atomic number both decreases

When a radioactive isotope ${}_{88}\mathrm{Ra}^{228}$ decay in series by the emission of 3$\mathrm{\alpha }$ -particles and a β -particle, the isotope finally formed is

• ${}_{82}\mathrm{X}^{216}$

• ${}_{83}\mathrm{X}^{215}$

• ${}_{88}\mathrm{X}^{216}$

• ${}_{83}\mathrm{X}^{216}$

The activity of a radioactive element decreases to one third of the original activity I₀ , in a period of 9 years. After a further lapse of 9 years, its activity will be

• I₀

• (2/3) I₀

• I₀ / 9

• I₀ / 6

If M_o is the mass of an oxygen isotope ${}_8\mathrm{O}_{17}$, M_p and M_n are the masses of a proton and a neutron, respectively, the nuclear binding energy of the isotope is

• (M_o − 8M_p) c²

• (M_o − 8M_p − 9M_n) c²

• M_oc²

• (M_o − 17M_n) c²