If half life of an element is $69.3 \mathrm{~h}$, then how much of its percent will decay in 10th to 11th $\mathrm{h}$. Initial activity $=50 ~\mu \mathrm{~Ci}$

Assertion : Heavy water is used to slow neutron in nuclear reactor.

Reason : It does not react with slow neutron and mass of deuterium is comparable to the neutron.

Assertion : For an element generally $N \geq Z$ ( $N=$ number of neutrons, $Z=$ atomic number)

Reason : Neutrons always experience attractive nuclear force.

The half-life of a radioactive substance is $20 \mathrm{~min}$. The approximate time interval $\left(t_2-t_1\right)$ between the time $t_2$, when $\frac{2}{3}$ of it has decayed and time $t_1$ when $\frac{1}{3}$ of it had decayed is

Assertion If electrons in an atom were stationary, then they would fall into the nucleus.

Reason Electrostatic force of attraction acts between negatively charged electrons and positive nucleus.

Assertion Radioactive nuclei emits $\beta^{-}$-particles.

Reason Electrons exist inside the nucleus.

A nuclear explosive is designed to deliver $1 \mathrm{~MW}$ power in the form of heat energy. If the explosion is designed with 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 $200 \mathrm{~MeV}$)

Assertion : A beam of charged particles is employed in the treatment of cancer.

Reason : Charged particles on passing through a material medium lose their energy by causing ionization of the atoms along their path.

Assertion : In He-Ne laser, population inversion takes place between energy levels of neon atoms.

Reason : Helium atoms have a metastable energy level.

Assertion : In $\alpha$-decay atomic number of daughter nucleus reduces by 2 units from the parent nucleus.

Reason : An $\alpha$-particle carries four units of mass.