In $\beta^{-}$decay, a neutron transforms into a proton within the nucleus according to the equation :

neutron $\rightarrow$ proton $+\beta^{-}+x$

In this equation the particle represented by ' $x$ ' is

Two radioactive substances $A$ and $B$ have same number of initial nuclei. If the half lives of $A$ and $B$ are 1.5 days and 4.5 days respectively, then the ratio of the number of nuclei remaining in $A$ and $B$ after 9 days is

$10^{10}$ electrons enter the emitter of a junction transistor in a time of $0.4 \mu \mathrm{~s}$. If $5 \%$ of the electrons are lost in the base, then the collector current is

An electron in $n$-region of a $p-n$ junction moves towards the junction with a speed of $5 \times 10^5 \mathrm{~ms}^{-1}$. If the barrier potential of the junction is 0.45 V , then the speed with which the electron enters the $p$-region after penetration through the barrier is

(Charge of the electron $=1.6 \times 10^{-19} \mathrm{C}$ and mass of the electron $=9 \times 10^{-31} \mathrm{~kg}$ )