During radioactivity why protons are not emitted by unstable nucleus but only alpha ,betta particle and gamma ray is emitted?​

Frequently, gamma ray production accompanies nuclear reactions of all types. In the alpha decay of U -238, two gamma rays of different energies are emitted in addition to the alpha particle.

U92238→He24+Th90234+2γ00(17.3.4)

Virtually all of the nuclear reactions in this chapter also emit gamma rays, but for simplicity the gamma rays are generally not shown. Nuclear reactions produce a great deal more energy than chemical reactions. Chemical reactions release the difference between the chemical bond energy of the reactants and products, and the energies released have an order of magnitude of 1×103kJ/mol . Nuclear reactions release some of the binding energy and may convert tiny amounts of matter into energy. The energy released in a nuclear reaction has an order of magnitude of 1×1018kJ/mol . That means that nuclear changes involve almost one million times more energy per atom than chemical changes!

photocells convert light energy to electricity