76. In a cloud chamber, a short, thick and continuously straight track is obtained on the photographic plate. The incident particle is (1) Neutron (2) -particle (3) B-particle (4) y-radiation 77. X-rays of 0.3A are incident on a crystal with lattice spacing 0.5. The angle at which second Bragg's diffraction maxima observed is (1) sin (3) sin- (13) (2) cos-1 (4) cos-1 78. The packing fraction of Body Centred Cubic Structure (BCC) is (1) 0.68 (2) 0.34 (3) 0.72 (4) 0.84 79. The energy of electron in s band of crystal is given by E-E-2A cos ka. The effective mass of electron is given by (1) 2Aa cos ka h (2) E-2A cos ka (3) 2Aa sin ka h (4) 2Aa cos ka 80. In Einstein's model of specific heat of the solids, which of assumptions is not true? following (1) A crystal consists of atoms which may be regarded as identical and independent harmonic oscillators. (2) Any number of oscillators may be present in the same quantum state. (3) The oscillators are quantum oscillators and have discrete energy. (4) All the oscillators vibrate with different frequency.