Chapter 7
Nuclear Physics – I
7.1 Basic Concepts and Formulae ................................
Solid angle
In two dimensions the angle in radians is defined as the ratio of the arc of the circle
and the radius, that isθ=s/r. In three dimensions, the element of solid angle dΩ
is defined as the elementary areaAat a distancedfrom a point, perpendicular to
the line joining the point and the area, divided by the square of the distance, that is
dΩ=ΔA/d^2. For a ring of radiirandr+dr, located on the surface of a sphere of
radiusR, the element of solid angle in polar coordinates is subtended at the centre
O is given by (Fig. 7.1)
dΩ= 2 πsinθdθ (7.1)We assume an azimuthal symmetry, that is scattering is independent of the
azimuthal angleβ.
Fig. 7.1Concept of solid
angle
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