Question

how to make parallel beam with an experiment???

Answer 1

The crystal detection cross section is a relatively smooth function of the incident angles and energy. We can create a look-up table using a set of discrete values 

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The crystal detection cross sections for any intermediate angles and energies can be obtained from this look-up table through interpolation.

Further reduction of the size of the look-up table can be accomplished using the symmetries of a square crystal array. To cover all possible incident angles, we need only the crystal detection cross sections for 

There are at least three possible ways to compute the proposed look-up table: 1) experimental measurement of the crystal detection efficiencies using a point source located at different positions around the crystal array, followed by conversion of the measured crystal detection efficiencies into the detection cross section using the known distance between source and crystals; 2) direct calculation of the line integrals (geometrical approach); and 3) MC simulation of the parallel beam of gammas illuminating the crystal array.

Answer 2

Answer:

Answer - Only converging lens and converging mirror can make a parallel beam of light incident on a point. Concave mirror is a converging mirror.