Which factor does NOT influence the geometric penumbra of a 60 Co unit?

The geometric penumbra of a radiation beam is the area at the edges of the radiation field where the radiation intensity gradually decreases to zero. Understanding what influences this geometric penumbra is crucial in medical dosimetry.

The source diameter impacts the penumbra because a larger source will produce a broader penumbra, resulting in less sharpness of the radiation beam's edges. This effect arises because a bigger source emits rays from more points, leading to greater spread.

The distance between the diaphragm and the patient is significant as it affects the attenuation and scattering of radiation. A greater distance generally results in a sharper penumbra since the rays have less chance to diverge significantly before they reach the patient.

The source to diaphragm distance also plays a critical role in defining the penumbra. A further distance tends to create a sharper beam edge because it allows for less lateral displacement of the rays, leading to a smaller penumbra.

However, the length of the source does not influence the geometric penumbra in the same way. The length of the source primarily determines the volume of radiation emitted and influences the dose distribution rather than the geometric spread of the beam. This is why the length of the source is the factor that does not affect the geometric