Question

*PET SCAN*

A typical kinetic energy of the positron emitted from F-18 is 0.2 MeV. What is a typical speed of positron in unit of c (=speed of light)? (Hint: The rest mass of positron is 0.511 MeV/c2 . The total energy = kinetic energy + rest mass).

c) The half-life of F-18 is roughly two hours. How long does it take to reduce the radioactivity down to 12.5% (= 1/8) level (so that a patient becomes safe to meet with his/her friends)?

Answer 1

PART A:

The kinetic energy is given by

PART B:

The radioactivity is directly proportional to the concentration.

The half-life is 2 hours.

So, every two hrs the concentration reduces to half and so the reactivity.

To reduce further 50% (to 25%) it takes another 2 hrs and to reduce further 50% (to 12.5%) it takes another 2 hrs.

So, in total it takes 6 hrs (3 half-lives) to reduce to 12.5%.