Joy of Sharing Science No 1 new Covid Special Edition

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Joy of Sharing ScienceT H E M E C H A N İ S MThe X-ray photon most often hits anelectron in the K-shell (lowest energyshell), knocking it off.An electron from either the L or M-shell drops down to K-shell. Theenergy difference between the energylevels is emitted as an X-ray photon.To fill the newly created vacancyin the L or M shell, an electronfrom higher shells drop down andrelease photons. As this processcontinues, the energy of thephotons decrease because thepotential difference between theshells decreases.
How are X-ray scans used to detect Pneumonia?The crucial point here is that the photons are emitted into random directions.Meaning that when photoelectric effect occurs, the X-ray is essentiallyabsorbed because it doesn’t continue in the same direction that it started withand doesn’t register in the X-ray detector.The probability of Photoelectric Effect occurring is proportional to Z3 pE 3 where Zis the atomic number of the element, p is the physical density of the medium,and E is the energy of the medium. This relation makes sense because if anatom has more electrons (a higher Z), photons have a higher chance of hittingthe electrons. If the atoms are closer to each other, the same logic applies.Finally, as the energy of the photons increases, it starts exceeding the energyneeded to knock the electrons in the K-shell. Then, Compton Effects startsdominating the interactions. Compton effect is a high energy photon givingsome of its energy to an electron and getting scattered into a random direction.As the Compton Effect doesn’t depend on the Z of an element and scatters X-rays in random directions, it causes noise in the x-ray tomography reading.Photoelectric EffectCompton ScatteringThe reason why your bones seemwhite in an X-ray while your lungsare pitch black is that the Calcium inyour bones has a high Z (20) whileNitrogen (7) and Oxygen (8) in the airhave low Z’s. That is the same reasonwhy Iodine (which has an atomic
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