Physics U30 Important

Explain how X-rays are produced?

• Electrons are emitted from a heated cathode filament.
• The potential difference between the cathode and the anode is very high.
• The potential difference accelerates electrons to high speeds, and electrons strike the anode and are suddenly decelerated.
• Their kinetic energy is converted into 1% x-rays and 99% heat.

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The emitted X-ray has energy (E = fh). A greater potential difference means a higher frequency means greater penetration of the beam. Therefore, greater hardness of the beam.

To increase the number of electrons emitted, increase the current in the cathode filament. More current means more heat produced. More electrons are emitted.

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PET Scan

A tracer is a substance introduced into the body. It is absorbed by the tissue being studied.

Process:

1. Tracer emits positrons.
2. Positron interacts with an electron in tissue.
3. When a positron and an electron meet, they annihilate each other.
4. Before annihilation, the total momentum of the positron and electron pair is extremely small. (They have negligible mass, and they are almost at rest relative to each other when they collide.) Total momentum before collision ~ 0.
5. On annihilation, 2 gamma photons are produced. Each gamma photon has momentum. So, to conserve momentum, the 2 photons must travel in opposite directions - exactly 180 degrees apart. Then the total momentum after the collision is equal to 0.
6. The PET scanner calculates that the annihilation event occurred along the line joining the 2 detectors of gamma photons that went 180 degrees apart. 

(This allows the computer to reconstruct the position of the radiotracer in the body)