FUNPHYS 15 - Production of X-Rays Flashcards
Recall atomic structure.
Atoms consist of a nucleus which is surrounded by electrons in orbits that have a specific energy value.
What happens when an atom moves between energy levels.
If an atom moves from a higher energy level to a lower energy level, it emits a photon. If an atom absorbs a photon, it can move from a lower energy level to a higher energy level. The photons emitted or absorbed have the same energy as the difference in energy between the two energy levels. To calculate the energy of a photon: E = E1 - E0 = hf = hc/wavelength
What are the 7 properties of x-rays?
- High-energy photons
- Can cause ionization in air and tissue
- Blacken photographic film
- Highly penetrating in tissue
- Absorbed by high-density materials
- Produce fluorescence
- Generally, travel in straight lines
How are X-Rays produced and the factors that affect the number and intensity of the x-rays produced?
X-rays are produced by accelerating electrons through very high voltages from a cathode and allowing them to strike a metal target called an anode. The number of electrons produced, therefore the number of x-rays produced depends on the filament temperature of the cathode. The anode material determines the intensity of the x-ray beam (the higher the atomic number of the anode material, the more efficient the production of x-rays because they will have higher melting points).
What is the range of voltage used for X-rays and what determines it?
20 - 200 kV and tube voltage determine how hard or soft an x-ray will be.
What is the formula for Photon Efficiency?
Photon Efficiency = Photon Energy / Heat Energy = 10^-19 ZV
What design features are used to prevent thermal damage of the anode?
These design features distribute the deposited energy over a bigger area to minimize damage:
- Rotate the anode (Rotation rates are around 3,600 rpm and any slight imbalance can cause large forces that will shatter the glass tube)
- Use an angled anode
What is an Evacuated Glass tube?
Air molecules slow down the accelerating electrons from the cathode to the anode due to Electrostatic Coloumb forces and collisions between the electrons and the electrons attached to air molecules. The evacuated glass tube evacuates air to 1 in 1 billion, allowing the accelerated electrons to travel unimpeded to the anode.
How much energy from the accelerating electrons is energy?
1% is energy that is used to produce X-rays.
How does the less than 1% energy from the accelerating electrons produce X-rays?
There are two mechanisms:
- Bremsstrahlung X-rays: The difference in kinetic energy between the incident electron before its interaction with the clouds of electrons around the anode atoms and after is what produces an X-ray photon.
- Characteristic X-rays: sometimes a fast incident electron collides with a ground state electron in one of the anode’s atoms and knocks it free of its orbital. The vacancy in the k-shell is immediately filled by an electron moving from a higher energy level to a lower energy level, causing the emission of an X-ray photon in the process. The X-ray photon’s energy is the difference in energy between the two energy levels. (K-alpha x-rays are from electrons moving from n=2 to n=1, K-beta x-rays are from electrons moving from n=3 to n=1). The K-alpha and K-beta x-rays will always be characterisitc to the anode material, hence the name. Characteristic x-rays are not used for medical purposes.
Explain maximum kinetic energy.
The maximum kinetic energy of the x-rays produced cannot exceed the maximum kinetic energy of the electron beam, and this maximum energy is achieved when the incident electron is stopped completely.
