1What is characteristic radiation and why is it called 'characteristic'?
• X-ray photons produced when inner shell electrons are ejected and replaced by outer shell electrons dropping down
• Called 'characteristic' because the photon energy is CHARACTERISTIC of the target material (tungsten)
• The energies are always the same specific values for tungsten — not random
• Different target materials produce different characteristic energies
2What are the two clinically useful characteristic X-ray photon energies from tungsten?
• K-alpha: 59.3 keV (L shell → K shell transition)
• K-beta: 67.0 keV (M shell → K shell transition)
• These are the only two that contribute to the diagnostic beam
• Lower energy cascade photons are absorbed by inherent filtration
3Is characteristic radiation heterogeneous or homogeneous? Why?
• HOMOGENEOUS — the beam has specific, fixed energies
• Characteristic photons are always produced at the same fixed energies (59.3 and 67 keV)
• There is no range or variation in the energies produced
• Opposite of bremsstrahlung which is heterogeneous
4Describe the production of characteristic radiation step by step:
• Step 1: Filament electron collides with a K shell electron of tungsten
• Step 2: Must have energy > 69.5 keV to eject the K shell electron
• Step 3: K shell electron completely removed from the atom (ionisation)
• Step 4: K shell vacancy is created
• Step 5: Electron from a higher shell drops down to fill the vacancy
• Step 6: Electron loses energy equal to binding energy difference between the two shells
• Step 7: This energy released as a characteristic X-ray photon
• Step 8: Cascade continues — electrons drop through multiple levels releasing further photons
5Why are the low-energy cascade photons (7.7 keV, 1.5 keV) not clinically useful?
• Produced when electrons drop between outer shells (M→L, N→M etc.)
• Their energies are too LOW to penetrate the patient
• They are absorbed by the tube's INHERENT FILTRATION before reaching the patient
• Only K-alpha (59.3 keV) and K-beta (67 keV) are energetic enough to be useful
6Calculate the energy of the photon produced when an electron drops from the L shell to the K shell in tungsten:
• Formula: photon energy = binding energy of RECEIVING shell − binding energy of DONOR shell
• Receiving shell = K shell = 69.5 keV
• Donor shell = L shell = 10.2 keV
• Calculation: 69.5 − 10.2 = 59.3 keV
• This is the K-ALPHA photon
7Calculate the energy of the photon produced when an electron drops from the M shell to the K shell:
• Formula: photon energy = binding energy of RECEIVING shell − binding energy of DONOR shell
• Receiving shell = K shell = 69.5 keV
• Donor shell = M shell = 2.5 keV
• Calculation: 69.5 − 2.5 = 67.0 keV
• This is the K-BETA photon