Bremsstrahlung

1
Bremsstrahlung

• Example of conservation of energy
• Radiative energy loss by fast electron when
  slowed near nucleus
• Results in spectrum of energies from many
  interactions

2
Diagnostic x-ray production
120kV
0V

• Electrons accelerated by E field
• Energies lt 120kV
• Can still generate therapy beams this way, but
  lack penetration
• Need MV, not kV!

3
MV x-ray production

• Carefully tuned microwave source
• 3 GHz 10cm wavelength

• Intense electric field
• Phase problem!

4
Microwave resonance cont.

• Sideline every second cavity
• Solves phase problem

5
Operation

• Inject bunches of electrons into cavity
• Time to coincide with pulses of microwaves

• Makes compact system

6
Waveguide for 4MV
7
Waveguide cont
8
Target and flattening filter

• Electrons bent through 270 degrees
• Collide with tungsten target
• Beam shaped for flatness

9
Linac
10
Linac
11
Vital statistics

• Output 6Gy/min at 1m. Lethal dose in 10 min.
• Weight 8 tonnes
• Cost 2.5m to 4m
• Lifespan 10y
• Facility 1.2m to 2.4m concrete as shielding for
  staff
• Support Maintenance contract gt100k per year.

12
The radiation beam
18MV
6MV
13
X-ray dose Vs Depth
14
Combining beams -- a pair
15
Combining beams three beams
16
A patient plan