Physics of heavy ions

Heavy-ion physics

To understand the effects of radiation on biological systems first the physical interaction processes involved in the passage of radiation through matter have to be considered. Here the absorbed dose, the amount of energy deposited in a small mass element of the absorber medium, is an important measurand (unit Gray, 1 Gy = 1 J/kg).

In contrast to electromagnetic radiation (X- and γ-rays), exhibiting an exponential decrease of intensity with penetration depth, heavy charged particles like protons or heavy ions have a well defined range in matter. Their energy deposition is characterized by a low entrance dose and a pronounced sharp maximum near the end-of-range (Bragg peak).

The slowing down process of heavy ions in matter is governed by inelastic collisions with atomic electrons of the absorber material and is very well described by the Bethe-Bloch formula. The incoming primary ions also undergo nuclear fragmentation reactions, resulting in a complex alteration of the composition of the particle field. Detailed data on these effects are indispensable for the calculation and optimization of the biologically effective dose for treatment planning.

The 'inverted' depth-dose profile (Bragg curve) of heavy charged particles represents a major advantage for the radiation therapy of deep-seated local tumors in comparison to conventional photon therapy. The favorable dose profile is additionally enforced by an enhanced biological effectiveness (RBE) of heavy ions in the Bragg peak region, which can be explained on a microscopic level by the high ionization density in the particle track (biophysical model).

Physics research programme related to radiation therapy with heavy ions:

  • Precision measurements of depth-dose profiles of ion beams and comparison with model calculations
  • Lateral beam spread and range straggling
  • Heavy-ion dosimetry
  • Measurements and model calculations for δ-electrons
  • Characterisation of the particle field as function of penetration depth of high-energy ion beams
    • Experimental studies of yields, energy- and angular distributions of nuclear fragments
    • Secondary neutrons
    • Bench mark data for validation of the physical model used for treatment planning