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Measurement of Absorbed Dose (4)

Measurement of Absorbed Dose (4). 參考資料： 1. The Physics of Radiation Therapy. Faiz M. Khan 2. Introduction to Radiological Physics and Radiation Dosimetry. Frank H. Attix. STOPPING POWER.

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Measurement of Absorbed Dose (4)

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  1. Measurement of Absorbed Dose (4) 參考資料：1. The Physics of Radiation Therapy.Faiz M. Khan2. Introduction to Radiological Physics and Radiation Dosimetry. Frank H. Attix

  2. STOPPING POWER • The expectation value of the rate of energy loss per unit of path length x by a charged particle of type Y and kinetic energy T, in a medium of atomic number z, • MeV/cm or J/m • 1 MeV/cm = 1.602 X 10-11 J/m

  3. STOPPING POWER • mass stopping power (dT/ρ dx) • MeV cm2 / g or J m2 / kg • 1 MeV cm2 / g = 1.602 X 10-l4 J m2 / kg • collision stopping power • the rate of energy loss resulting from the sum of the soft and hard collisions, which are conventionally referred to as "collision interactions.“ • radiative stopping power • owing to radiative interactions, bremsstrahlung

  4. STOPPING POWER • Energy spent in radiative collisions is carried away from the charged-particle track by the photons, • while that spent in collision interactions produces ionization and excitation contributing to the dose near the track.

  5. STOPPING POWER • Types of charged-particle coulomb-force interactions

  6. STOPPING POWER • "Soft" Collisions (b >> a) • When a charged particle passes an atom at a considerable distance, the influence of the particle's Coulomb force field affects the atom as a whole, thereby distorting it, exciting it to a higher energy level, and sometimes ionizing it by ejecting a valence shell electron. The net effect is the transfer of a very small amount of energy (a few eV) to an atom of the absorbing medium

  7. STOPPING POWER • Hard (or "Knock-On") Collisions (b~a) • the incident particle will interact primarily with a single atomic electron, which is then ejected from the atom with considerable kinetic energy and is called a delta (δ) ray. • δ-ray dissipates its kinetic energy along a separate track (called a "spur") from that of the primary charged particle.

  8. STOPPING POWER • mass collision stopping power • where subscripts c indicate collision interactions, s being soft and h hard.

  9. STOPPING POWER • Dependence on the stopping medium • decrease the mass collision stopping power as Z is increased

  10. STOPPING POWER • Dependence on particle velocity • decrease the mass collision stopping power as particle velocity increases. • The stopping power gradually flattens to a broad minimum of 1-2 MeV cm2 /g at T/m0c2 ≒ 3, and then slowly rises again with further increasing T.

  11. STOPPING POWER • Dependence on particle charge • Z2 , a doubly charged particle of a given velocity has 4 times the collision stopping power as a singly charged particle of the same velocity in the same medium. • an a-particle would have a mass collision stopping power of 200 MeV cm2 / g, compared with the 50 MeV cm2 / g shown for a singly charged heavy particle in water. • Dependence on particle mass • There is none.

  12. STOPPING POWER • Restricted Stopping Power • The mass collision stopping power expresses the average rate of energy loss by a charged particle in all hard, as well as soft, collisions. • The δ-rays resulting from hard collisions may be energetic enough to carry kinetic energy a significant distance away from the track of the primary particle.

  13. STOPPING POWER • Restricted Stopping Power • More importantly, if one is calculating the dose in a small object or thin foil transversed by charged particles, the use of the mass collision stopping power will overestimate the dose, unless the escaping δ-rays are replaced.

  14. STOPPING POWER • Restricted Stopping Power • The restricted stopping power is that fraction of the collision stopping power that includes all the soft collisionsplus those hard collisions resulting in δ rays with energies less than a cutoff value Δ. • symbolized here as

  15. STOPPING POWER • Restricted Stopping Power • An alternative and very important form of restricted stopping power is known as the linear energy transfer, LET, symbolized as LΔ ( ICRU, 1980) • LET is of greatest relevance in radiobiology and microdosimetry.

  16. STOPPING POWER • Restricted Stopping Power • If the cutoff energy Δ is increased to equal Tmax then

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