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I'm wondering about the equivalent dose and the radiation weighting factor of photons (according to ICRP 103).

Why is the weighting factor independent from the energy of the photons?

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The radiation weighting factors are used to compare the biological damage that different types of radiation do to human tissue.

Take an absorbed radiation dose of 1Gy for example (1Gy = 1 Joule/kg). This absorbed dose can be expressed in terms of the equivalent dose to tissue by multiplying by the radiation weighting factor; H = w * D

Damage to tissue is done only by charged particles when they interact with atoms through coulomb interactions.

Photons are not charged particles, then have to undergo multiple Compton scatterings in the body before they reach a low enough energy to undergo a photoelectric absorption event and release a secondary electron. It is this secondary electron that actually deposits the radiation dose in the tissue.

It is for this reason that photons are assigned a weighting factor of 1, no matter their energy, because they must undergo several interactions before depositing dose.

Electrons are also given a weighting factor of one, even though they are charged particles. This is because they loose their energy relatively slowly (compared to alpha particles or heavy ions) and also undergo a lot of interactions before depositing all their energy. They travel a relatively large distance while slowly depositing energy.

Alpha particles and heavy ions are given weighting factors of between 2 and 20, meaning they will deliver a dose that is biologically worse for the tissue by up to 20 times (depending on the weighting factor) for the same original absorbed dose.

This is because alpha particles and heavy ions interact very quickly and travel a very short distance in tissue, due to their weight and hence number of electrons in their atoms, which all interact with tissue. They deposit their energy in a very 'local' area in comparison to electrons or photons, meaning they do a lot more damage to that area of tissue.

tl;dr Photons travel a lot further in tissue (due to undergoing a lot of scattering events,no matter what their energy) so their dose is spread out over a larger area. Heavy ions and charged particles (alpha, beta) deposit their energy in a relatively local area compared to photons and so do a lot more damage to tissue, hence they are given higher weighting factors.

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  • $\begingroup$ Thank's for the answer, but i was really only interested in photons. If I understood correctly, the constant weighting factor of photons is caused by secondary electrons. This would match with my thoughts about the LET :-) $\endgroup$ Dec 23, 2015 at 11:15

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