7.2.3 Linear Energy Transfer

Created October 19, 1995

LINEAR ENERGY TRANSFER

The Linear Energy Transfer (LET) is similar to the stopping power except that it does not include the effects of radiative energy loss (i.e., Bremsstrahlung) or delta-rays. The difference between LET and S is that LET is local energy deposition only and S is concerned with the total energy lost by the particle. S and LET are nearly equal for heavy charged particles; for betas LET does not include delta-rays nor Bremsstrahlung.

The LET is related to Biological Damage. The severity and permenance of biological chages are directly related to the local rate of energy deposition along the particle track. The higher the LET, the higher the Q- quality factor in determining dose equivalent (Severts, where 1 Sv = 100 rem).

POSITRONS

Positrons behave nearly the same as electrons as they lose their energy, they are simply deflected in the opposite direction. As far as imaging is concerned, the 511 keV annihilation photons resulting from positrons come from the point at which the positron as slowed enough to engage in a prolonged encounter with an electron, i.e. it forms the unstable "positronium".

This places an isotope-dependent limit on spatial resolution in PET imaging, since each positron emitter has a different E(max) for positrons and hence a different range. The other consideration is the center-of-mass momentum of the positronium, which contributes to non-opposition of the annihilation photons in the laboratory frame of reference.

CHERENKOV EFFECT

When a charged particle is emitted in radioactive decay with velocity greater than c/n (the speed of light in the medium), where n is the index of refraction of the medium, a "shock wave" of photons is generated in the blue light range, analogous to the sonic boom of a jet flying faster than the speed of sound. This is applicable in nuclear medicine in the assaying of P-32, which in a water solution has a detectable (by liquid scintillator apparatus) Cherenkov emission.

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Douglas J. Wagenaar, Ph.D., wagenaar@nucmed.bih.harvard.edu