Ionising Radiation. Absorption, Attenuation and HVT

Radioactive material used in medicine can be used for either diagnosis or therapy. In both cases the primary directive is to minimize patient dose to non-critical regions. This is best achieved using where possible isotopes with short half lives and fast metabolic or physical elimination. The second directive is to use material indistinguishable metabolically from the stable version.

First, a quick look at ionizing radiations and their properties. Any charged massive particle can ionize atoms directly by fundamental interaction through the Coulomb force if it carries sufficient kinetic energy. In living tissue, macromolecular damage can lead to dysfunction – enzymes won’t work if their structure is changed by ionization, for example.Screen Shot 2014-04-14 at 8.22.55 PM

Alpha particles are monoenergetic, massive but slow, stopped by thin paper or a few cm of air, giving up all their energy in a short range, 105 ion pairs mm-1

Beta electrons are emitted with a range of energies, just shy of 2000x lighter than alphas, but 100 x less ionizing ability. Useful for imaging because the particles are penetrating enough to escape from the body and be externally detected. Stopped by thin Al or a few cm/1m of tissue, 103 ion pairs mm-1

Gamma photons – zero rest mass and v=c, stopped by thick (10cm) lead, range several m in air. Only 1 ion pair in each mm of path.

An exponential law of intensity v absorber thickness (x) c.f, law of radioactive decay shows how the intensity decreases with absorber thickness.If the absorber is homogeneous, we can define its absorptive ability by the ‘linear absorption coefficient’ ‘mu’, analogous to decay constant. The larger the value of ‘mu’ the better absorber of radiation the material is. ‘mu’ is related, obviously, to the density of the material but multiplied by a quantity describing how much of a target its atoms present to incoming radiation. We can keep it simple, however and just say:Screen Shot 2014-04-15 at 12.00.30 PMScreen Shot 2014-04-15 at 12.04.59 PMAttenuation or the ability of a material to absorb therefore weaken the radiation, is an exponential function, as shown. We can think about attenuation as ‘partial absorption’.

The HVT or ‘half value thickness’ is therefore the value of thickness x such that I drops to I/2, or

Screen Shot 2014-04-14 at 8.57.42 PMHVT is energy dependent, as one might expect – see fig I2.2, but this is less important than the exponential principle. What’s the HVT of this material? In what units is it measured? Suppose you had an energetic beta source, a detector and several identically thin squares of Al. You might think how you’d measure HVT for Al. Do you see why the source/detector distance would have to be constant throughout?Screen Shot 2014-04-14 at 9.09.51 PM

You should be able to find the HVT for this material hence its attenuation coefficient.

Look at Qs 3 and 4 on p702. These are important – exactly the kind of thing they might ask, so go through them carefully.





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