Roy and Niels

Roy and Niels

Monday, November 29, 2010

The Stopping Power of Frozen Water

In my last blog entry I commented on stopping powers of fast ions in medical physics, and announced the libdEdx library. (Stopping powers describe the energy loss of fast charged particles in material, thereby transferring energy to the target matter.) Stopping powers directly relate to the deposited dose, but there are plenty of more subtle effects where they may or may not have a profound influence:
  • Range of ions in matter. Often the mean excitation energy (not to be confused with the ionization potential or w value...) in the Bethe-equation is used as a macroscopic fitting parameter for the range of ions. Effects such as an primary particle dependent I-value are reported, even this is unphysical. Discussion is going on what the I-value for water actually is covering the 75 to 85 eV interval. PSTAR claims 75 eV. More recent studies seem to agree on a value close to 80 eV.
  • Ionization chambers rely on a solid assertion of stopping power ratios, since these detectors measure dose to air. In order to translate this to dose to water, you should know the particle spectrums and the stopping power ratio of water to air (see e.g. IAEA TRS-398 dosimetry protocol, so far one of the best out there, even though it has its flaws...) Or, you can use a parametrization, as Armin tries to show in our recent yet unpublished paper (pre-print).
  • Detector and biology response models such as the Katz or LEM model rely on the stopping power of ions. How large these effects are, is still to be investigated, and is something we want to look at using libdEdx and libamtrack.
There may be many more applications, whereas the first two mentioned here are quite well researched. Frustrating enough, if you have to calculate e.g. the stopping power ratios for a given particle spectrum, you have to rely on the ICRU49 (PSTAR/ASTAR) table and the ICRU73, and they are not calculated consistently. Ok, the errors may be minor for practical dosimetry purposes, but thinking of primary standard laboratories such as PTB in Braunschweig or NPL in London who try to increase the precision at least one order of magnitude, you may get into difficulties.

How can this be, don't we have a large data base on experiments for various ions on various targets? Well, yes, for some ion/target combination, but not for all of them. Peter Sigmund from University of Southern Denmark, (now Professor Emeritus), once showed a very nice matrix of combinations at our 4th Danish Workshop, where all the experimental gaps are.
Even worse is the situation for compounds, here no or very little data are available.

So, we decided to take this up a the 5th Danish Workshop on Particle Therapy, in order to sort out the field, and give the research some direction.
This brings me back to the title of this blog entry: The workshop was scheduled to take place tomorrow (30th November) in Aarhus, but exactly due to the stopping power of frozen water, we had to cancel it. Several key persons were stuck in various airports and could not make it because of snowstorms.

Now... this massive amount of snow in Aarhus at this time of the year is not common, and frankly, I wonder if I am going to make it to work tomorrow. Instead, I would like to invite you - dear reader of this blog - to stop a few minutes with me and silently enjoy the scenery below, accompanied with a piece of J.S. Bach.

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