Reference dosimetry in nonstandard beams

6 problems

• Beam-quality factors and $W_{\mathrm{air}}$ for ion dosimetry

  Partially resolved

  Converting a detector reading into dose in a particle beam needs the average energy spent to make one ion pair in air. Its uncertainty, especially for carbon ions, dominates the dose error.

• Chamber response in treatment-strength magnetic fields

  Partially resolved

  Machines that image with magnetic resonance while treating place the detector inside a strong magnet, which bends the electrons it is trying to count. The resulting response change is not fully predictable.

• Detector correction factors in millimetre-scale fields

  Partially resolved

  In beams only a few millimetres wide the detector itself disturbs the radiation it is measuring. The corrections needed are large and not known precisely.

• Traceable three-dimensional dose measurement

  Open

  Modern treatments shape dose in three dimensions, but routine checks measure it at points or on planes. A trustworthy full-volume measurement does not exist.

• Absolute dosimetry at ultra-high dose rate

  Open

  Standard dose meters saturate when the dose arrives in a single very intense pulse, because the charges they collect recombine before being counted. There is no accepted way to measure such doses absolutely.

• Mean excitation energy of liquid water

  Open

  How quickly a charged particle slows down in water depends on one material constant, the average energy needed to excite a water molecule. Its uncertainty alone produces millimetre-scale uncertainty in where a proton beam stops in water.