1 National Physical Laboratory, Medical Radiation Science, Teddington TW11 0LW, UK. Electronic address: anna.subiel@npl.co.uk.
2 St. Jude Children's Research Hospital, Memphis, TN 38105, USA.
3 Department of Physics, Scottish Universities Physics Alliance, University of Strathclyde, Glasgow G4 0NG, UK.
4 Department of Physics, Scottish Universities Physics Alliance, University of Strathclyde, Glasgow G4 0NG, UK; Diamond Light Source, Harwell Science and Innovation Campus, Fermi Avenue, Didcot OX11 0DE, UK.
5 Department of Radiation Oncology, Indiana University School of Medicine, Indianapolis, IN 46202, USA.
6 Department of Physics, Scottish Universities Physics Alliance, University of Strathclyde, Glasgow G4 0NG, UK. Electronic address: d.a.jaroszynski@strath.ac.uk.