Laser and Particle Beams

Research Article

Radiochromic film spectroscopy of laser-accelerated proton beams using the FLUKA code and dosimetry traceable to primary standards

D. Kirbya1 c1, S. Greena2, F. Fiorinia1, D. Parkera1, L. Romagnania3a4, D. Doriaa3, S. Kara3, C. Lewisa3, M. Borghesia3 and H. Palmansa5

a1 School of Physics and Astronomy, University of Birmingham, Birmingham, United Kingdom

a2 Dept of Medical Physics, University Hospital Birmingham NHS Trust, Birmingham, United Kingdom

a3 Centre for Plasma Physics, The Queen's University of Belfast, Belfast, United Kingdom

a4 LULI, Ecole Polytechnique, CNRS, CEA, UPMC, Palaiseau, France

a5 National Physical Laboratory, Acoustics and Ionizing Radiation, Teddington, United Kingdom

Abstract

A new approach to spectroscopy of laser induced proton beams using radiochromic film (RCF) is presented. This approach allows primary standards of absorbed dose-to-water as used in radiotherapy to be transferred to the calibration of GafChromic HD-810 and EBT in a 29 MeV proton beam from the Birmingham cyclotron. These films were then irradiated in a common stack configuration using the TARANIS Nd:Glass multi-terawatt laser at Queens University Belfast, which can accelerate protons to 10–12 MeV, and a depth-dose curve was measured from a collimated beam. Previous work characterizing the relative effectiveness (RE) of GafChromic film as a function of energy was implemented into Monte Carlo depth-dose curves using FLUKA. A Bragg peak (BP) “library” for proton energies 0–15 MeV was generated, both with and without the RE function. These depth-response curves were iteratively summed in a FORTRAN routine to solve for the measured RCF depth-dose using a simple direct search algorithm. By comparing resultant spectra with both BP libraries, it was found that the effect of including the RE function accounted for an increase in the total number of protons by about 50%. To account for the energy loss due to a 20 µm aluminum filter in front of the film stack, FLUKA was used to create a matrix containing the energy loss transformations for each individual energy bin. Multiplication by the pseudo-inverse of this matrix resulted in “up-shifting” protons to higher energies. Applying this correction to two laser shots gave further increases in the total number of protons, N of 31% and 56%. Failure to consider the relative response of RCF to lower proton energies and neglecting energy losses in a stack filter foil can potentially lead to significant underestimates of the total number of protons in RCF spectroscopy of the low energy protons produced by laser ablation of thin targets.

(Received January 04 2011)

(Accepted February 28 2011)

(Online publication April 08 2011)

Keywords

  • FLUKA code;
  • GafChromic film;
  • Linear energy transfer;
  • Optical density;
  • Radiochromic film spectroscopy;
  • Thin metallic targets

Correspondence:

c1 Address correspondence and reprint requests to: Daniel Kirby, School of Physics and Astronomy, University of Birmingham, Birmingham B15 2TT, United Kingdom. E-mail: djk191@bham.ac.uk

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