title: Energy dependent response of plastic scintillation detectors to photon radiation of low to medium energy.
abstract: OBJECTIVE
  Plastic scintillation detectors are promising candidates for the dosimetry of low- to medium-energy photons but quantitative knowledge of their energy response is a prerequisite for their correct use. The purpose of this study was to characterize the energy dependent response of small scintillation detectors (active volume <1 mm(3)) made from the commonly used plastic scintillator BC400.
  
  
  METHODS
  Different detectors made from BC400 were calibrated at a number of radiation qualities ranging from 10 to 280 kV and at a (60)Co beam. All calibrations were performed at the Physikalisch-Technische Bundesanstalt, the National Metrology Institute of Germany. The energy response in terms of air kerma, dose to water, and dose to the scintillator was determined. Conversion factors from air kerma to dose to water and to dose to the scintillator were derived from Monte Carlo simulations. In order to quantitatively describe the energy dependence, a semiempirical model known as unimolecular quenching or Birks' formula was fitted to the data and from this the response to secondary electrons generated within the scintillator material BC400 was derived.
  
  
  RESULTS
  The detector energy response in terms of air kerma differs for different scintillator sizes and different detector casings. It is therefore necessary to take attenuation within the scintillator and in the casing into account when deriving the response in terms of dose to water from a calibration in terms of air kerma. The measured energy response in terms of dose to water for BC400 cannot be reproduced by the ratio of mean mass energy-absorption coefficients for polyvinyl toluene to water but shows evidence of quenching. The quenching parameter kB in Birks' formula was determined to be kB = (12.3 ± 0.9) mg MeV(-1) cm(-2).
  
  
  CONCLUSIONS
  The energy response was quantified relative to the response to (60)Co which is the common radiation quality for the calibration of therapy dosemeters. The observed energy dependence could be well explained with the assumption of ionization quenching as described by Birks' formula. Plastic scintillation detectors should be calibrated at the same radiation quality that they will be used at and changes of the spectrum within the application need to be considered. The authors results can be used to evaluate the range of validity of a given calibration.
authors.affiliation: Fakultät Physik, Technische Universität Dortmund, Otto-Hahn Str. 4a, 44221 Dortmund, Germany.
authors.forename: Melanie
authors.initials: M
authors.lastname: Ebenau
authors.affiliation: Physikalisch-Technische Bundesanstalt, Bundesallee 100, 38116 Braunschweig, Germany.
authors.forename: Désirée
authors.initials: D
authors.lastname: Radeck
authors.affiliation: Physikalisch-Technische Bundesanstalt, Bundesallee 100, 38116 Braunschweig, Germany.
authors.forename: Markus
authors.initials: M
authors.lastname: Bambynek
authors.affiliation: Fakultät Physik, Technische Universität Dortmund, Otto-Hahn Str. 4a, 44221 Dortmund, Germany.
authors.forename: Holger
authors.initials: H
authors.lastname: Sommer
authors.affiliation: Universitätsklinikum Essen, Hufelandstr. 55, 45147 Essen, Germany.
authors.forename: Dirk
authors.initials: D
authors.lastname: Flühs
authors.affiliation: Fakultät Physik, Technische Universität Dortmund, Otto-Hahn Str. 4a, 44221 Dortmund, Germany.
authors.forename: Bernhard
authors.initials: B
authors.lastname: Spaan
authors.affiliation: Fakultät Physik, Technische Universität Dortmund, Otto-Hahn Str. 4a, 44221 Dortmund, Germany.
authors.forename: Marion
authors.initials: M
authors.lastname: Eichmann
mesh_terms: D000388:Air; D002138:Calibration; D003037:Cobalt Radioisotopes; D003198:Computer Simulation; D004583:Electrons; D004735:Energy Transfer; D008962:Models, Theoretical; D009010:Monte Carlo Method; D019047:Phantoms, Imaging; D017785:Photons; D010969:Plastics; D011145:Polyvinyls; D011874:Radiometry / Q000295:instrumentation* / Q000379:methods; D012588:Scintillation Counting / Q000295:instrumentation* / Q000379:methods; D014050:Toluene; D035501:Uncertainty; D014867:Water
chemical_list: D003037:Cobalt Radioisotopes; D010969:Plastics; D011145:Polyvinyls; D014867:Water; D014050:Toluene