Blazevic, A.

Brabetz, C.

Busold, S.

Deppert, O.

Roth, M.

Schreiber, A.

Schumacher, D.

GSI Repository

Coloration quenching of radiochromic films irradiated with proton energiesclose to maximum energy loss ∗O. Deppert1, C. Brabetz2, S. Busold1, A. Schreiber1, D. Schumacher3, A. Blaz˘evic´3, and M. Roth11Technische Universita¨t Darmstadt, Darmstadt, Germany; 2Johann Wolfgang Goethe-Universita¨t, Frankfurt am Main,Germany; 3Helmholtzzentrum fu¨r Schwerionenforschung, GSI, Darmstadt, GermanyRadioChromic Films (RCF) are widely used as beam di-agnostics in the context of laser driven proton acceleration,where typically angular divergent beams (up to 25◦ halfopening angle) with broad exponential energy spectra aregenerated. RCFs provide a precise measurement of the spa-tial beam profile and allow to determine the angular distri-bution. Moreover, used in a so called ”stacked“ configura-tion, where several RCFs are staggered behind each other,enables to fully reconstruct the spectral properties of theproton beam. This can be combined with special structuredlaser-targets to additionally reconstruct important beam pa-rameters like proton source size and transverse and longi-tudinal beam emittance. The methods to fully character-ize laser-accelerated proton beams were earlier publishedby our group as the so called Radiochromic film ImagingSpectroscopy (RIS) [1].In the scope of the ongoing research to further optimize theRIS, we have focused the attention to the response func-tion of the active layer of different RCF types. Therefore,we have carried out precise dose calibrations with 10 MeVprotons from the TANDEM accelerator at the Helmholtz-Zentrum Dresden-Rossendorf to reduce the uncertainty indeposited dose to be less than 5%. Besides this, we haveimplemented a multi-color-channel calibration [2] whichhandles each digitized RGB-channel independently. A dy-namic range weighting allows to choose the optimum ratioof each calibration to calculate a weighted mean dose fromthe three color-values. This method extends the dose rangeof the RCF by a factor of four while at the same time in-creases the calibration precision over the whole dose range.Furthermore, we have investigated in detail the dose-response function if irradiated by proton energies close totheir maximum energy deposition inside the active layer.The maximum energy deposition integrated over the thick-ness of the active layer material gets reached, if a pro-ton with certain energy gets fully stopped at the end ofthe active layer material. Therefore, we carried out mea-surements to scan the proton energy around its maximumenergy deposition and investigated its influence on the re-sponse function of the RCF. We used complementary scan-ning methods to vary the proton energy in front of the activelayer, either by reducing the proton energy of the accelera-tor (direct scan) or by using stacked RCFs with aluminumfoils of different thicknesses in front. The direct scan wasadditionally carried out for two different beam currents. Tomodel our experiment we have used the GEANT4 toolkit[3] to simulate the energy deposition inside the active layer∗This work is supported by HIC4FAIR.Figure 1: Relative effectiveness of RCF response functionfor different proton energies into active layer of HD-V2.material of HD-V2 RCFs. We numerically investigated aparameter study for different proton input energies usedin the experiment. It was found that the coloration offilms irradiated close to the maximum energy loss is signif-icantly reduced, resulting in a lower dose estimation fromour calibration compared to the simulated deposited dose ofGEANT4. The relative effectiveness ηeff (ratio of measuredto simulated dose) of the response function is shown in fig-ure 1. It can be clearly seen that the effectiveness follows aBragg-curve like slope with its minimum of (60.5± 0.3)%for input energies of (487± 22) keV which corresponds tothe input energy of maximum integrated energy loss in theactive layer. To fit the data-set (light-blue, 2σ confidenceblack-dashed) a parameterized Bragg-model was applied.In order to deconvolve our results from the layer thicknesswe have developed a proton tracking routine based on theSRIM tables. It was embedded inside a least-square fit-ting routine. The model to deconvolve from layer thicknessis derived from the derivative of the parameterized Bragg-model (self-similar ansatz) used before and optimized dur-ing the tracking. With this approach we were able to con-struct a relative differential effectiveness dηeff/dx whichcan be applied to any MonteCarlo or tracking routine tocalculate the response function of RCFs from the differen-tial energy loss including the quenching correction.Applied to laser-accelerated proton beams, the undervaluein calculated particle numbers will be approx. 20% if theeffectiveness of the RCF is not taken into account.References[1] F. Nu¨rnberg et al., Rev. Sci. Instrum. 80, 033301, 2009[2] O. Hupe et al., Med. Phys. 33, 4085, 2006[3] S. Agostinelli et al., Nuc. Instrum. Meth., 506.3, 250-303,2003GSI SCIENTIFIC REPORT 2013 PNI-PP-12doi:10.15120/GR-2014-1-PNI-PP-12 193

Coloration quenching of radiochromic films irradiated with proton energies close to maximum energy loss

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Coloration quenching of radiochromic films irradiated with proton energies close to maximum energy loss

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