The Response of ESR Dosimeters in Thermal Neutron Fields T. Schmitz1, E. Malinen2,3, N. Bassler4, M. Ziegner5,6, M. Blaickner6, H. Karle7, H. Schmidberger7, C. Bauer8, P. Langguth9, G. Hampel1 1 Institut for Nuclear Chemistry, University of Mainz, Mainz, Germany 2 Department of Medical Physics, Oslo University Hospital, Oslo, Norway 3 Department of Physics, University of Oslo, Oslo, Norway 4 Department of Exp. Clinical Oncology, Aarhus University Hospital, Aarhus, Denmark 5 AIT Austrian Institute of Technology GmbH, Vienna, Austria 6 Institute of Atomic and Subatomic Physics, Vienna University of Technology, Vienna, Austria 7 Department of Radiooncology, University of Mainz, Mainz, Germany 8 Max Planck Institute for Polymer Research, Mainz, Germany 9 Department of Pharmacy and Toxicology, University of Mainz, Mainz, Germany

Outline Short introduction of ESR dosimetry Motivation and aim Materials and Methods ESR detectors: Lithium formate and Calcium carbonate Experimental setup at the TRIGA Mainz Results ESR readout Comparison measured and calculated response Dose components Summary 16th International Congress on Neutron Capture Therapy, Helsinki, Finland / Tobias Schmitz, University of Mainz

Introduction: ESR dosimetry Alanine is the best known ESR dosimeter Through ionising particles or neutrons radicals are produced in the pellets Alanine radical L-α-Alanine The amount of radicals is measured by electron spin resonance (ESR) – spectroscopy and correlates to the absorbed dose 16th International Congress on Neutron Capture Therapy, Helsinki, Finland / Tobias Schmitz, University of Mainz

Relative Effectiveness - RE ESR dosimeters are calibrated against 60Co-Gamma-ray source Radical yield is particle and energy dependent ESR signal Z 𝑆 𝑅𝑒𝑓 (𝑅 𝑑𝑒𝑡𝑒𝑐𝑡𝑜𝑟 )= 𝑆 𝑍 (𝐷 𝑑𝑒𝑡𝑒𝑐𝑡𝑜𝑟 ) 𝑹𝑬= 𝑹 𝒅𝒆𝒕𝒆𝒄𝒕𝒐𝒓 𝑫 𝒅𝒆𝒕𝒆𝒄𝒕𝒐𝒓 (Isoresponse definiton) D – Dose R – Response Dose 𝑅 𝑑𝑒𝑡𝑒𝑐𝑡𝑜𝑟 𝐷 𝑑𝑒𝑡𝑒𝑐𝑡𝑜𝑟 16th International Congress on Neutron Capture Therapy, Helsinki, Finland / Tobias Schmitz, University of Mainz

RE calculation For alanine: Done with the Hansen & Olsen alanine response Model * Based on track structure theory by Butts and Katz (again based on Target theory) Implemented as user-written routine to Monte Carlo code FLUKA For Lithium formate: Track structure theory by Butts and Katz ** Build on the assumption that dose is always deposited by secondary electrons Difference in detector response due to different dose distributions * N. Bassler, J.W. Hansen, H. Palmans, M.H. Holzscheiter, S. Kovacevic, and the AD-4/ACE collaboration, “The Antiproton Depth Dose Curve Measured with Alanine Detectors,” Nucl. Instrum. Meth. B 266, 929–936 (2008). ** E. Waldeland, E.O. Hole, B. Stenerlöw, E. Grusell, E. Sagstuen, and E. Malinen, “Radical Formation in Lithium Formate EPR Dosimeters after Irradiation with Protons and Nitrogen Ions,” Rad. Res. 174, 251-257 (2010) 16th International Congress on Neutron Capture Therapy, Helsinki, Finland / Tobias Schmitz, University of Mainz

Alanine results 𝑅𝐸 (𝑛,𝑝) =0.56 𝑅𝐸 𝛾 =1 →𝟎.𝟖𝟔> 𝑹𝑬 𝒕𝒐𝒕𝒂𝒍 >𝟎.𝟗𝟑 (Presented at 15th ICNCT in Tsukuba, Japan, September 2012) Alanine measurement FLUKA dose response MCNP dose response 𝑅𝐸 (𝑛,𝑝) =0.56 𝑅𝐸 𝛾 =1 →𝟎.𝟖𝟔> 𝑹𝑬 𝒕𝒐𝒕𝒂𝒍 >𝟎.𝟗𝟑 16th International Congress on Neutron Capture Therapy, Helsinki, Finland / Tobias Schmitz, University of Mainz

Motivation Separation of dose components: Simulation – Use of Monte Carlo Codes as FLUKA or MCNP → Verification – Use of different phantoms and shieldings Alternative – Use of different ESR materials → Different elemental composition → Different detector response → Dose component identification in one irradiation combining multiple detectors → Can other materials be modelled as good as the alanine detector (incl. determination of RE factors)? 16th International Congress on Neutron Capture Therapy, Helsinki, Finland / Tobias Schmitz, University of Mainz

ESR dosimeters Dosimeter materials: Calcium carbonate 𝑪𝒂 𝑪𝑶 𝟑 Lithium formate 𝑳𝒊(𝑯𝑪𝑶𝑶)∙ 𝑯 𝟐 𝑶 Used as pressed pellets: Diameter: 5 mm Height: 2.5 mm Irradiation in PMMA phantom 10 Pellets on central length axis 16th International Congress on Neutron Capture Therapy, Helsinki, Finland / Tobias Schmitz, University of Mainz

TRIGA Mark II Mainz – Thermal Column Reactor: Isotropic field of thermal neutrons: Neutron flux (100 kW): 2 · 1010 n/(cm2s) Gamma flux (100 kW): 1 · 1010 γ/(cm2s) FLUKA plane source Simulated build-up Phantom position 16th International Congress on Neutron Capture Therapy, Helsinki, Finland / Tobias Schmitz, University of Mainz

ESR readout: Lithium formate Waldeland et al; Rad Meas 46(2011) Medium line width 60Co Photons: 1.49 mT Neutrons: 1.58 mT 16th International Congress on Neutron Capture Therapy, Helsinki, Finland / Tobias Schmitz, University of Mainz

ESR results: Calcium carbonate Identical dominant radical species No line-broadening: → No dose due to medium or high LET particle ESR measurement FLUKA absorbed dose 16th International Congress on Neutron Capture Therapy, Helsinki, Finland / Tobias Schmitz, University of Mainz

Lithium formate inside boron shielding ESR measurement FLUKA absorbed dose 16th International Congress on Neutron Capture Therapy, Helsinki, Finland / Tobias Schmitz, University of Mainz

FLUKA results: Lithium formate ESR measurement FLUKA absorbed dose ESR measurement FLUKA absorbed dose 16th International Congress on Neutron Capture Therapy, Helsinki, Finland / Tobias Schmitz, University of Mainz

FLUKA results: Lithium formate Calcium carbonate 16th International Congress on Neutron Capture Therapy, Helsinki, Finland / Tobias Schmitz, University of Mainz

FLUKA results: Lithium formate ESR measurement FLUKA absorbed dose ESR measurement FLUKA absorbed dose Calculated RE = 0.36 (Experimental RE = 0.42) t α Relative Fluence (cm-2) t α Particle spectra inside the detector RE factors according to track structure theory 16th International Congress on Neutron Capture Therapy, Helsinki, Finland / Tobias Schmitz, University of Mainz

Summary Aim: Evaluation of ESR detectors for a dosimeter set for the measurement of dose components Different ESR detector materials have been irradiated at the TRIGA Mainz. ESR readout has been compared to FLUKA calculations: Good agreement with: Calcium carbonate in PMMA phantom Lithium formate with boron shielding Slight Underestimation of response: Lithium formate in PMMA phantom Theories are not limited to the alanine detector Both detectors are potential materials for the dosimeter set 16th International Congress on Neutron Capture Therapy, Helsinki, Finland / Tobias Schmitz, University of Mainz

Acknowledgements 16th International Congress on Neutron Capture Therapy, Helsinki, Finland / Tobias Schmitz, University of Mainz

Thank you for your attention! Kiitän teitä huomiostanne! Thank you for your attention! Cathedral in Mainz, Germany

RE calculation Done with the Hansen & Olsen alanine response Model* Based on track structure theory by Butts and Katz (again based on Target theory) Build on the assumption that dose is always deposited by secondary electrons Difference in detector response due to different dose distributions Implemented as user-written routine to Monte Carlo code FLUKA Weighting of each dose deposited Depending on type and energy of the dose depositing particle * Hansen, J.W. (1984). Experimental Investigation of the Suitability of the Track Structure Theory in Describing the Relative Effectiveness of High-LET Irradiation of Physical Radiation Detectors. PhD thesis, Risø National Laboratory. Hansen, J. W. and Olsen, K. J. (1984). Experimental and Calculated Response of a Radiochromiv Dye Film Dosimeter to High- LET Radiations. Radiat. Res., 91:1–15. 16th International Congress on Neutron Capture Therapy, Helsinki, Finland / Tobias Schmitz, University of Mainz

Comparison of prim. Photon doses Calculated with FLUKA: Calcium formate Ammonium formate Lithium formate Mass Energy Absorbtion Coefficient Ratios: 𝜇 𝑒𝑛 𝜌 𝑥 𝜇 𝑒𝑛 𝜌 𝑦 Calcium f. / Ammonium f.: 1.36 Calcium f. / Lithium f.: 1.33 16th International Congress on Neutron Capture Therapy, Helsinki, Finland / Tobias Schmitz, University of Mainz

Primary particles in the thermal column Neutron flux and spectrum Gamma flux and spectrum GeV GeV 16th International Congress on Neutron Capture Therapy, Helsinki, Finland / Tobias Schmitz, University of Mainz

Pellet read-out Spectrometer: Bruker ESX Acquisition: 6 x 20 s with 90° rotation after 3. scan Analysis: Peak-to-Peak-amplitude Fitting functions for values below 5 Gy 16th International Congress on Neutron Capture Therapy, Helsinki, Finland / Tobias Schmitz, University of Mainz

Target theory and dose distribution Alanine dosimeters are calibrated by the NPL against 60Co-Gamma-ray source But radical yield is particle and energy dependend Photon irradiation Proton irradiation Target embedded in a passive matrix Hit: energy deposition sufficient for effect Effect Particle Track 16th International Congress on Neutron Capture Therapy, Helsinki, Finland / Tobias Schmitz, University of Mainz