Development of 6Li-loaded water-based liquid scintillator for the detection of secondary neutrons in proton therapy

High energy proton beam forms secondary neutrons with wide energy spectrum (keV to sub GeV scale) in proton therapy. These neutrons give unintended dose contribution to tissues outside of target volume, and then, may cause other cancer. That is why it is important to measure secondary neutrons produced by high energy proton beam. We are developing
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Li-loaded water-based liquid scintillator in order to measure secondary neutrons. Our liquid scintillator is the mixture of water, organic fluorescent material, lithium bromide (LiBr) and surfactant. In addition, we mix some samples with small quantity of organic solvent to obtain large amount of light yield. This scintillator is more safe and readily controlled than organic scintillators because large part of this scintillator is water. Moreover, since doped
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Li has large cross section of thermal neutron capture, our scintillator can detect neutrons effectively by discriminating neutron from other particles such as gamma-rays. The light from
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Li-loaded water-based liquid scintillator is detected by Multi-Pixel Photon Counter (MPPC). The hit information of MPPC includes the number of photons and timestamps. These are sent to data processing system via ethernet LAN. Then, we can observe the energy and position distribution of secondary neutrons produced by proton beam. This presentation reports how to fabricate water-based liquid scintillator. Then, we show the results of its optical properties and light yield measurement with various gamma-ray sources. Since the emission spectrum from our liquid scintillator imply the wavelength shifter (WLS) is needed to make the best use of MPPC, we introduce the plastic scintillation plate as the WLS and perform Monte Carlo simulation with Geant4 to optimize the width of WLS. Then, we reveal that the ideal width of WLS is more than 10 mm.