In recent years, Gas Electron Multiplier (GEM) neutron detector has been developing towards high spatial resolution and high dynamic counting range. A novel concept of the Al stopping layer was proposed to enable the detector to achieve sub-millimeter (sub-mm) spatial resolution. The neutron conversion layer was coated with the Al stopping layer to limit the emission angle of ions into the drift region. The short track projection of ions was obtained on the signal readout board, and the detector would get good spatial resolution. The spatial resolutions of the GEM neutron detector with Al stopping layer were simulated and optimized based on Geant4GarfieldInterface. When Al stopping layer was 3.0 μm thick, drift region was 2 mm thick, strip pitch was 600 μm, and digital readout was employed. The spatial resolution of the detector was 0.76 mm, and the thermal neutron detection efficiency was about 0.01%. Thus, the GEM neutron detector with a simple detector structure and a fast readout mode was developed to obtain a high spatial resolution and high dynamic counting range. It could be used for the direct measurement of a high-flux neutron beam, such as Bragg transmission imaging, very small-angle scattering neutron detection and neutron beam diagnostic.

In-beam PET is one of the imaging-based methods for monitoring carbon therapy by reconstruction of positron-emitters produced by incident particles. The accurate Monte Carlo simulation is necessary for range verification and therapeutic dose monitoring by means of the in-beam PET. GATE is capable of performing in-beam PET imaging but very few studies assess the accuracy of activity range measured from PET imaging with GATE simulated results. In this work, we present both experimental and GATE simulation studies of in-beam PET imaging of a phantom, which is irradiated by carbon ion beam, for range verification. The experiment data is acquired at Wuwei Heavy Ion Cancer Treatment Center in Gansu, China, with a dual-head plate PET prototype. The PET prototype consists of 2 panels, each arranged in 2 × 2 matrix. Each detector block is composed of 20 × 20 LYSO crystal array coupled with the position-sensitive photomultiplier tube (HAMAMATSU H8500). A homogeneous plastic phantom is irradiated with monoenergetic 131.05 MeV and 190.19 MeV 12C ion pencil beams. The irradiation and full response of PET prototype are simulated with GATE macros. A novel mathematical model is established, which is capable of calculating and revealing the variation of the positron activity. Our focus is on the reconstructed activity ranges obtained by the experimental measurement and GATE MC prediction combined with the positron activity distribution calculation mathematical model. Results show that the reconstructed activity distribution of experimental data and GATE MC prediction are in good agreement. The measured and simulated 1D activity peak and falling edge positions are within 1.0 mm in all cases. The 20%, 50% and 80% PET peak positions predicted by GATE are close to measurements. These results indicated that it is feasible to assess the accuracy of activity range measured from the dual-head plate PET system with the modeled GATE hadron-PET simulation.

In the heavy ion radiotherapy, the dose distribution is an important index to ensure the therapeutic effect. But it can not be directly measured during a treatment. Positron-emitting radionuclides (10 C, 11 C, and 15 O et al.) produced in the treatment process are deposited along the path and the end of the beam trajectory, so the positron activity distribution can be used to monitor the dose distribution of carbon ions. In this paper, we present results of measurements of positron activity distributions with a developed scanner system. Then the relationship between the activity distribution of positrons and the dose distribution of carbon ions is studied. The research shows that the activity distribution of positrons is quite similar to that of carbon ions in the direction of penetration depth. The activity peak position of positrons is smaller than the dose peak position of carbon ions, and this difference increases along with the increase of carbon ion beam energy. These experimental results are consistent with the GATE simulation results. It is found that the detector system can distinguish shift of peak position of 3 mm that corresponds change of a particle energy of 4 MeV/u. This indicates that the positron scanner system had been successfully developed for heavy ion therapy equipment.