The Research On Beam Dose-Depth Distribution Measurement Method Based On Topmetal

In carbon ion therapy,the accurate detection of dose-depth distribution of carbon ion beam is very important for precise delivery of beam dose to the tumor.At present,the beam dose-depth distribution can be measured by position emission tomography(PET)or prompt gammas.The detectors used in PET or prompt gamma scanner are usually scintillators combined with photomultiplier tubes(PMT)or avalanche photodiodes(APD),with low spatial resolution.Furthermore,there are undetectable areas caused by PMT-readout or APD-readout.Semiconductor CdZnTe pixel array detector has become a research focus due to the high spatial resolution,detection accuracy and the small size.Topmetal family is a series of pixel array readout chip with independent intellectual property rights which are able to receive charge directly.The main advantages are low noise and high sensitivity.This thesis studies from the scintillator detector to the CdZnTe detector based on Topmetal with high spatial resolution,and then focuses on the beam dose-depth distribution measurement methods,aiming at improving measurement accuracy and sensitivity.The detailed research contents include:Firstly,a collimated prompt gamma detector based on scintillators is proposed and designed for detecting the dose-depth distribution of carbon ion beam.The physical process of carbon ion beam injected to water phantom is simulated.The characters of the secondary particles are analyzed,including the position,angle and energy.Based on the analysis,a collimator system(lead layer,paraffin layer,boron carbide)is designed,and its structure parameters are optimized.The influence of neutrons on the measurement results is reduced by optimizing inclination of the detector.The results show that the depth-dose distribution of carbon ion can be measured by the detector of this structure,and the device designed according to this structure is preliminarily tested with the 300MeV/u argon ion beam in HIRFL.The detector works without shielding the neutron due to the site space limitation.The result shows that the detected peak is shifted backward compared to the theoretical value,which is consistent with the simulation result.Secondly,we propose a method to improve the detection accuracy of the carbon ion beam dose-depth distribution by using a CdZnTe pixel array detector based on Topmetal-?a.It is known that the accuracy of the detector depends on the size of the electrode.The sizes of traditional PMT and APD are centimeter magnitude order,while the pitch of Topmetal-?a is only 40/?m,which is able to achieve high spatial resolution.In addition,in order to ensure the consistency of pixel array,reset voltage of CSA in each pixel is designed to optimize independently to achieve consistency of the discharge time of the pixel array.Test result shows that the optimization of the reset voltage can increase standard deviationthe of pixel array discharge time from 8.8ms to 2.3ms,and the consistency increase also improves the spatial resolution of the detector.The detector gets a sub-pitch spatial resolution of 23.45?m.Thirdly,an AC coupling in pixel is proposed to solve the saturation problem of the output signal of Topmetal pixel array readout chip caused by the leakage current of CdZnTe.At present,the CdZnTe pixel array detector is DC coupled,the leakage current compensation circuit is required to improve the detector's insensitivity of the leakage current.The main drawback of this scheme is the circuit's noise essentially improved,and its typical equivalent noise charge(ENC)is about 100e-.The new Topmetal-?a uses AC coupling to eliminate the effects of DC leakage current.The circuit is simple and the noise is quite low(typical ENC is 12.4e-).The finite element method(FEM)is used to simulate the charge-receiving efficiency of AC coupling.The simulation result shows that the efficiency is about 45%of DC coupling.It can be found that this method is capable to improve the ratio of signal to noise and thus improve the detection sensitivity.The new CdZnTe pixel array detector based on Topmetal-?a has high spatial resolution and sensitivity,which is expected to be used in Compton-CZT PET or prompt gamma detection to play a positive role in improving the accuracy of beam delivery in carbon ion therapy.