the source code is adapted from the B1 example and TestEm7 of Geant4. Ionization_chamber_cyf_final and ionization_chamber_cyf_final1 is used to obtain the energy loss and gain of the proton beam in the plane-parallel ionization chamber. TestEm7 is used to calculate the equivalent water thickness (WET) of the plane-parallel ionization chamber. The following is the entire abstract of my graduation thesis. You can download and use my code if you meet with some similar problems, and you can also contact with me if you want some discussions. My email is chenyifanxjtu.phy@gmail.com and 2963645692@qq.com.
Proton therapy is an emerging tumor radiotherapy method, which has obvious advantages over conventional photon radiotherapy. A quality assurance (QA) system, to measure the performance parameters of the proton therapy system, mainly the Integral Depth Dose (IDD) curve of the proton beam in water, should be established for the proton therapy device. However, the current QA equipment completely relies on import. The plane-parallel ionization chamber is one of the key parts of QA equipment. Under the general trend of localization of proton therapy devices, it is crucial to develop and test plane-parallel ionization chambers by ourselves. This paper first introduces some related concepts on the basis of the working principle of the ionization chamber, and then completes the design of a plane-parallel ionization chamber and uses the Monte Carlo software Geant4 to simulate the ionization chamber. The energy loss and gain of the proton beam in the plane-parallel ionization chamber were successfully obtained. The calculation of the equivalent water thickness (WET) of the plane-parallel ionization chamber further shows that the influence of the ionization chamber on the energy and position of the proton beam is small. The maximum ionization current of proton beams with different energies (70-244MeV) in the plane-parallel ionization chamber is about 50nA-60nA, which provides an important basis for the design of the preamplifier. Since the plane-parallel ionization chamber needs to be used with a water tank, the third part of this paper obtains the Bragg curve and the Bragg peak position in the water tank through simulation calculations, and further obtains the energy deposition of the proton beam at different depths of the water tank. Based on the above results, the ionization current of the main measuring ionization chamber and the lower sensitivity limit of the preamplifier were calculated. This provides a numerical reference for measuring the integral depth dose curve by manipulating the plane-parallel ionization chamber in a water tank in reality. In the last part of this paper, the completed work is summarized and future work is prospected.