Research On Key Technologies For Gaseous Detectors Used In Biological Effects Of Radiation

Advanced radiotherapy such as proton and heavy ion therapy,shows a great advantage compare to the conventional radiation therapy,which is considered as an ideal method for cancer treatment.Unlike conventional high-energy photon therapy which only take into consideration the macroscopic absorb dose,measurements should be performed to assess the radiation damage to human tissue target in proton and heavy therapy,which take into account the random interaction between radiation and tissue in microscopic scale such as chromosomes or DNA.The weighting factor for the biological effect is based on these measurements.The development of the related tissue equivalent radiation detectors plays a key role in promoting the novel dosimetry concepts.The concept of microdosimetry and nanodosimetry has been established in the process of studyingthe microscopic radiation dosimetry for biological effect evaluation.The key obstacles of two gaseous detectors optimization for proton and heavy ion therapy are studied in this paper.Namely the miniaturization of the spherical tissue proportional counter?TEPC?for microdosimetrymeasurement in particle therapy and the method of direct detection of low energy ions in a novel optical avalanche chamber aiming for the nanometric spatial resolution for the measurement of particle track structure.The spherical tissue equivalent proportional counter is a detector used in the measurements of the microdosimetric spectrum which has an isotropic response to incident radiation.The detector filled with low pressuretissue equivalent gas to simulate the size of the cell.However,in the practical application of particle therapy,the sensitive volume is too large that the high intensity ion beam can cause pile-up effect of signal in the detector.One solution is to develop amini tissue equivalent proportional counter?mini-TEPC?of the 1mm in diameter in cylindrical shape.Due to the difficulty of constructing uniform electric field in mini-TEPC,the realization of spherical TEPC miniaturization is hindered.The optical avalanche imaging chamber?OPAC?,which has previously been established in PTB,which is capable of acquring particle track structure information from a single measurement.The optimal resolution of the detector was about 30nm,attribute to the effect of electron diffusion in the time projection chamer,which can notmeet the need of measuring particle track structure with nanometric resolution.One newly proposed approach is to innovate the working principle of time projection chamber,to detect the slow ions instead ofelectrons,which involves the effective conversion of ions to electrons using a conversion layer,is the key technology for releasing the concept of a novel detector with nanometric resolution for particle track structure.The main studies and conclusions of this paper are highlighted as follows:?1?The electric field of the Benjamin proportional counter is calculated using finite element method,aiming at the miniaturizationof the sphereical tissue equivalent proportional counter for the microdosimetryuses.The Benjamin-type counter is optimized in this study.The results show that the counter can maintains a uniform electric field along the surface of the anode wire when reducing to 1/10 of its current available size.?2?In order to overcome the effect of electron diffusion in conventional Opitcal avalanche chamber and to develop a TPC detector with nanometrical spatial resolution for heavy ion track structure.A novel method for the direct detection of low energy ion using a time projection chamber is proposed,which converts slow ions to secondary electrons proposed by Auger neutralization.This method is promising to be applied in the field of neutrinoless double beta decay detection.?3?In order to study secondary electrons induced by low energy ions by potential emission in the time projection chamber,a set of mini ion accelerator system wasndeveloped in the laboratory,at PTB,Germany.With the aid of SIMION simulation,the production and transportation of with energy from 20eV to1keVmonoenergiticAr⁺,Ar²⁺and He⁺ion beam is realized.A microchannel plate?MCP?and a secondary electron multiplier?EM?detector is calibrated.?4?Ultrananocrystalline diamond?UNCD?,nitrogen doped Ultrananocrystalline diamond??N?UNCD?,and hydrogen terminated nitrogen doped Ultrananocrystalline diamond??N?UNCD:H?samples were manufactured.The secondary electron emission porperties of these materials was measured in the low energy section after careful calibration of measurement system.The results shows that the Gamma value of UNCD,?N?UNCD and?N?UNCD:H induced by 20eV Ar⁺is 1.08%,0.53%and 0.74%,respectively.After heatingwith a moderate temperature,the Gamma value increased to2.08%,2.82%and 1.50%.The Gamma value of UNCD and?N?UNCD:H induced by40eV He⁺is 20.4%and 11.8%,respectively.After heating,the Gamma increased to 30.9%and 26.9%.The vacuum chamber is maintained in a vacuum degree of about 10^-6 mbar,Gamma decreases withexposure time.In summary,the possibility of miniaturization of the Benjamin proportional counter detector can be achieved and optimized detector is promising to be used as a mini tissue equivalent proportional counter.Experiments were performed using low energy ions and three kinds of diamond samples,slow He⁺with higher ionization potential energy and samples of diamond samples have excellent emission performance,while the Gamma of slow Ar⁺and diamond is only about 1%.Among these three diamond samples,UNCD exhibits the best secondary electron emission properties for low energy ions.Due to the presence of surface contamination,the?N?UNCD:H samples with negative electron affinity doesn't show high emission performance as expected.