A Beam Monitor Using Silicon Pixel Sensors Topmetal For Hadron Therapy

Cancer has always been a hard nut to crack for mankind.High energy X-rays are being rou-tinely used in most cancer treatments with radiation therapy.With the development of techno-logy,radiation therapy with hadrons,most commonly with protons or carbon ions,has been growing rapidly in recent years.The advantage of hadrons over X-rays is that energy deposition of hadrons takes place mostly in the last few millimeters before they come to rest(Bragg peak).Thus hadrons are especially useful for the deep-seated tumors and tumors close to critical or-gans.Today,about 70 proton and carbon-ion therapy centers are in operation all over the world.In hadron therapy,to ensure accurate delivery of a prescribed dose to the tumor zone,a beam monitor system is required to measure the beam’s intensity,position,and profile in real time.Currently,parallel-plate ionization chambers with one large electrode or electrodes segmented in strips or pixels are commonly used at hadron therapy facilities to monitor the beam online.However,restricted by the mm-scale width of strips and pixels,ionization chambers are not well suited for monitoring pencil beam of mm-scale size.Besides,the charge collection efficiency of ionization chambers is sensitive to the gas gap between electrodes.The nonuniform gas gap between electrodes will result to the inaccurate measurement of the position,profile and intensity of the beam.Topmetal series silicon pixel sensors are based on 0.35μm CMOS technology and are cha-racterized with naked metal in the top layer,which can collect charges drifting from the space directly.The first-generation sensor Topmetal-I contains 64 x 64 pixels with the pixel size of 80 x 80μm2.The equivalent noise charge of Topmetal-I in analog readout is 327e-.The second-generation sensor Topmetal-II-contains 72 x 72 pixels with the pixel size of 83 x 83μm2.The equivalent noise charge of Topmetal-II-in analog readout is 13e-.This paper provides a new type beam monitor based on Topmetal series silicon pixel sensors.The three main works are as follows:Firstly,we use 1 Topmetal-I sensor as anode to design one-dimensional beam monitor.In our design,the beam passes through the beam monitor without hitting the electrodes,making the beam monitor especially suitable for monitoring heavy ion beams.This design also reduces radi-ation damage to the beam monitor itself.The proton beam test at 88 inch cyclotron of Lawrence Berkeley National Laboratory(LBNL)shows the feasibility of the beam monitor.The test results show that position resolution is better than 35μm,intensity accuracy is better than 8%and the beam center position tested can reflect the movement of the beam.The position resolution and intensity accuracy aren’t the intrinsic resolution of the monitor,which include the fluctuation of the beam.Secondly,to enlarge the sensitive area of the monitor,we use 8 Topmetal-Ⅱ-sensors array as anode to design one-dimensional beam monitor.The equivalent noise charge of Topmetal-Ⅱ-is lower than that of Topmetal-Ⅰ.So the monitor based on Topmetal-Ⅱ-is more sensitive than that based on Topmetal-Ⅰ.The monitor is tested in the former superficially-placed tumor therapy terminal at the Heavy Ion Research Facility in Lanzhou(HIRFL)China.The carbon ion beam passing through the collimator is well centered on one sensor.The test results show that the beam monitor can measure position,incidence angle and intensity of the beam with a position resolution 17 μm,angular resolution about 0.5° and intensity accuracy better than 2%.Taking advantage of lower than 0.1 mm position resolution,the monitor is well suitable for radiation targets with small sizes and high dose gradients.But this monitor can only measure the one-dimensional profile of beams.Moreover,in the monitor a dead area of 5.4 mm exists between neighboring sensors.Thirdly,to measure the two-dimensional profile of the beam,we utilize two-dimensional silicon pixel sensor arrays(32 sensors in total)to design the two-dimensional beam monitor.In each dimension,two lines of eight sensors are aligned staggered to eliminate the dead area.The sensitive area of the monitor is 9 × 9 cm2.The data of the monitor is read out via TCP/IP continuously.The argon ion beam test in the External Target Facility at HIRFL shows that the monitor can measure the beam profile every 3.3 ms and the time structure of the spill.In hadron therapy,the position resolution of parallel-plate ionization chambers is sub-millimeter(>0.1mm&<lmm).The position resolution of the beam monitor based on Topmetal-Ⅱ-in the paper is better than 20μm,which can satisfy the requirement of pencil beam of mil-limeter scale(especially 1～2 mm).In the design of the beam monitor,the beam pass through the monitor without hitting the sensors,reducing radiation damage to the monitor and making it especially suitable for monitoring heavy ion beams.Moreover,the monitor contributes to reduce the recombination for the sharp pencil beam.