Comparison of two methods for assessing leakage radiation dose around the head of the medical linear accelerators

Objective:The aim of this study was to measure the leakage by two methods with ion chamber and ready packs film,and to investigate the feasibility and the advantages of using two dosimetry methods for assessing leakage radiation around the head of the linear accelerators.Methods:Measurements were performed using a 30 cm3ion chamber;the gantry at 0°,the X-ray head at 0°,the field size at between the central axis and a plane surface at a FSD of 100 as a reference,a series of concentric circles having radii of 50,75,and 100 cm with their common centre at the reference point.The absorbed dose was measured at the reference point,and this would be used as the reference dose.With the diaphragm closed,the measurements were taken along the circumference of the three circles and at 45°intervals.Results:Leakage radiations while the treatment head was in the vertical position varied between 0.016%–0.04%.With the head lying horizontally,leakage radiation was the same order magnitude and varied between 0.02%–0.07%.In the second method,the verification was accomplished by closing the collimator jaws and covering the head of the treatment unit with the ready pack films.The films were marked to permit the determination of their positions on the machine after exposed and processed.With the diaphragm closed,and the ready packs films around the linear accelerator the beam turned on for 2500 cGy(2500 MU).The optical density of these films was measured and compared with this of the reference dose.Leakage radiation varied according to the film positions and the magnitude of leakage was between 0.005%–0.075%.Conclusion:The differences between the values of the leakage radiation levels observed at different measurement points do not only reflect differences in the effective shielding thickness of the head wall,but are also related to differences in the distances between the target and the measurement points.The experimental errors involved in dosimetric measurement also contribute to such differences.

Measurement and analysis on distribution of radiation field dose inside thetreatment room of TaiChi Pro multimodal integrated digitalradiotherapy system

Objective:To examine the leakage radiation dose from the accelerator module's primary beam self-shielding,aswell as stray radiation levels and distribution in the treatment room across various planes for the innovative X/γmultimodal radiotherapy system,TaiChi Pro,which is the world's first digitally integrated system that combines alinear accelerator,multi-source rotational focused gamma knife,and multi-modal image guidance,in order toformulate treatment room safety strategies and estimate additional radiation doses to patients.Methods:The TaiChi Pro multi-mode integrated radiotherapy system was utilized with a 6 MV X-ray accelerator inthe flatten-filter free(FFF)treatment mode.The system has a primary radiation beam self-shielding,and anisocenter dose rate of 800 cGy/min.For detection points in various planes,an x,y,z coordinate system wasestablished with the isocenter as the origin.To simulate the situation of patient,water-equivalent phantom wasemployed.The ambient dose equivalent and ambient dose equivalent rate at detection points in the radiation fieldwere measured using thermoluminescence dosimeters.Results:The self-shielding area achieved a maximum dose of 114.0 mGy at 1 m distance from the target point after3 min of cumulative irradiation.The leakage radiation ratios at all points remained below 0.5%,fulfilling the selfshielding design requirements.Regarding the couch,the highest radiation was observed in the opposite directionof the beam at the target point.The stray radiation on the treatment bed plane resulted in an ambient doseequivalent that did not exceed 0.5%of the ambient dose equivalent at the central point.Conclusion:The levels and distribution patterns of stray and leakage radiation inside the TaiChi Pro treatmentroom meet the requirements of the relevant national standard.The measured values of indoor radiation levels anddistribution are very useful for room shielding design and personnel protection.

Performance improvement of a pulse tube cryocooler with a single compressor through cascade utilization of cold energy

The high-frequency pulse tube cryocooler(HPTC)has been attracting increasing and widespread attention in the field of cryogenic technology because of its compact structure,low vibration,and reliable operation.The gas-coupled HPTC,driven by a single compressor,is currently the simplest and most compact structure.For HPTCs operating below 20 K,in order to obtain the mW cooling capacity,hundreds or even thousands of watts of electrical power are consumed,where radiation heat leakage accounts for a large proportion of their cooling capacity.In this paper,based on SAGE10,a HPTC heat radiation calculation model was first established to study the effects of radiation heat leakage on apparent performance parameters(such as temperature and cooling capacity),and internal parameters(such as enthalpy flow and gas distribution)of the gas-coupled HPTC.An active thermal insulation method of cascade utilization of the cold energy of the system was proposed for the gas-coupled HPTC.Numerical simulations indicate that the reduction of external radiation heat leakage cannot only directly increase the net cooling power,but also decrease the internal gross losses and increase the mass and acoustic power in the lower-temperature section,which further enhances the refrigeration performance.The numerical calculation results were verified by experiments,and the test results showed that the no-load temperature of the developed cryocooler prototype decreased from 15.1 K to 6.4 K,and the relative Carnot efficiency at 15.5 K increased from 0.029%to 0.996%when substituting the proposed active method for the traditional passive method with multi-layer thermal insulation materials.