Background Research Of Tonne-scale Germanium Detector For Dark Matter Searches

The direct detection of dark matter has a great significance to the development of the field of fundamental physics.To improve sensitivity of the direct detection,it is a general trend to build low-background and large-mass detectors.CDEX projects to construct tonne-scale of germanium detectors and increase the sensitivity to 10^-45 cm² for search of light mass WIMPs（<10GeV/c²）.To achieve the goal,a series of scientific problems need to solve and one key problem is studying the background of tonne-scale detectors.Based on simulation and experiment,background of tonne-scale detectors is analyzed in this paper.Background of tonne-scale detectors can be classified as environmental background,background of shield,background from structures of detector and background of germanium crystal.The background from cosmic rays and its secondary particles,environmental gammas and neutrons in CJPL-II is obtained by measurements and simulations.A step by step simulation is developed to slove the problem of deep penetration when shielding the environmental background by liquid nitrogen.According to simulation,cosmogenic radionuclide of ⁷Be is found existing in liquid nitrogen and its concentration is obtained by energy spectrum of CDEX-10.The concentration of ²²²Rn（0.1 mBq/kg）in liquid nitrogen and its upper limit（1.3 mBq/kg）are analyzed by fitting peaks of primordial radionuclides in the spectrum of CDEX-10.Background from radionuclides in liquid nitrogen is evaluated.Copper is widely used in the tonne-scale detectors.While evaluations of cosmogenic radionuclides in copper are almost calculations and experimental measurement is few.Worse,calculations are not coincident.Measurement of cosmogenic radionuclides in copper is performed in this paper.Copper bricks are exposed in high altitude zone and yield of cosmogenic radionuclides is measured.The half-life of ⁵⁸Co is obtained by analyzing the change of radioactivity and it is consistent with nuclide database.Comparing to measurement,the calculation from physics list of Shielding in Geant4 is underestimated,especially for ⁵⁴Mn（5 times underestimation）.The difference can be reduce in the error range of 2times by selection of physics list of QGSP_INCLXX.An elaborate simulative model of tonne-scale detector unit is built by MC and backgrounds from kinds of structures are analyzed.The main background in copper structures is from copper shell,and insulating strip and circuit board constitute the major background in the non-copper structures.Peaks of X-rays from Cu in the low energy region are found for copper structures,while many peaks of X-rays from decay chains of ²³⁸U and ²³²Th exist for non-copper structures.Based on analysis of kinds of background,a background model for tonne-scale germanium detectors is built and methods of background control are proposed.