Experimental Study Of Accelerated Aging Of BESⅢ Drift Chamber Prototype

Beijing SpectrometerIII (BESIII) is a general-purpose detector located in the interaction region at the BEPC2 storage ring, where the electron and positron beams collide. The drift chamber (DC) is one of the most important sub-detectors in BESIII. It's very necessary to ensure that the drift chamber could run regularly after a long time because of the high expense and the long period of the construction. Usually, there are aging problems in the gas detectors. In order to ensure that the drift chamber could run regularly after a long time, it's necessary for us to perform the aging experiment before buiding the drift chamber.At first, The basic principle, structure, aging mechanism of the drift chamber and the aging parameter were described, then the structure of the aging prototype and preparatory work including wiring, sealing, wire tension and leak current measurement, gas system, high voltage system were introduced. The experimental study of accelerated aging of prototype with radioactive source ⁵⁵Fe 5.9 keV X-ray (1.85×10⁹ Bq) was done. The dependence of anode current and full energy photo-electron peak of ⁵⁵Fe 5.9 keV X-ray on the accumulated Q was measured. When total charge Q=0.07C/cm is accumulated (equivalent to 5 operation years of BESIII drift chamber) the anode current is 87% of primary value and aging ratio R=-0.19 %/(mC/cm) for small cell was measured. So we believe that the DC (operated with He/C₃H₈, 60/40) in the BESIII would survive the large doses expected at future BEPCII at lest 5 years in spite of the ageing.In order to choose the appropriate Propane for drft chamber, we have tested different C₃H₈ supplied by two companies (A and B), We did not observe any obvious differences in aging rate at our level of accuracy. This provides the references for us to choose the propane. Further more the experimental study of accelerated aging of prototype with radioactive source 10mCi ⁹⁰Srβray was done. When total charge Q=0.07C/cm is accumulated, the anode current is 83% of primary value and aging ratio R=-0.23 %/(mC/cm) for small cell was measured. The results are similar to the results of ⁵⁵Fe 5.9 keV X-ray. It ensures the reliability of the drift chamber further more.In order to prevent the aging of the prototype we added a water bubbler in our gas system and the results show that not only the water vapor can prevent the aging progress but also it can heal the aged chamber. This provide the references for us to heal the chamber when neccessary. After repeated several times aging experiments, the dependence of aging rate on the source activity was observed. The possible understanding for this dependence was discussed.