MEASUEEMENT OF MINUTE AMOUNT OF RADON AND TORON CONTAMINATION IN THE CHAMBER GAS FOR USE IN THE DOUBLE BETA DECAY EXPERIMENT

在地下实验中，例如寻找双β衰变的主要本底来源，被认为是环境中的放射性活性物质，特别是托克子核，以及探测器本身。为了测量这种污染，通常使用具有10MBq/m^3量级的氡灵敏度或检测下限的大容量静电收集型检测器。例如，在使用漂移室的实验配置的情况下，测量填充气体中的放射性污染是至关重要的。本探测器似乎在估计气体中的微量氡和托恩方面是有用的。为了了解这种类型的检测器，我们研究了1升体积检测器的一般行为。它的结构是一个能够让外部空气从底部进入的SUS容器，内部Conconine…的顶部安装了一个保持负电位的PIN型光电二极管在这种测量中，探测器首先被装在一个大的丙烯酸容器中，然后与外部空气隔离，并且观察到了计数率随时间的变化。由于我们的测量是针对自然氡和“地幔”的一个微弱的Toron源，所以与上面提到的大探测器和预备源相比，用来确定子核的观察到的半衰期不那么准确。我们的测量精度在3%~4%水平。然而，目前的检测器将更容易处理，以获得与其性能相关的各种信息。子核如^&lt；218&gt；Po很可能通过与氧反应而电离正极。这些离子也倾向于附着在水分子上，因此，由于寿命效应，离子可能在被检测到之前就丢失了。朱和霍普克过去曾研究过这种行为，我们通过离子迁移率的降低来考虑这一点。我们对α粒子产额的温度依赖关系的测量结果与预期行为以及过去的结果很好地一致。虽然目前的结果只针对水蒸气，但对于漂移室中常见的猝灭气体酒精蒸汽也可以得到同样的结果。需要对室内气体中的钍系列污染进行更仔细的研究，以评估短暂寿命的^&lt；208&gt；T1核的β衰变。另一个观察结果是稀有气体的环境。我们已经使用了一个斗篷来实现这一点。在氩气环境中的探测效率约为氦气的1/4。这一结果被认为是离子与Ar气体原子之间的电荷转移碰撞。观察到的结果与这一假设非常一致。较少

The major background source in the underground experiments, such as a search for the double beta decay, is recognized as the radio-active materials, specifically the daughter nucleus of toron, ^<208>T1, in the environment as well as the detector itself. To measure such contamination, a large volume electrostatic collection type detector having radon sensitivity, or the detection limit, of the order of 10 mBq/m^3 is commonly used. In case, for example, of an experimental configuration utilizing a drift chamber, the measurement of radio-active contamination in the filling gas is crucial the present detector seems to be useful in the estimate of minute amount of the radon as well as the toron in the gas. To understand such type of the detector, we have studied a general behavior of the 1 litter volume detector. Its construction was a SUS container capable to let the external air from the bottom and a PIN photodiode held at a negative potential was mounted at the top of the inside containe … More r. The radio-active ions, such as polonium and bismuth, are collected to the diode surface and their alpha decays are detected as dearly separated several peaks.In this measurement the detector was first housed in a large acrylic vessel, and next was isolated from the external air, and the change of counting rate with time was observed Since our measurements are for the natural radon and a weak toron source of the "mantle", the observed half lives to specify the daughter nuclei were less accurate compared with the case for large detector mentioned above and a prepared source. The precision of our measurement was at 3〜4 % level. However, the present detector would be much easier to handle to get various kinds of information related to its performance. The daughter nuclei such as ^<218>Po is likely to ionize positive by reacting with oxygen. These ions also tend to adhere the water molecules and as a result the ions may be lost before being detected due to the lifetime effect. Such behavior was studied in the past by Chu and Hopke, and we have considered this by the decrease in mobility of the ions. Our measurement of the temperature dependence of alpha particle yield was in good agreement with the expected behavior as well as the past result.Although the present result was only for the water vapor, about the same argument may hold for alcoholic vapor, which is a common quenching gas in the drift chamber. A more careful study will be needed for the thorium series contamination in the chamber gas to evaluate the beta decay of short lived ^<208>T1 nucleus. Another observation was for the environment of rare gases. We have performed this by using a mantle. The detection efficiency in argon gas environment was about 1/4 of the helium gas. This result was considered by a charge transfer collision of the ions with the argon gas atoms. The observation was in good agreement with this assumption. Less