Change of a Decay Constant of ^<40>K under Ultra High Pressure

超高压下衰变常数^&lt；40&gt；K随寿命1.28×10~(-9)年的变化引起了核物理和地球科学领域的兴趣。由于地球中含有大量的^&lt；40&gt；K化合物，因此很容易观测到1461keV的γ射线作为自然背景。为了排除这种自然本底的γ射线，进行了两种方法的测试。第一种方法是用铅砖屏蔽Ge探测器。第二种方法是使用康普顿相机，它可以使γ射线源的分布图像。通过选择仅从源位置发射γ射线的事件，发现背景γ射线可以减少约94%。这种效果与4.7厘米厚的铅砖的屏蔽效果相对应。前一种方法被采用，因为良好的统计数据至关重要。前者的光峰效率是后者的105倍。研制了一种新型的金刚石顶压室，可安装1mglt；40；在KEK-PF中进行了压力测量，认为高压会影响原子电子密度，电子俘获的衰减率也会发生变化。基于这一考虑，还从理论上估计了衰变常数的变化。在日研院的预制件建筑中，测量了^&lt；40&gt；K在8.7、11.4和20.1Gpa高压下电子俘获衰变后的γ射线。因此，测量的精度为~10^&lt；-2&gt；这被认为是由于Ge探测器、铅砖的屏蔽结构、数据采集的稳定性、γ光谱分析方法等问题造成的。我们将继续这项研究，以获得良好的统计和精度为10^&lt；-3&gt；。

A change of a decay constant of ^<40>K with life time of 1.28 x 10^9 year under ultra high pressure is interested in the region of nuclear physics and earth science.As much amount of ^<40>K chemical compound is included in the earth, the γ-ray of 1461keV is easily observed as a natural background. In order to exclude this natural background γ-ray, tests of two methods were carried out. First method is to shield Ge detectors by using lead bricks. Second method is to use Compton camera which can make the image of the distribution of the γ-ray source. By selecting events that the only γ-rays are emitted from the source position, it was found that the back ground γ-rays could be reduced about 94 %. This effect corresponds to that by a shield of the 4.7 cm thick lead bricks. The former method was adopted, as it is crucial the good statistics. Photo peak efficiency in former case is more by 10^5 times than that in the latter case.New type of diamond anvil cell was developed to install the amount of 1 mg ^<40>KCl powder. Measurements of pressure were carried out in KEK-PF.As high-pressure is considered to affect the density of atomic electrons, decay rate of electron capture is supposed to change. Based on this consideration, change of decay constant was also theoretically estimated.Measurements of γ-rays after electron capture decay of ^<40>K under high pressure of 8.7, 11.4 and 20.1 GPa were carried out in the prefab building at RIKEN. As this results, the accuracy of this measurements was 〜10^<-2>. This was considered to be caused by the problems of Ge detectors, shield configuration of lead bricks, stability of DAQ, method of analysis of γ-ray spectra and so on. We will continue this study in order to obtain the good statistics and the accuracy to be 10^<-3>.