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

超高压下寿命为1.28 x 10^9年的衰变常数^<40>K的变化在核物理和地球科学领域引起了人们的兴趣。由于地球中含有大量的^<40>K的化合物，1461keV的γ射线很容易被观测到作为自然背景。为了排除这种自然背景γ射线，进行了两种方法的试验。第一种方法是用铅砖屏蔽锗探测器。第二种方法是使用康普顿相机，它可以使图像的γ射线源的分布。通过选择仅从源位置发射γ射线的事件，发现背景γ射线可减少约94%。这种效果相当于4.7厘米厚的铅砖的屏蔽效果。采用了前一种方法，因为它对良好的统计数据至关重要。前一种情况下的光峰值效率是后一种情况的10^5倍。研制了一种新型金刚石顶砧槽，安装量为1 mg ^<40>KCl粉。在KEK-PF中进行压力测量。考虑到高压会影响原子电子的密度，因此电子捕获的衰减速率会发生变化。基于这一考虑，从理论上估计了衰减常数的变化。在日本理化学研究所的预制建筑中，测量了在8.7、11.4和20.1 GPa高压下^<40>K的电子捕获衰变后的γ射线。结果表明，测量精度为~ 10^<-2>。这被认为是由于锗探测器的问题、铅砖的屏蔽结构、DAQ的稳定性、γ射线谱的分析方法等原因造成的。我们将继续进行这项研究，以获得良好的统计量和精度为10^<-3>。

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>.