Reduction of $\gamma$-ray background using Compton camera

Development of the method to reduce the background $\gamma$- rays was carried out by using the Compton camera. Compton camera can make the image of the distribution of the $\gamma$-ray source. A change of a decay constant of $^{40}$K under high pressure is studied with interests in the fields of nuclear physics, nuclear astrophysics and earth science. As much amount of $^{40}$K chemical compound is included in the earth, the $\gamma$-ray of 1461 keV is easily observed as a natural background. It is crucial to reduce the natural background $\gamma$-rays in this experiment. RIKEN group developed the Compton camera which is called to be GREI (Gamma-Ray Emission Imaging). GREI consists of two double sided strip Ge detectors. By selecting events that the only $\gamma$-rays are emitted from the source position, it was found that the background $\gamma$-rays could be reduced about 94 \%. This effect corresponds to that by a shield of the 4.7 cm thick lead bricks.