The chamber contains a first and a second measuring electrode and a third electrode. The first measuring electrode is essentially a first flat ring portion. The second measuring electrode comprises an inner circular area to the periphery of which a second flat ring portion adjoins in an electrically conducting manner. The first and the second measuring electrodes are arranged in a first plane. The first and the second ring portion are combined approximately 360.degree.. The third electrode is arranged in a second plane parallel to and spaced from the first plane. When ionizing radiation (X-rays, electrons) enters the space between the first and second measuring electrodes on the one side and the third electrode on the other side, electrical signals will be derived from said respective first and second electrode.