The method for determining radiation stability of a crystal to radiation of a working wavelength to be employed in a subsequent application includes taking a first absorption spectrum of a cleaved piece of the crystal with a given thickness over a predetermined wavelength range from a first wavelength to a second wavelength by means of a spectrophotometer. A basket may be installed at the door and a pressure sensor may be provided on the basket to detect a pressure applied by a storage item stored in the basket. The stepped conductive layer includes a lower parts and an upper parts.