1. Field of the Invention
The present invention relates to a superconducting X-ray detection apparatus having a highly sensitive superconducting X-ray detector produced using a superconductive material, and to a method for mounting a low-temperature first-stage amplifier for amplifying a feeble signal output from the detector. The present invention also relates to a superconducting X-ray analyzer equipped with the superconducting X-ray detection apparatus.
2. Background Art
Recently, superconducting X-ray detectors called TES (Transition Edge Sensor) or STJ (Superconducting Tunnel Junction) have been developed. Currently, study on systemization of the superconducting X-ray detectors is made for meeting various requirements because of their high sensitivity and high energy resolution compared with conventional X-ray detectors. Since each of the superconducting X-ray detectors is a device utilizing superconduction, it needs to be cooled to a low temperature to be driven. Since drive at a lower temperature provides reduction in influence of thermal noise and expectable improvement in performance, the detectors are typically cooled to a very low temperature of about 100 mK.
TES is a temperature sensor utilizing change of a resistance value occurred when a material in a superconducting state transits into a normal conduction state following temperature rise, and is an X-ray sensor that converts energy of X-ray irradiation into heat and thus performs the detection. When a working point is fixed to a superconducting transition edge by applying appropriate bias current, abrupt temperature rise occurs when X-ray is irradiated on the TES, resulting in change of resistance. Since a current value flowing into the TES changes due to the resistance change, X-ray can be detected by monitoring the current value. Since the change of the current following the X-ray irradiation is feeble, several picoamperes, a SQUID array is typically used for signal amplification.
As a main application of the superconducting X-ray detector, an X-ray analyzer and a cosmic-ray detection apparatus are given. When used in the X-ray analyzer, for example, the superconducting X-ray detector is used in combination with a scanning electron microscopy (SEM). The superconducting X-ray detector is inserted into a sample room of the SEM, which is maintained in vacuum environment, at a normal temperature.
The detector needs to be placed near a sample to improve X-ray detection efficiency. X-ray generated from the sample by an electron beam irradiation is detected by the superconducting X-ray detector.
Near the sample, in addition to a sample stage for placing the sample, a lens barrel comprising an electron gun for irradiating an electron beam and an electron lens for focusing the electron beam, and a secondary electron detector for detecting electrons generated from the sample are placed, therefore a space for placing a refrigerator for cooling the superconducting X-ray detector to 100 mK or lower can not be secured. Therefore, the refrigerator is provided separately from the SEM, a cooling head is provided in a thin cylinder called snout constructed in a heat insulation structure, and the superconducting X-ray detector is fixed to an end of the cooling head. The sample is placed near the superconducting X-ray analyzer by inserting the snout from a port of the sample room of the SEM.
FIG. 5 shows a structure of an end portion of a conventional superconducting X-ray detection apparatus (for example, see non-patent literature 1). The end portion mainly comprises an outer layer of a snout 102 for maintaining vacuum, a radiation-heat shield plate 70 in about two layers for shielding radiation heat, a cooling head 103 for mounting the detector, and a super-insulation 80 applied to inner and outer layers of the radiation-heat shield plate 70. The outer layer of the snout 102 and the radiation-heat shield plate 70 have a certain thickness, and an appropriate interval is necessary, therefore even if the snout has a diameter of about 30 mm, diameter of the cooling head is small, about 10 mm.
A superconducting X-ray detector 10 such as TES is adhered to a sensor holder 30 produced using a material having high heat conductivity such as oxygen free copper or sapphire together with a connection pad 40, and mounted on an end section portion of the cooling head 30 by a screw clamp.
A low-temperature first-stage amplifier 20 is adhered to an amplifier holder 21 produced using the same material as the sensor holder 30 together with a connection pad 41, and mounted on the side near the end of the cooling head 30.
The superconducting X-ray detector 10 is connected to the connection pad 40 by wire bonding. Among electrodes of the connection pad 40, an output terminal of the superconducting X-ray detector 10 is connected to the connection pad 41 on the amplifier holder 21 using a lead, and as wiring for driving the superconducting X-ray detector 10, connection is made to a connector attached to outer packaging of the refrigerator using a lead.
On the other hand, the low-temperature first-stage amplifier 20 is connected to the connection pad 41 by the wire bonding. Among electrodes of the connection pad 41, an electrode for driving the low-temperature first-stage amplifier 20 is connected to a connector attached to the outer packaging of the refrigerator using a lead.
[Non-Patent Literature 1]
Y. Ishisaki, et al. “Proceeding of the Society of Photo-Optical Instrumentation Engineers (SPIE)”, Hawaii U.S., 2002, p831.
In the conventional superconducting X-ray detection apparatus, there has been a problem that since there is no space except for the space for mounting the superconducting X-ray detector, the low-temperature first-stage amplifier that amplifies an output signal from the detector is placed distantly from the detector, and the detector is connected to the amplifier using the lead having high electric conductivity, therefore wiring length is large, the output from the detector that is the feeble signal is influenced by disturbance noise. Furthermore, there has been a problem that handling performance is bad because connection of wiring is complicated.
The present invention overcomes the problems in the conventional art by providing a superconducting X-ray detection apparatus having a structure where the influence of the disturbance noise on the output from the detector is minimized by placing the detector and the amplifier in a same place and by providing a mounting method in which the connection of wiring can be easily made.