Infrared-Blocking Filters for Superconducting-Tunnel-Junction Particle Detector

摘要

The prevention of detector performance degradation due to infrared radiation is a serious issue in superconducting detectors. For this purpose, we designed and fabricated metal-mesh infrared-blocking filters. The infrared-blocking filters are extremely effective for superconducting-tunnel-junction (STJ) particle detectors. One of the best mesh structures consists of a freestanding 500-nm-thick Cr/Cu film, having a membrane size of 12 × 5 mm~2, and an array of circular holes of 2 μm diameter and 3.5 μm pitch. Infrared transmission spectra show that the transmission of the 300 K radiation is only 1%. On the other hand, the ion transmission efficiency is as high as 20%, which is consistent with the geometrically expected value. These properties produce good results in the sense that the holding time at 0.3 K in a ~3He cryostat is not affected and is as long as 130 h even under the operating condition, that is, with a through hole of a molecule flight path between the STJ detector at 0.3 K and room temperature. The subgap current of the STJ detector is not affected by the room-temperature radiation.