Mitigation of cosmic ray effect on microwave kinetic inductance detector arrays

Abstract

For space observatories, the glitches caused by high energy phonons created by the interaction of cosmic ray particles with the detector substrate lead to dead time during observation. Mitigating the impact of cosmic rays is therefore an important requirement for detectors to be used in future space missions. In order to investigate possible solutions, we carry out a systematic study by testing four large arrays of Microwave Kinetic Inductance Detectors (MKIDs), each consisting of ∼ 960 pixels and fabricated on monolithic 55 mm × 55 mm × 0.35 mm Si substrates. We compare the response to cosmic ray interactions in our laboratory for different detector arrays: A standard array with only the MKID array as reference; an array with a low T c superconducting film as phonon absorber on the opposite side of the substrate; and arrays with MKIDs on membranes. The idea is that the low T c layer down-converts the phonon energy to values below the pair breaking threshold of the MKIDs, and the membranes isolate the sensitive part of the MKIDs from phonons created in the substrate. We find that the dead time can be reduced up to a factor of 40 when compared to the reference array. Simulations show that the dead time can be reduced to below 1 % for the tested detector arrays when operated in a spacecraft in an L2 or a similar far-Earth orbit. The technique described here is also applicable and important for large superconducting qubit arrays for future quantum computers. PACS numbers: Valid PACS appear hereData loss caused by cosmic ray hits on the instruments' detectors is one of the main concerns for space observatories. Cosmic rays are so energetic that they penetrate the satellite structure, reach the detectors, and deposit a fraction of their energy through ionization and atomic excitation. The deposited energy causes a cascade of high-energy phonons (ballistic phonons) that spread inside the detectors, trigger the detectors' response, and create glitches in the data stream. These glitches lead to significant dead time of the detectors and loss of integration efficiency 1 . Although cosmic ray interactions are strongest in space, even on earth the interaction rate is about a few events/min/cm 2 , posing possible issues especially for cryogenic superconducting circuits with large areas, such as quantum computing chips.Microwave Kinetic Inductance Detectors (MKIDs) 2,3 are pair breaking detectors that sense a change in Cooper pair density due to radiation absorption, in contrast to bolometric detectors, such as Transition Edge Sensors 4 , which measure temperature. Hence, MKIDs only sense energies larger than the gap energy of a superconductor film (2∆). In the Planck satellite, in which bolometers a) Electronic mail: K.Karatsu@sron.nl are used, short and long glitches are observed due to cosmic rays 5 . Short glitches, with a decay of 4-10 ms, are determined to be direct hits in the bolometer. Long glitches, with a time scale exceeding 10 ms, are interpreted as cosmic ray hits in the Si wafer. The measurements of cosmic ray ...

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