Non-linearities in Superconducting Tunnel Junction Radiation Detectors and Their MCA Readout
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Non‑linearities in Superconducting Tunnel Junction Radiation Detectors and Their MCA Readout S. Friedrich1 · F. Ponce1,3 · J. A. Hall2 · R. Cantor2 Received: 18 July 2019 / Accepted: 21 January 2020 © This is a U.S. Government work and not under copyright protection in the US; foreign copyright protection may apply 2020
Abstract The response of cryogenic high-resolution detectors to a short-pulse laser consists of a Poisson-distributed set of equidistant peaks that correspond to integer numbers of absorbed photons. Since the laser has a negligible intrinsic line width, the peaks can be used for detailed characterization of both the detector and the data acquisition system. We have characterized our superconducting tunnel junction (STJ) photon detectors in the UV and soft X-ray range with a pulsed 355-nm laser at rates up to 5000 counts/s. The observed peaks are described by a Gaussian to very high accuracy, with a width between ~ 1 and ~ 3 eV FWHM depending on the detector area and the absorbed energy. For high statistics, centroids can be determined with a precision of order 1 meV over an energy range of several 100 eV. This allows identifying and correcting for non-linearities in the digitizer that can otherwise limit the calibration accuracy. Keywords Superconducting tunnel junctions · STJ radiation detectors · EUV detectors · MCA non-linearity · Integral non-linearity
1 Introduction The calibration accuracy of high-resolution spectra is ultimately limited by the uncertainties of the calibration energies, the predictability of the detector response function and the non-linearity of the multi-channel analyzer (MCA) of the data acquisition system. Pulsed optical or UV lasers are good sources for detector calibration because they have negligible intrinsic linewidth, their wavelength can be
* S. Friedrich [email protected] 1
Lawrence Livermore National Laboratory, 7000 East Ave., Livermore, CA 94550, USA
2
STAR Cryoelectronics LLC, Santa Fe, NM 87508, USA
3
Present Address: Stanford University, Stanford, CA 94305, USA
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Vol.:(0123456789)
Journal of Low Temperature Physics
measured with high accuracy and multi-photon absorption produces a distribution of equidistant peaks over a wide energy range. We have used a pulsed 355-nm UV laser to measure the response of our superconducting tunnel junction (STJ) radiation detectors and determine their calibration accuracy in the extreme ultraviolet (EUV). This paper shows that an accuracy of ± 1 meV is attainable and discusses the limiting contributions.
2 Experiment The detectors in these experiments were superconducting Ta–Al–AlOx–Al–Ta tunnel junctions with different areas fabricated at STAR Cryoelectronics [1]. They were operated at T ≈ 0.1 K in an adiabatic demagnetization refrigerator (ADR) with liquid N2 and He pre-cooling. The temperature was not regulated, since the STJ response is constant as long as the thermal quasiparticle density is negligible. The detectors were exposed to a pulsed frequency-tripled Nd:YVO4 laser (Spectra Physics, mode
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