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The Detection of alpha particles with superconducting tunnel junctions Wood, Gordon Harvey

Abstract

A superconducting thin film tunnel junction (Sn-Sn0₂ -Sn) of total thickness 4000 Å, area 7 x 10ˉ⁴ cm² and normal (4.2 K) resistance 77 mΩ was prepared on a glass substrate. When cooled to 1.2 K the junction was biased at 0.3 mV where, the Josephson supercurrent having been suppressed with a magnetic field, the junction dynamic resistance had its maximum value of 9.3Ω . The junction was then bombarded with 5.1 MeV alpha particles and the resulting pulses induced in the tunneling current were observed to have amplitudes up to 19 times the preamplifier-dominated rms output noise level. For purposes of analysis, it was assumed that the induced current pulse had the form i(t) = i₀ exp(-t/Ƭ), t ≥ 0. With this form of the current pulse and the known transfer function of the transmission line-amplifier system, it was calculated that for all pulses T = (1.38±.33)xl0ˉ⁷ sec and that for the largest amplitude pulses, corresponding to an energy loss ΔE⍺ ≤2.75 MeV, i₀ lay in the range 20 ≤ i₀ ≤ 26 μA with a most probable value of 22 μA. With this value of i₀ and ΔE⍺ = 2.75 MeV, an upper limit of 8.2 x 10ˉ³ eV has been assigned to the value of w(Sn), the average energy expended by the alpha particle to excite a quasiparticle pair in superconducting tin at 1.2 K. A tentative theory of the superconducting tunnel junction charged particle detector is given and the cryogenic and electronic apparatus required for the measurements are described. Details related to thin film junction fabrication technology and interpretation of dc experimental results are discussed in four appendices.

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