Superconducting calorimetric alpha particle sensors for nuclear nonproliferation applications
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Details of the microcalorimeter alpha detector. (a) Micrograph of the detector with the Sn absorber absent but outlined. The detector is thermally isolated from the surrounding Si chip by an insulating silicon nitride membrane. This isolation allows the TES to register a temperature change before the deposited energy exits the detector. The Si chip is heat sunk to a copper mount held at 80 mK in an adiabatic demagnetization refrigerator. The TES is heated from 80 mK into its resistive transition by the electrical bias. The Si chip is 6.35 mm on a side and 0.28 mm thick with an additional silicon nitride layer on top. The Si is removed from the center of the chip to form the freestanding SiN membrane. The Cu thermalization layer and TES thermometer on the membrane are thus thermally isolated from the Si frame. The Cu thermalization layer is and is attached to the TES by two Cu fingers. The TES film at the center of the membrane is . (b) Micrograph showing thermalization layer, TES, and surrounding epoxy posts. The eight posts are tall and in diameter. The Sn absorber is attached on top of the posts. The interdigitated features on the TES are Cu bars that control the width of the superconducting transition (Ref. 12). (c) Schematic (not to scale) of detector chip in profile. (d) Digitized record of the microcalorimeter response to a single alpha particle. The high signal-to-noise ratio of the measurement is obvious and corresponds to a temperature error significantly less than .
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Measured microcalorimeter alpha particle spectrum from (black), a fit to the data (red), and the same source measured with a Si detector (blue).
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Mixed isotope Pu alpha particle spectrum. (a) Pu alpha particle spectra taken with a state-of-the-art Si detector and (b) a microcalorimeter are shown in black and fit in red. The expected locations and relative heights of the (blue) and (green) peaks are also shown. The improved resolution and reduced straggling of the microcalorimeter greatly clarify the ratio. Both spectra are of the same source with the same integration time leading to about 4000 counts between 5000 and 5200 keV. [(b) inset] atomic ratios determined from fitting Si and microcalorimeter data with the line shape models of Bortels and Collaers (Ref. 19) (●), Westmeier (Ref. 18) with one straggling parameter (▲) and two straggling parameters (◼), Bland (Ref. 19) , and Hilton et al. (Ref. 6). (◆). All included fits have a reduced chi squared value close to one. Vertical error bars show statistical error for each model. The known atomic ratio is shown by two dotted lines that represent the error window. Red curves in (a) and (b) are fits with the line shape model of Bortels.
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