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(a) Phonon and ionization sensor layout for iZIP detectors deployed at Soudan. The Ge crystal is 76 mm in diameter and 25 mm thick. Both faces are instrumented with ionization lines (one face with +2 V and the other with −2 V) that are interleaved with phonon sensors (0 V) on a ∼1 mm pitch. The phonon sensors are arranged to give 4 phonon readout channels for each face, an outer sensor surrounding three inner ones. (b) Magnified cross section view of electric field lines (red) and equipotential contours (blue) near the bottom face of a SuperCDMS iZIP detector. The −2 V ionization electrode lines (yellow) are narrower than the 0 V athermal phonon collection sensors (green). (c) Fabricated iZIP detector in its housing.
Decay chain for 210Pb showing the most significant decays which end in a 206Pb nucleus from the 210Po alpha decay.
All panels show the same data from ∼900 live hours of detector T3Z1 with the 210Pb source facing side 1. Clearly visible are the symmetric charge events (large blue dots) in the interior of the crystal, and the events that fail the symmetric charge cut (small red dots) including surface events from betas, gammas, and lead nuclei incident on side 1 from the source. The two blue dots with circles around them are outliers that show a very low charge yield and just satisfy the symmetry requirement. (a) The symmetry cuts (dotted blue lines) flare out near the origin so that events are accepted down to the noise wall. The band just below 50 keV is from the 46.5 keV gammas from the source. (b) Ionization yield versus phonon recoil energy with ionization yield range of neutrons indicated (area within green lines). The hyperbolic black line is the ionization threshold (2 keVee—“ee” for electron equivalent); the vertical black line is the recoil energy threshold (8 keVr). Electrons from 210Pb (below ∼60 keVr) and 210Bi (mostly above 60 keVr) are distinctly separated from 206Pb recoils (low yield, below ∼110 keVr). (c) In addition to the data in (a) and (b) this panel also shows nuclear recoils from neutrons from a 252Cf source (green, low yield). As bulk events these show a symmetric ionization response between sides 1 and 2 like the bulk electron recoils at higher yield, and are thus nicely separated from charge-asymmetric surface events.
Comparison of data from the two detectors with a Geant4 simulation. (a) Gamma electron recoil band (data with ionization yield > 0.8, including the not simulated continuous spectrum from bulk gamma interactions), compared with the simulated X-ray peaks at 12 and 46.5 keV. (b) Beta band (data with yield 0.4–0.8), compared with the simulated beta decays from 210Pb or 210Po. (c) 206Pb recoil band (data with yield < 0.4), compared with the simulated 206Pb recoils from 210Po decay. Normalizations of the simulations in (a) and (b) were fixed by the normalization needed to match data and simulation in (c). Unlike the perfect simulation classifications, there is significant mixing in the data yield-based classification between “gammas” and “betas,” and between “betas” and “recoils of 206Pb” at energies below ∼20 keV.
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