SUPERCDMS SOUDAN OVERVIEW
2.2.3 Phonon Readout
As discussed above, during down-conversion, the phonon signal is localized and contains position information. At later times, the phonon distribution will thermal- ize and be uniformly distributed in the detector. Historically, most experiments of this type17, were specifically designed to measure this thermal-equilibrium popu- lation of phonons. Aside from the obvious shortcoming of ignoring all position
17Such as Edelweiss [38] and CDMS II [39]
Figure 2.7: Qualitative particle interaction overview for an iZIP detector. An in- cident particle (green) interacts with an electron or a bound nucleus. This pro- duces primary phonons (blue) as well as electron-hole pairs (black). By drifting the electrons and holes across the crystal Luke phonons are also produces (red). The oblique electron propagation is described in more detail in section 2.2.5.
information, this approach fundamentally limits the crystal size. Measuring the thermal phonon population is essentially measuring the change in temperature the entire crystal undergoes during a single particle interaction, which is inversely pro- portional to the total heat capacity of detector or∆T ∝ C1. At very low temperatures and substrate masses C is small enough that this temperature change can be ef- fectively measured. However, if the detectors are made too large, this change in temperature will be vanishingly small. To overcome these limitations, iZIP detec- tors are designed to resolve a portion of the early athermal phonon signal.
The iZIP detector detects phonons using quasiparticle-trap-assisted electro-thermal- feedback transition-edge-sensors (QETs), which are two-component phonon sen- sors. These consist of an Al absorber fin that acts as a quasiparticle funnel, cou- pled to a W transition edge sensor (TES). Each of the eight phonon detectors de- scribed in section 2.2.1 consist of 458 QETs that are read out in parallel. These QETs are spread throughout the pixel area. When an incident phonon encounters one of the Al absorber fins it can split a Cooper pair in the superconducting Al (Tc = 1.2 K) provided the phonon energy is greater than the Cooper pair binding energy (Ebinding = 2∆(0)Al = 340µeV). The resulting pair of quasiparticles18 can diffuse along the Al film until it encounters the W TES. Empirically, these quasi-
18These are similar to but distinct from electrons as we know them in the vacuum.
(a) QET detail. Al absorbers (silver) and
W TES’s (Orange) (b) Trapping Overview
Figure 2.8: Left: Detail of eight QET’s. Note the relative sizes of the silver Al absorber to the orange W TES. Right: A schematic depicting quasiparticle trapping.
Quasiparticles created in the large blue Al absorber fins have a ground state energy of 340µeV. The decrease in ground state energy in the W TES causes an increase in the number of low-energy quasiparticles that can’t transition back into the AL absorber film and become trapped in the TES [41].
particles have a characteristic diffusion length in our Al oflD =180µm [40], which sets the design length of the absorbers. Once they reach the TES, the quasiparti- cles can enter the W film and break additional Cooper pairs in the process. The critical temperature for W (Tc = 50 mK) is much lower than it is for Al, and as a result the gap energy is much lower in the TES than it is in the absorber fin (340 vs 20µeV). When these relatively high energy Al quasiparticles fall down into the relatively lower quasiparticle ground state energy of the W, the excess energy can go into breaking W Cooper pairs and creating more quasiparticles. Because these quasiparticles are all below the Al gap energy, they are effectively trapped in the W TES. In this way, the overall effect of this absorber fin/TES system is to col- lect quasiparticles over a large area of the detector surface (∼ 5% metal coverage) and funnel them into the TES where they are trapped and concentrated in a much smaller volume. A portion of the W TES is biased and heated into its superconduct- ing transition and this process can repeat causing in the biased (zero gap) portion of the TES. A QET overview is given in figure 2.8.
Our W TESs are voltage biased via a shunt resistor (Rsh = 22 mΩ << RTES ≈ 200 mΩ) and read out using the circuit show in figure 2.9. As it is biased, the Joule heating (P = Vb2/RTES) due to the bias current of the TES will cause it to self-heat
As a result the TES re-cools from the decrease in Joule heating rather than from heat flow to the substrate. This allows the substrate to be maintained at base tem- perature while the W TES is held in the middle of its superconducting transition.
The change in current through each TES is read out using a dedicated, low-impedance, SQUID amplifier circuit as shown in figure 2.9. The TES is biased in series to an in- ductor called the “input coil” (Li) which translates changes in current to changes in flux through the SQUID. Although the SQUID is a very sensitive magnetometer, it
19The exact point in the superconducting transition can be tuned by changing the bias voltage.
Practically, this is controlled by the current sent down the bias network. Typically, you first have to drive enough current to make the TES normal, then ramp it down to get the desired bias voltage.
Figure 2.9: Overview of the QET readout circuit. Any change in quasiparticle- sourced dissipation (RTES) will cause a change in the flux produced by the input coil (Li). This flux will induce op-amp G to provide current to the feed-back inductor LFBuntil the total flux through the SQUID is canceled.
operates most accurately in a closed-loop feedback circuit as shown in figure 2.920. The amplifier in the feedback loop supplies the feedback inductor (Lfb) with current until the field it produces cancels field from the input coil. BecauseLfb = Li/10, the small voltage signal produced by the SQUID will be amplified by a factor approxi- mately 10.
The noise from the phonon readout circuit has many different components, but is dominated by the Johnson noise from the shunt resister. This is due to the fact that the shunt resister is thermally tied to the still stage (T ≈ 600 mK), which is substantially warmer than the∼50 mK TES. The resulting white noise should be
≈ 10 pA√
Hz with a high frequency roll offset by Li/RTES. The measured noise spectrumin situhad much higher than expected noise at low frequencies (see fig- ure 2.10). This excess is believed to arise from 1/f noise from our electronics, together with vibrational coupling of our towers with external elements of the lab21. Although problematic for very low energy events, this analysis was relatively unaf- fected by the excess noise.