SUPERCDMS SOUDAN OVERVIEW
2.1 Experiment Installation
2.1.3 Detector Payload
For a complete overview of the SuperCDMS Soudan detector payload please see [33].
In brief, the thermal, electrical, structural and radiological needs of our detectors re- quire specialized hardware inside of the IceBox itself. This “cold hardware” was originally designed and built for the CDMS II experiment and subsequently modi- fied for use with SuperCDMS’ iZIPs, and can be broken down into five components.
• Detector Housing The iZIP disks themselves are mounted in hexagonal, high-purity copper housings via six Cirlex clamps. Electrical connections are made to each phonon and charge channel via Al wire-bonding to a small custom feed-through circuit mounded to the housing called a detector inter- face board (DIB). One side of the DIB contains bonding pads and a small LED8, and the other a Mill-Max connector. There are two DIBs per iZIP, each carrying 12 channels: 8 for phonon bias and readout, 2 for ionization, and 2 for the LED.
• Side CoaxThis is a thin copper plate which, on one end, connects to the DIB via its Mill-Max connectors and carries these signals in vacuum coax wires to the detector tower base. It also contains a small feedback circuit on the DIB side to correctly bias and read out the ionization electrodes.
• Tower Our iZIPs are mounted in five groups of three detectors in what is usually referred to as a “stack”. These stacks are mounted to the bottom of a
6Which were designatedchirpandthumpafter their coincident sounds.
7To characterize cryocooler noise for low mass WIMP searches an accelerometer was installed on the e-stem to record the timing of these noise events.
8This LED is used for detector neutralization.
amplified using a very sensitive type of magnetometer called a superconduct- ing quantum interference device (SQUID) which are thermally connected to ST temperature (about 800 mK). The primary charge amplification is done using JFETs which are at 4 K. These cards are named with a portmanteau of these two acronyms: “SQUET”11.
• Stripline Finally, the electrical signals are carried from the SQUET carts, through the e-stem, to 50-pin bulkhead connectors where they are externally accessible. The vacuum chamber at he end of the e-stem that contains these vacuum feed-through connections is called the “e-box”.
When talking about a particular detector, there are two different naming schemes that are used interchangeably. This first is to use the tower number followed by the stack position, where the first (top) detector of the fourth tower would be IT4Z1.
The detectors are also numbered consecutively starting at 1101, so IT1Z1 is 1101, IT1Z2 is 1102. . ., IT5Z2 is 1114 and IT5Z3 is 1115.
It was found in CDMS II that near-surface interactions, especially those from the
210Pb decay chain, were an important source of background interactions. To bet- ter characterize this background two210Pb sources were installedin situ. One was placed on the top face of detector IT3Z1, and one on the bottom face of IT3Z3.
These sources generated approximately 130 events per hour, and are of vital impor- tance for the construction of our surface background model (see section 4.3.4).
9The termstowerandstackare often used interchangeably in the CDMS collaboration to refer to the the stack, the tower, the combination of the two, or the entire assembly including the SQUET card. It is possible I will be similarly ambiguous in this dissertation.
10Starting from the stack side the stages are MC, CP, ST and 4K. They are connected thermally through the c-stem to the mixing chamber, cold plate, still and He bath of the dilution fridge respec- tively.
11As a people, experimentalists are nothing if not bad at naming things.
(a) Detector housing. (b) DIB connector
(c) Side Coax (d) Tower
(e) SQUET card (f) Stripline
Figure 2.5: Overview of cold electronics.
• Front-End Board (FEB)FEBs are custom 9U circuit boards that control the bias and readout of the phonon channels, charge channels, JFETs, SQUIDS, and LEDs. Each FEB is responsible for a single DIB, so each detector is read out with 2 FEBs. They are rack mounted and physically located near the e-box in the RF room itself.
• Receiver-Trigger-Filter (RTF) BoardRTF boards are also custom 9U cir- cuit boards, each one of which receives its input from a single FEB. As the name suggests these boards are used to set a trigger threshold so that only signals above a certain amplitude would be read out. This trigger is defined on the sum of all the phonon channels for a particular detector. In addition to issuing triggers, the RTF board also implements a couple of filters. A But- terworth band-pass filter with bounds of 300 Hz–3 kHz, is used on the signal that is sent to the trigger discriminator to prevent random noise from dominat- ing our data acquisition. The digitized signal is low-pass filtered to prevent aliasing. These boards are rack mounted in the electronics room which is above the RF anteroom.
• Trigger-Logic Board (TLB) The TLB has a global overview of the entire experiment and is responsible for issuing global triggers. The TLB collects triggering information from the RTF boards as well as the muon veto, and the main DAQ control computer12. When a global trigger is issued, what happens depends on the operational mode (see section 2.3.3). In short, while collecting WIMP-search data, a global trigger will cause the entire detector array to be read out. This allows for multiple-interaction events to be tagged, and for the iZIP array to act as an active self-shield. During calibration data taking, the trigger rate is much higher13 and as a result only detectors that issued a trigger are read out.
12Which is used to issue random triggers to better understand the noise environment.
13Around 20 Hz vs the 0.5 Hz for WIMP-search data.