Signal waveforms for one event obtained from a single detector. 

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Context 1

... electronic system of the PICASSO detector includes several functionally independent subsystems. Each one is dedicated to control specific detector parameters (e.g. temperature and pressure regulation, on-line detector calibration, etc.) or to collect and process acoustical signals. Experimental data consist of waveforms from acoustical piezoelectric sensors simultaneously acquired from the entire detector. In its current stage, the detector layout consists of a set of 8 clusters, each containing 4 detector modules adding to a total of 32 detectors ( Fig. 6 and 5). A group of 4 detectors is placed in an insulated metal box (Fig. 7) Temperature-Pressure Control Sysytem (TPCS), where the temperature can be set to a pre- determined value with a precision of about ±0.5 °C. While the temperature can be set individually to each TPCS, the pressure can be set for all detectors simultaneously only. Each detector has 9 piezoelectric sensors working as a group. There are three layers of sensors on the external surface of the acrylic container. Each layer contains three sensors distributed evenly along the detector’s perimeter covering 120° sectors. Layers of sensors are rotated by 60° relative to each other. This is done for complete coverage of the active volume and acoustical triangulation of the event position. Sensors are made with cylindrical shape piezoelectric transducers mounted inside brass containers designed as Faraday cages in order to reduce electrical noise (Fig. 8). Electrical signals from piezoelectric sensors are sent over coaxial cables to be amplified and digitized. Each detector module is equipped with its own subset of electronics (preamplifiers and ADCs) in order to operate independently from the other units. These electronic boards are located in a metal enclosure placed outside and above the cluster of 4 detectors. The location of these board enclosures is chosen so as to keep the length of the coaxial cables to the practical minimum in order to minimize any induced electrical noise. Special care is taken to reduce the microphonic effect of the cable as well. In case of the piezoelectric sensors, this microphonic effect of the cable can be quite problematic due to the very high impedance of both - the sensors and the preamplifiers. Although, just one sensor is required per detector to provide a minimum of meaningful information for the data analysis, generally all 9 channels per detector module are required to make detectors most efficient and to perform off-line analysis of event localization [Aubin (2007)] within the detector volume. The most important step taken during the development of the second stage of the experiment was the design of a scalable data concentration scheme. In order to collect data from different modules and to be able to relate them in time, a special VME data collector card was designed. This card is a multi-purpose system able to concentrate data from 12 independent data sources (in the case of PICASSO, one source is one detector with 9 sensor waveforms). It has already been successfully used for the TIGRESS experiment [Martin (2007)]. For the purpose of the PICASSO application, special firmware logic has been developed and custom tailored to acquire and record a large number of acoustical signals from a multitude of detectors. Detailed description of the collector card operation is given later in this chapter. Fig. 9 demonstrates the relationship between different parts of the data acquisition and Fig. 10 presents the block-diagram for one detector channel. The amplifier design for piezoelectric sensors present several challenges. First of all, the bandwidth and the gain have to be chosen in order to preserve the information about the time evolution of the pressure build-up from the evaporation of the superheated droplet. Unfortunately, a piezoelectric material might have a very irregular non-linear response to the applied force. To better understand the difficulties arising here, the reader can be referred to the recent second edition of [Arnau (2008)]. For the purpose of the current work it is necessary to mention the lack of sufficient understanding of all details in the process of bubble creation and evolution triggered by nuclear reactions in superheated liquids. Different Dark Matter nuclear recoil experiments use slightly different methods of acoustical detection. The PICASSO group’s study of acoustic signals shows that signals generated by rapid phase transition in the superheated liquid can be detected in the wide frequency region between ~1 KHz and ~200 KHz by different types of sensors. There is an indication that significant acoustic power is emitted in the low range of the frequency spectrum (Fig. 11). At the same time one can expect an external acoustical noise in the audio range at the low frequency end of the spectrum be quite significant and special care must be taken to reduce it. The second challenge is a very high impedance of piezoelectric material. It gets even more complicated if the behaviour of the piezoceramic at different temperatures is taken into account. Instead of individual preamplifier boxes used by PICASSO previously for each sensor, the new units combine 9 single channel boards carried on one motherboard. Each motherboard is assigned to one detector equipped with a full set of 9 sensors. This arrangement allows a considerable saving per channel on extra cables, connectors and enclosures. Overall view is presented in Fig. 12. The single channel board carries a two-stage preamplifier with a DC coupled input and a band-pass filter. This board has a single ended output with a total gain of both stages between 1000 and 4000 depending on the requirements of the experiment. Due to the very high impedance of the piezoceramic, the first stage of the preamplifier is designed using n- J FET transistors. There were several versions of this board, each used at different periods of time of the experiment. The first version of the preamplifier had a pair of low noise n-J FET transistors connected in parallel, in order to cope with the large capacitance piezoelectric sensors (Fig. 13). Later, when larger gain rather than ability to work with large capacitance of the signal source was requested, a second version of the preamplifier was built using an improved version of the microphone amplifier presented by A. Shichanov in 2002. Unfortunately, the original Internet link to his schematic is not active anymore. Therefore, we would like to present it here (Fig. 14) only with a minor modification. At the time of this writing, PICASSO is using this front stage in the single channel preamplifier boards. Each pair of J FET transistors has to be selected after closely matching by the value of saturated drain-to- source current and the pinch-off voltage. Such a selection was performed with the help of specially built hardware controlled by USB- 1408FS (Measurement Computing Corporation) - USB bus-powered DAQ module with 8 analog inputs, up to 14-bit resolution, 48 kS/ s, 2 analog outputs, and 16 digital I/ O lines. Software control was designed based on the National Instruments LabVIEW program. Nearly a thousand MMBFJ 309LT1 n-channel J FET transistors were measured, sorted and grouped into closely matching pairs. The preamplifier carrier board can hold up to 9 single channel boards. It also includes individual differential drivers for the next DAQ stage of digitizing circuitry as well as a reference source used by all of the differential drivers and the ADCs of the next stage. Differential drivers shift the bipolar range of acquired signals to a positive-only range of ADCs. Such a subdivision allows future trials of different amplifiers without changing the layout of the working detector modules. Any upgrade of the system in such a case will require less effort from the detector crew in the difficult underground working environment. The single channel preamplifier board can also be used separately from the current DAQ system with the same type of modules using the single ended data acquisition system which would not require a special data collection schema, i.e. in the stand-alone post-fabrication tests and calibration of the detector. Specifically for that purpose, the frequency range of the single channel amplifier is wider than required by described data acquisition system (Fig. 15). This allows it to be used with sensors which might have different ultrasound frequencies. Differential signals from the preamplifier carrier board are sent to the digitizer board through a short flat cable. Each digitizer board is equipped with 9 serial ADCs with 12-bit dynamic range (ANALOG DEVICES AD7450) controlled by one FPGA circuit (ALTERA Cyclone EPC6T144C8). The reference voltage on the preamplifier carrier board ...

Context 2

... The PICASSO group's study of acoustic signals shows that signals generated by rapid phase transition in the superheated liquid can be detected in the wide frequency region between ~1 KHz and ~200 KHz by different types of sensors. There is an indication that significant acoustic power is emitted in the low range of the frequency spectrum ( Fig. 11). At the same time one can expect an external acoustical noise in the audio range at the low frequency end of the spectrum be quite significant and special care must be taken to reduce it. The second challenge is a very high impedance of piezoelectric material. It gets even more complicated if the behaviour of the piezoceramic at ...