The Electron-Muon Ranger is a fully-active scintillator detector. It can be classified as tracking calorimeter as its granularity allows for track reconstruc-tion. The construction of the detector started in early 2011 and was placed at the end of the MICE beam line at RAL in September 2013 [189]. It has been operated without interruption since, with a significant hardware upgrade in October 2014. The detector hardware was thoroughly characterized in [190].
4.2.1 Design
The EMR is based on scintillator bars of triangular section arranged in a series of planes in an X–Y configuration [191]. One plane contains 59 bars and covers an area of∼1.1 m2. Each even number bar is rotated by 180 degrees with respect to the odd number one. A cross-section of bars and their arrangement in a plane is shown in figure4.3. This configuration does not leave dead areas in the detector, except for particles crossing a plane with angles equal to 45 degrees with respect to the beam axis.
Figure 4.3:EMR bar cross-section and their arrangement in a plane.
The light, produced when a particle crosses a bar, is collected by a wave-length shifting (WLS) fibre glued inside the bar [192]. At both ends, the WLS fibre is coupled to clear fibres that transport the light to a photomultiplier tube (PMT) [193, 194]. The clear fibres are protected with rubber sleeves and packed in aluminium fibre boxes. In order to increase the bending radius, which affects light attenuation, each fibre length is optimized. The two bunches of clear fibres coming from the two sides of a plane are glued into different types of connectors: one designed to interface with a multi-anode photomultiplier tube (MAPMT) and the other with a single-multi-anode photomultiplier tube (SAPMT). Figure4.4schematically represents the light transport procedure in a plane from scintillation to observation.
Figure 4.4:Scheme of the light transport in a plane of the EMR. At scintillation, light is transported by the WLS fibre to the connectors (2). It is transmitted through the clear fibres (1) to the fibre mask (3) that is fitted against the PMT. The mask represented in this figure is the MAPMT fibre mask.
Two successive perpendicular planes are attached to each other via aluminium profiles to form a rigid structure called a module. The full detector contains 24 modules as shown in figure4.5. Panels cover all sides of the detector in order to ensure light-tightness. The signals coming from an MAPMT are read out and processed by a dedicated front-end board attached directly to the fibre box. The SAPMTs are equipped with a voltage divider and the analogue signal is sent outside the detector for digitization.
Figure 4.5: (Left) 3D engineering view of the EMR detector and its mechanical support. The placement of the SAPMTs, MAPMTs, FEBs and fibre boxes are represented. (Right) Picture of the back of the EMR box placed in the beam line of MICE in its Step I configuration.
4.2.2 Readout chain
The EMR has a dual readout. Each scintillator plane is equipped with an MAPMT, measuring the light output of individual bars, and an SAPMT, recording the integrated response of all bars in the plane. The two read-out chains utilize different digitization techniques. This allows for a cross-calibration of the two measurements, and also provides a complementary piece of information, needed for particle identification and the measurement of the total energy deposited in the detector.
A schematic layout of the readout chain is shown in figure 4.6. The MAPMT is connected via a flex cable to a front-end board (FEB), which processes the signals and sends them to a piggyback digitizer-buffer board (DBB) for digitization and storage. The FEB is configured by the VME configuration board (VCB), which resides in the VME crate of the control rack. Each VCB is able to configure up to 16 FEBs, therefore three of them are required for the full detector. The DBBs are readout in groups of six. In each group the first DBB is a master and the other five are slaves. All the six boards are daisy-chained via an Ethernet cable and the master is connected
to a VME readout board (VRB), which transfers all the data from the six DBBs to the DAQ computer. In the whole detector there are 8 groups of DBBs, i.e. 8 VRBs are installed in the control rack. The SAPMT signal is integrated and digitized by a CAEN V1731 flash ADC [195].
Figure 4.6:Scheme of the electronics chain for the readout and setting of the EMR detector. The SAPMTs are connected to fADCs housed in the VME crate. The MAPMTs signals are sampled and digitized by the Front-End Boards (FEBs) and the Digital Buffer Board (DBBs).
The MAPMT is a 64-channel R5900-00-M64 PMT produced by Hama-matsu [196]. It is encased in aµ-metal tube acting as additional shielding against the fringe magnetic field. The PMT is aligned with respect to the fibre connector in such a way that each fibre shines on one PMT channel.
The MAPMT is readout by a dedicated front-end board equipped with a piggy-back digitizer-buffer board [197], which stores hit information through-out a spill. It consists of a PMT and its voltage divider is connected to an FEB through a flex cable.
The FEB processes the 64 MAPMT signals using a 64-channel ASIC2 called MAROC3 [198]. The analogue signals are fed into the chip where they are processed in parallel. Each channel consists of a pre-amplifier with a variable gain, a tunable slow shaper for analogue readout, a tunable fast shaper and a discriminator for the digital signal. The MAROC ASIC provides
2Application-specific integrated circuit
3Multi-Anode ReadOut Chip
parallel digital outputs forwarded to two high density connectors. The width of the discriminated signal represents the time-over-threshold measurement.
One multiplexed analogue output is also provided and this output is digitized by an external ADC. It takes 12.8µs (64 channels with a multiplexing clock of 5 MHz) to process the multiplexed signals, a time too long for the MICE DAQ duty cycle which foresees up to one particle every 5µs during∼1 ms spills. Only the fast time-over-threshold measurement, in the form of a leading signal time and trailing signal time, is used and recorded.
The SAPMT is also placed in a µ-metal casing. The EMR detector was initially assembled using second hand SAPMTs, available after the disassembly of the HARP experiment [199]. They were 10-stage, linear-focused XP2972 PMTs produced by Philips. A special selection procedure was developed in order to select the best samples for the assembly of the detector [200]. During the upgrade of the detector, all Philips SAPMTs were replaced by new R6427 tubes produced by Hamamatsu [201].
4.2.3 Internal calibration system
A calibration system is installed inside the enclosure of the detector. This system consists of an LED driver distributing light homogeneously, through a set of diffusers, to one hundred clear fibres, as depicted in figure4.7. Each fibre box is connected to one of the calibration fibres through a connector.
Inside a fibre box a clear fibre connects the calibration fibre to the PMT.
The light coming from the LED driver is blue in contrast with the green light coming from the WLS fibres glued inside of the scintillating bars.
The LED driver is connected to a low voltage power supply that produces voltages ranging 0–23 V and allows for the generation of a broad range of luminosity inside the detector. The system accepts an external trigger to generate short light pulses at regular intervals. It can be used to monitor the drift of the gain and the quantum efficiency of the PMTs.
Figure 4.7:Pictures of the LED driver system (left) and the output light of a single plane fibre bunch zoomed onto the blue calibration channel (right).