Tagger Microscope

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View of the tagger microscope from under the electron beam plane, with chamber walls removed
Main article: Tagger Microscope Contruction (UConn Wiki)

The Tagger Microscope is a movable, high-resolution hodoscope that counts post- bremsstrahlung electrons corresponding to the photon energy band of interest to the experiment in Hall D. While designed as a general-use device, it has been optimized primarily for use in the GlueX experiment, covering the Eγ range of 8.4-9 GeV (Ee 3-3.6 GeV)


Focal plane readout scheme

The design of the Tagger Microscope calls for the spectrally-analyzed electron focal plane to be instrumented with a detector array of scintillating fibers with axes oriented toward the oncoming electrons. This is done to maintain fine focal plane segmentation in two dimensions:

  • fine segmentation along the direction of electrons spread mitigates the rate and increases the energy resolution
  • segmentation in the y-directions allows selective readout to match the photon collimator acceptance.

To avoid placing photo-sensors along the path of the electronics, the scintillation light will be delivered to separately-mounted sensors and electronics via clear fiber waveguides.

The mechanical alignment structures allows assembly of the scintillating fibers for a wide range of crossing angles (β in the adjacent figure)

Alignment of the scintillating fibers with respect to the electron trajectories. The mounting rails for the scintillators has been designed with degrees of freedom necessary for online alignment to the electron plane as well as selection of the active fiber row.

Scintillator readout

The silicon photomultiplier (SiPM) has been identified as the appropriate photo-sensor for reading out the scintillating fibers. Its a novel technology that provides small, pixelated active windows (appropriate to the fiber cross-section) high efficiency and gain with sensitivities to perform single-photon counting at room temperature. It is competitive with traditional PMTs in terms of speed and do not require high voltage to operate (required bias voltages vary between 20-80 V). The solid state sensors are also much less sensitive to magnetic fields, making them a convenient choice for operating next to the tagger magnet.

The tagger microscope design employs custom-designed amplifier boards that can support up to 30 SiPM channels. Each board provides space for mounting the SiPM, initial signal amplification and summation circuitry. The amplifiers are equipped with online-selectable gain-control and online-controllable bias voltages. Thus, uniform quality of readout of all the optical channels can be maintained during run time.

Rendering of the silicon photomultiplier-based scintillating fiber readout electronics.

The adjacent rendering shows the amplifier board, mated with "backplane" used to patch signals to the outside of the chamber for readout, as well as the control board (topmost vertical board) which communicates with a computer via Ethernet interface. Bias voltages are set via this interface and various voltages and temperatures at different points of the electronics are queried.

Specifications

Microscope Construction

Microscope Fabrication Readiness Review (February 7, 2013)

View 1

View 2

Light yield tests

Today we collected ADC traces from the FADC250 and compared them with the same channel as viewed on the oscilloscope. There is a problem understanding this comparison, in that the amplitudes vary by 40% as viewed on the scope, but are all within 10% of each other as viewed by the FADC.

Scope chan1.pngRaw chan1.png
Scope (left) and FADC250 raw (right) pulse waveforms taken with laser diode pulser injecting light into the end of a fiber that has been painted end-to-end. The integral of the pulse shown on the right, in units of average ADC counts for a 20-sample gate, is 90.2 channels.


Scope chan2.pngRaw chan2.png
Scope (left) and FADC250 raw (right) pulse waveforms taken with laser diode pulser injecting light into the end of a fiber that has not been painted at all. This channel is split with a 50-50 passive splitter, with 50% of the signal going into the digitizer (scope or FADC) and the other half going to the discriminator. The signal is delayed by 80ns because of the extra cable leading to the splitter. The vertical scale has been increased by a factor 2 on the scope for this signal to facilitate comparison. The FADC250 vertical scale has not been altered. The integral of the pulse shown on the right, in units of average ADC counts for a 20-sample gate, is 95.8 channels.


Scope chan3.pngRaw chan3.png
Scope (left) and FADC250 raw (right) pulse waveforms taken with laser diode pulser injecting light into the end of a fiber that has been painted from the active end to the midpoint of the fiber, with the other half left bare. The integral of the pulse shown on the right, in units of average ADC counts for a 20-sample gate, is 99.2 channels.