Results of Prior Research

Tile-Fiber Optimization

Prototype cells of various shapes, sizes, thicknesses, surface treatments and fiber groovings were machined (see Fig. 2) and evaluated together with fibers of different shapes, dimensions and optical treatments to carry out a comprehensive study of the following:

  1. Cell processing
  2. Light response
  3. Response uniformity
  4. Efficiency
  5. Cross talk
  6. Ageing

The results of our studies, demonstrating that small scintillating cells are appropriate for a finely-segmented hadron calorimeter, are published in [5] and [6].

The different species of cells and grooves investigated.


Photodetectors

We are exploring the use of SiPMs/MRS [7] devices as the photodetectors for the hadron calorimeter. During the course of our investigations we also studied other solid-state photodetectors like APD's and VLPC's [8] but find that the SiPMs are the most suitable for the finely-segmented calorimeter we have in mind. SiPMs are multi-pixel photo-diodes operating in the limited Geiger mode. They have high gain ($\approx$$10^{6}$) but relatively modest detection efficiencies (quantum efficiency*geometric efficiency $\approx$ 15%) and therefore deliver performances similar to (or better than) a conventional PMT. They have a distinct advantage over the conventional PMTs however, due to their small size (1mm x 1mm), low operating voltages ($\approx$ 50V) and insensitivity to magnetic fields. On the 1$\rm {mm^2}$ sensor surface there are typically 1000-1500 pixels (see Fig. 3), each one of which produces a Geiger discharge when a photon impinges upon it. The energy is therefore proportional to the number of pixels fired. Typically a minimum ionizing particle (MIP) fires 15-20 pixels (or photoelectrons).

The mounting of the SiPMs on the scintillator tile (see Fig. 4) has a number of beneficial effects:

  1. Light Output: The light suffers little or no attenuation as it does not have to travel large distances in the fiber.
  2. Cost: The amount of fiber required (WLS or clear) is drastically reduced.
  3. Simplified Architecture: Since photo-conversion occurs right at the tile one can come out of the detector directly with electrical signals thus largely eliminating the problems associated with handling and routing of a large number of fibers.
  4. During the course of our investigations into these photodetectors the following characteristics were studied in detail:
    1. Working point
    2. Dark rate
    3. Linearity of response
    4. Temperature dependence
    5. Fiber alignment
    6. Medium-term stability
    7. Radiation damage
    8. Immunity to strong B-fields

The results of our studies, showing that SiPMs/MRS are suitable for a scintillator hadron calorimeter, are documented in [9] and [10].

Pixellated surface of the SiPM sensor (left) and single photoelectron separation observed with a SiPM (right).
Figure: The SiPM sensor mated with a 1mm WLS fiber and embedded in a 3cm x 3cm tile.

 

The SiPM sensor mated with a 1mm WLS fiber and embedded in a 3cm x 3cm tile.

Figure: The SiPM sensor mated with a 1mm WLS fiber and embedded in a 3cm x 3cm tile.


Test Beam Prototype

The prototyping studies summarized above have pinned down the configuration of the active layers of the scintillator HCal for us. In collaboration with our European colleagues we are now moving towards the construction of a 38 layer scintillator-steel prototype for the testbeam. The proposed prototype, the result of extensive hardware R&D and simulation studies, will address the following overall goals of our program:

  1. Technology demonstration
  2. Exploration of the full range of readout from purely digital to fully analog
  3. Validation of hadron shower models in MC
  4. PFA development

The active layers of the prototype consist of 5mm thick scintillator tiles sandwiched between 2cm thick steel absorber plates mounted on a movable table. In reality the absorber is split into three parts: 1.6cm absorber plate and two 0.2cm thick top and bottom skins of the ``cassette'' which houses the tiles. Each tile comes with its own 1mm diameter WLS fiber mated to a SiPM embedded in it. The tiles come in three granularities: 3cm x 3cm, 6cm x 6cm and 12cm x 12cm (see Fig. 5). The 3cm x 3cm cells form the inner core for thirty of the 38 layers while for the last eight layers only the coarser granularity cells are used. The granularity of the prototype has been optimized to achieve the goals listed above within a reasonable budget. As the initial proponents of the finer granularity we are responsible for the instrumentation of two-thirds (i.e. 20 layers) of the inner core. A 1mm thick co-axial cable runs from each photodetector to a charge integrating amplifier channel. This single co-axial cable carries both the bias (on its shield) and signal (on its core). The cables are supported on a G-10 plate which also has the reflective VM2000 glued to its tile-facing side.

Figure: Prototype geometry.

Figure: Prototype geometry.