Tagger Microscope

From GlueXWiki
Revision as of 10:38, 8 May 2014 by Jonesrt (Talk | contribs) (Light yield tests)

Jump to: navigation, search
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

mean cosmic yield = (8000 /MeV) * (1.6 MeV/cm) * (mean cosmic scint path length) * (SiPM pde) * (fiber transmission fraction) * (factor for reflection from upstream reflector, if present) mean cosmic scint path length = 0.21 mm (from Geant simulation of our setup) SiPM pde is 25% in this wavelength range, at the bias levels we are using factor for reflection from reflector on end of scintillating fiber was measured to be 1.7 for aluminized mylar minimum fiber transmission factor 1.8% needed to meet design goal of 200 pixels per 2cm electon track use measured cosmic yields to extract mean fiber transmission fraction for various configurations. 2.7% - prototype detector (double-clad, with paint, 60cm light guide) 1.9% - final production bundle 19 (single-clad, no paint, 160cm light guide, type "B" below) out of 22 final production bundles (30 fibers per bundle) with letter designation 1 has been straightened, fused, bent, painted, and glued - "G" 9 have been straightened, fused, bent, painted, but not glued - "P" 1 has been straightened, fused, bent, painted, and then had the paint removed - "R" 8 have been straightened, fused, bent, but not not otherwise touched - "B" 2 have been straightened, fused, but not bent or otherwise touched - "F" a few random sections of light guide remain that were not allocated to bundles 2 fibers have been straightened, but not fused or otherwise touched, ~160cm long - "S" 2 fiber sections remain as cut off the spool, never otherwise touched, ~80cm long - "C" We only have cosmic measurements so far for the prototype and for production fibers of type B. For the others, we have compared their response by injecting light from a pulsed laser diode through the end of the light guide with a type B fiber right next to it. There is a diffuser after the laser to make the beam spot > 5cm in size, to reduce position sensitivity and also the intensity of the pulses. There are variations from fiber to fiber, but here is a semi-quantitative comparison, in arbitrary units normalized to the type B pulse height. All SiPM gains have been equalized first. G : 30 - 60 P : 100 - 200 R : 10 - 30 B : 250 - 500 F : 350 - 700 S : 450 - 900 C : too short to measure in our setup For any given fiber we get consistent results from one pulser run to the next, but there is some question of exactly how these responses map onto scintillation transmission fraction. So far we have had time to measure only two bundles of production fibers with cosmics, both of them type B, one with pulser responses near the low end and the other near the high end of the range 250 - 500 shown above. Both bundles showed similar cosmic response within the 10% statistical precision of the measurement.

Summary of results from simulation and bench observations We have spent many hours visually comparing the light escaping from different types of fibers using a diode in a dark room. We also have written a stand-alone Geant simulations of light propagating in single-clad and double-clad fibers with different surface treatments (paint, air, abrasion, random cracks). Another stand-alone simulation is used to determine the path length distribution of cosmic tracks in our cosmic trigger configuration, to allow us to extract the fiber transmission fraction from measured pixel yields in cosmic runs. Here are some observations. Light is escaping all along the fibers, mainly from invisible defects in the structure (little micro cracks, speckles). To leading order they all look the same, including type C which looks especially bright and sparkling along its surface when illuminated from one end in a dark room. The edges of the square fiber are distinctly bright, as if light is getting trapped there and escapes preferentially from the corners, or perhaps the defect density is high there. Bent fibers are more bright along curved sections than straight, sometimes by a factor 2, not more. Painted fibers are more bright than unpainted, perhaps by a factor 2-3, especially in the bend regions. Overall it was very difficult to predict light yield ratios seen with the pulser by looking at escaping light visually. Even for fibers with almost no light transmission seen with the pulser, the visual exam seemed similar to others with a large pulser signal in side-by-side comparisons in a dark room. All of these observations only apply to single-clad fibers. To compare with double-clad we have to rely on simulation. Here are some conclusions from simulation. If all of the core and cladding surfaces are perfect, the transmission fraction is huge, like 25%, due to the large refractive index ratio between the core (1.6) and air. This disappears when almost any kind of imperfection is introduced in the outer surface (absorbing layer, abrasion and random defects) and the number quickly drops to the capture fraction implied by the core - cladding n-ratio. If the inner core-cladding interface is perfect, the transmission factor is 4.3% no matter what you do to the outer surface - paint, absorbing surface layer, surface abrasion and random defects. Under these conditions, bending has very little effect, only a few percent loss for bend radius down to 5 cm. If the inner core-cladding interface is imperfect, it is hyper sensitive to what you do there. Almost any imperfections make the result sensitive to conditions in the outer layers: painted vs. not, surface roughness and absorption. Adding a second cladding layer does not increase the capture angle of the transmitted light. That is what I thought it would do, but the simulation shows that the transmitted light after traversing 160cm of fiber is essentially all within the cone set by the ratio of refractive indices of the core and inner cladding, once you include imperfections in the boundary layers. The second cladding layer increases the transmission fraction by making it less sensitive to imperfections at the outer surface. Without the outer cladding, any light that escapes the core due to inner interface ripple reaches the outer surface has a good chance of being lost. If there is a second cladding, the light has a much better chance of being reflected back, after which it re-enters the core and is re-trapped inside. So with the second cladding layer, the chance of light loss is 1/N^2 instead of 1/N, where N is the mean number of boundary reflections before light that is nominally trapped inside the core hits a boundary defect and escapes. This last point explains why we didn't see any loss in yield from the paint with the double-clad light guide fibers that we used in the prototype, but we see this large factor 2 effect now in the production fibers which are single-clad.