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Simulation of the RICH-1 prototype

The RICH-1 prototype configurations are simulated to allow detailed comparisons of expected performance with data. The simulation is also used in the design of the prototypes themselves, to calculate the optimal position of the detector plane from the mirror. The program simulates photon emission in the radiators and assigns each a wavelength, Cherenkov angle and point of emission. Photons are then ray-traced to the HPD detectors and assigned to a hit. The program incorporates the following physical effects : An event display program is used to visualise the results of simulation and data events. An example of simulated events in RICH Configuration 2 is shown in Figure 9.

Figure 9: Event display showing the superposition of simulated events as in Configuration 2. The x and y axes are in units of mm.
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Contributions to the Cherenkov angle resolution for RICH Configuration 2 with a C4F10 gas radiator are shown in Figure 10. Assuming an emission point for the photons to be the mid-point of the radiator induces a systematic contribution to the Cherenkov angle resolution shown in Figure 10(a). The effect of chromatic aberration is to induce a spread in the reconstructed values of $ \theta_{\rm c}^{}$, and this contribution is shown in Figure 10(b). If in the reconstruction it is assumed that the beam particle follows the nominal trajectory, this results in an additional contribution to the Cherenkov angle resolution shown in Figure 10(c). Finally, the finite pixel size ( 2 xmm2) affects the reconstructed Cherenkov angle, as indicated in Figure 10(d). Here the precise nature of the modulation of the $ \theta_{\rm c}^{}$ distribution depends critically on the relative orientations of the Cherenkov ring relative to the line of the pixels in the HPD hexagonal structure. The effects of this contribution are simulated, however the precise alignment of the photon detectors relative to the ring remains a potential source of discrepancy between data and simulation. Results are averaged over compatible HPDs, in different orientations, in order to minimise this uncertainty.

Figure 10: Plots showing examples of reconstructed C4F10 Cherenkov angles when different contributions to the error are isolated in the simulation: (a) Emission point uncertainty only; (b) Chromatic aberration of the photons; (c) the particle position and direction uncertainty and; (d) the error due to the finite pixel size of 2 x 2mm2.
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The magnification of the incident position of photons arriving at the photocathode is measured to be 1.06 from laboratory studies using a scanning LED, and is included in the simulation. Finally, background signals from electronic noise are included in the simulation by smearing the expected photoelectron signal with a Gaussian whose sigma is determined by the pedestal width observed in data.


next up previous
Next: Estimates of the photon Up: Performance of a Prototype Previous: Alignment of the silicon
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