Silicon vertex detector for the KEK Super B factory
Section snippets
Detector configuration
Fig. 1 illustrates the configuration of the proposed SVD for the Super KEKB [2]. Six layers of silicon sensors enable an efficient reconstruction in SVD. The radii of the innermost and outermost layers are 1.3 and 14 cm and the angular acceptance is . The radius of 1.3 cm of the first measurement results in better vertex resolution than that of the existing SVD (SVD2) [4]. The innermost layer is a super layer of double sided silicon strip detectors (DSSD), or pixel sensors. The other four
Striplet DSSD and kapton flex readout
In Super KEKB, the background level at the innermost layer of the SVD will be about 20 times larger than that in the current KEKB. A significant occupancy reduction is necessary to keep the vertexing performances. The sensitive area of each readout channel of DSSD can be reduced by inclining the strips by . Since the size of the sensor in the innermost layer is , the strip length is reduced from 72 to 11 mm. A DSSD with 1024 inclined strips with spacing is shown in Fig. 3. A
APV25 front end ASIC
Among existing ASICs for silicon strip sensor readout, APV25 [8], designed for the CMS silicon tracker, satisfies the requirements: (1) radiation hardness, (2) negligible dead time at a trigger rate of 30 kHz, and (3) 50 ns shaping time. In the APV25, each of the 128 readout channels consists of preamplifier and shaper (50 ns peaking time) and a 192-stage analog pipeline of which up to 32 stages are used for the readout queue. The analog signals are serialized and the data sparsification
Hit time reconstruction
The APV25 can output the shaper output data in several pipeline cells around a trigger. Reconstruction of hit time using the wave form data has been tested in a series of beam tests in 2005 [10]. Fig. 6(a) shows a typical wave form from APV25. The hit time is reconstructed by fitting the data with a CR-RC shaper function. A clear correlation between the reconstructed hit time and event time is observed (Fig. 6(b)). The time resolution is found to be , 25 times shorter than the shaping time
Monolithic pixel option
The combination of the striplet DSSD and APV25 ASIC in the innermost layer satisfies the requirements for the SVD operated in the Super KEKB environment. It is however clear that monolithic pixel sensors could provide a better performance. An intense R&D for a monolithic active pixel sensor (MAPS) as a possible candidate sensor for the future SVD upgrade is being pursued [12]. The R&D is in the phase of testing a large size sensor with pipelined readout.
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