Beam test results of the Intermediate Silicon Tracker for sPHENIX

The Intermediate Silicon Tracker (INTT), a two-layer barrel silicon strip tracker, is a key component of the tracking system for sPHENIX at the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory. The INTT is designed to enable the association of reconstructed tracks with individual RHIC bunch crossings. To evaluate the performance of preproduction INTT ladders and the readout chain, a beam test was conducted at the Research Center for Accelerator and Radioisotope Science, Tohoku University, Japan. This paper presents the performance of the INTT evaluated through studies of the signal-to-noise ratio, residual distribution, spatial resolution, hit-detection efficiency, and multiple track reconstruction.

💡 Research Summary

The paper reports on a comprehensive beam test of the Intermediate Silicon Tracker (INTT), a two‑layer barrel silicon strip detector that will serve as a crucial component of the tracking system for the sPHENIX experiment at RHIC. The INTT consists of 56 ladders arranged in a cylindrical geometry around the beam pipe, with inner and outer radii of approximately 7.5 cm and 10 cm, respectively. Each ladder incorporates Type‑A and Type‑B silicon sensors (78 µm pitch, 16 mm or 20 mm strip length) read out by 52 FPHX ASICs. The FPHX chips employ a 3‑bit ADC implemented as eight programmable comparators; the digitized signal is the index of the highest comparator threshold exceeded.

The beam test was performed at the Research Center for Accelerator and Radioisotope Science (RARiS) of Tohoku University using a 1 GeV/c positron beam generated by a tungsten target. Four pre‑production INTT ladders were mounted in a dark box to form a telescope, with upstream and downstream scintillator paddles providing a coincidence trigger. Only the right halves of the ladders were powered (the most upstream ladder was excluded due to a bias‑voltage issue). Data acquisition employed a PHENIX FVTX front‑end module and a National Instruments PCIe‑6536B system running under Windows 10.

Signal‑to‑noise ratio (S/N) was measured by performing eight successive DAC scans, each shifting the comparator threshold by 4 DAC units, thereby sampling overlapping portions of the energy‑deposit spectrum. By merging the eight histograms for ladders L0 and L2, the most probable value (MPV) of the Landau‑shaped MIP signal was found to be 73.23 ± 0.20 (stat) ± 1.71 (syst) DAC units, while the combined noise width corresponded to 4.56 ± 0.16 DAC units. This yields an S/N greater than 15.1, comfortably exceeding the typical requirement for silicon trackers.

Spatial resolution was assessed through a residual analysis. For each cell, the y‑position measured on ladder L1 was compared to a linear interpolation of the positions measured on ladders L0 and L2. An initial vertical misalignment of 0.298 mm for L1 was corrected, after which the residual distribution’s width indicated a position resolution of roughly 70 µm, consistent with the design goal of 50–80 µm.

Hit‑detection efficiency was evaluated by counting events where a cluster appeared within 1 mm of the projected beam spot on each ladder. Even when a 1.3 m bus‑extender (BEX) cable was inserted between the HDI and the readout card, the efficiency remained above 99.3 %, demonstrating robust data transmission over long cables.

Multiple‑track reconstruction capability was tested by selecting events with more than one cluster in the same cell and reconstructing each cluster as an independent track. The distance between reconstructed tracks clustered below 0.2 mm, confirming that the INTT can resolve closely spaced tracks in high‑multiplicity environments, a key requirement for sPHENIX’s jet and heavy‑flavor physics program.

Although the test operated the sensors at 50 V—slightly below the full depletion voltage of ~57 V—the impact on performance metrics was negligible. The final sPHENIX configuration will run at 100 V, providing a safety margin. Overall, the beam test validates that the pre‑production INTT ladders and the associated readout chain meet or exceed the specifications for timing (≈0.1 µs), spatial resolution, and detection efficiency required for successful integration into the sPHENIX detector.