The timing upgrade project of the TOTEM Roman Pots detectors Nuclear Instruments and Methods in Physics Research A

We describe the upgrade project developed by the TOTEM Collaboration to measure the time of ﬂ ight (TOF) of the protons in the vertical Roman Pot detectors. The physics program that the upgraded system aims to accomplish will be addressed. Simulation studies allowed us to de ﬁ ne a geometry of the sensor such that the detection inef ﬁ ciency due to the pile-up of the particles in the same electrode is low even with a small amount of read-out channels. The measurement of the protons TOF with (cid:1) 50 ps time resolution requires the development of several challenging technological solutions. The arrival time of the protons will be measured by scCVD diamond detectors, for which a dedicated fast and low-noise electronics for the signal ampli ﬁ cation has been designed. Indeed, while diamond sensors have the advantage of higher radiation hardness, lower noise and faster signal than silicon sensors, the amount of charge released in the medium is lower. The sampling of the waveform is performed at a rate up to 10 GS/s with the SAMPIC chip. The sampled waveforms are then analysed of ﬂ ine where optimal algo- rithms can be implemented to reduce the time walk effects. The clock distribution system, based on the Universal Picosecond Timing System developed at GSI, is optimized in order to have a negligible uncertainty on the TOF measurement. Finally an overview of the control system which will interface the timing detectors to the experiment DAQ is given.


a b s t r a c t
We describe the upgrade project developed by the TOTEM Collaboration to measure the time of flight (TOF) of the protons in the vertical Roman Pot detectors. The physics program that the upgraded system aims to accomplish will be addressed. Simulation studies allowed us to define a geometry of the sensor such that the detection inefficiency due to the pile-up of the particles in the same electrode is low even with a small amount of read-out channels. The measurement of the protons TOF with $ 50 ps time resolution requires the development of several challenging technological solutions. The arrival time of the protons will be measured by scCVD diamond detectors, for which a dedicated fast and low-noise electronics for the signal amplification has been designed. Indeed, while diamond sensors have the advantage of higher radiation hardness, lower noise and faster signal than silicon sensors, the amount of charge released in the medium is lower. The sampling of the waveform is performed at a rate up to 10 GS/s with the SAMPIC chip. The sampled waveforms are then analysed offline where optimal algorithms can be implemented to reduce the time walk effects. The clock distribution system, based on the Universal Picosecond Timing System developed at GSI, is optimized in order to have a negligible uncertainty on the TOF measurement. Finally an overview of the control system which will interface the timing detectors to the experiment DAQ is given.

The TOTEM Roman Pots upgrade with diamond detectors
The TOTEM upgrade programme [1] focuses on improving the experiment's capability to explore and measure new physics in Central Diffractive (CD) processes: p þpp þ X þ p. Common CMS-TOTEM data taking are foreseen during the LHC Run 2, with a special LHC-optics configuration for which the proton acceptance is optimal (all ξ ¼ Δp=p for j tj 40:04 GeV 2 ). The installation of proton Time-Of-Flight (TOF) detectors in the TOTEM Roman Pots (RPs) allows us to reconstruct the longitudinal vertex position and thus to assign the proton vertex to the proper vertex reconstructed by the CMS tracker, even in presence of event pileup. Ultimately, 100 pb À 1 can be collected in runs with a pile-up μ ¼ 50%. Among the many Physics channels that can be studied with CD, the upgraded system aims to measure with an unprecedent sensitivity low mass resonances (with particular emphasis to Glueball candidates), exclusive CD dijets, charmonium states and events with missing mass signatures. Even at very low pileup, for inclusive CD events and in particular for events with missing momentum the association of the protons to the CMS vertex by using only the tracking variables is problematic. Here, as well as in all CD events measured at moderated pileup, the protons TOF measurement is crucial. As an example, at μ ¼ 50%, a factor $ 5 enhancement on the inclusive CD purity can be obtained from the installation of the TOF detector in the RP, assuming a time resolution of 50 ps. The TOF detector is based on scCVD diamond plates, chosen thanks to their proved radiation hardness and their fast response. Moreover, by using diamonds, it is possible to design a compact TOF that can be easily placed inside the existing vertical RP, without increasing the material budget for the LHC beam. Four TOF planes will be installed in each of the four vertical RPs. One plane is composed of eight 4.5 Â 4.5 Â 0.5 mm 3 sensors, provided by Element 6 LTD. After an extensive R&D on the Front End electronics a time resolution of the single sensor of the order of 100 ps has been proved. Therefore a TOF detector consisting of 4 diamond planes will have a time resolution of $ 50 ps, which means a resolution on the longitudinal interaction vertex (Z VTX ¼ cΔt=2) of σ Z $ 1 cm. To minimize the probability of particle pile-up in the same diamond pixel the design has been optimized in order to guarantee an uniform occupancy of the detector [2]. At μ ¼ 50% the inefficiency introduced by the pile-up is $ 0.6%.

Totem R&D on diamond detectors
The measurement of the protons TOF with 50 ps time resolution requires several challenging technological solutions. Indeed, while diamond sensors have the advantage of higher radiation hardness, lower noise and a faster signal than silicon sensors, the amount of charge released in the medium is lower. A SiGe BJT preamplifier with low-C feedback has to be placed at less than 1 cm from the diamond electrode: this increases the input impedance to few kΩ (therefore enhancing the S/N) while the straight capacitance seen by the signal is still small (which is important to keep the signal fast). This solution, already developed by HADES [3], was then further elaborated by TOTEM which designed an amplification chain able to keep the time resolution of the order of 100 ps also for the electrodes with larger capacitance (2 pF). The BJT-based amplification chain consists of the preamplifier followed by a single stage voltage amplifier and by a booster who also shapes the signal. Particular care was dedicated to optimize the bias network of the single stage amplifier in order to obtain undistorted phase and gain response. The amplification chain has $90 dB gain, with a maximum at 226 MHz. The power dissipated by each amplification channel is about 0.3 W. Both metallizations provided by GSI 1 (Cr-50 nm þ Au-150 nm) and by PRISM 2 (100 nm of TiW 10%90%) have been successfully tested. Detection efficiency is 4 98% in the bulk of the crystal with a negligible effect introduced by the unmetallized area between the strips (see Fig. 1). Depending on the electrode capacitance, the S/N ratio ranges between 20 and 25 and the risetime is $ 1.5-1.8 ns. The measurement of the time resolutions has been obtained from the difference of the arrival time of MIP particles (5.6 GeV electrons) traversing two diamond planes. An offline constant fraction discriminator was applied to reduce the time walk effects. The measurements were performed with an Agilent DSO9254A oscilloscope (8 bits, 20 GSa/s). A time resolution 80 o σ T o 108 ps was measured for electrodes capacitance 0:29 o C o2 pF. The analog signals will be digitized with the SAMPIC chip [4]: it acquires the full waveform shape of a signal, by sampling it through a 64 cell Delay Line Loop (DLL) based TDC and an ultrafast analog memory for fine timing extraction. Each channel has an input bandwidth of 1.5 GHz, an ADC precision of 11 bits and will operate with a sampling frequency up to 10 GHz. The sampled waveforms can be then analysed offline where optimal algorithms can be implemented to reduce the time walk and jitter effects. Measurement performed at the test beam shows that the time resolution obtained with the SAMPIC is compatible with the ones obtained with the high bandwidth oscilloscope.

Clock distribution and DAQ
In order to precisely measure the difference of the arrival time of the protons in the two sides of the interaction point, the crucial problem to be solved is to distribute common time signals to spatially separated detectors, with picosecond range precision. For this purpose TOTEM is adopting a clock distribution system based on an optical network, using dense wavelength division multiplex (DWDM) technique, transmitting two reference clock signals from the counting room to a grid of receivers in the tunnel. The system monitors the delays in the propagation of the signal to control all the transmission systematic effects that will affect the signal jitter. The TOTEM clock distribution is adapted from the "Universal Picosecond Timing System, developed for FAIR at GSI [5]. The digitized signal of the TOF detectors will be interfaced to the existing hardware, already used for the DAQ of the RP silicon detectors [6]. A dedicated board, hosting the radiation tolerant Microsemi Soc FPGA M2S150-FCG1152, has been designed in order to interface the TOF signal to the hardware already existing for the silicon detectors DAQ. Initially TOTEM will not use the timing detectors for the trigger generation, the expected trigger rate from the silicon sensors is o 100 KHz.

Conclusions
In this contribution we described the upgrade of the TOTEM vertical RPs with TOF diamond detectors. The sensors performance have been characterized in several test beams and satisfactory results on time resolution and detection efficiency have been achieved. The digitization of the signal has introduced a negligible deterioration of the time resolution. The solutions that will be adopted for the TOF clock distribution and for DAQ have been finally summarized.