ISSN 10637796, Physics of Particles and Nuclei, 2014, Vol. 45, No. 1, pp. 241–243. © Pleiades Publishing, Ltd., 2014.

A New FPGAbased Trigger/DAQ System for the MTV Experiment at TRIUMF1 R. Tanumaa, *, H. Babab, K. Ninomiyaa,b, Y. Nakayaa, S. Tanakaa, Y. Totsukaa, and J. Murataa a

Department of Physics, Rikkyo University, 3341 NishiIkebukuro, Tokyo 1718501, Japan b Nishina Center, RIKEN, 21 Hirosawa, Wako, Saitama 3510198, Japan *email: [email protected]

Abstract—A new FPGA based data acquisition (DAQ) and trigger combined system is developed for the MTV experiment (Mott Polarimetry for TViolation Experiment). The MTV experiment measures backward Mott scattering events by detecting the incident and scattered tracks using a drift chamber. For the backward scattering probability is as small as less than 0.1%, efficient triggering system is required to select the desired events. An intelligent Level2 trigger system reading hitting pattern together with its digital readout system are built in a custom FPGA system. In this paper, design overview and results from performance tests are described. DOI: 10.1134/S1063779614011036 1

1. INTRODUCTION

The MTV experiment at TRIUMF [1, 2] is aiming to search nonzero Tviolation in nuclear beta decay by measuring electron transverse polarization, which is measured as leftright scattering asymmetry in Mottscattering of the electrons by a thin metal foil. The trajectories of the scattering electrons are detected with a cylindrical drift chamber (CDC), which was commissioned in 2011 as a next generation tracking detector after performing physics production run in 2010 using a planar drift chamber. The CDC with 400 anode channels are used with 107cpps 8Li beam implanted on a surface of a stopper set at the center position of the CDC. The emitted electrons from the stopper is backwardly scattered by a thin metal foil set outside of the effective volume of the CDC. Then, the incident tracks and the backward scattered tracks are detected by the CDC. The scattering angles of these “Vtracks” are the measurement of this experiment. In order to achieve a high statistical precision, a high speedtriggering and DAQ system is required to exe cute the physics production using the CDC. In the triggering section, we have built an intelligent trigger ing system which can select the Vtrack events from the huge forward scattering events by performing online pattern recognition using CDC hitting pattern information. To achieve the requirement, a new FPGAbased system including both of the triggering and the DAQ part in a single FPGA chip is developed. The DAQ part can obtain the TDC information in order to select the real hit signal from accidental hits in the high counting rate environment. 1 The article is published in the original.

The Level1 trigger for this measurement is coinci dence logic of a plastic scintillation counter array sur rounding the CDC, which stops the scattered elec trons after penetrating the CDC. In a previous experi ment named RunII performed in 2010, we used the planer drift chamber with 104 readout anode chan nels. For the CDC has 400 anode channels, number of data words per event in a FIFO memory are signifi cantly increased, which reduces the DAQ speed. In order to achieve a high statistical precision, a new high speedtriggering and new DAQ system is required to execute the physics production using the CDC. In this experiment, position resolution inside the drift cell is not important because the track resolution is domi nated by physical multiple scattering. Therefore, only rough TDC resolution is requested to read the drift time. For example, conventional TDC such as CAENV1190A, which we used for the previous RunII experiment [2] shown in Fig. 1, has very high time res olution but is slow for our usage. Considering the requirements, we selected a general purpose VME board CAEN V1495 [3]. 2. NEW TRIGGER/DAQ SYSTEM The new FPGAbased system including both trig gering and DAQ part in a single FPGA chip is devel oped with the general purpose VME board. Schematic diagram of the new system is shown in Fig. 2. Require ment for the TDC is to decrease the number of words per event in the FIFO memory on the board, in order to increase the data taking event rate. The time interval between wires or PMT’s hit signals and Level1 start signal is measured in counter type TDC using internal clock, which users can customize the time resolution

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Fig. 2. Block diagram of the new DAQ system for the MTV–CDC setup.

by changing the clock speed. Our previous system with CAEN 1190A TDC uses two words for leading and trailing edge time information, on the other hand, the new TDC uses only one word for both of these time information. We also developed other readout mode without recording any timing information to suppress the data size, as a “coincidence register”. The number of words in an event depends on the hit multiplicity in the TDC mode, on the other hand, the event size is fixed in the coincidence register mode. Although we do not need the drift time information for this measurement, TDC information is useful in order to reject accidental coincident events. To achieve the accidental rejection, we developed an online timing cut function which is equivalent to set a coincidence window. In addition, sometimes it is use ful to measure the pulse time width in order to distin

guish real wire hit from cross talk or other electric noises. We have also developed the online time width selecting function in our system. In the coincidence register mode, 32 channels input correspond to a 32 bits fixed data. Performance tests for the TDC and the coinci dence register modes were performed with 16 simu lated stop signals with one start signal using a clock generator. TDC linearity was measured, which result is shown in Figure 3. We confirmed that the good TDC linearity is hold in the range of 400 to 2,400ns. In addi tion, DAQ speed is measured as the rate dependence of the live time for different readout modes. The results are shown in Figure 4. Both for the new TDC and coincidence register using V1495, live time keeps around 100% at 100 kHz, whereas previous system with V1190A TDC runs at around 80% live time. The

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A NEW FPGABASED TRIGGER/DAQ SYSTEM DAQ live time

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Fig. 3. Results of the TDClinearitty measurement.

Fig. 4. DAQ live time performances.

TDC mode have limits at around 170 kHz, on the other hand, the coincidence register’s limit is at around 475 kHz. Anyway, these new systems are shown to be fast enough to handle expected data tak ing rate in the coming MTV experiment using the CDC. Not only for the hardware development, but also a triggering logic is developed for the measurement with the CDC. Unlike to the RunII setup, we need to sup press straight line events without forming the Vtracks. It is because dominant events are these straight events. In the CDC setup, we have 12 trigger scintillation counters set inside the CDC, surrounding the stopper, which is counters surrounding outside of the CDC. The Level1 trigger is generated if there is coincidence between one of the trigger counters and stopping

counters, but rejecting straight coincidence between them. In order to keep good rejection factor of the online triggering, we need to apply Level2 trigger, which request CDC hit information. Current design of the Level2 trigger requests minimum number of the wire hit channels. Performance test of the triggering logic is performed with radiation source and test beam in 2012, showing its good enough ability to perform the physics data taking scheduled in 2013.

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REFERENCES 1. J. Murata, et al., J. Phys. CS 312, 102011 (2011). 2. J. Onishi, et al., J. Phys. CS 312, 102012 (2011). 3. See V1495 (General Purpose VME Board): http://www.caen.it/.

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