A Digital Trigger for the Electromagnetic Calorimeter at the COMPASS Experiment Stefan Huber, Jan Friedrich, Bernhard Ketzer, Igor Konorov, Alexander Mann, Thiemo Nagel and Stephan Paul
Abstract— At the COMPASS experiment at CERN’s SPS many physics channels containing neutral particles are studied. These particles are identified by two electromagnetic calorimeters covering different transfers regions. For certain physics the main signature is described by these neutral particles which requires a calorimetric trigger system. The method described here is fully based on the existing front end electronics and uses digital pulse shape analysis techniques. This approach allows to implement a flexible trigger system as well as to reduce cost by avoiding to produce new electronic components. The implementation of this new trigger as well as the performance measured during the 2009 run will be discussed.
I. I NTRODUCTION The two stage magnetic spectrometer COMPASS[1] (COmmon Muon and Proton Apparatus for Structure and Spectroscopy) performed in November 2009 a measurement of the Primakoff effect on pions. Therefore a π − beam was sent on a target consisting of nickel discs. This effect being radiative pion scattering on the targets nuclear coulomb field π − + Z → π − + Z + γ is characterized by high energetic photons which are due to a small momentum transfer scattered in very forward direction. The very small angle respectively to the incident beam direction requires to detect them at the Electromagnetic Calorimeter in the second spectrometer stage (ECAL2). In order to detect these events the new digital trigger
experiment. Dynamically adjusting calibration constants as well as changing trigger parameters are possible. II. T HE D ETECTOR ECAL2 placed 33 m downstream of the target has a size of 2.45 × 1.84 m2 divided into a matrix of 64 × 48 cells. Those cells have quadratic surface with a side length of 3.8 cm and consists in the peripheral region of GAMS modules, the more central region consists of radiation hard GAMS further the inner most part is equipped with modules of Shashlik type. The whole detector is read out by photomultiplier tubes (PMT) which are equipped with Cockcroft-Walton HV multipliers. The PM signals are fed into shaper cards which transform the fast signal from the PMTs into slower ones with a rise time of around 40 ns to avoid aliasing problems. An example pulse can be seen in figure 3. Due to the use of the shaper the pulse shape of GAMS and Shashlik modules is nearly the same. This signal is digitized by 12 bit ADCs running at a sampling rate of 40 MHz which is increased by a factor of two to 80 MHz by operating two of the ADCs in interleaved mode. 16 detector channels are read out via one Mezzanine Sampling ADC (MSADC) [2] card of which four are combined on one carrier card providing a total readout of 64 channels. III. T HE T RIGGER S YSTEM The trigger concept is to calculate a total energy released in the calorimeter within the 12 ns period of the 80 MHz clock which provides a common time reference system. If the total energy exceeds a certain threshold in coincidence with an incoming beam particle the trigger gets active. Most of the logic has been implemented into the FPGAs on the existing electronic boards. This electronics allows to sum 64 calorimetric channels within one ADC module. In addition to that a backplane card has been developed which combines eight of these boards together and makes the final trigger decision. The backplane also has additional interfaces which allow to interconnect it to further back planes. The trigger area can be flexible adjusted by configuration a channel mask from a database.
Fig. 1.
Setup of the COMPASS spectrometer
system which makes use off the existing readout electronics of this detector has been developed. It exploits the flexible FPGA based sampling ADC modules used at the COMPASS Physik Departement E18, Technische Universit¨at M¨unchen, 85748 Garching, Germany, contact:
[email protected]
978-1-4244-7110-2/10/$26.00 ©2010 IEEE
A. Signal Detection In order to find detector signals belonging pulse shape analysis is done in a pipelined way on the continuous samples coming from the detector without having an event preselection. Before this can be done the pedestal value for each channel has to be fixed. This is done taking advantage of the fact that
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the SPS provides the beam in spills of approximately 10 s with a 30 s gap in between. During a one second time intervall before the start of the spill many samples are accumulated and the mean value is calculated. This allows to get a very precise pedestal value which is robust against long term fluctuations. For online signal analysis the baseline is set to zero while for the readout it is set to 50 ADC counts in order to have room for negative values as well. In figure 2 the baseline distribution determined from LED calibration signals by taking the mean value of eight samples is shown. There the width is σ = 2 ADC counts. The pedestal corrected samples are fed into a digital constant fraction discriminator which detects pulses as well as determines the corresponding time. It is implemented in a way that the unmodified signal A and a signal B which is amplified by a factor of two and delayed by two clock cycles each are compared. One clock cycle has a length of 12.86 ns. A signal is accepted when the difference D = A - B is bigger than zero in the first clock cycle and smaller than zero in the following one (Fig. 3) in addition to that a per channel threshold is applied on the height of this transition. This threshold is set in a way that noise is suppressed but the loss of low energetic signals is minimal. For the 2009 data taking the value was set to 20 ADC counts corresponding to an energy of 0.8 GeV. The timing value of the CFD is used by calculating everything relative to the sample position where the zero crossing occurred and performing a linear interpolation between the difference value before zero crossing (D) and the one after (D’). In that way a time resolution of σ = 1 ns. is achieved. Where the fine time value corresponding to the sub clock cycle resolution is given by D . (1) D − D This interpolation is done using a look up table for 1/(D-D’). In order to avoid a to long look up table only the six most significant bits of the nominator and denominator are used to calculate the time. This limitation reduces the time resolution of the algorithm by at most 0.4 ns and that only for very t=
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Fig. 3. Principle of the CFD implementation used. In black the original signal coming from the detector is shown. The red curve represents the delayed signal while the blue one is the difference of the black and red curves.
high energetic signals where the width of the timing peak is only 0.8 ns. Due to that the performance of the trigger is not disturbed, because the resolution needed for further processing of the data is 6 ns as shown in section III-B. counts
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B. Determination Of The Total Energy For each channel time and energy calibrations are determined offline and then loaded into certain registers of the FPGA. In order to time in the individual channels the time calibrations are added to the timing values determined for the current event. To be able to sum up all corresponding energies for one event the signal is delayed by the coarse time what corresponds the number of clock cycles the signal is shifted respectively to the mean time. Further border effects have to be taken into account. These occur when the signal time is to close to a multiple of clock cycles. This is compensated by using the fine time. Signals which correspond to the first half of one clock cycles are extended by one clock cycle to the left while signals corresponding to the second half are extended to the right (figure 5).
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Fig. 5. Illustration how the binning is done for two example hits. Signal 1 detected first is active in the detected clock cycle and in the following one while signal 2 is issued in its detected clock cycle and extended to the previous one. This allows to align all detected signals.
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of around 30 kHz not the whole accessible area but 12 × 12 cells have been selected in the center of the calorimeter where a hole of 3 × 4 cells was excluded at the right to get rid of hits by elastically scattered beam particles at small angles. 1
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Combining many channels of the detector requires to interconnect the readout modules what is done via backplane summation cards. Having only a certain bandwidth available this requires to reduce the data rate as much as possible. This is done by consecutive summation of the individual energies before every interface where the dynamic range is also reduced to the threshold value. Energies above threshold are immediately saturated because they would fire a trigger in any way. C. Final Trigger Decision The total energy sum for every time slice is calculated at a VME backplane card which compares the sum with given thresholds. When the signal exceeds a threshold a TTL signal is sent synchronous with the 80 MHz readout clock where the length of the pulse depends on the number of clock cycles the threshold is exceeded. This signal is timed in respectively to the other triggers and brought into coincidence with the beam trigger signal which is also used as the time reference. The drawback of the digital implementation is the increased latency of 500 ns respectively to the analogue signals which have to delayed by that amount of time. Therefor special care was taken while developing the algorithm to avoid any unnecessary registers within the FPGAs. D. Monitoring One of the main advantages compared to an analogue implementation is the possibility to implement monitoring for every important quantity. For per event monitoring the amplitude determined by the CFD is stored into the data stream. This allows to tune the parameters in a way that always the maximum amplitude is selected. Further for each channel the baseline as well as a scaler value are stored into registers of the FPGA. This values are read out via the VME bus after each spill. E. Results During November 2009 the trigger was used for the first time where an area of 16 × 32 cells was connected by one backplane. Considering a maximal possible trigger rate
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Fig. 6. Fraction of events triggering both thresholds by events only triggering the lower threshold. This provides information about the trigger efficiency as well as its energy resolution.
In addition to this geometric arrangement the trigger has been set up with two different thresholds of 40 GeV and 60 GeV. The 40 GeV threshold had been prescaled with a factor of two what led to a trigger rate of 23 · 103 1/s for the high threshold and 20 · 103 1/s for the lower one. While the rate of incoming pions was 2.5 · 106 1/s. Due to the cinemtaic overlap of the two triggers the total trigger rate was 27 · 103 1/s. Harnessing the availability of two thresholds the energy resolution of the trigger can be determined by dividing the energy spectrum where both triggers were fired by the spectrum of the low threshold trigger. This is shown in figure 6 where the histogram was fitted with a Fermi function parametrized like in equation 2. f (x) =
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The resulting resolution is ΔE = 1.8 GeV what is caused mainly by the bandwidth which can be transmitted via the interfaces between the different FPGAs as well as the per channel threshold applied during signal detection. In addition to that several events were collected with beam trigger only from what rates could be extrapolated for different thresholds as well as bigger dimensions of the trigger area. IV. O UTLOOK The first data taking using the trigger has shown that the digital trigger concept is working and performing well. For the future several improvements are foreseen where the most important part is the extension to the whole calorimeter. In total six back planes covering 512 channels each will be interconnected and thus provide a trigger area covering all 3072 detector channels.
As an additional feature a dependance on the transverse momentum is considered. This could be done by modifying the energy calibration constants depending on the cell position. R EFERENCES [1] P. Abbon et al. The compass experiment at cern. Nucl. Instrum. Methods Phys. Res., A 577, 455–518 Jan 2007. [2] A. Mann, I. Konorov, and S. Paul. A versatile sampling adc system for on-detector applications and the advancedtca crate standard. In 15th IEEE-NPSS Real-Time Conference, 2007.
