Nuclear Instrumentsand Methods in Physics Research A 379 (1996) 545-547
NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH Section A
The SND calorimeter first level trigger D.A .. Bukin, T.V. Dimova, V.P. Druzhinin, V.B. Golubev, A.V. Gritsan , Yu.V. Yu.S . Velikzhanin *
Budker Institute of Nuclear Physics, Novosibirsk State University Novosibirsk 630090, Russia
Abstract The first-level trigger (FLT) of the 1632 channel NaI(T1) scintillation calorimeter of the SND detector at the VEPP-2M collider is described. The FLT utilizes signals from individual calorimeter towers to check the event topology for particular combinations of hit towers with certain distances between towers, number of clusters and their sizes in different parts of the SND calorimeter. The pipeline FLT logic works with a clock rate of 16 MHz which is the collider beam crossing frequency. The latency time of the FLT decision is 500 ns. During the 1995 SND data taking run, the FLT reduced the 100 kHz background rate to 40 Hz.
(9 the total energy deposition in the calorimeter for physi-
The Spherical Neutral Detector (SND) was built for the VEPP-2M electron-positron collider [ 11, operating in the energy range 2E from 0.4 to 1.4 GeV. The main part of the SND [ 2,3] is a three-layer spherical calorimeter consisting of 1632 NaI(T1) crystals. The goal of the experiment with the Spherical Neutral Detector is to study electron-positron annihilation into hadrons in the energy range from 0.4 to 1.4 GeV. The maximum event rate of hadron processes at the current collider luminosity is a few Hz only, while the rate of Bhabha scattering within the detector solid angle is 20 Hz. The main sources of background are the electromagnetic showers produced in the collider magnets and lenses by particles lost from the beam. The absence of magnetic field in the detector makes the background situation even more complicated. In the real experimental conditions, the total counting rate can reach a level of 100 kHz in the calorimeter. The first-level trigger has to reduce the trigger rate down to a level of less than 50 Hz while retaining high efficiency for events of the physical processes under study.
2. First-level trigger 2.1. Requirements
to the FLT
The FLT design is based on the following differences properties of physical and background events:
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cal events is usually larger than for background events; (ii) hit counters in physical events form two or more compact clusters in the calorimeter; (iii) particles in physical events originate from the collider interaction region; (iv) in background events, the main part of the total energy deposition in the calorimeter is localized at relatively small angles with respect to the beam axis. Taking all that into account, the following parameters were used for event identification in the calorimeter FIX (0 total energy deposition in the calorimeter; (ii) energy depositions in certain parts of the calorimeter; (iii) number of clusters and their relative and absolute locations; (iv) existence of a cluster with an energy deposition above a certain threshold in at least two calorimeter layers.
2.2. FLT organization The special crate of the SND first-level trigger has an additional fast (60 ns) ECL-bus for FLT components. Logical signals, collected by the first-level trigger interfaces (IFLT) and “OR” modules, are applied to modules forming the FLT components: Track Finder, Calorimeter Logic, Layers Logic, Energy Deposition Logic. These modules produce 48 different FLT components; 14 of them come from the calorimeter. The trigger decision is formed by ten Mask Modules, each having 48 inputs for trigger components from different systems of the SND detector. It is possible to define 10 different trigger types by writing appropriate masks into the Mask Modules.
1996 Elsevier Science B.V. All rights reserved VI. DATA ACQUISITION
D.A. Bukin et al./Nucl.
Instr. and Meth. in Phys. Res. A 379 (1996) 545-547
OR Modules r---l
TS Rings +
Fig. 1. Front-end electronics.
Fig. 3. Calorimeter
3.1. Front-end electronics For FLT needs, 1632 calorimeter counters are grouped into 160 towers (Fig. 1). Each tower consists of 12 counters in all three layers (6 counters per tower at small angles). The signals from phototriodes after the charge-sensitive amplifiers are applied to the analog F12 modules (Fig. 1) , containing shaping amplifiers and programmable attenuators needed for the equalization of transmission coefficients in different channels. Each F12 module processes signals from counters of one tower and forms a fast total energy deposition signal and three logical signals, corresponding to energy depositions above 5 MeV in separate calorimeter layers. 3.2. The calorimeter FLT interface module Signals from groups of 16 towers each, forming two calorimeter sectors, are collected by IFLT modules (Fig. 2). From logical signals of the tower layers, combinations of two of three layers are constructed. Then for each tower two logical signals are produced: a “soft tower” (TS) with a total energy deposition higher than 25 MeV, and a “hard tower” (TH) - the same as a TS but with an energy deposition in 2 of 3 layers larger than 5 MeV in each. Then the IFLT signals are forwarded to the modules of the final analog and logical summation to form the “sector” and “ring” signals. The analog partial sums of the energy depositions in the calorimeter crystals go to the Energy Logic module containing discriminators with different thresholds. This permits the application of requirements on energy de-
3.3. Calorimeter Lugic module
Fig. 3 shows the organization of the Calorimeter Logic. Two groups of 20 signals each from sectors at different angles, produced by the summation of tower signals in the polar direction, and 8 signals from rings - the sums of towers iu the azimuthal direction - form the set of input signals for the calorimeter Logic module. The ring signals and the partial sector signals from towers at large angles (TSLA) produce the signal “two towers at large angle” (TDLA) . Then full sector signals (TS) go to the address bus of 1 MB static RAM, where the look-up table is stored. Then the information from rings contained in another 32 kB RAM is added. So, according to the memory contents, the components of the ELT defining the number of towers and their relative positions are produced. The passage of signals through the pipeline FLT logic is clocked with a beam collision frequency of 16 MHz.
xl ----6 channels
positions in different parts of the detector. The analog signals of the total energy depositions in the towers go to the 25 MeV constant fraction discriminators which form strobe signals from the towers. An “OR” of these strobes from the whole detector is used as the strobe for the Mask Modules. It allows one to have a time resolution for trigger signals better than 5 ns, which permits synchronization with beams crossing the collider (60 ns) .
Fig. 2. IFLT module.
At present, me SND is taking data in the energy region of the +-meson. The total counting rate in the calorimeter reaches 100 kHz. The total rate on the FLT output is less than 40 Hz, including 15 Hz from events with neutral particles only. The main features of the SND FLT are the following: - The pipeline logic of the FLT allows a decrease of the trigger dead time down to less than 60 ns, which allows working under hard background conditions. - The use of programmable trigger decision modules and static RAMS in all modules forming FLT components
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Instr. and Meth. in Phys. Res. A 379 (1996) 545-547
makes the SND trigger sufficiently flexible to provide an optimal operation in varying background conditions.
[ 11 G.M. Tumaikin, Proc. 10th Iot. Conf. on High Energy Particle Accelerators, Serpukhov, 1977, vol. 1, p. 443 Acknowledgements This work was supported by Russian Foundation of Basic Researches, grant No.93-02-19192.
I21 V.V. Anashin et al., Proc. 5th Int. Conf. on Instrumentation for Colliding Beam Physics, Novosibirsk, March 1990 (World Scientific) p. 360. [31 V.M. Aulchenko et al., Proc. Workshop on Physics and Detectors for [email protected]
, Fmcati, April 1991, p. 605.
VI. DATA ACQUISITION