1. General Consideration 2. Choice of Crystal 3. Structure of EMC 4. Quality control of CsI(Tl) crystals 5. CsI(Tl) counter 6. Readout 7. Counter testing 8. Calibration and monitoring 9. Mechanical Structure 10. Question of EMC design 11. Summary Preliminary Design of Electromagnetic Calorimeter (EMC) September Lu Jun-guang

EMC plays an important role in the BES. The primary functions of calorimeter are to provide precision measurement of energies and positions of electrons and Photons. The general physics requirements of BESIII lead to EMC based on CsI(Tl) crystals over the entire available solid angle with performance targets, * Energy region: 20 MeV to 2 GeV. key energy region < 500MeV. * Energy resolution: * Spatial resolution: * Reconstruction of π 0 and η * Contributes to e/πand e/μ separation * Provide neutral energy trigger * With electronic noise: 200kev/each crystal 1. General Consideration

2. Choice of Crystal Energy resolution of calorimeter: *  EC is the intrinsic resolution due to fluctuations of the energy deposition and the photon statistics;  rl is from the shower leakage including contributions from “dead material” in front of the calorimeter and the supporting structure;  PD is from photodiodes directly hit by charged particles;  noise is from electronic noise including “pile up” at high luminosities;  cal is from errors of calibration and non-uniformity of the system.

The Properties of several inorganic crystal scintillators Crystal NaI(Tl) CsI(Tl) BGO PbWO 4 Density (g/ ㎝ 3 ) Radiation length ( ㎝ ) Molière radius (cm) DE/dX(Mev/cm)(per mip) Nucl. Int. length ( ㎝ ) Refractive index (480 nm) Peak emission (nm) Relative light output (PMT) (PD) Light yield temp.coef.(%/ 0 C) ～ Decay time (ns) Hygroscopic strong slight no no Referral cost($/ ㎝ 3 )

Figure 1. Effect of electronics noise on the energy resolution from Monte Carlo simulation.γrays pass through MDC and TOF. Energy is obtained by the sum of 5 x 5 CsI(Tl) crystals, and the sum of direct energy deposit in photodiodes with a factor of 40.

The position resolution of photon showering Figure 2. The average position resolution of photon showering in the calorimeter vs. photon energy.

mass resolution Figure 3. mass resolution vs. momentum

The efficiency of detection for photon and Figure 4. (a) photon detection efficiency vs. photo energy. (b) detection efficiency vs. momentum.

Figure 5. (a) shows a measurement of a CsI(Tl) crystal Crystal: 3.5cm x 3.5cm—4.5cm x 4.5cm,25cm long coupling two PDs(S ) using 60 Co source with 1.17 and 1.33 MeV γ - rays. (b) the amplitude of the signal output from a photodiode directly exposed by 60 keV-rays from an 241 Am source light output (2 x PD): ~ 5000 e /MeV

Effect of energy deposit in the photodiode From measurement: It is ~45 times of the energy deposit in the crystal. GEANT simulation shown the energy deposit of photodiodes for a shower leakage is about 240 keV which corresponds to energy deposit of 11 MeV of CsI(Tl) crystal, With a small rate: 0.5 GeV 1.GeV rate 2% 5% So this effect is negligibly small compared to the expected energy resolution at 1GeV ( 〜 3%).

3. Structure of EMC Figure 6. Configuration of the electromagnetic calorimeter

Calorimeter is composed of a barrel and two endcaps. barrel : inner radius: 94 cm inner length: 276cm polar angle 33.5 o —144.7 o (cosθ ～ 0.82). 56 rings ( z direction), each with 144 CsI(Tl) crystals All crystals point to the collision point with a small tilt of 1 o ～ 3 o in  and 1.5 o in the  directions. Endcap: at 138 cm from the collision point, polar angle: 21.3 o —34.6 o and o — o (cos θ ～ 0.93). Each endcap consists of 8 rings, and vertically splits into two half in order to open horizontally. The entire calorimeter have 9600 CsI(Tl) crystals with a total weight of 22 tons.

Dimension of CsI(Tl) crystals Figure 7. Shape CsI(Tl) crystals. Basic size of one CsI(Tl) crystal: 4cm x 4cm —5cm x 5cm, 24 cm (L) According to GEANT simulation, about 73% of incident energy is deposited in one segment when a ray with energy above 100 MeV enters at the center of the segment.

4. Quality control of CsI(Tl) crystals the tolerance of the crystal dimension as +0, -200 µm for all side 1mm for length light output :(200 µm Teflon sheet, two PD (S ) and 1 µs shaping time) 5000 e /MeV light uniformity (200 µm Teflon sheet and 2-inch PMT,testing eight point in length of 24cm). Radiation hardness of crystal can be reached: 5% decrease of light output per krad

Figure 8. The setup for measuring the light output and the uniformity of the crystal.

Light output for few vendor crystals

Figure 9. The light output nonuniformity in a 25cm-long crystal

Effect of non-uniformity of light output of crystal on energy resolution

5. CsI(Tl) counter Figure 11. Assembly of a CsI(Tl) crystal module.

Wrapping: 200μm Teflon sheet + 25 μm Al +25 μm Mylar sheet Figure 12. Light output versus thickness of Teflon and Tyvek

6. Readout Photodiode Hamamatsu S photosensitive area: 1cm x 2cm Thickness of Wafer: 300μm Quantum efficiency(560nm): 80% Supplies Reverse Voltage : 70 V Capacitance : 85 PF Dark current: 4 nA Temp. dependence for noise: 10 %/ 0 C Uniformity of q. e. : 1% Difference of q. e. : 10%

Preamplifier and amplifier For each counter: Two PD + Two preamplifier + One amplifier Preamplifier noise: ~1000 e (~200kev)/counter Shaping time of amplifier: 1μs Figure 13. Light output and noise due to the shaping time of amplifier

Figure 14. Block Diagram of the readout From Detector Post amplifier Q Module Test Controller Fan-out Trigger TEST, DAC CLK L1 VMEVME L1 reset Buffer full CLK L1 L1 reset Buffer full L1 L1 reset Buffer full SCLK, DIN, SCV Analog Sum Preamplifier

Gain 1mV/fc ENC 0.16fc (80pF input capacitance) Dynamic Range 0.5fc ~ 1500fc Output decay time 50  s Max linear output 2V Low noise charge sensitive amplifier 1 AMP/diode, 2 AMPs/crystal Average of 2 AMP outputs to improve S/N Calibration circuit at the input 20 wire twisted cable/Ch to Post AMP Preamplifier Specification

Figure 15. Block diagram of the post amplifier ½(A+B), A, B can be selected CR-(RC) 2 with pole-zero cancellation shaping,  =1  s Gain adjustable with digital potentiometer Analogue sum for trigger Differential connection with Pre-AMP and Q module A+B A B CR (RC) 2 From Test Controller To Q Module To Trigger ∑ A B From Preamplifier Post Amplifier

Q Module 3 FADCs sample signals from 3 different gain AMPs Delay samples for 1.7  s with pipeline to wait for 3.2  s trigger latency L1 Find peak within 3  s time window after L1 arrival Select peak, make range encoding & compression, store data in buffer Inner trigger for radiation source calibration & adjusting gain 9U VME module, 32ch/module ×.25 Pipeline ×1 ×8 Pipeline Peak Selec. Range Encoding Compress Peak Buffer From Post AMP. FADC Disc. Delay L1 Out. Trig Inn. Trig Thr. Register Figure 16. Block diagram of Q module

7. Counter testing Before construction of CsI(Tl) module Light output of Crystal will be tested by γ source Difference ~50%, Uniformity ~ 5% (PMT) ~10% (PDs) PD will be burned at 80 0 C for 600 hours,dark current and sensitivity checking by χ source and light pulse. Difference ~10% Preamplifier and amplifier will be tested by χ source irradiate reference PD, noise and the gain difference checked Then match them in order to have similar signal amplitude between crystals for digitization.

Before installing the calorimeter structure Each counter will be tested by using cosmic-ray. It will provide a pre-calibration of counters, Cosmic- ray measurement is one way to reach a required 1% accuracy. Beam test of a CsI(Tl) matrix We plan to perform a beam test of a 6 × 6 crystal matrix when all elements of a detector module is ready. It will help us to debug the system, obtain the first hand experience for calibration, cross check Monte Carlo simulation, and finalize the system design.

Figure 17. The setup for the cosmic-ray measurement

7. Calibration and monitoring Environmental control and monitoring Temperature of preamplifier will be controlled by the cooling pipe system :~25 o C ± 1 o Humidity will be controlled by flushed with dry air or nitrogen ~5% ± 3% Temperature monitoring About 600 LTM8802 temperature sensors distributed around the calorimeter,using precision of 0.5 o C Humidity monitoring About 200 sensors are distributed and linked to the slow control system, using precision of 3% Radiation dose monitoring (50 ～ 300 rad/year) About 80 sensors distributed around the calorimeter, using a sensitivity level of 0.5 rad

Calibration Before calorimeter assembling, each counter will be pre-calibrated using cosmic ray in the Lab. Has cosmic ray running for 1 month after the installation of BESIII finished At normal performance: 1.Calibration system of electronics every day (gains, pedestals and linearity) 2.Temperature corrections for CsI crystal with temperature coefficient of 0.3%/C o 3.Cosmic-ray muons can be used periodically to check absolute light yield and the detector performance. 4.The ultimate energy calibration using e + e -,γe + e - and π o events. Bhabha event rate: ~0.6 kHz events/counter-day. 5. The effects of radiation damage of crystals have been monitored by a Xe lamp-fiber system

Figure 18. Overview of the Xenon lamp-fiber monitor system

parameter of light output of crystal fitted use bb events is the data of light output of the crystal i. is the fraction of light output of the crystal i. are the fraction and error of energy deposited in,defined by MC,

9. Mechanical Structure 72 slots in φ 14 in Z each compartment: 8 crystals Steel bar 144 piece Inner wall (Al) : 1.6 mm (T) compartment wall: 0.5mm (T) Figure 20. Support structure of the barrel calorimeter

Figure 21. Assembly structure of the crystal module in the compartment

Process to assemble crystals will start at the top of platform. 1. Uninstall two reinforce steel bars and plastic super-Modules 2. Insert two rows of crystals one by one. 3. Install the two reinforce steel bars,bridge bars and fix the elastic jig, then press each crystal tightly. 4. Install the cooling pipes and inserts cables and fibers. 5. Using the Xe lamp-fiber system to test the signal of each crystal. 6. Install the bar of the outer wall

10. Question of EMC design It is short that CsI(Tl) calorimeter with a length of 13X 0. suitable length: 15X 0 (28cm) cost: ~8.64 M$ (13X 0 ) +1.87M$ (add 2X 0 ) energy resolution: 3.7% to 2.6% for 1GeV There are not compartment wall in between crystals for calorimeter support. effect of 0.5mm Al fins for 2x4 crystal module on energy resolution: ~ + 0.5%.

Figure 24-a The shower leakage and the leakage fluctuation for CsI(Tl) calorimeter with a length of 24cm. Figure 24-b The contribution to the energy resolution for length of CsI(Tl) crystals.

Figure 25. relative energy deposition and energy resolution for different thicknesses of the air and materials in the inter-crystal

Fig 26. Relative energy deposition and energy resolution for different tilt angle of crystal to point the interaction point

11. Summary A basic design of BEMC is to use CsI(Tl) crystals. suggested use 13X 0 in length. Covering the polar angle of cosθ ～ Expected performance:  E /E ~ 1%/√E + 2.7%,  x,y ≤ 5mm/ √E Quality of crystal: light output: 5000 e /MeV, uniformity: ~10% wrapping: 200μm Teflon sheet Readout: adopt two PD S in each crystal. Electronics noise: ~1000 e (~200 keV)/counter shaping time: 1 μs Single crystal calibration will used Bhabha event and Xenon flusher for monitoring Thanks