Hit-Rate Dependence and Gas Mixture of Prototype MRPC for LEPS2 Chia-Yu Hsieh 謝佳諭 Institute of Physics , Academia Sinica , Taiwan For LEPS2 Collaboration Hello , everyone . I'm Chia-Yu Hsieh from Academia Sinica,Taiwan. Today I am going to talk about the prototype MRPC we made for the LEPS2 experiment.

Outline Stack Dependence Hit Rate Dependence Gas Mixture The LEPS2 Experiment at SPring-8 Beam Test of Prototype MRPC Results of MRPC#1-4: Geometry of Pad /Strip Readout Stack Dependence Hit Rate Dependence Gas Mixture Summary First I will give you some brief introduction about the LEPS2. Then talk about how we made and tested the prototype MRPCs. We made four kinds of MRPC. I will show you the result on these dependences. Finally I will give a conclusion and what we are going to do in the future.

Super Photon ring-8 GeV BL31 LEPS2 BL33 LEPS raise up the intensity of beam increase the angle acceptance of detector develop the new and faster DAQ system Study hadron-photon production Sprin-8 This is the sky view of SPring-8. SPrin-8 locate in west Japan and the energy of electrons in the storage ring is about 8GeV . About LEPS LEPS is located at beam line33 of Spring-8. It’s derived from “Laser Electron Photon at Spring-8”. We use the laser photon inject to the storage ring to obtain the high energy photons with 1.5 to 2.4 GeV. The experiment started the data taking from 2000 and it has been 10 years. We study the photo-production of vector-mesons and hyperons to understand the strangeness production in the strong interaction. About LEPS2 Now we want to upgrade it to the 2nd generation, named LEPS2. We use one of the long straight beam line of Spring-8 ,BL31. We will raise up the intensity of beam(10^7), increase the angle acceptance of detector, and develop the new and faster DAQ(data acquisition) system.

LEPS2 Detectors TOF seperation of π/k up to about 1.1GeV while traveling in 100cm. Goal σTOF <50 ps Efficiency>99% Rate capability >1 Hz/cm2 Electronics<1000ch =>MRPC LEPS2 detector Here shows the sketch of the LEPS2 detector. It is designed to detect both charged particle and photons. The BCS photons collide with the target and produced some mesons and hadrons. We measure the momentum and time-of-flight (TOF) of charged particle to determine its mass for identification. The solenoid magnet provides a 1-T magnetic field. The track of charged particle is constructed by the time projection chamber(TPC) and multi-wire drift chamber(MWDC). The time information of them in the low momentum region is given by the TOF detector. TOF for LEPS2 In the LEPS2 experiment we want to separate kaon and pion with energy up to 1.5GeV while traveling in a distance of 100m. We need a TOF detector with (1)Time resolution better than 50ps (2)High efficiency (3)Rate capability higher than 1Hz/cm2 Multi-gap resistive plate chamber is chose

Beam Test &Data Analysis The trigger rate is adjusted by the intensity of laser. Scintillators Chamber Pad Holder TDC=trf-tmrpc σ 2rf-mrpc= σ 2rf+ σ 2mrpc , where σ rf =14~18ps Laser BCS γ Pb B(X) e+ MR P C S 1 RF 2 3 4 e- Holder S C3 Pad SC4 SC1 SC2 RPC e- 1x2cm2 Setup This is one of prototype MRPC we made. The blue frame show s the position of chamber which is supporter by the holders. There are four scintillators. Two of them were in the front of chamber, and two of them were behide. We tested the MRPCs in the BL33 at SPring-8. Beam test setup The BCS photons was produced by laser photons collide with electrons with 8GeV in the storage ring. The photon beam hit converter(Pb) produced the electron-positron pairs in 1-T magnetic field. The MRPC prototype were placed on the same side with elecctrons. Efficiency The trigger was defined as the four scintillators with 1*2 cm^2 beam area. Time resolution We use TDC to measure the TOF. Start is provided by the RF signal from the storage ring. Stop is provided by the MRPC. We use the time of RF signals as the time reference. The time resolution is contributed by both RF and MRPC. The time resolution of RF signals is about 12-18ps depends on the bunch mode. In general it is neglectable. Due to the time delay of small signals, a slewing correction has to be made according to the pulse-height spectrum. The distribution of corrected time is fit by Gaussian. We can derive its time resolution of RF plus MRPC from the standard deviation.

Pad Readout (Readout on One Side) MRPC#1_148 μm x 6 gaps x 2 stacks glass= 100x500 mm2 , tinner=0.4 mm;touter=0.5 mm PC board= 130x570mm2 , tinner=0.4mm carbon tape=0.13 mm copper=0.035 mm fishing line 2.5X7.4cm2 1.5X5.5cm2 5.0X7.4cm2 10.0X7.4cm2 Parameters Number of gap: 6 Number of stack:2 • Gap width: 0.148 mm Resistivity of carbon tape : 500 Ω/sq • Glass thickness: inner:0.4 mm;outer:0.5 mm • Gas mixture: R134a:iso-butane:SF6=90%:5%:5% MRPC#1 We made 2 stacks MRPC#1 has 12 gaps with 148um gap with. The active area is 10*50cm2. The gas mixture of chamber is 90% R134a, 5% SF6, and 5% Iso-butane. This is the PCB board we ordered from the company. There were four kinds of pad size, because we want to reduce the electronics to 1000ch.

2.5X7.4 cm2 1.5X5.5 cm2 5.0X7.4 cm2 10.0X7.4 cm2 Pad Size Dependence 99% Efficiency Time resolution We measured four kinds pad size with the same readout shape. The position of beam is in the center of pad. But the slewing correction didn’t work well for the biggest pad,so I didn’t show the result. All of them can 99% efficiency above the knee of the plateau. We need to apply higher voltage to reach the high efficiency for the bigger pad because the signals of it are small. The time resolution was getting worse when the pad size becoming bigger. Only 1.5*5.5cm2 pad achieve the request of LEPS but we need about 7000ch to cover the area of TOF detector. We can not afford it. So we gave up pad read-out, try strip readout. Only the small-size pad satisfies Eff.>99% and σ~50ps,satisfied the request of LEPS2 .However it takes 7000chs to cover the area of TOF detector. =>Strip readout

Strip Readout (Readout on Both Side) MRPC#2 148 μm x 6 gaps x 2 stacks L3:2.5x40cm2 L2:1.5x40cm2 L1:2.5x20cm2 Smallest Pad 1.5x5.5cm2 Size HV Channel Efficiency Resolution SP:1.5x5.5 cm2 12 kV ~ 7000 ch 99% 50±3 ps L1:2.5x20cm2 ~ 2000 ch 93% 62±2 ps L2:1.5x40 cm2 96% 61±3 ps L2:2.5x40 cm2 ~ 1000 ch 90% 63±2 ps We made MRPC#2 with strip readout. The configuration is same as MRPC#1. But this time we didn’t use the PCB board made by company, we taped the cooper tape on the surface of plastic plate by ourselves. We made 3 different size strips. For comparison we also import the smallest pad for the refernce. //Except for the strip readout we also made the 1.5*5.5cm2 small pad. //We can measure the time resolution and efficiency of it, and compare the results with MRPC#1 to make sure the condition and noise level of chamber is O.K. The strip was read out at both side. Efficiency was obtained from “and” of both sides. Time resolution was the standard derivation of mean time of both sides. The time resolution is independent of the position for the strip. Table This table shows the results at 12kV. For the smallest pad, the result is consistent with MRPC1. It means the hand made readout works well. Fort the biggest strip, it can achieve 99% efficiency and the time resolution is in the range of 55~65ps. The performance of the biggest strip is worse than the smallest pad. But we only need 1000ch to cover the whole area of TOF detector. So now 2.5*40cm2 strip is the best strip size for us. Because the time resolution didn’t change so much when we make the strip longer. We will try longer strip next time when we make real size MRPC with area of 10*100cm2. (1)The efficiency of all the pad can achieve 99% above the knee of plateau. (2)The time resolution of smallest pad is consistent with MRPC#1. It means the chamber works well. (3)The time resolution of 2.5*40cm2 is in the range of 55~65ps along the strip. It’s worse than 50ps,but we only need about 600ch to cover the area of TOF detector. We have to make a compromise between money and accuracy. Finally we chose 2.5*40cm2 strip. The performance of the smallest pad is consistent with the result of MRPC#1. The efficiency of long strip is not enough.=>wider gap MRPC

Strip Readout MRPC#3 260 μm x 5 gaps x 2 stacks L3:2.5x40cm2 L2:1.5x40cm2 L1:2.5x20cm2 Smallest Pad 1.5x5.5cm2 Size HV Channel Efficiency Resolution SP:1.5x5.5 cm2 12 kV ~ 7000 ch 99% 50±3 ps L1:2.5x20cm2 ~ 2000 ch 56±2 ps L2:1.5x40 cm2 98% 64±3 ps L3:2.5x40 cm2 ~ 1000 ch 61±2 ps We made MRPC#2 with strip readout. The configuration is same as MRPC#1. But this time we didn’t use the PCB board made by company, we taped the cooper tape on the surface of plastic plate by ourselves. We made 3 different size strips. For comparison we also import the smallest pad for the refernce. //Except for the strip readout we also made the 1.5*5.5cm2 small pad. //We can measure the time resolution and efficiency of it, and compare the results with MRPC#1 to make sure the condition and noise level of chamber is O.K. The strip was read out at both side. Efficiency was obtained from “and” of both sides. Time resolution was the standard derivation of mean time of both sides. The time resolution is independent of the position for the strip. Table This table shows the results at 12kV. For the smallest pad, the result is consistent with MRPC1. It means the hand made readout works well. Fort the biggest strip, it can achieve 99% efficiency and the time resolution is in the range of 55~65ps. The performance of the biggest strip is worse than the smallest pad. But we only need 1000ch to cover the whole area of TOF detector. So now 2.5*40cm2 strip is the best strip size for us. Because the time resolution didn’t change so much when we make the strip longer. We will try longer strip next time when we make real size MRPC with area of 10*100cm2. (1)The efficiency of all the pad can achieve 99% above the knee of plateau. (2)The time resolution of smallest pad is consistent with MRPC#1. It means the chamber works well. (3)The time resolution of 2.5*40cm2 is in the range of 55~65ps along the strip. It’s worse than 50ps,but we only need about 600ch to cover the area of TOF detector. We have to make a compromise between money and accuracy. Finally we chose 2.5*40cm2 strip. 10x50 cm2 chamber=> 2.5x40 cm2 260 μm x 5 gaps x 2stacks The time resolution doesn’t change so much for the longer strip. =>We will try longer strip(> 2.5x40 cm2) with real size chamber with area of 10x100 cm2 .

Hit Rate Dependence Biggest Strip 2.5x40cm2 98% Efficiency Time resolution 60ps Condition We also measured performance of 260um MRPC with strip size of 2.5*40cm2 under different hit rates. The limitation of the hit rate is 150Hz/cm2 in our beam test. Results Efficiency achieve more than 98% under the rate below 150Hz/cm2. With the increase of rate the time resolution is getting worse. We need to apply higher voltage to achieve the best resolution under higher rate. 2.5 x 40 cm2 strip 260 μm x 5 gaps x 2stacks The hit rate of LEPS2 experiment is below 1Hz /cm2. The high enough voltage is necessary to achieve a good time resolution and high efficiency at a higher hit rate.

Hit Rate Dependence Biggest Strip 2.5x40cm2 Higher rate Arriving time of signals Average charge of signals Higher rate  Lager current flows into the resistive plates Bigger Voltage drops on the electrodes Smaller signals Lower efficiency Slower drift velocity Later arriving time Averaged time This plot shows the average charge under different rates. With the increase of hit rate the average charge of signals became smaller. The reason is: The current flowing into the glass increases when the hit rate increases. It leads to a bigger voltage drop on the electrodes. So the induced signals are smaller. That’s why we need to apply higher voltage to achieve high efficiency and good time resolution. Arriving time For the same reason because of the voltage drops on the electrodes, the drift velocity of electrons and ions became smaller. That’s why the arriving time of signals later under higher rate. For a given hit rate signals arrived earlier with the increase of high voltage. Principle The electrons and positive ions produced deposited on the surface of plates reduced the electric field. The full field is restored by current flowing through the resistive plates. This mechanism leads to a fall-off voltage on the resistive plates.

Gas Mixture Smallest Pad 1.5x2.5 cm2 Butane (C4H10) Iso-butane Smallest Pad 1.5x2.5 cm2 Efficiency 99% Time resolution ~50ps Butane and Iso-butane Formally the gas mixture of chamber was 90% R134a, 5% SF6, and 5% Iso-butane before. This time we replaced Iso-butane to Butane. Both Butane and Iso-butane are quencher but the structure of Iso-butane is more symmetric than Butane. It can absorb more photoelectron. Theoretically Iso-butane is better than Butane. But Butane is cheaper than Iso-butane. If we replace Iso-butane to Butane, we can save money. Result The results shows there is not big difference between Butane and Iso-butane at 13kV. The performance of Butane seems fine. But we don’t know the aging effect of Butane. We will try it in the near future. Temperately we still use Iso-butane as quencher. R134a : SF6 : Iso-butane = 90 : 5 : 5 R134a : SF6 : Butane = 90 : 5 : 5 The small pad : Eff.~99% and Res~50ps at 13.5 kV But the aging effect of Butane has to be investigate.

Summary & Future work MRPC for LEPS2 We almost achieve the request of LEPS2 experiment. Future work (1) Impedance of strip and amplifier (2) Longer strip( > 2.5 x 40 cm2) (3) 10x100cm2 glass Configuration 260 μm x 5 gaps x 2 stacks Strip size 2.5 x 40 cm2 (~1000 channels) Gas mixture R134a:iso-butane:SF6 =90%:5%:5% Efficiency ~99 % (~20 Hz/cm2) Time resolution ~60 ps (~20 Hz/cm2)  improve to 50ps MRPC for LEPS2 After investigating many things, we chose MRPC has 10 gaps with 260um gap width. The strip size is 2.5*40cm^2 The gas mixture of chamber is 90%R134a,5%iso-butane,and 5%SF6 is in the ratio of 90 ,5,and 5. Under these conditions the 98% efficiency and about 60-65ps time resolution can be achieved under hit rate of 20Hz/cm2. Future work In the future we will study further the effect of the impedance of pad and develop the amplifier for the readout of MRPC. And we will try longer strip with a real size chamber with total area of 10*100cm2.

Thank you for your attention. Acknowledgement Natsuki Tomida (Kyoto University , Japan) Hiroaki Ohnishi (RIKEN , Japan) Masayuki Niiyama (Kyoto University , Japan) Wen-Chen Chang (IPAS ,Taiwan) Thank you for your attention.

Stacks Dependence Biggest Strip 2.5x40cm2 99% 98% Efficiency Time resolution MRPC#3_260 μm x 5 gaps x 2 stacks MRPC#4_260 μm x 5gaps x 1 stack The request of LEPS2 experiment is 99% efficiency. =>2.5 x 40 cm2 strip 260 μm x 5 gaps x 2stacks Reason In the design of LEPS2 detector we will put the energy calorimeter outside the TOF detector. It’s better to make the thickness of chamber as thin as possible. We made 1 stacks MRPC with gap size of 260um. Results The efficiency of 1 stacks is 1% worse than 2 stacks, because the signals of two stacks is bigger than one stacks. The time resolution of both of them can achieve 60-65ps at 14kV. Even though the time resolution is good,the request of LEPS2 experiment is 99% efficiency. Finally we still decided to use 2 stacks MRPC.

Back Up

Trigger rate We adjust the laser to change the trigger rate. Pad S4 S1 S2 RPC e- 70 100 600 RPC (center) *trigger rate The laser is unstable. =>The trigger rate are fluctuated with time. =>The trigger rate is a range. The electron beam is uniform along the chamber. =>The trigger rate is uniform along the chamber. The electron beam is bended by magnetic field =>The overlap are of four scintillators affect the trigger rate. =>We use the front two scintillators close to the chamber to decide the trigger rate. The higher rates can be obtained by moving the RPCs up closer to the main beam or adjust the intensity of injected laser. We adjust the laser to change the trigger rate. The unit of trigger rate: Hz/cm2