Calibration of the RPC charge readout in the ARGO-YBJ experiment

1. Xiangdong Sheng for the ARGO-YBJ CollaborationX. Workshop on Resistive Plate Chambers and Related DetectorsFeb. 9-12, GSI, Darmstadt, Germany Calibration of the RPC charge readout in the ARGO-YBJ experiment

2. Outline • Introduction • Experiment setup • Data analysis and Results • Summary and discussion

3. Introduction 24 Clusters (guard ring) 130Clusters (central carpet) 8 Strips = 1 Pad (56 × 62 cm2 ) 2 Big Pads = 1 RPC (1.4 × 1.25 m2 ) • The usage of Resistive Plate Chambers (RPCs) in ARGO-YBJ experiment, • High Altitude (Yangbajing, Tibet, P. R. China, 4300m a.s.l.); • Full coverage of a single layer ,operated in streamer mode (-7200V) with the gas mixture made of 15% Argon, 10% Isobutane and 75% R134A. • * 18480 PADs and 3696 Big Pads in Total; • * with a total active area (about 6700m2) • 3. BigPads are implemented in each RPC to measure particle density of EAS events

4. The measurable energy of ARGO array: * Digital read-out, performed by “strips” to measure the particle number of small size showers (limitation to the energy of a few hundreds of TeV). And its saturation is at ~22Particles/m2. * Analog read-out, performed by “Big Pads” to extend the size (or the energy range) up to the “Knee” region. And its capability needs to reach 104Particles/m2. Number of Particles

5. Experiment setup Angular resolution： 0.7（nhit>60） Time resolution : < 2.0ns 90o31’50” E, 30o6’38” N 4300m a. s. l., 606g/cm2 1. The Calibration Aims: To get the linearity between the particle densities and the corresponding analog read-out of RPCs 2. The telescope was setup with 2 scintillation detectors and 5 PRCs. * To perform a co-site calibration, acting as Cluster 231 in ARGO array; * New electronics and DAQ independent, GPS-based timing system equipped; * Taking data since Dec. 2008. Now it is still in progress.

6. Fig. A single scintillation tile with size of 25cm×25cm×2 cm Scintillator : 275cm×125cm RPC : 280cm×125cm The scintillation detectors can be described as below, 1. Each detector: 5 ×11 the so-called scintillation Tiles, with a size of 275cm×125cm. 2. Each Tile’s readout is performed using the fibers (1) 8 WLS fibers (BCF92single cladding fiber Ф1.5 mm × 30 cm) * WLS fibers were glued into grooves of 1.6×1.6 mm2 at the “Tile” surface. * One end, coated with an aluminum layer * the another end, connected to one clear fiber (BCF98) (2) clear fiber (BCF98single cladding fiber,250cm length), guide the light to PMT (3) the surface of each Tile was coated by a layer of Tyvek. (4) Detection efficiency (>95%) and Time resolution (<1.8ns) are similar to the ones of RPC. 3. The area of Each scintillation detector is only 2% less than the one of RPC, and a total 440 fibers in each detector guide the light to one PMT.

7. Fig. Sketch map of the calibration telescope. Some comments on the calibration experiment, 1. Charged particle beams ——— Secondary particles in EAS 2. RPC2, 3, 4 act as the ones to be calibrated, while Scintillation detector 0 and 1 are used to measure the number of injected charged particles. 3. All the outputs from Big Pads of RPC2,3,4 and scintillation detectors are picked up with new electronics and DAQ. 4. The digital readouts of all 5 RPCs in the telescope are merged into the ARGO-YBJ data acquisition system (DAQ).

8. Fig. Schematic diagram of the electronics and DAQ FPGA: Field programmable gate array FADC system continuously digitizes all analog signals at 50 MHz The functions of the electronics and DAQ are as below, 1. all 8 input Signals are filtered and shaped. 2. the shaped signals are split into two channels with low gain and high gain amplifiers and digitized by 16 Flash ADCs (FADCs, 10bits, 50MHz). 3. The resulting digital data from FADCs are collected and analyzed by FPGA. * A channel is fired if the amplitude exceeds the preset corresponding threshold. * If RPC3 and both scintillation detectors are fired, a trigger is generated and the data are pushed into a buffer in the FPGA. * The data are transferred to An embedded computer or one PC finally. 4. a GPS-based timing system will record the event time with a precision of 100ns. Time message recorded could be used to make a off-line coincidence with ARGO data.

9. The linearities and High Voltage (HV) responses of the PMT (CR105-1) used in each scintillation detectors and the amplifier gains of the electronics have been calibrated in detail. Such as, (1). The HV responses of PMTs show a nonlinearity less than 1% from 350V to 750V. (2). The nonlinearities of PMTs’ outputs are less than 5% in 2.5 orders of magnitude. Fig. High Voltage response of one PMT

10. Data analysis and Results 1. Offline Events coincidence A match are performed off-line between the telescope events and the ARGO-YBJ ones in a time window of 1 microsecond,basing on the event times (Recorded by the corresponding GPS-based timing systems). Only matched events are used in the following data analysis and the ARGO data provide all the fired Pads and Strips, reconstructed primary direction and core location of these matched events. When charged particles pass through a RPC, the charges are induced in both BigPads of the RPC. So in the following data analysis, the charges shared by 2 BigPads are summed as the total charges induced in one RPC. 2. Data analysis When n (n>20) charged particles impinge on the scintillation detector, according to the Central Limit Theorem, its amplitude distribution well follows a Gaussian one. (1). the RPC charges are binned (increasing bin width for statistical consideration) (2). Chosen the events in each RPC charge bin, the number of particles and the error can be calculated from the scintillation amplitude distribution as follow, where μ0 and σ0 are the mean value and RMS of the single particle amplitude distribution of scintillation detector, while μ,σ represent the mean and RMS of the scintillation amplitude distribution, and N is the number of events in this bin.

11. Events with only one particle passing through the telescope are picked up by using the pad information (digital readout). * For Scintillation detectors: the amplitude distribution induced by single particles approximately follows a Landau distribution. * For RPCs: in the amplitude distribution, the first peak corresponds to the cases with one streamer generated in the gas chamber, here, the single streamer peak follows a Gaussian distribution and contains about 90% events with a resolution of about 14%, while the tail is from the cases with multiple streamers generated by single particles. Fig. Amplitude distribution of one scintillation detector in case of single particles. Fig. Charge readout of one RPC in case of single particles.

12. Fig. Number of charged particles VS. RPC charge readout, a linear fit is performed to the data. Fig. Nonlinearity of the calibration. With the PMTs working at 500V and data acquired in about 10 days the calibration reached a particle density of about 200particles/m2 with errors less than 3% and a nonlinearity of less than 5%, which show that there is a good linearity between the particle densities and the corresponding analog readout of RPCs.

13. Summary and discussion 1. Preliminary results show good linearity between the particle densities and the corresponding analog readout of RPCs. 2. The calibration (up to 1000particles/m2) would be available in next step. * To extend the calibration up to high particle densities, the PMTs working High Voltages and Electronics Gains should be adjusted to satisfy the further calibration requirements. * Since the dead time of the telescope DAQ are caused by the high trigger rate of low particle density events, the single channel thresholds will be increased to accumulate the statistic at high particle densities. * The effects of PMT from the temperatures and High Voltages have been concerned and now there are the outputs of the corresponding sensors to send to the DAQ. * The injection angle effect on the scintillation detector and RPCs will be corrected. 3. A method has been developed to propagate the absolute calibration scale to the whole array. Once one calibrated RPC provides an absolute calibration scale to all RPCs in whole array, basing on the axial symmetry of the shower lateral distribution, the uniformity of the RPCs can be retrieved by ARGO shower events. Thus the calibration scale can be propagated to the whole array.

14. Thank you!