ME instrument and in-orbit performance Meng Bin(IHEP)

Outline ME Introduction In-orbit Performance Simple introduction ME structure, mainly on MED Collimators Detectors Data acquire process Dead time In-orbit Performance Spectrum of on-orbit calibration sources In-orbit time interval spectrum In-orbit count rate curve In-orbit housekeeping data

Introduction The Medium Energy X-ray Telescope (ME) is one of the three main payloads of HXMT. Contains 1728 channels of Si- PIN detectors Energy range: 5-30 keV Detection area : 952 cm2 Time resolution: 255μs Energy resolution: 2.96keV@20keV ASICs are used for readout

Basic Structure Consists of three detector boxes (MED) and one electric control box (MEB). MED: Detecting the X-ray photons Converting the photon energy and arrival time information into digital signals Sending them to the MEB MEB: Data exchange Power supply

MED Structure Each MED contains three identical detection units. A detection unit is divided into six detection modules sharing one control logic. Each module has 32 Si-PIN detectors and one ASIC. Collimators are mounted on top of the detectors.

Collimators Collimators are mounted on the top of the detectors to limit the FOV. 3 FOVs are provided: 1°×4°, 4°×4°, and the blocked ones. The collimator adopts an aluminum alloy frame with tantalum foils inserted inside. Two on-orbit calibration radioactive sources are installed in one MED.

Detectors Pixel size: 56.25 mm2 (12.5 mm × 4.5 mm) Pixel thickness: 1mm One guard ring is shared by two pixels on a silicon chip Junction capacitance: ~ 5 pF Leakage current: <10pA(-25℃, depletion voltage: 150 V) Two silicon chips are packaged in a ceramic shell A 50 μm beryllium (Be) window is pasted to the ceramic package to shield the optical light and protect the surface of silicon chips.

Signal Acquisition The X-ray photon hits the detector and produce a group of charge. The charge group is amplified by a Charge Sensitive Amplifier. The output of the CSA is sent to a slow shaper and a fast shaper. When the output of the fast shaper is higher than a specified value, a trigger is sent to FPGA. FPGA controls the ASIC to sample and hold the slow shaper output after a peaking time it received the trigger. All the 32 channels of the slow shaper output are digitalized and read out by FPGA.

Dead Time Source: Single ASIC trigger Multi-ASIC trigger ADC Dead Time, about 162.1μs Readout Dead Time, about 93.7μs(One ASIC) Single ASIC trigger Timing resolution: 6μs. Total: (162.1+93.7) μs->252~258μs Multi-ASIC trigger FPGA sampling period is 62.5ns. Several ASICs in one Unit trigger in one period Total: (162.1+n*93.7) μs

Time Interval Spectrum Single ASIC trigger Multi-ASIC trigger The time interval spectrum of an MED obtained in thermal vacuum test. The horizontal axis is in units of 6 μs The time interval spectrum of MED when more than one ASICs in one unit are triggered simultaneously

In-orbit Performance

Spectrum Spectrum of on-orbit calibration source can be acquired on ground and in-orbit Red line: On ground Blue line: In-orbit ME performs well both on ground and in-orbit

Time Interval Spectrum The on orbit count rate of ME is usually very low Only one ASIC is triggered in most cases. Dead Time: 255μs

Count Rate The original count rate of each unit keep steady. In SAA, the count rate falls to 0. Peaks appear near SAA and higher latitude.

Housekeeping Data Voltages provided by MEB

Housekeeping Data Voltages received by MED

Housekeeping Data High voltage used by MED ME is working in a steady state

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