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

2. 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

3. 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:
ASICs are used for readout

4. 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

5. 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.

6. 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.

7. 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.

8. 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.

9. 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: ( ) 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

10. 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

11. In-orbit Performance

12. 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

13. 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

14. 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.

15. Housekeeping Data
Voltages provided by MEB

16. Housekeeping Data
Voltages received by MED

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

18. Thank you