Medium Energy X-ray Telescope

  The Medium Energy X-ray Telescope (ME), whose basic function is to observe the 5-30 keV emission from the sky, is one of the three main payloads of the Hard X-ray Modulation Telescope (HXMT). The scientific objectives of ME are:

  1. To cover the gap between the energy bands of the high energy X-ray telescope (HE, 20-250 keV) and the low energy X-ray telescope (LE, 1-15 keV) of HXMT, so that the properties of the point sources discovered in the scan observations can be better constrained.
  2. To measure the medium energy X-ray emission of sources, so as to obtain their broad band spectra and multi-wavelength variabilities, together with HE and LE.

  In order to meet the requirements of these objectives, ME uses a Si-PIN detector array. The whole array includes 1728 pixels, 4 pixels (with an area of 56.25mm2 each) are in one ceramic package, and so 432 Si-PIN detector monomers are used. The front-end electronics and forming electronics are realized by using Application Specific Integrated Circuit (ASIC) chips. The data acquisition circuit is controlled by Field - Programmable Gate Array (FPGA).
  ME consists of three medium energy detector boxes (MED) and one electric control box (MEB). Data collected by the three MEDs are sent to the MEB through LVDS interface, while the communication between the MEB and the satellite platform uses both the LVDS interface that is mainly for the scientific data and the 1553B interface that is mainly for engineering data and commands.
  The main technical specifications of ME are summarized in Table 1.

Table 1 Main features of ME

 

  According to the function of ME, it can be divided into three parts, which are collimator, detectors and electronics. The collimator defines three kinds of field of views (FOVs), i.e., 1°×4°, 4°×4°, and full-blocked, which can be used to estimate the different background components in orbit. The Si-PIN detectors are developed by the Institute of High Energy Physics, which are mainly used to detect the X-ray photons in the energy range of 5-30 keV. The ME electronics uses ASIC chips to collect, amplify, digitize and transmit the output signals of the detectors.

  The structure of MED is shown in Figure 1.

 

Figure 1 The structure of ME detector box

  In each MED there are nine independent plug-in units, and on each plug-in unit two detector components are installed, including 16 4-pixel Si-PIN detector,two electronics boards each with a piece of 32-channel ASIC.

  A series of experiments and tests have been carried out on ME, which include the calibrations, environment tests, as well as some function and interface tests. The energy response, temperature response and dead time of ME are calibrated with radioactive sources and double crystal monochromator. The radioactive sources are Am-241 and Co-55. A typical energy spectrum of Am-241 is given in Figure 2.

 

Figure 2 A typical energy spectrum of Am-241fitted with a few Gaussian functions

  The typical energy spectrum of Co-57 is given in Figure 3. In this figure the central energies of the profiles are 6.4 keV and 14.4 keV.

 

Figure 3 A typical energy spectrum of Co-57 with two Gaussian functions fitted

  The E-C relations of one pixel at temperatures from -45℃ to 0℃ with a step of 5℃ are given in Figure 4, which shows that the linearity is quite good and the signal amplitude increases with temperature as expected.

 

Figure 4 The E-C relations of one Si-PIN pixel at temperatures from -45℃ to 0℃ with a step of 5 ℃ (from up to the bottom)

  The energy resolution of ME is mainly determined by the noise of the detector and readout electronics. The FWHM values of two pixels at different energies are given in Figure 5. The mean values are about 2.8 keV and 3.0 keV, respectively.

Figure 5 The FWHM of F4A0 CH0 and CH15 in MED at different energy

  The detection efficiency has been simulated from 0 keV to 40keV, according to the results of calibration module tested in PANTER and the simulation data is shown in Figure 6.

 

Figure 6 The simulated detection efficiency according to the results of calibration module tested in PANTER

  The dead time of ME is affected by the ASIC number of simultaneous triggering. The count rate of ME in orbit is very low under normal work condition and the system is based on single ASIC trigger. In case of single ASIC trigger, the dead time is about 42 or 43 time code unit, which is about 252~258 us at normal temperature.