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  • ReceivedAug 30, 2019
  • AcceptedDec 3, 2019
  • PublishedFeb 21, 2020
PACS numbers

Abstract

The Insight-Hard X-ray Modulation Telescope (Insight-HXMT) is a broadband X-ray and γ-ray (1-3000 keV) astronomy satellite. One of its three main telescopes is the High Energy X-ray telescope (HE). The main detector plane of HE comprises 18 NaI(Tl)/CsI(Na) phoswich detectors, where NaI(Tl) is used as the primary detector to measure ~ 20-250 keV photons incident from the field of view (FOV) defined by collimators, and CsI(Na) is used as the active shielding detector to NaI(Tl) by pulse shape discrimination. Additionally, CsI(Na) is used as an omnidirectional γ-ray monitor. The HE collimators have a diverse FOV, i.e. 1.1°×5.7° (15 units), 5.7°×5.7° (2 units), and blocked (1 unit). Therefore, the combined FOV of HE is approximately 5.7°×5.7°. Each HE detector has a diameter of 190 mm resulting in a total geometrical area of approximately 5100 cm2, and the energy resolution is ~15% at 60 keV. For each recorded X-ray event by HE, the timing accuracy is less than 10 μs and the dead-time is less than 10 μs. HE is used for observing spectra and temporal variability of X-ray sources in the 20-250 keV band either by pointing observations for known sources or scanning observations to unveil new sources. Additionally, HE is used for monitoring the γ-ray burst in 0.2-3 MeV band. This paper not only presents the design and performance of HE instruments but also reports results of the on-ground calibration experiments.


Funded by

China National Space Administration(CNSA)

the Chinese Academy of Sciences(CAS)

the Strategic Priority Research Program of the Chinese Academy of Sciences(Grant,Nos.,XDA04010202,XDA04010300,XDB23040400)


Acknowledgment

This work was supported by China National Space Administration (CNSA) and the Chinese Academy of Sciences (CAS), the National Key Research and Development Program of China (Grant No. 2016YFA0400800), and the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant Nos. XDA04010202, XDA04010300, and XDB23040400).


Interest statement

These authors contributed equally to this work


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  • Figure 1

    Illustration of the HE telescope onboard the Insight-HXMT astronomy satellite. The HE telescope is composed of main detectors, collimators, auto-gain control detectors (not shown), anti-coincidence shield detectors (HVT) and particle monitors.

  • Figure 2

    Distribution pattern of 18 HE collimators. The numbers beginning with “15” stand for collimators, and the numbers beginning with “12” stand for the corresponding HGC detectors. The numbers beginning with “13” stand for the corresponding lateral HVT detectors.

  • Figure 3

    A cutaway view of an HED.

  • Figure 4

    A structural sketch of the HE collimator. Left: wide-FOV collimator; middle: narrow-FOV collimator; right: blocked collimator.

  • Figure 5

    The superposed FOVs of the HE telescope. The three solid rectangles stand for the FOVs of the three narrow-FOV collmator sets; the two dashed squares represent the FOVs of the two wide-FOV collimators.

  • Figure 6

    A diagram to show the structure and installation positon of HGC. (a) An overall configuration of a main detector and its corresponding collimator and HGC; (b) top view to show the HGC position in the collimator; (c) a sketch map of HGC structure.

  • Figure 7

    Schematic of AGC windows.

  • Figure 8

    A sketch of a top HVT.

  • Figure 9

    A sketch of a lateral HVT.

  • Figure 10

    (a) The overall appearance of HPM; (b) the inner components of HPM.

  • Figure 11

    A sketch of the double-crystal monochromator.

  • Figure 12

    Position distribution of incident X-ray beam.

  • Figure 13

    Pulse width spectrum of 133Ba in normal mode. There is an apparent separation of the NaI(Tl) events and CsI(Na) events. (a) A scattering plot of pulse height versus pulse width. We can see a bridge connecting the two full-energe peaks of NaI(Tl) and CsI(Na). It is corresponding to Compton events. (b) A count distribution plot of pulse width. It also shows an excelent pulse shape discrimination capability.

  • Figure 14

    Pulse width spectrum of 137Cs in low gain mode. Similarly to Figure 13, there is an apparent separation of the NaI(Tl) events and CsI(Na) events. (a) A scattering plot of pulse height versus pulse width. We can see a bridge connecting the two full-energe peaks of NaI(Tl) and CsI(Na). It is corresponding to Compton events. (b) A count distribution plot of pulse width. It also shows an excelent pulse shape discrimination capability.

  • Figure 15

    Position of the pulse width peaks of NaI and CsI events as functions of PHA derived from background data. The error bars stand for the 2×FWHM of the pulse width of these two types of events.

  • Figure 16

    Nonlinearity response of NaI(Tl) for HED Z01-25, normalized to unity at 60 keV. It is consistent with some of previous studies, except for the low energy band of GBM.

  • Figure 17

    E-C relationship and the corresponding residuals of detector Z01-25. (a) Low gain mode; (b) normal mode.

  • Figure 18

    Energy resolution of detector Z01-25 as a function of energy. (a) Normal mode; (b) low gain mode.

  • Figure 19

    Efficiency of HPGe. Dashed line: the total efficiency curve from simulation; solid line: the full-energy-peak efficiency curve from simulation; solid circles: the full-energy-peak efficiency data from experiments. Cited from Liu et al. [22].

  • Figure 20

    Detection efficiencies of 18 HEDs derived from calibration experiments; the red curve is from simulation. (a) Normal mode; (b) low gain mode.

  • Figure 21

    Comparison of efficiency between HED with and without HVT.

  • Figure 22

    Peak mapping of 50 keV photons on 192 positions for HED Z01-5. The value of a color bar indicates the signal amplitude of 50 keV photon in ADC channel.

  • Figure 23

    Energy resolution mapping at 50 keV photons on 192 positions for HED Z01-5. The value of color bar indicates the energy resolution in terms of percentage.

  • Figure 24

    Efficiency mapping at 50 keV photons on 192 positions for HED Z01-5. The value of color bar indicates the detection efficiency in terms of percentage.

  • Figure 25

    Histogram of intervals (the theoretical value subtracted) between two consecutive GPS events.

  • Figure 26

    Statistics of the time interval between adjacent photons. It is well fitted with a line as expected in a logarithmic coordinate system.

  • Table 1   Main characteristics of HE

    Parameter

    Values

    Energy band

    20-250 keV (normal mode)

    0.2-3 MeV (low gain mode)

    Geometric area

    about 5100 cm2

    Main detector

    NaI(Tl)/CsI(Na)

    ~3.5 mm/40 mm

    Timing accuracy

    <10 μs

    Deadtime

    <10 μs

    Combined FOV (FWHM)

    5.7°×5.7°

    Energy resolution (FWHM)

    ~15%@60 keV

    Maximum count rate

    > 30000 cnts/s

  • Table 2   Working modes of HE telescope

    Item

    Normal mode

    Low gain mode

    NaI(Tl) energy range

    20-250 keV

    100-1250 keV

    CsI(Na) energy range

    40-600 keV

    0.2-3 MeV

    HV of PMT

    normal

    reduced

    Auto-gain control

    enabled

    disabled

    The above energy ranges are approximate; the exact value varies among the 18 detectors.

  • Table 3   Radioactive sources used during on-ground calibration campaign

    Operation mode

    Nuclide

    Half-life period

    Energy (keV)

    Radioactivity (Bq)

    Normal mode

    139Ce

    137.6 d

    165.9

    3.004×105

    133Ba

    10.5 yr

    81

    1.188×106

    356

    57Co

    271.7 d

    122

    8.699×105

    241Am

    432 yr

    59.5

    8.809×105

    109Cd

    461 d

    88

    6.184×105

    Low gain mode

    137Cs

    30 yr

    662

    2.233×106

    113Sn

    115 d

    392

    1.519×106

    60Co

    1925 d

    1173

    4.729×105

    1332

    88Y

    107 d

    898

    7.697×104

    1836

    40K

    1.28×109 yr

    1461

    (from environment)

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