The High Energy cosmic Radiation Detection facility (HERD)
Baseline design of HERD

HERD(High Energy Cosmic Radiation Detection) facility is one of the Cosmic Lighthouse Program onboard China’s Space Station, planned to be launched and assembled in 2020. The main science objectives of HERD onboard china’s space station are detecting dark matter particle, study of cosmic ray composition and high energy gamma-ray observations. The main constraints imposed on HERD are: total weight less than around 2 tons and total power consumption less than around 2 kilowatts.


To achieve HERD’s science objectives, HERD must have the capability of accurate electron and gamma-ray energy and direction measurement (tens of GeV – 10TeV), adequate cosmic ray energy measurement with charge determination (up to PeV).

Figure 1: Schematic diagrams of HERD.


The baseline design includes two main parts to achieve HERD’s measurement objectives: the micro silicon strip tracker (orange) covering the top and fours sides of the instrument, for incident particle trajectory and nucleon charge measurement; the LYSO crystal calorimeter (blue), for the incident gamma-ray, electron and cosmic ray nucleon energy measurement.

Figure 2: Schematic diagram of the top tracker design.


The top tracker is constituted of micro silicon strips and thin tungsten foils; the latter act as converter for gamma-rays. A gamma-ray continues until it interacts with the thin tungsten foils, producing an electron and a positron, and the primary direction can be reconstructed from the electron/positron tracks. There are seven tracking planes of x-y layers of silicon strips, each with the dimension of 65 cm × 65 cm, and five converter layers of tungsten sandwiched between layers 2-6, with thickness 3×1 mm + 2×2 mm = 2 X0. The first layer of silicon can be used to determine if the incident particle is a gamma-ray or a charged particle. The top tracker provides charge identification, trajectory measurement, back scatter rejection and some early shower development of gamma-rays and electrons. The other 4 sides are covered with silicon strip detector the same as the top one, only with a dimension of 65 cm × 50 cm and three layers without tungsten foils, for nucleon charge and the trajectory measurement.


The calorimeter(CALO) is a stack of almost 10k LYSO cubic crystals, with each cell of 3 cm×3 cm×3 cm read out by wavelength shifter fiber coupled with ICCD. CALO has a dimension of 63 cm× 63 cm× 63 cm. The total thickness of CALO correspond to about 55 radiation lengths(3 nuclear interaction lengths), making it the deepest calorimeter ever used in space. The full active CALO measures the total energy deposited of electromagnetic shower induced by gamma-ray or electron or the energy deposited of hadronic shower induced by cosmic ray nucleons. Its theoretical energy resolution is shown in Figure 3, which is less than 1% for electrons and around 20% for protons in its full energy band. The geometrical factor is about 3 m2sr for electrons and gamma-rays, and beyond 2 m2sr for cosmic ray nuclei up to PeV, as shown in Figure 4.

Figure 3: Energy resolution for electron and proton.

Figure 4: Geometrical factor for electron and proton.


Table 1: Characteristics of all components of HERD.


structure

size

thickness

unit/cell size

readout

main functions

tracker

(top)

silicon strip

65 cm ×

65 cm

2X0

7 x-y layers

(tungsten foils interspersed)

--

charge identification.

trajectory.

early shower.

tracker

(side×4)

silicon strip

65 cm ×

50 cm

--

3 x-y layers

--

charge identification.

nucleon track.

CALO

homogeneous

LYSO crystal

~10k cubes

63 cm ×

63 cm ×

63 cm

55 X0

3 λ

3 cm ×

3 cm ×

3 cm

wls fiber

+ ICCD

γ/e energy.

e/p separation.

nucleon energy.


Table 2: Expected performance of HERD.

γ/e energy range(CALO)

tens of GeV-10TeV

nucleon energy range(CALO)

up to PeV

γ/e angular resolutiontop Si-strip

0.1 degree

nucleon charge resolutionall Si-strip

0.1-0.15c.u

γ/e energy resolution(CALO)

<1%@200GeV

proton energy resolution(CALO)

20%

e/p separation power(CALO)

<10-5

electron eff. geometric factor(CALO)

3.8 m2sr@200 GeV

proton eff. geometric factor(CALO)

2.6 m2sr@100 TeV

HERD is a powerful space telescope. It will collect sufficient high energy gamma-ray, electron and cosmic rays with high energy resolution and energy reach than achievable with existing space experiments, and with great potential in advancing the understanding of the origin and propagation of high energy cosmic rays, as well as in identifying possible Dark Matter signatures and new discoveries in high energy gamma astronomy.


Figure 5: gamma ray smoking gun(dark matter annihilation signal) observation of HERD(preliminary).

Figure 6: HERD 2 yr proton observation(preliminary)

Figure 7: HERD 2 yr Helium observation(preliminary)

Figure 8: HERD 2 yr Iron observation(preliminary)