Study of the performance of the HEPD apparatus for the CSES mission

TL;DR

This paper evaluates the performance of the HEPD instrument on the CSES satellite, focusing on its ability to distinguish particles and identify nuclei, supporting space-based geophysical and radiation studies.

Contribution

It presents a detailed analysis of HEPD's particle separation and nuclear identification capabilities, a novel assessment for this space mission instrument.

Findings

Effective electron-proton separation demonstrated

Nuclear identification up to iron achieved

Instrument performance supports geophysical research

Abstract

The High-Energy Particle Detector (HEPD) is one of the payloads of the CSES space mission. The CSES (China Seismo-Electromagnetic Satellite) mission will investigate the structure and the dynamic of the topside ionosphere, will monitor electric and magnetic field and high energy particle fluctuations, searching for their correlations with the geophysical activity, in order to contribute to the monitoring of earthquakes from space. The HEPD is built by the Italian collaboration and has different goals. It will study the temporal stability of the inner Van Allen radiation belts, the precipitation of trapped particles in the atmosphere and the low energy component of the cosmic rays (5 - 100 MeV for electrons and 15 - 300 MeV for protons). Here is presented a study of the performance of the apparatus to separate electrons and protons and identify nuclei up to iron.

Tables

Table 1. Table 1: CSES payloads.

Category	Payloads
ElectroMagnetic field	Electric field detector
	High precision magnetometer
	Search coil magnetometer
In-situ plasma	Plasma analyzer package
	Langmuir probe
Plasma construction	GNSS occultation receiver
	Tri-band beacon
Energetic particle	High energy particle package

Table 2. Table 2: HEPD main technical characteristics.

Parameter	Value
Energy Range	Electrons: 3-100 MeV
Energy Range	Protons: 30-200 MeV
Angular resolution	at 5 MeV
Energy resolution	at 5 MeV
Particle identification	$>90\%$
Free field of view	$\ge {70}^{\circ }$
Pointing	Zenith
Operative temperature	-10∘+45∘

Full text

I The CSES Mission

The China Seismo-Electromagnetic Satellite (CSES) is a space mission dedicated to Wang :

measurement of signals from electromagnetic emission and its disturbances in ionosphere; 2. 2.

measurement of background magnetic field in space; 3. 3.

measurement of the disturbance of plasma in ionosphere, such as contents, density and temperature of the ions, density and temperature of the electron; 4. 4.

measurement of energetic particles precipitation.

Some studies Zhang ; Sgrigna ; Alex show a correlation between the electromagnetic anomalies from Earth and ionosphere perturbations and the earthquakes. In DeSantis , a review of the searches for correlation between phenomena in the atmosphere, ionosphere, magnetosphere and seismic events is reported. Different observables are studied: the precipitation of low energy electrons (E $>$ 0.3 MeV) trapped within the Van Allen Belts Battiston ; the depletion in the intensity of VLF electric field at satellite altitudes in Nemec ; Pisa ; time/space localized OLR anomalies Guo .

CSES will also complement the cosmic ray measurements in an energy range up to few hundreds of MeV and will contribute to the study of the solar-terrestrial environment with the observation of Coronal Mass Ejections.

The satellite is based on the Chinese CAST2000 platform. It is a 3-axis attitude stabilized satellite and will be placed in a 97,4*∘* inclination Sun-synchronous circular orbit, at an altitude of $\sim$500 km. The working zone is in the latitude range [-65*∘;+65∘*], where there is no rotation of solar panel and no action for attitude and orbit control system of satellite.

The launch is scheduled for 2017, July-August and the expected lifetime is 5 years.

CSES hosts several instruments on board, as reported in Tab. 1.

II The High-Energy Particle Detector

The High-Energy Particle Detector (HEPD) is one of the payloads of the CSES space mission. The HEPD is built by the Italian collaboration; its main characteristics are reported in Table 2.

To study the low energy component of the cosmic rays, the HEPD detector is composed by different instruments: a tracker, made of two planes of double-side silicon micro-strip sensors, at the top of the instrument; the trigger plane made by a plastic scintillator read by 2 PMTs and divided into 6 segments; a calorimeter, composed by 16 plastic scintillators with dimensions (15x15x1) cm3 and a layer of 9 LYSO cubes, for a resulting plane of dimension (15x15x4) cm3; a scintillator veto system, 5 mm thick, at sides and at the bottom of the instrument.

III MC Simulation

A Monte Carlo simulation based on Geant4 toolkit was developed in order to evaluate the HEPD performances. In Fig. 1A one veto plane has been removed to show the calorimeter inside, the silicon planes are on the left; in Fig. 1B the electronic box is showed; in Fig. 1C the 16 scintillator planes are showed, the first plane is involved in the trigger configuration.

The interaction of particles inside the silicon tracker, the trigger plane, the calorimeter planes and the veto systems, togheter with the mechanical structure and the HEPD box were simulated.

In order to estimate the performances of HEPD, 109 events have been uniformly generated according to a power law spectrum $E^{-\gamma}$, where:

• •

$\gamma=2.2$, for electrons in the energy range [1-200] MeV;

• •

$\gamma=2.7$, for protons in the energy range [10-500] MeV;

To ensure an uniform exposure for the detector, a spherical surface around HEPD, with a different bin size at the poles and the equator, has been considered. The simulation has been used to study the trigger mask efficiency, the energy resolution of the calorimeter and the electron/proton discrimination.

In Fig. 2 the trigger rate is reported with the basic trigger both for electrons and protons. It requires the coincidence between the trigger plane (T1) and the first calorimeter plane (P1) (blue line). The red line represents the trigger rate for those events without a signal into the lateral veto; the black line represents the trigger rate for events completely contained into the detector. In Fig. 3 the trigger rate for a different mask is reported; it requires the coincidence of the trigger plane (T1), of the first (P1) and the second (P2) calorimeter plane.

III.1 Energy resolution

The HEPD calorimeter is composed by 2 different materials: the upper calorimeter is made by plastic scintillator planes, while the lower calorimeter is made by Lyso cristals. To study the different energy losses in each detector, simulated samples at fixed energies have been used. The minimum energy required for particles to entirely cross the 3.5 mm aluminium thickness of the satellite wall is $\sim$2 MeV for electrons and $\sim$30 MeV for protons.

The simulated energy range is [3-150] MeV for electrons and [30-200] MeV for protons. In Fig. 4 the energy losses of electrons with energy E=100 MeV into the upper and lower calorimeter are reported. The green line represents the total energy released by the particle; the magenta line represents the energy deposited into the upper calorimeter, while the blue line is the energy released into the Lyso cristals. In Fig. 4 the profile of the energy released into the upper calorimeter (CALO) vs the lower calorimeter (LYSO) is also reported. In Fig. 5 the same plots are reported for protons with energy E=200 MeV.

In Fig. 6 the energy resolution of the HEPD calorimeter is reported. Only events completely contained into the calorimeter have been considered. The energy resolution has to be $<10\%$ at 5 MeV. Both for electrons and for protons it is in good agreement with these initial requests. The energy losses inside the material of the mechanical structures are responsible of the low energy tails observed in the distributions.

III.2 Electron/Proton discrimination

One of the most important features of HEPD is the separation between electrons and protons.

Since, at these energies protons are slow and not relativistic within the HEPD operational range, the dE vs E method for discriminating electrons against protons can be used. To normalize the simulated proton and electron spectra, the results in lpsc have been used. The normalization factor is chosen at 200 MeV, where proton spectrum is $\sim$45 times greater than the electron spectrum.

The dE is measured within the two layers of silicon tracker, while the sum of the energy released in the detector provides the E measurement. The veto system allow to select events completely contained into the calorimeter. The resulting plots for electrons and protons are shown in Fig. 7. Electron and proton distributions are very different and particles lay in separate energy bands.

IV Conclusions

In this paper the study of the performances of the HEPD detector have been described. The HEPD project involved the construction of 4 detector versions: the Electrical Model, the Structural and Thermal Model, the Qualification Model and the Flight Model.

The Qualification Model have been tested in April 2016 at the Beam Test Facility (BTF) of the “Laboratori Nazionali di Frascati” of INFN. The Flight Model has also been tested at the BTF in Frascati in September 2016 where it was irradiated with electrons and positrons from 30 to 150 MeV. In November 2016, the Flight model has been also tested at Trento Facility with a proton beam with energies in the range [35 - 220] MeV. The beam test data are under analysis.