A Compact Particle Detector for Space-Based Applications: Development of the Low Energy Module (LEM) for the NUSES Space Mission

1. Introduction: The NUSES space mission

NUSES is a planned space mission aiming to test innovative observational and technological approaches related to the study of low-energy cosmic rays, gamma rays and high-energy astrophysical neutrinos. The satellite will host two payloads, named Terzina [1,2] and Zirè [3,4,5,6,7]. Terzina will be a pathfinder for future missions devoted to observing Ultra High Energy Cosmic Rays (UHECRs) and neutrino astronomy using space-based instruments[8,9,10,11].

Zirè is a particle detector measuring electron, proton and light nuclei fluxes from a few up to hundreds MeV, also testing new tools for the detection of $\gamma$-rays. The main purpose of the detector is to measure the counting rates of the trapped particles precipitating from the Van Allen Belts (VABs), looking for eventual anomalies occurring in the proximity of catastrophic events such as earthquakes or volcanic activities involving the lithosphere. Another essential science objective of ZIRÈ is to monitor solar activity and its periodical cycle characterised by a duration of about 11 years. In particular, it is useful to monitor the occurrence of events such as Solar Flares (SFs) or Coronal Mass Ejections (CMEs) during solar maxima[12]. The investigation of energetic particles in the magnetosphere will improve the understanding of the acceleration mechanisms involved during those events with the possibility of space weather broadcasts. The other science objective is to detect photons with energies up to tens of MeV. This allows the study of some of the most violent and energetic events in astrophysics which are the so-called Gamma Ray Bursts (GRBs) [13,14], rapid and intense pulses of gamma rays coming from very far sources.

Zirè will be equipped with an additional sub-detector: the Zirè-Low Energy Module (LEM). The LEM will be attached to the external structure of the NUSES satellite (see Figure 1). It will fit within a volume of 10x10x10cm³ and it will be able to perform event-based Particle Identification (PID) for low-energy charged particles, such as electrons and protons.

2. The need for a Low Energy Module

As mentioned before, one of the NUSES mission goals is the monitoring of particle precipitations from Van Allen Belts (VABs) investigating for possible correlations with earthquakes and validating Lithopsphere-Atmosphere-Ionosphere-Magnetosphere coupling models. Several processes can cause a precipitation of the trapped charged particles from the radiation belts. These processes are not fully understood yet, but the primary cause is thought to be electromagnetic fluctuations in the radiation belt. These electromagnetic fluctuations can be produced through solar-magnetic storms, lightning storms, man-made electromagnetic emissions and seismic activity. It is known that the measurement of the fluxes of trapped charged particles can improve the models of the coupling between the lithosphere, atmosphere, ionosphere, and magnetosphere [15]. In particular, statistical evidence of a temporal correlation between particle precipitations from Van Allen Belts and strong seismic events has been pointed out [16]. These observations, motivate the interest in further, detailed, measurements of electron fluxes in the energy window $0.1-7\mathrm{MeV}$ that could be a promising channel to identify hypothetical seismic precursors.

Therefore, with the aim to extend the measurements to lower energies, a Zirè Low Energy Module (LEM), has been introduced in the design of the NUSES satellite. The Low Energy Module will be a compact spectrometer, fitting within a volume of 10x10x10cm³ (right panel of Figure 1 for the volume envelope of the LEM), able to perform an event-based measurement of energy, direction and composition of low energy charged particles, in particular down to 0.1 MeV for electrons. The main goal of this detector is the monitoring of the magnetosphere and ionosphere environment. Other goals of the LEM detector will be the study of Space Weather, monitoring the flux of low energy protons, and the investigation of the particle composition within the harsh environment of the South Atlantic Anomaly (SAA), measuring the isotopic abundances of H, He and eventually the fraction of heavier nuclei.

3. Geometry and detection concept of the LEM

In a particle spectrometer (like e.g. Zirè) the measurement of the particle direction is usually performed by tracking technique; however, for low energetic particles, the standard tracking approach fails due to large multiple Coulomb scattering within the first detector layer. For the measurement of the arrival direction of low energy particles, a collimation technique must be adopted [17]. In the collimation technique, a shaped passive shield must be thick enough to stop energetic particles from “unknown”/random directions. The passive collimator allows the detection of particles arriving from the "known" directions. To avoid bulky and heavy passive shields, the technique of “active collimation” is developed for the LEM spectrometer. This technique relies on the use of shaped plastic scintillators as a veto, tagging the particles that are crossing only a relatively light passive shield. To limit the occupancy of the veto detectors in the SAA and in the lower Van Allen belt near the poles, permitting a reliable measurement of the particle composition there, a 0.8 cm thick aluminium shield is considered for the LEM. The particle identification capability of the LEM relies on the consolidated $\Delta$E-E spectrometric technique [18,19,20], performed by five pairs of thin Passivated Implanted Planar Silicon (PIPS) detectors placed in a telescopic configuration. The typical resolution of the PIPS detector is ≃10 keV.

In Figure 2 an exploded view of the detector shows the components and a preliminary assembly of the detector. On the other hand, in Figure 3, a schematic representation of the instruments explains its adopted detection scheme. Coming from the zenith, a particle can enter inside the LEM through the holes drilled in the top shield avoiding being tagged by the drilled top veto. Depending on the particle direction, the nature of the charged particle is identified by one of the five $\Delta$E-E spectrometers. Each spectrometer is composed of a 100$\mu$m thin silicon detector superimposed to a 300$\mu$m thick silicon detector. To extend the energy range capability for the LEM particle identification, a lower calorimeter is placed below the PIPS. A lower calorimeter made by a 2 cm thick plastic scintillator allows a flux measurement of up to 10 MeV for electrons, thus a reasonable overlap is expected with the Zirè flux measurements [21]. Finally, a bottom veto is identifying particles of relatively large energy, that are not contained by the lower calorimeter. A good particle identification capability is expected for contained particles that are crossing one of the 100$\mu$m top PIPS.

Assuming a low energy non-relativistic charged particle passing through the thin PIPS detector, both the energy deposited, $\Delta E\propto \frac{Z^2}{{\beta }^2}$, and the total kinetic energy, $E_k\approx \frac{1}{2}m{\left(\beta c\right)}^2$, are velocity dependent. Combining these quantities, a particle classifier can be defined: PID = ${log}_{10}\left(\Delta E·E_k\right)\approx {log}_{10}\left(Z^{2}m\right)+const.$, that is mainly dependent on the particle mass, m, and charge, Z, but is almost energy independent.

For the characterisation of the detector’s performances, a GEANT4 Monte Carlo simulation [22] was appositely developed. For the simulation of the geometry reported in Figure 2, developed with a parametric computer-aided design (CAD) software, we adopted the Geometry Description Markup Language (GDML) [23]. We generated the GDML file (compatible with the GEANT4 toolkit) using the GDML Workbench [24] for FreeCAD 0.20.

The PID classifier is shown in the left plot of Figure 4. The non-relativistic approximation is not valid for electrons, however they are identified thanks to the very low mass. The poor energy resolution expected by the plastic scintillator calorimeter is responsible for the PID performance degradation at relatively large energy where the particles are crossing the thick PIPS stopping in the plastic calorimeter.

To estimate the particle identification efficiency, it is possible to define some specific intervals for the PID for each particle: [-3, 0] for electrons, [0.7, 1.4] for protons, and [1.6, 2.5] for alpha particles. In the right panel of Figure 4, the particle identification tagging efficiencies, for the three families of particle, is higher than 90 % in the three respective PID proxy intervals. In particular, we observed that for accepted electrons, protons, and alpha particles, respectively the $99\%$, $94\%$, and $96\%$ were correctly tagged.

In Figure 5, the characterisation of the LEM Field of View (FoV) and angular resolution for protons and electrons is shown. The scatter plot shows the incoming direction of the particle (at the Monte Carlo truth level) projected on the plane. Here, the zenith direction is assumed to be encoded by the origin of the plot. The colour identifies which $\Delta$E-E channel has been triggered. The overall LEM FoV is ≈ 45°. The obtained angular RMS is ≈ 6° for proton and $\alpha$ particles while a worse resolution (≈ 12°) is expected for electrons due to interactions with the inner fringes of the LEM aperture.

The overall LEM geometrical factor1 is ≈ 0.2-0.3 cm²sr. It is approximately constant for electrons in the range 0.2-5 MeV, for protons in the range 3-50 MeV and for $\alpha$ particles in the range 20-200 MeV. In Figure 6, the geometric factor estimated for electrons, protons, and alpha particles is shown.

Knowing the orbit parameters of the NUSES mission (Sun-synchronous, 97 degrees, LEO 550km), a preliminary map of the expected rates of the LEM can be evaluated using the model International Radiation Environment Near Earth AE9/AP9 (IRENE-AE9/AP9) [26]. In the LEO environment, the most impacting populations of charged particles are trapped protons and electrons. With IRENE-AE9/AP9 we could estimate the differential omnidirectional/isotropic fluxes of those particles.

In Figure 7 it is shown that the LEM will experience a high acquisition rate (≈ 50 kHz) in the SAA, thus a twofold data transmission approach is in preparation (“event-based” for rates below 1kHz and “histogram based” for larger rates) to fulfil the data bandwidth assigned to LEM in the NUSES mission.

4. Preliminary test on PIPS sensors.

The heart of the LEM detector is composed of the PIPS spectrometer. The spectrometer will use four circular PIPS with an area of 150 mm² surrounding a central one with an area of 55 mm² (see Figure 8). The central PIPS diameter is smaller to equalize the geometrical acceptance among the five channels. The five top sensors, with thickness 100$\mu$m, will be the R-series (ruggedized) PIPS manufactured by ORTEC/AMETEK [27].

The two sides of the 100$\mu$m PIPS are covered by an aluminium and a gold layer with a thickness of ≈50$\mu$g/cm² and ≈40$\mu$g/cm², respectively. These layers ensure that the PIPS detectors are light-tight and “ruggedized”. The five bottom sensors, with thickness 300$\mu$m, will be manufactured by Canberra/MIRION [28]. The two sides of the 300$\mu$m PIPS are covered by aluminium with a thickness of ≈70$\mu$g/cm² and ≈250$\mu$g/cm², respectively, to ensure the detector is light tight.

A preliminary set-up for the measurement of the performances of a PIPS detector was tested in the INFN-TIFPA laboratory. The depletion voltage of the PIPS sensor was 60V and the preliminary measurement of the power budget of the used charge amplifier is below 100mW/ch. The sensor was tested acquiring atmospheric muons in telescopic configuration, $\gamma$-rays from ¹⁷⁶Lu source and $\alpha$-particles and $\gamma$-rays from ²⁴¹Am source. Excellent linearity of the energy scale was obtained. Moreover, the PIPS response to particles with very different specific ionization (muons, recoiling electrons and $\alpha$) has been found to be compatible within a few per cent, as expected.

Two key requirements for the LEM project are guaranteeing a low energy threshold and a relatively fast response. The acquired measures verify the feasibility of the 40 keV energy threshold adopted in the LEM simulations as well as the 10 keV energy resolution.

Another important factor for the LEM is the measurement of the signal decay time; this is related to the detector occupancy which could be an issue in the harsh environment of the SAA. The ≈ 200 ns measured signal decay time prevents the possible pile-up of overlapping signals from different particles in the SAA.

5. Conclusion

The project of a compact (10x10x10cm³) particle spectrometer, the Low Energy Module (LEM) as part of the Zirè instrument on board the NUSES space mission is in the construction and testing phase. Prototypes of PIPS detector readout have been tested in the INFN-TIFPA laboratory, confirming ≈ 10 keV energy resolution and ≈ 100 ns signal decay time. The LEM will be able to perform measurements of energy, direction, and composition of low-energy charged particles down to 0.1 MeV kinetic energy, studying the coupling between the lithosphere and magnetosphere, measuring the particle fluxes in the SAA and monitoring the Space Weather.