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
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
E-E spectrometers. Each spectrometer is composed of a 100
m thin silicon detector superimposed to a 300
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
m top PIPS.
Assuming a low energy non-relativistic charged particle passing through the thin PIPS detector, both the energy deposited, , and the total kinetic energy, , are velocity dependent. Combining these quantities, a particle classifier can be defined: PID = , 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
,
, and
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
E-E channel has been triggered. The overall LEM FoV is ≈ 45°. The obtained angular RMS is ≈ 6° for proton and
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 factor
1 is ≈ 0.2-0.3 cm
2sr. It is approximately constant for electrons in the range 0.2-5 MeV, for protons in the range 3-50 MeV and for
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.
Full NUSES Author List
R. Aloisio 1,2, C. Altomare 3, F. C. T. Barbato 1,2, R. Battiston 4,5, M. Bertaina 6,7, E. Bissaldi 3,8, D. Boncioli 2,9, L. Burmistrov 10, I. Cagnoli 1,2, M. Casolino 11,12, A.L. Cummings 13, N. D’Ambrosio 2, I. De Mitri 1,2, G. De Robertis 3, C. De Santis 11, A. Di Giovanni 1,2, A. Di Salvo 7, M. Di Santo 1,2, L. Di Venere 3, J. Eser 14, M. Fernandez Alonso 1,2, G. Fontanella 1,2, P. Fusco 3,8, S. Garbolino 7, F. Gargano 3, R. A. Giampaolo 1,7, M. Giliberti 3,8, F. Guarino 15,16, M. Heller 10, R. Iuppa 4,5, J. F. Krizmanic 17,18, A. Lega 4,5, F. Licciulli 3, F. Loparco 3,8, L. Lorusso 3,8, M. Mariotti 19,20, M. N. Mazziotta 3, M. Mese 15,16, H. Miyamoto 1,7, T. Montaruli 10, A. Nagai 10, R. Nicolaidis 4,5, F. Nozzoli 4,5, A. V. Olinto 14, D. Orlandi 2, G. Osteria 15, P. A. Palmieri 6,7, B. Panico 15,16, G. Panzarini 3,8, A. Parenti 1,2, L. Perrone 21,22, P. Picozza 12,11, R. Pillera 3,8, R. Rando 19,20, M. Rinaldi 11, A. Rivetti 7, V. Rizi 2,9, F. Salamida 2,9, E. Santero Mormile 6, V. Scherini 21,22, V. Scotti 15,16, D. Serini 3, I. Siddique 1,2, L. Silveri 1,2, A. Smirnov 1,2, R. Sparvoli 11, S. Tedesco 7,23, C. Trimarelli 10, L. Wu 1,2,†, P. Zuccon 4,5, S. C. Zugravel 7,23.
1 Gran Sasso Science Institute (GSSI), Via Iacobucci 2, I-67100 L’Aquila, Italy
2 Istituto Nazionale di Fisica Nucleare (INFN) - Laboratori Nazionali del Gran Sasso, I-67100 Assergi, L’Aquila, Italy
3 Istituto Nazionale di Fisica Nucleare, Sezione di Bari, via Orabona 4, I-70126 Bari, Italy
4 Dipartimento di Fisica, Università di Trento, via Sommarive 14 I-38123 Trento, Italy
5 Istituto Nazionale di Fisica Nucleare (INFN) - TIFPA, via Sommarive 14 I-38123 Trento, Italy
6 Dipartimento di Fisica, Università di Torino, Via P. Giuria, 1 I-10125 Torino, Italy
7 Istituto Nazionale di Fisica Nucleare (INFN) - Sezione di Torino, I-10125 Torino, Italy
8 Dipartimento di Fisica M. Merlin, dell’Università e del Politecnico di Bari, via Amendola 173, I-70126 Bari, Italy
9 Dipartimento di Scienze Fisiche e Chimiche, Università degli Studi di L’Aquila, I-67100 L’Aquila, Italy
10 Département de Physique Nuclèaire et Corpusculaire, Université de Genève, 1205 Genève, Switzerland
11 INFN Roma Tor Vergata, Dipartimento di Fisica, Universitá di Roma Tor Vergata, Roma, Italy
12 RIKEN, 2-1 Hirosawa, Wako, Saitama, Japan
13 Departments of Physics and Astronomy & Astrophysics, Institute for Gravitation and the Cosmos, Pennsylvania State University, University Park, PA 16802, USA
14 Department of Astrophysics & Astronomy, The University of Chicago, Chicago, IL 60637, USA
15 Istituto Nazionale di Fisica Nucleare, Sezione di Napoli, via Cintia, I-80126 Napoli, Italy
16 Dipartimento di Fisica E. Pancini dell’Università di Napoli Federico II, via Cintia, I-80126 Napoli, Italy
17 CRESST/NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA
18 University of Maryland, Baltimore County, Baltimore, MD 21250, USA
19 Università di Padova, I-35122 Padova, Italy
20 Istituto Nazionale di Fisica Nucleare (INFN) - Sezione di Padova, I-35131 Padova, Italy
21 Dipartimento di Matematica e Fisica “E. De Giorgi", Università del Salento, Via per Arnesano, I-73100 Lecce, Italy
22 Istituto Nazionale di Fisica Nucleare - INFN - Sezione di Lecce, Via per Arnesano, I-73100 Lecce, Italy
23 Department of Electrical, Electronics and Communications Engineering, Politecnico di Torino, Corso Duca degli Abruzzi 24, I-10129 Torino, Italy
† Now at Institute of Deep Space Sciences, Deep Space Exploration Laboratory, Hefei 230026, China