The Cryogenic AntiCoincidence detector for ATHENA X-IFU: a scientific assessment of the observational capabilities in the hard X-ray band

ATHENA is a large X-ray observatory, planned to be launched by ESA in 2028 towards an L2 orbit. One of the two instruments of the payload is the X-IFU: a cryogenic spectrometer based on a large array of TES microcalorimeters, able to perform integral field spectrography in the 0.2-12 keV band (2.5 eV FWHM at 6 keV). The X-IFU sensitivity is highly degraded by the particle background expected in the L2 orbit, which is induced by primary protons of both galactic and solar origin, and mostly by secondary electrons. To reduce the particle background level and enable the mission science goals, the instrument incorporates a Cryogenic AntiCoincidence detector (CryoAC). It is a 4 pixel TES based detector, placed <1 mm below the main array. In this paper we report a scientific assessment of the CryoAC observational capabilities in the hard X-ray band (E>10 keV). The aim of the study has been to understand if the present detector design can be improved in order to enlarge the X-IFU scientific capability on an energy band wider than the TES array. This is beyond the CryoAC baseline, being this instrument aimed to operate as anticoincidence particle detector and not conceived to perform X-ray observations.Comment: Accepted for publication on Experimental Astronom

Prototipo di Piattaforma LDB

Il documento descrive un sistema progettato e realizzato per rispondere alle necessità di equipaggiamenti a bordo di palloni di lunga durata (Long Duration Balloon: LDB) che si effettuano necessariamente da regioni Artiche ed Antartiche. Le caratteristiche del sistema, denominato Prototipo Piattaforma LDB (PPLDB), lo rendono inoltre utilizzabile per ogni genere di voli di pallone stratosferico, per il controllo di payload sub-orbitali, o per il controllo remoto di qualsiasi strumentazione quando siano assenti collegamenti terrestri. Le caratteristiche del sistema lo rendono compatibile o facilmente adattabile alle specifiche proprie sia di un equipaggiamento primario (per il controllo del volo) sia di un payload. Il sistema PPLDB é stato testato in condizioni simulate di voli stratosferici, pertanto qualificato in un ampio intervallo operativo di temperatura e pressione, in tutti i suoi componenti

Geant4 simulations of soft proton scattering in X-ray optics. A tentative validation using laboratory measurements

Low energy protons (< 300 keV) can enter the field of view of X-ray space telescopes, scatter at small incident angles, and deposit energy on the detector, causing intense background flares at the focal plane or in the most extreme cases, damaging the X-ray detector. A correct modelization of the physics process responsible for the grazing angle scattering processes is mandatory to evaluate the impact of such events on the performance of future X-ray telescopes as the ESA ATHENA mission. For the first time the Remizovich model, in the approximation of no energy losses, is implemented top of the Geant4 release 10.2. Both the new scattering physics and the built-in Coulomb scattering are used to reproduce the latest experimental results on grazing angle proton scattering. At 250 keV multiple scattering delivers large proton angles and it is not consistent with the observation. Among the tested models, the single scattering seems to better reproduce the scattering efficiency at the three energies but energy loss obtained at small scattering angles is significantly lower than the experimental values. In general, the energy losses obtained in the experiment are higher than what obtained by the simulation. The experimental data are not completely representative of the soft proton scattering experienced by current X-ray telescopes because of the lack of measurements at low energies (< 200 keV) and small reflection angles, so we are not able to address any of the tested models as the one that can certainly reproduce the scattering behavior of low energy protons expected for the ATHENA mission. We can, however, discard multiple scattering as the model able to reproduce soft proton funneling, and affirm that Coulomb single scattering can represent, until further measurements, the best approximation of the proton scattered angular distribution at the exit of X-ray optics.Comment: submitted to Experimental Astronom

Updates on the background estimates for the X-IFU instrument onboard of the ATHENA mission

ATHENA, with a launch foreseen in 2028 towards the L2 orbit, addresses the science theme "The Hot and Energetic Universe", coupling a high-performance X-ray Telescope with two complementary focal-plane instruments. One of these, the X-ray Integral Field Unit (X-IFU) is a TES based kilo-pixel array providing spatially resolved high-resolution spectroscopy (2.5 eV at 6 keV) over a 5 arcmin FoV. The background for this kind of detectors accounts for several components: the diffuse Cosmic X-ray Background, the low energy particles (<~100 keV) focalized by the mirrors and reaching the detector from inside the field of view, and the high energy particles (>~100 MeV) crossing the spacecraft and reaching the focal plane from every direction. Each one of these components is under study to reduce their impact on the instrumental performances. This task is particularly challenging, given the lack of data on the background of X-ray detectors in L2, the uncertainties on the particle environment to be expected in such orbit, and the reliability of the models used in the Monte Carlo background computations. As a consequence, the activities addressed by the group range from the reanalysis of the data of previous missions like XMM-Newton, to the characterization of the L2 environment by data analysis of the particle monitors onboard of satellites present in the Earth magnetotail, to the characterization of solar events and their occurrence, and to the validation of the physical models involved in the Monte Carlo simulations. All these activities will allow to develop a set of reliable simulations to predict, analyze and find effective solutions to reduce the particle background experienced by the X-IFU, ultimately satisfying the scientific requirement that enables the science of ATHENA. While the activities are still ongoing, we present here some preliminary results already obtained by the group

Thermal simulations of temperature excursions on the Athena X-IFU detector wafer from impacts by cosmic rays

We present the design and implementation of a thermal model, developed in COMSOL, aiming to probe the wafer-scale thermal response arising from realistic rates and energies of cosmic rays at L2 impacting the detector wafer of Athena X-IFU. The wafer thermal model is a four-layer 2D model, where 2 layers represent the constituent materials (Si bulk and Si$_{3}$N$_{4}$ membrane), and 2 layers represent the Au metallization layer's phonon and electron temperatures. We base the simulation geometry on the current specifications for the X-IFU detector wafer, and simulate cosmic ray impacts using a simple power injection into the Si bulk. We measure the temperature at the point of the instrument's most central TES detector. By probing the response of the system and pulse characteristics as a function of the thermal input energy and location, we reconstruct cosmic ray pulses in Python. By utilizing this code, along with the results of the GEANT4 simulations produced for X-IFU, we produce realistic time-ordered data (TOD) of the temperature seen by the central TES, which we use to simulate the degradation of the energy resolution of the instrument in space-like conditions on this wafer. We find a degradation to the energy resolution of 7 keV X-rays of $\approx$0.04 eV. By modifying wafer parameters and comparing the simulated TOD, this study is a valuable tool for probing design changes on the thermal background seen by the detectors.Comment: accepted for publication in the Journal of Low Temperature Physic

The mechanical and EM simulations of the CryoAC for the ATHENA X-IFU

The design phase of the CryoAC DM for the ATHENA X-IFU has concerned numerical simulations to exploit different fabrication possibilities. The mechanical simulations have accounted for the peculiar detector structure: 4 silicon chips asymmetrically suspended by means of 4 microbridges each. A preliminary study was performed to analyze the response to acceleration spectra in the frequency domain, shocks and time domain random displacement, prior to a real vibration test campaign. EM simulations to spot unwanted magnetic fields have been conducted as well. In this work we will show the latest advance in the design of the new detectors, showing the main results coming from various simulations

Large Area ?-thermal Phonon TES Detector Mediated by the quasi-particle Diffusion Signal for Space Application

Low temperature detectors operated at about 0.1K have achieved excellent spectral performances in the soft X-rays, becoming appealing for new challenging measurements with space missions in Astrophysics. In order to exploit their full sensitivity, it is necessary to minimize the background signals generated by the cosmic rays, i.e., high energy protons and light nuclei, that leave sizable amounts of energy in the same spectral window of the astrophysics signals. Detectors for GeV protons and nuclei operating few millimeters from the X-ray detector at 0.1K can act as anti-coincidence to disentangle the fake signal of cosmics. Fast and large detectors are designed and fabricated. These operate by mixing the fast a-thermal phonon signal with the slow diffusive thermal ones. A greater uniformity in the response should be obtained using large shaped superconducting aluminium films that acts as phonon collectors: the quasi-particles created by high energy phonons diffuse along the film toward a small Ir TES sensor giving out to a fast rise time. Here we present the measurement of an operating prototype of a superconducting anticoincidence detector for the proposed space mission ATHENA+