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The X-IFU sensitivity is degraded by the particles background expected at L2 orbit, which is induced by primary protons of both galactic and solar origin, and mostly by secondary electrons. To reduce the background level and enable the mission science goals, a Cryogenic Anticoincidence (CryoAC) detector is placed < 1 mm below the TES array. It is a 4- pixel TES based detector, with wide Silicon absorbers sensed by Ir:Au TESes.
The CryoAC development schedule foresees by Q1 2017 the delivery of a Demonstration Model (DM) to the X-IFU FPA development team. The DM is a single-pixel detector that will address the final design of the CryoAC. It will verify some representative requirements at single-pixel level, especially the detector operation at 50 mK thermal bath and the threshold energy at 20 keV.
To reach the final DM design we have developed and tested the AC-S7 prototype, with 1 cm2 absorber area sensed by 65 Ir TESes. Here we will discuss the pulse analysis of this detector, which has been illuminated by the 60 keV line from a 241Am source.
First, we will present the analysis performed to investigate pulses timings and spectrum, and to disentangle the athermal component of the pulses from the thermal one. Furthermore, we will show the application to our dataset of an alternative method of pulse processing, based upon Principal Component Analysis (PCA). This kind of analysis allow us to recover better energy spectra than achievable with traditional methods, improving the evaluation of the detector threshold energy, a fundamental parameter characterizing the CryoAC particle rejection efficiency.
The background for this kind of detectors accounts for several components: the diffuse Cosmic Xray 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. In particular, these high energy particles lose energy in the materials they cross, creating secondaries along their path that can induce an additional background component.
Each one of these components is under study of a team dedicated to the background issues regarding the X-IFU, with the aim 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 XMMNewton, 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. The L2 environment characterization activities, and the analysis and validation of the physical processes involved in the Monte Carlo simulations are the core of an ESA activity named AREMBES (Athena Radiation Environment Models and Effects), for which the work presented here represents a starting point.
The X-IFU operates at temperatures below 100 mK and thus requires a sophisticated cryostat. In order to allow the beam focused by the telescope to reach the X-IFU detector, windows need to be opened on the cryostat thermal and structural shields surrounding the cold stage. X-ray transparent thermal blocking filters need to be mounted on such open windows to make the radiation heat-load onto the detector array negligible with respect to conduction heat load and dissipated electrical power, and to minimize photon shot noise onto the detector. After a brief survey of the heritage from space satellite and sounding rocket experiments on thermal filters operated at cryogenic temperatures, we present the selected baseline design of the thermal filters for the ATHENA X-IFU detector, show the performances, and finally discuss possible improvements in the design to increase the X-IFU quantum efficiency at low energies.
A cryo-amplifier working in a double loop-flux locked loop scheme for SQUID readout of TES detectors
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