X-IFU background Simone Lotti INAF-IAPS Roma

Summary Background assessment and associated uncertainty level  background requirements Updates Outline: Particle background components CGR induced background Updated background estimates and analysis Simulations status and open issues Soft protons focussed background Expected flux at focal plane Required magnetic diverter efficiency Ci sn stati updates che hanno dato consolidamento, risultati ancora parziali

The particle background Induced by 2 populations of charged particles able to reach the focal plane, depositing part or all of their energy - Solar protons - Cosmic Rays High energy (>100 MeV) CR dominated in stationary conditions Low energy (<100s keV) Both components depend on the solar cycle, and they are anticorrelated.

Preliminary background estimates These were the preliminary bkg estimates, you all have seen this plot, it was based on a preliminar geometrical configuration that you can see on the left, and the results are consistent with the current requirement of 5x10^-3 cts/cm2/s/keV

New FPA mass model (CAD) Preliminary cryostat mass model Updates New FPA Geant4.9.4  Geant4.10.1 New detector Updated physicslist/settings New FPA mass model (CAD) Preliminary cryostat mass model From these results there were several updates to the simulations. We received a new more detailed model of the FPA, which as you can see is much more complex than the previous one. We also moved to a newer version of Geant4,and we rediscussed the software settings. I also had to rework the analysis software to adapt it to the new detector shape and to include the time flag of the events, so taking into account the temporal responseof the detectors. What is missing at least from the mass model point of view is the cryostat and the satellite models Updated analysis Updated cryostat and satellite mass models not yet inserted

Updated background estimates So with the new setup this is the background level foreseen. If we refer to the .2-10 keV band the background is a factor of 2 higher, while the situation is better if we exclude the part of the spectrum below 2 keV which will be however dominated by the diffuse component

Background origin and composition L’electron shield potrebbe essere migliorato facendolo più spesso Ci sono incertezze riguardo la fisica dei fotoni e del backscattering

Secondary electrons

Open points New cryostat model New satellite model New settings are still preliminar Relevant processes (backscattering) reliability

Uncertainty Due to GCR flux we expect a variability of a factor ~2 during the nominal lifetime of the mission For E<2 keV the background is by far dominated by the CXB. The energy range for defining the particle background requirement should be 2-12 keV With the current simulation results we could consider adopting 10-2 cts cm-2 s-1 keV-1 as best guess for the XIFU background, and associate to this an uncertainty of a factor of ~3. (mass model + physics + settings, to be revised after AREMBES) Present uncertainty associated to the background ~3 (mass model + physics + settings)

Cannot cross the spacecraft, concentrated by the optics Low energy particles Cannot cross the spacecraft, concentrated by the optics XMM and Chandra shown that if unhandled: Can compromise up to 40% of obs. time Additional bkg component, poorly reproducible We need: external fluxes, focalization efficiency, magnetic diverter efficiency, dE/dx Issues: Low energy external fluxes in L2 are poorly known, are variable spatially and with time, and depend on the chosen orbit Focalization efficiency determined with MC simulations, different treatments give different results However: Several data from existing satellites New experimental data to validate simulator Heritage from already flown X-ray missions

Magnetic diverter dimensioning 4 detector Geant4 simulations Magnetic diverter required transmission efficiency L2 low energy environment analysis Ray-tracing simulations Geant4 simulations

External soft protons flux The low energy L2 environment is currently poorly known, complex, and highly dynamical OoM approximation: F∝E-1.5 Fluxes @80 keV: We set two limits which will encompass the ion fluxes that ATHENA will experience:   t>90% outside the plasma sheet: flux @80 keV ~ 10.5 p cm-2 s-1 sr-1 keV-1 t<90% outside the plasma sheet: flux @80 keV ~ 105 p cm-2 s-1 sr-1 keV-1

Mirrors focalization efficiency Two independent estimates of the mirrors focalization efficiency:   Ray-tracing simulations for 100-150 keV protons : Where Ω 𝜃 is the solid angle of the optics, 𝐴 𝑜𝑝𝑡 is the optics area in 𝑐 𝑚 2 , 𝐴 𝑑𝑒𝑡 is the detector area in 𝑐 𝑚 2 , 𝑛 𝑑𝑒𝑡 = 𝑁 𝑑𝑒𝑡 𝐴 𝑑𝑒𝑡 is the number of particles impacting on the detector per unit area in 𝑝 𝑐𝑚 −2 , and 𝐼 𝑖𝑛𝑐 = 𝑁 𝑖𝑛𝑐 Ω 𝜃 𝐴 𝑜𝑝𝑡 the intensity of the proton flux on the optics in 𝑝 𝑐𝑚 −2 𝑠𝑟 −1 . Where 𝑓 𝑐𝑥𝑏 𝑥𝑖𝑓𝑢 1 𝑘𝑒𝑉 is the flux of CXB photons impacting on the detector in 𝑝 𝑐 𝑚 −2 𝑠 −1 𝑘𝑒 𝑉 −1 and 𝐼 𝑐𝑥𝑏 𝑒𝑥𝑡 1 𝑘𝑒𝑉 is the CXB intensity outside the optics, and 𝑝 𝑐𝑚 −2 𝑠 −1 𝑠𝑟 −1 𝑘𝑒 𝑉 −1 , respectively. assume that the protons will behave like 1 keV photons: Referred to different energy ranges Assuming that protons will behave like 1 keV photos is VERY conservative We take the second estimate as upper limit We need ray-tracing simulations in the 4-80 keV energy range

Magnetic diverter efficiency The magnetic diverter efficiency as function of the proton energy has been calculated for few energies, and relative only to the WFI FoV. We lack a real modelization, we use 3 possible trends for the diverter transmitted fraction: (extremely conservative) (optimistic) These are referred to 40x40 arcmin2 FoV  conservative for X-IFU Assume the proton flux to be extremely collimated (~0.1°)

Energy loss: X-IFU filters In first approximation, we can conservatively assume that the protons do not lose energy in their interaction with the mirrors. For XIFU the filters baseline currently consist in a total 0.28 um Kapton + 0.21 um Aluminum, divided in 5 filters of identical thickness.   For such configuration, from Geant4 simulation we have the transmission function shown in Figure, and the distribution of initial energies of protons that reach the focal plane with a residual energy in the range 0.2<E<10 keV shown in Figure, which can be taken as the transmission function for background-inducing protons. f we assume an impacting spectral shape 𝐸 −1.5 , a reasonable assumption for the SP, we can find the same distribution in a more realistic case (see Figure 5, left). Furthermore, from the corresponding cumulative distribution (Figure 5, right) it is easy to see that if we want to deflect 99% (99.9%) of protons that arrive at the focal plane with energies between 0.2 and 10 keV we must deflect protons with initial energies up to ~70 keV (80 keV). To deflect 99% (99.9%) of background protons, we must deflect up to ~70 keV (80 keV)

Requirement on the magdiv efficiency Stima flusso senza diverter Putting everything together we can estimate the fluxes of background-inducing protons on the focal plane spectra for particles that reach the FPA with energies inside the instruments sensitivity band (extremely conservative) The real number is somewhere between these two We want these fluxes to be T2 already satisfies our requirement. For T1 and T3 the efficiency must be improved by R1~600 and R3~40

Open points related to SP and magdiv Proton fluxes Proton reflectivity model Need to estimate η and angular spread of the proton beam with raytracing for E<100 keV Ions Magnetic diverter efficiency

Other grades grade 1 grade 2 grade 3 grade 4 19

Primary particles

Energy loss: WFI filters 400 Å of Al 2000 Å of Kapton 700 Å dead Al layer on-chip. of 400 Å of Al and 2000 Å of Kapton, plus a 700 Å dead Al layer on-chip. To deflect 99% (99.9%) of background protons, we must deflect up to ~48 keV (55 keV)