1. X-IFU background
Simone Lotti
INAF-IAPS Roma

2. 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

3. 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.

4. 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

5. 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

6. Updated background estimates
So with the new setup this is the background level foreseen.
If we refer to the 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

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

8. Secondary electrons

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

10. 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)

11. 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

12. Magnetic diverter dimensioning4
detector
Geant4 simulations
Magnetic diverter required transmission efficiency
L2 low energy environment analysis
Ray-tracing simulations
Geant4 simulations

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

14. Mirrors focalization efficiency
Two independent estimates of the mirrors focalization efficiency:
Ray-tracing simulations for 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

15. 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°)

16. 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 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)

17. 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

18. 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

19. Other grades
grade 1
grade 2
grade 3
grade 4 19

20. Primary particles

21. 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)