laboratorio in banda x v. fioretti - inaf/iasf bologna the simbol-x mission and the background...

Laboratorio in banda X V. Fioretti - INAF/IASF Bologna

The Simbol-X mission and the The Simbol-X mission and the background rejectionbackground rejection

Corso di Strumentazione per l'Astrofisica 21/05/08


Corso di Strumentazione per l'Astrofisica 21/05/08 Outline

• The sensitivity of a focusing X-ray telescope

• The background problem:

The photonic component

The hadronic component

• Active and passive shielding

• The case of the Simbol-X mission:

Background evaluation from balloon experiments

The passive shielding design (material, thickness, mass)

Soft proton flares detected by XMM/Newton

The magnetic proton diverter


Corso di Strumentazione per l'Astrofisica 21/05/08 The sensitivity – 1

• The sensitivity is the minimum detectable flux

• A fundamental parameter in the case of far and faint sources (as in the majority of extragalattic X-ray sources)

• In X-ray Astronomy, the standard flux unity is the Crab:

- Power law F(E) = k×E-Γ (k=9.7 ± 1, Γ=2.1 ± 0.03) ph. cm-2 s-1 keV-1

- Integrating in the energy range….

Energy range [keV] 1 μCrab [erg cm-2 s-1]

2 – 10 2.16 × 10-14

10 – 30 1.28 × 10-14

20 – 40 7.71 × 10-14


Corso di Strumentazione per l'Astrofisica 21/05/08 The sensitivity – 2

• The sensitivity is different for collimated (A) and direct (B) imaging X-ray telescopes

Collimated system

Focusing system

A1

A2

B1

B2

d

minAET

BcostF

ETA

ABcostF

eff

dmin


Corso di Strumentazione per l'Astrofisica 21/05/08Telescopes with collimated or indirect imaging system

•BeppoSAX/PDS

Field of view = 1.3°

Sensitivity = 1 mCrab

•INTEGRAL/IBIS

Angular resolution = 12’’

Field of view = 9° x 9°

Sensitivity = 0.5 mCrab


Corso di Strumentazione per l'Astrofisica 21/05/08Telescopes with direct imaging system

•Chandra

Angular resolution = 0.5’’

Field of view = 30’ x 30’

Sensitivity = 0.1 µCrab (@2 keV)

•XMM-Newton

Angular resolution = 15’’

Field of view = 30’ x 30’

Sensitivity = 1 µCrab (@6 keV)


Corso di Strumentazione per l'Astrofisica 21/05/08 The sensitivity of a focusing X-ray telescope

ETA

ABcostF

eff

dmin

22C

2geomeff ReflFLReflAA

The effective area:

EC

The critical reflecting angle:

The detection area: 2d HPDtanFL4

A


Corso di Strumentazione per l'Astrofisica 21/05/08 The background

Main components

Photonic

(induced by CXB photons)

Hadronic

(caused by hadronic interactions)

• “Prompt” emission

• “Delayed” emission

• Solar flares

• Particles trapped in the Earth magnetosphere

• Aperture component

• Shield leakage

• Scattering Compton

• Stray-light


Corso di Strumentazione per l'Astrofisica 21/05/08 The shielding system

Passive shielding:

- An high Z material (more efficiency to photoelectric absorption) that, with a specified geometry, shields the focal plane from photons coming from outside the telescope field of view

- Materials (high Z, low cost) = Lead, Tungsten, Tantalum

- Often a grading is applied (a set of materials with decreasing atomic number until the fluorescence of the last layer falls below the telescope energy range)

Active shielding

- It is a detector itself, posed around the focal plane

- When an hadronic particle interacts with the active shield, i.e. deposits an energy above the operative lower energy limit (500 keV, 1 MeV), it causes a detected event.

- All the coincidence events with the telescope detectors are rejected

- Material = BGO, NaI, Plastic


Corso di Strumentazione per l'Astrofisica 21/05/08 The photonic background

Aperture component:

• The unfocused Cosmic X-ray photons arriving at the focal plane cause background events

• Important at E < 100 keV (power law of index ~1.4)

• The CXB is isotropic and it increases for greater aperture angle

• Solution = passive shield


Corso di Strumentazione per l'Astrofisica 21/05/08 The photonic background

The shield leakage:

• Photons that don’t interact with the passive shield or deposit an energy below the active shield threshold

Scattering Compton:

• High energy photons (E > 100 keV) can be deflected and arrive at the focal plane

Production of couples:

• Photons with E > 1 MeV interact with the material producing electrons and positrons


Corso di Strumentazione per l'Astrofisica 21/05/08 The hadronic component

• It is mostly caused by incident cosmic rays (~ 90 – 95 % protons)

• It depends on the type of orbit and mission duration

• Prompt emission:

Secondary particles and gamma photons produced by cosmic rays interactions that arrive to the focal plane.

Solution = Active shielding

• Delayed emission:

Cosmic rays can also produce radioactive nuclei, that decay with times longer than active shielding coincidence time.

This so calles spallation effects increases with mission duration.


Corso di Strumentazione per l'Astrofisica 21/05/08 The hadronic component

The Soft Proton flares:

• Protons trapped by the Earth magnetic field (E < 500 keV) can be funnelled by the grazing optics of an X-ray telescope and arrive to the focal plane, causing CCD damage and increasing the background level

• Damage occurred in ACIS instrument of Chandra

• Rapidly increasing background flares are detected by XMM-Newton

• Solution = magnetic diverter


Corso di Strumentazione per l'Astrofisica 21/05/08 Background detected by balloon experiments

)keV 30 a(E167 -2.38

• Objective: evaluate how the background flux depends on aperture angle

• Energy range = [20 - 40] keV


Corso di Strumentazione per l'Astrofisica 21/05/08 A case study: Simbol-X

• Energy range = from ~0.5 keV to ~100 keV

• Angular resolution: ~15 - 20 (HPD)

- IBIS ->12′

• Field of view: ≥ 10′

• Sensitivity Fmin ≤ 1 µCrab (~ 20 keV)

- A factor of ~ 500 respect to SAX/PDS and INTEGRAL/IBIS


Corso di Strumentazione per l'Astrofisica 21/05/08 A case study: Simbol-X

• E > 15 keV

• Fmin < 1 µCrabm 20 FL

“Formation flight tecnique”


Corso di Strumentazione per l'Astrofisica 21/05/08 Simbol-X : the background as a critical point

2d HPDtanFL4

A The fraction of background

increases

• The formation flight tecnique doesn’t allow the use of an optical tube

• ΔE = 0.5-100 keV implies the use of active shielding

Total maximum background flux:

B = 110-4 count cm-2 s-1 keV-1


Corso di Strumentazione per l'Astrofisica 21/05/08 Simbol-X : the passive shielding

Dopt

Ddet


Corso di Strumentazione per l'Astrofisica 21/05/08 Simbol-X : the passive shielding

Dopt

DMSC

Ddet



Simbol-X : the shielding composition

Material ZDensity [g/cm3]

Kα

[keV]

Lead 82 11.35 74.97

Tungsten 74 19.30 59.32

Tantalum 73 16.65 57.5

Material ZDensity [g/cm3]

Kα

[keV]

Tin 50 7.31 25.37

Copper 29 8.96 8.05

Aluminum 13 2.70 1.49

Carbon 6 1.70 0.28


Corso di Strumentazione per l'Astrofisica 21/05/08 Simbol-X : the mass

Collimator mass Skirt mass



…..end?


Corso di Strumentazione per l'Astrofisica 21/05/08 Acronyms:

• FL = focal length

• HPD = half power diameter (angular resolution)

• Refl = reflectivity

• B = background

• CXB = cosmic X-ray background

• BGO = Cristallo di Germanato di Bismuto

• NaI = Ioduro di Sodio


Corso di Strumentazione per l'Astrofisica 21/05/08 Appendix:

• FL = focal length

• HPD = half power diameter (angular resolution)

• Refl = reflectivity

• B = background

• CXB = cosmic X-ray background

• BGO = Cristallo di Germanato di Bismuto

• NaI = Ioduro di Sodio

laboratorio in banda x v. fioretti - inaf/iasf bologna the simbol-x mission and the background...

Documents

banda x

kev slide

simbolx mission

passive shield slide

mcrab slide

background rejection

plastic slide

hadronic component active