1. E.Costa – Anticoincidence for NHXM Frascati 12-13 November 2009
Anticoincidence for a the New Hard X-ray Imaging and Polarimetric Satellite Mission: possible solution and a few firm points
Enrico Costa IASF-INAF Roma
Presentation at the Meeting on November
At ESRIN Frascati
E.Costa – Anticoincidence for NHXM Frascati November 2009

2. Need for Anticoincidence?
This is one of the points to be assessed.
GOALS:
To minimize the background
To minimize the background modulation
To minimize the background anisotropy
Trade-off ingredients:
Dead Time
Weight
[Power]
E.Costa – Anticoincidence for NHXM Frascati November 2009

3. Sources to drive the design
Simulations
Experience from previous missions
Testing on ground
Major Inpacts on design
Design (i.e. choice of materials and thicknesses) of Shields and Anticoincidence
Design of Baffle
Mission analysis and profile
E.Costa – Anticoincidence for NHXM Frascati November 2009

4. E.Costa – Anticoincidence for NHXM Frascati 12-13 November 2009
Source of Background
Prompt Background
Charged Particles: Primary CR, Splash Albedo electrons. Mainly rejected by Pulse Height
Neutrons: elastic scattering
Photons:
CXB
Earth Albedo
Secondary Products of CR interaction with S/C
Delaied Background
Activation of Detectors at the SAA passage
E.Costa – Anticoincidence for NHXM Frascati November 2009

5. E.Costa – Anticoincidence for NHXM Frascati 12-13 November 2009
SIMBOL-X
SIMBOL-X preliminary Design
E.Costa – Anticoincidence for NHXM Frascati November 2009

6. Simulations (chipaux 2008)
Simulations performed for SIMBOL-X showed that the proton component is prevailing. Passive shielding and plastic scintillator as AC seems the best choice.
E.Costa – Anticoincidence for NHXM Frascati November 2009

7. E.Costa – Anticoincidence for NHXM Frascati 12-13 November 2009
The main stream
Hard X-Ray Astronomy in the balloon era evolved to the base-line configuration of thin scintillator detector with an active shintillator AC (possibly in the Phoswich configuration).
Frontera (1985) flew a balloon payload including detectors with passive shielding (+ plastic AC) and phoswich detectors of the same size. The latter had a background rate 5 times lower. This experiment supported by simulations based on atmospheric and cosmic X/γ ray BKG and on accelerator testing of activations has driven the design of SAX/PDS.
All narrow field hard X-ray detectors have been based, more or less, on active inorganic AC shielding: OSO7, OSO8, HEAO-1/A4, SAX, SUZAKU.
E.Costa – Anticoincidence for NHXM Frascati November 2009

8. E.Costa – Anticoincidence for NHXM Frascati 12-13 November 2009
The Others: Nustar
Active anticoincidenceof CsI 1.5 cm thick (≈7 g/cm2).
Also acting as baffle
E.Costa – Anticoincidence for NHXM Frascati November 2009

9. The others: Astro-H HXI
The Low energy Imager and the High Energy Imager are surrounded with a thick BGO shield (4 cm ≈ 30 g/cm2) read with APDs (following Suzaku experience).
E.Costa – Anticoincidence for NHXM Frascati November 2009

10. E.Costa – Anticoincidence for NHXM Frascati 12-13 November 2009
AC and LEO
Do not forget that in LEO:
Photon spectrum is much more relevant. In fact the Earth is occulting a high fraction of CXB but is contributing a strong albedo. The latter above keV prevails on the former.
On the contrary CR protons are cut-off by earth magnetic field. Low energy protons of solar origin are almost absent. High energy particles are, in the large majority, rejected by pulse height and impact on BKG through γ’s produced in the satellite structure.
In LEO (as in balloons) the photonic component, dominated by earth albedo, is the principal source of BKG.
This can be substantially suppressed by anti-coincidence. Inorganic scintillators are the best candidates because of high light output and because combine with a good total absorption.
E.Costa – Anticoincidence for NHXM Frascati November 2009

11. E.Costa – Anticoincidence for NHXM Frascati 12-13 November 2009
Choice of AC material
CsI: is the baseline.
Advantages: Good light yield. High Z. Easy to manufacture.
Disadvantages: Phosphorescence. Flurescence in the middle of the band.
BGO: the most fashionable so far:
Advantages: High density. High Z (good photoabsorption).
Disadvantages: Poor light yield (high threshold). Flurescence in the middle of the band
LaBr3: The new wave.
Advantages: High Light yield, High Z.
Disadvantage: Radioactivity
More ….. TBV
Choice of read out device
PMT’s
Vantages: robust, reliable (new models)
Disadvantages: HV (high weight)
APD:
Vantages: light
Disadvantages: stability
SiPMT: a more interesting variant with low voltage and better tability
SDD
Vantages: light low consumption gain independent on HV
Disadvantages: readiness
E.Costa – Anticoincidence for NHXM Frascati November 2009

12. Interference AC - imager
Anticoincidence must be effective, light, leak tight
But it surrounds the two imagers , tightly packed, that have serious problems of:
Thermal control (very serious for CCDs)
Cable routing
Mechanical mounting
The design of AC will follow the design of the imager and will be taylored on it
E.Costa – Anticoincidence for NHXM Frascati November 2009

13. E.Costa – Anticoincidence for NHXM Frascati 12-13 November 2009
A few firm points
An anticoincidence is needed for the High Energy Detector.
The need for anticoincidence for the Low Energy Detector is to be confirmed. Only some configuration is compatible (DePFET, CDD).
Anticoincidence must be combined with passive shielding.
From experience anticoincidence must include high Z inorganic detector.
Anticoincidence/shield is tightly connected with baffling.
Anticoincidence will be taylored to imager that will drive the design.
E.Costa – Anticoincidence for NHXM Frascati November 2009