Design of multilayer coatedgamma-ray telescopes

Ernst-Jan Buis, M. Beijersbergen, G. Vacanticosine Science & Computing

M.Bavdaz, D. Lumb Office of advanced concepts and payloads

ESTEC-ESA

Content

_ Design of a gamma-ray telescope

_ Software

_ Free parameters

_ Possible implementations

_ Geometry

_ Effective area

_ Modifications

_ Conclusions

Requirements

_ Effective geometrical area of 1 m2

_ Mirror configuration optimized for 158keV and 511 keV

Technology

Design of a gamma ray telescope

_ Nested mirrors

_ Formation flying

_ Thin mirrors in combination with small mirror spacing

_ Multilayer coatings for reflectivity 'beyond' critical angle

Comparison: XEUS

_ Energy range 1-30 keV

_ Focal length 50 m

_ Geometrical area 30 m2

_ Effective area 10 m2@ 1keV

_ Mirror area 3000 m2

_ Mirror thickness 200 !m

_ Mirror spacing 800 !m

_ Launch date > 2015

Detector spacecraft

Mirror spacecraft

Software

_ Software used to design grazing angle opticsis “Xraytracer”

_ “Xraytracer” is modified version of MSIM,which is used for calibration of the XMM-Newton mission

_ Introduced multilayer reflectivity

_ Use optical constants for higher energies

_ “Xraytracer” is now also used to design Xeus

Procedure

1. Initial requirements (effective area, mirrorspacing and thickness);

2. Choose absorber and spacer material;

3. Calculate average reflectivity in an interval oftwo grazing angles for several points inparameter space;

4. Determine the number of nests and theirsizes;

5. Implement geometry in “Xraytracer”;

6. Determine final (on and off axis) effective area.

Multilayer coatingIndex of refraction of the absorber material should differ from unity as much as possible:

For the absorber we have selected: W, Ni and Mothe spacer is C

Gamma ray reflectivity

Free parameters:

_ Grazing angle and 'angular'bandwidth

_ Fractional thickness in bi-layers

Optimal number of layers and fractional thickness done using the methode by Kozhevnikov et al. (2001)

Tuning the layer thickness of each layer to get a flatdistribution of the reflectivity

Estimation of effective area

First approximation of the effective area canbe done using geometric considerations:

effective area(1 m2 required)

aperture utility(70% assumed)

reflectivity(calculated)

aperture

Once the reflectivity is known (between the two angles), the focal length derived from the required effective area.

Scan parameter space

_ For 158 keV no focal length shorter than 180 m.

_ Clear correlation between number of layers in thecoating and focal length.

Scan of the parameter space

Short focal length is achieved with reflectivity over largeangular range, but it needs many layers in the coating toestablish this..

Reflectivities

158 keV

511 keV

Reflectivity (../..)

541 layers W-C 4852 layers Ni-C

Models_ Picked four models:

_ Two models optimized for 158 and two for 511keV

_ Models with low and high number of layers

Model Design Energy Material #layers

I 158 W/C 541

II 158 Ni/C 4852

III 511 W/C 4798

IV 511 Mo/C 1419

Model Focal Length Radii Mirror area Weight

I 372.6 1.64-1.95 977 456

II 189.1 0.13-0.99 1633 761

III 384.6 0.13-0.94 3116 1452

IV 833.5 0.58-0.87 2438 1136

Models layout

Model I: W/C 158 keV

Model II:Ni/C 158 keV

Effective area

Surface roughness included using a Debye-Waller factor.

Off-axis respons

_ Focal plane is not included

_ Off-axis angles correspondto very large distances inthe focal plane:

_ 7 m @ 0.5 deg

_ 3 m @ 0.5 deg

Robustness of the results

Larger energy bandwidth

Total mirror area

997 m2

2720 m2

Smaller telescopes

_ mirror area starting of 120 m2 will give 0.14 m2 effective area_ Focal length above 100 m

Summary and conclusions

_ Software for gamma ray telescope is exists

_ Large number of possible implementationspossible, only four were addressed in this talk

_ Reflectivity can be reached up to 511 keV

_ Multilayer mirrors can have large energybandwidth

_ Effective area of 1 m2 with a mission whichcomparable to XEUS

_ Technology should be directed to highnumber of layers.