Design of CAM & Collimator for the ASTROSAT CZT Imager

1. Design of CAM & Collimator for the ASTROSAT CZT Imager Sushila R. Mishra ASTROSAT Lab Raman Research Institute Bangalore 560 080

2. The CZT Array CZT Detector Plane (nominal) size of 32 cm × 32 cm 64 units of 16 cm2 CZT detector modules For ease of fabrication these 64 units are sub-divided into four identical quadrants consisting of 16 CZT modules each.

3. Detector Board

4. Detector Board The CZT modules are pixellated by design and each module has 256 pixels. Each pixel is of size 2.5mm × 2.5mm and thickness of 5 mm.

5. Background (area : 500 cm2) ENERGY RANGE FLUX Counts for opening angle E1 E2 cm-2 s-1 str-1 3°× 1.5° 6°× 6° keV keV 5 10 4.9 3.5 27.72 10 25 2.4 1.7 13.58 25 50 0.61 0.43 3.45 50 100 0.25 0.18 1.41 46.16 5.81

6. Background (area : 500 cm2) ENERGY RANGE FLUX Counts for opening angle E1 E2 cm-2 s-1 str-1 3°× 1.5° 6°× 6° keV keV 5 10 4.9 3.5 27.72 10 25 2.4 1.7 13.58 25 50 0.61 0.43 3.45 50 100 0.25 0.18 1.41 46.16 5.81 LE :- 3.5 + 1.7 = 5.2 cnt/s HE :- 3.45 + 1.41 = 4.9 cnt/s Combination = 10.1 cnt/s

7. Low Energy Collimator Slats Single unit of the collimator of size 4 cm × 4 cm and height 40 cm. For 6°× 6° FOV, all 4 sides covered by a tantalum sheet of 0.2 mm thickness.

8. Low Energy Collimator Slats Single unit of the collimator of size 4 cm × 4 cm and height 40 cm. For 6°× 6° FOV, all 4 sides covered by a tantalum sheet of 0.2 mm thickness. For 3°× 1.5° FOV, each such unit further sub-divided into eight partsby aluminium sheets of 0.25 mm thickness.

9. Efficiency of CZT Detector as a function of energy

10. Absorption by 0.25 mm thick aluminum sheet at various angles as a function of energy

11. Absorption by 0.2 mm Tantalum sheet at various angles as a function of energy

12. Absorption by 0.5 mm Tantalum (CAM) sheet at various angles as a function of energy

13. Design of Coded Aperture Mask (CAM) Mask pattern of length, n = 2m – 1 = 255 m = 8 Generating Function : Irreducible primitive polynomial of degree m P = p0 x0 + p1 x1 + p2 x2 + ........ + p8 x8 pj's are only 0 or 1 ( j = 0,1,2,........,8 ) The elements of the mask pattern: ai i = 0 , ...... , 254 ai = 1 (open) ai = 0 (closed) First m values of ai, i = 0, 1, ...... , 7 arbitrary rest generated using the shift register algorithm: ai+8 = p0 ai + p1 ai+1 + ........ + p7 ai+7 (mod 2) i = 0, 1, .... , 246

14. Design of Coded Aperture Mask (CAM) Start with all 128 possible polynomials. The mask patterns generated are subjected to Cyclic Autocorrelation. Patterns showing a single peak and flat side-lobes in the Cyclic Autocorrelation are chosen as URAs. 16 mask patterns could be generated. no. of open elements in each pattern = 128 To make 16 × 16 pattern one closed element is appended to the array, which is then wrapped. Transparency = 128 / 256 = 50%

15. Generating Functions

16. CAM patterns generated using 16 × 16 Linear Wrap. Black regions represent open mask area.

17. CAM patterns generated using 16 × 16 Linear Wrap. Black regions represent open mask area.

18. Low Energy Collimators Parallel Case Perpendicular case

19. 1 – D CACF for regions 1 to 8 for mask pattern 8 for parallel and perpendicular divisions.

20. Figure of Merit (FOM) Cyclic Autocorrelation performed for each sub pattern to assess the FOM, defined as where a --> average a --> the standard deviation of the CACF of the 32 element pattern unwrapped into a 1-D array (1 – a) FOM = a

21. Pattern 1 Parallel sub-divisions of pattern 1, 8 and 13 selected. 1 13 8 Parallel Perpendicular 11 10

22. 2 – Dimensional Linear Autocorrelation Function Pattern 1, 8 and 13 are subjected to linear autocorrelation Amplitude k m LACF of Mask Pattern 8

23. Second order Polynomial fit for the LACF as a function of √(k2+m2)

24. Loosely held elements Pattern 1 Pattern 8 Pattern 13