1. DEVELOPMENT OF PIXELLATED SEMICONDUCTOR DETECTORS FOR NEUTRON DETECTION Prof. Christer Fröjdh Mid Sweden University

3. THE MEDIPIX COLLABORATION Development of hybrid pixel detectors for single photon processing Coordinated by CERN Around 20 member institutes MEDIPIX1 64 x 64 pixels, 170 um MEDIPIX2 256 x 256 pixels, 55 um Window mode MEDIPIX3 256 x 256 pixels, 55 um Spectral mode Charge summing

4. MEDIPIX2 Sensor materials Si GaAs CdTe … Response from a CdTe detector connected to a MEDIPIX2

5. DETECTOR RESEARCH AT MIUN Detectors for photons, neutrons and charged particles Simulation Processing Characterization Applications Spectral imaging Phase contrast imaging Radiation detection and characterization - security

6. NEUTRON DETECTION

7. PROJECT PROPOSAL Proposal to develop semiconductor position sensitive pixellated neutron detectors for future ESS applications Proposers: Mid Sweden University, Sundsvall, Sweden ACREO AB, Kista, Sweden Institute of Experimental and Applied Physics (IEAP), CTU in Prague, Czech Republic Nuclear Physics Institute (NPI) AS CR, Czech Republic Collaborator: Paul Scherrer Institute (PSI) Villigen, Switzerland

8. OBJECTIVES The aim of the project is to investigate different options for high performance neutron detectors and to develop reliable processes for production of such detectors with following parameters: Position sensitive slow (resonance, thermal, cold, ultra-cold) neutron detectors with high micrometric spatial resolution (1 ‑ 3 µm). Slow neutron detectors with high quantum efficiency for neutron imaging and radiation monitoring. Investigate options for position sensitive neutron detectors for Time-of-Flight applications with medium spatial resolution (approximately 20 µm) and time resolution on the level of 20 ns adaptable to flight times up to ms. These detectors should be optimized to medium/high efficiency according to demands on position resolution

9. BASIC TECHNOLOGY The TIMEPIX readout chip Single photon processing 256 x 256 pixels 55 x 55 um 2 pixel size Special readout modes Time over threshold Time of arrival ToT and ToA require low flux Pulse height Pulse Threshold

10. HIGH MICROMETRIC SPATIAL RESOLUTION

11. THE DETECTOR MODULE Si sensor Converter layer Bump bonding Readout chip

12. POSITION RESOLUTION Use centroiding to calculate position Gaussian shape Sub pixel resolution Improve resolution by reducing the pixel size in future readout chips

13. IMPROVED SELECTIVITY A dE/E structure can be used to improve selectivity. Only hits in coincidence are counted. (The dE sensor is not pixellated) Si sensor Converter layer Bump bonding Readout chip dE E

14. HIGH QUANTUM EFFICIENCY

15. THE STUFFED DETECTOR Pores are etched in the silicon sensor and filled with a neutron converter.

16. IMPROVED EFFICIENCY

17. WHAT CONTROLS EFFICIENCY ? Pore size Number of particles reaching the active volume of the sensor Energy of the particles when they reach the sensor Wall thickness Mechanical stability – control over the etching process Amount of energy deposited in the wall from each particle Electrical field inside the wall Summary For sufficiently thick pores it is mainly a geometrical factor If energy discrimination has to be used to improve selectivity then narrower pores and thicker walls are needed

18. SPECTRUM AS A FUNCTION OF WALL THICKNESS Pore diameter is 8 um, LiF filling

19. PROCESSING Pore etching in Si is a mature technology DRIE Photochemical etching Sidewall processing in critical Low leakage current Resistant to the neutron converter Other challenges Bump bonding Large area structures…

20. TIME RESOLUTION ON THE LEVEL OF 20 NS

21. TIME RESOLUTION Current solution Use TIMEPIX in ToA-mode Dynamic range limited by counter length (11810 counts) Resolution limited by maximum clock frequency (and time walk) The next generation: TIMEPIX3 Simultaneous ToT and ToA in the pixel Increased counter capacity Clock frequency? The maximum clock frequency depends on noise and heat dissipation Expected to be available in about two years

22. COLLABORATION Development of detectors for specific applications Characterization of detectors and sensor materials