1. RAD2012 Ultra-Thin 3D detectors Celeste FletaInstituto de Microelectrónica de Barcelona Celeste Fleta Instituto de Microelectrónica de Barcelona Centro Nacional de Microelectrónica - CSIC Spain Celeste.Fleta@csic.es Ultra-Thin 3D Silicon Detectors for Active Neutron Detection

2. RAD2012 Ultra-Thin 3D detectors Celeste FletaInstituto de Microelectrónica de Barcelona Motivation Radiotherapy linacs  Working at > 8MeV generate neutrons by fotonuclear reaction  Pulsated radiation  Usually dosemeters  passive  Real time counting and high gamma rejection factor required Other applications: nuclear security, avionics, space, …

3. RAD2012 Ultra-Thin 3D detectors Celeste FletaInstituto de Microelectrónica de Barcelona Neutron detection with silicon sensors Silicon radiation sensors  Low bias (few V)  Low battery consumption for transportability  Compact: small sizes (mm) and weight (g)  Resistant to shock and inmune to magnetic fields  Fast response (ns) Adaptation for neutron detection  Converter with high thermal neutron cross section  Reaction products with enough energy to reach the detector  Will use 10B-based compounds t I particle Maximum detection efficiency 4.7%

4. RAD2012 Ultra-Thin 3D detectors Celeste FletaInstituto de Microelectrónica de Barcelona Traditional “planar PIN” silicon sensors  Thickness: 300 – 1000 µm  Depletion voltage: 80-200 V Low charge collection time: ns  High γ-ray absorption probability Standard sensors  Thickness: 10-20 µm Depletion voltage < 10 V Low charge collection time: ns Low γ-ray absorption  High capacitance and electronic noise Thin sensors gamma rejection/capacitance trade-off

5. RAD2012 Ultra-Thin 3D detectors Celeste FletaInstituto de Microelectrónica de Barcelona New “Ultra-thin 3D” silicon sensors 3D is advantageous if thickness < 50 µm  Thickness: 10-20 µm  Columnar electrodes passing through substrate Depletion voltage: few V Low charge collection time: ns Low γ-ray absorption Resistant to radiation damage Capacitance lower than the planar equivalent

6. RAD2012 Ultra-Thin 3D detectors Celeste FletaInstituto de Microelectrónica de Barcelona Design and fabrication  Design and fabrication done in-house  0.5 cm 2 active area, window <400nm SiO 2  Electrode fabrication: 1. ICP etching of the holes: ALCATEL 601-E 2. Holes partially filled with LPCVD polysilicon 3. Holes doped with P or B 4. Holes passivated with TEOS SiO2 Electrodes: 5µm diameter, 10µm deep SOI wafer 10µm active thickness Detail of a sensor design n-contact n-holes connected together with thin metal lines p-contact on other side p-holes

7. RAD2012 Ultra-Thin 3D detectors Celeste FletaInstituto de Microelectrónica de Barcelona In pictures

8. RAD2012 Ultra-Thin 3D detectors Celeste FletaInstituto de Microelectrónica de Barcelona Electrical test  Sensors depleted at ~5 volts  50-70 nA/cm 2 at 10V  60-80 pF/cm 2 Capacitance vs. voltageCurrent vs. voltage

9. RAD2012 Ultra-Thin 3D detectors Celeste FletaInstituto de Microelectrónica de Barcelona Electronics The detector is mounted on a separated board to allow testing different detectors with the same system  Compact (50g, 13x2.5cm)  Fast (80000 counts/s)  Cheap (<100€)  towards a portable system

10. RAD2012 Ultra-Thin 3D detectors Celeste FletaInstituto de Microelectrónica de Barcelona 137 Cs 10µm sensor  Threshold = 100keV  1 count/10 min  1 gamma counted every 10 8 300µm sensor  29.0±0.2 c/s 90 mCi (2.8x10 9 γ/s in 4π) 1m distance Lab source tests 241 Am-Be + 137 Cs 40 mCi AmBe (88000 n/s) 8 mCi 137 Cs 10cm polyethilene 800µm sensor  204.0±0.6 c/s

11. RAD2012 Ultra-Thin 3D detectors Celeste FletaInstituto de Microelectrónica de Barcelona Hospital tests sensor  Elekta Synergy (Hospital General de Catalunya) and Siemens Primus (Hospital de Santiago de Compostela)  6MV (γ only) and 15MV (γ + n)  10x10 cm 2 field  50 to 500 MU/min* *MU: a Monitor Unit is a measure of the machine output of a linac which is calibrated to deliver an absorbed dose under particular conditions, e.g. 100 MU gives 1 Gray in water at 100 cm SSD for a 10x10 cm 2 field

12. RAD2012 Ultra-Thin 3D detectors Celeste FletaInstituto de Microelectrónica de Barcelona Hospital tests  No pile-up counts observed  At 6 MV (γ only): 3.8 counts/min  1E6 γ/cm 2 s  Gamma rejection factor: 2x10 -9  At 15 MV: γ/n counts = 0.002 (sensor without/with H 3 10 BO 3 ) Fixed rate: Elekta Synergy, 400MU/min H 3 10 BO 3

13. RAD2012 Ultra-Thin 3D detectors Celeste FletaInstituto de Microelectrónica de Barcelona Hospital tests Variable rates: Siemens Primus, 15 MV  Linear response: no pile-up up to 500 MU/min 50 MU/min: 4010±100 counts in 10 minutes 500 MU/min: 4062±48 counts in 1 minute  γ/n rate = 0.02 System works well in pulsated gamma/neutron environment H 3 10 BO 3

14. RAD2012 Ultra-Thin 3D detectors Celeste FletaInstituto de Microelectrónica de Barcelona Summary and outlook Summary  Innovative ultra-thin 3D silicon sensors with 10 µm thickness have been successfully fabricated and adapted to detect neutrons with 10 B-enriched compounds.  The detectors show a gamma rejection factor higher than 10 -8 for 137 Cs for a threshold of 100 keV, and 2x10 -9 in a radiotherapy field.  Preliminary tests of the detectors in clinical linacs show their usefulness in these complex gamma-neutron pulsated radiation fields: low gamma count rate, linear up to at least 500 MU/min. Ongoing work  Working to integrate the system in a fully portable dosemeter.  Developing 10 B- based converter deposition tecniques.  Working to obtain absolute efficiencies/gamma rejection factors with calibrated sources.  Developing microstructured detectors for higher neutron detection efficiency. C. Guardiola et al., “Ultra-thin 3D silicon sensors for neutron detection”, 2012 JINST 7 P03006

15. RAD2012 Ultra-Thin 3D detectors Celeste FletaInstituto de Microelectrónica de Barcelona Thanks for your attention! Full author list: C. Fleta, C. Guardiola, D. Quirion, J. Rodríguez, G. Pellegrini, J.P Balbuena, M. Lozano Instituto de Microelectrónica de Barcelona, Barcelona, Spain F. Gómez, X. González, D. González, J. Pardo Universidad de Santiago de Compostela, Santiago de Compostela, Spain F. García Helsinki Institute of Physics, University of Helsinki, Helsinki, Finland