Celeste Fleta Instituto de Microelectrónica de Barcelona

1. 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. Motivation Radiotherapy linacs • Usually dosemeters  passive • Real time counting and high gamma rejection factor required • Working at > 8MeV generate neutrons by fotonuclear reaction • Pulsated radiation Other applications: nuclear security, avionics, space, …

3. Neutron detection with silicon sensors particle I t Silicon radiation sensors 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 • 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) Maximum detection efficiency 4.7%

4. Traditional “planar PIN” silicon sensors Standard sensors • Thickness: 300 – 1000 µm • Depletion voltage: 80-200 V • Low charge collection time: ns • High γ-ray absorption probability Thin sensors • Thickness: 10-20 µm • Depletion voltage < 10 V • Low charge collection time: ns • Low γ-ray absorption • High capacitance and electronic noise gamma rejection/capacitance trade-off

5. New “Ultra-thin 3D” silicon sensors • 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 3D is advantageous if thickness < 50 µm

6. Design and fabrication Detail of a sensor design n-contact • Design and fabrication done in-house • 0.5 cm2 active area, window <400nm SiO2 • Electrode fabrication: • ICP etching of the holes: ALCATEL 601-E • Holes partially filled with LPCVD polysilicon • Holes doped with P or B • Holes passivated with TEOS SiO2 Electrodes: 5µm diameter, 10µm deep SOI wafer 10µm active thickness n-holes connected together with thin metal lines p-holes p-contact on other side

7. In pictures

8. Electrical test Capacitance vs. voltage Current vs. voltage • Sensors depleted at ~5 volts • 50-70 nA/cm2 at 10V • 60-80 pF/cm2

9. Electronics • Compact (50g, 13x2.5cm) • Fast (80000 counts/s) • Cheap (<100€)  towards a portable system The detector is mounted on a separated board to allow testing different detectors with the same system

10. Lab source tests 137Cs 10µm sensor • Threshold = 100keV • 1 count/10 min • 1 gamma counted every 108 300µm sensor • 29.0±0.2 c/s 90 mCi (2.8x109γ/s in 4π) 1m distance 800µm sensor • 204.0±0.6 c/s 241Am-Be + 137Cs 40 mCi AmBe (88000 n/s) 8 mCi 137Cs 10cm polyethilene

11. Hospital tests • Elekta Synergy (Hospital General de Catalunya) and Siemens Primus (Hospital de Santiago de Compostela) • 6MV (γ only) and 15MV (γ + n) • 10x10 cm2 field • 50 to 500 MU/min* sensor *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 cm2 field

12. Hospital tests Fixed rate: Elekta Synergy, 400MU/min H310BO3 • No pile-up counts observed • At 6 MV (γ only): 3.8 counts/min • 1E6 γ/cm2s  Gamma rejection factor: 2x10-9 • At 15 MV: γ/n counts = 0.002 (sensor without/with H310BO3)

13. Hospital tests Variable rates: Siemens Primus, 15 MV H310BO3 • 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

14. Summary and outlook Summary • Innovative ultra-thin 3D silicon sensors with 10 µm thickness have been successfully fabricated and adapted to detect neutrons with 10B-enriched compounds. • The detectors show a gamma rejection factor higher than 10-8 for 137Cs 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 10B- 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. 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