1. 8 th World Conference on Neutron Radiography  Gaithersburg, MD USA  October 19 2006 Imaging of Neutron Fields with Submicron Resolution R. Gregory Downing Inorganic Chemical Metrology Group Analytical Chemistry Division Chemical Science and Technology Laboratory National Institute of Standards and Technology

2. 8 th World Conference on Neutron Radiography  Gaithersburg, MD USA  October 19 2006 The Challenge: develop a new concept for neutron radiography detectors An imaging technique that would … break the 10 micrometer spatial resolution barrier

3. 8 th World Conference on Neutron Radiography  Gaithersburg, MD USA  October 19 2006  basically limited by the size of the electron charge cloud produced following a neutron-atom reaction… Existing Technologies: Phosphors  light cones Pixels  Capacitance well size Gaseous amplifiers  electron clouds

4. 8 th World Conference on Neutron Radiography  Gaithersburg, MD USA  October 19 2006 a paradigm shift in how we capture the information on where the neutron reaction occur! What really is required is…

5. 8 th World Conference on Neutron Radiography  Gaithersburg, MD USA  October 19 2006 n E = 2054 keV 6 Li E = 2727 keV 4 He 3H3H 3 He5333b 3 He(n,p) 3 H 6 Li940b 6 Li(n, 3 H) 4 He 10 B3837b 10 B(n, 4 He) 7 Li xxx Gd >>10000b xxx Gd(n,  e) xxy Gd Particle emission is diametrical

6. 8 th World Conference on Neutron Radiography  Gaithersburg, MD USA  October 19 2006 Neutron Converter Encoder Encoder Time-of-Flight (ToF) Coincidence Neutron Beam

7. 8 th World Conference on Neutron Radiography  Gaithersburg, MD USA  October 19 2006 The reaction gives a unique coordinate solution Known: Mass of each particle Initial energy of each particle Stopping power of converter Stopping rate for each particle is different Measure: The unique time of flight (ToF) for each particle pair Two PSD encoders establish the x-y coordinates for each pair Calculate: TOF  Residual energy for each particle pair  unique depth (x) of each reaction Position sensitive encoder establishes a unique (y,z) position for the reaction Variation in time/energy/stopping power/x-y position give spatial uncertainty List mode output Impose conditions: Min./Max. delta time window for the coincidence pair Line segment must pass through detector volume Particle pair must yield a unique depth A Jacobian Transformation defines unique angular emission & confirms measured angle t1t1 t2t2

8. 8 th World Conference on Neutron Radiography  Gaithersburg, MD USA  October 19 2006 Charged particle ranges in the converter material for the 6 Li(n, 3 H) 4 He reaction defines the optimal thickness ParticleEnergy (keV) 3 H 2727.877 4 He 2055.515 3 H 4 He LiF 33.456.07 Aluminum 37.946.73 Silicon 42.957.51 Lithium 132.8523.29 Max. Range (micrometers) Substrate Gold1002 Å1060Åfor 45/22.5 keV

9. 8 th World Conference on Neutron Radiography  Gaithersburg, MD USA  October 19 2006

10. High-speed encoder electronics n 3H3H 4 He 3H3H Li - rich converter 6 Li(n, 3 H) 4 He Assume a 5  m thickness of LiF, Encoders are 5 cm away from LiF Then the particle TOF differences are either 118 ns (T/  or 10 ns (  /T) Thus for 50 nm position resolution in the converter thickness dimension: The timing resolution required is varies from 5.6 ns to 0.22 ns Resolution along length of a plate converter is < 40  m

11. 8 th World Conference on Neutron Radiography  Gaithersburg, MD USA  October 19 2006 Encoder response time to an MCP electron pulse

12. 8 th World Conference on Neutron Radiography  Gaithersburg, MD USA  October 19 2006 Example of an encoder - > 45 mm x 45 mm active area Therefore the converter must be thin (few  m), but can be several cm in length, giving good solid angle coverage to collect charged particles emerging from the converter.

13. 8 th World Conference on Neutron Radiography  Gaithersburg, MD USA  October 19 2006 Encoder shown mounted MCP electron amplifier

14. 8 th World Conference on Neutron Radiography  Gaithersburg, MD USA  October 19 2006 … but a progressive loss of resolution due to L/D beam broadening Off-set stacking to gain increased imaging area … Top view Side view

15. 8 th World Conference on Neutron Radiography  Gaithersburg, MD USA  October 19 2006 Potential Error Sources: Beam Divergence Surface Roughness

16. 8 th World Conference on Neutron Radiography  Gaithersburg, MD USA  October 19 2006 Sample for Study Intense Neutron Beam Detector System Resulting Real-Time High-Resolution Radiograph

17. 8 th World Conference on Neutron Radiography  Gaithersburg, MD USA  October 19 2006 into screen is the direction of neutron travel An alternative converter geometry leading to more precise X-Y definition? Encoder Cylindrical Converter

18. 8 th World Conference on Neutron Radiography  Gaithersburg, MD USA  October 19 2006 Top View Alternative Converter Concepts with Increased Area Side View

19. 8 th World Conference on Neutron Radiography  Gaithersburg, MD USA  October 19 2006 Weaknesses: Accuracy of stopping power tabulations Getting sufficient neutrons on the detector to take full advantage of resolution! Challenges: Alignment of detector with beam direction Alignment of sample with detector Good quality material and precise production of the converter shape

20. 8 th World Conference on Neutron Radiography  Gaithersburg, MD USA  October 19 2006 Strengths: Resolution in one dimension: 0.1 to << 0.01  m Resolution in 2 nd dimension: > 1) 40  m or 2) << 0.01  m Speed – Real-time image building Reasonable coverage area  3 orders of magnitude Continuous coverage – virtually nonpixilated detector Position uncertainty determined with each data point Encoders are out of the neutron/gamma beam. Scanning of sample by moving it over the converter and time tagging each event with the known sample location.

21. 8 th World Conference on Neutron Radiography  Gaithersburg, MD USA  October 19 2006 Extra Slides

22. 8 th World Conference on Neutron Radiography  Gaithersburg, MD USA  October 19 2006 Sum along the length of the converter to increase statistics while at high resolution for 1D studies. Also gives realistic possibility for real-time analysis Single or few cell studies in biological specimens Grain boundaries in geological, metallurgical or biological specimens Potential applications

23. 8 th World Conference on Neutron Radiography  Gaithersburg, MD USA  October 19 2006 Channel blocking Random Channeling Charged-Particle Channeling Possible measured energy variations due to converter crystal structure