8 th World Conference on Neutron Radiography  Gaithersburg, MD USA  October 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

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

8 th World Conference on Neutron Radiography  Gaithersburg, MD USA  October  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

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

8 th World Conference on Neutron Radiography  Gaithersburg, MD USA  October 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

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

8 th World Conference on Neutron Radiography  Gaithersburg, MD USA  October 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 th World Conference on Neutron Radiography  Gaithersburg, MD USA  October 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 He H 4 He LiF Aluminum Silicon Lithium Max. Range (micrometers) Substrate Gold1002 Å1060Åfor 45/22.5 keV

8 th World Conference on Neutron Radiography  Gaithersburg, MD USA  October

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

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

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

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

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

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

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

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

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

8 th World Conference on Neutron Radiography  Gaithersburg, MD USA  October 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

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

8 th World Conference on Neutron Radiography  Gaithersburg, MD USA  October Extra Slides

8 th World Conference on Neutron Radiography  Gaithersburg, MD USA  October 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

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