LDRD Seminar Series: ‘3D Printing of Microchannel Plates for Photon and Neutron Detectors’
Physicist/Instrumentation Group Leader Bob Wagner (HEP) will discuss his Laboratory-Directed Research and Development (LDRD) sponsored work at the LDRD Seminar Series presentation Tuesday, Jan. 23, 2018. “3D Printing of Microchannel Plates for Photon and Neutron Detectors” begins at 12:30 p.m. in the Building 203 Auditorium. All are welcome to attend.
Microchannel plates (MCPs) are used for electron gain multiplication in a variety of applications from mass spectrometers to particle detectors. Traditionally these have been lead glass capillary arrays with millions of pores of size from a few to tens of microns diameter. Firing of the capillary array plate in a hydrogen furnace produced a resistive structure through the pores allowing them to be biased for electron multiplication. A plate of approximately a millimeter thickness can support 1000 volts and provide a gain of 103 – 104. A pair of plates mounted in package under ultra-high vacuum with a photocathode window is the basis for photodetectors that provide time resolution of tens of picoseconds and sub-millimeter spatial resolution. Fabrication of microchannel plates and the lead glass used as a substrate is costly and generally has limited the use of microchannel plates to small areas incorporating a few detectors. In 2009, a collaboration of physicists and materials scientists including members of the High Energy Physics, Energy Systems, Materials Science, and X-ray Science divisions began a program to reduce the cost of microchannel plate photodetectors, improve their performance characteristics, and scale the detectors to large active areas of hundreds of cm2. By changing the capillary substrate to borosilicate glass and replacing functionalization by hydrogen firing with resistive and emissive coatings applied by atomic layer deposition (ALD), the collaboration reduced the cost of the detectors by an order of magnitude, improved to the timing resolution to 30ps, and produced photodetectors with 400 cm2 active area. The project received an RD100 award in 2012 for the work.
During this same time, helium-3 which was used in most thermal neutron detectors was becoming scarce and expensive. Boron-10 was a relatively inexpensive alternative having about 75 percent of helium-3’s efficiency for thermal neutron absorption. However, it is not a gas and incorporating it into large detectors was difficult. The LDRD proposed by the Argonne MCP group envisioned enriching the boron in borosilicate glass with boron-10 and producing microchannel plates capable of absorbing and detecting thermal neutrons. While it ultimately proved too inefficient and expensive to produce the glass, our collaborator, Mike Pellin (MSD), proposed trying to 3D print capillary arrays with enriched boron-10 using a recently acquired 3D printer with sub-micron resolution. 3D printing the capillary arrays proved to be a success and we have used ALD to turn these into MCPs that produce a reasonable gain.
This presentation will discuss the development of 3D printing of capillary arrays via the two photon polymerization technique and the future of the program to scale from centimeter size MCPs to meter size MCPs. The technique allows novel geometry MCPs to be printed. The photoresist which forms the substrate material is inexpensive and the potential exists to reduce the cost of MCPs yet another order of magnitude. Incorporation of boron-10 into the photoresist and fabrication of a “thermal neutron photocathode” will be discussed.
During his 40 years at Argonne, Bob Wagner has worked as an experimenter on a number of particle and astroparticle physics experiments such as the Collider Detector at Fermilab, the VERITAS gamma-ray telescope array, and most recently, the direct muon to electron conversion (Mu2e) experiment at Fermilab. His background in photodetector development led to his selection as the project physicist for the Large Area Picosecond Photodetector collaboration that developed the new generation of MCP photodetectors. He is currently the Detector R&D group leader in the High Energy Physics Division. The group is working to develop detectors to enable future particle and cosmology experiments including development of improved silicon pixel detectors for colliding beam experiments, superconducting detectors for Cosmic Microwave Background observations and optical ring resonator filters for high redshift supernova observations, as well as improving MCP photodetectors and advancing their 3D printing.