Summary:
Neutron sources are an essential component for the detection and identification of nuclear materials, mineral and petroleum exploration, nuclear reactor analysis, and nuclear waste monitoring. Neutron spectroscopy techniques allow for a nondestructive and remote measurement of these materials [1]. Conventional sources are radionucleides such as 252Cf . However, these isotopes do not lend themselves to pulsed neutron applications and present many safety issues. Accelerator based neutron generators have been developed, but they tend to be large, expensive, and unreliable. The device reported here is capable of being turned on and operated in pulsed neutron mode for the safe and reliable production of neutrons. Our MEMS neutron gun uses an inexpensive ceramic injection micro-molding and metallizing paste process for the batch fabrication of many neutron guns with standard industrial fabrication processes.
Background:
Currently, the most common neutron source is a 252Cf sample. These sources are highly radioactive and toxic and would be a perfect ingredient in a dirty bomb. As a consequence, the department of energy plans to take them off the market for commercial use. Accelerator based neutron sources have many advantages to address this problem. They rely on accelerating deuterium ions into deuteriated targets where a fusion reaction produces neutrons (Fig. 1). These tube based sources are commercially available [2,3]. However, the tube based sources cost hundreds of thousands of dollars and are extremely fragile not lending themselves to under sea oil field exploration. A micro-fabricated neutron source would replace the toxic califorinium, would be appreciably less expensive, more durable, and small enough to be placed under sea submarine drones. They would also lend themselves better towards covert elicit nuclear material as their smaller size lend them towards concealment. We have produced a small, accelerator based neutron source using a new ceramic/metal bonding micro-fabrication process.
Neutron Gun Physics:
Neutron generators operate by first ionizing deuterium gas (2H) with a penning trap and accelerating these charged ions into a deuteriated or tritiated (3H) metal hydride target to produce neutrons with energy 2.5 MeV or 14.1 MeV (Fig. 2). Our neutron gun is hermetically packaged using alumina ceramics which are inexpensive, can be cast into micro-structures, and are ideal for high voltage packaging (Fig. 3).
Fabrication:
An injection mold for the ceramic micro-casting is micro-milled in teflon (Fig.4). A nano-composite alumina ceramic (Al2O3) mixture is then poured, cured, and released from the teflon mold producing the top and bottom substrates. A Ag and organic binder paste is painted onto the ceramic surfaces and fired at 9500C to activate the ceramic surface for bonding (Fig. 5). Feed-through holes are milled in the top ceramic substrate to allow for electrical connections to the penning trap and target. Finally, the top and bottom ceramic substrate are solder bonded along metallized surfaces in a vacuum to hermetically seal the neutron generator.
Experimental Results:
Ceramic devices were packaged in a helium gas ambient so that helium leak tests could be performed to determine their ability to retain fill gases and hold vacuum seals. Under a range of temperatures, the devices’ leak rates increased only slightly with temperature, remaining low for extended neutron gun operation (Fig. 6). Confined micro-plasmas were observed by operating the penning trap under vacuum. Ionized gas particles are shown fluorescing in a circular orbit (Fig. 7). Tensile strength tests show that the ceramic/metal package provides the durability required for a mobile neutron gun that can be used on site (Fig. 8). Characteristic pulses were detected by operating the neutron gun under vacuum with the penning trap biased at 1800Vdc and the target biased at -20KVdc. Neutron scintillator coupled with a photomultiplier tube allowed for the detection of these pulses (Fig. 9).
