In a truly scintillating set of experiments, scientists at NIST and the University of Maryland have demonstrated that a process called excimer* scintillation can be controlled and characterized precisely enough to serve the pressing national need to detect neutrons with high efficiency.
This animation depicts the progressive stages in END systems. (1) A neutron strikes a boron atom, which undergoes fission, producing an alpha particle and a lithium ion. (2) Those reaction products excite surrounding atoms of noble gas, which form temporary molecules called excimers. (3) As the excimer atoms fall back to their ground state, far-ultraviolet photons are emitted. The photons signal the presence of a neutron.
Detecting neutrons emitted by radioactive materials is of critical importance to homeland security and counter-terrorism activities, such as screening cargo containers, as well as other vital applications in nuclear power instrumentation, workplace safety, and industry. Not surprisingly, demand for detectors has risen dramatically over the past decade.
Nearly all existing detectors rely on a Geiger counter-like cylinder filled with helium-3 (3He), a rare isotope that exhibits a distinctive reaction when struck by a neutron. Historically, the sole source of 3He has been the radioactive decay of tritium, a key component in nuclear weapons. As production of these weapons has declined over the past three decades, so has the inventory of tritium – and hence 3He. The shortage has prompted a worldwide search for another method or material to fill the gap.**
That is a formidable challenge. "Detection of low-energy neutrons is called 'the dismal science' by many of its practitioners for good reason," says Charles Clark of the PML's Quantum Measurement Division. "Neutrons are, of course, electrically neutral. You can't accelerate them, and they don't generally have enough energy to excite anything. Fortunately, one way they can be detected is when they prompt a fission reaction and the reaction products interact with a surrounding noble gas."
For the past several years, Clark, Michael Coplan of the University of Maryland's Institute for Physical Science and Technology, and colleagues have been investigating a process called excimer-based neutron detection (END): The use of excimer scintillation – the emission of radiation from short-lived, exotic molecules created when a noble gas is excited by fission products – as an appropriately sensitive, accurate, and repeatable measure of incident neutrons.
Few previous studies have reported absolute numbers of photons emitted per fission event, and the data from those studies are inconsistent. The NIST work, reported in the Journal of Applied Physics , brings a new degree of experimental control and repeatability that is available in the United States only at NIST. There, the scientists were able to draw on the resources of the NIST Center for Neutron Research (NCNR), the Synchrotron Ultraviolet Radiation Facility (SURF III), and the Center for Nanoscale Science and Technology (CNST). The Maryland University Training Reactor (MUTR) was also used in the course of this work.
"It is very important that we know precisely and very accurately every element of the experiment," says Coplan. "That extends down to the wavelengths of the neutrons, the neutron flux, the precise dimensions of the boron film, and a number of other variables, and our instruments have to be calibrated to the minimum achievable uncertainty."
The action (see animation above right) takes place in a 7 cm square hollow reaction cell with a passage for the incoming neutron beam and windows for the outgoing scintillation light. The neutron target is a thin film of boron-10 (10B) – one of just a handful of materials suitable for low-energy neutron fission reactions – surrounded by a volume of ultrapure argon, xenon, or krypton gas.
When a neutron strikes a 10B atom, the atom splits into a positively charged alpha particle (two protons and two neutrons) and a lithium-7 (7Li) ion, and a gamma ray. The fission products have approximately 2.5 million electron volts of kinetic energy that is deposited in the gas through excitation and ionization.
When a noble-gas atom is thus excited, it is temporarily capable of binding to an adjacent, unexcited atom – something that cannot occur in the gas' normal state. The resulting excimer molecule dissociates on a time scale of nanoseconds), releasing far ultraviolet (FUV) photons with wavelengths in the range of 120 nm to 200 nm, depending on the gas. The FUV photons then travel to a photomultiplier, where they produce an electrical charge that is amplified, recorded, and stored for analysis.
"From the carefully calibrated neutron beam, we detect a few neutrons per second," Coplan says, "and for every neutron captured, we get tens of thousands of photons. That's an extraordinary signal gain." Indeed, about 30 percent of the reaction energy is channeled into the FUV emissions.
"What we're trying to do right now is establish absolute numbers against which we can measure anything," Clark says. "We're in this business to improve the accuracy of primary measurements." Their conclusion was that excimer-based neutron detection is a viable option for practical detectors.
The researchers experimented with four different thicknesses of boron films (fabricated at CNST), weighing trade-offs: As films get thicker (up to 1200 nm thick), they capture a larger number of neutrons, but the fission products have more trouble getting out, losing energy before reaching the gas. The scientists also examined the effect of different gas pressures on scintillation rate, which increases with pressure up to about 80 kPa, after which photon count rates are relatively constant.
In the next stage of work, the researchers will explore what occurs when the boron is more symmetrically arranged within the chamber, and reaction products have more freedom to travel isotropically. They obtained three-dimensional samples of "carbon foam" that has been coated with boron carbide (see photo at left). Sample densities vary from 10 to 40 pores per inch. Carbon has almost no interaction with neutrons and the open structure of the foam allows the emerging fission products to interact more efficiently with the surrounding noble gas, circumventing some of the ion-trapping limitations of the thin-film configuration.
This part of the project is pursued in collaboration with the Johns Hopkins University Applied Physics Laboratory (APL). APL scientists Chris Lavelle and Ryan Deacon were the lead authors of "Characterization of boron coated vitreous carbon foam for neutron detection" (2013); Daniel Hussey of NCNR, Clark and Coplan were co-authors.
"We're also looking at ways to convert the FUV photons to a wavelength compatible with photomultiplier tubes that have much greater efficiency than the one we're using now," Coplan says. "Eventually, we may want to use solid state silicon photomultipliers."
Because the work requires high-precision contributions from fields as diverse as neutron beam control, ultraviolet measurement, and nanofabrication, Clark says, NIST is the ideal place for the project.
"At NIST," he says, "you can integrate these very different modalities into one package. We have the expertise in the FUV and world-class calibration resources at SURF III. We have expertise in neutron dosimetry and an excellent source of cold neutrons at the NCNR. And we have expertise in fabrication of thin films at the CNST."
NIST's collaboration with the University of Maryland also contributed to the University's educational mission. Jacob McComb, the first author on the Journal of Applied Physics paper, received a Ph.D. in Nuclear Engineering for his work on this project, and is now employed as a nuclear engineer by the Defense Nuclear Facilities Safety Board.
* An excimer (excited dimer) is a short-lived molecule formed from two atoms, one of which has its valence shell completely filled with electrons. (e.g., a noble gas.) When one atom is sufficiently excited, it is boosted to a high-energy condition that allows it to form a dimer with an adjacent atom. As the excited molecule decays back to its ground state, an ultraviolet photon is released and the two atoms of the molecule dissociate. This process is the basis for excimer lasers used in microelectronics manufacturing.
** For more information, see the Government Accountability Office Technology Assessment titled Neutron Detectors: Alternatives to Using Helium-3.
Reference: "Noble gas excimer scintillation following neutron capture in boron thin films," Jacob C. McComb, Michael A. Coplan, Mohamad Al-Sheikhly, Alan K. Thompson, Robert E. Vest and Charles W. Clark, J. Appl. Phys. 115, 144504 (April 14, 2014)