Edward Hartouni (16-FS-004)
In the coming decade, the current state-of-the-art detector systems used to search for the elusive dark matter component comprising 23% of the matter of the universe will reach the noise limit for candidate dark matter masses above 10 GeV/c2 due to background neutrinos. The prospect for searches of lighter-mass candidates rests on as-yet-to-be-invented detector technologies.
In a preprint by Lopez-Suarez et al.1 titled "New dark matter detector using nanoscale explosives," the authors suggest a unique detector technology that would have the required low mass sensitivity and would also scale up to the large detector volumes required to search to the neutrino background limits. This proposed detector concept uses thermite nanoparticles with dimensions “tuned” to contain all of the energy of a recoiling nucleus hit by a dark matter particle. The kinetic energy of the recoil, lost in the slowing down collisional process in the thermite, would result in the increase of the temperature of the nanoparticle exceeding the melting temperature of the material, initiating the exothermic reduction-oxidation reaction between the metal fuel and oxide.
In this feasibility study, we set out to identify the knowledge gaps in our understanding of nanoparticles relevant to thermites. Specifically, we want to know what the controlling parameters are that govern the ignition behavior when a thermite is subjected to radiation, and we want to specify candidate nanoparticle attributes that are relevant to dark matter detection. Once identified, an assessment can be made of the likelihood of success for the class of radiation detectors suggested by Lopez-Suarez for a number of detection applications. This assessment, if positive, would lead to a technology research and development program to inform future directions for this technology.
Background and Research Objectives
Lopez-Suarez et al. propose using radiation-initiated deflagration (combustion that propagates through a gas or across the surface of an explosive at subsonic speeds) of thermites as a tool for detecting dark matter. They also address the requirements for "light mass" dark matter searches (≤10 GeV/c2), the most important being low detection signal threshold with low noise. The solution recognizes the high density of energy deposition expected for nuclei recoiling in a collision with a dark matter particle moving at speeds of hundreds of km/s. This energy is deposited locally and "heats up" the material. Lopez-Suarez et al. calculate the rise in local temperature and point out that it could exceed the melting point of various metals, which is a requirement for initiating thermite redox reactions. They then use the energy deposition length scale to set a thermite granule size of order 1 nm. This size provides sensitivity to dark matter while remaining insensitive to higher energy ionization, which tends to have a much lower energy-density deposition in the granule. Exploiting the characteristics of the high energy-density and limited deposition range is the innovation of this proposal. The use of engineered energetic materials points towards the possibility of constructing large-mass inexpensive detectors required for "next-generation" dark matter searches.
The range of application of this innovative idea seems to be broader than the important scientific issue of the existence of dark matter. Thermites are well-known materials that have been engineered for a large number of applications. Recent interest in thermites has also begun to focus on nanomaterial properties. Thermites have certain attributes that make them a good option for radiation detection: For instance, there's a range of materials that can be used to produce thermites. Also, it is possible to design thermite reaction products, specifically the energetic chemical reaction that follows the reaction's initiation, known as "chemical amplification."
A deeper analysis of the behavior of nano-thermites as radiation detectors reveals the lack of knowledge of the essential underlying science. There is no model of radiation initiation in these materials, at any scale. While anecdotal accounts of radiation initiation of thermite samples in beams of x rays are known, there is only one published paper analyzing the radiation initiation in high explosives. The ionization energy deposited in the material will also diffuse in the material, making an understanding of the thermodynamic properties of the materials an important factor in the calculated temperature rise. The thermodynamics of nanomaterials, a new area of research, has not yet produced a way of calculating the heat flow in these materials, which characteristically exhibit "surface" behavior with little "bulk." These properties also create a challenge for fabricating and characterizing the materials. Finally, nano-thermites are an equally new area of research
In characterizing the desired properties for a chemical amplifier, a set of criteria were developed. Designs that would address and meet these criteria would be productive avenues of future detector development.
The basic features of the Weakly Interacting Massive Particle (WIMP, a leading candidate for dark matter) interaction with ordinary matter are postulated as follows:
- Interactions are assumed to occur with nuclei, not shell electrons.
- The induced recoil energies are on the order of 1 to 100 keV, depositing energy over a small (~0.1 μm) region with relatively high energy density.
- For equivalent energy depositions, non-nuclear interactions deposit energy over much larger distances (>1 μm), and thus lower energy density, potentially permitting a means to distinguish WIMP interactions from one important class of backgrounds.
This study developed a set of desired properties that the chemical amplifiers would have to meet to be competitive with ongoing dark-matter detector development:
- Total background rejection from all sources at the level of 1 part per about 1 to 3 million triggers for 10 to 100 kg detectors, or 1 part per 100 million for multi-ton detectors.
- Energy thresholds at the keV level or below.
- Target masses of 1 to 10 tons (for a rate-based search for ~100 GeV WIMPS), or target masses of a few tens of kg (for a rate-based search for ~10 GeV WIMPS).
- Discrimination power against electromagnetic recoils of one contaminating event per thousand electromagnetic recoils—or better.
- Higher density to increase the efficacy of self-shielding.
In this the study, we developed two simple experiments that demonstrate the minimum requirements for chemical amplifiers to proceed to a next phase of development. These two experiments are the logical next step in the development of this class of detectors.
- Sensitivity to individual nuclear recoils. The success of the chemical amplifier concept depends on the registration of a signature from individual WIMP particles. The closest Earthly analog to a WIMP is the neutron. An early research goal should be to irradiate candidate materials with neutrons and determine the strength and characteristics of the signal generated. A first goal could be bulk irradiation with a pure beam of neutrons, possibly resulting in multiple nano-explosions, followed by gradual thinning of the target sample, to the point where single interactions could be verifiably registered. This is analogous to the progress in the realm of ionization detectors. There, current-based approaches, in which a large number of electron–ion pairs are generated by bulk irradiation, eventually gave way to more sensitive individual particle detectors.
- Selectivity for nuclear recoils against other backgrounds. With success in step one, the next important step is to differentiate WIMP-like recoils from electromagnetic interactions. This calls for irradiation of a demonstrated neutron-sensitive material with gamma-rays and/or electrons, either to confirm the expected absence of a signal, or alternatively, to determine whether the electromagnetically induced signal can be differentiated from the nuclear recoil signal.
Scientific Approach and Accomplishments
The kinematics of an elastic collision of a dark matter particle with an atom in a nanoparticle depends on the velocity of the dark matter particle and its mass, along with the mass of the atom. The velocity range of interest is the galactic "in-fall" velocity of the order of hundreds of kilometers per second.
The maximum kinetic energy transfer results in the atom recoiling along the incident dark matter direction. For these velocities, the nucleus is accelerated in the collision at a rate that is slow compared to the electron motion, resulting in the acceleration of the entire atom. For lighter atoms, there is some possibility of electron ionization. The recoiling atom then moves through the nanoparticle undergoing energy loss in the atomic collisional process. For the purpose of this study, the treatment of Lindhard and Scharff2 is sufficient to capture the important physics.
The general features of the energy loss are that the range of the recoiling atom is proportional to its initial energy, and the angular scattering distribution is very forward-peaked. Figure 1. shows the scattering process for a lithium nanoparticle. In that picture, the energy transfer results in a recoiling lithium atom range that exits the particle.
The energy lost by the recoiling atom heats the nanoparticle. However, the bulk thermodynamics of the material are not appropriate for this treatment. The thermal energy loss is a phonon collision process, and the largest scattering source for the phonons in this problem is the nanoparticle surface. The details of the loss may be an important consideration in detailed detector designs.
The heat capacities and the melting temperature depend on the relative scale of ratio of atomic radius to nanoparticle diameter. Melting the nanoparticle initiates the thermite reaction. The melting temperatures decrease with decreasing nanoparticle diameters; the heat capacity increases mildly for decreasing diameters. While the practical literature provides empirical rules for how these thermodynamic quantities are affected in the nanoparticle regime, it is likely that a detailed design will require a more detailed scientific foundation.
Based on the current knowledge, the likely candidate metals can be compiled. For a 3-nm diameter nanoparticle, four metals would achieve melting temperatures for the lightest, slowest dark matter particle candidate: lithium, tin, cesium, and bismuth. This is shown in Figure 2.
The initiation metric is the nanoparticle temperature resulting from the recoiling atom's energy loss divided by the melting temperature. Contours for which this quantity equals 1 are shown. These contours exclude smaller diameters—for example, a 2-nm diameter tin nanoparticle will not ignite if struck by a 1 GeV/c2 dark matter candidate travelling at 300 km/s. Figure 3 shows a detection sensitivity plot for lithium. These initiation studies provide an estimate of the sensitivity of the chemical amplifiers as well as a set of material attributes.
Energetic materials are a class of materials with stored chemical energy, which can exothermically react to liberate energy under certain conditions and are commonly subdivided into three categories; explosives, pyrotechnics, and propellants. Explosives are materials in which the fuel (e.g., carbon, hydrogen) and the oxidizer (e.g., NO2) are intermixed at the molecular scale. These are usually polycrystalline solids, but can be single crystal or amorphous solids or liquids, as well as fuel–oxidizer composites. Explosives are often formulated with a binder, such as Kel-F or Viton, to improve their safety characteristics for processes such as pressing and machining. In terms of power, explosives outrank other energetic materials due largely to the atomic mixing scale resulting in greatly reduced kinetic time scales. Also, these materials form permanent gaseous products, leading to very high, local overpressures that assist in the ignition and propagation of a detonation front up to several kilometers per second.
Reactive materials may be considered as a fourth category of energetic materials, referring to metastable mixtures of two or more constituents that can undergo self-sustaining exothermic reactions through oxidation–reduction or alloying reactions. The two most common types of reactive materials are metal/metal-oxide (thermite) and metal/metal (intermetallic) mixtures. Other systems such as aluminum/Teflon or silicon/sodium perchlorate may also be considered as reactive materials. Depending on the pressure and temperature pulse as chemical energy is released, along with the application, the classification of reactive materials may change. For example, a mixture of aluminum with ammonium perchlorate may be used as a propellant. However, if the particle size is reduced and the reactivity increases, that same mixture may be categorized as a reactive material. There is a wide array of formulations, tunability of energy release rate within a formulation, and applications being considered for reactive materials.
One of the most interesting features of reactive materials is that, unlike explosives, the rate of chemical energy release can be tailored by several orders of magnitude through selection of a variety of parameters, such as the particle size, morphology, formulation, composition, density, confinement, and the addition of diluents (diluting agents). A partial list of binary thermite systems, along with their thermodynamically predicted equilibrium properties, can be found in Fischer and Grubelich, which also includes several calculations for intermetallic systems as well.3 These calculations are often used when screening for formulations to produce a desired output, such as temperature, pressure, or a specific product. While there are some correlations between these thermodynamic predictions and the energy release rate, one must exercise caution since, in reality, the reaction of a reactive material is a highly dynamic process. These calculations, for example, don’t include particle size or morphology, which are known to affect the reactivity.
Nanoparticles are commonly defined as particles with at least one measurable dimension less than 100 nm. These materials have characteristically high surface areas, leading to enhancements in a variety of applications with surface-dependent phenomena. Furthermore, classical theories of thermodynamic and transport phenomena may need modifications as the particle size decreases.
Regarding the fabrication of nanoparticles of reactive materials, the choice of reactive materials for any particular application often starts with thermodynamic calculations. In general, a system of materials A and B exothermically reacts to form a product C plus some amount of energy released by the rearrangement of chemical bonds. The total amount of energy released is based on the reduction in free energy achieved in going from reactants to product(s). While the total energy release is often relatively fixed based on the choice of reactive system, the rate of this energy release can change significantly depending on the configuration of the reactants. In general, if the reactants are physically closer together, the reaction should be able to occur more quickly. This points to the fabrication method as a critical determinant of the expected reactivity.
A number of fabrication approaches have been developed. The most significant of those reviewed were: particle mixing, mechanical fabrication, thin film deposition, self-assembly, porous silicon, and three-dimensional printing. These approaches could produce nanoparticle systems with the relevant dimensions.
Impact on Mission
The exploration of new, innovative detectors for rare events has direct relevance to both the scientific mission of the Laboratory as well as the national security mission. The feasibility of developing a new class of radiation detectors with high sensitivity to a selectable energy window supports the Laboratory's strategic focus area in stockpile stewardship science. Additionally, the exploration of thermite properties at the nanometer scale, with an eye to possible manufacturing processes for these energetic materials, enhances the Laboratory's advanced materials and manufacturing core competency.
While there are a number of unmet needs in the area of neutron detection, the field is relatively mature, and the main lines of research—robust replacement for helium-3 and improved directional detection—do not appear especially promising in the context of chemical amplifiers. We have identified one possible exception, involving a variation on neutron dosimetry that might in principle permit remote interrogation. Current detectors, such as thermoluminescent detectors and bubble detectors all require collection and interrogation by various means in order to extract relevant information on neutron exposure. A neutron detector that could be remotely interrogated to determine exposure could find application in areas where it is otherwise difficult to recover the detector, including post-accident scenarios or other situations where physical access to the detector is limited. Notionally at least, chemical amplifiers as a class may be amenable to development in this direction. The basic idea is that the reaction induced by the neutron would create a new population of chemical states, whose presence and number could be remotely detected by interrogation with lasers or other light source. This still requires line of sight access to the detector, but inducing the signal using a laser (for example) permits in principle recovery of a signal, proportional to neutron exposure, at a much greater standoff distance than is possible for existing detectors.
The results from this feasibility study are being prepared as a document to be used to assess and guide future research and development for chemical amplifiers for radiation detection.
- Lopez-Suarez, A., et al., “New dark matter detectors using nanoscale explosives.” arXiv (2014). arXiv:1403.8115v2
- Lindhard, J., and M. Scharff, "Energy dissipation by ions in the keV region." Phys. Rev. 124(1), 128 (1961).
- Fischer, S., and M. Grubelich, A survey of combustible metals, thermites, and intermetallics for pyrotechnic applications. 32nd Joint Propulsion Conf. and Exhibit, Lake Buena Vista, FL, July 1–3, 1996. http://dx.doi.org/10.2514/MJPC96