Positronium Annihilation Lifetime Spectroscopy Study of SBA-15 Tracy K. Steinbach Grand Valley State University April 22, 2010 Abstract Positronium annihilation lifetime spectroscopy (PALS) is a technique used in the characterization of porosity in nanostructured materials. Positrons, the antiparticle to the electron, enter a sample and either immediately annihilate with an electron or they can capture an electron to form Positronium (Ps), the hydrogen-like bound state. Ps then annihilates with a lifetime corresponding to the pore size. This annihilation converts the total mass of the positron and electron into high-energy photons. These annihilations are detected to measure Positronium lifetime. SBA-15, a mesoporous silica that consists of a two dimensional hexagonal array of cylindrical mesopores with interconnecting micropores, is a system of interest as a catalysis support and low- dielectric constant material. Nitrogen adsorption, x-ray diffraction, and small angle neutron scattering (SANS) have been used to characterize the pore structure in SBA- 15, but none of these techniques have been able to yield a complete picture. PALS is a technique that has the potential to yield insight of the pore structure in SBA-15. 1 1 Introduction The porosity of a material is the volume within a substance not taken up by the mate- rial itself, i.e. the free volume. The porosity plays a key role in determining a material’s macroscopic physical properties. In polymers, the porosity of the polymer plays a key role in the mechanical, electrical, and transport properties. For example, one specific physical property of polymers defined by the free volume is the glass transition temperature. This is the temperature at which a polymer transitions between a “glassy” phase where the mobil- ity of the polymer chains is highly frustrated and the “rubber” phase where they are highly mobile and able to relax to equilibrium. For example, at room temperature, the polymer in your eye glasses is in the glassy phase while saran wrap is in the rubber phase. Porosity also plays a major role in determining many important properties of porous low dielectric con- stant (low-k) materials. The density, stiffness/strength, thermal conductivity, and chemical reactivity of a dielectric material all depend in part on its porosity [3]. Amaterialthatfallsinthelattercategoryisaparticularcompositionofmesoporoussilica called Santa Barbara A 15 (SBA-15). SBA-15 is made up of a two dimensional hexagonal array of cylindrical mesopores with interconnecting micropores, as shown schematically in Figure1. ByvaryingthetemperatureandtimeusedtosynthesizeSBA-15, thewallthickness and pore radius can be varied. The typical pore diameters seen for SBA-15 range from 3 to 30 nm [8]. There are many applications for which SBA-15 may be applied, including but not limited to catalysis support, low-k dielectric, gas adsorption, and drug delivery [4]. An application of SBA-15 is for use as a low-k dielectric material. As the porosity of a material increases, the dielectric constant is driven down from that of the base material, makingthematerialusefulinmicroelectronicsamongotherapplications. Thelowerdielectric constant materials help increase the complexity of computer chips, while at the same time minimizing the size of the chip, and improving circuit speed and memory capacity within 2 Figure 1: Schematic drawing of SBA-15. SBA-15 consists of a two dimensional hexagonal array of cylindrical mesopores with interconnecting micropores. computers and other electronic devices [3]. One area of active research is in the use of SBA-15 as a catalysis support in Fischer- Tropsch synthesis, which is being studied by a group from the University of Maine [6]. Fischer-Tropsch synthesis is a catalyzed chemical reaction through which hydrocarbon fuels are produced from carbon monoxide and hydrogen. The reaction requires the use of cata- lysts such as iron, cobalt, nickel, or ruthenium to produce synthetic fuels. SBA-15 can be used as a catalysis support to balance metal dispersion in the reaction as well as limiting the transportation of reactants (CO and H ) into the reaction and the products (normally 2 alkanes) out of the reaction. To be able to take full advantage of SBA-15 in any application, the details of the pore structure must be better understood. The pore structure of SBA-15 has been previously studiedusing severalmetrologies, includingsmall angleneutronscattering (SANS)and small angle x-ray scattering (SAXS). The techniques have proposed a model for the structure of SBA-15, as shown in Figure 1 [12]. However, these techniques have limitations that may have yielded an incomplete model for the structure of SBA-15. SANS measures the momentum transfer that occurs when neutrons pass through a sam- ple. In SANS we are only concerned with elastic scattering (no energy change of the neutron) 3 𝑘 ′ 𝑞 scattered beam 𝑘 2𝜃 incident beam sample Figure 2: Small angle neutron scattering momentum transfer diagram. and so the incident momentum vector, (cid:126)k, and the neutron’s scattered momentum vector, (cid:126)k(cid:48), are equal in magnitude. However, there is a change in direction, with θ as the scattering angle, due to the inhomogeneities in the sample. Because the neutron also acts as a wave, we can write the magnitude of the momentum as p = mv = h¯k = 2πh¯/λ, and the magnitude of the momentum transfer (also known as the scattering angle) (cid:126)q =(cid:126)k(cid:48) −(cid:126)k is given as 4πsinθ q = |(cid:126)k(cid:48) −(cid:126)k| = λ where m is the neutron mass, v is the speed of the neutron, and λ is the wavelength. From Bragg’s law, nλ = 2dsinθ, with n being the order of the reflection and d being the spacing between lattice planes, we can find the location of scattering peaks from the constructive interference of wave interacting with periodic structures [11]. Through these methods SANS has provided some very important information about the structure of SBA-15, but it has its limitations. One major limitation encountered in SANS is that the results found using SANS are very model-dependent when converting from the scattering angle to the pore size. If the model used in analyzing the scattering angle does not match completely with the actual structure of the pores the final results may be inaccurate. Additionally, SANS frequently requires the use of a solvent (typically H O, D O, 2 2 or a mixture of both) in order to provide contrast matching of certain areas of the sample. 4 Contrast matching is used to mask the scattering signal from certain parts of a sample, so that the signals from other parts of the sample can be seen more clearly. However, many samples do not retain their original pore structure when a solvent is added, thus causing SANS to produce results that may not completely match the pore structure actually present in the sample. Positronium annihilation lifetime spectroscopy (PALS) is a metrology that has been used tostudyporousmaterialsforseveraldecades[7]andisacomplementarytechniquethatcould yield additional insight about the pore structure of SBA-15. Details on the PALS technique will be presented shortly. In contrast to SANS, PALS is much less model-dependent and does not require a solvent, thus it should give complementary results about the structure of SBA-15. Another advantage of PALS is that it requires smaller scale equipment compared to SANS (the PALS instrumentation can be set up on a bench top). 2 Background Theory PALS is an experimental technique that utilizes antimatter, namely the positron. The positron, the antiparticle to the electron, is produced in two different ways. Positrons can either be emitted in radioactive decay of certain nuclei, including 22Na, 58Co, 64Cu, and 68Ge (which is convenient for use in a research lab), or are formed through pair production. Pair production occurs when a high-energy γ-ray having energy greater than 1022 keV interacts with a heavy nucleus to convert energy into mass and produce a positron and an electron (as well as a photon that is absorbed into the nucleus to conserve momentum). Pair production is essentially the reverse process of positron/electron annihilation. When a positron enters a material it will eventually annihilate with an electron. This annihilation converts the total mass, m, of the positron and the electron into energy, E, 5 in the form of high-energy photons. This conversion of mass into energy, in the form of photons, follows Einstein’s equation, E = mc2. In many cases the positron and electron do not immediately annihilate; instead they may first form the hydrogen-like bound state of a positron known as positronium (Ps). The time from when Ps is formed until it annihilates is known as its lifetime. The lifetime of Ps is dependent upon the environment in which it is found, as well as its state. Positronium has two states, singlet (para-) and triplet (ortho-), that are dependent upon the relative spin state of the positron and electron. Para-Ps has a spin state of 0, while ortho-Ps has a relative spin state of 1. The annihilation of para-Ps occurs with the emission of two back-to-back γ-rays of 511 keV, with a short vacuum lifetime of ∼125 ps [7]. This is a 2γ annihilation that conserves both momentum and energy. To conserve angular momentum and charge conjugation ortho-Ps is required to annihilate into at least three photons ( 3γ annihilation). Ortho-Ps has an odd charge conjugation, as do photons, so when ortho-Ps annihilates it must annihilate into an odd number of photons; to conserve momentum it must annihilate into at least three photons [10]. The lifetime of ortho-Ps in vacuum is ∼142 ns [7]. These characteristic lifetimes for para- and ortho-Ps are dependent upon the environment surrounding the Ps. This dependence is the driving principle behind the PALS technique, which uses measured lifetimes in a material to discern the structure of porous materials. Once formed, positronium tends to locate within the voids, or pores, in the material where the net Ps energy is the lowest. When positronium is within the pores its energy is low because the electrons in the material are far away. Though Ps locates in the voids of a material, it still interacts with the walls of the pores. The more often it interacts with the material, the more likely it is to annihilate. The measured decay rate, λ , is the sum of the m 6 Figure 3: Positronium formation in porous materials. Positrons that enter a sample form Ps, which tends to localize in pores if it does not annihilate immediately. The Ps interacts with the pore walls, shortening its lifetime. From reference [7]. vacuum decay rate, λ , plus the quenching rate, λ . vac quench λ = λ +λ m vac quench (Note that the decay rate, λ, is inversely proportional to the lifetime, τ.) The smaller a pore is the more the Ps will interact with the walls (quenching) and thus the sooner it will annihilate, yielding a shorter lifetime. Quenching is annihilation with an electron the Ps is not bound to. The idea of this Ps interaction in a material is depicted in Figure 3. By measuring the shortened lifetime of Ps the average pore size can be determined [7]. 3 Methods A schematic of the PALS apparatus used to measure the lifetime of Ps in the material is shown in Figure 4. A sealed 22Na radioactive positron source is embedded in a bulk sample of SBA-15 powder. Two fast plastic scintillators are used to detect γ-rays. The start detector 7 Start Detector Stop Detector Sample Positron Source Figure 4: Schematic drawing of PALS apparatus is setup to be sensitive to the 1270 keV γ-ray emitted concomitantly with the emission of a positron from the 22Na source. These positrons thermalize in the material and form Ps. The stop detector is setup to detect the 511 keV annihilation γ-ray. The stop and start pulses are presented to a time to analog converter (TAC). A multichannel analyzer (MCA) is used to record the TAC timing pulse in a computer. This process is repeated many times, and all the lifetimes are used to generate a lifetime histogram similar to the spectra seen in Figure 5. This histogram is then fitted to one or more Ps lifetime(s) and intensity(ies). From these fitted lifetimes the pore diameter(s) within the sample are determined. The sample cell used, shown in Figure 6, was machined from stock aluminum. A 1” x 1” x .8” aluminum block was machined for the main component of the cell and a 1” x 1” square of sheet aluminum was cut to create a lid. The central hole drilled in the block is designed to hold the 22Na source surrounded by the SBA-15 power (using approximately a cubic centimeter of sample). The lid is screwed to the base of the cell. Due to time constraints we were not able to construct a sample cell that could would allow direct vacuum pumping, 8 Annihilations from positrons Annihilations from orthopositronium Figure 5: Typical lifetime spectra for Ps in porous material (not SBA-15). Characteristic lifetime, τ, is shortened by Ps quenching. From reference [1]. Figure 6: Sample Cell. The sample cell was machined from aluminum to hold the positron source and the sample of SBA-15 9 Figure 7: Set of phototubes tested. Photo- Figure 8: Three of the five bases tested. tubes 3 and 4 were used in the final appa- Bases A and B were used in the final appa- ratus. ratus instead the sample cell was placed in a desiccator that could be pumped down to a few microns of vacuum. After the desiccator was evacuated the detectors were brought in as close to the cell as possible and data was run. Photomultiplier tubes are photon detectors that utilize the photoelectric effect. When radiation falls upon a metal surface, electrons are emitted if the photon energy is greater than the work function of the material being irradiated. The work function is the minimum energy needed to remove an electron from an atom. These initial electrons that are emitted are then accelerated from the photocathode into a series of dynodes. Each dynode emits multiple electrons for each one that hits it, increasing the number of electrons after each dynode. [9] A set of five phototubes (shown in Figure 7) and five bases (shown in part in Figure 8) were tested to determine which worked the best and would then be used in the final apparatus. A block of fast plastic scintillator was milled and polished to use in this testing process. Each phototube was tested with two different bases at three voltages on each base to give a rough view of which phototubes provided the most gain at the lowest voltage input. An example of the pulse output directly from the phototubes is seen in Figure 9. The two 10