Newswise – In January 2020, the United States Department of Energy (DOE) selected the Brookhaven National Laboratory as the site to build a multibillion dollar project Electron-ion collider (EIC). This new generation facility for nuclear physics research will consist of two intersecting accelerators: one producing an intense beam of electrons and the other a beam of protons or heavier atomic nuclei. Scientists will study the frontal collisions of these beams, which will produce precise three-dimensional (3D) snapshots of the internal arrangements of the quarks and gluons that make up the protons and neutrons of atomic nuclei. The purpose of these studies is to deepen our understanding of the fundamental building blocks of visible matter in today’s universe and the force that holds it together.
The construction of this machine over the next decade will require several technical advances, including a way to cool the ion beams. At the EIC, ion beams heat up as they move and separate by intra-beam scattering, making collisions with electrons less likely. The EIC team plans to cool the ion beams using high-brightness, high-current electron beams. For this electronic cooling system, photocathodes, materials that convert light (photons) into electrons, will serve as the electron source. In particular, they selected photocathodes made up of alkali metals – those that occupy the first column of the periodic table (lithium, sodium, potassium, rubidium, cesium and francium) – and antimonides, or compounds with antimony (a semi-metallic element).
“Alkaline antimonides are a group of cathode materials with photoemissive properties, which means they generate electrons when struck by light,” explained Mengjia Gaowei, associate researcher at Brookhaven Lab Collider-accelerator department (GOUJAT). âThese photocathode materials emit electrons because their surface has a relatively low energy barrier for electrons with enough energy to exit. They have high quantum efficiency at wavelengths of visible light, generating more electrons per photon of light than many other materials.
To meet EIC electronic cooling design parameters, the photocathode will need to operate for an extended period (several days) while producing a high current (100 milliamps), exceeding the performance of the cathode previously reported by the scientific community (65 milliamps for several hours for several alkaline photocathodes). As Gaowei explained, multi-alkali photocathodes can degrade due to various factors. For example, the heat generated by the laser (light source) can cause material loss when operating at high current. Another problem is back-ion bombardment, which occurs when the electron beam ionizes the residual gas in the vacuum chamber, creating ions that bombard the surface of the cathode. The surface can also be chemically contaminated; alkaline antimonides are very sensitive to water and oxygen.
Ao Liu, Deputy Director of the Research Department of Euclid Techlabs, LLC, is Euclid’s Principal Investigator (PI) on a DOE Small Business Innovation Research (SBIR) price of the Office of Nuclear Physics to prepare a layer on top of the photocathode to protect it from this damage and prolong its life. The Brookhaven PI is Gaowei, who joins forces with Liu to lead a collaborative effort comprising colleagues from C-AD at Brookhaven, Center for Functional Nanomaterials (CFN), Instrumentation Division, and EIC Directorate.
Liu started collaborating with Brookhaven as PI on another related SBIR price, granted in 2020, with physicist Brookhaven EIC Erdong Wang. The objective of this SBIR is to construct a radiofrequency (RF) magnetron sputtering source for hexagonal boron nitride (hBN), a promising two-dimensional (2-D) material for the protective layer. Sputtering is a technique used to deposit thin films of a material on a surface or substrate. In RF magnetron sputtering, an RF electric field accelerates ions toward a target containing the material to be deposited, ejecting (sputtering) its atoms from the surface. These atoms are deposited on the substrate to form a thin film.
âOne of the main goals of these SBIR projects is to find innovative ways to improve the performance of various beamline components for major DOE accelerator programs such as the EIC,â said Liu.
For this SBIR project, the Gaowei and Liu teams use a different technique, 2D thin film wet transfer, to obtain thin layers of hBN on an antimony substrate and then deposit the alkaline cesium on them. The team will attempt to force cesium to move under the hBN layer to react with the antimony layer in a controlled manner – a process known as intercalation – to form the alkali antimonide photocathode. But the first candidate coating they’ll explore is graphene, or 2-D carbon, on antimony. Euclid prepares and pre-characterizes samples at Center for Nanoscale Materials (CNM) at the DOE Argonne National Laboratory as part of an active user proposal. Then they will bring the samples to Brookhaven for cesium intercalation and final characterization.
“We chose to start with graphene because it has already been proven that the intercalation of cesium occurs on a monolayer of graphene,” Gaowei explained. âFor hBN, there are no such published results. Our calculations suggest that cesium will behave similarly on hBN, but we need to test it experimentally. Ultimately, hBN will be the final coating material because graphene will not create an emissive surface for the photocathode; this will prevent the electrons from going out.
To track the growth of the photocathode under the 2D protective layer, the team will use X-ray photoemission electron microscopy (XPEEM) / low energy electron microscopy (LEEM). terminal station of National synchrotron light source II (NSLS-II) Electron spectromicroscopy light line at Brookhaven. The CFN operates this terminal station through a partner user agreement with NSLS-II. Jurek (Jerzy) Sadowski, materials scientist at CFN Interfaces and Catalysis Science Group who directs the research program in electron spectromicroscopy at CFN, is the contact person for the terminal station. Last year, Gaowei, Sadowski, Wang and their colleagues performed an experiment at this terminal station, activation Gallium arsenide photocathodes with cesium, tellurium and oxygen to improve their quantum efficiency. Gallium arsenide is the cathode material of choice for generating the polarized electron beams that collide with the ion beams in the EIC.
âThe XPEEM / LEEM terminal station is a comprehensive spectromicroscopy tool that provides information about the local elemental distribution and electronic structure of a sample,â Sadowski said. âFor this project, we will use this tool to observe cesium intercalation in real time. These real-time observations will help us understand how to adjust the experimental conditions to achieve the desired structure. In addition to watching the process unfold, we can do elemental surface mapping to see how uniform the intercalation is at the scale of a few tenths of a nanometer.
Beyond the beamline experiments, the team will work with scientific staff Xiao Tong and use a multi-probe surface analysis system in proximal CFN leads Establishment characterize oxidation states. When graphene is transferred to antimony, the antimony inevitably oxidizes (combines with oxygen) and forms various oxides of antimony. Previous studies have indicated that these oxides dissociate at high temperatures. The team will investigate whether they can completely remove antimony oxides, which would otherwise compromise the performance of cesium antimony. They will also use the multisensor system to image the surface structure of graphene on antimony.
After this proof of concept demonstrating the coating system for cesium antimonide, the next step would be to determine if it works for cesium potassium antimonide, the photocathode material that will ultimately be needed. Cesium potassium antimonide has higher quantum efficiency (generating more electrons with the same laser) in the visible light range and is the material of choice for most accelerators that use alkali antimonide photocathodes .
âThere are a lot of technical challenges to overcome,â Gaowei said. âOur goal through this SBIR award is to test our proposed method of cathode encapsulation, which could increase cathode robustness against poor vacuum while maintaining high quantum efficiency. These improvements would extend cathode life for applications in nuclear physics facilities, such as electron cooling at the EIC. “
This research is supported by the DOE Office of Science. The CFN, CNM, and NSLS-II are all facilities for DOE Office of Science users. Gaowei is the recipient of a 2021 Early Career Research Program price of the Nuclear Physics Program of the DOE Bureau of Science to perform “Cathode R&D for a High Intensity Electron Source in Support of the EIC.” The SBIR project is supported by the DOE SBIR program under contract number DE-SC0021511.
To learn more about how to partner with CFN, contact CFN Deputy Director for Strategic Partnerships, Priscilla Antunez at [emailÂ protected].
The Brookhaven National Laboratory is supported by the Office of Science, US Department of Energy. The Office of Science is the largest supporter of basic research in the physical sciences in the United States and works to address some of the most pressing challenges of our time. For more information visit https://energy.gov/science.