How Does It All Work? A Summary of the IDEAS Beamline at the Canadian Light Source

Transcription

1 How Does It All Work? A Summary of the IDEAS Beamline at the Canadian Light Source

2 What Makes Up The Canadian Light Source? 4. Storage Ring 5. Synchrotron Light 6. Beamline 1. Electron Gun 2. Linear Accelerator 3. Booster Ring 7. Optics Hutch 8. Experimental Hutch 9. Control Station

3 An Overview The process of light production begins with the Electron Gun. A tungsten button is heated to 1000 C, causing electrons to boil off. A periodic high voltage between the cathode and anode pulls the electrons off in bunches, and causes them to accelerate towards the Linear Accelerator (LINAC).

4 The LINAC uses a series of Radio Frequency Cavities (RF Cavities). Inside these cavities are waveguides with a travelling electric field. The electrons are further bunched and accelerated by these fields to c with an energy of 250 MeV.

5 The electron bunches are sent to circulate the Booster Ring. Another RF Cavity provides a boost in energy each time the electrons orbit the ring. The boost is from a short, ultra high voltage travelling electric wave. It takes.6 seconds for the electrons to circulate the ring 1.5 million times, reaching c with an energy of 2.9 GeV. RF Cavity in the Booster Ring

6 The electrons are then sent to the storage ring where massive dipole or bending magnets, or insertion devices, cause the electrons to laterally accelerate and emit light energy. The electrons lose energy from this, therefore a Superconducting RF cavity (SRF) is used to replace the lost energy, keeping the electrons energy roughly 2.9 GeV. Bending Magnet Storage Ring SRF Cavity

7 Light Production When a charged particle accelerates, its electric field has to shift (correct) positions with it. Einstein discovered that the fastest anything, even information, can move is c. The electric field far away has no idea the charged particle has accelerated yet. This means that the electric field correction takes time to travel to the end of the electric field. This travelling electric field correction is a wave in the electric field, which is light.

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9 Vacuum Everything from the electron gun to the beamline is under vacuum. A vacuum is an area with a pressure lower than atmospheric pressure, 760 torr. The storage rings vacuum pressure is torr. Beamline pressures vary. There are less than 10 particles per cubic centimeter in the storage ring, which is less than around the International Space Station. Vacuums are needed to reduce the number of particles in the beams path so the electrons or the light do not collide with them, and become lost.

10 Beamline The electrons are travelling in a circular path. Since light propagates rectilinearly, it comes off in a fan tangential to the ring. This fan of light is broad spectrum, from infrared to x-ray. The light passes through a port in the ring wall towards the beamlines.

11 Optical Hutch The incident light is initially diverging. To fix this, the first thing the light interacts with is a toroidal mirror. A toroidal mirror is a concave mirror cast on the side of a large torus. It is curved in both the x and y planes, which allows the mirror to focus both planes at the same time. The mirror conditions Converging Mirror the light to either converge or collimate, depending what the experiment needs. Torus Collimating Mirror Toroidal Mirror

12 Monochromator The conditioned light then passes to the Monochromator. The name is of Greek roots, Mono meaning single and Chroma meaning colour. The Monochromator is used to select the energy of light that passes through to the experiment. It contains two flat crystals and mechanical hardware to change the angle between them and the beam. Crystals

13 Energy/Wavelength selection is done by Bragg Diffraction. Light rays reflecting from different layers of a crystal travel different distances. If the difference in the distance travelled is an integer multiple (n) of the wavelength, then the waves constructively interfere, creating a monochromatic beam. All other wavelengths destructively interfere, filtering them out. Constructive Destructive nλ = 2dsin(θ)

14 The experimental hutch is where the experiment takes place. In this hutch are the detectors, sample stages, and their related electronics. Experimental Hutch Sample Vacuum Chamber Vacuum Ion Chamber To Optical Hutch Ion Chamber Sample X-rays Sample Stage KETEK Detector

15 The experimental hutch houses a vacuum sample chamber. Experiments that are air sensitive or require low energies are conducted here. Following this is a motorized sample stage for experiments that are not air sensitive or require higher energies. The stage can move the sample in and out of the beam remotely. Phosphor Card Typical Sample Sample Vacuum Chamber

16 Detectors There are two types of detectors used. The first is for Fluorescence based experiments, which measures the spectrum given off by the sample after it absorbs x-rays from the beamline. The other is for Transmission based experiments, which measures how much light the sample absorbs. Fluorescence Transmission G

17 KETEK SDD Detector The Fluorescence detector is called a KETEK Silicon Drift Detector. This detector is placed outside of the beams path, but faces the sample. This collects the light emitted by, and scattered from, the sample and surroundings. Sample Sample Vacuum Chamber KETEK Detector X-rays Vacuum Ion Chamber Sample Stage

18 The KETEK detector works similar to solar panels. Light shines on the silicon layer, knocking electrons off. The more energetic the light, the more electrons are knocked off. A voltage is set up causing the electrons to drift to the anode. The cathode position changes, moving inward along the drift rings to speed things up.

19 The detector then measures the charge on the anode and converts it to photon energy. This is then sent to a computer and the process repeats. A spectral graph is created by plotting the number of photons detected vs the energy they were detected at. The spectrum below shows the elemental composition of a sample.

20 X-ray Fluorescence, XRF, is a common technique that uses these detectors. X-ray photons are used to excite electrons in the sample from low energy states to higher ones, leaving a hole for other electrons to fill. As the other electrons fall to fill the hole, they fluoresce. The energy fluoresced depends on the energy difference between the initial and final energy states, which are unique to each element. The incident x-ray energy is fixed, usually the maximum energy available, ensuring that as many elements as possible absorb the x-rays below that energy.

21 FMB Oxford Ion Chamber The transmission detectors are called Ion Chambers. These are located along the lights path, one before the sample and two after. These detectors measure the intensity of light before and after a sample to see how much is absorbed. Ion Chamber Vacuum Ion Chamber

22 The ion chambers are filled with a gas that is ionized when light passes through. These ions are separated by a voltage and the resulting current is measured to determine the beam s intensity This is a light intensity measurement. Intensities are continuously measured as they change frequently.

23 The first chamber measures the light s intensity before the sample. This one is built into the vacuum system as it also lies before the vacuum chamber. The sample sits between the first and second chamber. The second chamber measures how much light the sample has absorbed. The third chamber is for a reference foil. The foil has known absorption features and can be used to calibrate the samples absorption features.

24 An intensity measurement is taken at a lower energy than the sample is expected to absorb. The energy is increased slightly and the process repeats. At certain critical energies, the photons have just enough energy to cause new electron transitions. This is seen by a large increase in the amount of light absorbed, called an absorption edge. This process is called X-ray Absorption Near Edge Structure, XANES.

25 The position and shape of this edge tells us the element, the oxidation state, and the chemical state being measured. The post edge features are due to the emitted photo-electron scattering off surrounding atoms back towards the central atom. This changes the absorption behavior and allows us to determine where the surrounding atoms are. Edge Post Edge Pre Edge

26 Operating Funding Partners

27 Capital Funding Partners

28 Acknowledgments David Muir who provided information about the beamline and synchrotron. Tracy Walker and Anna-Maria Boechler who provided information about the synchrotron and facility. CLS Staff who answered questions about everything. Created by Tylor Sove, IDEAS Summer Student Thank You!