Electron Time-of-Flight End Station

 The Electron Time-of-Flight End Station. The five analyzers can be seen protruding from the chamber, along with a multitude of cables connecting analyzers and detectors to electronic equipment.

Rotating Vacuum Chamber
The vacuum chamber supporting the analyzers can be rotated about the x-ray beam by 90 degrees while under vacuum. This allows the collection of spectra at many different angles, which increases the accuracy of angular-distribution measurements and allows for the calculation of additional angular-distribution parameters.

 View of the inside of the chamber. The analyzers are all focused on the interaction region from different angles.

A gas needle ejects the gas being used perpendicular to the photon beam. The space where the photons and gas interact is called the interaction region. There, photoemission occurs due to the collision of the photons from the beam and the gas particles. These electrons can go into an analyzer and must travel a distance of 437.5mm and be within ± 2.7 degrees cone relative to a straight flight path in order to be detected.

 Diagram of the gas needle and the outstreaming gas which is interacting with the photon beam
Analyzers
Electrons arrive at the analyzers at a minimum kinetic energy of 5 eV after passing through a retarding lens system. Analyzers 2 and 3 are positioned 54.7 degrees out of the plane perpendicular to the x-ray beam in order to study nondipolar angular-distribution effects. Analyzers 1, 4, and 5 are used to measure dipolar angular distributions and cross-section ratios.

 Experimental schematic of the electron time-of-flight system. Light from the ALS storage ring passes through beamline optics into a differential-pumping section. The chamber and analyzers can rotate around the photon beam for more accurate electron angular-distribution measurements.

 

 Detail of a Time-of-Flight Analyzer.

 
Detectors
Electrons entering each analyzer are detected by two Micro-Channel Plates (MCPs) in a Chevron arrangement. MCPs are thin glass disks with thousands of microscopic tubes. When electrons collide with the walls of the tube, they produce additional electrons which cascade down the tube. This effect occurs because a voltage is placed across the MCPs. The voltage accelerates the electrons and improves chances that the pulse will be detected. The efficiency of the detectors is based on the area ratio of all pores to the total active area.

Electronics
Once an electron hits the Micro-Channel Plates, a cloud of electrons is made that hits an anode which charges a capacitor that produces pulses each time it discharges. From there, the pulses are amplified. Afterwards, a Constant Fraction Discriminator inverts the signal, shifts it to the right a little bit, and adds the original signal with the inverted and shifted signal. This new signal marks the start time for the time-to-amplitude converter/biased amplifier while the end time is marked by the ALS Bunch Marker signal that is produced every 328 ns. The time signal is converted into a voltage and the different voltages correspond to specific channel numbers. The voltage is converted into this channel number using an Analog-to-Digital Converter and stored as a count in a multichannel analyzer. Each of the Multi Channel Analyzers is read through a computer-interface board using software written in LABVIEW programming language from National Instruments. Analog signals from the beam monitor, the chamber pressure, and the gravitational sensors (which are used to determine the chamber angle) are also converted to digital signals which are linked to the data-acquisition computer to be viewed and stored. A spectrum is made up of all the counts produced over all the channel numbers. The peaks in the spectrum correspond to electrons with certain kinetic energies.

 Flow Chart of how the signal given by an electron becomes part of a spectrum

 

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