The art of measuring the speed of a flying electron

The art of measuring the speed of a flying electron

Exploring flying electrons is called electron spectroscopy. When a material is irradiated with intense light, the electrons are forced to leave the surface. Their direction of movement and speed can tell us what the material looks like on a microscopic level. Electron spectroscopy demands two things: a light source and an electron detector. My thesis discusses their interaction. On one side, the brightest light source in the world: the synchrotron radiation-producing storage ring. On the other, a measuring tool: the time-of-flight spectrometer. Both of them are important to electron spectroscopy; the problem is they do not fit very well together.

The storage ring is a particle accelerator in which electrons are circulated close to the speed of light. While circulating, they create intensive rays of X-ray light or ultraviolet light. We call it synchrotron radiation. At MAX IV there are three storage rings in which electrons are circulated in bunches with 3-metre spacing. Since the electrons move at close to the speed of light, there are just ten nanoseconds between each pulse of light. Ten nanoseconds is a very short time, but other storage rings could have even closer spacing between the light pulses. The BESSY storage ring in Berlin, for example, has only two nanoseconds between pulses.

The time-of-flight spectrometer is an instrument to measure the electron speed and direction of movement. Measuring an electron’s speed with a time-of-flight instrument is similar to timekeeping for a sprinter on a running track. The track always has a fixed length. The starter provides a start signal by firing a starting pistol, which is also the signal for the timekeeper to start the clock. The timekeeping ends when the runner crosses the finish line. Knowing the measured time, we can calculate the mean speed of the runner. In the time-of-flight spectrometer, the electron is the runner and the running track is a vacuum tube. An electron, which has been forced out of the sample surface by a light pulse, is guided through the vacuum tube to the detector. The light pulse acts as the starting pistol and the detector as the timekeeper.

This is where our problems start. All electron spectrometers with good energy resolution have flight times of at least 100 nanoseconds, sometimes many microseconds. Since we cannot see the electron flying, the only thing we can measure is when the electron reaches the detector. The start signal must come from the light pulse. However, these pulses are much closer to each other than required by the time-of-flight instrument. This would be the same as firing the starting pistol at the running track many times during each race. For the timekeeper, it is impossible to know when the runner started the race. For the timekeeping to work, there can only be one shot in each race. In the same way there can only be one light pulse each time an electron flies through the spectrometer. When each electron is followed by a lot of shots we have to ask ourselves: Which is the smoking gun?

In my thesis, I discuss different ways to solve the problem of the smoking gun.

There are a number of solutions. One way is to change the settings of the storage ring to ensure the light pulses arrive less often. This is similar to ensuring the pistol is fired less often. Many storage rings around the world have this feature, but it just so happens that this is very hard to do at storage rings such as MAX IV. Attempts to change the position of the electrons in the storage ring could make the whole accelerator unstable. My wise colleagues, the accelerator physicists, are nevertheless attempting to create this feature at MAX IV, which would be very beneficial to the time-of-flight instrumentation.

Photo finish in the 100 metre sprint at the Olympic Games in Athens 2004. The camera acts a a detector and makes the timekeeping exact. (Image: Public Domain)
Photo finish in the 100-metre sprint at the 2004 Olympic Games in Athens. The camera acts a detector and makes the timekeeping exact. (Image: Public Domain)

Yet another way to reduce the separation of light pulses before they reach the experiment is to physically block them. The starting pistol is fired, but the shot is not heard. A device created to block light pulses is called a chopper. The chopper is a rotating wheel with very small openings along its periphery, allowing only a single light pulse to pass. Making the choppers necessary for the time-of-flight spectrometer requires advanced engineering skills. We need a wheel with a one-metre circumference that rotates a thousand times per second, and has openings just a few micrometres wide. Such choppers are commercially available today and I propose that these can be used at MAX IV.

If it is impossible to change the properties of the light pulses, it becomes necessary to work on the detector. If you cannot stop the starting pistol, you have to stop the runners after they have started. I have worked with colleagues in Berlin to develop two kinds of electronic gate, which block electrons emitted from the sample.

We filter out all electrons that we don’t want to detect with an electric field applied to a very thin transmission mesh made from pure gold. We utilise an electric pulse to open the gate as often as necessary for the spectrometer. During that short period, the electrons can pass without problem. The difference between the two gates is that the first version, which we call the detector gate, stops the electrons just before the finish line, whereas the second version, which we call the front gate, applies the electric field at the start. The challenge is to create an electronic pulse that is sufficiently strong to block the electrons we don’t want to detect, but can also act very rapidly and exactly. We know that some electronic pulses create radio waves that can blind the detector. To solve this problem we had to study many different kinds of electric pulses and their effects on the spectrometer. In the end we managed to create electric pulses suitable for the BESSY storage ring and the light pulses created there. Thanks to our detector gate, we could detect electrons 30 times more effectively. An experiment that would otherwise take a full day could now be performed in 20 minutes. The time-of-flight spectrometer at BESSY has now become much more versatile, and detector gates will be used at new experimental stations.

An equally good result at MAX IV will require the front gate. It is a bigger challenge for us, since the electric pulses have to be both stronger and shorter. In my thesis I discuss the properties of such a gate. Some tests have been conducted, but more work is required before the gate can be used in a proper experiment at MAX IV. For the upcoming experiments I will redesign the gate in order to achieve better steering of the electrons. I will find a new pulse generator to produce pulses with higher quality and shorter pulse lengths. Our goal is to use gates during standard operation of time-of-flight instruments at MAX IV. If we can provide researchers with access to state-of-the-art light quality and state-of-the-art instruments, future research in Lund will be able to achieve truly great things.

The windling “race tracks” in a time-of-flight spectrometer; from the start in a very small region, to the finish at the larger detector. The complete flight takes place in a vacuum tube.
The winding “running tracks” in a time-of-flight spectrometer; from the start in a very small area to the finish at the larger detector. The complete flight takes place in a vacuum tube.

Christian Stråhlman

PhD student at Max IV Laboratory