New detector triples the speed of SLAC’s electron camera, enabling higher sensitivity

Researchers have conducted the first experiment using a new detector, installed in the megaelectronvolt ultrafast electron diffraction (MeV-UED) instrument, at the Linac Coherent Light Source (LCLS) at the Department of Energy’s SLAC National Accelerator Laboratory. This detector is the first to keep pace with the MeV-UED’s maximum electron production rate of 1,080 electron pulses per second. Compared to the previous detector’s maximum rate, the new detector collects three times more data over the same time span, drastically improving the instrument’s efficiency and sensitivity.

“With this new detector, we’re able to read out each individual pulse of electrons from the instrument,” said Alexander Reid, MeV-UED facility director. “That gives us a much more powerful way of examining the experimental data to answer our science questions.”

Improved sensitivity enables new experiments

MeV-UED tracks how electrons scatter when passing through samples. The scattered electrons produce diffraction patterns on a detector – signals that encode the positions of atoms and electrons in the sample and reveal structural changes that occur within a tiny fraction of a second. These structural changes, in turn, help researchers decode photochemical reactions and other important processes in chemistry, biology, materials science and other fields.

With its higher data collection rates and sensitivity, the new detector allows researchers to better observe subtle chemical processes that produce weak diffraction signals and have been difficult to study before, such as proton-transfer reactions. Proton transfers are involved in many reactions, including the catalysis of biochemical reactions, but they occur within one millionth of a billionth of a second, making them challenging to catch.

“This improved detection efficiency will not only shorten experiments but also give us even finer details of these molecular changes as a function of time,” Reid said. 

Because the detector will measure every available pulse of electrons – three times more than before – teams will also be able to vary experimental conditions, such as light and temperature, from pulse to pulse to learn more about how these changes affect the chemical reaction pathway. The pulse-by-pulse detection will allow researchers to use new measurement strategies that sharpen structural features, boost time resolution and enhance signal quality.

“The difference between this detector technology and the old one is like the difference between a car and a spacecraft. It's really a game changer for this experimental setup,” said Thomas Wolf, head of the LCLS Chemical Sciences Department and principal investigator at the Stanford PULSE Institute. “The new detector will also tremendously benefit more sophisticated data analysis methods based on machine learning.”

Wolf led the first test experiment with the new detector, in which researchers measured the changing structure of a gas molecule, iodochloromethane, that plays a role in the depletion of ozone in our atmosphere. The new setup is now available to MeV-UED’s user community.

Cross-team collaboration achieves upgrade

MeV-UED’s new detector is a version of the ePix10k, a detector pioneered by engineers at SLAC to detect X-rays. Introduced in 2020, the ePix10k was ten times as sensitive as its predecessor and was rapidly adopted by X-ray facilities around the world. Researchers at MeV-UED were eager to apply the technology to electron detection. However, while the X-ray detector works in ambient air, high-energy electrons are easily scattered by air molecules, so teams across SLAC reengineered the detector to function in the MeV-UED instrument’s vacuum.