From SLAC: “How do you catch femtosecond light?”


SLAC Lab

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Gabriella Carini
Staff Scientist
Joined SLAC: 2011
Specialty: Developing detectors that capture light from X-ray sources
Interviewed by: Amanda Solliday

Gabriella Carini enjoys those little moments—after hours and hours of testing in clean rooms, labs and at X-ray beamlines—when she first sees an instrument work.

She earned her PhD in electronic engineering at the University of Palermo in Italy and now heads the detectors department at the Linac Coherent Light Source (LCLS), the X-ray free-electron laser at SLAC.

SLAC/LCLS

Scientists from around the world use the laser to probe natural processes that occur in tiny slivers of time. To see on this timescale, they need a way to collect the light and convert it into data that can be examined and interpreted.

It’s Carini’s job to make sure LCLS has the right detector equipment at hand to catch the “precious”, very intense laser pulses, which may last only a few femtoseconds.

When the research heads in new directions, as it constantly does, this requires her to look for fresh technology and turn these ideas into reality.

When did you begin working with detectors?

I moved to the United States as a doctoral student. My professor at the time suggested I join a collaboration at Brookhaven National Laboratory, where I started developing gamma ray detectors to catch radioactive materials.

Radioactive materials give off gamma rays as they decay, and gamma rays are the most energetic photons, or particles of light. The detectors I worked on were made from cadmium zinc telluride, which has very good stopping power for highly energetic photons. These detectors can identify radioactive isotopes for security—such as the movement of nuclear materials—and contamination control, but also gamma rays for medical and astrophysical observations.

We had some medical projects going on at the time, too, with detectors that scan for radioactive tracers used to map tissues and organs with positron emission tomography.

From gamma ray detectors, I then moved to X-rays, and I began working on the earliest detectors for LCLS.

How do you explain your job to someone outside the X-ray science community?

I say, “There are three ingredients for an experiment—the source, the sample and the detector.”

You need a source of light that illuminates your sample, which is the problem you want to solve or investigate. To understand what is happening, you have to be able to see the signal produced by the light as it interacts with the sample. That’s where the detector comes in. For us, the detector is like the “eyes” of the experimental set-up.

What do you like most about your work?

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There’s always a way we can help researchers optimize their experiments, tweak some settings, do more analysis and correction.

This is important because scientists are going to encounter a lot of different types of detectors if they work at various X-ray facilities.

I like to have input from people who are running the experiments. Because I did experiments myself as a graduate student, I’m very sensitive to whether a system is user-friendly. If you don’t make something that researchers can take the best advantage of, then you didn’t do your job fully.

And detectors are never perfect, no matter which one you buy or build.

There are a lot of people who have to come together to make a detector system. It’s not one person’s work. It’s many, many people with lots of different expertise. You need to have lots of good interpersonal skills.

What are some of the challenges of creating detectors for femtosecond science?

In more traditional X-ray sources the photons arrive distributed over time, one after the other, but when you work with ultrafast laser pulses like the ones from LCLS, all your information about a sample arrives in a few femtoseconds. Your detector has to digest this entire signal at once, process the information and send it out before another pulse comes. This requires deep understanding of the detector physics and needs careful engineering. You need to optimize the whole signal chain from the sensor to the readout electronics to the data transmission.

We also have mechanical challenges because we have to operate in very unusual conditions: intense optical lasers, injectors with gas and liquids, etc. In many cases we need to use special filters to protect the detectors from these sources of contamination.

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And often, you work in vacuum. With “soft” or low-energy X-rays, they are absorbed very quickly in air. Your entire system has to be vacuum-compatible. With many of our substantial electronics, this requires some care.

So there are lots of things to take into account. Those are just a few examples. It’s very complicated and can vary quite a bit from experiment to experiment.

Is there a new project you are really excited about?

All of LCLS-II—this fills my life! We’re coming up with new ideas and new technologies for SLAC’s next X-ray laser, which will have a higher firing rate—up to a million pulses per second. For me, this is a multidimensional puzzle. Every science case and every instrument has its own needs and we have to find a route through the many options and often-competing parameters to achieve our goals.

X-ray free-electron lasers are a big driver for detector development. Ten years ago, no one would have talked about X-ray cameras delivering 10,000 pictures per second. The new X-ray lasers are really a game-changer in developing detectors for photon science, because they require detectors that are just not readily available.

LCLS-II will be challenging, but it’s exciting. For me, it’s thinking about what we can do now for the very first day of operation. And while doing that, we need to keep pushing the limits of what we have to do next to take full advantage of our new machine.

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SLAC LCLS-II

See the full article here .

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SLAC is a multi-program laboratory exploring frontier questions in photon science, astrophysics, particle physics and accelerator research. Located in Menlo Park, California, SLAC is operated by Stanford University for the DOE’s Office of Science.
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