Sensing the Energy: Calibrating the LCLS

Oct 01, 2008
The LCLS total energy sensor will sit directly in the X-ray beam during commissioning to help calibrate diagnostic equipment. Photo by Brad Plummer

The Linac Coherent Light Source will generate X-rays 10 billion times brighter than any source before it. Being the first of its kind, the LCLS has presented engineers with a number of unique technical hurdles. Measuring just how much punch the LCLS beam actually packs has proved especially challenging. But a team of LCLS scientists and engineers led by Stephan Friedrich at Lawrence Livermore National Laboratory has solved the problem with a tiny sensor designed to confront the beam head on.

The power contained in a single pulse from the LCLS is estimated to be in the neighborhood of a few billion watts—a tremendous amount to be sure. However, that power will be compressed into an unimaginably tiny sliver of time: around 100 femtoseconds, or one-tenth of a quadrillionth of a second. Because the pulses are so short, the total energy of the beam is relatively low, about 2 millijoules.

Despite their brevity, the momentary burst of X-rays still packs enough punch to instantaneously destroy most materials, a critical consideration for sensor design. Additionally, the sensor must be able to measure pulses at the full repetition rate of 120 per second.

Measuring X-ray beam power is typically a routine undertaking for X-ray physicists. At synchrotron labs, as with the LCLS, scientists prefer indirect measurements that don’t destroy the beam. These are usually accomplished with devices that absorb only a small part of the beam, which would otherwise taint sensitive experiments. One approach uses a device called an ionization chamber, in which the beam passes through a hollow cavity filled with a gas such as helium. The passing X-rays strip electrons from the helium atoms, generating a signal proportional to the beam energy, which gives scientists an indirect way to calculate the energy without disturbing the X-rays.

For an ionization chamber to work, researchers must first know how X-rays of different energies affect helium atoms. That has been well established in synchrotron labs. In the case of the LCLS, laser power of this magnitude has never been created. No one knows what exactly such a beam would do inside an ionization chamber.

Engineers have overcome this challenge with a new sensor that will measure the power directly by intercepting the beam. This will make the beam useless for doing science. However, the information will be used to calibrate diagnostic equipment—such as ionization chambers—during the commissioning phase of the LCLS, before scientists begin conducting experiments. Once commissioning is complete, the sensor will be retired in favor of indirect methods.

Directly calibrating ionization chambers first requires a device that can withstand the X-ray power. Friedrich's team solved the problem using a combination of commonly available materials, starting with a silicon wafer with a thermometer affixed to it. The silicon absorbs the X-rays, converts the energy to heat, and the onboard thermometer responds to that heat with a change in electrical resistance.

The device is mounted to a heat exchanger that cools the sensor back down within a few milliseconds, readying it for the next pulse. In this way, the new sensor can measure the full power of the LCLS running as designed at 120 pulses per second.

Friedrich says his team looked at a variety of different exotic materials that could withstand the X-rays, but settled on silicon because it has been thoroughly researched. "In a project like this, you don't want to reinvent the wheel," he said. "We wanted to work with a material that people have a lot of experience with and that lots of other smart people have thought about. Silicon is a great example."

Source: by Brad Plummer, SLAC

Explore further: Physicist's Nobel Prize up for auction, $325,000 to start

Related Stories

Architects to hatch Ecocapsule as low-energy house

3 hours ago

Where people call home depends on varied factors, from poverty level to personal philosophy to vanity to community pressure. Ecocapsule appears to be the result of special factors, a team of architects applying ...

California farmers agree to drastically cut water use

7 hours ago

California farmers who hold some of the state's strongest water rights avoided the threat of deep mandatory cuts when the state accepted their proposal to voluntarily reduce consumption by 25 percent amid ...

Apple may deliver ways to rev up the iPad, report says

7 hours ago

MacRumors last month said that the latest numbers from market research firm IDC's Worldwide Quarterly Tablet Tracker revealed Apple stayed on as the largest vendor in a declining tablet market. The iPad ...

Recommended for you

On-demand X-rays at synchrotron light sources

17 hours ago

Consumers are now in the era of "on-demand" entertainment, in which they have access to the books, music and movies they want thanks to the internet. Likewise, scientists who use synchrotron light sources ...

New model sheds light on 'flocking' behaviour

May 26, 2015

Understanding how turbulence can alter the shape and course of a flock of birds, a swarm of insects or even an algal bloom could help us to better predict their impact on the environment.

User comments : 1

Adjust slider to filter visible comments by rank

Display comments: newest first

superhuman
1 / 5 (1) Oct 02, 2008
Measuring just how much punch the LCLS beam actually packs has proved especially challenging.


Judging by the solution it wasn't that challenging.

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.