8. December 2015

Measuring the simultaneity

Research Using Muons Particle Physics

What does a physicist do when his experiment needs an extremely precise time measurement? So precise that existing electronics cannot help him? Stefan Ritt simply decided to develop his own solution. The result is called DRS4, a high-precision electronic chip that could unlock the physics of our entire universe. As an additional benefit, the chip is already helping doctors to localise brain tumours with great accuracy.

How long is a moment? What exactly is now? Who decides whether two events are taking place simultaneously? Stefan Ritt has found a pragmatic answer to these questions: he breaks down time into units of a billionth of a second each. Thus, for him, simultaneously means within the same billionth of a second.

By using this definition, Ritt, a physicist at PSI, has certainly not put scores of philosophers and psychologists out of a job. This definition of simultaneity applies only in the context of his electronic chip.

Stefan Ritt is a physicist and research group leader at PSI, but the right-hand side of his office looks like an electronics workshop. He describes himself as an electronics enthusiast, and when he encountered a problem with the electronics in an experiment he was working on, he set about finding the solution himself.

Millions of muons for universal questions

The experiment in question concerns exotic elementary particles known as muons. Almost as soon as they are formed, muons decay into three other particles, some more exotic than others: an electron, a muon neutrino and an electron antineutrino. However, some physical theories suggest that there is another decay mode, albeit extremely uncommon, where a muon decays into precisely one electron and one light particle. For years, the physicists at PSI have been searching for this second type of decay, or rather for the products: matching pairs of one electron and one light particle. They want to determine whether the decay is only very rare or whether it is entirely impossible.

For the research community, this muon experiment is crucial: if this decay were to exist, it would lend credence to a whole family of physical theories. If it does not exist at all, this would confirm a different school of thought. And this has far-reaching consequences, because these theories describe nothing less than our universe.

30 million muon decays per second: that is the number of incidents measured by Ritt and his colleagues when their experiment is running. It is only through this sheer mass that the fine line that separates extremely seldom from entirely impossible can be drawn with more and more precision. The process is similar to searching for a needle in a haystack, except that no one knows whether the needle even exists.

The pair in the mass

As a result, the researchers have to pull out all the stops. In doing so, they make use of the special pairing of electron and light particle. Stefan Ritt therefore explains his experiment with a better analogy than the haystack: Our data is like a bustling crowd of people. And in that crowd we are looking for that one couple holding hands and moving along together.

Of course moving along together has to be taken in a figurative sense. In practice, the two products of the decay would actually spin off in opposite directions. And that is the first in the set of tricks the physicists use: They search for one electron and one light particle that hit the detectors on exactly opposing points of their large-scale experiment. The second trick is that these two products must each have precisely half the energy of the original muon. And the third and final trick is that the electron and the light particle could only belong together if they are detected at precisely the same time.

This is where Ritt’s definition of simultaneity and the billionth of a second come into play. Of course it is ultimately a computer that evaluates the data and a piece of software that decides whether the condition of simultaneity is met. But in order to do so, the data first has to be entered in the computer.

This is because the experiment produces analog data. Computers on the other hand can only handle digital data. Nowadays almost every physics experiment requires an analog-to-digital translator, which technicians refer to as an A/D converter. Normally a standard solution such as an oscilloscope is sufficient for this purpose, and scarcely any researcher has to worry about this component in their experiment.

He plans on the left, builds on the right

Not so in the case of Ritt’s muon experiment. With 30 million muon decays per second and timing precision down to a billionth of a second, ready-made electronics would have been completely inadequate. We knew from the very beginning that we would have to develop something special, something that didn’t yet exist, explains Ritt.

That is how it came about that Ritt took a peek over the shoulders of the electronics technicians at PSI in 2003. Soon, circuit boards and plug housings, electronics components and soldering irons began to accumulate in one part of his office. Standing in the middle of the room now feels like standing between two halves of a brain: Ritt does the theoretical deskwork on the left and builds electronic components on the right.

After several years of development, the result is now an electronic chip called DRS4. This electronic component is just about one centimetre by one centimetre in size and has a flat design. If conventional A/D converters can be described as a translator between the experiment and the computer, then DRS4 is a multiple simultaneous interpreter at the World Championships for fast speaking people: On eight channels at the same time, each chip processes an extremely quick succession of data. Because several thousand detectors are used in the muon experiment, several hundred of these chips are needed.

In short, the chip senses each particle signal and finds its precise temporal position. The latter can thus be determined precisely to within a billionth of a second. This is a huge technical feat. Stefan Ritt describes it in a more matter-of-fact way: I am not aware of any other electronic chip in the world that can offer such precise time measurement.

The DRS-chip did not fail to catch the attention of the Institute of Electrical and Electronics Engineers (IEEE), leading to Stefan Ritt recently being awarded the institute’s prestigious fellow status.

Locating brain tumours through time measurement

The highly precise time recording makes the chip interesting for other scientists as well. While talking with colleagues from other specialist fields at international conferences, Ritt found that they were confronted with exactly the same problem as he was. PSI now supplies DRS4 all over the world. For example, a medical research group in Tübingen in southern Germany is using the chip to localise brain tumours with a maximum degree of accuracy. The procedure used is known as Positron Emission Tomography and involves placing the patient’s head in a ring of detectors. The detectors register the decay particles of a radioactive material that has been introduced into the tumour cells. If a particle reaches a detector at the top of the ring sooner than the corresponding particle at the bottom of the ring, the tumour must be located above the centre of the ring. In order to localise the tumour precisely using this procedure, doctors need the exact time measurement provided by Ritt’s chip.

The electronic chip is sold through PSI’s Technical Transfer program at cost price. As an advocate of Open Source at all levels, Ritt is wholeheartedly in favour of this approach; the software he himself wrote for the circuit board is of course also provided free of charge.

An electronics technician knocks on Stefan Ritt’s door and enters his office. He explains that programming of the circuit board is stuck and he doesn’t really know what to do next. No problem, says Ritt, Leave it here, I’ll take a look at it in a minute.

Physicist, electronics technician, programmer – one thing is clear: Being a research group leader alone would not be enough for this man.

Text: Paul Scherrer Institute/Laura Hennemann

The domino effect

Ritt explains the technical details of his electronic chip using dominoes: After all, the name DRS stands for Domino Ring Sampler. Each detector supplies a constant electrical signal to the chip. If a particle hits the detector, this signal curve displays a corresponding peak. The task of the chip and the related circuit board is to determine the precise temporal position of that peak.

For this purpose, the output of each detector is broken down continuously in the chip into 1,024 time intervals. The corresponding measurement points are written to 1,024 memory cells on the chip. From a technical perspective, these memory cells are capacitors, but for simplicity Ritt talks about 1,024 little buckets into which the information is filled. In order to fill each bucket, a corresponding switch has to be actuated.

The Domino Ring Sampler

The problem is the data flood of the experiment, which is why these buckets have to be filled extremely quickly. This is why Ritt created a virtual row of dominoes on the chip. Each falling domino uses a switch to fill a bucket and triggers the next domino at the same time, which in turn actuates the corresponding switch and so on. In addition, the dominoes are arranged in a circle. This means that all 1,024 buckets are filled over and over again from the beginning, so their information is updated in a continuous loop. This explains the term Ring Sampler in the chip’s name.

If an electron or light particle now hits the detector, the domino effect only needs to be stopped in order for the signal of the detector to be distributed and stored to the buckets. There is scarcely any faster method available.

The information can then be read more slowly and at leisure from these buckets. The measurement curve distributed to the 1,024 buckets, i.e. comprising 1,024 storage points, is transmitted to the electrical circuit board on which the chip is located. This circuit board is roughly the size of a paperback book, but is able to replace an expensive oscilloscope the size of a shoebox which the researchers would otherwise have to use. Later in the process, the circuit board itself is responsible for converting the analog data into digital data and transmitting the digital signal to the computer. This is where the final calculations are made. Stefan Ritt programmed the software needed for this purpose himself.

Additional information
DRS chip website
Popular article about research into rare muon decay

Dr. Stefan Ritt, Laboratory for Particle Physics,
Paul Scherrer Institute, 5232 Villigen PSI, Switzerland,
Telephone: +41 56 310 37 28, e-mail: stefan.ritt@psi.ch