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Ingrid Gregor | DESY | Germany

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Star Trek IV

Tuesday, October 13th, 2009

Vertex detector geometries of the two design options. Left: 5 single ladders (VTX-SL). Right: 3 double ladders (VTX-DL).

Yesterday I heard a talk about transparent aluminum, and the speaker actually referred to the aluminum Scotty created in Star Trek IV. Transparent aluminium is a state of aluminium achieved by bombarding a very thin (50 nm = 0.000050mm) Al foil with soft X-ray laser radiation (wavelength 13.5 nm). Such a laser source can be found here at DESY – the FLASH facility. The short laser pulse knocks out a core L-shell electron from every aluminium atom without breaking the crystalline structure of the metal making it transparent to X-rays of the same wavelength. More details can be found here.

Unfortunately the aluminium is only transparent for a very short moment, so there is still a long way until the material scientists can compete with Scotty. And 50nm is also not really a thickness one can handle, and of course not build a basin to transport a pot whale through time….
But why am I interested in thin material? Something really thin but very stable is what we need to build the vertex detector at the Linear Collider. The vertex detector is the innermost detector directly around the beam pipe measuring the first few cm of the created particles’ tracks (vertex). We need to build this really thin to allow high precision tracking also with low momentum tracks. If we chose the material to thick, this for our studies very important particles loose too much energy before they can reveal their secrets to us.

And by “thin” we do not mean only the thickness measured in mm, but also the material properties are counted. We usually speak of radiation length X0, a characteristic of a material, related to the energy loss of high energy particles when passing through the material. X0 is both, the mean distance over which a high-energy electron loses all but 1/e* of its energy by bremsstrahlung, and 7/9 of the mean free path for pair production by a high-energy photon. We usually do not give the length itself, but we rather say that a certain thickness of a material is x % of a X0. This helps us to compare material directly. The radiation length X0 for Silicon is 93.7 mm so we say 100micrometer of Silicon is 0.106% of an X0. But for example 100 micrometer of lead are 1.9% of X0, so it is way less “transparent” to particles than silicon and we of course would not build the vertex detector out of lead.
Typically the sensor material for a vertex detector is Silicon due to the very nice possibilities to measure particle tracks with it, as for example described by Frank or me earlier on this blog.

The vertex detector for the Linear Collider should be in the order of 0.8% of material. Looking at the numbers above one can see that this is a real challenge as we would like to have 5 layers of silicon. The silicon needs to be thinned to about 50 micrometer, so half the thickness of a standard piece of paper.
But a detector is not only made out of silicon. The silicon has to be mounted around the beam pipe by some mechanical structure. Carbon fiber composite is a nice material, but still too “thick” to build the vertex detector. New ideas are carbon foam making a real light-weight detector.
The biggest challenges in the material business are the so-called services. We need to get power and cooling to and from the detector, as well as digital signals to steer the electronics. And of course we want to get the data out of the detector. All of this means cables, hoses, and pipes, and this of course means material.
So you can see, building such a detector is a real challenge and we need some time to handle all this problems. Some of the problems can’t be solved; then we have to look into the impact on what we want to measure and maybe have to adjust elsewhere. And this is also an answer to Paul’s comment some time ago “It *still* seems odd to be upgrading something that isn’t running yet?”

Maybe we should try to find Scotty, he always had a solution for this kind of problems, and I would love to own a tricorder!

A tricorder

A tricorder

*e = 2.71828…, the Base of Natural Logarithms

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The Matrix

Thursday, October 8th, 2009

2009 Nobel Prize Winning Physicists From L to R: Willard S. Boyle, Charles K. Kao, and George E. Smith.

2009 Nobel Prize Winning Physicists From L to R: Willard S. Boyle, Charles K. Kao, and George E. Smith.

The 2009 Nobel Prize in Physics went to Charles K. Kao Willard, S. Boyle, and George E. Smith for revolutionizing the way in which information can be transmitted globally. Charles K. Kao received the Prize for groundbreaking achievements concerning the transmission of light in fibers for optical communication. S. Boyle and George E. Smith for the invention of an imaging semiconductor circuit – the CCD sensor.

A CCD sensor can be found in scanners, digital cameras, video cameras, and mobile phones. The CCD sensor consists of a matrix of light sensitive photo diodes converting the light into electric charge. The little photo diodes are coupled electrically together (charged coupled device = CCD) and this enables that the charge can be transported from one pixel to the next. This is done column-wise. A good picture describing this process is transporting water via a chain of buckets; the water is always put from one bucket to the next. Outside of the sensor the information is put back into a picture like a mosaic. The invention of this device was 1969, but it took 12 years until the first device was on the market, with 0.3 Megapixels. But they became only a real success in the last 10 years when the world wide digital data transfer picked up speed.
And did this have an impact on particle physics? Yes, definitely! I think the development of the pixel sensors for particle physics definitely profited from the advancements in this field. Semiconductor processes in general had a great impact on the detectors for high energy physics. Only because the market was always asking for even faster chips, with more functionality integrated, the structures within the chips were decreased in the size. And this small size structures give the possibility to design radiation tolerant electronics with a lot of functionality integrated. And with these new technologies we can get the best out of our detectors.

A good example for this development is shown in the picture below. The device is a front end card from the ZEUS calorimeter, developed at the end of the 80s, at a time when not every body had a computer on the desk and long before we thought of mobile phones.
This front-end board digested the signals from 12 channels. Nowadays the functionality of this board could be integrated in a chip of the size of the nail of my small finger, and probably not only for 12 channels, but for 100 channels. One very nice development in the field of pixel detectors was described by Frank just yesterday.

I was really happy to see that the Nobel committee selected this topic and awarded the fathers of the CCD sensors and optical data transmission.

A front-end card from the ZEUS calorimeter for the readout of 12 channels. Development of the end of the 80s.

A front-end card from the ZEUS calorimeter for the readout of 12 channels. Development of the end of the 80s.

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Into Thin Air

Wednesday, September 23rd, 2009

On quantum.diaries a lot of different sides of the life as a particle physicist were covered so far and I think a rather complete picture of this profession was drawn. But one tedious task was not yet mentioned (at least I can not recall), but we have to do it all the time – writing proposals.
We have to write proposals and submit it to the ministry of science to secure the budget for the institute, proposals to the EU to get additional funding, proposals to our directors to convince them about the importance of a new study, and, and, and ….
Of course it is very important that we carefully sketch our scientific plans and how we want to spend the money, also to avoid that the tax-payer’s money is wasted. But there are days, when I really do not like this part of the job.
A proposal has to be clear and complete; that sounds reasonable. But not only the actual scientific work has to be described, but also details on who should do the job, what is needed to do the job, and what technical support is needed. Sometimes one has to give an exact number of how many hours will be needed to get a job done. But this is the hard part about it. As we are doing science, we sometimes do not know how long it will take to reach a certain goal, as it was not done before. I really do not know how long it takes to design a new electrical board, program it and test it. It is a new board, otherwise we would buy it somewhere. All little details have to be checked carefully, and therefore a proposal can eat up a lot of time. And depending of the kind of proposal, it is send to its destination, and after some weeks the only thing one hears is, that the proposal was not accepted.
On the other hand all the work can be worth it. A few years back our proposal to the EU was accepted and the EUDET consortium was started. The EUDET telescope was partially financed by this consortium and therefore EU money is in it. It is a very nice project and it seems that the telescope was also needed, as it is now used by a lot of groups. So I will be keeping this in mind when working on a new EU proposal in the next 2 month ….

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Ingrid vs. InGrid

Wednesday, September 16th, 2009

As parents think about a name for a new baby, experimentalists give their experiments names. Very often one finds unpronounceable 3-letter names, but sometimes particle physics collaborations find nice names, such as ZEUS or ATLAS (ok, I am biased here 😉 ). Also smaller devices or subsystems get a name, giving us the possibility to speak in a terrible slang outsiders don’t understand anymore. Some of my colleagues are real masters in speaking this slang making sure, that not only outsiders do not understand them.
In the recent years I realized that also my first name, Ingrid, ended up in the community as name for different devices. And this can actually lead to very funny situations. For example a more recent idea to read this micro pattern gas detectors (MPGD) with a pixel anode. In order to accurately control the alignment of the grid geometry, the integration of the MPGD amplification and the pixel chip is done by means of ’wafer post-processing’ technique. With this technology, the structure of thin (1 μm) aluminum grid is fabricated on top of an array of insulating pillars of typically 50 μm height, which stand above the pixel chip. This forms then an integrated readout of the gaseous detector. This technology is called Integrated Grid – abbreviated as InGrid.
Whenever I listen to a talk where these integrated grids are discussed, I have to smile. For example the InGrid experts like to show the behavior of the system at different high voltages and then discuss “Ingrid at 400V” versus “Ingrid at 600V”. The thought of being tested at these high voltages is not very appealing, but the InGrids they talk about seem to be fine with that.
Another device, also called Ingrid, is a scintillator subsystem of the ND280 detector at T2K, a long-baseline neutrino oscillation experiment in Japan. In this system Ingrid is consisting of 16 modules interleaved with iron. So this is a rather big device. Two years back I was at a conference where somehow a presentation about this rather big device slipped into a session about micro pattern gas detectors (as described above); the convener probably had so many talks with “InGrid” in the title, he misplaced the talk about the neutrino facility. The poor speaker! He had to present this completely out of topic presentation in front of tracking people (tracking detectors are usually comparable small and light weight). Everybody knew that this is the wrong session and we were already slightly amused. And then the speaker said “Ingrid weighs 7 tons”, so all my friends and myself started to laugh. And the speaker had no clue what he said to make us laugh.

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In Vino Veritas

Sunday, September 6th, 2009

Yesterday I went to a very important event, at least according to my friends here at CERN – the wine market (Salon de Vins) in Thoiry. In a small hall and an attached tent 42 winegrowers presented their wines. Of course with the possibility to taste samples of the different sorts and vintages. We started out with testing champagne, went then through a number of white wines before we worked through all the red wines. As I was the designated driver, I mainly watched the tasting or smelled at the noble liquid to get an idea about the differences. But I also tried here and there.

But this was not a big party where everybody got drunk (my first understanding of this event) but rather serious business. We had a long list of orders from friends. And they did not order only single bottles but always boxes of 6. For every ordered wine type we had to find the correct booth in the exhibition, greet the winegrower (the friend I went with was very well known), taste the different vintages, select, negotiate, wait for the bill, pay, collect the wine, say goodbye and bring the lot to the car. And this for something like 15 different wine sorts…. During the afternoon the trunk of the car filled significantly and my friend had more problems to keep the oversight of what we had to look at next. Sitting down, eating some bread and drinking some water always helped to refocus.

But even so this was serious business, I had a real fun afternoon. Not drinking did not spoil the event at all. I learned a lot about wine and French wine growers and have now a better idea of what kind of wine I like. At the same time I met a lot of nice people from CERN (somehow most of the people we bumped into used to be working for OPAL). And the car did not break down, even so we put 189 bottles of wine into the trunk.

All together 189 bottles in the trunk....

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Gone with the Wind

Sunday, August 30th, 2009

Last week I went camping on a northern German island – Spiekeroog. It is a beautiful little East Frisian island just off the German coast where no cars are allowed. The campground was very natural with little dunes and sandy – just beautiful even so it was difficult to find a flat spot for the tent. In the first night I was sleeping not too well as I was always sliding towards the far end of my sleeping mat. But this is normal when camping and part of the game. During the days we did long hikes along the beaches, went swimming or when it was windier, played with the stunt kite. This mini vacation was only a few days, but really nice and great fun.

In the end the wind increase quite a bit and my tent started to have problems. As we were planning to leave in the evening, I decided to take it down and pack it up. But somehow the tent decided to crash before I was done with packing….

My tent in the wind

My tent in the wind

... and after it collapsed while I was removing my stuff.

... and after it collapsed while I was removing my stuff.

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Voodoo Lounge

Thursday, August 20th, 2009

Last week I was working together with “my” summer students Jeanette and Silvia at the DESY test beam. We looked into the electronic noise of the “old” ZEUS strip telescope.

The ZEUS MVD strip telescope at DESY test beam

The ZEUS MVD strip telescope at DESY test beam

This is a relative old telescope build something like 10 years ago to characterize the ZEUS MVD silicon strip modules (MVD stands for Micro Vertex Detector and was an upgrade silicon strip detector of the ZEUS tracking system). Since then many different groups used the telescope as a tool to define the tracks of the test beam particles at the DESY test beam. But as you can imagine, the system is getting old after 10 years of operation. And recently the performance is not as it used to be. Mainly the electronic noise increased quite a bit. Electronic noise is a random signal characteristic of all electronic circuits and a constant offset underlying the real signal. Therefore the signal to noise ratio is a crucial performance parameter of our detectors. If the noise increases too much, the signal disappears in this carpet of noise and we have a hard time to track the particles. There are of course books on how to design and set up the readout system to reduce the noise to a minimum, but somehow noise is  most of the time  tricky. This I experienced many times before.

When Jeanette, Silvia and I were investigating the noise of the telescope last week we checked if the grounding was done according to the book (no loops, connections from on ground to all component etc.). Everything seemed to be ok, but somehow we still had a high noise level. So we started to change the system, just to see what happens. Whenever we expected the system to improve, it got worse and vise versa. In the end we found a configuration with really good noise behavior, but we absolutely did not understand why it improved.

So this was a really nice demonstration for the summer students, that an exact science like physics can also be something not at all exact, but rather like voodoo…

Silvia and Jeanette taking data

Silvia and Jeanette taking data

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Rockin’ in the free world

Sunday, August 16th, 2009

I actually wanted to write about summer students and voodoo, but I first have to report on an absolutely great concert I saw last night.

Pear Jam on stage August 15th 2009 in Berlin, Germany.

Pear Jam on stage August 15th 2009 in Berlin, Germany.

Pearl Jam played in the Wuhlheide in Berlin last night. The Wuhlheide is an Open Air stage in the middle of a forest with a capacity for 17000 people. It is a beautiful location I love to go to for concerts. Going to concerts there means, taking the local train to a station close to the place and then walking through the forest to get to the entrance. Left and right from the way are people selling drinks and Bratwurst, or trying to buy or sell the last tickets. It is a nice walk for about 15 minutes and somehow is part of the whole event as one walks with thousands of people going to the same nice event. To get in to the arena itself we had to wait quite a while, but we could enjoy it by standing in the sun and listening to what the people around were talking about. As this was one out of only 4 concerts in Europe on this tour, the crowd around us was very international. I heard many different languages, from Italian, Spanish to Polish and Swedish. Finally inside, we, a group of six, found a nice spot in front of the sound mixing guy (usually the spot with the best sound). Already the first band, Gomez, was amazing! I didn’t know them before, but I guess I have to add them to my collection.

At about 9pm, Pearl Jam came on stage, and the atmosphere exploded! 17000 people were singing along from the first tune on. It seems that Eddie Vedder, the singer, had a good monitor system, otherwise he wouldn’t have heard himself anymore. The play list of the evening was a nice mix of songs from many of their 9 albums. It was really really nice. Here an example song, some people are really quick to upload material.

One of the encore songs was “Keep on rockin’ in the free world”, written by Neil Young. Pearl Jam said, that the played this song often before, but never in Berlin. Due to the history of Berlin this song gets a very special meaning! All in all it was one of the best concerts I ever went to and I enjoyed a lot.

The next concert I am planning to go to is Billy Talent in Hamburg!

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In 80 days around the world

Thursday, August 13th, 2009
statistik

My flight activities (long distance) of the recent years ... It looks similar or with more flights for most of my colleagues.

…. ok, it took me a few days more to get around the world, but in principle this is possible as a particle physicist. If you followed this blog, you will have realized by now, that traveling is one major activity of this community. Working in worldwide collaborations means, that we need to travel to meet our collaborators. Of course, also mentioned in other entries in this blog, we use phone meetings, video meetings, and other means to avoid traveling. But in the end we travel around the world quite a bit. Whenever a bunch of particle physicists is hanging around together, like in the evenings during a conference, sooner or later they start to talk about airlines, airports and travel adventures. Each of us has a repertoire of stories about getting stuck at the remotest places on the earth, missed planes, the most bumpiest flight etc.


As the community is not so large, people meet not only ones in the life. We actually bump into each other at different places in the world. I have actually a number of friends I meet only ones per year during one specific conference. We always have a nice time, having fun during coffee breaks or meeting in the evening for a beer. At the end of the week we say goodbye and say something like “See you next year in Orlando”.

Even so the community is small and after some years you know a lot of the people, we don’t know everybody. So usually name batches are given to the attendees of larger meetings. I started to collect this name batches when I started my first post-doc position. In the picture you can see the mountain of name batches I collected in the last 7 years… I wonder how big this will be at the end of my working life.

Probably by then we will have some implanted mini LCD name batch…

My name batch collection of the last 7 years.

My name batch collection of the last 7 years.

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Zeus and Hera

Thursday, August 6th, 2009

ZEUS at Hera with open shells to see the calorimeter.

ZEUS at Hera with open shells to see the calorimeter.

Today I received a present from a colleague who is cleaning out his office. It is actually a mobile (something to hang underneath the sealing moving slowly) with a number of HES sensors from the ZEUS detector at HERA. This reminded me, that we report in this blog a lot on collider experiments just about to be turned on, or experiments to be constructed in the distant future. But ZEUS for example is a beautiful experiment constructed almost 20 years ago, running successfully for 15 years and turned off about 2 years ago. The analysis of the data is still in full swing and we expect many more beautiful papers.

The HERA storage ring was the only one in the world in which two different types of particles were accelerated separately and then brought to collision. In the 6.3-kilometer-long tunnel electrons collided with hydrogen nuclei, i.e. protons from the hadron family, nearly 2000 times heavier. In these electron-proton collisions, the point-like electron acts like a tiny probe that scans the inside of the proton and reveals its inner structure. ZEUS and H1 were the two experiments used to reveal these interactions. A very nice summary of the most important results can be found here.

ZEUS was located in the south hall at the HERA ring and was 10m x 12m x 19m large and had a total weight of 3900 tons. The heart of the ZEUS detector was the uranium scintillator calorimeter (CAL), which measured energies and directions of particles and particle jets with high precision. The CAL was surrounding the tracking detectors consisting of a vertex detector, the central drift chamber, the forward and backward drift chambers, and in the forward direction a transition radiation detector to identify high-energy electrons.

From 2005 to the end of ZEUS data taking in 2007 I was part of the team keeping the detector alive and running, namely the Uranium calorimeter. It was a rather interesting challenge for me, as until then I was working mostly with new detectors in the development phase. But ZEUS was at that time 13 years old, using electronics developed at the end of the 80’s, a complete different time when looking at electronics. No mobile phones, no desktop computers etc. I had to learn how to get the best out of the detector. Very quickly I realized that the calorimeter had a personality. It was like an old car, which stops moving when you shout at it (ok, I admit that my car also has a personality…).

Of course phone calls in the most inappropriate moments, or in middle of the night, were daily business. It happened many times that I had to go to the hall around 3 am and restart the calorimeter as it “tripped” due to a power glitch. A very likely event in the summer time with a lot of thunder storms around. Or some other, for the calorimeter vital system, had a hick-up. But this is in the end a very rewarding job. At least if I got the problem fixed (luckily most of the time the case), I felt very good when going back to bed. And I of course I shared this task with other people and did not have to leave my bed every time there was a problem.

The ZEUS collaboration about 5 minutes after HERA was switched off.

The ZEUS collaboration about 5 minutes after HERA was switched off.

At the end we had to turn ZEUS off, after 15 years of successful operation. This was done with a nice big party. All current and former ZEUS members were invited to do the last shift together. As this many people, about 300, did not fit into the rather small control room, we put up a stage in front of the experimental hall, a beamer, and a screen. All the online plots were displayed on the screen so that we could watch the data taking and the moment of the last beam dump. A tent was organized, and food and drinks were served. We actually had a great time; even so the moment of turning off the detector was a sad moment. You can find a funny summary of this event also at the Symmetry magazine.

By now the south hall is completely empty, but I think I will report on the disassembly of the detector another time.

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