Test-beam tales: Days 4 and 5

Saturday, 20 August

8am: Another early start – meeting the two physicists and taxiing them to the beam area to be on shift for the next eight hours. Both are at the same institute – NCU Taiwan – but, as is often the case in our field, only one is from Taiwan – the other is from India.

Things began well: the data taken overnight looked good so we changed back to electrons and started to take data for different energies, to get an understanding of how “linear” our system is: if the electron energy increases by a factor of two, our signal should also increase by a factor of two. We used electrons at 20, 35, 70, 100, 150, 200 and 250 GeV. This is one of the advantages of the CERN test-beam facility compared to other places – this large energy range.

5pm: The second shift crew of the day took over, another two people from NCU. But this time both were Taiwanese – a faculty member and his new student. We finished taking data with the electrons incident on one region of the sensor so decided to move the setup, to see if another part of the sensor showed the same behaviour. The table we are on can move vertically or horizontally, with good accuracy, even though it can support objects of up to 10 tonnes. We moved the table 3cm vertically downwards, and then for the horizontal movement we simply pushed our setup to the side by 3cm! And then we started to take data again.

The 10-tonne-capacity table supporting our prototype. The beam enters from the left

A quick analysis of some of the data showed that the movement of 3cm vertically was a bit too much, so we increased the table height by 1cm and tried again. This time it looked fine, as evidenced by the “event displays” below. These are pictorial representations of the signals in our sensor. The hexagonal sensors are divided into 128 “cells”, most of which are also hexagons. Each of these cells is an individual readout element, with an area of about 1cm2. In the pictures the signal size in each cell is shown as a colour and a corresponding number. The incoming electrons “shower” in a lead block and these showers leave energy deposits in many cells. It is by combining the information from many cells that we estimate the energy of the electron, and also get a measurement of where the electron hit the lead block, in the vertical and horizontal directions.

Two “event displays” showing the signal in our sensor due to a 150 GeV electron showering in the lead near the centre of the sensor and in one corner

1am: After taking some data in the new position with electrons we again switched to pions for the overnight run. A quick test showed everything looked OK so we headed home. No mammals – apart from some humans – were seen on the journey home today!

Sunday, 21 August

7am: The day started so well (as well as can be expected at that unearthly hour of the morning). The overnight data looked good and we could continue taking data with electrons. The shifters this morning come from India and Turkey.

But just as we arrived in the beam area (just before 8am), the accelerator complex at CERN suffered a power cut. Such power cuts are brutal and can damage electronics on some occasions. This was one such occasion. Experts had to be called in from various locations, to come to fix some problems and get the system up and running. It looked like it would take a couple of hours so I went home!

2:30pm: The experts were still working on recovering from the power cut, and no good time estimate was available so I sent the shifters home. For the next few hours updates came, to say that things were improving, but it wasn’t until much later that the beam eventually came back.

9pm: The beam finally came back! The new shift crew (the same guys as the previous evening) started our system again but there was now a problem with our data acquisition. Some expert help was required, so a few of us went to the area and the problem was fixed. Then we ran into a “standard” problem that everybody faces – a hard disk was full! However, the symptom of this was not obvious and it took some tracking to identify and then fix it. By 11pm we were again ready to take data, which we did until about 1am and then set a long run going overnight.

No mammal photos today, although two mammals decided to try to commit suicide in front of my car. The first, a rabbit, decided to have a race with me: she was pretty fast and, fortunately, decided she had had enough and swerved back onto the field. The other, a badger, just sauntered across the road, without a care in the world. And hitting it was not an option as badgers are hard and it would have damaged my car. After braking she gave me a look of contempt and wandered off. Unfortunately I didn’t get photos of either animal, but the badger looked a bit like this:

"Angry All the Time" by Thomas Hawk (CC BY-NC). Source – https://www.flickr.com/photos/thomashawk/119223684/

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