Gain scanner

The CERN ALICE detector complex will undergo a major upgrade, part of which the complete readout system of its central barrel (the Time Projection Chamber detector) will be replaced with GEM (Gas Electron Multiplier) foils (details in Ref. [1, 2]). Currently there is a relatively large dead time in the readout (up to 500 microseconds), to avoid ions flowing back into the detector area because those would distort further measurements. This new technology will enable ``streaming'' of the data instead of taking snapshots without giving up precision, and this feature makes it extremely useful in the upcoming higher luminosity runs.

The Wigner Institute in Budapest is one of the two Advanced Quality Assurance Centers, we test and evaluate the quality of the foils in a clean room environment. An important part of this process is to measure directly the gain of these individual foils. The gain scanner, shown in the photo below, was designed and built by the detector team here to do just that.

The scanner is basically a Multiwire Proportional pad that is able to locate the hits from the source with a few millimeter accuracy in both directions. The charged particles are provided by a positive beta-decaying source, 55Fe, which emits Auger electrons as well in addition to the X-ray radiation. Those electrons will ionize the Na gas with which the gain scanner's box is filled, and the GEM foil will amplify this signal. The measurement itself is done twice, first we measure the amplification when 0V is applied at the GEM, and then we repeat it at 500V. The gain, which is also shown below, is the ratio of these two.

Knowing the gain might be the most important thing to know about the GEM foils which will be built into the TPC detector. However, this measurement is long and tedious. According to [3] the gain is correlated with the geometry of the holes of the foil. With another equipment we measure each hole (typical diameter is 60 micron), and comparing that to the gain measurement we can test this assumption. If it holds with the required precision of 5%, we can use the much faster optical measurement to qualify the foils. The results are quite promising as shown in the figure below.

[1] F. Sauli, GEM: A new concept for electron amplification in gas detectors, Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, 386 no. 2 (1997) 531-534
[2] F. Sauli, The gas electron multiplier (GEM): Operating principles and applications, Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, 805 (2016) 2-24
[3] T. Hildén, E. Brücken et al., Optical quality assurance of GEM foils, Nuclear Instruments and Methods in Physics Research Section A, 770 (2015), 113-122

Author: Márton Vargyas