New PMTs for the ALICE V0-C detector

Panos Charitos

During the 2011 proton-proton run it was observed that the efficiency of the PMTs used in the V0 detectors had started to deteriorate. Gerardo Corral says: "At that time time we still had to take data and we could make no intervention but we continued monitoring the performance of the PMTs. Based on a series of measurements, it was clear in 2012 that the effect was probably due to radiation and ageing effects. Our colleagues from the University of Lyon had the chance to remove a number of PMTs from the C side of the V0 detector and make some more detailed testing and measurements." He continues: "They took out 6 PMTs out of the 32 that are used in the C side and confirmed that their performance was reduced. We knew that we had to deal with this problem and the LS1 was the perfect opportunity”. For the A side of V0 things were more difficult as this side of the detector lies closer to the region of the beam pipe and one had to be very careful to avoid damaging the pipe."

The new PMTs arrived near the end of 2013 and were calibrated to be ready for installation. Gerardo explains that this is not just a replacement but also an upgrade as the new PMTs can operate at lower voltage. Reducing the high voltage means that one is also reducing the after-pulse signal which was one of the problems that V0 faced from the first runs. With the replacement of the PMTs the team is able to tackle these two issues: “We have PMTs with better gain that also work with lower voltage and thus reduce the after-pulse effect”.

Installing the new ALICE PMTs: Solangel Rojas Torres, Ildefonso Leon Monzon, Gerardo Herrera Corral, Arturo Tauro, Werner Riegler, Pieter Ijzermans and Elisa Laudi.

The installation of the new PMTs took place during the second week of April. All the PMTs on the A side have been replaced and soon the team will start working on replacing the PMTs on the C side of V0. Gerardo explains: “The A side is much more complicated, though it lies 3.3 m from the IP it is still in the beam-pipe. We had to move the V0-A 30 cm away on the region where a very delicate beryllium pipe sits. It was a very slow process during which we took a lot of precautions”. Gerardo Herrera Corral with Ildefonso Leon Monzon and a PhD students Solangel Rojas Torres worked closely with members of the ALICE Technical Management team, namely Werner Riegler, Arturo Tauro, Corrado Gargiulo, Pieter Ijzermans and Elisa Laudi.

The installation has been very successful since we had no accidents or other causes of delay. The PMTs have now been tested and a very good signal has been measured in all of them.

PMTs in V0

The V0 detector is a disk with 42 cm diameter and 2.5 cm thick. It is segmented in 32 cells with each cell linked to readout with optical fibres. When a charged particle crosses the plastic light is produced by scintillation. The fibres take out the light and shift the wavelength from blue to the green part of the spectrum. So green light arrives to the PMTs since their photocathode is more sensitive to these wavelengths. Through photoelectric effect electrons are emitted from the photo-cathode and travel through an elecromagnetic field; they hit a series of dynodes, amplifying the number of electrons and in that way the signal is strengthened.

V0 is very important for ALICE as it provides level – zero triggering. The new PMTs will give better efficiency but also allow reducing the after-pulsing signal. When you hit the window you have a pulse of electrons coming from the dynodes and a few nanoseconds later you have a second pulse that is not authentic but is created in the PMTs due to the ionization of the gas. The vacuum in the PMTs is not perfect; the gas atoms are ionized by the electrons and go to the opposite direction as they have positive charge, they hit again producing more electrons and give a second pulse. This is very bad for triggering as these signals are fake triggers that we have to suppress. With the new PMTs the after-pulse probability is ten times lower and V0 will be better equipped to play its triggering role in the forthcoming run in 2015.

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