In the spirit of the ongoing LS2, the time without particle beams is currently used to significantly upgrade the two CERN Irradiation facilities run by EP-DT, namely IRRAD and GIF++.  Both facilities deliver essential services to the HEP community, with the focus on validating and optimising detector technologies for the High-Luminosity upgrade of the LHC in 2025 and CERN projects beyond.  In this series of articles, we will describe the different upgrade projects based on very different requirements. While the IRRAD upgrades need to fit within the overall East Area renovation project, the Gamma Irradiation Facility GIF++ needs to be maintained in operation throughout the whole LS2 to support the ongoing mass-production and ageing tests for muon detectors of the LHC Experiments.

Located on the H4 beam-line in EHN1 North Area, the GIF++ is a unique place where a high-energy muon beam (150 GeV/c) is combined with a strong 137Cs gamma source. The facility has been built in 2014 in a shared effort by the EN and EP department and was specifically designed to test real size detectors of several square meters, as well as to host a variety of smaller prototype chambers. Equipped with a comprehensive gas distribution system, a wide range of available gases, mixer units as well as the possibility to use premixed gas bottles, the facility regularly hosts muon detectors from all four LHC experiments as well as from outside collaborations.

CERN’s Irradiator (¹³⁷Cs, 14 TBq as of 2014) is operated throughout the year, independently of the SPS beams, and provides two independent gamma irradiation zones with a total floor space of more than 100 m². Two attenuator filter systems allow the gamma field in each zone to be tailored to the needs of the setups under test. It can provide high fields (≤ 2 Gy) for ageing studies or can be used to simulate the expected background conditions for which each detector has been designed. The muon beam available during specific runs (6-8 weeks per year) can then be used to show the tracking performance under these expected conditions. The GIF++ is therefore a vital test facility to prepare the LHC experiments for the upcoming HL-LHC.

Replacement of the original GIF (West Area) in 2014, the introduction of two independent irradiation zones, and more than doubling the existing area, allowed the GIF++ to host many more setups throughout the year and increase its user base. However, in 2018 it became clear that the success of this facility was pushing it to its operational limits. With more than 20 large scale setups hosted throughout the year and up to 11 setups tested simultaneously in the muon beam (Fig.1), the situation became critical and called for major upgrade plan.

Figure 1: The Irradiation bunker during the muon beam time in October 2018.

To overcome the impelling space problems, an extension of the irradiation bunker was proposed. With the help of EN-EA, part of the upstream area on the H4 beam-line could be freed, allowing an extension of the irradiation bunker by 9.6 meters, adding nearly 60 m² of radiation field. This one-sided extension would also create a dedicated low-radiation zone (by up to 14 m distance from the source), making the test of multiple chambers with different gamma field requirements possible at the same time. This project was supported by all four LHC experiments as well as the EN and EP department. In a combined effort, the required funding was allocated, and the preparation work could start in late 2018.

One of the main challenges of the extension project was that the mass-production tests for the ATLAS New Small Wheel (NSW) chambers could not be delayed, and several long-term ageing studies (ATLAS, CMS) needed to be continued. In addition to the scheduled tests, ALICE made an urgent request to test TPC upgrade chambers in the GIF++ following the discovery of a hardware fault. All chambers needed to be tested, and in case necessary be repaired and re-tested in time for the installation in Point 2. Therefore, the GIF++ facility needed to be kept operational for a maximum of days during 2019. On the other hand, the parallel mass production test required by ATLAS for the sTGC and MicroMegas chambers could greatly benefit from the additional space of the extended bunker.

By carefully rearranging the corner blocks of the existing bunker shielding, it became feasible to prepare the extended bunker area independently from the facility operation. Over several weeks, the bunker walls, cable trays, false floor etc. could be installed (Fig.2). Finally, in July 2019, the facility was stopped during only 2 additional weeks to finalize the extension followed by one week to perform the compulsory annual maintenance of the ¹³⁷Cs Irradiation system.  

Figure 2: Preparation of the shielding wall for the bunker extension (top left). Installation of the false floor (top right). Combining the bunker parts by removing the separation wall (bottom left). The extended irradiation bunker during commissioning (bottom right).

To optimise the available space in both irradiation fields, the Irradiator was also displaced by 90 cm towards the upstream wall (Fig.3). This freed the necessary space in the downstream area for the mass-production chamber tests, while still preserving the low-irradiation field in the far upstream. On 19th of July, the DSO test of the extended facility was successfully past, and the facility could restart is full operation on time providing now more than 100 m² of irradiation floor space.

Figure 3: To optimise the usage of both irradiation fields, the Irradiator has been relocated to 90 cm upstream (left). Installation of the cosmic tracker support (right top) and the first 3 muon chambers of the roof tracker based on ATLAS RPC technology (right bottom).

In the shadow of the main extension project, multiple improvements could be accomplished in 2019, including a doubling of the primary gas distribution panels. Thanks to the work from a summer student, the operation web page of the facility (https://gif-irrad.web.cern.ch) has been completely redesigned, and it is now possible to retrieve all centrally stored operation values (e.g. Irradiator status, applied filter, environmental and gas parameters, etc.) via a simple web interface. Several days were spent for removing no longer used cables to guarantee enough free space in the cable pass-throughs for future installations.

Since the muon beam, produced by the SPS, will not be available until the second quarter of 2021, we took an extra effort to improve the facility capabilities to trigger on cosmic muons. Built from ATLAS RPC muon chambers, the first 3 (out of 4) roof chambers have now been installed and – with the previous installed ground confirmation chambers - complete the cosmic muon tracker in the downstream area. To provide additional trigger capabilities also in other spaces inside the bunker, a prototype of a mobile cosmic trigger has been developed (Fig.4). Build from 4 recuperated muon chambers (M1R3, double layer) that became available from LHCb during LS2, the frame is designed to fit around installed setups and can be tilted as needed to get the best compromise of cosmic muons and gamma irradiation. The first results of the cosmic trigger being able to work in the variable gamma background are very promising. If the need arises, the system can easily be extended or duplicated. 

Figure 4: Prototype of a mobile cosmic trigger build from recuperated LHCb muon chambers. The frame is designed to fit around a typical muon chamber under test inside the irradiation bunker.

With the majority of the upgrade work completed in the GIF++, the focus in 2020 will now shift to the upgrade of the CERN proton IRRADiation facility (IRRAD) in the East Area (www.cern.ch/ps-irrad). The main LS2 projects for IRRAD concern the extension of the storage and sample-handling area and the renewal of the beam instrumentation of the T8 beam line in order to cope with the increasing demand for running the East Area facility with Heavy Ion beams.