WA105 experiment (ProtoDUNE-DP)

In order to gain experience in building and operating large-scale LAr detectors, an R&D programme is currently underway at CERN. Such programme will operate two prototypes by 2018, with the specific aim of testing the prototypes design, assembly, and installation procedures; the detectors operations; as well as data acquisition, storage, processing, and analysis under beam conditions and with cosmic ray data. The two prototypes will both employ LAr TPCs as detection technology, with one prototype only using liquid argon, called ProtoDUNE Single-Phase, and the other using argon in both its gaseous and liquid state, thus the name ProtoDUNE Dual-Phase (DP). Both detectors will have similar sizes.

In particular, ProtoDUNE-DP, also known as WA105, will have an active volume of 6×6×6 m3 and an active mass of 300 t. In ProtoDUNE-DP the charge charge is extracted, amplified, and detected in gaseous argon above the liquid surface allowing a finer readout pitch, a lower energy threshold, and better pattern reconstruction of the events.

The light signal is used as trigger for non-beam events, timing for both beam and non-beam events, cosmic background rejection, and there is also a possibility to perform calorimetric measurements and particle identification. The Photon Detection System of ProtoDUNE-DP is formed by 36 8-inch cryogenic photomultipliers (PMTs) placed at the bottom of the detector. As wavelength-shifter, tetraphenyl butadiene (TPB) is coated directly on the PMTs. Each photomultiplier has a voltage divider base soldered to it. In addition, a light calibration system checks the optimum operation of the PMTs, and an external DAQ system records the light signal.

(Left) Internal view of the protoDUNE-DP cryostat installed at the EHN1 area at CERN. (Right) Drawing of the WA105 detector with 36 PMTs placed at the bottom.

Following a staged approach, prior to the ProtoDUNE-DP construction, the WA105-3×1×1 m3 dual phase LAr TPC demonstrator was assembled at CERN. Its construction and operation aims to test scalable solutions for the crucial aspects of this technology: ultra high argon purity in non-evacuable tank, large area dual phase charge readout system in several square meter scale, and accessible cold front-end electronics. Five PMTs coated with the wavelength shifter, TPB, are fixed under the cathode. They are sensitive to the 128 nm scintillation light from the argon scintillation and provide the reference time for the drift as well as the trigger. The detector took cosmic ray data during 2017, and the results are being analysed.

(Left) Diagram of the WA105-3×1×1 m3 dual phase LAr TPC demonstrator. (Center) The WA105-3×1×1 m3 prototype being installed. (Right) PMTs installed at the bottom of the WA105-3×1×1 m3 cryostat.

We also work on R&D for large scale cryogenic liquid detectors, participating in the AIDA-2020 European Project (Advanced European Infrastructures for Detectors and Accelerators), funded by the Horizon 2020 European Programme.

CIEMAT contributions to WA105

Cryogenic test facility at CIEMAT to test 10 PMT at the same time inside a vessel filled with LN2.CIEMAT, together with IFAE, is responsible for the light detection system of the ProtoDUNE-DP experiment, including the acquisition and characterization of 40 8” cryogenic photomultipliers, coating with wavelength-shifters, design and production of the voltage circuit and HV splitters, production of the associated mechanics, PMT calibration system, and final installation and commissioning of the light detection system in the detector. CIEMAT also contributes to the development of the simulation and reconstruction software for both charge and light readout and the study of the detector performance for low energy interactions. The group is participating in the analysis of data from cosmic muons taken with the WA105-3×1×1 m3 demonstrator and in simulations of the ProtoDUNE-DP light signal.

A cryogenic test facility has been developed at CIEMAT where 40 PMTs have been characterized at room and LN2 temperature inside a 300 litre vessel, including final mechanical structures, HV dividers and cables. The dark current and gain as a function of the applied voltage are measured for each PMT. The response of the PMTs with respect to the amount and frequency of the light received is also studied. A publication is under preparation.

A PMT calibration system integrated in ProtoDUNE-DP detector has been designed to calibrate the response of the PMTs installed inside the LAr volume. The goal is to determine the PMT gain and study the PMT stability. An optical fiber will be installed at each PMT in order to provide a configurable amount of light from a LED source.

The CIEMAT group has important responsibilities inside the WA105 experiment as leaders of the Photon Detection subgroup and members of the Technical Coordination Group and Dissemination Board.

Cryogenic test facility at CIEMAT to test 10 PMT at the same time inside a vessel filled with LN2.