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UK-ATLAS Irradiation Facility

Introduction

The High Luminosity upgrade of the LHC (HL-LHC) will begin in 2024, during long shutdown 3, to allow LHC experiments and the LHC to prepare for proton-proton collisions at an unprecedented luminosity, aiming to deliver an additional integrated luminosity of approximately 2500 fb^-1 over 10 years of operation.

The increased luminosity and accumulation of radiation damage within the ATLAS detector requires the existing materials within the detector to be re-examined to evaluate their durability and performance at high fluences. The inner detector, closest to the interaction point, will be replaced with an all silicon tracker to maintain performance and cope with the increase of total radiation fluence by approximately a factor of ten. A new inner tracker layout, improved cooling system and streamlined power cable layout has been proposed and it is essential to test the radiation hardness of the new components and materials.

The Birmingham Cyclotron

The Birmingham MC40 Cyclotron is situated in the School of Physics and Astronomy at Birmingham University and has been in operation since 2004. The primary function of the cyclotron is the production of radio-isotopes for use in medical imaging but is also used for general research. Although the cyclotron can produce a range of ions at various energies, for irradiation studies of materials for the HL-LHC we are interested in using protons at an energy of 26-27 MeV. Although this energy is an order of magnitude less than the energy of the beams in use at the CERN irradiation facility, the required fluence is measured in units of 1 MeV neutron equivalents (1MeV n_eq). This means that the Birmingham Cyclotron can irradiate the silicon samples to HL-LHC fluences, equivalent to that at the CERN facility, in 80 seconds per cm². A comparison between charge collection measurements of a sensor irradiated at Birmingham and other irradiation facilities is presented below, with comparable results.

The Scanning Table

A pre-configured xy-axis cartesian robot system has been built for the irradiation facility, in order to move the cool box containing samples to be irradiated, directly into the beam. The table typically moves at 4 mm/s along the x-axis and upto 25 mm/s in the y direction, this is controlled by a NI compact RIO Real-Time programmable controller, with a third party servo drive to control the y movement. Using LabView, a GUI has been created which allows interaction with the robot to set the scan path. Monitoring of the temperature and humidity within the cold box is measured during the scan, and the box is flushed with dry nitrogen to ensure constant humidity with an electrical fans for good air circulation. The box was initially cooled by forced convection using an 800 W chiller, circulating glycol however this was upgraded to an evaporative cooling system using liquid nitrogen in 2014.


The cool box is made from radiation hard material so that it is not affected by the high fluences incident on the box. The window allows exposure of the sample to the beam, where the samples are attached to a detachable rail system which is fixed to the underside of the lid. The samples are mounted on to custom-made carbon fiber frames, which do not melt or disintegrate when exposed to high fluences. Nickel activation foils are placed in front of the sample for dosimetry.

Dosimetry

Once a sample has been irradiated, it is necessary to measure the dose which it has received. Prior to the irradiation, the beam profile and position is checked using gafchromic film. The beam is a 1 cm × 1 cm square beam. The algorithm which calculated the beam path requires the beam current to be input. The default scan achieves a fluence of 1 × 10¹⁵ 1MeV n_eq cm^-2, and by adjusting the beam current or number of runs, a range of target fluences are possible. In addition to scanning through the beam, point-to-point irradiations are possible for samples approximately the same size or smaller than the proton beam, which can achieve a range of fluences.
During irradiations, a Faraday cup is used to collect the charge which has passed through the sample. This can then be cross-checked with a gamma spectra of the nickel activation foils. The nickel produces an isotope of ⁵⁷Ni which decays with a lifetime of 35.7 hours to ⁵⁷Co. This produces a gamma ray peak at 1377 keV and using the cross-section for ⁵⁷Ni production, the number of incident protons can be determined and compared to the other dosimetry methods.

Irradiation tests

A range of materials and sensors have been irradiated since 2013 at the UK-ATLAS Irradiation Facility. Whilst the system is optimised for silicon sensors, being the majority of irradiations, there is flexibility to irradiate larger samples of different materials, see AIDA2020 website for further details.
Final qualification of the facility is scheduled for May 2016.

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