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ILL and ESS: Securing the future of neutron science through pioneering multi-grid detection

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ILL and ESS: Securing the future of neutron science through pioneering multi-grid detection. 26.07.2013

For at least three decades the Institut Laue-Langevin (ILL) in Grenoble has pioneered the field of neutron detectors. However in 2009 a global shortage of Helium-3 threatened instrument development in neutron scattering science at ILL, and facilities around the world.

Helium-3 is a vital isotope in instrumentation for neutron detection. It has a high absorption cross section for thermal neutron beams and is therefore used as a converter gas in neutron detectors. It does this via a nuclear reaction, absorbing neutrons to produce a charged Tritium and a proton, which go on to produce a charge cloud that can be detected electronically.

To secure the future use of neutrons in research areas spanning the traditional and applied sciences, an alternative technique, known as multi-grid, has been introduced by the ILL. This new approach has been led in recent years by collaboration between the ILL and the European Spallation Source (ESS). Now evidence of multi-grid performance has been clearly demonstrated.

What is a multi-grid?

In this unique set up the detection elements, called grids, are electrically insulated and stacked in columns to make proportional counter tubes, typically 3metres in length. Several of these columns are mounted in a large gas vessel to fill the detection volume.
Each grid contains approximately 15 aluminum substrates coated on both sides with a thin neutron convertor film, such as Boron Carbide (10B4C) that provides a detection efficiency of ~50% for thermal neutrons.

The position of each neutron is measured in three directions (X, Y and Z for TOF correction). Even if the number of anode wires is 15 times higher compared to a solution based on 3HE PSDs, the number of electronics readout channels is only a factor of 4 higher thanks to the electronics readout of the grids. One further advantage of this technique is that its construction is well suited for large area detectors operated in a vacuum chamber.
The performance of this type of detector, in particular its detection efficiency, depends on the use of good quality convertor films requiring high expertise and expensive equipment to develop and install. Since there was no known company to produce 10B4C films, ESS agreed a partnership with the Linkoping University to develop a coating process compatible with the production of large surfaces.

With this in place, a second agreement was signed in April 2010 between ILL and ESS to support the development of the Multi-grid technique; this collaboration was reinforced in October 2011 by the participation of both labs in the CRISP project.

The project represents a true partnership between the two institutes. Whilst the ILL has developed the mechanics and the electronic readout scheme of several prototypes, ESS and LiU have continuously improved the process of B4C coating. Both ILL and ESS together then characterized the convertor films, mounted the prototypes, and tested them with neutrons.

This commitment to thorough testing and trialing has allowed their team to uncover and solve new challenges. The very low noise environment of the instrument brought to light background interference ten times higher compared to 3He counter tubes. It has been shown that the origin of this noise is the alpha particles emitted by Thorium and Uranium components within the Aluminium.

Previous test run at the ILL highlighted the problem of alpha noise and the negative impact on the performance of the multi-grid detector. However project scientists have since shown how this can be solved by plating the Aluminium with ~20 micrometers of Nickel.
Through this process of rigorous testing and refinement the project has been able to significantly improve the performance of their multi-grids set ups. The results from the latest test come from new measurements on the IN6 instrument at ILL and show that this multi-grid technique can deliver a similar detection efficiency and gamma sensitivity to 3He tubes. The good detection efficiency of the Multitube tested on IN6 results from an optimization of the film thickness by MC simulation and from a high ratio between sensitive area and dead space provided by the mechanics.

The final step in this project to fully demonstrate the relevancy of this technique is to build a large size demonstrator, and test it in real conditions. This will be done in 2014 following the latest agreement by ILL and ESS to continue their contribution to the project. After this, the possibility of technology transfer to the industry may also be considered.


"10B Multi-Grid proportional gas counters for large area thermal neutron detectors": published in Nucl. Inst. & Meth.

"10B4C Multi-Grid as an alternative to 3He for large area neutron detectors": published in IEEE Transactions on Nuclear Science

Contacts: Charles Simon, ILL Director for Projects and Techniques and Dimitri N. Argyriou, ESS Director for Science

Notes to editors

Notes to editors

1. About ILL – The Institut Laue-Langevin (ILL) is an international research centre based in Grenoble, France. It has led the world in neutron-scattering science and technology for almost 40 years, since experiments began in 1972. ILL operates one of the most intense neutron sources in the world, feeding beams of neutrons to a suite of 40 high-performance instruments that are constantly upgraded. Each year 1,200 researchers from over 40 countries visit ILL to conduct research into condensed matter physics, (green) chemistry, biology, nuclear physics, and materials science. The UK, along with France and Germany is an associate and major funder of the ILL. 
2. About ESS - The European Spallation Source is a Partnership of 17 European Nations committed to the goal of collectively building and operating the world's leading facility for research using neutrons by the second quarter of the 21st Century