Portugal: RPC Results so far

30 November 2018

Detail of the inside of the Multilayer Double-gap RPCs detector ready to be closed (back side, i.e. opposite side of the neutron entrance window).

Resistive Plate Chambers are being developed as neutron detectors as part of SINE2020's Detectors work package. You can read more out more about them and how they work in our other articles.

Luís Margato, Andrey Morozov and Alberto Blanco from LIP Coimbra in Portugal have been working on the project and below you can find out what they have been doing.

Step 1: Conceptual design

Luís Margato and his team initially used Monte Carlo simulations to investigate design concepts for 10B RPCs. Using open source codes (ANTS2 toolkit) they assessed the effects of changing the detector parameters and materials: for example the width of the gas gap, converter layer thickness or angle of incidence of the neutron beam on the detector. Once explored, it was time to make some prototypes.

Step 2: Proof-of-concept

As a result of the simulations, a hybrid RPC prototype was constructed at the lab in LIP Coimbra, with the help of C. Hoglund at the ESS detector coatings workshop who was responsible for the production of the coatings. It was tested at ILL in France (as part of an exploratory project funded by FCT). Comparing two RPC prototypes, one with the converter layer and one without, showed that the neutron converter allows neutrons to be detected and at a good spatial resolution too. The concept works!

Multilayer Double-gap RPCs detector on TREFF neutron beamline at FRMII (GE) for the tests with neutrons (4.7 Å); from left to the right: Karl Zeitelhack, Alberto Blanco and Luís Margato.

Step 3: Prototypes

Next, two more prototypes with different gas-gap widths (0.35mm and 1mm) were made and tested, in collaboration with Karl Zeitelhack, at FRMII on the TREFF beamline as part of SINE2020. Results showed a spatial resolution better than 0.25mm FWHM and 12.5% detection efficiency for 4.7 angstroms neutrons. These were in good agreement with the MC simulations, including the expected better performance and resolution of the thinner (0.35mm) gas-gap. But can it be improved further by providing several opportunities for neutrons to be captured?

Step 4: Multilayers

Using the better performing gas-gap of 0.35mm, a detector with double-gap RPCs in a multilayer architecture was then designed and assembled at LIP and tested at FRMII. The prototype contained 10 double-gap 10B RPCs (comprising of 20 neutron converter layers) and the spatial resolution performance is maintained as good as for the case of one of the single-gap hybrid RPC. The measured detection efficiency was about 60% making a multilayer design very encouraging. Both results were again in good agreement with simulations.

RPCs: Multilayer Double-gap ready
RPCs: Multilayer Double-gap ready

Multilayer Double-gap RPCs detector ready for the tests with neutrons.

RPCs: Multilayer Double-gap ready TREFF
RPCs: Multilayer Double-gap ready TREFF

Multilayer Double-gap RPCs detector on TREFF neutron beamline at FRMII (GE) for the tests with neutrons (4.7 Å); view from the backside of the detector showing the Front End Electronics (FEEs) and the DAQ system (TRB3).

RPCs: Multilayer Double-gap ready
RPCs: Multilayer Double-gap ready

Detail of the inside of the Multilayer Double-gap RPCs detector showing its backside (opposite side of the neutron entrance window), the header connectors to plug the Data Acquisition Systems and the gas inlet and outlet valves.

RPCs: Multilayer Double-gap ready
RPCs: Multilayer Double-gap ready

Front view of the Multilayer Double-gap RPCs detector in TREFF neutron beamline with a cadmium slit collimator at the front for the spatial resolution measurements.

RPCs: CAD of detector
RPCs: CAD of detector

CAD Project of the detector; view of the Aluminium gas tight chamber showing details of the neutron entrance window, gas inlet and outlet adaptors, HV feedthroughs and the openings for the header connectors.

RPCs: Figure 1 Results
RPCs: Figure 1 Results

Figure 1: Results obtained with the Multilayer Double Gap RPCs Detector at TREFF neutron beamline.
Left: Neutron detection efficiency versus the high voltage applied to the RPC cathodes (anodes were kept at ground); the overall efficiency is represented by the top curves; the curves in the bottom depict the detection efficiency only for the first double-gap RPC.
Right: Detection efficiency for each individual double-gap RPC measured for HV of 2300 V; RPC nº 1 is facing the Al neutron entrance window. The experimental curve (red) show an exponential decrease from the first to the last RPC as expected from the neutron absorption along the depth of the detector. The experimental results show to be in good agreement with the simulations (blue and green curves) performed with the ANTS2 toolkit (https://github.com/andrmor/ANTS2)

RPCs: Figure 2 Results
RPCs: Figure 2 Results

Figure 2: Spatial resolution results obtained with the Multilayer Double Gap RPCs Detector at TREFF neutron beamline. It was measured a spatial resolution better than 250 μm FWHM (figures on the left side).
The figure in the middle show the 3D capability of this type of detectors, with the third coordinate (Z) being determined by the neutron time of flight (TOF); the timing uncertainty introduced by the flight time of thermal neutrons in any one boron layers is less than 1 ns.
On the right side, is show an image acquired with a cadmium mask. The neutrons hitting the open letters will pass the mask and will be detected with an efficiency greater than 50%; the remaining ones hitting the cadmium will be absorbed. The acquired image with the Multilayer Double Gap RPCs Detector demonstrates a good spatial resolution and good image homogeneity.

The preliminary results show that for a double-gap RPC, irradiated by an Na-22 gamma source, the sensitivity of the RPC to the Na-22 gamma rays, and in the high voltage (HV) region of the plateau for neutron detection can go down to ~10^-6 for the 511keV photons and may go bellow 10^-5 when the 1.27 MeV are taken into account.

Step 5: Gamma sensitivity

Unfortunately, gamma rays emitted from a sample or by other materials in the neutron beam path can disturb the detectors response contributing false events to the results and so it is important to understand and reduce their effect on the RPCs being developed. Using Co-66 and Na-22 gamma sources, the 10B RPCs are being characterized for their gamma sensitivity. Then when the parameters are evaluated, the design to improve it.

The preliminary results show that for a double-gap RPC, irradiated by an Na-22 gamma source, the sensitivity of the RPC to the Na-22 gamma rays, and in the high voltage (HV) region of the plateau for neutron detection (see figure right), can go down to ~10-6 for the 511keV photons and may go below 10-5 when the 1.27 MeV are taken into account. These results were obtained without any optimization of the detector regarding the gamma sensitivity so by optimizing the detector design concerning this aspect, in principle it will be possible to reduce these values.

Next steps:

With such a promising detector technology in the making we need to work out how to improve the current designs and materials even further, for example optimizing the 10B4C layer thicknesses in the multilayer device to increase the counting rate capability. So for now, Luís is back to the virtual world of simulations using information learned from prototype testing.

In particular, the team are looking at improving the counting rate of the detector, very important as we want these new detectors to count as many neutrons per second per square millimeter as possible.

Other areas for future investigation include modelling of the detector considering neutron scattering in the detector materials and varying the angle of incidence of the neutron beam from the perpendicular normal incidence.

RPCs are a robust and established technology that are accessible and not expensive. We have proved the concept of using RPCs to detect neutrons with high precision in 3D but in which concerns aspects, such as, e.g. the counting rate capability, we are walking in the unknown as this detectors in neutron detection operates in a different range of HV than standard RPCs and they have never been developed for neutron scattering applications” – Luís Margato
RPCs: coating
RPCs: coating

Float glass plate coated on one side with a 1 μm thick layer of 10B4C at the ESS detector coatings workshop in Sweden.

RPCs: multigap RPCs
RPCs: multigap RPCs

Assembly of a multigap RPC with five gas-gaps for the feasibility tests of the multigap RPC design; six plates of glass are needed to form the stack, with five of them coated on one face (cathode side) with a 1 μm thick layer of 10B4C. This corresponds to five layers of converter per multigap RPC. In this particular RPC design, all electrodes must be made from a resistive material e.g. float glass. Moreover, the converter must have a high surface resistivity in order not to shield the induction of the signals on the external signal pickup strips.

RPCs: multigap RPCs
RPCs: multigap RPCs

Two multigap RPCs stacked one on top of the other. The 10 mm diameter pen on the face shows that is needed a very small thickness to comprise 10 layers of converter by using this RPC design. Unluckily, despite the several attempts, we have not yet been able to produce converter layers with sufficiently high surface resistivity for this RPC design to work as a position-sensitive neutron detector.

RPCs: Multilayer Double-gap ready
RPCs: Multilayer Double-gap ready

Front view of the Multilayer Double-gap RPCs detector in TREFF neutron beamline with a cadmium mask with the words LIP and FRM II engraved; the letters have a width of 0.4 mm.

References:
L. M. S. Margato, A. Morozov, Boron-10 lined RPCs for sub-millimeter resolution thermal neutron detectors: Conceptual design and performance considerations
L. M. S. Margato, A. Morozov, A. Blanco, P. Fonte, F. A. F. Fraga, B. Guerard, R. Hall-Wilton, C. Höglund, A. Mangiarotti, L. Robinson, S. Schmidt, K. Zeitelhack
Boron-10 lined RPCs for sub-millimeter resolution thermal neutron detectors: Feasibility study in a thermal neutron beam

Acknowledgements: Luís Margato, LIP