Instrument Service Group Detectors

Highly efficient, performant detectors are a key component of state-of-the-art scientific instruments at neutron scattering facilities. The task of the Instrument Service Group Detectors is to design such detector systems and the associated readout electronics, install them on the instruments at the MLZ and permanently keep these systems at the cutting edge of technology. Commercially available as well as in-house developed detectors are used to provide tailor-made solutions for the instruments.
It takes care in maintenance, trouble shooting and calibration of the installed detector systems. In addition, the group is actively involved in investigating new detector technologies as part of international collaborations.

In 2021, with the completion of the UYL laboratory building, the service group detectors was able to move into two new, well-equipped laboratory rooms with a total area of 180 m², thereby significantly improving the possibilities for developing and maintaining modern large-area detectors.
The first laboratory shown in Fig.1 is dedicated to the mechanical production and integration of detector components and the testing of entire detector systems. The focus is on the production and maintenance of large ³He and other gaseous detectors, in which the detector service group has specialized. The core of the laboratory is a 30m² clean room in which the highly sensitive internal components of the detectors are manufactured. Fig.2 shows a view of the clean room with the laminar flow cabinets and detector manufacturing facilities installed therein.
The second laboratory shown in Fig.3 is a state-of-the-art electronics laboratory that focuses on the development, examination and testing of electronic components for the readout of detector systems.
Finally, for testing of detectors, the group operates a 1mCi ²⁵²Cf-neutron source located in a dedicated laboratory within the neutron guide hall West.

The SAPHiR instrument, currently under construction in the Neutron Guide Hall East, is dedicated to the investigation of liquid and polycrystalline samples under extreme pressure and temperature conditions. For the three detector banks perpendicular to the primary beam and in the forward scattering regime a total of 648 linear position-sensitive ³He detector tubes (PSD) are arranged along an arc with a radius of 1500 mm and the sample position at its centre. Fig.4 shows a front view of the 0°-forward bank with the beam transmitted through the centre of the detector bank.
The detector tubes each have an active length of 400 mm, a diameter of 8 mm and are filled with 20bars of ³He gas. For the SAPHiR tubes the efficiency varies between approximately 50% at 1 Å and 70% at 2.4 Å. Both ends of the anode are connected to charge sensitive preamplifiers and the ratio of the two signal amplitudes gives the location along the tube at which the neutron was detected. A resolution of 2.8mm (FWHM) has been confirmed in earlier test measurements using a slit screen.

The probably most demanding development project carried out by the detector service group was the design and production of two curved, large-area two-dimensional position sensitive ³He-filled Multi-Wire Proportional Chambers (MWPC) in collaboration with the Paul-Scherrer-Institute (PSI) as part of the CHARM project [1]. The detectors, with a design based on a concept developed at Brookhaven National Laboratories (BNL), were developed for the new high through-put powder diffraction instrument ERWiN at MLZ and the upgrade of the existing powder diffractometer DMC-II at PSI.
Each detector, with a curvature of 800mm, provides 130° horizontal and 14° vertical acceptance with 0.115° angular resolution (FWHM) in both dimensions. It consists of nine individual curved MWPC-segments mounted seamlessly inside a common pressure vessel filled with 6.5bar ³He + 1.5bar CF⁴, resulting in 75% detection efficiency for thermal neutrons. Each segment consists of a bent printed circuit board with 128 horizontal cathode strips and 128 vertically stretched anode and cathode wires with 1.6 mm strip/wire pitch, respectively. A total of 24 of these MWPC-segments were built at MLZ, whereof 12 are in use for the PSI detector. Fig.5 shows the installation of the nine MWPC-segments into the pressure vessel during the integration of the ERWiN detector in the clean room of the FRM II detector laboratory.
Single wire/strip readout using the Time-over-Threshold method with a centre-of-gravity algorithm is applied to determine the arrival time and impact position of each individual neutron. The compact front-end and signal processing electronics [2], developed entirely by the service group detectors, are mounted directly on the detector vessel. Fig.6 shows the completed ERWiN detector during the installation at the instrument ERWiN at FRM II.
The detector for DMC II was assembled in the PSI detector laboratory. Following installation and commissioning on the instrument, the detector went into user operation in September 2022.

In the course of the CHARM project, the service group detectors developed a modular readout and data acquisition system for Multi-Wire Proportional Chambers with individual channel readout. The system is designed to handle almost any number of individual MWPC-segments mounted inside a common pressure vessel. The analogue Front-end consists of charge sensitive preamplifiers with 350ns peaking time followed by comparators to deliver a Time-over-Threshold (ToT) signal. The ToT-signals are encoded and timestamped at 80MHz by FPGA-based boards. The signal processing includes a cluster recognition in x- and y-position as well as in signal arrival time. The neutron impact position is then determined using a centre-of-gravity algorithm based on the ToT-information. A further FPGA-based board gathers and post-processes data from all the segments for reconstructing neutron events occurring at the boundary of two adjacent segments. Finally, the reconstructed events are delivered to the DAQ system with 10-bit position resolution, timestamping with 100 ns precision and total charge deposit with 8-bit resolution.