HAL-9500 Developments

The instrumentation for HAL-9500 (sample environment, detector systems, front-end electronics) has been specifically developed to match the highly demanding requirements for low temperatures in high magnetic fields. Ongoing developments based on the ‚standard‘ equipment for user operation are listed below.

A Coulomb-barrier thermometer (primary thermometer) is permanently mounted on the silver cold finger coupling between mixing chamber and sample rod and allows the calibration of secondary thermometers correcting for their magnetic field dependence. In addition, a superconductive reference fixpoint thermometer (mutual inductance) and a CMN thermometer (paramagnetic susceptibility) can be mounted on the mixing chamber flange for zero-field thermometer calibration.

The timing detector systems consist of fast plastic scintillators read out by SiPMs from both sides. The scheme used in HAL-9500 results in an unrivaled time resolution of 60 ps (sigma) for the 33 mm diameter system.

Decay positrons originating from muons not stopping in the sample are rejected by using scintillation detectors in close proximity of the sample (i.e., at cryogenic temperatures). The scintillators are coupled to an optical system using WLS fibers, the photodetector (SiPM) is at ambient temperature outside the OVC. Refining the present rejection scheme should allow to remove the presently existing distortions after ca. 20 ns.

The compact axial timing detector geometry as presently used for TF is not suitable for LF measurements. For measurements using the DR a redesign of the (not used) validation detectors which are mounted on the still shield might allow to perform LF measurements (although anti-cipating a very large alpha variation with increasing magnetic field).

The high-field setup requires the use of horizontally mounted cryostats. In the presently used Janis cold-finger cryostat the sample is in vacuum, thermal contact is made by the glue to the cold finger. Especially for powder samples this leads to some uncertainty of the ‚real‘ sample temperature and thermalisation times. A horizontal flow cryostat developed by LIN with the sample in He gas flow is ready for testing (status 09/2021).

The presently used TDC from CAEN (V1290) is based on the CERN HPTDC chip, which is not available anymore. Future DAQ systems might be based on its successor (picoTDC) or on FPGA based systems, eventually waveform digitizers. Potential candidates will be tested as they become available.

The histograms recorded in HAL-9500 have a length of 400‘000 bins, bin width is 25 ps. Due to the low count number at longer times, histogram fitting requires the use of negative log-likelihood minimization. Defining a goodness-of-fit criterion for this turns out to be non-trivial, several approaches are under development.