VMS: Versatile Muon Spectrometer

• Location: area piE1.
• Positive muons, standard momentum 28 MeV/c (other momenta possible).
• Degree of polarization >95%.
• Direction of spin ~8-45° with respect to beam axis (depending on the mode of the spin-rotator).
• Positron detectors: 2 longitudinal, 2 transverse.

Note: VMS is the upgraded version of the Dolly spectrometer.

• A muon detector (M).
• Four positrons detectors (with respect to the beam direction) :
  • Forward (F),
  • Backward (B),
  • Left (L) - split into three subdetectors,
  • Right (R) - split into three subdetectors.
• A backward veto detector (B_veto), consisting of a truncated hollow cone scintillator with a 12-mm hole facing the M counter. The purpose of B_veto is to collimate the muon beam to a 12-mm spot by rejecting the muons (and their decay positrons) missing the aperture ("active collimation").
• A cone-shaped forward veto detector (F_veto) for use with samples that do not cover the entire beam spot. Its purpose is to reject muons (and their decay positrons) that have missed the sample. When the sample/holder assembly stops all muons, the F_veto signal may be added to the F signal to increase the forward solid angle.

Note: To analyze data, use 1 for B, 2 for F, 3+4+5 for L and 6+7+8 for R.

A pdf drawing of the VMS detectors is available.

Cryostat

• Type: Oxford Variox (top loading, sample in He-4 exchange gas).
• Temperature range: 1.6-300 K.
• Sample holders: Two sample sticks are available.
• Sample rotation: Remote-controlled rotation of the sample stick for orientation-dependent measurements can be made available on request. Please contact the instrument scientist.
• Reference Manual: is available (but more up-to-date information is available in the QUICK REFERENCE MANUAL).
• Liquid He is supplied in two 100 litre dewars. While one dewar is in use, the second one is being refilled by PSI staff at the filling station in the experimental hall (west gallery). Therefore it is vital that the empty dewar is immediately brought back to the filling station and not left in the area. Contact the instrument scientist for help. The dewars should not be emptied completely. Leave about 10% level.

In test operation: Oxford Heliox He3 insert for the temperature range down to 0.25 K (see a dedicated HELIOX QUICK REFERENCE MANUAL).

Sample Size and Recommended Sample Mount

• Sample chamber (Variox): Free diameter 31 mm, distance from bottom of sample chamber to center of beam spot is 41 mm. A sample of 10 mm diameter and a thickness of 0.2 g/cm^2 at the correct position will stop the majority of the incoming muons.
• Drawings of the sample region (Variox with Heliox insert) are available here.
• Sample holder: Either single-window or two-window sample holders (recommended) are available on site. In both cases, the distance from the center of the top sample to the holder base is 27 mm. The distance between centers is 35 mm.
  If possible, users can already glue their sample using GE varnish (IMI 7031) on 21 x 29 mm² copper foil (25 µm thick), before coming to PSI. This will speed up the sample mounting and cooling.

Please note: Users are expected to bring their own sample holders and to make sure that the samples are safely sealed and fixed on the holder. Users who intend to bring hazardous sample materials (radioactive, toxic, flammable, etc.) to PSI should read the instructions on our safety page well in advance of their scheduled beam time.

Temperature Control (see VARIOX QUICK REFERENCE MANUAL and VARIOX INITIAL COOLDOWN MANUAL and HELIOX QUICK REFERENCE MANUAL )

The temperature sensors currently in use are:

Variox cryostat:
- Cernox X89194 (Variox 55441)
- Cernox X36049 (Sample stick)

Heliox insert:
- Cernox X182092 (He-3 High-T range)
- RuO₂ (Rox) U09912 (He-3 Low-T range)

Orange cryostat:
- Cernox X208883 (Heat exchanger 1)
- Cernox X208884 (Heat exchanger 2)
- Cernox X224954 (Sample stick)

Magnetic Fields

• Main field (WEU): 0-0.8 T parallel to the beam.
• Main field (WEUL): 0-50 mT parallel to the beam (high precision over restricted range).
• Auxiliary field (WEV): 0-15 mT perpendicular to the beam.
• Earth-field compensation: Two pairs of coils for the components perpendicular to the beam (horizontal and vertical) and one pair for the component parallel to the beam (longitudinal). Compensation is usually better than 0.001 mT for all components.

Data Format and Storage

Users are responsible for storing their own data in a safe place. Every week a data backup is performed in the PSI archive system and in the specific μSR FTP-server. You are strongly advised to retrieve your own data (in the native "root" format, or exported to "bin") from the web page musruser.psi.ch.

A manual for the data-acquisition software deltat is available in PDF format

Logic Diagram

The TDC electronics is characterized by a logic performed exclusively at the software level. A manual is available in PDF format. The electronic logic is similar to that of the GPS instrument (see Manual of GPS).

The old pTA (9308 picosecond Timing Analyzer) logic diagram is available as a pdf-file (780 kB).

The area is equipped with an experiment console (running RedHat Linux) pc12017. The µSR data acquisition system consists of this console, a Linux back-end server (psw418) located in the computing building (Hauptgebäude) and a front-end PC (pc12022) running Linux controlling the VME electronics. A number of different devices (temperature controllers, magnetic-field power supplies, etc.) are used for the slow control of the experiment.

These devices are mainly accessed via GPIB (IEEE-488) bus, RS-232 serial line, TCP port, or EPICS Process Variables.

• GPIB: The GPIB devices are accessed through Agilent LAN/GPIB Gateways E5810A.
• RS-232: The RS-232 devices are controlled through RS-232 terminal servers. Moxa NPort 5110 or NPort 5150 1-channel, Lantronix ETS8PS 8-channel, Moxa NPort 5650 8-channel or Lantronix EDS16PR 16-channel RS232 terminal server.

The acquisition software deltat is based entirely on the DAQ software package MIDAS.

A Linux workstation (pc13369) is also available for data analysis and can be used through a usual PSI account or a local account (ask the instrument scientist for information).

The HIPA secondary beam-line control system controls the beam-line elements (magnets, slit systems, etc.) between the target and the experiment. The area is equipped with an EPICS Console (hipa-pie1-02) to set the EPICS Process Variables of the beam-line elements, to optimize beam settings, and for diagnosis.