Avoided Level Crossing Spectrometer (ALC)

In 2009 the ALC instrument was commissioned with a new detector system and a new cryogenic environment. The new ALC allows for both time-integral and time-differential measurements in the range of magnetic fields 0-5 T and temperatures 3-470 K.  The spectrometer is located in the experimental area piE3 and uses surface positive or cloud negative muons from Target E.  

General Specifications:

• Location:             area piE3

• Magnet:              Oxford Instrument superconducting solenoid (length 1 m;  warm bore diam. 20 cm;  max. field 5 T)

• Cryostat:             Janis He-flow cold-finger (3 K -- 470 K)

• Samples:             min diam.10 mm, max diam. 56 mm;  for liquid and powder samples sample cells are available

• Detector:             20 positron counters arranged in two rings + optional muon-counter

• Beam:                 positive surface muons (28 MeV/c);  negative cloud muons (33 MeV/c)

• Polarization m⁺:   90% longitudinal (with separator),  60% transverse (spin-rotator, 42 degrees)

• Polarization m^-:   30% longitudinal (with separator),  20% transverse (spin-rotator)
  

• Modes:                time-integral or time-differential 
  

The following detailed information is available:

• panorama view of the spectrometer in the piE3 experimental area

• design drawing of the Janis cryostat

• sample holder (hollow cylinder extending the cryostat cold finger)

• sample plate (for mounting solid samples up to diam. 56 mm)

• sample cells d26 and d36 (for liquid and powder samples)

• detector system is realized on Geiger-mode Avalanche Photodiodes instead of conventional Photomultiplier Tubes. The detector is compact, insensitive to magnetic fields, and is operated at a low voltage of about 40 V

Time-integral ALC operation mode (positive muons)

• longitudinal polarization:  ~90%

• collimators:  diam. 10 -- 20 mm

• muon rate: 1'500'000 -- 3'500'000/s at H = 0,  increasing to max. 8'000'000/s up to 4 T

• samples:  min diam. 10 mm, max diam. 56 mm;   for liquid and powder samples sample cells are available

• asymmetry vs. field scans:  field increment  -- min 0.0003 T;  typical time at a field point -- 10-20 s

Data Acquisition ALC-mode:  ALC User Guide

Time-differential mode (positive muons)

• longitudinal polarization ~90%

• transverse polarization ~60% (spin rotation ~42 degrees)

• detector grouping for LF or TF measurements done in software

• collimators: diam. 10 -- 20 mm

• incoming muon rate: sufficient (reduced to 30'000/s)

• samples:  min diam. 10 mm, max diam. 56 mm;  for liquid and powder samples sample cells are available

• very low background level:  typical ~0.3%;   min ~0.02%

• data taking rate: ~30'000'000 events/hour in 1.5T (sum over all 20 positron counters)

• time resolution: s = 0.5 ns (1.2 ns FWHM)

Time-differential mode (negative muons)

• longitudinal polarization ~30%

• transverse polarization ~20% (spin rotation)

• detector grouping for LF or TF measurements done in software

• collimators:  diam. 10 -- 20 mm

• incoming muon rate: 8'000/s -- 30'000/s

• samples:  min diam. 10 mm, max diam. 56 mm;  for liquid and powder samples sample cells are available

• background level:  typical ~1%

• time resolution: s = 0.5 ns (1.2 ns FWHM)

Data Acquisition TD-mode: standard DeltaT software (html, pdf)