Faraday Cup Measurment Notes¶

Picoammeter setup:

• plug in
• wait 1 hour
• activate ZCHK (maybe already when turned on) (should say ZC on screen)
• plug into faraday holder
• set to lowest range (2nA)
• activate ZCOR to set zero (should say ZZ on screen)
• set auto range if desired
• deactivate ZCHK to begin measuring
• focus beam to a spot
• pull out holder until spot hits the faraday cup (max current) (This is a negative value)
  Examples
  • 200keV
    • 1.21 nA
  • 120 keV
    • 2.53 nA

Exposure Time to Screen Current measurment¶

• refer to Tecnai Help for "Exposure Time, Plate Camera CP"
• formula: beam current (nA) = 2.15 * emulsion / measured_exposure_time
  • exp time is between 3.1 and 3.5 depending on size of beam
  • emulsion = 1
  • beam current is between 0.61 and 0.69 nA

Leginon screen scale factor measurement¶

• adjust scale factor until beam current is same as faraday cup
  • 200 keV
    • 0.725
  • 120 keV

Camera sensitivity¶

• 200 keV
  • 45-50 count/e
• 120 keV
  • 107 count/e

2011-02-02 calibration on Tecnai 2:¶

• faraday measurement: 1.52 nA
• Tecnai main screen measurement (emulsion = 1): 2.15 / 2.6 = 0.83 nA
• Leginon scale factor to match faraday current: 0.793
• Legion measures Tietz sensitivity: 68.7 counts/e-

2013-02-28 Calibration on T12 Spirit¶

• 120kV, Filament Heat= 29, step 1, Spot 3, mag 46K, Beam Current = 5.5-7.5 µA, LaB6 installed 2013-02-22
• TVIPS F416 camera
• Technai main screen measurement (emulsion = 1): 2.15 / 0.11 = 19.54 nA
• Leginon scale factor to match Faraday current: 0.991 (was 0.80)
• Leginon measure Tietz sensitivity: 38.8294 counts/e- (was 32.5211)

Finding the Faraday Cup¶

• See picture below
• Center to cup is 4mm
• Use a 6mm or 7mm Allen (hex) wrench as a wedge (spacer) between the holder end cap and stage. Use the wrench as a lever to fine tune the position