Project

General

Profile

Faraday Cup Measurment Notes » History » Revision 9

Revision 8 (John Crum, 08/27/2012 04:29 PM) → Revision 9/11 (John Crum, 08/27/2012 04:54 PM)

h1. 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) 
 ** 200keV 
 *** 1.01 nA 
 ** 120 keV 
 *** 2.53 nA 


 h1. 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 

 h1. Leginon screen scale factor measurement 

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

 h1. Camera sensitivity 

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



 h1. 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- 


 h1. 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