Quality Assurance / CEM Lifetime
 
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CEM lifetime

The term lifetime describes the behavior of the gain at a fixed operating voltage under a constant particle exposure. Measurements show that the gain is not stable, but features characteristic profiles in different regions that are generally applicable to all CEMs.

The initial gain drop in the region < 10e9 pulses that is common to all multipliers after they have been exposed to air, is due to a loosely-bound gas cover at the surface of the channel. It reduces the work function of the glass, so that the secondary electron emission is higher than for a gas-free surface. Under the onset of particle bombardment this gas is desorbed. This initial rapid drop in gain which all multipliers exhibit after exposure to air has been called "clean-up phase" (see Burn-in procedure).

After this rapid decrease the gain levels at a plateau value. The operational characteristics of the CEMs then remain stable until about 10e10 counts. A second type of gain fatigue becomes observable after 10e10 accumulated counts. This type of loss in gain is permanent and due to non-recoverable surface damage. For a certain time this loss in gain can be compensated by a higher applied voltage.

The decrease in gain during the lifetime of a CEM can be observed by the fact that the optimal working point moves always further toward the right toward higher applied voltages. This is shown in Fig. 2. This can be proven with routine measurements such as with the MEASAR measuring system.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

         
 
User Report
  
Courtesy of SPECS GmbH, Berlin
  
         
         
 
Lifetime test of the PHOIBOS EDR CEM
 
         
 
The standard detector systems of the PHOIBOS analyzer series are based around the extended dynamic range CEM. Because of the low resistance the extended dynamic range CEMs are suitable for extremely high count rates. The gain is governed by the detector voltage and the condition of the emissive

 
layer. The condition of this layer changes with usage and to compensate for a drop in emissive quality of the surface an increased detector voltage can be applied, thus keeping the overall gain constant. If the detector voltage has reached the limit of 3.5 kV at this point the CEM is at the end of its lifetime and needs replacing.
 
         
         
         
 
 
         
         
 
Ageing of the PHOIBOS Extended Dynamic Range CEMs. The detector voltage is increased if the counted rate decreased.
 
         

 

 

 

 

 

 

 

 

 

 

 
 
Fig. 1 Measuring set up for CEM lifetime measuring
 
 
 
Fig. 2 The optimal working point moves with the accumulated load always further to the right
 
 
Fig. 3 The KBL505/45 operated non-stop over 15 years on board the ULYSSES probe
 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Dr. Sjuts Optotechnik GmbH

Rudolf-Wissell-Str. 14, 37079 Goettingen, Germany Tel. + 49 (0) 551 209 95 62, Fax. + 49 (0) 551 209 95 63, Email: sjuts@t-online.de