[RASMB] max out photomultiplier

Tom Laue tml at cisunix.unh.edu
Tue May 15 05:36:25 PDT 2007


Hi Hermes-
I agree with John. You certainly will damage the photocathode if you 
expose the PMT to high light levels with the high voltage on. The 
default for the XLI is to have the voltage off until just before a scan, 
so you may not have damaged it. Nonetheless, it is a very good idea to 
keep the power off whenever servicing the instrument.
I also agree that it sounds more as though the slit assembly may be 
plugged. If you hold the slit assembly up to the light, peering through 
the lens, you should see a uniformly-shaped rectangle of light along 
with some diffraction fringes along the length of the image.
There is an amplifier in the base of the PMT. However, failure of this 
component is very rare and marked by a complete loss of signal.
Best wishes,
Tom

John Philo wrote:
> Hermes,
>  
> Definitely you should always power down the centrifuge before the slit 
> assembly is removed! Yes I believe photomultipliers can show some slow 
> recovery after excessive light exposure, but if the intensity has not 
> recovered by now I don't think it will without intervention, and you 
> will probably need to bring in the service engineer.
>  
> However it is possible that the problem is not in the PMT 
> itself. The intensity loss might have been caused by the attempted 
> cleaning of the slit assembly---it is very easy to block the slit 
> itself with debris, or possibly you did not put the two little lenses 
> back in quite correctly. Also my understanding is that there is an 
> integrated circuit amplifier located within the base of the PMT 
> housing, and this too can be damaged if there is excessive PMT current 
> (too much light). So unfortunately you might need an entire PMT 
> assembly, not just a new PMT.
>  
> Good luck,
>  
> John
>
>     -----Original Message-----
>     *From:* rasmb-bounces at rasmb.bbri.org
>     [mailto:rasmb-bounces at rasmb.bbri.org] *On Behalf Of *Hermes Reyes
>     Caballero
>     *Sent:* Saturday, May 12, 2007 8:05 AM
>     *To:* rasmb at rasmb.bbri.org
>     *Subject:* [RASMB] max out photomultiplier
>
>     Dear RASMB members,
>
>      
>
>     I was cleaning the slit ensemble in the XL-A and I though of
>     cleaning the PMT too. So I did it with ethanol and expose the PMT
>     to the light when the machine was turn on. I read somewhere in
>     this forum that can cause the PMT to max out, and in fact the
>     intensity was around 6-fold lower than before the cleaning
>     procedure. I turn off the machine for 12 hrs and the intensity is
>     still very low, with no apparent recovery. Does anybody know if
>     this PMT will comeback?
>
>      
>
>     Have a nice day!!
>
>      
>
>     Hermes
>
>      
>
>      
>
>     Hermes Reyes_Caballero PhD. Candidate
>     **The Giedroc Lab***
>     *Department of Biochemistry & Biophysics
>     Texas A&M University
>     2128 TAMU Room 419
>     College Station, Texas 77843-2128
>     Phone: (979) 845-6828
>
>      
>
>      
>
> ------------------------------------------------------------------------
>
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>   

-- 
Department of Biochemistry and Molecular Biology
University of New Hampshire
Durham, NH 03824-3544
Phone: 603-862-2459
FAX:   603-862-0031
E-mail: Tom.Laue at unh.edu
www.bitc.unh.edu
www.camis.unh.edu




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