Is this something that can be replaced manually or do I just have to accept that this is the window provided for the PMT I'm using (UV-transmitting glass on an R446)?
I could employ more filters to block out as much light as possible but I was just wondering if this approach is possible.
PMTs work at high vacuum. If you remove the quartz window you'll loose the high vacuum. Perhaps it will implode in your face. Why don't you put the filters before the PMT (i.e., in the optical path of the light)?
Derek,
No, PMT cannot be dis- or reassembled on site. I even doubt it can be factory-repaired in case, e.g. it is broken. So you usually just accept the PMT you have or buy a new one. In this sense, PMT is a solid piece of the equipment and it requires carfeful consideration when it is bought. Sometimes you can ask the factory to attenuate the individual PMT properties that suit you the most. For example, they can select PMT from the assembly line to most accommodate your request. For instance, you can order an extended UV sensing, or extended NIR sensing.
Note however: you are free to filter light passing on to a PMT's photocathode: just use some appropriate light filters.
- with best regards.
please tell us more about your application. Sounds like you want to add filters not change the window.
I did not take the high vacuum into consideration. Thank you for your replies. I will just add more filters.
Thierry, I am measuring chlorophyll fluorescence. Right now, my set-up is fairly primitive and my current filter configuration is not filtering out all stray and scattered light which is affecting my fluorescence signal. My current PMT (Hamamatsu R446) has a UV window but I was considering whether a borosilicate glass would be a better window material as it cuts out light
I used to do something similar, Derek. I mean, setting up optical filters before the photomultiplier tube. Finally, I ended up placing a filter in an unconventional position (at the entrance of the chamber that contained the PMT). I stuck the filter to the wall by using sticky tape. If you decide to do something similar, be careful not to damage the surface of the filter with the sticky tape (perhaps a filter holder will prove useful) and also avoid placing the sticky tape in the beam path. Another thing that must be taken into account is that the Quartz casing of the PMT must NEVER be touched with bear hands, since this will leave a grease film over the Quartz and it will be the end of the PMT. The grease film can be removed with optical paper + ethanol, though. I wish you all the best in your experiments.
Derek, to remove stray and scattered light you don't need a filter or alter the window of the PMT.
1) collect the light with a lens or mirror with a long focal length
2) then focus the light through a fairly long tube (you can use a cardboard tube from a paper roll or something similar)
3) place your PMT on the other end of the tube
4) IF you need to reject light from your source (a laser I am guessing), place a long pass filter (with a cut-off wavelength longer than your source) at the entrance of the tube or in front of the PMT.
This method will remove tremendously the stray/scattered light from hitting your PMT.
Please see Figure 1 in the attached paper. We did not draw the tube in front of the PMT on the Figure but there is one in our set up which is about 40 cm long. We spray painted it black to adsorbed as much as possible any stray light.
The dark current is the current out of the PMT when there is no light. Cutting out any light such as you suggest will not reduce your dark current.
If you are struggling with low signal to noise ratio, use a chopper and detect the signal with a lock-in amplifier set up at the frequency of the chopper. This will select out the noise and will allow you to see very small signals from your samples. This method is common for infra red photo luminescence. Not so much for visible light as one normally can got good signal from source/detector combination in this wavelength range.
Article Differential Reflectance Spectroscopy in Analysis of Surfaces
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