Closed Cycle Coolers at LHe temperature?

[Massimo Robberto]

Hi friends.
I'm working in the mid-IR with two cameras based on closed cycle 
coolers (CTI 350 and 1020 models). A problem with 
these cooling systems is that they cannnot go below 11-12K,
a temperature a bit to high for certain type of detectors. In fact 
in one of the two instruments we have added a small helium vessel
in order to cool down the detector to about 5K.
It would be very nice to reach the LHe temperature with a modified 
closed cycle cooler having a "third stage". A low cooling capacity 
should be sufficient to cool the detector alone. 
 
Recently I heard that this kind of modification is not impossible to do
and has been succesfully made in some US astronomical institute.
Is it true? Is there something published? May I have some more info?
Thank you all in advance
Massimo