Background radiation can be reduced by installing the HPGe detector in an underground laboratory, which is often expensive and inconvenient. The other possibility is to operate the spectro- meter with an anticoincidence system, in which a scintillator detector is used in an anticoincidence circuit with the germanium detector. Various factors in the shielding design of the HPGe detector also need to be considered for background reduction, (optimum thickness of Lead shielding, endcap material selection, inner lining of Lead shielding by copper etc). Installation of Charcoal filters for adsorption of radon in ventilation system has also been recommended.

Thank you Sir Ahmad

I am strongly interested in radon-background subtraction. 

Charcoal filter sounds nice because we currently do not have high budget for building a radon-background subtraction. I will try to find more related documents about this

Your comment about "subtraction" gives a hint to a possible technique. Perhaps you should measure a background spectrum for a long time in order to minimize the noise. And then during an experimental measurement you could subtract this noise free spectrum with the relative intensity proportional to the acute measuring time.

Just an idea!

Dear Krieger, thanks for your comment.

More precisely, we would like to develop a live-time subtraction spectroscopy. This means, the accumulated spectrum has ultra-low background due to the active shielding (by anticoincidence techniques) and passive shielding (lead, radon-background subtraction, etc. )

However, we want to gain more idea and knowledge in this feature.

I had a short look on your work (double beta decay etc). Possibly you should contact MPI Heidelberg, but I think you already did it. Sorry, no new ideas from my side.

The background problem is not the radon, but the radon progeny. Circulating filtered air through the system will reduce the progeny. An electrostatic filter works best. The progeny collects on ungrounded plastic. Any plastic in your system should be conductive and connected to ground. Electrical wires will collect progeny. Wiring should be isolated from the circulating air. You can reduce the radon in the air by charcoal filters on the incoming air and recirculating filters in the room.

Added note: Use charcoal on room air as a last resort. You can excessively dry the air causing other problems like electrostatic surfaces, static discharges, and dry skin.

From our experience, two very simple additional procedures can help:

- close the shield doors also when the detector is not in use, this might help to avoid dust deposition;

- check the materials you use to protect the detector from contamination, for static electricity (for example, kitchen foil can increase Rn daughter background a lot).

Hope this helps!

There is a very easy way that we use to reduce radon progeny background, if you use a nitrogen cooled detector: flush the measuring chamber (the inside of the shield with the nitrogen that evaporates from the cryostat Dewar. Take care to circulate it through a longer duct, otherwise due to the low temperature of the vapors you might end up with condensation problems.

This is a solution used, for example, in the underground lab at Felsenkeller.

Sorry, I did not read the question all the way down. It is clear you already use the method. But it is quite effective, though...

usually 2 methods to reduce the radon background. 1) high pressure N2 flush the chamber from another LN2 container, not use the HPGe Dewar.  2) air-tighter shielding design.  

if you want to subtract the radon background from the spectrum, you can try Monte Carlo method.In CDEX background analysis, we do some study.  the problem is that you should discriminate the air-radon background and the U-238 background from the materials. The full peak from radon and its progeny  are 242\295\352\609 keV, and the peak of 352keV is higher than that of 609keV, and, there is no peak of 186keV (which come from Ra-226).  the full peak from U-238 inside the surrounding materials are 186\242\295\352\609keV , and in this situation, the peak of 352keV is less than 609keV, and especially, the peak of 186keV is obviously.  From these two characterics, you maybe can find some way to subtract the air-radon background from measurement  gamma spectrum.

As an alternative to permanent nitrogen flushing,, it is possible to fill the empty space within the shielding with "blocks" of a light material like polysterene foam, although I am not sure that it will be sufficient for ultra-low background spectrometry. 

We simply fill the space with plastic bags filled with air and it works nicely.

Nitrogen flushing helps displace "external" radon daughters, but doesn't really help when the nuclides are in the lead shielding itself (from nuclear testing fallout). In this case you need to use low-activity (which usually means pre-WWII) material for your shield.

For example in one of the NAA laboratories in Hungary they use pre-WWII steel rather than lead, recovered from bridges destroyed during the war. Several companies supply "ultra low background lead" shields which use at least some low-activity, pre-WWII lead.

http://www.bnc.hu/?q=node/13

http://ortec-online.com/download/HPULBS.pdf

http://canberra.com/products/detectors/pdf/777-SS-C40116.pdf

http://www.fonderie-de-gentilly.com

Article Evaluation of lead shielding for a gamma-spectroscopy system

As the colleague Tondeur, we fill empty space within the shielding with blocks of polysterene foam. This reduces not only the background, but also its variation.

Dear all,

Thanks.

Actually, this is not related to "double beta decay". This is a research work of a PhD student, my friend and colleague in Viet Nam. I joined the discussion with him, and I am also interested in this technique. Thus, I would like to ask people that have experience. 

Thanks for all suggestions. These help a lot !

We are considering to apply economical ones according to the budget limit :D

Best regards,