150% - do You believe in fairy tales?

@Marek_Brancewicz : http://www.ortec-online.com/download/gem.pdf

Apart from that I have visited labs having HPGe of this efficiency.

Why you have some contradiction to this please tell.

First question is: How this efficiency is estimated? I think this is probably kind of relative efficiency, just to make a marketing gimmick.

Detectors can't create photons, You will not register 150 photons if only 100 hit the crystal. You can only gain the output signal, but it doesn't mean that You improve Your efficiency.

HPGe detector efficiency (number in %) is usually compared the efficiency of 3 inch NaI(Tl) crystal @ Co-60 photopeak (1332.5 keV). So sometimes this value may be higher than 100%. 

@Marek what Jan is saying is right. 150 % efficiency is relative to 3inch x 3inch NaI(Tl).

Sorry for framing the question without "relative". Now I have modified it.

Hi @ all,

efficiency is always given as relative efficiency. It will be hard to find single HPGe crystalls with more than 30-40 percent rel. efficiency. The capsule is larger than the crystall by some millimeters (thickness of Al capsule and vacuum isolation). I do not know what you intend to do with a big crystal but usually to gain efficiency HPGe detectors are packed to a bigger ensemble like in Clover detectors with total relative

efficiency in “add-back” mode between 130 and 140% (See Duchene et Al NIM A432-1999-90) Or look for GRETINA or AGATA detectors as examples for segmented high-efficiency HPGe detectors.

I want to use it for whole body counting (internal contamination monitoring) for high energy photon emitters such as Co-60.

Ok. In that case where you ask for a high detection efficiency I would not recommend using a super big crystal but use a small number of standard crystals or detectors or if you prefer to use a single device I would recommend clover detectors. Larger crystals are much more expensive than packing together small ones. I suppose you need Ge detectors also for the identification of isotopes otherwise you could also use NaI(Tl) which can be produced as very large and efficient crystals not requiring any cooling.

Actually we have been using NaI(Tl) detectors as well as phoswich detectors.

HPGe is thought of due to better resolution. Small crystals won't suffice for high energy photons as activity is low in the body and the exit spectrum is further modified by body attenuation.

For low energy photons array of smaller detectors are being used by us.

A single crystal detector can usually go as high as 150% efficiency, but I've heard the guys from Canberra talking about 200% relative efficiency - never saw it, though. he largest multiple crystal detector I have seen has 1300% relative efficiency (yes, no mistake in the number of zeroes), and is meant for airborne gamma spectroscopy mapping.

OK, so what is the real efficiency of 3-inch NaI(Tl) for 1332.5 keV?

I recall it is about 0.0012 counts per photon if the Co-60 source is kept at 25 cm from detector surface.

DEEPAK, to estimate real efficiency in this case I need also active area of detector. Can You give me this value?

Marek, I can't understand what do you mean by active are/volume for NaI(Tl). In my opinion there is some confusion with HPGe's active volume.

@ Marek: for the NaI the active volume is the entire crystal, you do not have dead layers like for HPGe. For a NaI 3 inch detector the volume is typically given from 3" diameter by 3" length.

Active area - area from which photon can be detected. I need it to estimate the real efficiency of 3 inch NaI(Tl). So, correct me if I'm wrong, but my fast estimation based on Deepak's data shows that the real efficiency of 3 inch NaI(Tl) for 1332.5 keV is around 5%. Do You agree?

I understand, you wanted to rule out the geometrical efficiency and then remain only with the quantum efficiency. Yes, at a first glance it does seem correct. This would mean that a HPGe with 150% relative efficiency would have a 7.5% efficiency at Co-60, which is plausible.

Yes Sir!