1. We know the energy resolution of HpGe detectors is the best out there, better than NaI/TI and other detectors. WHY is this the case? What is happening at the atomic or molecular level that makes this so?
2. All radiation detectors have similarly low intrinsic efficiencies of much less than 5% (1% - 2% is typical in my experiments). Again, what is happening at the atomic or molecular level that makes this so?
Ionizing radiation creates electrons/holes in the detector material. The higher the number of generated (primary) electron/hole pairs, the higher is the energy resolution. The number of pairs depends on the band gap of the matrial and is therefore higher in semiconductor than in an insulator (as NaI).
The intrinsic efficiency depends on what is the probability that a gamma quant is completely absorbed in the detector. This probability is higher for heavier elements and bigger detectors, so quite limited for Ge detectors with relatively low atomic number and limited crystal size. Detector materials like NaI(Tl), BGO can reach higher efficiencies.
Hello David,
The total photon cross section usually decreases with increasing photon energy. For low Z (atomic number) materials, the total photon cross section decreases with increasing photon energy until it reaches a fairly low constant value at very high photon energies. For high Z materials such as lead and above, the total photon cross decreases with increasing photon energy until it attains a minimum, after which it rises somewhat and plateaus. You can find very detailed charts of this effect in the following article: J. H. Hubbell, H. A. Gimm, I, Overbo; Pair, triplet, and total atomic cross sections (and mass attenuation coefficients) for 1 Mev-100 GeV photons in elements Z = 1 to 100; Journal of Physical and Chemical Reference Data; Vol. 9; No. 4; October 1980; pp. 1023-1147. Note, the title of this article notwithstanding, the curves cover the photon energy range 10 to 1011 eV.
Regards,
Tom Cuff
Dear David Musoke ,
the type of creation of the detector signals of HPGe and NaI(Tl) is very different.
In the fist case (HPGe) you create electron-hole pairs (as Erik mentioned already above), the charge of which is passed*) to the electrodes of the detector crystal and is subsequently processed by appropriate electroncis. However during the time of charge collection recombinations of some electrons end holes take place so that not all of them contribute to the detector signal.
This lack of charges is taken into account by the so-called Fano factor F when estimating the intrinsic energy resolution FWHMi:
FWHMi = 2*sqrt(2*ln2)*sqrt(F*e*E)~ 2,35* sqrt(F*e*E) with E as the energy of the x-ray photon and e as the mean energy necessary for the creation of an electron hole pair.
For HPGe we have e ~ 3eV and F ~ 0,1.
In the case of NaI(Tl) we have luminescence processes yielding photons in the optical regime which are further processed by PMTs and subsequent electronics . Here the Fano factor is F=1 and the e is about 30eV per luminescent photon.
So the large differences of the intrinsic energy resolutions are due to very different Fano factors F and the differences in e.
*) by the aid of high voltage applied to the electrodes
Thank you all for the helpful and excellent answers you've given to my question. It's hard to mark out the best answer, since I found ALL of them excellent... Thank you very much for being willing and taking the time to answer my question.
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