Like other solid state scintillation detector viz NaI(Tl), CsI(Tl) etc detector why ZnS(Ag) detector can't be used for Gamma counting also? Why ZnS(Ag) is used specifically or can be used specifically for alpha counting only? Can we used NaI(Tl) detector with suitable modifications in the design as alpha counting?
My first answer is: no commercial usual NaI(Tl) for alpha counting because the NaI material ist hygroscoppic and therefore needs a strong sheat to keep the water vapour away. This sheat is thicker than the range of the alphas. Every alpha detector must permit the free access of the low ranged alpha particles.
The use of ZnS for photons depends an the geometry. Because of the low interaction rates of gammas you need voluminous detectors to achieve a sufficient efficiency. .
@Hanno Krieger Sir, The first part seems ok. But photoelectric effect cross section is prop to Z^5. Zav: NaI: 38 compared to 48 for ZnS. Then ZnS detector needs lesser size than NaI.
Your comment is right, but I didn´t compare ZnS with NaI, NaI can be produced as large transparent detectors, where you can collect the produced scintillation light very well. To improve this technique NaI detectors are surrounded by a reflecting layer, which prevents alpha transmission. ZnS is normally produced as thin not transparent layer with light emitted from the surface. You see it´s not only matter of high Z, but there are additional factors (geometry, transparency etc).
@Hanno Krieger Sir,That means sir, "in principle" both ZnS(Ag) and NaI(Tl) detectors can be used for detecting alpha and gamma rays! Is it right sir?
@Debasish, if you manage to create a geometry, where the alphas can directly impact NaI material (vaccum chamber...) than both materials are usable. If you want to use commercial detectors NaI is unsuitable.
ZnS is a polycristalline phosphor/scintillator. Largest available grain size are some 40-50 um. Hence, one is directed to screen use only. NaI can be made in large bulk material. ZnS has among the highest intrinsic efficiencies available ( approx 20%) but has a low transparency to its own light. High extrinsic efficiency for gamma for polycrystalline (powder) phosphors can be obtained for instance by Gd2O2S:X (X=Tb, Eu,Sr).
@Jan Lindström sir, would you please suggest any reference material for the answer, which will help me to understand better...
Dear Debasish, You can take a look at my profile here at Researchgate. The introductory text in my Licentiate thesis contain some information and further references. Free download. (Many of the references are available free as pdfs under for instance Google Scholar)
Two questions were asked, Can ZnS(Ag) be used as a gamma scintillator? and Can NaI(Tl) be used as an alpha scintillator. The answer to both is yes, but the practical use is very difficult.
ZnS was one of the first detectors for x-rays, but required considerable effort with viewing cones and dark adaptation. Photo-sensitive devices have greatly reduced the effort. Nevertheless, the construction of a practical gamma counter is limited by the thickness of the detecting layer and the transmission of light through the layer. Normally produced ZnS is white and is used as an opaque pigment in paints. Transparent ZnS can be produced and is used for instrument windows and lenses. Transparent ZnS emits in far blue and near UV and requires an activator, like Ag to shift and concentrate the emissions to an acceptable wavelength. It is difficult to produce a transparent, activated ZnS.
NaI can be packaged with a sufficiently thin, reflecting window that allows alpha particle detection. The resulting detector is very fragile. If the NaI detector was intended for alpha spectrometry, much better resolution at much lower cost can be achieved with a silicon diode.
The nice thing is that the lifetime of the light emission from ZnS is longer than that from many organic materials. This can allow you to get alpha / beta discrimination in a contamination probe. Here is a flim I made some years ago which shows the different lifetimes.
I used UV light to excite the scintillators
https://www.youtube.com/watch?v=5xdJmCvqd9U
We have recently demonstrated that ZnS can be a scintillator AND be transparent to its own scintillation light.
http://www.sciencedirect.com/science/article/pii/S0022231314005237#
I'm not agree that ZnS:Ag can be obtained in policrystalline state only. We grow singlecrystalline ZnS:Ag by Bridgman method. Diameter of ingots is about 25 mm and 30 mm length. It is water clear crystals light green color. These crystals are efficient alfa detectors but sensitivity to gamma radiation is too low. For thin samples ~1 mm almost zero signal for gamma. Obtained FWHM resolution for Am alfa particles ~5.5 MeV is 5%.
Of course single crystal Zn is attainable, and is available commercially, usually produced by a Bridgeman process. Doping with Ag is straightforward through diffusion typically. Bulk polycrystalline CVD ZnS is much more widely used in general, as an infrared window. Commercially available scintillator ZnS:Ag is powder.
What happens when ZnS is exposed to gamma radiation, whose energy is in the order of 1 MeV?
ZnS(Ag) detectors can not be fabricated beyond 25 mg/cm2 due to loss of opacity. With this thickness gamma detection efficiency is very less. NaI(Tl) detector is hygroscopic in nature and hence can not be useful for alpha detection
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