Since environmental samples have volume, hence point source efficiency calibration curves are not appropriate.

In order to obtain correct results, the samples shall be counted under the same measuring conditions as those under which the system has been calibrated.  The detection efficiency curve depends not only on a detection system but also on a sample shape and a sample matrix with different density and height of environmental samples. Variability in the sample chemical composition is not major problem since mass attenuation coefficients (µm) differ only slightly from one environmental sample to another. The preparation of the standards can also be made using "in house procedures". This can be done by homogeneously incorporating of certified and traceable solutions of radionuclide into inactive matrices with the same composition and density as the sample to be assayed.

Energy calibration: Cobalt -60 or any point source can be used for energy calibration. It shows the energy peak location on certain channel.

Resolution of a detector is the ability of the detector to distinguish between two energy peaks which are very close in range. This means that the detector must produce sharpener energy peaks so that two peaks very close can be distinguished. The value of resolution can be calculated by dividing FWHM by Energy.

 Detector efficiency: The efficiency of a detector is the proportionality constant which relates the activity of the source being counted and the number of counts observed.

 There are various (Absolute efficiency, Intrinsic efficiency, Relative efficiency, and Full energy photo peak efficiency) kinds of efficiency, which are in common uses for gamma ray detectors. Here we are focusing on the relative efficiency of the detector. Relative efficiency is the ratio of area of the photo-peaks to the intensity of the source

Relative efficiency = area under photopeaks / intensity of the source

Two articles are attached which show intercomparison in preparation of reference material and advantages of IAEA (or any other organization) reference material for detector calibration.

Thank you very much sir for your answer.

Ideally, I would prefer to develop a calibration independent of certified reference materials - for example using in-house standards produced using a known quantity of spike, or by detector response simulation (ideally, reconciling several different calibration methods). I would then use the certified reference materials as a confirmation cross check, running them within each batch of unknown samples to confirm satisfactory quality of results. 

A lot of reference materials are certified to relatively low precision (for example IAEA375, which I use regularly, has 137Cs activity concentration to 1.5%, and natural activity concentrations even worse - 238U to 25%). If you measure a sample to high precision, you don't want the calibration uncertainties to be significant. Also the quantity of reference materials is limited, and they go out of stock (as is the case of IAEA375) - you don't want to establish a calibration on a material which you can't get more of if you change geometry. Finally, you can spike your own materials to much higher concentrations than most reference materials, allowing you to run very short calibration runs, and of course can make whatever quantity you require for the particular medium you're working with.

In principle, the most accurate analysis can be only achieved if you have an identical calibration source that matches your unknown sample. In this ideal case you will eliminate any correction related to the differences of the sample characteristics. However, you have to be aware about the difference between the reference materials and the certified reference materials. Basically, the certified reference materials can be used for calibration purposes where you can proof the traceability; other samples can be used for quality control and verification purposes. The point sources are very helpful for calibration purposes when using the simulation software where you can generate, by simulation, different geometrical efficiency curves for different types of detectors.

For more information on calibration source preparation, you can download a publication about the preparation of in-house calibration source for gamma spectrometry measurements. http://download-v2.springer.com/static/pdf/706/art%253A10.1007%252Fs10967-012-1893-9.pdf?token2=exp=1428817824~acl=%2Fstatic%2Fpdf%2F706%2Fart%25253A10.1007%25252Fs10967-012-1893-9.pdf*~hmac=d5f80f8a16fac96d9e4318681ed3575869dd48a0b05630914cec23b4fed96af9