Chapter IV Experimental Technique 43-76
4.2 Experimental Procedure 54
4.2.7 Optimization of Useful Parameters of the HPGe Detectors 64
The outstanding advantage of gamma ray spectrometry is the ability to measure gamma emitters directly in the samples without adopting any chemical separation. Gamma ray spectrometry allows both qualitative identification and quantitative determination of the radionuclides in the sample. A modern digital gamma ray spectrum is in essence a list of numbers of pulses measured within small consecutive pulse height ranges. So the most important parameters of a detector are energy calibration - the relationship between channels and energy, energy resolution and efficiency calibration - the relationship between number of counts and disintegration rate. All these parameters are the function of gamma ray energy. The performance of the detector depends on the high resolving power and the higher value of efficiency. The measurements of detector parameters of the HPGe detector used in the present experiment are given below [6].
4.2.7.1 Efficiency Calibration of HPGe Detector
The efficiency of a detector is a measure of the number of gamma rays detected out of the total number of gamma rays that are actually emitted by the source. In order to quantify the activity present in an activated sample an accurate calibration of efficiency of a detector as a function of gamma ray is necessary. For this reason, the efficiency calibration should be known before use. The accuracy of all quantitative results will depend on the efficiency calibration. The efficiency of a detector changes with the physical change of counting system and the environment that surrounds it. So, counting must remain constant throughout the experiment.
Efficiency can be expressed in several ways; some common ones of them are:
a. Absolute efficiency, b. Intrinsic efficiency, and c. Relative efficiency.
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a. Absolute Efficiency
Absolute efficiency is the ratio of the number of counts (gross counts) produced in a detector divided by the number of gamma rays actually emitted by the source in all directions [5]. It depends upon the distance between the source and the detector and includes the solid angle subtended by the detector. Therefore, the absolute efficiency of a detector can be written as,
b. Intrinsic Efficiency
Intrinsic efficiency is the ratio of gross counts produced in a detector divided by the number of gamma rays striking the detector. It can be written as,
The relation between the absolute efficiency and intrinsic efficiency is given as follows,
(4.1) where, Ω = the solid angle subtended by the detector at the source position and f = the percentage yield for any source.
c. Relative Efficiency
It is the efficiency of a detector relative to efficiency of another standard detector. It is the ratio of the absolute efficiency of an HPGe detector to a 3″ diameter by 3″ long NaI scintillation detector, each at 25 cm from a point source and specified at 1332.51 keV.
Knowing the intensities and the peak areas of the gamma rays, the relative efficiency for the full energy and the escape peaks have been calculated using the following relation,
(4.2)
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d. Construction of Full Energy Photo Peak Efficiency
Full energy photo peak efficiency is the most significant parameter in practical gamma spectrometry. This efficiency is a basic parameter of the detector and is independent of the detector geometry. This efficiency is used for producing full energy peak pulses only, rather than a pulse of any size for the gamma ray.
In this experiment an accurate calibration of the efficiency of the detector is necessary.
Great care was taken in calibrating the efficiency because the overall uncertainty of the measurement would be limited by the uncertainty of the efficiency value. HPGe detector is widely used for gamma ray spectroscopy measurements primarily because of their superior resolution compared with NaI(TI) crystal [7].
In the present work only the intrinsic full energy photo peak efficiency is calculated. The resolution of HPGe detector offers the possibility of more precise measurement of gamma ray yields. Accurate knowledge of efficiency of the detector as a function of gamma ray for a particular experimental geometry is needed.
Calculation of the efficiency of HPGe detector requires precise definition of the geometry of the system. However, the germanium used for fabricating these detectors varies over a wide range of types, shapes and sizes and the spectrometers usually have to be cooled by LN2 ( Liquid nitrogen) temperature. The efficiency of the HPGe detector is determined from the following relation,
( )
(4.3) where, Iγ = Absolute gamma ray intensity of the standard source and
CPS= Net peak area per unit time
A calibrated source is specified by its activity on certain date. The correction for decay since that day can be ascertained through the equation:
At = A0 e-λt
where, At = Present activity of the source, A0 = Initial activity of the source,
67 t = Decay time, λ = Decay constant
2 / 1
693 . 0
T and T1/2 = Half-life of the source.
In the present work Al2O3 based 226Ra has been used as standard material for determining efficiencies of the detectors. Gamma ray counts of the standard were recorded for 5000 seconds at the surface of the detector. A standard material contains 226Ra, which decays successively by emitting gamma rays. The daughter nuclides of the material are 214Pb,
214Bi and others which were characterized by their gamma energy. The experimental data for efficiency measurements are given in Table D1. Experimental efficiency data points were fitted using Eqns. 4.4 and 4.5. Efficiency curves (Figs. 4.11 and 4.12) so developed were used to calculate the efficiencies of gamma energies emitted from different radionuclides studied in the present experiment.
Fig. 4.11 Efficiency curve of the HPGe detector-2
y = 800.11x-0.967 R² = 0.9796 0
1 2 3 4 5 6 7 8
0 500 1000 1500 2000 2500
Efficiency(%)
Energy (KeV)
D-1
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Fig. 4.12 Efficiency curve of the HPGe detector-2
The empirical equation for the relations between the efficiency (%) and the gamma ray energies are therefore,
Y= 800.1×X-0.96 (4.4) Y= 1086×X-1.00 (4.5)
Where, Y, is efficiency expressed in percentage, and X is gamma-ray energy in keV.
Eqn. 4.4 was the working equation for detector-1 and Eqn. 4.5 was that for the detector- 2.