Full energy peak

Although the source full energy peak efficiency [after Crouthamel... 

Unfortunately, Ge detectors have poor response functions. For a 5 x 5 cm coaxial detector (nominal 20%), only about 3/4 of impinging 1.33 MeV y rays do interact, and, of these, only 15-20% give useful full-energy peaks. 

In this experiment, the Ge detector with spectrometer is calibrated for its efficiency, s, with a standard that emits a set of gamma rays at energies that span the range of interest, usually from a few keV to 1.5 MeV. The counting efficiency is calculated from the ratio of the net count rate to the reported disintegration rate at each full-energy peak in the spectrum. A correction for radioactive decay is needed. 

Step 1. Count the quality control source under specified conditions for a brief period (e.g., 500 s) to obtain count rates in the channels beneath the full-energy peaks. Count the radiation background for a long period (e.g., 200,000 s). 

Carefully swirl to mix. Be certain to cover the bottom evenly, but do not swirl the solution up onto the container walls. The total initial volume is 5.0 mL. Screw the lid on the container and carefully place in position in the germanium gamma-ray detector counting chamber it should be centered and level. Count twice for a sufficient time period to accumulate 2000 counts (typically 100 s). Check to confirm that at least 2,000 counts have been accumulated at each of the peaks used for calibration. Collect the gross gamma-ray count rates for the full-energy peaks in Data Table 2B.1. 

Full-energy peaks usually are measured - as in this experiment - with a Ge detector and spectrometer system to identify the energy at the peak maximum and determine the intensity given by the integrated area beneath the peak. 

Full-energy peak (or photopeak) efficiency The efficiency for producing full-energy peak pulses only, rather than a pulse of any size for the gamma ray. 

An example of a full-energy peak efficiency curve for a germanium detector is shown in Fig. 5.19. 

Figure 12.3 shows the source spectrum. In the case of perfect energy resolution, this monoenergetic source produces in an MCA the measured spectrum shown by Fig. 12.4. Some photons produce pulses that register in channel Cq, corresponding to the source energy Eq, and thus contribute to the main peak of the spectrum, which is called the full-energy peak. The Compton...

The full energy peak, corresponding to E (this is the highest energy peak)... 

Full-energy peak efficiency is defined as follows ... 

The efficiency of Ge detectors quoted in the list of specifications by the manufacturer may be a relative full-energy peak efficiency or an absolute... 

For < 2 MeV, the full-energy peak is intense and almost Gaussian in shape. 

Assume that the mass of an element in the sample will be determined from the full-energy peak at Using the notation of Sec. 14.4, the mass m is given by (see Eq. 14.22)... 

NaI Tl) scintillation-detector. The energy resolution of the detector is measured by the width (W) of the full-energy peak (FEP) at one-half the maximum height H/2) of the FEP. 

Figure 19.8. A composite gamma-ray spectrum of Cs, Mn, and Co. The net area of the full-energy peak (FEP), as obtained by baseline subtraction, is proportional to the activity of the radionuclide.

The absolute efficiency in this calculation is the ratio of the net number of counts measured in the 1332-keV full-energy peak divided by the number of gamma rays emitted by the °Co source at this energy during the same time interval. 

The spectral response of a detector is more complex than described in Section 2.4.4 because of the bulk of the detector. The observed Compton continuum consists of single plus multiple successive scattering interactions. When such multiple Compton scattering interactions are terminated by a photoelectric interaction, the pulse is added to the full-energy peak. Most of the counts in a full-energy peak for gamma rays above 100 keV are due to such multiple scattering plus a final photoelectric interaction. 

The radionuclide is identified by the energy at the midpoint of the characteristic full-energy peak. It is quantified in terms of the count rate in the channels that define the full-energy peak. Subtracted from this count rate is the count rate in these channels due to other gamma rays discussed in Section 10.3.7 in simple cases, the background count rate per channel is the average of the count rates in one or more channels on each side of the peak. 

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