Peak-to-Compton ratio

Figure 12.26 A sketch of the Co spectrum, indicating how it is used for efficiency and peak-to-Compton ratio determination.

Another parameter specified by the manufacturer of Ge detectors is the peak-to-Compton ratio (PCR). Looking at Fig. 12.26, the PCR is defined by the equation... 

Some operating parameters can be considered in common for all these detectors, notably detection efficiency and the radiation background. Additional parameters pertain only to detectors with associated spectrometers. These parameters concern the radiation energy peaks that identify and quantify radionuclides, and include energy calibration, energy resolution, peak-to-Compton ratio, and peak shape. 

Selection of detector type and size should be controlled by its application, both immediate and long-term. In addition to cost, important performance characteristics of a semiconductor detector for gamma-ray spectrometry are the resolution, efficiency and peak-to-Compton ratio. 

Spectrometers have additional parameters for which response should be tracked. These include the count rate in spectral energy regions of interest, peak channel numbers at selected energies, difference in channel numbers between two specified energy peaks, peak energy resolution at specified energies, and the peak to Compton ratio (see Section 8.3.4). A QC chart can be established for each parameter by replicate measurements. This chart will display each data point as well as lines at the mean value and at 2a and 3a values. 

Detectors for gamma-ray spectrometry are usually characterized by their resolution, efficiency, and peak-to-Compton ratios. 

The peak-to-Compton ratio refers to the ratio of the number of counts in the 1332 keV peak of cobalt-60 compared to the number of counts in the Compton region. Germanium detectors usually have a resolution better than 2keV at Cobalt-60 1332 keV, relative efficiencies are mostly smaller but can be 100% or better, and peak-to-Compton ratios show values up to 50. 

The detection limits can be further improved by lowering the background activity using a Compton suppression spectrometer (CSS). A fairly simple CSS would consist of a high resolution Ge detector surrounded by a Na guard detector and an array of electronic modules. The peak-to-Compton ratio can be improved to 550 to 650. The cost of a CSS can vary between US 75 000-100 000. 

Germanium detectors are characterized by three parameters resolution, peak-to-Compton ratio, and efficiency. The resolution is typically given for the 1332-keV Co line and varies from 1.8 keV for the very best to 2.3 keV for the very large detectors. The peak-to-Compton ratio is measured as the ratio of the number of counts in the 1332-keV peak to the number of counts in a region of the Compton continuum. Values vary from 30 to 90 for the most expensive model. The efficiency is expressed as a relative efficiency compared with the 7.5x7.5-cm Nal(Tl) scintillation detector. Relative efficiencies of HP-Ge detectors vary from 10% up to 150%. The dead time of semiconductor detectors is low, so the count rate is limited largely by the electronic circuit. 

The larger the detector, the greater the probability of complete absorption of the gamma-ray and hence a larger full energy peak and lower Compton continuum (i.e. higher peak-to-Compton ratio). 

The warranted parameters — resolution at particular energies, peak width parameters, peak-to-Compton ratio and relative efficiency. This is the specification on which you purchased the detector and expect to be achieved. 

Such a specification would refer to a standard coaxial detector. Specification sheets for other types of detector will contain other infomiation and not all the parameters noted in Table 11.3 will be warranted. For example, for a well detector geometric details of the well and the active volume will be quoted. It is likely that only the detector resolution will be warranted. Peak-to-Compton ratio may be measured, but not warranted. For low-energy detectors, only the resolution wiU be warranted. 

From the point of view of checking the detector specification, this single spectrum can be used to check all warranted parameters - resolution, peak shape parameters, relative efficiency and peak-to-Compton ratio. 

Figure 11.9 Peak-to-Compton ratio - P/C ( Co source), (a) The general location of P and C (counts on a logarithmic scale), and (b) the constant nature of the continuum in the B region (counts on a linear scale)...

We would expect a more efficient, larger detector to have a larger background count rate. The proportion of gamma-rays striking the detector that end up on the Compton continuum is related to the peak-to-Compton ratio (P/C). Figure 13.1 shows how this varies with relative efficiency. The data in this figure were derived... 

Figure 13.1 Peak-to-Compton ratio as a function of relative...

A well-designed system should be able to achieve reductions in the Compton continuum of a factor of 8 to 10, improving the peak-to-Compton ratio of a detector with, from say 50 to 1 to 500 to 1. ORTEC claim peak-to-Compton ratios of up to 940 to 1. Equation (13.8) told us that the MDA is proportional to the square root of the background continuum level. Reducing that by a factor of 9 improves the MDA by a factor of 3. 

PEAK TO COMPTON RATIO In germanium detectors, the ratio of the maximum count in the 1332.5 key peak from Co to the average count per channel in the range 1040 to 1096 key. 

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