Gamma-Ray Spectroscopy

The NRL gamma spectroscopy lab has three high-resolution gamma spectroscopy detectors. All of them are high purity germanium detectors (HPGe) housed in copper-lined lead caves to reduce background, and are used to identify and quantify radioisotopes in samples. A typical GRSS detector channel consists of a high-purity germanium (HPGe) semiconductor detector, a pre-amplifier, an amplifier, a high-voltage power supply, a multi-channel analyzer (MCA), and a computer-based acquisition and analysis system. In modern systems, many of these components are combined into integrated units. At the NRL, the Canberra Lynx system is employed, which integrates the power supply, digital amplifier, and MCA into a single box. The Lynx units are networked, allowing one computer to control multiple Lynxes.

Gamma rays emitted from a radioactive source that are absorbed in the HPGe detector produce electrical pulses, and  the pulse amplitude is proportional to the energy deposited in the detector, which allows for measurement of gamma ray energies. The MCA sorts these pulses by amplitude, and computer software displays a plot of the number of pulses received at each pulse amplitude. Such a plot is called a spectrum because it shows the spectrum of energies emitted by the source. Comparison of the peaks found in a spectrum against a library of known radionuclide energies and abundances allows identification of the radioactive components of a sample. If the system efficiency is calibrated using a source with traceable activity, the activity of those radionuclides can be quantified.

Data acquisition and control, as well as quantitative analysis of identified radionuclide activity, is performed by the software package Genie 2000 from Canberra Industries. The software provides for spectrum acquisition, storage, isotope identification, and activity quantification, as well as detector system energy and efficiency calibration.

Example Spectrum Plot
Example Spectrum Plot

The figure below shows a picture of a GRSS system at the NRL. On the desk are the computer used for analysis and display as well as the Lynx MCA (seen behind the keyboard), and to the right is the detector shield that minimizes counts from background radiation and a vacuum dewar filled with liquid nitrogen for keeping the HPGe detector at its operating temperature.


Gamma-ray spectroscopy system setup

GRSS Calibration

Calibration of a gamma-ray spectrometer involves placing a traceable source, often with emissions at multiple gamma-ray energies, in a repeatable position relative to the detector and acquiring a spectrum. Using the measured spectrum in conjunction with the source activity and date from the source calibration certificate, the analysis software computes the efficiency of the detector at each of the source energies for the source in that position. A polynomial curve fit provides an efficiency curve as a function of energy.

The HPGe detectors of the GRSS at the NRL are calibrated using a NIST-traceable mixed-nuclide point source. The first detector is a Canberra GC5019 HPGe, which has an efficiency of 50% relative to a standard 3 inch x 3 inch NaI detector at 1332 keV, and has full width at half max (FWHM) of 1.9 keV for peaks measured at 1332 keV. The second detector is a Canberra GC1419 HPGe, which has an efficiency of 14% relative to a standard 3 inch x 3 inch NaI detector at 1332 keV, and has full width at half max (FWHM) of 1.9 keV for peaks measured at 1332 keV. The third detector is a Canberra GC1420 HPGe, which has an efficiency of 14% relative to a standard 3 inch x 3 inch NaI detector at 1332 keV, and has full width at half max (FWHM) of 2.0 keV for peaks measured at 1332 keV. The calibration source contains nine radionuclides, with gamma emissions ranging from 88 keV to 1836 keV. This provides a calibration curve that covers all the major emissions from 22Na and 154Eu (123 keV - 1596 keV). For each peak in the source, the stated 3-sigma uncertainty (99% confidence) in the emission rate is 3%.

Calculations of sample activity take into account the efficiency of the detector system as a function of energy, the gamma-ray emission probability for the nuclide/energy, and correction for radioactive decay during the count. The figure below shows a sample spectrum from the calibration measurement. The nine nuclides result in eleven full-energy peaks.

Example calibration spectrum
Example calibration spectrum