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Prompt gamma activation analysis
The approximate detection limits of the chemical elements assuming a typical environmental sample under usual measuring conditions using PGAA © TUM
PGAA as a method
Prompt Gamma Activation Analysis was recommended by Heinz Maier-Leibnitz in 1963. He immediately recognized the importance of low spectral background which is still the most important step in the instrument development. The first PGAA experiments were performed by Henkelmann and Born at FRM and ILL, Grenoble in 1968. FRM II also established a PGAA facility, it is located at the strongest neutron beam of the world opening possibilities to measure low-mass and/or low-cross-section materials.
Neutrons and gamma rays have a large penetration depth even in condensed matter, so samples with thicknesses up to a couple of centimeters can analyzed. The samples can be measured without any specific preparations, and the method yields the bulk composition of the irradiated volume. PGAA is non-destructive. The sample can be in any physical form: in proper containers liquids and gases can also be measured. Also the chemical form of the sample has no influence on the analysis. For each element, the result of the non-destructive analysis is an averaged concentration (mass fraction) over the irradiated sample volume, i.e. a bulk analysis.
In principle, all the chemical elements can be analyzed with PGAA. The analytical sensitivity of the method mainly depends on the neutron capture cross section of the elements which typically varies in the millibarn-kilobarn range. Certain elements can be determined only as major components (such as O, C, Be, Bi), some as trace elements (B, Cd, rare-earths) with detection limits down to nanograms, while most elements can be analyzed in the microgram range. The major advantage of the method is its capability in the analysis of light elements. It has a unique sensitivity for hydrogen or boron, which can be determined in the ppmw and ppbw level, respectively.
Gamma spectra have an inherent dynamic range, i.e. the smallest detectable peaks have an area of 10–100 counts. However, the area of the same peaks have a practical upper limit, too, which is about six orders of magnitude higher, than the smallest detectable area. Hence, the elements detectable on the ng level can be determined up to mg level, i.e. as minor elements, and never major components, in much higher quantities than a milligram, because the peaks of the sensitive element alone will mask those from all other elements.
The approximate detection limits of the chemical elements assuming a typical environmental sample under usual measuring conditions using in-beam NAA. © TUM
In-beam Neutron Activation Analysis (ibNAA)
The beam is so strong at PGAA (4–6×1010 cm–2 s–1) that its flux is comparable to those in small reactors fully dedicated to NAA. Activating in beam is more advantageous, because the neutron self-absorption can be corrected for in a much simpler way, than in an isotropic neutron field. Guided beams do not contain epithermal or fast neutrons, thus the activation process can also be described much simpler. Thanks to the high flux irradiation, many elements are activated, i.e. the induced radioactivity can be counted with much better sensitivities, than when using PGAA alone. Activity counting takes place in a low-background counting chamber with higher detector efficiencies. Even short-lived nuclides (with half-lives of a few seconds) can be detected here.
As in traditional NAA, the light elements are normally impossible to analyze based on their induced radioactivities. In the first two periods, the only exception is fluorine whose half-life is just 11 s, and its cross section is also low (9.6 mb), but right after the irradiation, it can be measured with a fair sensitivity. Cyclic irradiations/measurements can help in this context. The real strength of NAA is the analysis of elements in the fourth period of the Periodic Table and above, however, a few lighter ones are detectable with high sensitivities (like Na, Mn, Sc), too. The typical detection limit is around or even below the μg level.
Classic NAA, without chemical treatment also called instrumental NAA (INAA), can also be performed at FRM II. The activation takes place in the irradiation channels of the reactor in a much higher flux (>1013 ~1014/cm²s) . The highly thermalized neutron field of FRM II reactor offers unique possibilities for the classical instrumental NAA, too.
PGAA and NAA complement each-other. PGAA is best used for the determination the major components of the samples, while NAA analyses the (minor and) trace elements.
The samples are located in the beam in aluminum frames among Teflon strings. Their typical diameter is a few centimeters with thicknesses in the millimeter range. 16 such samples can be placed in the automatic sample changer. The sample chamber can be evacuated to exclude the possibly interfering prompt gamma lines of nitrogen (and argon).
The PGAA facility is located in the neutron guide hall at the end of the curved neutron guide 4B with the length of 55 m. The last 6.9 m of the guide is elliptically tapered. The thermal equivalent flux (or the capture flux) of the focused neutron beam in the focal point is 6×1010 cm–2 s–1, the highest beam flux recorded. The last 1.1 m of the elliptical guide can be replaced by a set of collimators keeping the beam uniform and parallel.
Using a set of beam attenuators, the flux can be adjusted between 3×107 cm–2 s–1 and 4×1010 cm–2 s–1 (the maximum value at sample position is somewhat lower than that in the sample position) to find the best irradiation conditions matching the sample.
Cold neutron spectrum of guide NL4b (last 5.8 m elliptically focused) with a mean energy of 1.83 meV (6.7 Å).
Two different beam conditions:
Detectors #1 and #2 are on the two opposite sides of the PGAA sample chamber to detect the prompt gamma rays from the irradiated samples. Detector #3 is located outside the concrete bunker of the facility in a low-background chamber dedicated to counting the activated nuclides for iBNAA.
The shielding is made of mainly lead against the high-energy gamma rays, lined with boron- and lithium-containing layers to shield against neutrons. The components can be easily moved or modified, so that the shielding accommodates other setups (NDP, Prompt Gamma Activation Imaging + Neutron Tomography (PGAI-NT) or gamma-gamma coincidence system).
Data recording system
The spectra are evaluated using the Hypermet code5, the list of the peaks are then compared to the spectroscopic data library6, finally the qualitative and quantitative analysis is performed based on statistical methods.7
 Zs. Révay, T. Belgya, in: Handbook of Prompt Gamma Activation Analysis with Neutron Beams (Ed. G.L. Molnár) Kluwer, Dordrecht, 2004, p 1.
 M. Crittin, J. Kern, J.-L. Schenker, Nucl. Instrum. and Meth. A449 (2000) 221.
 P. Kudejova et al, J Radioanal Nucl Chem 265 (2005) 221.
 Zs. Révay, P. Kudějová, K. Kleszcz, S. Söllradl, Christoph Genreith: In-beam activation analysis facility at MLZ, Garching, Nucl Instrum Meth A799114–123.
 B. Fazekas, J. Östör, Z. Kis, G. L. Molnár, A. Simonits: The new features of Hypermet-PC, in: Proceedings of the 9th International Symposium on Capture Gamma-Ray Spectroscopy and Related Topics, Budapest, Hungary, October 8-12, (Eds. G. Molnár, T. Belgya, Zs. Révay) Springer Verlag, Budapest/Berlin/Heidelberg, 1997, p. 774.
 Zs. Révay, R.B. Firestone, G.L. Monár, T. Belgya, in: Handbook of Prompt Gamma Activation Analysis with Neutron Beams (Ed. G.L. Molnár) Kluwer, Dordrecht, 2004, p. 173.
 Zs. Revay, Anal. Chem. 81 (2009) 6851.
Dr. Zsolt Revay
Phone: +49 (0)89 289-12694
Dr. Christian Stieghorst
Phone: +49 (0)89 289-54871
Phone: +49 (0)89 289-14906
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Find the latest publications regarding PGAA in our publication database iMPULSE:
Heinz Maier-Leibnitz Zentrum. (2015). PGAA: Prompt gamma and in-beam neutron activation analysis facility. Journal of large-scale research facilities, 1, A20. http://dx.doi.org/10.17815/jlsrf-1-46
Zs Revay, P Kudejova, K Kleszczm S Sölradl, Ch Genreith, In-beam activation analysis at Heinz Maier-Leibnitz Zentrum, Garching. (2015). Nucl Instrum Meth A799, 114-123. http://dx.doi.org/10.1016/j.nima.2015.07.063
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