Volume 98, Number 1, January-February 1993 Journal of Research of the National Institute of Standards and Technology [J. Res. Natl. Inst. Stand. Technol. 98, 127 (1993)] Prompt-Gamma Activation Analysis Volume 98 Number 1 January-February 1993 Richard M. Lindstrom A permanent, full-time instrument for thermal-neutron instrument at this re- prompt-gamma activation analysis is actor. Hydrogen backgrounds of a few National Institute of Standards nearing completion as part of the Cold micrograms have already been achieved, and Technology, Neutron Research Facility (CNRF). which promises to be of value in nu- The design of the analytical system has merous applications where quantitative Gaithersburg, MD 20899 been optimized for high gamma detec- nondestructive analysis of small quanti- tion efficiency and low background, ties of hydrogen in materials is neces- particularly for hydrogen. Because of sary. the purity of the neutron beam, shield- ing requirements are modest and the Key words: activation analysis; cold scatter-capture background is low. As a neutron beams; elemental analysis; neu- result of a compact sample-detector ge- tron capture gamma rays; nuclear ana- ometry, the sensitivity (counting rate lytical methods; prompt gamma-rays. per gram of analyte) is a factor of four better than the existing Maryland-NIST Accepted: August 10, 1992 1. Introduction The nuclei of some elements of a sample placed ence between the masses of the separated neutron in a field of neutrons absorb neutrons and are and proton and that of the ground state of the transformed to an isotope of higher mass number. deuteron. For a slow neutron this energy is 2224.6 Conventional neutron activation analysis employs keV. The only available deexcitation mode of this the radiations emitted during the decay of radioac- compound nucleus is by the emission of a gamma tive products for elemental analysis. Some elements ray of 2223.23 keV energy, the balance of the reac- do not produce radioactive capture products, but tion energy being carried off as recoil by the do emit prompt gamma rays at the time of neutron deuteron. The presence of a gamma ray of this en- capture. If the sample is placed in an external neu- ergy in the spectrum of a specimen during neutron tron beam from a reactor and viewed by a high-res- irradiation indicates the presence of hydrogen in olution gamma-ray spectrometer, these gamma the sample, and the intensity of this gamma ray rays allow qualitative identification and quantita- relative to a standard is a quantitative measure of tive analysis of the neutron-capturing elements the amount of hydrogen present. This analytical present in the sample. technique has been given a number of names, most As the simplest example, 'H captures a neutron often neutron-capture prompt-gamma-ray activa- to produce an excited nuclear state of deuterium tion analysis, which we abbreviate as PGAA. Cold (Fig. 1). The energy of this state is precisely deter- neutrons offer substantially improved analytical mined through the Einstein relation by the differ- sensitivity over thermal neutron beams. 127 Volume 98, Number 1, January-February 1993 Journal of Research of the National Institute of Standards and Technology 'H + n pulse height analyzer and computer, measures the energy and intensity of the prompt gamma radia- tion emitted. The apparatus is completed by a beam stop to absorb the neutrons which are not absorbed by the sample, and the shielding neces- sary to protect the detector and the experimenters Y 2223.23 keV from stray gamma rays and neutrons. 2.2 Applicability The use of neutron-capture gamma rays as a method of elemental analysis was introduced many ^H years ago [1-3]. With the development of large, Fig. 1. Energy level diagram of the.^ =2 system. high-resolution gamma-ray detectors in the past decade, PGAA has taken its place as a comple- mentary technique alongside conventional neutron activation analysis [4,5]. This method is particularly 2. Principles useful for determining nondestructively elements 2.1 Experimental which absorb neutrons but do not produce radioac- The apparatus is conceptually simple (Fig. 2): A tive products. The PGAA method analyzes the en- collimated beam of neutrons is extracted from the tire sample, including any substrate or container by reactor and the sample inserted into the beam. A which it is supported in the beam. The values of germanium detector, coupled to a multichannel the nuclear parameters and the abundances of the Neutron guide and sample assembly (side view) Neutron Beam Cold collimator Sample stop neutrons from filter C r:> Shutter Gamma detector and lead shield (top view, cross section) 1.0 m Fig. 2. Schematic of the PGAA apparatus. 128 Volume 98, Number 1, January-February 1993 Journal of Research of the National Institute of Standards and Technology elements in common materials are such that peaked and continuum background caused by all PGAA finds its greatest applicability in the deter- components of the sample. mination of nonmetals that form the major and mi- nor elements of geological and biological materials 2.3 Sample Considerations (H, C, N, Si, P, S), or trace elements with high thermal capture cross sections (B, Cd, Gd) that are For a successful PGAA measurement, the sam- not readily determinable by other techniques. ple must be large enough for the analyte to give a PGAA has been used alone to measure up to 21 usefully strong signal, and small enough that the elements in standard rocks [6,7], and in combina- total capture rate is not too high for the detector tion with conventional instrumental neutron activa- and that neutron and gamma-ray scattering and ab- tion analysis^ (INAA) to measure as many as 48 sorption within the sample gives acceptably small elements in coal without chemical separation [8]. errors. For many materials the optimum sample These two complementary techniques have been size lies between 0.1 and 10 g. Samples with special extensively used in the study of natural and man- geometry such as entire silicon wafers can be ac- made atmospheric aerosols [9]. Two bibliographies commodated. Ready access to the sample position of PGAA applications have been compiled [10,11]. may make feasible the nondestructive analysis at Partly because of the need for continuing access low temperature of volatile materials such as solid to a reactor neutron beam, application of PGAA as cometary samples [14]. a routine method of elemental analysis has been In the analysis of plant and animal tissue, both pursued to date at only a few laboratories on a full- detection limits and accuracy of PGAA measure- time basis (for reviews see [12,13]). Because of ments are often determined by the amount of hy- lower neutron fluence rate and (usually) lower drogen in the sample. The strong hydrogen capture gamma-ray detection efficiency, the sensitivity of gamma ray at 2223.2 keV is accompanied by a high the method for most elements is two to three or- Compton continuum, which makes the detection ders of magnitude worse than INAA, which limits limits of other elements below 1995 keV poorer most routine applications to the determination of than they would otherwise be. Active Compton the above mentioned elements. Irradiation times of suppression can reduce this baseline substantially, at least several hours are required for most multi- but not eliminate it. Because of neutron scattering element analysis, hence the throughput is low be- in the sample, a high concentration of hydrogen in cause only one sample can be irradiated and the analytical matrix may lead to either larger or measured at a time. smaller signals per gram of the elements of interest The sensitivity of PGAA for a given element, ex- [15,16]. This source of bias is minimized for spheri- pressed in counts*s"'g"', is given by cal or near-spherical samples [17,18]. ^ NAl(r4>re(E) ^~ M (1) 3. PGAA at the CNRF 3.1 Cold Neutrons where For chemical analysis, the ideal neutron field has S = sensitivity, counts-s"'g"' the largest possible number of activating particles //A =Avogadro's number (slow neutrons) per second per unit area at the / = fractional abundance of the capturing iso- sample, and the smallest possible number of inter- tope fering particles (fast neutrons and background a = neutron capture cross section, cm^ gamma rays) at the detector. A narrow beam is de- </> = neutron fluence rate, cm"^s~' sirable so that the gamma-ray detector can be r = gamma ray yield, photons per capture moved near the sample and the size of the shield- e(£) = gamma ray detection efficiency at energy £, ing may be minimized. The beam need not be par- counts/photon allel, but its intensity should be uniform across the M = atomic weight target. A guided beam of cold neutrons meets these re- The useful detection limit in practice is set by quirements very well [19]. Cold neutrons have been the sensitivity, the counting precision required, the applied to PGAA in only a few laboratories to date blank (signal in the absence of a sample), and the [20-23], though several instruments are under 129 Volume 98, Number 1, January-February 1993 Journal of Research of the National Institute of Standards and Technology construction or active planning [24-26]. As Maier- ature, is installed in the guide 3.1 m upstream from Leibnitz pointed out long ago, the reduction in the PGAA sample position. background by use of a high-quality beam is more The apparatus is shown in Fig. 2. The lower important for analytical purposes than is an in- 50x45 mm of the 50x110 mm NG-7 guide is ex- crease in the capture rate [19]. Henkelmann and tracted into air through a window of magnesium Born have demonstrated this by collecting spectra 0.25 mm thick.