87 1 IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 41, NO. 4, AUGUST 1994 Detector Systems for Imaging Neutron Activation Analysis Yuni K. Dewaraja, Ronald F. Fleming The University of Michigan, Dept. of Nuclear Engineering, 2301 Bonisteel Blvd., Ann Arbor, MI 48109-2100 Martin A. Ludington Albion College, Dept. of physics, Albion, MI 49224 Ronald H. Fleming Charles Evans & Associates. 301 Chesapeake Drive, Redwood City, CA 94063 Abstract This paper compares the performance of two imaging detector systems for the new technique of Imaging Neutron Activation Analysis (Imaging NAA). The first system is based on secondary electron imaging, and the second employs a position sensitive charged particle detector for direct lbcalization of beta particles. The secondary electron imaging system has demonstrated a position resolution of 20 pm. The position sensitive beta detector has the potential for higher efficiencies with resolution being a trade off. Results presented show the feasibility of the two imaging methods for different applications of Imaging NAA. I. INTRODUCTION Imaging NAA is capable of discerning 2-D elemental distributions in heterogeneous solids as well as identifying and quantifying the elements in the sample [l]. In conventional neutron activation analysis (NAA), elements are identified and the volume averaged concentrations are determined by detecting the characteristic gamma rays emitted during the decay of neutron activated radionuclides. In Imaging NAA we additionally obtain position information by locating the beta @articles that are often emitted in coincidence with the y-rays. For each disintegration, if the p r a y and the p particle are detected in coincidence, the y-ray energy identifies the rtldionuclide and the f3 position determines its location in’the simple. Imaging NAA is also applicable to nuclides such as %r that decay purely by electron capture (EC) since low energy Auger electrons that follow EC can be imaged [2]. The technique has many potential applications in such fields as geology, biology, and material sciences. In biological applications the concept of Imaging NAA can be ~tilized to simultaneously image radiopharmaceuticals labeled with multiple radionuclides. The long counting periods associated with conventional NAA of small particles can be substantially reduced by Imaging NAA. Instead of y r a y oounting individual particles separately, we can count an array of particles simultaneously using a single imaging system. The Imaging NAA apparatus consists of a gamma ray spectroscopy system, an electron imaging system, a coincidence circuit and a computer. There are several detector slystems described in the recent literature that can be used for eiectron localization in Imaging NAA. These systems, based on scintillators, Si strip detectors and gaseous detectors, have been used in such applications as digital beta autoradiography and radiochromatography. The beta imaging detector descriW in [3] combines a charge coupled device with a scintillator qnd photocathode. The reported spatial resolution is 20 pm qnd the efficiency is 100 8 (relative to the measured s t a n w source). A 2-D imaging system combining scintillating optical fibers and multianode photomultipliers has been developed for DNA sequencing [41. Si strip detectors Mve been successfully used to determine the distribution of beta emitters such as 14C, 32P, and 35S. Efficiencies close? to 50% and resolution better than 0.5 mm has been reported B]. A parallel plate gas avalanche chamber has been usedhto optically image the 2-D distribution of beta particles with 0.5 mm resolution [6]. This method is based on the emission of light from molecules excited in the avalanche process. We report here two approaches to electron localization in Imaging NAA based on: A) imaging low energy second electrons produced by primary beta particles within the sample; B) direct detection of the primary beta particles us ng a position sensitive Si beta detector. I II. INSTRUMENTATION AND METHODS A. Secondary Electron Imaging System Fig. 1 is a schematic of the prototype instrument built by Charles Evans & Associates [l]. The imaged area and h e h the ideal sample size is about 1 mm in diameter, and typip sample thickness is tens of nms. Secondary electrons $re (few ev) secondaries can be imaged if they escape from the P p created by the p particles within the sample, and low ene surface of the sample. Using electron optics, the seconw electrons are accelerated in an electric field, focused at a crossover point, and projected onto a dual microchannel p te (DMCP) image intensifier. A resistive anode enco er determines X,Y positions for each secondary electron casc by the sample are detected by the Ge(Li) detector. The inpbts to the coincidence circuit are the electron position signal +d the y-ray energy signal. The energy, position, and tifne stamp associated with all coincidence events are stored in e computer in the form of a list file. The coincide ce obtain element maps and local area y-ray spectra. During the analysis real time images and spectra can be observed. Since the sample is very thin, the displacement or p particles from their point of origin prior to producgg secondary electrons is minimal and contributes little to the emerging from the DMCP. At the same time y rays emi k information stored for all nuclear decays can be organized nuI to 0018-9499/94$04.00 Q 1994 IEEE