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Phương pháp trùng hợp cc (phát hiện hai ctia phát ra gần như đồng thời) đã được sử dụngtrong nghiên cứu cấu trúc hạt nhân 1, và cũng là một điểmsible cách để cải thiện độ nhạy cảm của dấu nhắckỹ thuật phân tích kích hoạt gamma (PGAA) 2.Là một phần của cơ sở cho các neutron gây raquang phổ tia gamma (NIPS) tạiCổng chùm PGAA của Budapest Reac nghiên cứutor một cc coincidence bộ máy đã được phát triển.Trong các bài viết trước của chúng tôi, đồng xu cc khu vựcphương pháp cidence được giới thiệu 2, và meathiết lập bảo đảm đã được mô tả chi tiết 3. Trongbài báo hiện tại chúng tôi trình bày một phép đo clo để minh họa chất lượng của hệ thống, và một phép đo của một loạt các mẫu thủy tinh như làsử dụng thực tế của nó.
Nuclear Instruments and Methods in Physics Research B 213 (2004) 406–409 www.elsevier.com/locate/nimb A practical test of a c–c coincidence measurement setup for PGAA P.P Ember *, T Belgya, J.L Weil, G.L Moln ar Institute of Isotope and Surface Chemistry, Chemical Research Centre of the Hungarian Academy of Sciences, P.O Box 77, H-1525 Budapest, Hungary Abstract A second cold-neutron beam experimental station has been built as part of the renewal of our prompt gamma activation analysis facility at the Budapest Research Reactor This new station has been instrumented for neutroninduced prompt gamma-ray spectroscopy, involving c–c coincidence measurements The c–c coincidence arrangement is introduced briefly, and its usage is illustrated with a practical example of the inactive tracer method for samples from glass furnace Ó 2003 Elsevier B.V All rights reserved PACS: 25.40.Lw; 81.70.Jb; 82.80.Ej; 82.80.Jp Keywords: n; cị; En ẳ cold; Prompt gamma activation analysis; Coincidence; Spectral interference Introduction The c–c coincidence method (detection of two c rays emitted almost simultaneously) has been used in nuclear structure studies [1], and is also a possible way to improve the sensitivity of the prompt gamma activation analysis (PGAA) technique [2] As part of the facility for neutron-induced prompt gamma-ray spectroscopy (NIPS) at the PGAA beam port of the Budapest Research Reactor a c–c coincidence apparatus has been developed In our previous articles the regional c–c coincidence method was introduced [2], and the measurement setup was described in detail [3] In our present article we show a chlorine measurement to * Corresponding author Fax: +36-1-392-2584/2222 E-mail address: ember@alpha0.iki.kfki.hu (P.P Ember) illustrate the quality of the system, and a measurements of a series of glass samples as the first practical usage of it Experimental work All measurements were made using the same arrangement, as detailed in our most recent article [3] 2.1 Detector parameters and positioning Two coaxial HPGe detectors were used for the measurements described here Detector HPGe-I 00 00 had 30% efficiency relative to a · NaI detector and 1.8 keV resolution at 1332 keV c-ray energy Detector HPGe-II had 15% efficiency and 1.9 keV resolution HPGe-II was placed horizontally at a 0168-583X/$ - see front matter Ó 2003 Elsevier B.V All rights reserved doi:10.1016/S0168-583X(03)01663-X P.P Ember et al / Nucl Instr and Meth in Phys Res B 213 (2004) 406–409 distance of 4.3 cm from the target, and HPGe-I vertically, about 2.6 cm from the target Lead blocks of cm thickness were used around the crystals as gamma shields, and plates of 1.5 mm thick lead were placed between the detectors and the tube to decrease the number of back-scattered gamma photons, and to filter out the X-rays 2.2 Test samples The chlorine sample was a PVC foil of 0.11 g mass and 1.8 · 2.5 cm2 surface area It was irradiated in the neutron beam for 23 h to excite the 36 Cl nuclei via the 35 Cl(n,c) reaction, which has a large cross section The de-exciting c-rays have been measured in coincidence mode The glass samples were borrowed from a series of industrial measurements carried out with the Compton-suppressed spectrometer at our PGAA facility [4,5] Recently a series of experiments were performed to measure the uniformity of mixing of input ingredients in a glass furnace with an inactive tracer method This furnace works continuously, and the tracer was injected in an instant with the ingredients, and the samples were taken at certain times from the outcoming melted glass The process is shown in Fig Important aspects of the tracer selection were: it should not affect the technology, nor change the parameters of the produced glass Furthermore it has to have a large neutron capture cross section Boron was selected for normal glasses, and gadolinium for the borosilicate glasses containing approximately 5% of boron Our samples were taken from a measurement series of borosilicate glasses The original samples were of several tens of grams, thus we cut from them about a half gram for our purpose Fig Draft of the inactive tracing of a glass furnace 407 Table Data of the glass samples Sample number Sample mass (g) Measuring time (min) Gd concentration (ppm) Uncertainty of Gd concentration (ppm) 0.529 1052 0.87 0.1392 0.536 1179 1.4 0.106 0.449 1220 11.9 1.547 0.48 325 21.9 1.533 Gd concentration was measured by the Compton-suppressed PGAA spectrometer of the NIPS facility Table contains the data of the four glass samples: the number of the sample for reference, the mass of the borated glass sample (used for the coincidence measurement), the measuring time (equals with the irradiating time), and the Gd concentration with its uncertainty The concentration data were calculated from the Comptonsuppressed measurement Sample was taken just before the gadolinium tracer was put into the furnace, so this measurement represents the natural gadolinium concentration of the glass Results and discussion 3.1 Chlorine sample 48 million coincidence events were collected in the chlorine experiment The upper half of Fig shows the total projected time spectrum, which means the histogram of all time values from the list Fig Time spectrum of 60Co c–c coincidence events Gates were set on the 1173 and 1332 keV c peaks in the HPGe-I and HPGe-II energy spectra respectively 408 P.P Ember et al / Nucl Instr and Meth in Phys Res B 213 (2004) 406–409 file The time spectrum has a peak to background ratio of 150, and FWHM of 30 ns The structure of any time spectrum depends on many factors, such as the detector dimensions, the threshold setting in the CFD electronics, and the variety of transition energies and life times of levels involved in the deexcitation process of the target nuclei In the case of a broad energy range of transitions, one cannot obtain as good time resolution as can be obtained with e.g a 60 Co-calibration source [3] and narrow energy gates, due to the time-walk of the detector output signals [6] In addition, finite lifetimes cause centroid shifts or exponential tails that also broaden the time peak The lower half of Fig illustrates the time walk by showing the time peak of three full energy peak-to-peak coincidences In detector HPGe-I a gate on the 517 keV energy peak was set for all three cases, while in HPGe-II gates on the 6110, 1951 and 786 keV peaks were applied respectively The effect of the time walk is clearly visible; the stop signals of higher energy (higher amplitude) pulses come earlier, while the lifetimes of the intermediate levels are negligible The measured FWHM values of the three time peaks are 13 ns, which is the same as was found for the 60 Co time peak [3] Fig presents the total projection of the HPGe-I detector energy signals from the 35 Cl(n; c) capture reaction As can be seen, a few very strong lines dominate the spectrum, including the strongest primary transition of 6110 keV energy For demonstration of the off-line coincidence analysis method, we set a gate on the 5086 keV double escape peak, instead of the 6110 keV full energy peak, because the latter and its single escape peak form doublets with the single- and double-escape peaks of a weaker, higher energy c ray To estimate the background, we set background regions on both sides of the time and energy peaks The peak and background gate settings are shown as gridded and crosshatched areas, respectively, in Fig The two background regions combined cover the same number of channels as the peak gates The analysis program subtracts the projection of the backgrounds automatically from the coincidence projection spectra and also calculates the uncertainties in the gated spectrum In Fig the spectrum of the c-rays measured by the HPGe-I detector with the gate settings explained above is shown The strongest coincidence peaks are labeled with their energies As can be seen, the signal to noise ratio is very good The random coincidences have almost completely disappeared, as illustrated by the magnified insert of the energy-gate setting area Fig shows the corresponding decay scheme (taken from ENSDF [7]) for the 35 Cl(n; c)36 Cl reaction, retaining only those transitions which are expected to be in coincidence with the strongest 6110 keV primary capture c-ray For easier reading the c-ray energies and their absolute intensities in percentage are rounded off The c transitions with intensity less than 1% are drawn with dotted lines Fig Total projection energy spectrum of the HPGe-I detector from the chlorine experiment Fig Spectrum of c-rays in coincidence with 5086 keV, the double escape peak for 6110 keV P.P Ember et al / Nucl Instr and Meth in Phys Res B 213 (2004) 406–409 409 tration calculated from the Compton-suppressed measurements Each point has the uncertainty marked in both directions The graph also shows the fitted linear function that was forced to cross the origin The r2 value of the fit is also given on the graph As can be seen in Fig the calibration curve is linear, which means the method is applicable for concentration determination Fig Partial decay scheme of 36 Cl Only the lines in coincidence with the 6110 keV peak are shown Conclusions In this article we have studied the applicability of c–c coincidence for the determination of concentrations with PGAA method We found that the coincidence reduces spectral background and interference, but preserves the linear relationship between signal and concentration A possible way of further improvement is the use of a digital signal analyzer, which would allow larger count rates, and hence better precision for a given measuring time Fig Calibration curve for the 182 keV chlorine peak In agreement with our expectations, the gated energy spectrum of HPGe-II contains only the peaks expected from the known level scheme of 36 Cl The weak 2467 keV line can be easily identified, but the 511 keV annihilation peak masks the similarly weak 508 keV line 3.2 Glass samples The above outlined c–c coincidence technique was applied for the analysis of the glass samples The peak areas were normalized for each sample to g sample mass and 1000 of measurement time Fig shows the calibration curve for the coincidence measurement: the calculated and normalized peak areas for the 182 keV gadolinium peak are plotted versus the gadolinium concen- References [1] A.H Wapstra, Alpha and Gamma-ray Spectroscopy, Vol I, fifth printing, North Holland Publishing Company, Amsterdam New York Oxford, 1979, Chapter VIII/C, p 539 [2] P.P Ember, T Belgya, G.L Molnar, Appl Radiat Isot 56 (2002) 535 [3] P.P Ember, T Belgya, J.L Weil, G.L Molnar, Appl Radiat Isot 57 (2002) 573 [4] T Belgya, Zs Revay, B Fazekas, I Hejja, L Dabolczi, G € st€ L Molnar, Z Kis, J O or, Gy Kaszas, in: Proceedings of the Ninth International Symposium on Capture GammaRay Spectroscopy and Related Topics, Vol 2, Budapest, Hungary, 1996, p 826 [5] G Molnar, T Belgya, L Dabolczi, B Fazekas, Zs Revay, Veres, I Bikit, Z Kiss, J O € st€ A or, J Radioanal Nucl Chem 215 (1997) 111 [6] G.F Knoll, Radiation Detection and Measurement, third ed., John Wiley & Sons, New York, 2000, Chapter 17/IX, p 438 [7] Evaluated Nuclear Structure Data File (ENSDF) produced by members of the International Nuclear Structure and Decay Data Network, and maintained by the National Nuclear Data Center, BNL, USA Also available online from IAEA Nuclear Data Section Vienna