NSDUAZ unfolding package for neutron spectrometry and dosimetry with Bonner spheres

Transcription

1 NSDUAZ unfolding package for neutron spectrometry and dosimetry with Bonner spheres Vega-Carrillo, H.R. 1, Ortiz-Rodríguez J.M. 2, Martínez-Blanco M.R 1 1 Unidad Académica de studios Nucleares Universidad Autónoma de Zacatecas Calle Ciprés 10 Fracc. La Peñuela Zacatecas, Zac. México 2 Unidad Académica de Ingeniería léctrica Universidad Autónoma de Zacatecas Av. Ramón López Velarde 801 Col. Centro Zacatecas, Zac. México fermineutron@yahoo.com Abstract NSDUAZ (Neutron Spectrometry and Dosimetry from The Universidad Autónoma de Zacatecas) is a user friendly neutron unfolding package for Bonner sphere spectrometer with 6 LiI(u) developed under LabView environment. Unfolding is carried out using a recursive iterative procedure with the SPUNIT algorithm, where the starting spectrum is obtained from a library initial guess spectrum to start the iterations, the package include a statistical procedure based on the count rates relative to the count rate in the 8 inches-diameter sphere to select the initial spectrum. Neutron spectrum is unfolded in 32 energy groups ranging from 10-8 up to MeV. Keywords: Neutron, Unfolding, Bonner sphere, Spectrometry, Dosimetry 226

2 INTRODUCTION In 1960 was reported a neutron spectrometer made with a set of polyethylene spheres and a thermal neutron detector (Bramblett et al., 1960). This is known as Bonner sphere spectrometer, BSS, or Multisphere spectrometer and nowadays is the most used spectrometer for radiation protection purposes. With some modifications to the original design BSS can unfold neutron spectrum from thermal up to several GeV, it can be used with active or passive detectors and its operation is simple (Bedogni et al., 2007). However, the weight, wasting time to perform the measurements, low resolution, the need of a response matrix and an unfolding procedure are some of drawback pointed out for BSS (Vega-Carrillo and Iñiguez, 2002). Basic BSS includes seven spheres 0, 2, 3, 5, 8, 10 and 12 inches-diameter, where Ball 0 means the bare thermal neutron detector. The count rate of i-th detector, C i, hereafter detector is use to describe the thermal neutron detector located at the centre of a polyethylene sphere, is related with the neutron spectrum, (), and the response matrix, R (), through the Fredholm integral equation of the first kind shown in equation 1. i max i (1) C R d min The amount of detectors and energy groups is finite, and the discrete version of equation 1, shown in equation 2, becomes an ill-conditioned problem where the amount of equations is small compared with the amount of unknown. N G (2) C R i 1,..., m i i,j j j 1 227

3 Here, N G is the number of energy groups and m is the amount of detectors. Due to NG > m equation 2 has an infinite number of solutions where few has physical meaning. Codes like BUNKI, LOHUI, GRAVL, MAXD, BUMS, MITOM, FRUIT and BSPOK, have been used to unfold the neutron spectrum (Danyluk, 2010, Bedogni et al., 2007, Tómas et al., 2004, Sweezy, Hertel and Veinot, 2002, Reginatto and Goldhagen, 1999, Matzke, 1994, Routti and Sandberg, 1985). These codes use different algorithms to unfold the spectrum from the BSS count rates; some derive the final spectrum by perturbing an initial guess spectrum using mathematical rules, and others model the initial spectrum using a set of physical parameters. In the codes, quality of the initial guess spectrum could affect the final solution To overcome the need of an initial guess spectrum the Artificial Intelligence Technology has been used (Ortiz-Rodriguez et al., 2011). Mostly of the available codes is not user friendly and an initial guess spectrum, to start the unfolding process, must be provided by the user being some of the problems associated with the unfolding process with BSS. To overcome the problem of providing the initial guess spectrum and in order to perform the unfolding in an intuitive and user-friendly code, the aim of this work is to present the NSDUAZ unfolding package for neutron spectrometry and dosimetry with Bonner Sphere Spectrometer. 228

4 MATRIALS AND MTHODS BUNKI code was developed in Fortran IV to be run in a DC-10 computer (Lowry and Johnson, 1983), later this code was adapted to run under DOS in a personal computer, the response matrix was improved and a subroutine to plot the spectrum was included, this code was named BUNKIUT developed at The University of Texas at Austin. This code has two algorithms, BON31G and SPUNIT, for unfolding, and user can select different options to provide the initial guess spectrum, one of this is parametric through the MAXIT option where a mathematical spectrum is built using a thermal component, (1/) x component a high energy Maxwellian peak. Through recursive iterations the initial spectrum is multiplied times the response matrix and a set of count rates are obtained which are compared with the experimental set of count rates until a user-defined convergence criterion is reached or the maximum amount of iterations are achieved. Due to advantages of SPUNIT algorithm (Brackenbush and Scherpelz, 1984, Doroshenko et al., 1977) it was included in NDSUAZ code. ach neutron spectrum produce different count rates in BSS, to overcome the need of input an initial guess spectrum, have been suggested to use a catalog of neutron spectra and their BSS count rates (Vega-Carrillo and Iñiguez, 2002). The catalog contains the IAA s neutron spectra (IAA, 1990) and their respective BSS count rates. In the catalog BSS counts are normalized to counts in 5 inches-diameter detector (Ball 5). In NDSUAZ, inputted count rates are normalized to count rate measured with Ball 5 and compared to count rates in the catalog, once the closest relative BSS counts are identified its corresponding spectrum is used to start the unfolding. With the unfolded spectrum, total fluence rate,, spectrum mean energy, Av, and the ambient dose equivalent rate, H*(10), are calculated using the discrete versions of equations 3, 4 and 5 respectively. 229

5 max d (3) min Av max min d (4) max * H*(10) h d (5) min The use of discrete version of equations 4 and 5 in NSDUAZ code allows that Av and H*(10) being calculated using the lower, middle, upper and the median value of each group. NSDUAZ was developed in an intuitive and graphical environment using LabView software, the BSS count rates are saved in a TXT file writing the count rates of Ball 0, 2, 3, 5, 8, 10 and 12 in a single column. If several count rates, from different experiments, are available they can be saved in the same TXT file is blank-spaced columns, the first column is, by default, identified as 0. Two sections are provided in the code in order to write free notes by the user. Once the user call the TXT file, a relative convergence error and a maximum amount of iterations must be input by the user, by the fault the code has 1% and 1000 respectively. To unfold the spectrum the user clicks the Unfold spectra key, and unfolded results are shown on the screen, that can be saved in a report by pressing the Save data key. In figure 1 is shown the NSDUAZ initial screen, in the Test operator and Notes and comments blanks, user can add notes to record the unfolding. The file with the count rates is uploaded pressing the folder icon. 230

6 Figure 1.- NSDUAZ package start screen In order to test the code the neutron spectrum of a 241 AmBe source was obtained. The source is inserted in a moderator surrounded by aluminum. A BSS with a cm 2 6 LiI(u) scintillator was located at 1 m from the source. NSDUAZ code was used to unfold the neutron spectrum and to estimate the total fluence rate and the ambient dose equivalent; this last was compared with the ambient dose equivalent measured with a neutron area monitor Berthold LB6411. Measuring conditions are far from free-field condition due to room-return [Vega-Carrillo et al., 2007]. 231

7 RSULTS The BSS count rates measured at 1 m from the 241 AmBe are shown in figure 2; count rates were typed as a single column in a file named cps.txt. Convergence percent was 2% and the default 1000 iterations were used; then the Unfold spectra key was pressed resulting in figure Count rate [ cps ] m Sphere diameter [ inches ] Figure 2.- BSS count rates NSDUAZ output shows the plot of unfolded spectrum (SPUNIT) and the initial guess selected with the criteria of the relative BSS count rates, spectra numerical values are also shown. The output screen also shows the total fluence rate, mean energy and the ambient dose equivalent. 232

8 Figure 3.- NSDUAZ output screen In figure 4 the 241 AmBe lethargy spectrum is shown, total flux is 3 cm -2 -s -1, the ambient dose equivalent rate is 2.43 ± 0.18 Sv-h -1. The H*(10) measured with the neutron area monitor is 2.70 ± 0.30 Sv-h -1, being similar to the dose obtained by the NSDUAZ package, probable explanation of the difference between both H*(10) values is due to differences in the ICRP 74 (1996) fluence-to-h*(10) conversion coefficients, used in NDSUAZ, and the Berthold LB6411 response. 233

9 m () [ cm -2 - u -1 ] NSDUAZ Neutron energy [ MeV ] Figure AmBe lethargy spectrum CONCLUSIONS NDSUAZ is a computer package to unfold neutron spectrum from the BSS count rates. Unfolding is carried out using recursive iterations with SPUNIT algorithm without the need of input the starting spectrum. The package is intuitive and user-friendly providing on screen the neutron spectrum, total fluence rate, mean energy and ambient dose equivalent. Unfolding results can be saved in a full report. NDSUAZ performance was shown by measuring the BSS count rates from a 241 AmBe isotopic neutron source. 234

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