29th Monitoring Research Review: Ground-Based Nuclear Explosion Monitoring Technologies

Transcription

1 TESTING THE SPECTRAL DECONVOLUTION ALGORITHM TOOL (SDAT) WITH XE SPECTRA Steven R. Begalsk, Kendra M. Foltz Begalsk, and Derek A. Haas The Unversty of Texas at Austn Sponsored by Army Space and Mssle Defense Command Contract No. W9113M ABSTRACT The Spectral Deconvoluton Analyss Tool (SDAT) software was developed to mprove countng statstcs and detecton lmts for nuclear exploson radonuclde measurements. SDAT utlzes spectral deconvoluton spectroscopy technques and can analyze both β-γ concdence spectra for radoxenon sotopes and hgh-resoluton HPGe spectra from aerosol montors. The SDAT tool has been ntegrated nto a standalone graphcal user nterface (GUI). Ths GUI may mport spectra, analyze the data for Xe concentratons, and graphcally dsplay the results. Ths tool has been tested wth data generated va MCNPX models as well as real data. 729

2 OBJECTIVES Envronmental xenon samplng and measurement unts are used n nuclear weapons test montorng networks because radoxenon sotopes may be all that s released from tests performed deep underground or underwater. Some envronmental xenon samplng and measurement unts, lke the Automated Radoxenon Sampler-Analyzer (ARSA) developed by Pacfc Northwest Natonal Laboratory (PNNL) and the Swedsh Automated Noble Gas Unt (SAUNA) developed by the Swedsh Natonal Defense Research Establshment (FOI), use β-γ concdence detectors that are energy dspersve on both the β and γ energy axes. Sgnals from four radoxenon sotopes ( 131m Xe, 133 Xe, 133m Xe, 135 Xe) comprse a sample spectrum. Under poor operatng condtons, a few radon daughters ( 214 Pb, 214 B) may nterfere wth the sample spectrum. Applyng conventonal regon-of-nterest (ROI) spectrum analyss algorthms to such 3-D spectra results n relatvely hgh mnmum detectable concentratons (MDCs) due to the subtractve process of determnng net counts n the ROI. To overcome ths problem, a project team from The Unversty of Texas n Austn s developng software to deconvolve the 3-D sample spectra nto the most probable combnaton of sgnals usng the non-negatve least-squares method. Ths method can use the entre sgnal from each radonuclde and consequently may mprove the sgnal to nose rato. It has been postulated that the use of such an algorthm wll result n a better ablty to resolve spectral nterferences and thus mprove countng statstcs and detecton lmts for nuclear exploson radonuclde measurements. The SDAT GUI, expands upon a prevously developed algorthm, the Multple Isotope Component Analyss (MICA) tool. The SDAT tool was tested aganst the CORIANT code (Foltz Begalsk, 2001) on data produced from an automated radoxenon sampler/analyzer (ARSA) unt when t was statoned n Guangzhou, Chna. The CORIANT tool utlzes a regon of nterest approach to the spectral analyss whle SDAT utlzes a spectral deconvoluton approach. Samples were selected that contan clear radoxenon sgnals. Whle a true test of SDAT wll be the analyss of samples wth low count rates, ths test was conducted as a frst step to show that the two methods produce smlar results. RESEARCH ACCOMPLISHED Ths occurred n three steps. The frst step was the creaton of the SDAT tool. The second step was the creaton of standard lbrary nputs for the SDAT code, whch was accomplshed va MCNP smulatons. The thrd step was to analyze samples wth SDAT and compare the results to those produced wth CORIANT. Step 1: Creaton of SDAT GUI The detector response for a sample conssts of a 255x255 matrx of numbers. Each entry represents the number of counts regstered n a certan Eγ, E β bn. Each row represents a γ-channel bn and each column represents a β-channel bn. We wll refer to ths structure as a hstogram. Other nformaton contaned n the sample fle ncludes calbraton nformaton and other sample characterstcs, e.g., the sample Xe gas volume from whch the total sampled atmospherc volume s calculated. The sample hstogram wll be deconvolved nto ndvdual sotopc responses usng the SDAT concept to determne atmospherc actvty concentratons for each radoxenon of nterest. To do ths, however, we must have calbrated hstograms of all the possble ndvdual sgnals that can make up a sample hstogram. These hstograms should have the same sze and calbraton characterstcs as the sample hstogram n addton to good countng statstcs. Therefore, we need the followng detector response matrces wth ther assocated actvtes: 131 [ ] m 133 Xe 255x255, [ ] m Xe 255x255, [ Xe ] 255x255, and [ Xe ] 255x

3 Each detector-response hstogram can be generated usng a detector modelng program lke MCNP or acqured by countng a calbraton source on the detector. Snce the ARSA has four beta detector cells, the above detector response hstograms would need to be generated or acqured for each beta cell. In addton, the energy, resoluton, and effcency calbratons have to be determned for each beta cell. Wth all of ths nformaton at hand, and knowng n whch detector cell the sample was counted, the followng algorthm can be appled to a sample for determnng the atmospherc radoxenon concentratons. The SDAT code was developed to nput the lbrary and sample fles. These fles are text fles. The current verson of SDAT just reads the sample spectrum and does not read any header nformaton that may be assocated wth Sample Pulse Heght Data (SPHD) formatted fles. The open sample wndow s shown n Fgure 1. Once the proper fles have been selected, SDAT analyzes the data and dsplays the results. Fgure 2 shows the results dsplay. The sample s dsplayed as a 2-D plot n the top left corner. Below the sample plot are three tables. The top left table s the Axs Parameters table whch allows users to modfy the mnmum and maxmum x and y ntercepts of all the plots n the SDAT wndow. By default, they are set at 0 and 250. The Parameter/Value Table s stuated to the rght of the Axs Parameters table and contans a summary of the parameters specfed n the Open Sngle wndow,.e., the senstvty number, the sample fle name, and the lbrary fle names for each radoxenon of nterest. Ths table s located n the bottom left corner of the SDAT wndow. It contans the concentraton coeffcents and errors of each radoxenon of nterest calculated wth and wthout the use of data weghtng va the weghtng matrx descrbed earler. The resultng multpler coeffcents can be used to determne the actvty of each radoxenon usng equaton 1. M A C = (1) V where C = Sample actvty concentraton of radoxenon sotope I; M = Multpler coeffcent of radoxenon sotope as determned by SDAT; A = Actvty represented by lbrary fle of radoxenon sotope I; and V = Ar volume sampled. The actvty concentraton uncertantes of each radoxenon can be determned usng equaton 2. 2 ( A σ ) + ( M σ ) 1 2 M Aσ V σ C = M A + (2) V V where σ x s the uncertanty of varable x. All other varables have been defned above. 2 The weghted result and unweghted result plots are located at the top mddle and top rght corner of the SDAT wndow. The plots are calculated as shown n equaton 3. ( [ ] ) [ ξ ] C lb, (3) where C are the concentraton coeffcents for radoxenon sotope, [lb] are the lbrary fles for radoxenon sotope, and [ξ] s the weghtng matrx for the weghted result plot and the dentty matrx for 731

4 the unweghted result plot. These plots may be compared wth one another and the orgnal sample plot to see how well the SDAT calculated results model the sample data. The Weghted Resdual and Unweghted Resdual Plots are located drectly below the Weghted Result and Unweghted Result Plots. The plots are calculated shown n equaton 4. ( ) [ ξ ]) [ ] C [ lb] Sample, (4) where [Sample] s the orgnal sample data and all other symbols have been prevously defned. These plots show the dfferences between the orgnal sample data and the calculated models of the sample as shown n the weghted result and unweghted result plots. SDAT also allows for sample re-analyss may wth dfferent nput data. Results may also be saved. Over tme, the energy vs. channel calbraton of both beta and gamma detectors can change. If the changes are sgnfcant, the SDAT program wll return large errors because the sample sgnal wll no longer match those n the lbrary fles. In such cases, the sample spectra should be rescaled to match the energy calbraton propertes of the orgnal lbrary fles. Alternatvely, the lbrary fles can be rescaled to match the sample calbraton propertes and saved. Ths program can also be used to compress or expand the data as desred. Fgure 3 shows the SDAT rescalng wndow. Step 2: Creaton of Xe Lbrary Fles va MCNP Lbrary fles were created for 131m Xe, 133m Xe, 133 Xe, and 135 Xe va a combnaton of MCNPX and MATLAB post processng. Ths s the same method as reported n Haas et al. (2007). The lbrary fles were created for each of the four cells n the β γ concdence detector system utlzed n the ARSA system. The lbrary fles were then adjusted to match the energy calbraton of the samples collected n the feld. Fgure 4 shows a comparson between the synthetcally generated 133 Xe lbrary fle and a sample wth 133 Xe collected by the ARSA n Guangzhou, Chna. Step 3: Comparson of SDAT wth CORIANT Results As an ntal qualty control check, fve spectra were analyzed wt SDAT. These samples collected wth an ARSA unt n Guangzhou, Chna were ntally analyzed by CORIANT at the Center for Montorng Research n Arlngton, VA. Fgure 5 llustrates the comparson that was made between the SDAT results and the CORIANT results. The 133 Xe results compare very well. Snce these results are from samples collected n the feld, the correct quantty of 133 Xe n the sample s not known. Ths just serves as a check to make sure that the SDAT tool produces acceptable results for samples wth strong Xe sgnals. The analyss of 131m Xe, 133m Xe, and 135 Xe were also performed. These results dd not compare as well as the 133 Xe results shown n fgure 5. Ths ssues wth the 131m Xe and 133m Xe was prmarly a detecton ssue. CORIANT reported 131m Xe and 133m Xe present n all of the samples analyzed. However, SDAT calculated 131m Xe to be present n four out of the fve samples and dd not calculate 133m Xe to be present n any of the samples. Dscrepances n these results are largely attrbuted to havng correct β channel calbratons, whch are questonable for both the CORIANT and the SDAT analyss. Vsual nspecton of the data does not show any clear 131m Xe or 133m Xe sgnals. Another ssue rased by ths comparson s the error calculaton performed n SDAT. The error bars produced for ths comparson appear to be smaller than expected. Whle the error calculatons may be correct, efforts should be made to revew the error calculaton n SDAT. 732

5 CONCLUSIONS AND RECOMMENDATIONS The frst verson of the SDAT code was completed. Samples were analyzed wth the code usng MCNPX/MATLAB generated lbrary fles. Results from ths comparson agreed wth CORIANT results from the Center for Montorng Research. Future work for the SDAT code wll nclude an ncorporaton of actvty calculatons as well as an ablty to read SPHD fle headers. The hghest prorty work wll ental the creaton of radoxenon sotopes va The Unversty of Texas at Austn TRIGA reactor. These spectra wll be mportant to use as lbrary fles for SDAT. They wll also allow for advanced testng of SDAT wth low countng statstcs samples. Snce the xenon concentratons may be expermentally controlled, the accuracy and precson of SDAT may be determned. REFERENCES K.M. Foltz Begalsk (2001). J. Radoanalytcal Nucl. Chem. 248: (3), pp D. A. Haas, S. R. Begalsk, and K. M. Foltz Begalsk (2007) Modelng β-γ concdence spectra of 131m Xe, 133 Xe, 133m Xe, and 135 Xe Sgnals, Journal of Radoanalytcal and Nuclear Chemstry (n press). 733

6 Fgure 1. SDAT GUI open sngle wndow. 734

7 Fgure 2. SDAT GUI sample analyss results dsplay. 735

8 Fgure 3. Dsplay after rescalng a sample. 736

9 Fgure 4. Comparson of β and γ compressed spectra from MCNPX/MATLAB generated 133 Xe lbrary fle and from sample collected n Guangzhou, Chna. 737

10 Concentraton (mbq m -3 ) Corant SDAT FEB-11 07: MAR-18 07: MAR-27 15: MAR-28 23: APR-12 07:00 Sample Collecton Start Day and Tme Fgure 5. Comparson of results between SDAT and CORIANT for fve samples from the ARSA n Guangzhou, Chna. 738