Multisphere Neutron Spectrometric System with Thermoluminescent Dosemeters: Sensitive Improvement

Transcription

1 Multisphere Neutron Spectrometric System with Thermoluminescent Dosemeters: Sensitive Improvement Gregori, B.N.; Papadópulos, S.B.; Cruzate, J.A. and Kunst, J.J. Presentado en el 13 th International Conference in Solid State adiation Dosimetry, Atenas, Grecia, 9-13 julio 2001

2 MULTISPHEE NEUTON SPECTOMETIC SYSTEM WITH THEMOLUMINESCENT DOSEMETES: SENSITIVE IMPOVEMENT Gregori, B.N.; Papadópulos, S.B.; Cruzate, J.A. and Kunst, J.J. Autoridad egulatoria Nuclear Argentina ABSTACT In this work, a neutron spectrometric system based on a set of moderating spheres with thermoluminescent detectors (TLD) is presented. The system at the Nuclear egulatory Authority (AN) Dosimetry Laboratory consists of 12 solid spheres made of high-density polyethylene (δ=0.95 g/cm 3 ), with diameters ranging from 2 to 12 and TLD sensitive to thermal and gamma radiation, namely TLD-600 and TLD-700, located at the centre of the spheres. The neutron response matrix for this Bonner Sphere Spectrometer (BSS) was calculated using the MCNP-IVB code and the library ENDF/B-VI in the energy range between thermal neutron and 100 MeV. The neutron spectrum was obtained using the LOUHI82 unfolding code. The improvement in sensitivity of the system is based on the election of a different heating cycle of the TLD that allows the increase in sensitivity by a factor 2.6 compared with the standard laboratory treatment. The system response for the calibration with AmBe source is presented. I. INTODUCTION Suitable knowledge of the neutron spectrum is necessary to determine radiation protection quantities like ambient dose equivalent, H*(10), among many others. A neutron spectrometer, Bonner Sphere Spectrometer (BSS) (1,2) is used in neutron radiation protection measurements. It is based on the ability of some materials to moderate and capture the neutrons and is made up of a thermal detector; a set of polyethylene spheres and the associated electronics. This applies for an active detector like 10 BF 3 o 3 He, or scintillators (eg. 6 LiI(Eu)). The BSS has a good performance for radiation protection purposes. It is important to point out that a net limitation appears in the case of active detectors in the environmental of high intensity or pulsed radiation fields, because of pile up effects or undesirable dead times. The best solution for this problem is using passive detectors like activation (3)(4)(5), track (6), and thermoluminescent (6) detectors. They have already been applied in aircrew measurements and pulsed fields from medical and research accelerators. In this work, the AN-BSS based on TLD was developed and tested. The TLD-BSS consists of 12 high-density polyethylene spheres (0.95 g/cm 3 ), with diameters ranging between 2 and 12 and Bicron TL detectors (TLD-600 and TLD-700) placed both at the centre of the spheres. The system uses the difference between these types detectors readings as a measure of the thermal neutron fluence. The TLD-BSS energy response was calculated with MCNP-VIB code (7).The system was calibrated with an AmBe source and the measurements were unfolded with the LOHUI82 code (8)(9). 223

3 II. ESPONSE MATIX CALCULATION The neutron response matrix for TLD-BSS was calculated using the MCNP Monte Carlo code version IVB from Los Alamos National Laboratory, using the cross sections tables obtained from the library ENDF/B-VI, included in this code. The energy range covers from thermal neutron up to 100 MeV. To consider the effect of chemical binding and crystalline structure during the scattering of thermal neutrons the S(α,β) tables are used. The TLD-BSS response was defined as the thermal neutron fluence integrated between 0 ev and the cut off-energy of Cd (0.512 ev) by incidence neutron fluence. The statistical error was less than 5%. A parallel monoenergetic neutron beam on a sphere with a TLD placed in the centre of the sphere and bare was modelled. The physical characteristics of the TLD-600 were obtained by Harshaw-Bicron (3x3x0.9 mm 3, density of 2.64 g/cm 3 ; mass composition of % F, 25.59% 6 Li and 1.17% 7 Li.). The polyethylene density is 0,95 g/cm 3. In Figure 1, the sphere response is shown. III. SPECTOMETIC SYSTEM SETUP The experimental TLD-BSS described in this work consist of polyethylene spheres with diameters of 2.5, 3, 4.2, 7, 8, 9, 10 and 12 and TLD-600 and TLD-700 (Bicron) both placed on the centre of the sphere. This tandem is used because both detectors present the same response to gamma fields and the TLD-600 thermal neutron response is one thousand times the TLD-700 (10) response, then, the difference in readings reflects the effect of the thermal neutron fluence. The detectors were selected with sensitivities between 0.95 to 1.05 and with a coefficient of variation smaller than 2%. They were calibrated in energy response in the egional eference Centre for Gamma and X-ray Dosimetry of Argentine. The TLD holders were positioned in a polyethylene stent inside the sphere, designed especially for that task. The system sphericity was measured at the Instituto Nacional de Tecnología Industrial (INTI) of Argentine. To determine the reading of the TLD-600 to thermal neutron in a mixed (neutron and gamma) field, the TLD-700 readings are subtracted. The relationship between the two TLD gamma responses can be described as k = γ 700 γ 600 It was assumed that the TLD-700 thermal neutron response is negligible compared to TLD-600. Then, the total TLD-600 thermal reading can be expressed as 224

4 γ n n+ γ 700 = k Where n+ 600 and n 600 are the TLD-600 total and neutron readings. System Calibration In order to obtain the thermal neutron fluence in terms of the TLD response, the system was irradiated at the neutrons laboratory. The irradiation facility is 6 x14 x 4m 3, with walls made of zinc-plated corrugated steal sheet to minimise the backscatter. The spheres were positioned in the calibration bench 1.5m over the floor and 0.70m from the centre of the spheres. The 241 AmBe source has a flux of 1.14x10 7 s -1 (±10%). The TLD-600 and TLD-700 gamma response ratio, k, was determined with a 137 Cs source. The ISO8529 (11) AmBe spectrum was used and the planned experience was simulated taking into account the floor effects. The statistical error was less than 3%. The calibration factor (cm 2 of thermal fluence/nc) was obtained by fitting the response data by the method of least squares. The TLD response has been shown to be strongly dependent on the heating treatment selected. Heating cycle process The standard laboratory treatment (SLT) consists in using two external ovens, one for recovery during 1h at 400 C and another during 3h at 100 C. Preheating in an external oven at 100 C during 20 min is also used. Heating rate is 10 C/s from 23 C up to 300 C in the TLD reader. It is a Harshaw3500QS one. In this case the calibration factor was cm -2 / nc with a 1.5%. of error. In the new treatment implemented (NT) (12) at our laboratory the reader is used for the TLD recovery. A preheating of 120 C during 10s, with the same heating rate 10 0 C/s till 300 C for TLD-600 to include the thermal neutron dosimetry peak and till 270 C for TLD-700, and an annealing process of 300 C during 5 s was used. The calibration factor obtained was cm 2 /nc with an error of 2.2%. Spectrometry system validation In order to validate the spectrometric system, the LOUHI82 unfolding program was applied to the previously described TLD-BSS data. The LOUHI82 is a general purpose least squares unfolding program. It is based on the minimisation of the weighted square sum of the fitting error of activities and several alternative terms related to the smoothness and assumed shape of the solution, allowing varying degrees of a priori information. The response matrix is the parameter needed to unfold the actual spectrum, where the initial data is the thermal neutron fluence inside TLD-BSS obtained from the calibration factor. In Figure 2, the TLD-BSS measurements unfolded with a LOHUI82 and the AmBe spectrum from ISO is shown. They are expressed as lethargy in terms of neutron energy, where B is the neutron fluence in the interval ln(e/e o ), E is the neutron energy and Eo is equal to 1MeV. They have a good agreement, the difference in the normalised fluence is of 3%; in the ambient dose equivalent, H*(10) is of 2.6%, meanwhile in the mean energy determination is less than 1%. 225

5 Φ th (n/cm 2 ) / Φ 0 (n/cm 2 ) 10 0 TLD-BSS response " 2" 2.5" 3" 3.5" 4.2" 5" 6" 7" 8" 9" 10" 12" E (MeV) Figure 1. TLD-BSS response vs neutron energy obtained for the different spheres diameter and bare detector B/ ln(e/e 0 ) ISO 8529 LOUHI Energy (MeV) Figure 2. Comparison between the AmBe spectrum from ISO8529 ( ) and the TLD-BSS measurements unfolded with LOUHI82 (...). 226

6 IV. CONCLUSIONS The spectrometric system developed is a suitable device for measurements in pulsed and high intensity fields. The TLD-BSS has the possibility to make measurements in order to optimise radiation protection in working places. The theoretical and experimental results show the possibility of using the response matrix of the system in the evaluation of the measured spectrum. The integrated neutron fluence between 0 ev and the cut-off energy of Cd is an adequate quantity to be considered as a TLD-BSS response, because it gives a direct relation with TLD measurements. The heating cycle proposed improved the system sensibility in a factor 2.6. Using the TLD reader equipment as a unique processor obliged to perform an exhaustive quality control. It is necessary to clarify that the heating treatment proposed has a limit at 4Gy. The curve glow change towards the high temperature peaks and the normal curve could not be restituted with that treatment. The measurement with the sphere of 7 diameter has added importance because it has a maximum in the response to the high neutron energy present. This kind of neutron spectrum is found for instance in aircrew dosimetry (13). V. EFEENCES (1) Bramblett,.L., Ewing,.J. and Bonner, T.W. A New Type of Neutron Spectrometer. Nucl. Instrum. Methods 9,1-12 (1960). (2) Awschalom, M. and Sanna,.S. Applications of Bonner Sphere detectors in neutron field Dosimetry. adiat. Prot. Dosim. 10 (1-4), (1985). (3) Sanna,.S. Thirty One Group esponses Matrices for the Multisphere Neutron Spectrometer over the energy ange Thermal to 400 MeV. Health and Safety Laboratory, ep. HHASL-267(1995). (4) Caizergues,. and Pollout, G. Calcul de la esponse des Spheres de Bonner pour les Detecteurs Li, He et Mn Comparaison avec les Donnees Experimentales pour l Iodure de Lithium. ep. CEA (Commissariat a l Energie Atomique, Centre d Etudes Nucleaires de Saclay, Gif-sur-Yvette) (1977). (5) Alberts, W.G., Cosack, M., Klunge, H., Lesiecki, H., Wagner, S. and Zill, W. European Workshop on Neutron Dosimetry for radiation Protection. Tep. PTB-ND-17 (Physikalisc Technische Bundesanstalt, Braunschweig) (1979). (6) Dhairyawan, M.P., Nagarajan, P.S. and Venkataraman, G. Further Studies on the esponse of Spherical Moderated Neutron Detectors. Nucl. Instrum. Methods 175, (1980). (7) Briesmeister, J.F., MCNP A General Monte Carlo Code for Neutron and Photon Transport. Version 3A. LA-7396 (September 1986). (8) outti, J.T., Ph. D. Thesi: High Energy Neutron Spectroscopy with Activation Detectors, Incorporating new Methods for the Analysis of Ge(Li) Gamma ay Spectra and the Solution of Fredholm Integral Equations, Lawrence adiation Laboratory, University of California, Berkely, UCL

7 (9) outti, J.T. and Sandberg, J.V., General Purpose Unfolding Program LOUHI78 with Linear and Nonlinear egularisations. Comput. Phys. Commun. 21 (1980) (10) Horowitz, Yigal S. Thermoluminescence and Thermoluminescent Dosimetry. CC Press, Inc. Boca aton, Florida, USA. (11) ISO 8529: eference Neutron adiations. Part (12) Papadópulos, S.; Gregori; B, López González, F. Aplicaciones de un nuevo tratamiento térmico en detectores TL de uso en dosimetría ambiental. Proceeding of the IPA-egional Congress, ecife, Brasil (13) Alevra, Alexandru NEUTON SPECTOSMETY Section de Protection Technique de la Société Française de adioprotection, á l INSTN- Centre de Cadarache Allée du Château Saint Paul lez Durance, sur LA DOSIMETIE DES NEUTONS, 5 novembre