Proccedings of the ISSSD 2012 ISBN 978-607-00-6167-7 Characterization of a 239PuBe Isotopic Neutron Source H.R. Vega-Carrillo1/*, V.M. Hernández-Dávila1, T. Rivera-Montalvo2, A. Sánchez3 1Unidad Academica de Estudios Nucleares de la UAZ, C. Cipres 10, Fracc. La Peñuela, 98068 Zacatecas, Zac., Mexico 2CICATA-Unidad Legaria del IPN, Av. Legaria 694, 11500 Mexico, DF, Mexico 3ESFM-IPN, Av. Instituto Politecnico Nacional, 07738 Mexico DF, Mexico Abstract: A Bonner sphere spectrometer was used to determine the features of a 239PuBe neutron source used to oper- ate the ESFM-IPN Subcritical Reactor. The spectrometer is a 6LiI(Eu) scintillator and 2, 3, 5, 8, 10 and 12 inches-diameter polyethylene spheres, that was located 100 cm from the neutron source. The count rates obtained with the spectrometer were unfolded using the NSDUAZ code and neutron spectrum, total fluence, and ambient dose equivalent were deter- mined. A Monte Carlo calculation, using the MCNP5 code, was carried out to estimate the spectrum and integral features being less that values obtained experimentally due to the presence of 241Pu in the Pu used to fabricate the source. Using the experimental information the actual neutron yield and the mass fraction of 241Pu was estimated. Keywords: Isotopic; Neutron source; 239PuBe; Bonner spheres; MCNP 1. Introduction Environmental neutrons are produced during cosmic The Q-value of this reaction is 5.7 MeV that is shared rays interactions with atmospheric nuclei and from nuclear among the reaction products. reactions occurring between particles and some nuclei in the environment. Artificially, neutrons are produced in The intermediate state of this reaction is the formation fission and nuclear fusion, in nuclear reactions in accelera- of the compound nucleus of 13C, and neutrons are pro- tors, with nuclear reactions induced with alpha particles or duced through different channels. gamma-ray or through spontaneous fission. [1, 2] The alpha-neutron sources are produced by mixing the Neutrons have an energy distribution, or spectrum, 9Be with the α-emitter radioisotope, nevertheless 9Be is ranging from few tenths of eV to several GeV. The radiobi- widely used other target nuclei like 10B, 7Li, 19F, 13C and 18O, ological effectiveness strongly depends upon the energy are also used. Among α-emitting nuclei used in al- distribution therefore the knowledge of neutron spectrum pha-neutron sources are 210Po, 226Ra, 227Ac, 228Th, 238Pu, is important for dosimetry. 239Pu, 241Am, 242Cm, 244Cm. The isotopic neutron sources are small, compact, port- Isotopic neutrons sources are widely utilized in several able, easy to handle and do not require of high voltage. In activities including neutron activation [4], teaching [5], and these sources neutrons are produced by (γ, n) or (α, n) ex- monitor calibration [6]. For calibration purposes the Inter- oenergetic nuclear reactions. In this group are also those national Organization for Standardization (ISO) recom- neutron sources produced during the spontaneous fission mends to use 241AmBe, 241AmB, 252Cf and D2O-moderated of heavy nuclei like 252Cf. [3] 252Cf sources [7]. Alpha-neutron sources produce neutrons through The 239PuBe sources containing small quantities of 241Pu the (α, n) reaction like that shown in equation 1. tend to increase the neutron yield because 241Pu decays to 241Am that also emits alpha particles. Thus the neutron 9 Be n 12 C* (1) source will tend to increase its neutron yield with time [8]. * E-mail corresponding author: [email protected] Telephone: +52-(492)-922 7043 X 118 Fax: +52-(492)-922 7043 x 120 64 Proccedings of the ISSSD 2012 ISBN 978-607-00-6167-7 Calibration, safety and security protocols, and potential 2.00e+7 applications of neutron sources require knowing the phys- ical features of neutron source like the source strength (neutron yield), spectrum, and the effect of laboratory 1.80e+7 ] conditions upon neutron spectrum and the dosimetric -1 1.60e+7 quantities. This information is also useful to design the 241Pu weigth fraction source storage, shielding and neutron source´s handling 0.1 1.40e+7 0.5 procedures. 1.5 2.0 The ESFM-IPN has a subcritical reactor Nuclear Chicago Source strength [ s 1.20e+7 model 2000 that uses a 239PuBe to start the neutron multi- plication [9]; in the Figure 1 is shown the subcritical reac- 1.00e+7 tor. 0 50 100 150 200 Time [ years ] Figure 2. 239PuBe strength for different 241Pu concentration The 239PuBe source will reach the maximum strength in a time given in equation 3. 1 ln 2 T (3) 1 2 Variations in the source strength change the integral features of the neutron source; on the other hand, the dimensions of the hall, and materials within, where the source is used change the neutron spectrum and the neu- Figure 1. Subcritical reactor trons mean energy. All these changes modifies the ambient dose equivalent, H*(10), and the total neutron fluence, , that are important for safety, security and radiation pro- During fabrication of this type of neutron sources there 241 tection issues. are small amounts of Pu impurities that modify the neu- 241 tron yield. The Pu is a beta emitter with a half-life of 241 The aim of this work is to determine the neutron spectrum 14.33 years, decays to Am that is an alpha particle emit- 239 241 of the PuBe neutron source of the ESFM-IPN Subcritical ter. Gradual accumulation of Am, whose half-life is 432 y, Nuclear Reactor. The actual neutron yield and the probable will cause an increase in the yield of neutrons according to amount of 241Pu present in the Pu used during the source equation 2. [8] fabrication were also estimated. t t Qt Q 7.4x106 w f e 1 e 2 (2) o 2. Materials and Methods 2.1. Neutron source Here, Q is the neutron emission rate, in neu- 239 o The ESFM-IPN PuBe isotopic neutron source has trons/second, at the time of fabrication, w is the mass of 239 79.9950 grams of Pu and 40 grams of Be; the Pu atomic Pu in the source, f is the 241Pu mass fraction during source 241 fraction is 0.9301. The source was made by Monsanto Re- fabrication, and are the decay constant of Pu and nd 239 1 2 search Corporation in June 22 , 1967 with a nominal Pu 241Am respectively. activity of 0.185 TBq; its initial neutron yield was 9.04E(6) -1 s . Unfortunately, uncertainties and details about the In Figure 2 is shown the source strength in function of 241 amount of Pu are not available. time for different concentrations of 241Pu in mass fraction. 65 Proccedings of the ISSSD 2012 ISBN 978-607-00-6167-7 The 239Pu has a half-life of 2.41E(4) years [10, 11] thus Calculated and measured total fluence were compared and the reduction of neutron yield due to 239Pu decay is sur- used to estimate the amount of 241Pu present in the pluto- passed by the neutron yield increase due to the 241Pu con- nium used during the source fabrication. tent in the Pu used during source fabrication. This source will reach its maximum neutron yield in 2040. In 2011 this source was 44 years old, for 241Pu impurities from 0.1 to 2 3. Results and Discussion w/o the source strength varies from 9.55E(6) to 1.92E(7) s-1. 3.1. Measurements In Figure 4 the pulse height spectra of a-peak for each 2.2. Measurements sphere are shown. Using a Bonner sphere spectrometer, BSS, with a 0.4 Ø × 0.4 cm2 cylindrical scintillator of 6LiI(Eu) and 0, 2, 3, 5, 8, 100 10 and 12 inches-diameter polyethylene spheres the neu- 239 0 tron spectrum of PuBe to 100 cm was measured. Count- 2 3 ing time for each sphere was large enough to have a un- 5 8 certainty less than 5% in the net counts under the al- -1 10 ] 10 pha-peak. The count rates were used to unfold the neutron -1 12 spectrum, E(E), using the UTA4 response matrix [12] in the NSDUAZ unfolding code [13]. With the E(E) the total fluence rate, , the H*(10) and the neutron mean energy, 10-2 Count rate [ s EAv, were calculated using the discrete versions of equa- tions 4, 5 and 6. 10-3 50 100 150 200 250 E E dE (4) Channel number E Figure 4. BSS pulse height spectra to 100 cm The count rates were used as input in the NSDUAZ un- H * (10) h * E E E dE (5) folding code; the unfolded spectrum in shown in Figure 5. E 50 1 40 E E E dE (6) Av m E ] -1 E u - 30 -2 In equation 5 h*(10) are the neutron flu- 20 (E) [ cm ence-to-ambient dose equivalent conversion coefficients E from ICRP74 [14]. In equation 6, Em is the median value of E the energy groups. 10 0 2.2. Monte Carlo calculation 10-8 10-7 10-6 10-5 10-4 10-3 10-2 10-1 100 101 102 Energy [ MeV ] Using the MCNP5 code [15] a model of the neutron source was built in order to estimate the neutron spectrum Figure 5. 239PuBe neutron spectrum to 100 cm and the H*(10) to 100 cm from the source. In this calcula- tion the neutron spectrum was estimated using the same In figure 5 can be noticed that below 0.1 MeV there are energy groups used by the NSDUAZ unfolding code. few neutrons due mainly to room-return [16, 17]. The larger amount of neutrons is from 0.1 up to 14 MeV; this With the calculated spectrum the value of H*(10) was spectrum is alike to another source having roughly the determined using the ICRP 74 neutron fluence-to-ambient same age and activity [18, 19].