Characterization of a Pube Isotopic Neutron Source

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 determined. 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

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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 Qt  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.

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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

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 calculation 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]. dose conversion coefficients.

Proccedings of the ISSSD 2012 ISBN 978-607-00-6167-7

The Chi-square of experimental count rates, Mc, and 3.2. Calculations those folded with the spectrum, Fc, is 0.0195, this value is In figure 6 is shown the neutron spectrum calculated 2  below the critical -value for = 0.95 and 6 degrees of with the MCNP5 code; in the same figure there is the freedom. This value was calculated using equation 7, measured neutron spectrum.  where i is the error propagation of Mc + Fc for the i-th sphere, and NS is the amount of spheres used during the measurement. 102

Measured Calculated

NS 2 ] Fc  Mc -1 2  i i  u 1    (7)  10

- i i -2

(E) [ cm 0 Integral features, obtained with the spectrum infor- E 10



mation, are shown in Table 1. E

239 Table 1. PuBe integral features to 100 cm 10-1 -2 -1 -1 10-8 10-7 10-6 10-5 10-4 10-3 10-2 10-1 100 101 102  [cm -s ] EAv [MeV] H*(10) [Sv-h ] Energy [ MeV ] 101 ± 6 3.55 133 ± 8 239 Figure 6. Calculated and measured PuBe neutron spectrum It can be noticed that above 0.02 MeV both spectra are The uncertainties in the fluence and the ambient dose alike; in the measured spectrum there are epithermal and equivalent were obtained adding the square of errors in thermal neutrons coming from those high-energy neutrons the matrix response (3%) and the maximum error of the that collide with laboratory walls lose energy and return measured count rates (5%). back to the laboratory.

2 The ambient dose equivalent factor is 365.78 pSv-cm ; Calculated neutron spectrum does not show epithermal according to the IAEA [20], for this type of source, this val- neither thermal neutrons because during calculations la- ue is 395 pSv-cm2 which is larger because this last value is boratory laboratory´s walls were not included, and how it for a point-like source in a free field [21]. has been pointed out in literature [16, 17], their presence is the reason of room-return neutrons. Using the neutron fluence and the neutrons mean ener- With the calculated spectrum and the original neutron gy the Ambient dose equivalent was calculated using equa- yield the H*(10) was also calculated being 116.5 mSv-h-1. tion 8. The H*(10) measured is 14.2% larger than that obtained

with the Monte Carlo calculations, this difference is proba- H * (10)   h10, EAv  (8) ble because the neutron yield is larger due to the presence of 241Pu during the source fabrication and because the ambient dose-to-fluence conversion coefficients are function

In this equation, h(10, EAv) is the fluence-to-ambient of neutron energy. dose equivalent factor for EAv MeV neutrons, this factor was linearly interpolated from the fluence-to-ambient dose In order to estimate the source´s neutron yield the measured neutron spectrum was integrated from 0.451 up equivalent coefficients take from the ICRP74 [14]. Using to 14.9 MeV giving a fluence of 87.1 cm-2. The same calcu- this crude procedure the H*(10) is 148.3 Sv-h-1, being lation was carried out with the spectrum calculated by the 11.5% larger than the H*(10) calculated using the spectrum MCNP5 code and the fluence is 77.7 cm-2. This experi- data. Using the same procedure the Personal dose equiva- mental fluence is 12.11% larger than the calculated fluence. o lent Hp,slab(10, 0 ) and the Effective dose in antero-posterior Using this difference the actual neutron yield was esti- -1 -1 geometry, EAP, were estimated whose results are Hp,slab(10, mated being 1.01E(7) s instead 9.03E(6) s that should be, o -1 -1 0 ) = 154.9 pSv-h and EAP = 162.3 pSv-h . These values, 44 years later due to Plutonium activity decay, if during even being crudely estimated, can be used to define con- source fabrication the Plutonium had been free of 241Pu servative source handling protocols. impurities.

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Using these neutron yields the probable amount of 241Pu Acknowledgements present during the source fabrication was 0.23 w/o. This work is part of the LINAC and MONITOR projects which are partially supported by the Consejo Zacatecano 4. Conclusions de Ciencia, Tecnología e Innovación.

Using a Bonner sphere spectrometer the neutron spectrum of the ESFM-IPN 239PuBe was measured to 100 cm. With the spectrum, integral features such as total fluence, neutron mean energy and the ambient dose equivalent References were determined. Using the MCNP5 the spectrum of the source was also determined using Monte Carlo methods. [1] Vega-Carrillo HR; Manzanares-Acuña E. (2004). Background These values were used to estimate the actual neutron neutron spectrum at 2420 m above sea level. Nuclear In- struments and Methods in Physics Research A 524: 146-151. yield and the probable concentration of 241Pu present in [2] Vega-Carrillo HR; Torres-Muhech C. (2002). Low energy the source during its fabrication. 239 neutrons from a PuBe isotopic neutron source inserted in moderating media. Revista Mexicana de Fisica 48: 405-412. Measured neutron spectrum has the characteristic peak, from 0.1 up to 14 MeV, of this type of sources when the [3] Garg AN; Batra RJ. (1986). Isotopic neutron sources in neu- spectrum has low resolution. The spectrum also shows the tron activation analysis. Journal of Radioanalytical and Nu- clear Chemistry, Articles 98: 167-194. presence of epithermal and thermal neutrons due to the [4] Naqvi AA; Al-Nagadi MM. (2004). Performance comparison room-return produced by the laboratory walls. 241 of an AmBe neutron source-based PGNAA set up with the KFUPM PGNAA set up. Journal of Radioanalytical and Nu- To 100 cm from the source, the neutron fluence rate is clear Chemistry 260: 641-646. 101 cm-2, whose mean energy is 3.55 MeV. The 91.5% of these neutrons have energies larger than 0.45 MeV while [5] DuBard JL; Gambhir A. (1994). Dust off the neutron howitzer to teach nuclear physics!. American Journal of Physics 62: the 3.4% have energies less than 1.5 eV. 255-257.

[6] To 100 cm from the source, the ambient dose equivalent Józefowickz K; Golnik N; Tulik P; Zielczynski M. (2007). Ra- -1 dionuclide neutron sources in calibration laboratory - Neu- is 133 pSv-h . Using a crude procedure the resulting am- tron and gamma doses and their change in time. Radiation -1 bient dose equivalent is 148.3 Sv-h being 11.5% larger. Protection Dosimetry 126: 134-137.

o [7] Conservative Personal dose equivalent H (10, 0 ) and ISO. Reference neutron radiations. Characteristics and p,slab methods of production. International Organization for the Effective dose in antero-posterior geometry, EAP, are Standardization. ISO-Standard 8529. (2001). 154.9 pSv-h-1 and 162.3 pSv-h-1 for H (10, 0o) and E p,slab AP [8] NCRP. Calibration on survey instruments used in radiation respectively. protection for the assessment for ionizing radiation field and radioactive surface contamination.National Council on Ra- The neutron spectrum calculated with Monte Carlo diation Protection and Measurements. Report No. 112. methods using a model without the laboratory walls has Bethesda MD. (1991). the peak noticed in the measured spectrum. The ambient [9] Vega-Carrillo HR. TEORÍA DE REACTORES NUCLEARES: dose equivalent and the total neutron fluence are less than REACTOR NUCLEAR SUBCRÍTICO. Saarbrücken, Germany. values obtained experimentally, mainly due the neutron Editorial Académica Española. (2012). yield. [10] Hamilton T. Radiation fallaout-Guam. Lawrence National Laboratory. Report UCRL-TR-204361. (2001). Actual neutron yield is 1.01E(7) s-1, this is larger than -1 [11] de Bievre P; Verbruggen A. (1999). A new measurement of 9.03E(6) s that should be, 44 y later, due to the amount of the half-life of 241Pu using isotope mass spectrometry. 241Pu present in the Pu used when the source was fabri- Metrologia 36: 25-32. cated. [12] Hertel NE; Davidson JW. (1985). The response of Bonner spheres to neutrons from thermal energies to 17.3 MeV. The probable mass fraction of 241Pu present during the Nuclear Instruments and Methods in Physics Research A source fabrication was 0.23%. 238: 509-516. [13] Vega-Carrillo HR; Ortiz-Rodriguez JM; Martinez-Blanco MR. (2012). NSDUAZ unfolding package for neutron spectrometry and dosimetry with Bonner spheres. Applied Radiation and Isotopes, doi: http//dx.doi.org/10.1016/j.apradiso.201 2.04.020.

Proccedings of the ISSSD 2012 ISBN 978-607-00-6167-7

[14] ICRP74. (1996). Conversion coefficients for use in radiologi- cal protection against external radiation. Annals of the ICRP 26: 199.

[15] X-5 Monte Carlo team. MCNP-A general Monte Carlo N-particle transport code. Version 5. Los Alamos National Laboratory Report LANL-UR-03-1987. (2004). [16] Vega-Carrillo HR; Manzanares-Acuña E; Iñiguez MP; Gallego E; Lorente A. (2007). Study of room-return neutrons. Radia- tion Measurements 42: 413-419.

[17] Vega-Carrillo HR; Manzanares-Acuña E; Iñiguez MP; Gallego E; Lorente A. (2007). Spectrum of isotopic neutron sources inside concrete walls spherical cavities. Radiation Meas- urements 42: 1373-1379. [18] Vega-Carrillo HR; Manzanares-Acuña E; Hernández-Dávila VM; Ramírez-González J; Hernández-Villasana R; Ruiz-Chacón A. (2009). Spectrometry and dosimetry of a neutron source. Radiation Effects & Defects 164: 218-223.

[19] Vega-Carrillo HR; Manzanares-Acuña E; Becerra-Ferreiro AM; Carrillo-Núñez A. (2002). Neutron and gamma ray spectra of 239PuBe and 241AmBe. Applied Radiation and Isotopes 57: 167-170.

[20] IAEA. Compendium of neutron spectra and detector re- sponses for radiation protection purposes. International Atomic Energy Agency Technical report series No. 318. (1990). [21] Raimonidi N; Tournier B; Groetz JE; Piot J; Riebler E; Cro- visier P; Chambaudet A; Cabanné N. (2002). Assessment of neutron dosemeters around standard sources and nuclear fissile objects. Radiation Protection Dosimetry 101: 197-200.