Applied Radiation and Isotopes 57 (2002) 167–170 . Neutron and gamma-ray spectra of 239PuBe and 241AmBe H!ector Ren!e Vega-Carrilloa,b,c,*, Eduardo Manzanares-Acu *naa, Ana Mar!ıa Becerra-Ferreiroa,b, Aureliano Carrillo-Nu *neza,d aUnidad!e Acad!emica!e, Estudios Nucleares, Universidad Aut !onoma de Zacatecas, Apdo. Postal 336, 98000 Zacatecas, Zac. Mexico bUnidad!e Acad!emica!e, Ingenier!ıa El!ectrica, Universidad Aut !onoma de Zacatecas, Apdo. Postal 336, 98000 Zacatecas, Zac. Mexico cUnidad!e Acad!emica!e, Matem !aticas, Universidad Aut !onoma de Zacatecas, Apdo. Postal 336, 98000 Zacatecas, Zac. Mexico dUniversidad Tecnol !ogica del Estado de Zacatecas, Mexico Received 1 May 2001; received in revised form 17 December 2001; accepted 30 January 2002 Abstract Neutron and gamma-ray spectra of 239PuBe and 241AmBe were measured and their dosimetric features were calculated. Neutron spectra were measured using a multisphere neutron spectrometer with a 6LiI(Eu) scintillator. The 239PuBe neutron spectrum was measured in an open environment, while the 241AmBe neutron spectrum was measured in a closed environment. Gamma-ray spectra were measured using a NaI(Tl) scintillator using the same experimental conditions for both sources. The effect of measuring conditions for the 241AmBe neutron spectrum indicates the presence of epithermal and thermal neutrons. The low-resolution neutron spectra obtained with the multisphere spectrometer allows one to calculate the dosimetric features of neutron sources. At 100 cm both sources produce approximately the same count rate as that of the 4.4MeV gamma-ray per unit of alpha emitter activity.r 2002 Elsevier Science Ltd. All rights reserved. Keywords: Neutron spectra; Gamma-ray spectra; Isotopic neutron sources; Background 1. Introduction Many heavy nuclides decay by spontaneous fission. When undergoing fission, nuclei generally emit two fission products per fission and several neutrons and gamma rays. Spontaneously fissioning radionuclides are often used as convenient sources of neutrons or fission products.The best example of this is 252Cf (Mann et al., 1991). Other frequently used isotopic neutron sources are based on (g; n) and (a; n) reactions in beryllium and other light elements. The main features of these neutron sources have been compiled (Hoste, 1988; NCRP, 1991). Isotopic neutron sources are portable, easy to shield, and they are widely used. Some applications are activation analysis (Lewis et al., 1997), as calibration source (Mukherjee, 1995) and in several industrial uses (Szabo and Boutaine, 1997). Some drawbacks of these sources are: they produce few neutrons, have short half- lives and leakage tests should be periodically performed. Neutron source applications and their dosimetric features are energy dependent. 239PuBe and 241AmBe have been widely studied with respect to their neutron spectra (Griffith et al., 1990) but information about their gamma-ray emission is scarce. Here, neutron and gamma-ray spectra were measured and the dosimetric features of the sources calculated. 2. Materials and methods Two isotopic neutron sources, 239PuBe and 241AmBe, were used. Their activities were 1.85� 1011 and *Corresponding author. Unidade´ Acade´micae´, Estudios Nucleares, Universidad Aut !onoma de Zacatecas, Apdo. Postal 336, 98000 Zacatecas, Zac. Mexico. Tel./fax: +52-492-2-70-43. E-mail address:
[email protected] (H.R. Vega-Carrillo). URL: http://cantera.reduaz.mx/Brvega. 0969-8043/02/$ - see front matter r 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 9 6 9 - 8 0 4 3 ( 0 2 ) 0 0 0 8 3 - 0 3.7� 109 Bq, respectively. 239PuBe is a bare source, while 241AmBe is inserted in a cylindrically shaped enclosure made of 3-cm-thick polyethylene. To measure the neutron spectra, a Bonner spheres spectrometer (BSS) was utilized (Vega-Carrillo, 2001). The spectrometer has a cylindrical shape, 0.4� 0.4 cm2, 6LiI(Eu) scintillator that is located at the center of several high-density polyethylene spheres of different diameters. Seven spheres were used, whose diameters are 0 (bare detector), 5.08, 7.62, 12.7, 20.32, 25.4, and 30.48 cm. Neutron spectra were unfolded using the BUNKIUT code and the UTA4 response matrix (Hertel and Davidson, 1985). During unfolding, the initial guess spectra were selected using the CATALOG program (Vega-Carrillo and I *niguez, 2002). For the 241AmBe neutron source, the BSS was located at 13071 cm above the floor level. The source-to- detector distance was 6371 cm, and the measurements were carried out in a closed environment. The 239PuBe neutron source was measured in an open environment with the BSS located at 23071 cm above floor level, and at 10071 cm source-to-detector distance. For both neutron sources the gamma-ray spectra were measured using a gamma-ray spectrometer with a cylindrical shape, 7.62� 7.62 cm2, NaI(Tl). The source was located at 10072 cm from the center of the scintillator, and at 11671 cm above the floor level. To avoid scintillator activation, a neutron shield made with a mixture of boric acid, plaster and polyethylene was located between the source and the detector. A gamma- ray background spectrum was measured for 24 h. 3. Results and discussion Normalized neutron spectra are shown in Fig. 1. The neutron flux at 100 cm from the 239PuBe, was 7275%cm�2 s�1, while the neutron flux at 63 cm from the 241AmBe neutron source was 672.5%cm�2 s�1. Above 0.2MeV, the neutron spectra of both sources are similar. This is in agreement with Griffith et al. (1990). Below 0.2MeV, the 241AmBe source shows the presence of epithermal and thermal neutrons. This is attributed to the neutron room return, where some of the neutrons emitted by the source interact with the room walls and are returned back with less energy. The neutron spectra were used to calculate the neutron doses. These were 11.170.6 mGyh�1 and 9374 mSv h�1 for 239PuBe and 0.5470.14mGyh�1 and 4.470.1mSv h�1 for 241AmBe. Neutron fluxes and neutron dosimetric features are in agreement with the data published at NCRP 112 (NCRP, 1991). Gamma-ray spectra of both sources, corrected by background, and background are shown in Fig. 2. All spectra are normalized to the multichannel pulse-height spectra acquisition time. In this figure, the following peaks can be observed in the background spectrum: Neutron energy [ MeV ] 10-8 10-7 10-6 10-5 10-4 10-3 10-2 10-1 100 101 102 N or m al iz ed le th ar gy fl ue nc e [ c m− 2 − ∆u − 1 ] 0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 241 AmBe 239 PuBe Fig. 1. 241AmBe and 239PuBe normalized lethargy fluence spectra. H.R. Vega-Carrillo et al. / Applied Radiation and Isotopes 57 (2002) 167–170168 0.912MeV from 228Ac, 1.12MeV from 214Bi, 1.46MeV from 40K, 1.76MeV from 214Bi, 2.12 and 2.2MeV overlapped from 214Bi and 2.6MeV form 208Tl. This spectrum is in agreement with a gamma-ray background spectrum measured by Petrasso et al. (1989). Both neutron sources show peaks at 0.2, 0.5, 3.4, 3.9 and 4.4MeV. The 4.4MeV photon is produced during the decay of the bound-excited state of 12C* that is produced in the 9Be(n; a)12C* nuclear reaction, mean- while 3.9 and 3.4MeV peaks are the single and double escape of the 4.4MeV photon. A small peak at 2.2MeV is observed in the 241AmBe source, which is due to photons produced by the neutron capture, H(n; g)D, in the polyethylene neutron source enclosure. The net count rate of the 4.4MeV peak produced by 239PuBe was 5670.7%s�1, while the 241AmBe net count rate was 175%s�1. The net count rates were divided by the activities of the 239Pu and, this results in 11.2 and 10 photonsBq�1 for 239PuBe and 241AmBe sources, respec- tively. 4. Conclusions Neutron and gamma-ray spectra of 239PuBe and 241AmBe sources have been measured. The Bonner spheres spectrometer has a low resolution for the neutron spectra. Nevertheless, the resolution is good enough to calculate the neutron doses. The use of closed environments during the measure- ments of neutron spectrum produces room return that is shown by the presence of epithermal and thermal neutrons. The count rates, at 100 cm, of the 4.4MeV gamma rays in both neutron sources produce approximately the same photon strength per unit of the source activity. Acknowledgements This work was suported by CONACyT (Mexico) under contract 31288U. References Griffith, R.V., Palfalvi, J., Madhvanath, U., 1990. Compen- dium of neutron spectra and detector responses for radiation protection purposes, IAEA Technical Report Series No. 318, pp. 70–79. Hertel, N.E., Davidson, J.W., 1985. The response of Bonner spheres to neutrons from thermal energies to 17.3MeV. Nuclear Instruments and Methods A 238, 509–516. Hoste, J., 1988. Isotopic neutron sources for neutron activation analysis, International Atomic Energy Agency TECDOC- 465, pp. 12–17. Lewis, D.G., Natto, S.S.A, Ryde, S.J.S, Evans, C.J., 1997. Monte Carlo design study of a moderated 252Cf source for in vivo neutron activation analysis of aluminum. Medical Physics and Biology 42, 625–636. Photon energy [ MeV ] 0 1 2 3 4 5 6 7 8 N (E ) d E [ c ps /M eV ] 0.001 0.01 0.1 1 10 100 239PuBe 241AmBe Background Fig. 2. 239PuBe, 241AmBe and background gamma-ray spectra. H.R. Vega-Carrillo et al. / Applied Radiation and Isotopes 57 (2002) 167–170 169 Mann, W.B., Rytz, A., Spernol, A., 1991. Radioactivity Measurements: Principles and Practice, Pergamon Press, Elmsford, NY, USA, p. 24. Mukherjee, B., 1995. Development of a simple neutron irradiation facility with variable average energy using a light water moderated 242Am/Be source. Nuclear Instru- ments and Methods A 363 (3), 616–618. NCRP, 1991. Calibration of survey instruments used in radiation protection for the assessment of ionizing radiation fields and radioactive surface contamination. National Council on Radiation Protection and Measure- ments, NCRP Report No. 112, Bethesda, MD, USA, p. 85. Petrasso, R.D., Chen, X., Wenzel, K.W., Parker, R.R., Li, C.K., Fiore, C., 1989. Problems with the g-ray spectrum in the Fleischmann et al. experiments. Nature 339 (6221), 183–185. Szabo, J.L., Boutaine, J.L., 1997. Some examples of industrial uses of neutron sources. Radiation Protection Dosimetry 70 (1/4), 193–197. Vega-Carrillo, H.R., 2001. Neutron energy spectra inside a pet cyclotron vault room. Nuclear Instruments and Methods A 463 (1–2), 375–386. Vega-Carrillo, H.R., I *niguez, M.P., 2002. Catalogue to select the initial guess spectrum during unfolding. Nuclear Instruments and Methods A 476 (1–2), 270–272. H.R. Vega-Carrillo et al. / Applied Radiation and Isotopes 57 (2002) 167–170170 Neutron and gamma-ray spectra of 239PuBe and 241AmBe Introduction Materials and methods Results and discussion Conclusions Acknowledgements References