2 KEV FILTERS OF QUASI-MONOCHROMATIC EPITHERMAL NEUTRONS

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1 Journal of Nuclear and Radiation Physics, Vol. 6, No. 1&2, pp KEV FILTERS OF QUASI-MONOCHROMATIC EPITHERMAL NEUTRONS H. N. Morcos and M. Adib Reactor Department, Nuclear Research Center, EAEA, Cairo, Egypt Rec. 18/10/2010 Accept. 29/11/2011 A simulation study of monochromatic beam filters based on selecting suitable materials for the production of quasi-monochromatic neutron beams in the energy range of 2 KeV are given. The simulation allows estimating the purity of the filtered neutron beam versus its intensity. Calculation shows that filters based on 54 Sc can produce a quasi monochromatic neutron beam at 2 KeV at low background. The main parameters of the suggested filtered quasi-monochromatic neutron beam were compared with those available in literature. For the same incident neutron flux, the present neutron filters are found to have higher intensity at low accompanying background than the others. Keywords: quasi-monochromatic beams: neutron filters. INTRODUCTION Filtered neutron monochromatic beams in KeV region nowadays have wide applications. Such beam is required for high precision measurements of the total and partial cross-sections for fundamental neutron-nuclear investigations and for precise determination of average nuclear parameters [1, 2]. It is also used in Boron-Neutron Capture Therapy (BNCT) [3, 4] and more recently in Gadolinium Neutron Capture Therapy (GdNCT) [5]. The main idea of neutron filter technique is the use of large quantities of a certain material which have the deep interference minima in its total neutron cross-section. By transmitting reactor neutrons through bulk layer of such material, one can obtain the quasi-monochromatic neutron lines instead of white reactor spectrum. To select only

2 70 H. N. Morcos etal one quasi-monochromatic line (neutron filter) with high purity, a composition filter of some selected elements is usually used. The composition filter consists of the "main filter material", and additional materials, for which resonance maxima in their total neutron cross-sections coincide with interference minima for filter material, with the exception of the selected line producing the quasi-monochromatic beam. Recently Gritzay [1] reported a neutron technique and components selected for forming filtered neutron beams at 2 KeV. The main filter material for 2 KeV was 45 Sc and the additional materials were 60 Ni, 54 Fe, 59 Co, 10 B, S and 27 Al. The results reported by Gritzay [1] showed that the filter beam at 2 KeV is strongly disturbed at KeV and at KeV. These may be due to the selected additional elements by Gritzay. Therefore in the present work, the additional elements for filtered beam at 2 KeV, reported by Gritzay are replaced by other ones to obtain higher intensity of the main peak at low background from the accompanying neutrons at higher neutron energies. MODELING CALCULATION OF NEUTRON FILTERED SPECTRA A computer code QMENF (Quasi Monochromatic Epithermal Neutron Filter) in the FORTRAN language was developed to calculate the neutron spectra formed by filters. The filter components and their amounts are optimized to get highest possible intensity without disturbed lines of the main energy line and the lowest one of the parasitic energy lines in filtered neutron spectrum. It is a complete system with input preparation, running calculations and output results. The input preparation contains the total cross-section data for different materials calculated by the PrePro2007 code using the JENDL-3.3 and ENDF/B-6 libraries [ 6 ]. The incident neutron spectrum is also considered as input and can be taken either constant, 1/E or reactor spectrum. The reactor spectrum is considered as Maxwellian for thermal neutrons, 1/E- dependence for resonance one and at high energies as a fission spectrum. The main running function of the QMENF code is to search for the elements of the required neutron filter energy, by fixing the energy E within range of ΔE. The result of such search is a list of all elements having a deep interference minimum in the total cross-section at the given neutron energy range. Then, the most promising filter elements are chosen from this list as they have the lowest value of the cross-section at interference minimum within the given energy width. Moreover the accompanying other minima at energies not equal to E+ΔE is not so pronounced. Consequently, each such element is considered as the "main element" of the filter.

3 2 KEV FILTERS OF QUASI-MONOCHROMATIC 71 Then the program searches for other elements which have resonance maxima in their total cross-section coincide with all interference minima for chosen filter material with the exception of the line producing the quasi-mono-energetic beam at energy E, in order to eliminate the parasitic energy lines. The result of the search is a list of elements. Such list may contain several elements, since it may be not available for one element, to have resonance maxima at all minima of the main filter element. These elements are called "additional elements" of the filter. The output results are the transmitted neutron spectrum through the filter. The spectrum was normalized to unity in the energy range from 10-5 ev to 20 MeV. The result of calculation includes transmitted flux versus energy and material thickness. Moreover the program can calculate the area under the main peak and other parasitic ones, and determine the ratio which corresponds to the purity of the filter. The purity versus intensity can be defined as required. More details of the computer code are given elsewhere [7]. RESULTS AND DISCUSSION The composition of 2 KeV filter consists of the 45 Sc as the main filter material and additional materials, for which resonance maxima in their total neutron cross sections coincide with interference minima for filter material, with the exception of the most deep interference minimum energy. Fig. 1 shows the calculated total cross-section of 45 Sc versus neutron energy at temperature 300 K. From Fig. 1 one can see that the main wide dip is at 2KeV, while other narrow dips are at higher energies. Such dips limited the use of 45 Sc as a neutron filter. Therefore, other filter components must be added to eliminate these dips. The selected components for forming the filtered neutron beam at 2 KeV along with those reported by Gritzay [1] are listed in Table 1.

4 72 H. N. Morcos etal Figure Sc total neutron cross-section. Table 1: Components of selected filters. Element Components gm/cm 2 Gritzay [ 1 ] Present work Filter 1 Present work Filter 2 45 Sc Fe Ni Co V (natural) Ti (natural) B 0.2 (85%) 0.2 (85%) 0.2 (85%) S (natural) Al

5 2 KEV FILTERS OF QUASI-MONOCHROMATIC 73 Using the computer code QMENF the intensity of the filtered beam at 2KeV for the composition reported by Gritzay [1] and assuming that the reactor spectrum at KeV region follows the 1/E law is calculated and displayed in Fig 2. Figure 2: Neutron transmission Gritzay [1] filter. Fig. 2 shows that the main peak at 2 KeV is strongly disturbed by dips at KeV and KeV. These dips are caused due to neutron resonances of 59 Co and 60 Ni respectively. While the two small disturbing dips at 1.06 KeV and KeV are due to the neutron resonances of 45 Sc at these energies. To eliminate such disturbing dips in the main peak at 2 KeV in the present work, the components 60 Ni and 59 Co are replaced by natural vanadium (filter I). Fig. 3 shows the filtered neutron beam at 2 KeV when replacing 60 Ni and 59 Co elements by only natural V.

6 74 H. N. Morcos etal Figure 3: Neutron transmission (Filter I). One can see that the dips due to 60 Ni and 59 Co are eliminated. Moreover the intensity of the peak at 2 KeV is two and half times than that by Gritzay filter [1]. However, the contribution of parasitic peaks is slightly higher than that by Gritzay. To improve the quality of the filtered beam at 2 KeV, another composition was suggested by replacing natural V by natural Ti (Filter II). Moreover, the amount of 54 Fe is taken ten times less than that given by Gritzay. The composition of (filter II) is also listed in Table 1. Using QMENF code the filtered neutron beam was calculated for this case and the result of calculation is displayed in Fig. 4.

7 2 KEV FILTERS OF QUASI-MONOCHROMATIC 75 Figure 4: Neutron transmission (Filter II). From the figure one can see that the intensity of the main peak is increased at lower background than that with vanadium. For comparison the parameters of the main peak along with the parasitic ones for different filter components are listed in Table 2. Table 2. The parameters of the main and parasitic peaks. Filter Energy KeV of main peak FWHM KeV Main peak Parasitic peaks Relative intensity % Gritzay[1] , Filter I Filter II *) I intensity of main peak present work I o intensity of main peak Gritzay [ 1 ]

8 76 H. N. Morcos etal The filter is usually installed at the exit of the horizontal reactor channel, consequently neutrons emerging from a steady state reactor with energies less than 1 ev obey the Maxwellian energy distribution and their intensities are higher than the 1/E spectrum. Such neutrons are parasitic for the filter at 2 KeV. To remove their contribution 10 B (85%) with thickness 0.2 gm/cm 2 was added to the filter material. Such addition decreases the intensity of the main peak by a factor 8%. Gritzay added natural S with thickness 56 gm/cm 2 to the filter component. However, our calculation shows that such thickness decreases the intensity of the main peak by a factor 65% while the decrease of the parasitic peaks 48%. Therefore, S was not added to our filter composition. Since the selected elements for the filter composition are chemically active, therefore they were packed in an aluminum container. It was found that an 27 Al thickness of 0.54 gm/cm 2 satisfies the safety of the filter elements. In the same time such thickness almost has no effect on altering the main peak (less than 1 %). However, in the present work the thickness of 27 Al is increased to be gm/cm 2 and gm/cm 2 for composition of (Filter I) and (Filter II) respectively. Such increase was found to decrease the parasitic background at energies from KeV without noticeable decrease of the intensity of the main peak at 2 KeV. CONCLUSION The developed computer code QMENF was found to be sufficient for calculating the quasi-monochromatic neutron beam at 2 KeV. Based on 45 Sc and V (filter I) and 45 Sc and Ti (filter II), the obtained quasi-monochromatic beams were found free from the disturbing dips at KeV and KeV. Moreover, the intensity of the main peak is about three times higher than that calculated from Gritzay s filter. Moreover, the advantages of the selected additional elements in the present work are natural elements. Consequently, they are cheaper. However, the contribution of parasitic peaks is slightly lower than our filters. REFERENCES [1] O. Gritzay, V. Kolotyi, V. Psheniehnyi, M. Gnidak, O. Kalchenko, N. Klimova, V. Libman, V. Razbudeyi, A. Kyslytskyi, V. Venediktov, O. Korol, "Neutron Filter technique and its use for Fundamental and applied investigations." 6 th conference on Nuclear and particle physics NUPPAC'07, Luxor, Egypt, Nov (2007). [2] R. Moreh, R. C. Block, Y. Danon; "Generating a multi-line neutron beam using an electron Linac and a U-filter." Nuclear Instruments and Methods in Physics Research A, 562, pp , (2006).

9 2 KEV FILTERS OF QUASI-MONOCHROMATIC 77 [3] M. Viaggi, M. A. Dagresa, J. Longhino, H. Blaumann, O. Caletta, S. B. Kahl, G. J. Juvenal, M. A. Pisarev; "Boron neutron capture therapy for undifferentiated thyroid, carcinoma: Preliminary results with the combined use of BPA and BOPP." Applied Radiation and Isotopes 61, pp , (2004). [4] E. Bisceglie, P. Colangelo, N. Colonna, P. Santorelli and V. Variale; "On the optimal energy of epithermal neutron beams for BNCT. Phys. Med. Biol. 45, pp , (2000). [5] Gelsomina Detasio etal; "Are gadolinium contrast agents suitable for gadolinium neutron capture therapy " Neurological Research, Volume 27, pp , June (2005). [6] Cullen D. E.; "PREPRO2007 (2007 ENDF/B Pre-Processing codes". IAEA-NDS- 39, Rev. 13 March 17, (2007). [7] H. N. Morcos and K. Naguib" QMENF- A computer package for Quasi- Monochromatic Epithermal Neutron Filter calculations." Under publication (2010). مرشح ٢ ك.إ.ف للنیوترونات فوق الحراریة شبھ وحیدة الطاقة حنان نحیب مرقص و ممدوح أدیب شحاتھ شعبة المفاعلات مركز البحوث النوویة ھیي ة الطاقة الذریة القاھرة مصر دراسة تشبیھیة معطاه لموحد فیض مرشح نیوترونى على أساس اختیار المواد المناسبة لتكوین فیض نیوترونى شبة موحد الطاقة فى حدود طاقة ٢ ك.إ.ف. التشبیة یسمح بتحدید نقاوة الفیض النیوترونى للمرشح فى علاقة مع شدتھ. الحسابات أظھرت أن المرشح على أساس سكاندیوم- ٥٤ یمكن أن ینتج فیض شبة موحد للطاقة عند ٢ ك.إ.ف. بخلفیة صغیرة.أھم البارامترات المقترحة للمرشح شبة وحید الطاقة قورنت بالمتاح فى النشر. و قد وجد أنھ لنفس الفیض النیوترونى الساقط. المرشح النیوترونى المعطى فى البحث لھ فیض أعلى عند خلفیة مصاحبة أصغر من المرشحات الا خرى.