Acta Polytechnica CTU Proceedings https://doi.org/10.14311/APP.2022.37.0038 Acta Polytechnica CTU Proceedings 37:38–42, 2022 © 2022 The Author(s). Licensed under a CC-BY 4.0 licence Published by the Czech Technical University in Prague SPECTROMETRY OF NEUTRON FIELD BASED ON P(20)+BE SOURCE REACTION Ján Kozica,b,∗, Milan Štefánika,b a Czech Technical University in Prague, Faculty of Nuclear Sciences and Physical Engineering, Department of Nuclear Reactors, V Holešovičkách 2, 180 00 Prague 8, Czech Republic b Nuclear Physics Institute of The Czech Academy of Sciences p.r.i., Řež No. 130, 250 68 Řež, Czech Republic ∗ corresponding author: kozic@ujf.cas.cz Abstract. The research deals with a spectrometry of neutron field based on the p(20)+Be source reaction. The p(20)+Be interaction stands for a nuclear reaction where protons accelerated to energy of 20 MeV bombard Be target. Produced neutron field has broad energy distribution up to 18 MeV. The research was carried out at the Nuclear Physics Institute of the Czech Academy of Sciences and was motivated by extension of experimental possibilities of the NG-2 target station. To determine the neutron spectrum, the activation foil technique was used. During the experiment, ten activation foils were irradiated. Activated foils were measured using the HPGe detector to determine the reaction rates of observed reactions. The neutron field was simulated in the MCNPX code, and the simulation served as a priori information for the unfolding of the neutron spectrum in the SAND-II code. A determined neutron spectrum is essential for various experiments, e.g., material research, measurements of nuclear data, etc. Keywords: Accelerator-driven neutron source, beryllium target station NG-2, activation foil technique, neutron energy spectrum, SAND-II. 1. Introduction Fusion reactors such as ITER [1], DEMO [2], are ex- pected to use a d+T reaction which produces neutrons with energy distribution up to 20 MeV and with a peak at an energy of 14 MeV. The p(20)+Be source reac- tion presented in this paper produces a neutron field with a broad energy spectrum up to 18 MeV which covers almost the whole energy range of the d+T reaction. This fact allows the usage of the neutron field produced by the p(20)+Be source reaction for the material research for future fusion reactors. The new neutron field from the p(20)+Be source reaction can also be used for experiments of measurements, benchmarks of nuclear data and fast neutron activa- tion analysis which extends experimental possibilities of beryllium target station NG-2. 2. Materials and methods 2.1. Activation foil technique An activation foil technique is a method which is used to characterize an energy spectrum of neutron sources. To determine a neutron spectrum using this method a set of spectroscopically thin foils or wires are ir- radiated in an unknown neutron field. The foils or the wires mentioned are called activation detectors. Reactions of neutrons with the activation detectors produce radioactive nuclei which emit gamma radi- ation. The gamma radiation is detected using, for example, a HPGe detector. From the measurements of the gamma-ray spectra of the irradiated activation de- tectors, it is possible to determine the reaction rates per target nucleus of the observed reactions using an equation RR = λS(Eγ) treal tlive I(Eγ)ε(Eγ)N0(1−e−λtirr )e−λtcool (1−e−λtreal ) , (1) where λ is a decay constant of the produced radioac- tive nucleus, S(Eγ) is an area of a full energy gamma- ray peak, tlive is a duration of the measurement of gamma spectrum, treal is a duration of the measure- ment of gamma spectrum with consideration of a dead time, I(Eγ) is an intensity of the gamma ray, ε(Eγ) is a detection efficiency of the apparatus for a given gamma-ray energy, N0 is the number of target nu- clei, tirr and tcool is an irradiation and a cooling time respectively. Correction factors are also applied. A rel- ative uncertainty of the reaction rate is determined by an equation [5] δRR = √( 0.03 ( treal tlive − 1 ))2 + (tcoolλδT1/2)2 + δ2 S(Eγ) + δ2 I(Eγ), (2) where δj is a relative uncertainty of j-th quantity. The reaction rate is defined by an equation RR = ∫ +∞ 0 φ(En)σ(En)dEn, (3) where φ(En) is a neutron spectral flux density, σ(En) is a microscopic cross-section of respective reaction and En is an energy of neutron. From the measure- ment of the irradiated activation detectors, a set of reaction rates for the observed reactions is obtained 38 https://doi.org/10.14311/APP.2022.37.0038 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 37/2022 Spectrometry of Neutron Field Based on p(20)+Be Source Reaction Reaction Energy of reaction [MeV] Threshold energy [MeV] 9Be(p,n)9B −1.85 2.06 9Be(p,pn)8Be −1.66 1.85 9Be(p,nα)5Li −3.54 3.93 9Be(p,pα)5He∗ → α+n −2.31 2.57 9Be(p,npα)4He −1.57 1.75 9Be(p,α)6Li∗ → p+α+n −2.19 2.31 9Be(p,α)6Li∗ → p+5He∗ → p+α+n −3.25 3.61 9Be(p,α)6Li∗ → 5Li+n −3.53 3.92 9Be(p,p’)9Be∗ → α+5He∗ → α+n −2.46 2.74 Table 1. Reactions of protons with beryllium which produce neutrons [3, 4]. and a process of unfolding the unknown neutron en- ergy spectrum is based on a solution of equations resulting from the definition of the reaction rate. 2.2. p+be as a neutron source reaction Based on the suitable physical properties of beryllium, mainly a melting point of 1287 °C [6] and thermal conductivity of 190 Wm−1K−1 [6], it is possible to use it as an accelerator-driven external neutron source target, despite its toxicity [7]. Reactions of protons with beryllium which produce neutrons are summarized in Table 1. The most important neutron producing reaction is 9Be(p,n)9B which contributes to the fast component of the neutron energy spectrum. The energy of this reaction is −1.85 MeV [3], and it has threshold energy equal to 2.06 MeV [3]. The rest of the reactions listed in Table 1 produce neutrons with energy of the order of MeV units. Depending on the beryllium target’s energy thick- ness, the external neutron source can produce a white or semi-monoenergetic spectrum. Neutron field with white energy spectrum is a neutron field with broad energy distribution. Although the beryllium target is mostly used as an energetically thick white neutron source, Uwamino et al. [8] have studied an energeti- cally thin beryllium target as a semi-monoenergetic neutron source. The energetically thick accelerator- driven beryllium neutron source has been intensively studied by Lone et al. [9] and Brede et al. [10]. Exper- imental data measured by Lone et al. and Brede et al. are shown in Figures 1 and 2 respectively. Brede et al. derived an empirical equation for the determination of a fluence averaged neutron energy [10] En = 0.433Ep − 1.39, (4) where En is the fluence averaged neutron energy and Ep is an energy of incident protons. The equation is usable for neutrons with an energy higher than 2 MeV. 2.3. Experimental apparatus At the Nuclear Physics Institute of The Czech Academy of Sciences (NPI CAS) an isochronous cy- clotron U-120M and neutron target stations NG-2 are Figure 1. Neutron energy spectra from the source reaction p+Be measured by Lone et al. for proton energies of 14.8 MeV, 18 MeV and 23 MeV [9, 11]. Figure 2. Neutron energy spectra from the source reaction p+Be measured by Brede et al. for proton energies of 19.08 MeV, 19.92 MeV and 22.01 MeV [10, 11]. operated. The cyclotron U-120M is a multi-purpose device which can accelerate the positive and nega- tive ions. It was manufactured at the Joint Institute for Nuclear Research in Dubna Russia. At the be- ginning of the operation, the cyclotron was able to accelerate only positive ions. An option to accelerate negative ions was added during a modernization of the cyclotron between 1996 and 1998. The beam of accelerated ions could be extracted to a hall where the cyclotron is located or to an external experimental hall. Parameters of the ion beams extracted from isochronous cyclotron U-120M are listed in Table 2. The Department of Nuclear Reactions of the NPI CAS operates two accelerator-driven neutron produc- ing target stations NG-2. One of them has embedded 39 Ján Kozic, Milan Štefánik Acta Polytechnica CTU Proceedings Energy Maximal current Ion [MeV] [µA] H+ 6 − 25 5 D+ 12 − 20 5 3He+2 18 − 52 2 4He+2 24 − 38 5 H− 6 − 35 13 D− 11 − 20 10 Table 2. Parameters of the extracted ions from the cyclotron U-120M [12]. Figure 3. The beryllium target station NG-2 at the Nuclear Physics Institute of The Czech Academy of Sciences. an energetically thin lithium target and the other has an energetically thick beryllium target. The beryl- lium target has dimensions of 8 mm in thickness and 50 mm in diameter. Because the beam of protons is fully stopped inside the beryllium target, cooling of the target is needed and it is cooled by liquid alco- hol at a temperature of 5 °C. This target is used to produce a neutron field with broad energy distribu- tion. The beryllium target station NG-2 is shown in Figure 3. 2.4. Data acquisition To determine an energy spectrum at a position P14 (see Figure 4) of the neutron field produced by an accelerator-driven external neutron source based on a source reaction p(20)+Be, an irradiation experiment has been carried out at the Nuclear Physics Institute of The Czech Academy of Sciences. The experimental arrangement is illustrated in Figure 4. The activation foil technique has been used and a set of various high purity metallic spectroscopic foils, which has been delivered by GoodFellow Company, have been irradiated. The set of foils has contained materials Au, In, Fe, Al, Lu, Y, Co, Ti, Nb, and Ni (see Figure 5). During the experiment, the proton beam has been accelerated to the kinetic energy of 20.134 MeV and a mean current on the surface of the beryllium target has been 12.3 µA. The irradiation lasted for 11 hours. After the irradiation has ended, the activation detec- tors have been transferred to a gamma spectrometric Figure 4. A schematic of the experimental arrange- ment. Figure 5. The set of activation detectors. laboratory where their gamma spectra have been mea- sured on a HPGe Canberra semiconductor detector for four weeks. 3. Results From the measured gamma-ray spectra of the irradi- ated set of activation detectors the necessary quanti- ties for determination of reaction rates of the observed reactions, which have been identified based on charac- teristic gamma-ray energy, a half-life of their products and with respect to their activation cross-section, have been obtained. Using Equations (1) and (2), a set of reaction rates and their relative uncertainties for the observed reactions, which are summarized in Table 3, have been determined. The reaction (n,x) denotes a combination of reactions for the natural atomic abundance of the element. A priori information about the neutron spectrum has been obtained from simula- tion in MCNPX v.2.7.0. code [13] using LA-150h [13] and ENDF/B-VII.1 [14] nuclear data libraries. The geometry in the simulation has been the same as has been shown in the schematic in Figure 4. The neu- tron energy spectrum has been successfully unfolded in SAND II code [15, 16]. During the unfolding pro- cess, 22 out of a total of 24 reaction rates have been successfully used with respective cross-sections from the EAF-2010 nuclear database [17], and the neutron energy spectrum obtained from the simulation in the MCNPX code has served as an initial guess spectrum. The unfolded neutron energy spectrum shows good agreement with the MCNPX predictions. Both simu- lated and unfolded neutron energy spectra from the source reaction p(20)+Be at the position P14 are il- lustrated in Figure 6. During the process of neutron spectrum unfolding, calculated over experimental re- action rate ratios (C/E) of all used reactions have been monitored and they are listed in Table 4. As can be seen from Table 4 all of the C/E ratios are close to 40 vol. 37/2022 Spectrometry of Neutron Field Based on p(20)+Be Source Reaction Reaction RR [s-1] Relative uncertainty [%] natLu(n,x)176mLu 4.10×10−16 0.52 natLu(n,x)174mLu 8.69×10−17 1.84 93Nb(n,α)90mY 7.13×10−19 1.04 93Nb(n,2n)92mNb 5.46×10−17 0.72 natNi(n,x)58Co 1.29×10−16 0.33 natNi(n,x)57Ni 2.41×10−18 0.78 natNi(n,x)57Co 4.38×10−17 0.97 89Y(n,γ)90Y 3.34×10−19 2.85 89Y(n,2n)88Y 8.09×10−17 0.47 27Al(n,α)24Na 1.60×10−17 0.62 197Au(n,γ)198Au 3.97×10−16 0.11 197Au(n,2n)196Au 2.89×10−16 0.19 197Au(n,2n)196mAu 1.39×10−17 0.72 59Co(n,γ)60Co 7.04×10−17 1.23 59Co(n,α)56Mn 4.18×10−18 1.45 59Co(n,p)59Fe 1.02×10−17 0.83 59Co(n,2n)58Co 7.40×10−17 0.24 natFe(n,x)56Mn 1.51×10−17 1.02 natIn(n,x)115mIn 1.65×10−16 0.41 natIn(n,x)116mIn 1.23×10−16 0.62 natIn(n,x)114mIn 1.54×10−16 0.74 natTi(n,x)46Sc 6.12×10−18 1.12 natTi(n,x)47Sc 4.88×10−18 0.33 natTi(n,x)48Sc 6.42×10−18 0.36 Table 3. The set of reaction rates and their uncertainties for observed reactions. unity which confirm the correctness of the unfolding process. The unfolded neutron energy spectrum has been compared to neutron spectra reported by other authors to check the correctness and to show good agreement in shape. Moreover, the mean energy of the unfolded neutron spectrum is 7.3 MeV which is in agreement with the computed mean energy from the empirical Equation (4). The neutron flux density for a neutron energy higher than 1 MeV reached a value of 5.6×109 cm-2·s-1 at the position P14. Figure 6. The unfolded and determined neutron energy spectra from the source reaction p(20)+Be at the position P14. 4. Conclusions At the Nuclear Physics Institute of The Czech Academy of Sciences, an energy spectrum of the Reaction C/E ratio 197Au(n,2n)196Au 0.98 197Au(n,2n)196mAu 1.12 197Au(n,γ)198Au 0.96 27Al(n,α)24Na 1.19 115In(n,n’)115mIn 0.96 59Co(n,p)59Fe 0.93 59Co(n,α)56Mn 1.05 59Co(n,γ)60Co 1.04 59Co(n,2n)58Co 0.90 89Y(n,2n)88Y 1.03 89Y(n,γ)90mY 0.8 natFe(n,x)56Mn 1.08 natTi(n,x)48Sc 0.98 natTi(n,x)47Sc 1.07 natTi(n,x)44Sc 0.92 93Nb(n,α)90mY 1.08 93Nb(n,2n)92mNb 1.04 natLu(n,x)176mLu 0.99 natLu(n,x)174mLu 1.03 natNi(n,x)58Co 1.01 natNi(n,x)57Ni 0.91 natNi(n,x)57Co 0.96 Table 4. Calculated over experimental reaction rate ratios. new neutron field based on the p(20)+Be source reaction at the position P14 has been measured. 41 Ján Kozic, Milan Štefánik Acta Polytechnica CTU Proceedings The p(20)+Be source reaction produces a neutron field with a broad energy distribution up to 18 MeV and mean energy of 7.3 MeV. The neutron energy spectrum has been determined using the activation foil technique. For proton beam current 12.3 µA the neutron flux density for a neutron energy higher than 1 MeV reached a value of 5.6×109 cm-2·s-1 at the position P14. Furthermore, the determined neutron energy spectrum shows good agreement in shape with neutron spectra reported by other authors for similar energies of protons. The neutron field from the source reaction p(20)+Be has an energy distribution which covers almost the neutron energy spectrum from the d+T reaction and it is suitable for material research for future fusion reactors. It also extends experimental possibilities of beryllium target station NG-2, mostly for measure- ments, benchmarks of nuclear data, and fast neutron activation analysis. Acknowledgements This publication and irradiation time at the NG-2 neutron source was supported by OP RDE, MEYS (Ministry of Education, Youth and Sports), Czech Republic under the project CANAM, CZ.02.1.01/0.0/0.0/16_003/0001812. 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Journal of the Korean Physical Society 59:1100– 1103, 2011. https://doi.org/10.3938/jkps.59.1100. 42 https://www.iter.org/ https://www.euro-fusion.org/programme/demo/ https://www.nndc.bnl.gov/qcalc/ https://doi.org/10.1016/j.radphyschem.2018.06.046 https://doi.org/10.1023/a:1010937301028 https://www.webelements.com/ https://doi.org/10.1002/14356007.a04_011.pub2 https://doi.org/10.1016/0168-9002(88)90318-X https://doi.org/10.1016/0029-554X(77)90616-4 https://doi.org/10.1016/0168-9002(89)90399-9 https://www-nds.iaea.org/exfor/ http://www.ujf.cas.cz/en/ https://doi.org/10.1016/j.nds.2011.11.002 https://doi.org/10.1016/0029-554X(72)90482-X https://doi.org/10.3938/jkps.59.1100 Acta Polytechnica CTU Proceedings 37:38–42, 2022 1 Introduction 2 Materials and methods 2.1 Activation foil technique 2.2 p+be as a neutron source reaction 2.3 Experimental apparatus 2.4 Data acquisition 3 Results 4 Conclusions Acknowledgements References