Neutron Source Calculation in Nuclear Physics using SOURCES 4A Code, Lecture notes of Physics

Download Neutron Source Calculation in Nuclear Physics using SOURCES 4A Code and more Lecture notes Physics in PDF only on Docsity! LA-13639-MS SOURCES 4A: A Code for Calculating (α,n), Spontaneous Fission, and Delayed Neutron Sources and Spectra Los N A T I O N A L L A B O R A T O R Y Alamos Los Alamos National Laboratory is operated by the University of California for the United States Department of Energy under contract W-7405-ENG-36. Approved for public release; distribution is unlimited. An Affirmative Action/Equal Opportunity Employer This report was prepared as an account of work sponsored by an agency of the United States Government. Neither The Regents of the University of California, the United States Government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise, does not necessarily constitute or imply its endorsement, recommendation, or favoring by The Regents of the University of California, the United States Government, or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of The Regents of the University of California, the United States Government, or any agency thereof. Los Alamos National Laboratory strongly supports academic freedom and a researcher's right to publish; as an institution, however, the Laboratory does not endorse the viewpoint of a publication or guarantee its technical correctness. Edited by Maco Stewart, Group CIC-1 2. Sample Problem #7 .................................................................................... 56 VII. Acknowledgments .......................................................................................... 58 VIU. References .................................................................................................... 59 Appendix A- Output Files for Example Problem ................................................... 62 Appendix B - Energy-Dependent, Thick-Target Yields for Various Target Materials ......................................................................................... 97 Appendix C - General Bibliography ...................................................................... 109 Appendix D - Los Alamos Scientific Laboratory Report LA-8869-MS .................. 117 vi SOURCES 4A: A Code for Calculating (cx,n), Spontaneous Fission, and Delayed Neutron Sources and Spectra by W. B. Wilson, R. T. Perry, W. S. Charlton, T. A. Parish, G. P. Estes, T. H. Brown, E. D. Arthur, M. Bozoian, T. R. England, D. G. Madland, and J. E. Stewart ABSTRACT SOURCES 4A is a computer code that determines neutron production rates and spectra from (cx,n)reactions, spontaneous fission, and delayed neutron emission due to the decay of radionuclides. The code is capable of calculating (u,n) source rates and spectra in four types of problems: homogeneous media (i.e., a mixture of et-emitting source material and low-Z target material), twe-region interface problems (i.e., a slab of ct- emitting source material in contact with a slab of low-Z target material), three-region interface problems (i.e., a thin slab of low-Z target material sandwiched between a- ernitting source material and low-Z target material), and (a,n) reactions induced by a monoenergetic beam of et-particles incident on a slab of target material. Spontaneous fission spectra are calculated with evaluated half-tie, spontaneous fission branching, and Watt spectrum parameters for 43 actinides. The (cx,n) spectra are calculated using an assumed isotropic angular distribution in the center-of-mass system with a library of 89 nuclide decay a-particle spectra, 24 sets of measured and/or evaluated (qn) cross sections and product nuclide level branching fractions, and functional u-particle stopping cross sections for Z<106. The delayed neutron spectra are taken from an evaluated library of 105 precursors. The code outputs the magnitude and spectra of the resultant neutron source. It also provides an analysis of the contributions to that source by each nuclide in the problem. I. INTRODUCTION .In many systems, it is imperative to have accurate knowledge of all signit-icantsources of neutrons due to the decay of radionuclides, These sources can include neutrons resulting from the spontaneous fission of actinides, the interaction of actinide decay cx- particles in (a,n) reactions with low- or medium-Z nuclides, and/or delayed neutrons fi-om 1 u the fission products of actinides. Numerous systems exist in which these neutron sources could be important. These include, but are not limited to, clean and spent nuclear i%el (UOZ, ThOz, MOX, etc.), enrichment plant operations (UFfj, PuF1), waste tank studies, waste products in borosilicate glass or plutonium (WPu), in storage containers. calculate neutron sources (magnitude aforementioned interactions and decays. glass-ceramic mixtures, and weapons-grade The SOURCES 4A code was designed to and spectra) resulting from any of the The spontaneous fission spectra are calculated with evaluated half-we, spontaneous fission branching, and v data using Watt spectrum parameters for 43 actinides. The (a,n) spectra are calculated with a library of 89 nuclide decay et-particle spectra, 24 sets of evaluated (a,n) cross sections and product nuclide level branching fractions, and 105 functional u stopping cross sections using an assumed isotropic neutron angular distribution in the center-of-mass system. A maximum u-particle energy of 6.5 MeV is allowed by SOURCES 4A. This restriction is required because of the limitations of the cross section libraries. The delayed neutron sources are calculated horn a library of evaluated delayed neutron branching fi-actionsand half-lives for 105 precursors. The SOURCES 4A code is capable of calculating neutron sources in homogeneous problems (i.e., homogeneous mixtures of a-emitting and low-Z materials), interface problems (i.e., composite material consisting of two separate slab regions), et-beam problems (i.e., a monoenergetic a-beam incident on a low-Z slab), and three-region interface problems (i.e., a thin slab of low-Z target material sandwiched between a- emitting source material and low-Z target material). However, systems that include combinations of these problems must be run separately and then compiled by the user. SOURCES 4A consists of a FORTRAN 77 (F77) source code, a user-created input file, up to five output ffles, and four library files. The SOURCES 4A code has been under development for several years with continuing improvements made in methods and data. The original version of SOURCES (SOURCES lx) was actually named POFEAL and was primarily used for calculating Pi OF E-ALpha [i.e., the probability of an (cc,n) interaction with nuclide i by an cx-particle prior to stopping in the material]. 1 SOURCES 2x was an improvement of the original POFEAL code which included spectra calculations.2 Also, 2 which yields the energy loss of a-particles of energy E per unit path length X.3 The energy loss of an et-particle of initial energy E. in traveling a distance L can be determined from the stopping power as Similarly, the distance traveled in slowing from Ea to E; is ‘=k5dE=i”i)dE” (2) (3) During the slowing down of the a-particles within the material, neutrons may be produced by (ct,n) reactions with the nuclides contained in the material. The probability of an (ec,n)interaction with nuclide i by an et-particle of energy E traveling from x to x+dx is Nisi (E)’E NiCTi(E)dx = ‘E (Hdx (4) where Ni is the atom density of nuclide i and ~i is the microscopic (ct,n) cross section for nuclide i. The probability of (ct,n) interaction with nuclide i by an cx-particle that slowed from E. to E; is then (5) Thus, the probability of an a-particle undergoing an (cx,n)reaction with nuclide i before stopping in the material is given by the thick-target neutron production function, The stopping cross section (e) is defined as, (6) (7) wh&-eN is the total atom density of the material. The quantities pi and pi Cm now be expressed in terms of the stopping cross section 5 N. ‘a J Oi (E) PJEa+E’a)=# —dE ~,a s(E) and N. ‘“ q(Ea)=; j cri(E) —dE. ~ s(E) (8) (9) In general, any material involved in a homogeneous problem will be composed of any number of different elements (e.g., H, C, and O). The stopping cross section &(E) of a material composed of J elemental constituents may be calculated using the Bragg- Kleeman4relationship 4E) +,%(E) i– where N=@j” (10) (11) A fraction of the decays of nuclide k within a material may be via u-particle emission. This fraction (~k) of alpha decays may occur with the emission of one of L possible particle energies. The intensity PMis the fraction of all decays of nuclide k resulting in u-particle of energy El; and thus, cx- an (12) Therefore, the fi-action of nuclide k decays resulting in an (cx,n)reaction in a thick-target material containing I nuclides with non-negligible (ct,n) cross sections is Rk(a,n)=~f:~PJE1). 1=1 i=1 The value for Pi(El) will be determined using the discrete form of Eq. (9), (13) (14) where ~li=~i(0), ~Gi=~l(E1), &l=&(O),and &G=&(El)(i.e., the energy range has been discretized into G-1 energy groups). It is important to note that to calculate the (cx,n) 6 neutron source per decay of nuclide k, it is necessary to have accurate knowledge of the discrete-energy (ct,n) cross section for each target nuclide (d,), discrete energy stopping cross section (&g)for all elemental constituents, atom fraction (NilN) for each target nuclide, the intensity for emission of each of L a-particles (~kl), and the energy of each of the L a-particles (El). The atom fractions are provided by the user in a file named tapel. The other quantities are available to SOURCES from a number of library files (see Section m). The (ct,n) spectra are determined assuming an isotropic neutron angular distribution in the center-of-mass (COM) system5 with a library of 89 nuclide decay et spectra and 24 sets of product-nuclide level branching fractions. Figure 1 shows an illustration of a general (u,n) reaction in the laboratory system where any associated gamma ray is assumed to be emitted after the neutron is emitted. This assumption is identical to neglecting the momentum of any associated gamma ray, but it accounts for its energy. v,mn / ---S-lx- “-”-~ Vo,ma W Before ,-,-,....... ---- ~ "-'---"-"-"-"-"w-'-"-""`-"-`-`-`-`-`-`-"-`-"-" VC,mC Compound Nucleus 7 After Fig. 1. (ct,n) Reaction in the Laboratory System. Eq. (26) can be expressed easily in terms of the square root of the neutron kinetic energy as J%=’/g’ (27) where En,~ is the neutron kinetic energy in the laboratory frame of reference Ilom an incident et-particle of energy Eu and generating a product nuclei of level m. Thus the neutron kinetic energy is “.=[’=(*)’FZ-Z=31’28) where we have defined mn al=— ma and mn a~ –—— mr “ (29) (30) (31) Equation (28) relates the maximum (+ second term) and minimum (– second term) permissible neutron kinetic energies from an incident et-particle of energy E. generating a product nuclide with level m. For each target nuclide and each source et-particle, the code can read the number of product-nuclide levels (Mi), the number of product level branching data points (Mi), the (u,n) reaction Q-value (Qi), the excitation energy of each product-nuclide level [Ei.,(m)], ancl the fraction of (cx,n)reactions at energy E(m) resulting in the production of product level m [fi(um’)] from the library fdes. The neutron energy spectra will be discretized into a user-defined energy group structure. The fraction of target i product level m reactions of source k u-particles occurring in et-particle energy group g is 10 (32) where Pi(El) was defined in Eq. (14). The branching fraction of et-particles at Ea reacting with target nuclide i and producing product level m is ‘i,~ (m)= f~(“?m’–l) + (f/ (M’,.W’)– f~(Vi>.VZ ‘a - -’:1) . (33)‘–l)) ~(m~) – E(m–1) Thus, the fraction of et-particles at E. reacting with target nuclide i and resulting in product level m reactions occurring in cc-particle energy group g is simply the product of Eq.’s (32) and (33): ~:k (m)= Si,k(ln)H;,k(m) . (34) It will be assumed that the neutrons are isotropically emitted ftom the compound nucleus; therefore, they will contribute evenly to all groups between E+n,~and E-n,~. The contribution per decay of source nuclide k to neutron energy group g is given by (35) where E~+land E~are between E+~,~and E-~,~. 2. Spontaneous Fission Sources The spontaneous fission of an actinide nuclide k is accompanied by the emission of an average vk(SF) neutrons. The fraction of nuclide k decays that are spontaneous fision events are given by the SF branching fraction F? r ‘x” (36) Thus, the average number of SF neutrons emitted per decay of nuclide k (by any mode) is Rk(SF)= F~ Vk(SF) . (37) Therefore, to compute the neutron production due to spontaneous fission per decay of nucl.ide k, the SF branching fi-action and average number of neutrons per spontaneous fission must be known. These quantities are available to SOURCES from a library tile named tape5 (see Section III). The spontaneous- flssionmmtrcm.sra using two evaluated parameters (a and b): X~ (E)= R~(SF)e-~’a sinh~. are-approximated by-a Watt k -ilssionspectra (38) 11 Evaluated parameters areprovidedfor43 fissioning nuclides inthetape51ibrary file (see Section III below). 3. Delayed Neutron Sources During the fissioning process, a number of products are formed including neutrons, gamma rays, beta rays, neutrinos, fission products, and an appreciable amount of energy. Some of the fission products formed as a result of fission can decay by ~- emission to a highly excited state, which can then decay by emitting a neutron. These neutrons are called “delayed neutrons” because they appear within the system with some appreciable time delay. The nuclide emitting the neutron is referred to as the “delayed neutron emitter,” and the nuclide which @ decays to the emitter is referred to as a “delayed neutron precursor. ” It is customary to assume that one neutron is emitted per decay and that the emitter decays almost instantaneously. Thus, the fraction of decays by nuclide k (by any mode) leading to the emission of a delayed neutron is given by the product of the DN branching fraction (FDNk): Rk (DN) = FkDN. (39) Computiug the neutron production rate due to delayed neutron emission requires knowledge of the DN branching ffaction. The value for FDNkis provided to SOURCES in a library fde (see Section III). A series of evaluated delayed neutron spectra are provided in a library file for 105 precursor nuclides [~@)]. These evaluated spectra are provided in a discretized form. They are read directly into SOURCES and then adjusted so that the default spectra energy mesh correlates with the user-desired energy mesh. The energy spectra is then renormalized by multiplying through by the quantity Rk(DN), such that X~N(E) = R, (DN)P~ (E) . (40) 4. Total Neutron Source The average total number of neutrons per decay emitted due to (a,n) reactions, spontaneous fission, and delayed neutron emission is given by Rk = R, (a!,n)+Rk (SF)+Rk (DN). (41) 12 : I ~. .. 1 ... . . dA I -.. .... (w,s.,t ... :- ““”””””.. I I 4\ “A 1 1 /% &:-.--m---. .“ EO .-. *Y / . . .-. .. . . . .’ / . . . .“ . . . . .“:. ,“’L._— ~~“””””....:..””. . /. /. /. b’ x Fig. 5. The cx-ParticleSolid Angle to Differential Area Due to Generalized a Source. To derive the a-particle source rate at the interface, consider the half-space above the x-y plane shown in Fig. 5. There exists a uniform volumetric source (Sv) of a-particles in this half-space. The a-particles are assumed to be emitted isotropically with initial energy Eo. The differential area dA subtends a solid angle dfl when viewed from the source point dV. Thus, Cos@ dKl=-dA r2 and dV=r2sin@. dO. d@.dr. The rate at which u-particles are born in dV is equal to SvdV = SVr2sinq$”dO.d@.dr. (46) (47) (48) 15 The solid angle subtended by dA relative to the total solid angle into which cx- particles are emitted is given by dfi COSfb-t&i ‘= 4z”r2 “4Z (49) Multiplying Eq. (48) by Eq. (49) yields the number of et-particles per unit time originating within dV that can pass through dA provided that r is less than the u-particle range, or s, dU=— 4?r sin@cos@.dA. dO. d@.dr. (50) The rate at which u-particles pass through dA as a result of having been born in a hemispherical shell centered about dA whose radius is r and thickness is dr is acquired by integrating Eq. (50) over 6 and Q, or Performing the integration yields dU’= ~dA “dr. From Eq. (7) we see 1 dE dr = –—— N s(E) (51) (52) (53) where E(E) is the stopping cross section and N is the total atom density of the material in the region. Thus, the rate at which &-particles pass through the interface per unit area is given by dU’ SV 1—= _ dA ——dE . 4N s(E) (54) Therefore, the rate at which a-particles with energies between Eg and Eg+Ipass through the interface per unit area (Q) is S.. ?: dE-@-z=!L=_ _ A 4N j &(E) “ ~ The volumetric source (Sv)can be expressed as (55] 16 $ ‘4Nkf: where & is the decay constant for source nuclide k, Nk nuclide k, and PH is the fraction of all decays of nuclide energy Ekl. Thus we see that (56) is the atom density of source k resulting in an cx-particle of (57) This quantity (Qg) is the source of a-particles between energies Eg and E~+lpassing into the low-Z target material (Region II) per unit area and per unit time. The quantity @gis then used by SOURCES as the source strength of a monoenergetic beam with energy: Eg + Eg+l E:m = 2“ (58) SOURCES can then use the same procedure developed in Section 11.B to solve for the neutron production rate due to the a-particles crossing the junction with energies between E~ and Eg+l. SOURCES then repeats this procedure for all a-particle energies and all source nuclides. D. Three-Region Interface Problems A three-region (cx,n)problem consists of an u-emitting slab (such as Pu, Po, or Am) in direct contact with a thin slab of low-Z target material (such as Be, C, or Al) which is itself in contact with a thick (a,n) target (Fig. 6). In this particular problem, u-particles born in region A, can slow through region A to interface ab, slow through region B to interface bc, and slow to a stop in region C. Thus, neutrons can be produced in both region B and region C due to the slowing cx-particles. 17 calculate the neutron production rates and spectra from the u-particle source at interface be due to material B assuming region C is infinitely thick (Ygt!@). Then, we will calculate the neutron production rates and spectra from the a-particle source at interface be due to material C assuming region C is ‘infinitely thick (Ygh,c). The total neutron production rates and spectra due to the interface is then given by: Yg = !f?~,c+ (Y:b,,–Y:,B). These multigroup neutron source rates are then output to a file for the user. 20 (64) 111. FILE STRUCTURE The SOURCES 4A code system is composed of an F77 source code, an executable, an input file, several output files, and a series of library files. All of these fdes (except for the output files which SOURCES will generate) are necessary for proper execution of the SOURCES code. The name and a short description of each fde included in the SOURCES 4A code system are included below: tapel = user input file tape2 = stopping cross section expansion coefficients libra~ tape3 = target (qn) cross section library tape4 = target (~n) product level branching library tape5 = sources decay data library tape6 = neutron source magnitudes output file tape7 = absolute neutron spectra output file tape8 = normalized neutron spectra outputfile tape9 = neutrons spectra output file by product level Outp = summary output file SO URCE4A.for = F77 source code SOURCE4A.exe = executable code. Fig. 7 illustrates the sources code structure and how each fde interacts with the executable file. vtapel Fig. 7. Schematic Diagram of the SOURCES 4A File Structure. The data necessary for computing the magnitude of the neutron source due to (ct,n) reactions, spontaneous fission, and delayed neutron emission are: 1. The energy-dependent a-particle stopping cross section for all elemental constituents (&~i). 2. The energy dependent (cx,n)cross section for all target nuclides (di). 3. The intensity for emission of each of the L cx-particles (FN). 4. The energy of each of the L cx-particles(El). 5. The SF branching fractions for each source nucl.idek (l?F~). 6. The average number of neutrons per SF of nuclide k [v@?)]. 7. The DN branching fraction for each nuclide k (FDNk). 8. The source nuclide decay constants (Xk). To calculate the neutron source spectrum it is necessary to have data for: 1. The number of product nuclide levels (M) for all target nuclides. 2. The number of product nuclide level branching data points (M’) for all target nuclides. 3. The (cx,n)reaction Q-value for all target nuclides. 4. The excitation energy [E.X(m)]of product nuclide level m for all target nuclides. 5. The fraction of (cz,n)reactions with target i at energy E(m) resulting in the production of product level m. 6. The a-particle, neutron, target, and product nuclei masses. 7. Watt’s fission spectrum parameters (a and b) for each source nuclide k. 8. Delayed neutron energy spectrum for each source nuclide k. AUof these parameters are included in the library files. The library files contain (a,n) target nuclide cross section parameters for all the nuclei listed in Table I and source parameters for all the nuclei listed in Table II. These nuclides are listed in ZAID format and are defined as ZAID = state + (10A) + (1OOOO’Z),where Z is the atomic number, the atomic mass, and the state is either Oor 1 for ground or metastable, respectively. A is 22 Stopping-power coefhcients (which are a function of atomic number only) are included for all elemental constituents with Z <105. The data by Ziegler et d. 13was used for all Z <92. The stopping power coefficients calculated by Perry and Wilson2 were used for 92< Z S 105. 25 IV. INPUT AND EXECUTION L The SOURCES input is designed to be relatively simple; however, its length can vary over a wide range from exceptionally short (S 10 lines) to very long (2 50 lines). This large range depends on the number of nuclides (source and target) contained in the problem Appropriate knowledge of the physics (both macroscopic and microscopic) present in a problem is vitaI for proper execution of SOURCES (see the Po-Be sample problems in Section VI). All SOURCES input is free format with spaces as delimiters (or corns; however, spaces look better). The input deck should be created in a file named tapel for use by the SOURCES executable. Every SOURCES problem begins with the Same two cards: card 1: title card 2: idd id The first card is a title card with a maximum length of 77 characters. The second card contains two records (idd and i~, which define the type of problem to be considered (homogeneous, interface, or bea@ and the type of neutron source output to be produced (magnitudes only or magnitudes and spectra), respectively. The record idd can be either a 1 (for a homogeneous problem), a 2 (for an interface problem), or a 3 (for a beam problem). The record id can be either a 1 (for magnitudes only) or a 2 (for magnitudes and spectra). The remaining input cards depend upon the type of problem being considered. A. Homogeneous Problems (idd=l) A homogeneous problem must contain at least 8 cards which describe the elemental constituents in the material, the neutron energy group structure to be used in the output, the source nuclides present, the type of stopping cross sections to be used, and the (ct,n) target nuclides present. If multiple materials are present or neutron energy spectra are requested, then more cards can exist in the input deck. Cards 1-9 for a homogeneous problem areas follows: card 1: title card 2: idd id card 3: card 4.1- 4.nz: card 5: card 5.1- 5.nng: card 6: card 7.1- 7.nq: card 8: card 9.1- 9.nt: Multiple cards are designated nz isg jzm(j) azm(j) nng enmax enmin (if necessary) en(n) (if necessary) nq jq(k) aq(k) nt nag idt(i) at(i) by subcards (i.e., if nz=3, then the input deck would include card 3 and subcards 4.1, 4.2, and 4.3). Each card must be entered on a new line (the exceptions are subcards 5.1 through 5.nng where all records en(n) can be entered on the same line or multiple lines). Several example input decks are included below to illustrate the procedure described above. Each record is defined as follows: nz = isg = jzm(j) = azm(j) = nng = enmax = the number of stopping cross section elemental constituents present in the material (must be an integer between Oand 20). the type of stopping cross sections to be used (O for solid stopping cross sections, 1 for gas stopping cross sections). the atomic number of each stopping cross section elemental constituent from j=l to nz. the flaction of all atoms that are element j. the number of neutron spectrum energy groups (integer between 1 and 750 or between -1 and -750); read only if id=2, otherwise omitted. If nng is positive, then the energy group structure will be determined by a linear interpolation between enmax and enmin (cards 5.1 through 5.nng are omitted). If nng is negative, then the energy group upper bounds must be specitied on cards 5.1 through 5.nng (however, enmax and enmin must still be included). the maximum neutron energy in MeV (read only if id=2, otherwise omitted). 27 Exanmle Problem #3 - PuF4 Gas for Neutron Source Magnitude and Spectra. _le 3 - pI-@’4GzIsemsProbkn 12 21 9 0.8 94 0.2 20 15.0 0.0 6 9423802.13e+17 9423902.54e+21 9424001.65e20 9424107.39e18 9424208.99e17 9524104.37e18 12000 901900.8 Example Problem #3 illustrates the usage of a linearly interpolated energy structure (nng>O) and gas stopping power coefficients (isg=l). In this problerq the neutron source spectra and magnitudes (id=2) are determined for a PuF4 gas. This problem is reminiscent of possible criticality conditions in enrichment operations. The energy spectra have been established to include 20 groups (nng=20) linearly interpolated from 15.0 (emnax) to 0.0 (enmin) MeV. The problem includes five isotopes of plutonium (Pu-238, Pu-239. Pu-240, Pu-241, and Pu-242) and one isotope of americium (Am-241) as sources (nq=6). Also one isotope of fluorine (F-19) is included as an (ct,n) target (n*l). Note that due to its low concentration, Am was neglected as an elemental constituents. Thus, only two elemental constituents are present (Pu and F, nz=2) to slow the a-particles. Both of these elements will use gas stopping power coefficients in all calculations. The number of cx- particle energy groups (nag) used was 2000. The free-form input allows for the atom densities to be entered in any format (i.e., decimal, scientific with the + or – sign on the exponent, or scientific without the+ or – on the exponent) and for spaces to be used fi-eely to allow for easier reading by the user. Also, nuclides can be included as sources or targets and not appear as an elemental constituent. ‘-~he Outputs-fd$ some Of-the above ex~ple problems will be presented in Appendix A. Also, several sample problems that show other input decks are described in Section VI. 30 B. Interface Problems (idd=2) An interface problem input deck is divided into two sections, one for the source side and one for the target side. The deck must contain at least 13 cards that are used to describe the material constituents of both the source and target sides, the source nuclides present, the target nuclides present, the types of stopping cross sections to be used, the u- p@icle energy group structure to be used at the interface, and the neutron source energy group structure to be used in the output. The cards are described as follows: card 1: title card 2: idd id card 3: nzq isgq eamax eamin card 4.1- 4.nz: jzq(i) azq(l card 5: naq card 6: nq card 7.1- 7.nq: jq(k) aq(k) card 8: title2 card 9: nzt isgt card 10: jzt(k) azt(k) card 11: nng enmax enmin (ti.necessary) card 11.1- 11.nng: en(n) (if necessary) card 12: nt nag card 13.1- 13.nt: idt(i) at(i). Multiple cards are designated by subcards (i.e., if nz=3, then the input deck would include cards 3, 4.1, 4.2, and 4.3). Each subcard must be entered on a new line (the exception being cards 11.1 through 11.nng where all records en(n) can be entered on the same line or multiple lines). Each record is defined as follows: nzq = the number of stopping cross section elemental constituents present in the source material (must be an integer between O and 20). 31 lsgq = eamax = eamin = jzq(k) = azq(k) = naq = nq = jq(k) = aq(k) = title2 = nzt = isgt = jzt(k) = azt(k) = nng = the type of stopping cross sections to be used for source side (O for solid stopping cross sections, 1 for gas stopping cross sections). the maximum et-particle energy for u-particle source at interface. the minimum a-particle energy for u-particle source interface. the at the atomic number of each stoppiug cross section elemental constituent from j=l to nzq for the source side. the fraction of all atoms on source side that are element k. the number of u-particle energy groups (integer between 1 and 4000) for u-particle source at the interface. the number of source nuclides to be evaluated (integer value between 1 and 300). the source nuclide k identiilcation in ZAID format (see Section III). the fi-action of all atoms on source side that are source nuclide k. title record for the target side (maximum of 77 characters). the number of stopping cross section elemental constituents present in the target material (must be an integer between O and 20). the type of stopping cross sections to be used for the target side (O for solid stopping cross sections, 1 for gas stopping cross sections). the atomic number of each stopping cross section elemental constituent from j= 1 to nzt for the target side. the fraction of all atoms on the twget side that are element k. the number of neutron spectrum energy groups (integer between 1 and 750 or between -1 and -750); read only if id=2, 32 Exanmle Problem #5 - Am-A1132Interface Calculation for Neutron Source Magnitudes and Spectra. _le 5 - &n-AIB2IhterfaceFroblem 21 1 0 6.50 0.000001 95 1.0 52 1 9524101.00 targetis ccnpased 20 of AlB2 13 0.333333 5 0.666667 3 4000 501000.132667 501100.534000 1302700.333333 C. Beam Problems (W=3) The input deck for abeam problem is traditionally simpler than that for an interface or homogeneous problem because the problem is devoid of any source nuclides to describe. A beam problem must contain at least eight cards that describe the elemental constituents in the material, the neutron energy group structure to be used in the output, the et-particle beam energy, the type of stopping cross sections to be used, and the (ct,n) target nuclides present. If multiple materials are present or a neutron energy spectrum is requested, then mo~e cards can exist in the input deck. Cards 1–8 are as follows: card I: card 2: card 3: card 4.1- 4.nz: card 5: card 5.I - 5.nng: card 6: card 7: card 8.1- 8.nt: title idd id nz isg jzm(j) azm(j) nng enmax enmin en(n) (if necessary) ebeam nt nag idt(i) at(i) (if necessary) Note that multiple cards are designated by subcards (i.e., if nz=3, then the input deck would include cards 3, 4.1, 4.2, and 4.3). Each subcard must be entered on a new line 35 (the exception being cards 5.1 through 5.nng where all records en(n) can be entered on the same line or multiple lines), Each record is defined as follows: nz = isg = jzm(j) = azm(j) = nng = enmax = enmin = en(n) = ebeam = nt = nag = idt(i) = the number of stopping cross section elemental constituents present in the material (must be an integer between Oand 20). the type of stopping cross sections to be used (Ofor solid stopping cross sections, 1 for gas stopping cross sections). the atomic number of each stopping cross section elemental constituent from j = 1 to nz. the fkaction of all atoms that are element j. the number of neutron spectrum energy groups (integer between 1 and 750 or between -1 and -750); read only if id=2, otherwise omitted. If nng is positive, then the energy group structure will be determined by a linear interpolation between enmax and enmin (cards 5.1 through 5.nng are omitted). If nng is negative, then the energy group upper bounds must be specilled on cards 5.1 through 5.nng (however enmax and enmin must still be included). the maximum neutron energy in MeV (read only if id=2, otherwise omitted). the minimum neutron energy in MeV (read only if id=2, otherwise omitted). the upper energy bound in MeV of neutron groups, listed in descending order (must contain nng records in any format); read only if id=2 and nng is negative (otherwise omitted). the cx-particlebeam energy in MeV. the number of target nuclides (integer value between 1 and 20). the number of cx-particleenergy groups to be used in calculation (integer value between 1 and 4000). the target nuclide i identflcation in ZAID format (see Section III). 36 Exanple6 - AlphaRam (5.5MeV) on Si02 32 20 8 0.666667 14 0.333333 -22 10.0 0.0 10.007.00 6.005.50 5.00 4.50 4.00 3.50 3.25 3.00 2.752.502.252.00 1.751.50 1.25 1.00 0.75 0.50 0.25 0.10 5.5 44000 801700.000253 801800.001333 1402900.015567 1403000.010333 D. Three Region Interface Problems (zW=4) Athree-region interface problem input deck is divided into four sections. The first section contains information regarding the energy and angular grids to be used in the calculations. The rernaining three sections pertain toeach ofthethree slab regions. The deck must contain at least 15 cards that are used to describe the u-particle energy grid at each interface, the neutron energy grid for the output, the angular grid, material constituents for all regions, the source nuclides present, the target nuclides present, and thetypes of-stoppingcross-sections to+beuseci. Tiiecarcis are-descrill-d”as to~ows:.-—- card 1: title at(i) = theflactionofall atoms thataretarget nuclidei. Example Problem#6 below illustrates the procedure described above. This beam problem (idd=3) consists of a slab of silicon dioxide bombarded by 5.5 MeV a-particles (ebeam=5.5). Solid stopping cross section values (isg=O) are used for the two elemental constituents (rzz=2)present h the problem (Si and O). The problem solves for the neutron source magnitudes and spectra (id=2) resulting from four (rzt==)target isotopes (O-17, O- 18, Si-29, and Si-30). The outputs for this problem will be presented in Section V. Also several sample problems that show other input decks are described in Section VI. Examrde Problem #6 -5.5 MeV et-particle Beam Incident on a Slab of Silicon Dioxide. 37 n.zb= isgb = anumb = t = jzb(k) = azb(k) = ntb = idb(i) = atb(i) = title3 = nzc = isgc = jzc(k) = azc(k) = ntc = idc(i) = the number of stopping cross section elemental constituents present in region B (must bean integer between Oand 20). the type of stopping cross sections to be used for the target side (O for solid sections). the atomic stopping cross sections, 1 for gas stopping cross number density of all materials in region B (in atoms/b-cm) the thickness (in cm) of region B. the atomic number of each stopping cross section elemental constituent from j= 1 to nzb in region B. the tiaction of all atoms in region B that are element k. the number of target nuclides (integer value between 1 and 20). the target nuclide i identtilcation in ZAID format (see Section III) for targets in region B. the fraction of all atoms in region B that are nuclide i. title record for the region B (maximum of 77 characters). the number of stopping cross section elemental constituents present in region B (must bean integer between Oand 20). the type of stopping cross sections to be used for the target side (O for solid stopping cross sections, 1 for gas stopping cross sections). the atomic number of each stopping cross section elemental constituent from j=l to nzb in region B. the fraction of all atoms in region B that are element k. the number of target nuclides (integer value between 1 and 20). the target nuclide i identtilcation in ZAID format (see Section III) for targets in region B. the fiwticm .ofall atoms in-region ‘Bthat arenuclkk-i. I Two example input decks are listed below. These examples illustrate the proper usage of the cards and records described above. 40 The f~st example problem consists of a slab of weapons grade plutonium (WPu) adjacent to a slab of Be with a thin layer of Al for region B. The problem has idd=4 to signify a three-region interface problem and id=2 for a magnitudes and spectra solution. The WI% consists of 5 isotopes of Pu (Pu-238, Pu-239, Pu-240, Pu-241, and Pu-242) and one isotope of Am (Am-241) as a contaminant. Thus, rzzq=2 (for Pu and Am), and nq=6 (for the six isotopes of Pu and Am). Solid-stopping cross sections were used for all regions (isga=isgb=isgc= O). The a-particle energy structure at each interface consists of 400 groups linearly interpolated between 6.50 and O.0000001 MeV. Region B is composed of Al metal [nzb=l and ntb=l (for Al-27 only)] with a density of 0.15 atoms/b- cm and a thickness of 1 mm. Region C is composed of beryllium metal, thus nzc=l and ntc=l (for Be-9 only). The neutron energy group structure is defined to contain 20 groups linearly interpolated between 10.0 and 0.0 MeV. Forty angular groups are used at each interface (ncg=40). Example Problem #7 - Weapons Grade Pu-Al-Be Interface Source Calculation for Magnitudes and Spectra. _le +17(Wu-Xl-Ee) 42 400 6.5 0.0000001 20 10.0 0.0 40 WPu region 20 94 0.9998 95 0.0002 6 9423800.0005 9423900.9233 9424000.0650 9424100.0100 9424200.0010 9524100.0002 Al interface 1“0 0.5 0.1 13 1.0 1 1302701.0 Be reflector 10 4 1.0 . Example Problem interfaced with an AlB2 #8 models a problem with a pure plate with a small (3.0 cm thick) COZ 41 Am-241 source material gap. This example solves for the neutron source magnitude and spectra (id=2) using only Arn-241 as the source material (~z~= 1). Region B consists of two elements (rzzb=2) and three (cqn) target nuclides (ntb=3). Gas stopping powers are used in region B (isgb= 1). The target material in region C is made of Al and B (nzc=2). Note that the elemental constituents can be entered in any order [i.e., Al (Z= 13) before B (2=5)]; however, the target isotopes must be in increasing ZAID order. Three (a,n) target isotopes are present in region C: B-10, B-l 1, and Al-27. The outputs for all of the above examples will be presented in Appendix A. Example Problem #8 - Arn-C02-AJB2Interface Calculation for Neutron Source Magnitudes and Spectra. E=m@e 8 - Arn-CD2-AlB2InterfaceI?mblen 42 400 6.5 0.0000001 20 10.0 0.0 60 PureM-241 in regicmA 10 95 1.0 4 -L 9524101.0 C33.2gasinregim B 2 1 0.0043.0 6 0.333 8 0.667 3 601300.0073333 801700.0002667 801800.0013333 AI.B2shieldin regionC 20 13 0.33333 5 0.66667 3 501000.132667 501100.534000 1302700.333333 E. Execution To execute the SOURCES 4A code, one needs to run the executable file. The input deck must be named tapel, and the library files must be named tape2 through tape5. After execution, SOURCES 4A will display a STOP message informing the user whether 42 The tape7 fde lists the absolute neutron spectra (i.e., neutrons per second per unit volume per unit energy). This file fust lists the multigroup neutron spectra (i.e., the energy bounds) used in the calculations in decreasing order. The absolute neutron spectra listed by neutronhrget combination is then displayed in order coinciding with the group structure speciiled at the beginning of the file. Totals per target nuclide for (ct,n) reactions, totals for all (a,n) reactions, totals for spontaneous fission neutrons, and totals for delayed neutrons are akio listed (if applicable). The file tape8 is similar to tape7 except that normalized neutron spectra are reported. Tape9 lists the energy dependent neutron production rates by target nuclide and by product nuclide level. Again, the neutron energy group structure is first listed and then a breakdown of the neutron energy spectra with target nuclide totals is reported. The tape7 output files corresponding to the input files listed in Example Problems #2, #4, and #6 are included in Appendix A, as well as the tape9 output files for Example Problems #2 and #4. 45 I W. SAMPLE PROBLEMS Several sample problems are listed below. These problems are available to aid the user in construction of the input deck (tapeI). The problems were executed to model experimental arrangements, and measured data are reported with the SOURCES 4A calculation when available. A. Homogeneous Mixtures I. Sample Problem #I This problem illustrates the neutron source magnitudes and spectra from a PuBel~ source (elemental constituents are 13/MBe and l/M Pu) with six isotopes of Pu (Pu-237, Pu-238, Pu-239, Pu-240, Pu-241, and Pu-242) and one isotope of Be (Be-9) present. The ex~ple solves for the magnitude and spectra (id=2) and uses a 48 group neutron energy structure (nng=48) which is linearly interpolated between 12.0 (erzmax) and 0.0 (erzmin) MeV. The six Pu isotopes are used as sources (nq=6), and the one beryllium isotope is the target (rzt=1). The atom fractions and densities can be entered in scientiilc notation or decimal notation. This input deck is an appropriate model of the experimental measurement performed by L, Stewart. 14 Sanple 1 - PuBe13 .S’Ource (Stewart,1953) 12 20 4 0.928571 94 0.071429 48 12.0 0.0 6 94237013144.0 9423807.08e+17 9423905.82e+21 9424003.74e+20 9424101.69e+19 9424201.22e+18 1 4000 400900.928571 Fig:-8;-s&npEPrObk?nI-#Hn-puFWCk: 46 1.6E-I-04~ .............. .”..-— ”.”.... ,,-’, .. ...... .... , 1.4Et04 2.0E+03 O.OE-I-00~/, ,,, ,,, .,, ,,, ,,, ,,, ,,, ,\,, ,,, ,t 0.0 2.0 4.0 6.0 8.0 NeutronEnergy(MeV) Fig. 9. Energy-Dependent Neutron Source Strength 10.0 12.0 14.0 inPuBe13Homogeneous Problem as Calculated by SOURCES 4A and Compared with Measured-Data. A comparison of the data measured by the experimenters and the SOURCES 4A calculation is presented in Fig. 9. To construct this plot, the histogram output from SOURCES 4A was converted to a continuous distribution using the midpoint energy for each energy group. This conversion was repeated for all energy-dependent neutron source plots in this section (i.e., Figs. 8, 11, 14, 16, 18, and 20). The total neutron source magnitude calculated by SOURCES 4A was 2.69x105 neutrons/s-cm3, whereas the experimenters reported a total neutron source rate of 2.28x105 neutrons/s-cm3. This magnitude of agreement (A17$ZO)is standard for a SOURCES 4A calculation. From Fig. 9, reasonable agreement between the SOURCES 4A spectrum calculation and the measured values is found. The calculation neglected any source contaminants (esp., Am- 241), because they were not specitled in the published experiment. 47 An analysis of the data in Fig. 12 shows that the SOURCES 4A calculation appears to overestimate the average neutron energy produced tiom the sample. On further analysis, it can be found that a Po-Be source, though a mixture of ot-ernitting material and (ct,n) target material, is composed of grains of Po and grains of Be. These grains have an average diameter signitlcantly larger than the cx-particlerange. Thus, it is postulated that a Po-Be source is more properly modeled as an interface problem of Po and Be. This theory is supported by the calculations performed in Sample Problem #5. The outcome of this problem is extremely important. A SOURCES 4A user must be aware of the physics inherent in any problem Mng modeled. In the case of Sample Problems #1 and #2, the materials were compounds (PuBel~ or U02FZ). Thus, the u- emitting nuclides and (cx,n)target nuclides are intimately mixed. For Po-Be, the material has a tendency to clump into grains, and the grain size of the metals can and will affect the outcome of the calculations. Therefore, it is imperative that a user analyze the nature of any problem under consideration prior to constructing the input deck. B. 1. Interface Problem Examples Sample Problem #4 In 1944, a study was conducted by Perlman et aL17 at Los Alamos chemical National Laboratory to explore the possibility of using an (a,n) neutron source to simulate a fission neutron spectra. In this study, a series of platinum foils (3 x 3 cm in size) were coated with 180 mCi of Po and then interleaved between sintered B4C slabs. The entire PO-B4C ass~mbly was placed in a brass box and sealed under a slight vacuum. The resultant neutron energy spectra from the source was measured using the photographic emulsion met,hod. A schematic of the experimental setup is shown in Fig. 13. 50 Brass To model Po Foil’ B4C Fig. 13. Po-BICSource Arrangement for Sample Problem #4. this arrangement, a SOURCES 4A input deck (Fig. 14) was constructed which employed the interface problem capabilities of SOURCES 4A (k?d=2). The atomic fraqtions listed in the input deck show that natural boron (19.9% B-10 and 80.1% B-11) and carbon (98.9% C-12 and 1.1% C-13) were used. Also the a-particle source was set to include 100% Po-2 10. This input deck was executed to solve for the neutron source spectra and magnitudes (id=2) resulting from the (ct,n) interactions in the boron carbide. San@e 4- W-MC Interface Experiment (Perlm3n, 1944) 22 1 0 10.0 0.0000001 841.0 50 . .L 8421001.00 tametba Wsl* 20 5 0.8 6 0.2 50 10.0 0.0 3 4000 501000.1592 501100.6408 601300.0022 Fig. 14. Sample Problem #4 Input Deck. 51 4.0E+05— ----...”...”--...” -..—...”-..-—— —-—— -------------- 3.5E-t05-- c g 3.OE-I-05 g 1 i! $ 2.5E+05-- -F G 2.0E+05-- % if’ $ 1.5E+05-- 8 s $. 1.0E+05-- [ 5.0E+04-- EEH O.OE+OCI 1 I , 1 0.0 1.0 2.0 3.0 4.0 5.0 6.0 NeutronEnergy(MeV) Fig. 15. Energy-Dependent Neutron Source Strength in PO-B4CInterface Problem as Calculated by SOURCES 4A and Compared to Measured Data. J The energy spectrum of the calculated PO-B4Cneutrons is plotted in Fig. 15, along with the measured data. The agreement between the measured data and the SOURCES 4A calculation is reasonable and typical of an interface problem. The addition of contaminants and important to note the samples. other Po isotopes could greatly affect the shape of this spectrum. It is that the researchers did not specify the presence of any contaminants in 2. Sample Problem #5 In Sample Problem #3, a Po-Be source was investigated as a possible homogeneous problem. It was discovered that the energy spectrum of the neutrons calculated by KJUIRXK 4A was shifted-to a higher average energy than what was reported by the experimenters. 16 It was suggested that this shift was due to the grain structure of the materials in Po-Be sources. To verify this hypothesis, SOURCES 4A was used to model 52 45000.O 40000.O 35000.O 2 1000O.O 5000.O 0.0 L E#/<‘ I1 1 11 -jlf’-11 ‘[i 1 1 , 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 NeutronEnergy(,MeV) Fig. 19. Energy-Dependent Neutron Source Strength from 5.0 MeV cx-Particles Incident on Aluminum Oxide Slab as Calculated by SOURCES 4A and Compared to Measured Data. The total neutron yield per incident a-particle was reported by the experimenters to be 1.58x10-7 neutrons/a-particle.18 The SOURCES 4A calculation output a value of 1.63x10-7 neutrons/cx-particle. The measured average neutron energy was 1.14 * 0.04 MeV whereas SOURCES 4A reported an average energy of 1.35 MeV. As can be seen, the total neutron yields agree to within 470. The energy-dependent neutron spectra are plotted in Fig. 18. The neutron spectra have a few small discrepancies, and the average neutron energies show an 1870 difference; however the agreement is generally very good. The SOURCES 4A total neutron yields consistently have better agreement to measured data than the spectral calculations. However, as is shown here and in Sample Problem #7, for beam problems the SOURCES 4A calculations are excellent for both magnitudes and spectra. 55 2. is Sample Problem #7 The input deck used for modeling the magnesium irradiation by 5.5 MeV a-particles shown in Fig. 20. The magnesium sample contains two naturally occurring isotopes (Mg-25 and Mg-26) as (cx,n)target nuclides. The isotope Mg-24 was neglected due to its negligible (a,n) cross section. The neutron energy group structure consisted of81 energy groups in 0.1 MeV bins. Sanple7- AlphaE?ean(5.51.@V)onE@ 32 10 12 1.0 81 8.15 0.05 5.5 2 4000 1202500.10 1202600.1101 Fig. 20. Sample Problem #7 Input Deck. 1.6E+051 1.4E-I-05I ~1.2E+05 4.0E+04 2.OE-I-04 L t O.OE+OO — I , , m,1 1 # I o 1 1 0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 NeutronEnqy (MeV) Fig. 21. Enagy-llepaukmt ..NeutronSource Strength from 5.5 -N%VW%rtMes- Incident on Magnesium Slab as Calculated by SOURCES 4A and Compared to Measured Data. 56 The total neutron yield per incident a-particle was reported by the experimenters to be 1.33x104 neutrons/cwparticle.18 The SOURCES 4A calculation output a value of 1.27x104 neutrons/a-particle. The measured average neutron energy was 2.85 & 0.12 MeV, where as SOURCES 4A reported an average energy of 3.04 MeV. The total neutron yields agree to within 5%. The energy-dependent neutron spectra are plotted in Fig, 21. The measured and calculated neutron energy spectra have excellent agreement (within experimental error). 57 8.’ J. K. BAIR and J. GOMEZ DEL CAMPO, “Neutron Yields from Alpha Particle Bombardment,” NUCLSci. and Eng. 71, page 18 (1979). 9. J. K. BAIR and F. X. HAAS, “Total Neutron Yield from the Reaction 13C(~,n)1b0and 17’lsO(ct,n)20’21Ne,”Phys. Rev. C 7, page 1356 (1973). 10. D. L. LESSOR and R. E. SCHENTER, “Neutron Spectra from (ct,n) Reactions in Plutonium Compounds Calculated from Hauser-Feshbach Reaction Theory,” Brookhaven National Laboratory report BNWL-B- 109 (1971). 11. M. BALAKRISHNAN, S. KAILAS, and M. K MEHTA, “A Study of the Reaction 19F(~,n)22Nain the Bombarding Energy Range 2.6 to 5.1 MeV,” Pramana 10, page 329 (1978). 12. S. E. WOOSLEY, W, A. FOWLER, J. A. HOLMES, and B. A. ZIMMERMAN, OAP-422 (1975). 13. J. F. ZIEGLER, “Helium Stopping Powers and Ranges in All Elemental Matter,” Vol. 4 of The Stopping Power and Ranges of Ions in Matter Series, Pergammon Press, New York (1977). 14. L. STEWART, “Determination of the Neutron Spectrum and Absolute Yield of a Plutonium-Beryllium Source,” Master’s Thesis, University of Texas (1953). 15. R. L. SEALE and R. E. ANDERSEN, “Intrinsic Neutron Source Strengths in Uranium Solutions,” Trans. Am. Nucl. Sot. 63, page 226 (1991). i ~-o”L. S~ECK ~“c--H:~C~Ej”mfiergy S“pectmmof~o-~e Neutrons,” Los Alamos Sci@fic Laboratory report LA-111 (1944). 60 17. T. H. PERLMAN, H. T. RICHARDS, and J. H. WILLIAMS, “The Energies of the Neutrons horn Polonium Alphas on Boron,” Los Akunos Scientific Laboratory report LA- 66 (1944). 18. G. J. H. JACOBS and H. LISKIEN, “Energy Spectra of Neutrons Produced by a- Particles in Thick Targets of Light Elements,” Ann, Nucl, Energy 10, page 541 (1983). 61