LA--UR--98--5379

White Rock Science

Los Alamos National Laboratory, Group T-2

Introduction

The widespread use of radionuclides in medical and industrial applications is steadily increasing, leading suppliers to seek out new production facilities. A reliable supply chain is necessary to both encourage new applications and to replace aging production sources. The United States, in particular, faces a domestic production shortfall. Among the possibilities for radionuclide production are high power accelerators, either purpose built or alongside existing applications. As an example of the latter, a recent study by the Medical University of South Carolina¹ discussed the production of medical radioisotopes at the proposed Accelerator Production of Tritium Facility (APT). The report considered a large number of radioisotopes for present or future applications in medical treatment and diagnostic procedures. Many of these radioisotopes could be produced in the intense and energetic neutron and proton fluxes characteristic of the APT target and blanket assembly.

We have undertaken a study to see to what extent existing nuclear data models are applicable to calculations of radionuclide production in a high energy, high power environment. We chose the APT target/blanket assembly as a typical environment in which to study isotope production. In addition to the availability of existing input models for our Monte Carlo flux calculations, the high energies of the neutron and proton fluxes offer a formidable test of the nuclear data. In a previous report², we considered the production of two radioisotopes---¹⁸F and ¹³¹I---at two locations in the APT blanket. We have extended that study to look at the production rates of 22 isotopes in nearly 500 locations throughout the APT target and blanket. In addition to the 100 milliamp 1.7 GeV proton beam energy assumed in the previous study, we also treat beam energies of 1.0, 1.2, 1.4, 1.6, and 1.8 GeV (all at 100 milliamps).

The production rate of a radioisotope can be obtained from the integral of the flux and cross section leading to the direct production of the radioisotope as a reaction product. Additional production is realized from other radionuclides that decay to the desired product. Evaluation of production rates then requires knowledge of the neutron and proton fluxes at some position in the APT assembly and cross sections leading to production of the desired radionuclide and its progenitors. Ideally, one would use a cross section library that includes data for all nuclides in the neighborhood of the desired product in a transmutation code. Construction of such a comprehensive library is beyond the scope of the present study. Instead, we evaluated cross sections for reactions most likely to lead to our desired products.

Figure 1 is a 3-dimensional rendering of a computer model of the target/blanket assembly. The model is based on the Todosow geometry with a 16 cm x 160 cm beam area. The beam enters the assembly through the green window on the right. The beam strikes tungsten-dominated ladders in the center of the assembly; the position of these ladders correspond to the pipes extending from the yellow manifolds. The lateral blankets extend to either side of the beam line and ladder area. The downstream blanket encompasses the region to the left of the ladder region, while the upstream blanket is the narrow area between the entrance window and the ladder region.

We used MCNPX version 2.1.1 to calculate neutron and proton fluxes throughout the APT model, excluding pipes and ladders within the beam cavity and some regions at the bottom of the model. We made runs for beam energies of 1.0, 1.2, 1.4, 1.6, 1.7, and 1.8 GeV, assuming a 100 milliamp beam in each case. For each case, the runs followed 120,000 incident protons.

Tally Locations. Subdivision of cells in the upstream, downstream, and lateral blanket regions and in the beam cavity between ladders yielded approximately 183 cells in which the fluxes were tallied. Figure 2 shows the cell locations in a slice in the X-Z plane through the middle of the target model. We did not tally fluxes in the ladders (red areas in Figure 1), nor in the coolant pipes (green areas).

Assuming reflective symmetry in the X direction (where Z is the beam direction and Y is the vertical direction), tallies were taken over the union of a cell on the -X side of the beam with its +X counterpart. Three such pairs of symmetric cells are shaded light gray, yellow, and light blue in Figure 2. Accounting for cells symmetric about X = 0, there were 102 tallies.

Vertical Segments. We segmented the tallies into five equally spaced vertical segments. Figure 3, a cross section through the upstream blanket, shows the segmenting. Segment 1 is the topmost segment, Segment 3 is in the middle position, and Segment 5 is the lowest. With a few exceptions, all cells in the blankets extend the full height of the target, while the cells in the beam line do not extend to the topmost and lowest segments. The fluxes, and hence production rates, were greatest in the middle segment (3), decreasing towards the top and bottom. Unless otherwise stated, the results presented here are for the middle segment.

Locations for Detailed Results. We present detailed summaries of the production rates at four locations. Two of these locations, in the upstream (light blue cells in Fig. 2) and downstream (orange cell) blankets, were chosen to overlap the locations used in the previous study. The other two locations, one behind the fifth ladder in the beam cavity (dark blue cell in Fig. 2) and the other opposite the sixth, seventh, and eighth ladders in the first lateral blanket row (yellow cells), are the locations of maximum production rates for a great majority of reactions. For each reaction, we found the cell with the maximum rate over our entire set of tallies, and the cell with the maximum rate over all tallies excluding the beam cavity cells. In approximately 80% of the reactions, this procedure picked our two selected locations. For all but a handful of the remaining reactions, the picked cells were a neighbor of the selected locations.

We did not model any fixtures, such as irradiation tubes, that would be required to produce radioisotopes in any location. Our results thus assume any such fixtures would have no effect on the fluxes.

Figures 4a, 5a, 6a, and 7a show the neutron fluxes for the locations in the upstream blanket, downstream blanket, beam cavity, and lateral blanket, respectively; Figures 4b, 5b, 6b, and 7b do the same for the proton fluxes. Each panel contains a flux plot for each of the five vertical segments (except for Fig. 6 where the beam cavity cell does not extend to the top and bottom segments). The vertical scale is the same for all plots on a panel. Each plot shows the flux for each of the beam energies we considered according to the legend at the top of the panel. In the upper left of each panel is a slice in the X-Z plane through the center of the model, similar to Figure 2. The diagram shows the current location filled with red.

Flux Color Contours on Planes. We prepared a variation of the target geometry for display of the fluxes (and production rates) as color-coded contours on a plane. We introduced 5 planes to represent the 5 vertical segments; the vertical position of the plane lies at the center of the corresponding segment. Each plane thus represents the flux averaged over a segment. For visual orientation, we included the ladders, top manifolds, and other elements in the revised geometry; these elements, show n in gray in the plots, represent cells in which we did not take a flux tally. (This revised model is used for visualization only; it was not used in the flux calculations.)

Figure 8 is a view of the target from the side where the planes are seen edge-on as 5 horizontal lines. Figure 9 is a view of the full target model (without surrounding shielding) from the same perspective as is used in the contour plots that follow.

Figures 10 through 21 are color contour plots of the neutron and proton fluxes for each of the beam energies.

Our earlier work involved the modeling of neutron- and proton-induced reactions on isotopes of O, F, Ne, Na, Mg, Al, Xe, Cs, Ba and La for energies to 1.7 GeV. We have now added reactions on isotopes of S, Cl, Ar, K, Zn, Ga, Ge, As, Zr, Nb, Mo and Hg.

In the earlier and present efforts, neutron and proton reactions below 150 MeV were modeled with the HMS-ALICE code^3,4; at higher energies, the CEM95 code^5,6 and the LAHET code system^7,8 were used. For neutron reactions below 20 MeV, the data of the European Activation File EAF-97, Rev. 1^9,10 and certain improvements by M. Herman¹¹, labeled as Mike, were also used where applicable. The resulting compilations typically show significant disagreements at energies where the available data progresses from one source to another, as seen in the graphical cross-section data representations for neutrons in Appendix A and for protons in Appendix B. There we use a broad gray line to indicate our choices of the energy-dependent cross section; these choices were made with the assistance of M. B. Chadwick.

Cross-sections for production of the isomeric states ^193mPt and ^195mPt are problematic. Only the EAF-97, Rev. 1 tables give explicit values for excited state end products. (We will explore methods of obtaining isomer production data with the LAHET code system in future work.) The EAF-97 tables are the basis of our neutron data for these two isotopes. For proton reactions, we scaled the ¹⁹³Pt and ¹⁹⁵Pt CEM95 cross sections by a factor 0.5. This scaling factor was suggested by the EAF-97 data and agrees with an experimental data point for ¹⁹⁷Au ^195mPt.

Measured proton cross-section data, where available, are included in the representations and were used in the evaluation considerations. Although we considered experimental data from a large number of sources, the data suitable for the reactions of interest are from the NUCLEX¹² compilation. Some of the data are for "cumulative" reactions that include more than a single end product. We did not factor the cumulative data points into our evaluations. Much of the experimental data is for natura l element targets. Comparing such data with synthesized natural element cross sections would serve as a check on the cross-section calculations for a wider range of reactions than the 14 for which we show data points.

Our cross section tables reach to a maximum energy of 1.7 GeV. Between 1.7 and 1.8 GeV, we used the value at 1.7 GeV. Because most cross sections are relatively independent of energy at these high energies, this assumption should not be far removed from reality.

We consider production rates for the following 20 end product nuclides:

To produce these nuclides, we calculated neutron and proton reactions on the stable, naturally occurring isotopes of elements in the neighborhood of the targets investigated. These 70 nuclides of 25 elements are:

For each reaction, we

1. constructed a continuous energy representation of the cross section from the evaluation tables;
2. formed a flux-weighted average cross section for each particle flux at each location;
3. computed the one-hour irradiation end product P production rate per gram of target to each target nuclide / reaction product p radionuclide combination;
4. formed the one hour irradiation production rate per gram of target nuclide or naturally occurring element.

The flux-weighted cross section _tp for each target t and reaction product p is found by

The cross section _tP leading to end product P is taken as the sum of all cross sections for the direct production of P and products p decaying to P. The cross section _ZP for element Z leading to end product P is obtained as the natural-abundance-weighted sum of the cross sections _tP of the various naturally occurring target nuclides of the element. For intermediate nuclides that have multiple decay paths, we multiply the rate to account for the branching factors to the desired end product.

The production rate R_tP (Ci/g-hr) for each target t --- end product P combination is

,

where is the decay constant (s^-1) of end product P, T = 3600s corresponds to a one-hour irradiation, N_t = N_o/A_t is the atom density (atoms/g) of the target material, N_o is Avogradro's number (6.022 x 10²³ atoms/mole), and A_t is the atomic weight of the target. A_t is taken as the integer mass number for isotopic targets and as the atomic weight for the elements.

Table 1 lists the reactions we included, grouped by end product P. On each line, the target nuclide is followed by "n", "p", or "n p", to denote neutron and proton reactions. The decay path to the end product follows. Between each nuclide in the decay we list the branching fraction. The "Factor" on the left-hand side is the product of all branching fractions.