Design and validation of a fission-product source for radioactive beams

A neutron-generator-based source of fission products (nuCARIBU) is being developed to replace the existing ${}^{252}\mathrm{Cf}$ source at Argonne National Laboratory. Neutrons generated by ${}^7\mathrm{Li}(p,n){}^7\mathrm{Be}$ reaction are thermalized in a compact polyethylene [(${\mathrm{C}}_2{\mathrm{H}}_4{)}_n]$ moderator and impinge on a thin ${}^{235}\mathrm{U}$ foil to produce fission products. Experiments have been conducted to verify the nuCARIBU concept and simulation results using a 4 MeV, $0.16\mu \mathrm{A}$ proton beam from the ATLAS accelerator irradiating a LiF neutron production target and two different simplified versions of the nuCARIBU (${\mathrm{C}}_2{\mathrm{H}}_4{)}_n$ moderator with thicknesses of 3.81 cm and 1.91 cm separating it from the enriched ${}^{235}\mathrm{U}$ fission foil. The emitted $\gamma$-ray peaks of 1383.93 keV (${}^{92}\mathrm{Sr}$), 358.0 keV (${}^{104}\mathrm{Tc}$), 565.992 keV (${}^{134}\mathrm{Te}$), and 258.411 keV (${}^{138}\mathrm{Xe}$) obtained from fission products are used to determine the neutron-induced fission rates in the fission foil. The measured fission rates are in good agreement with those simulated using the PHITS Monte Carlo code with user-defined neutron production cross sections based on well-known cross section data. These experimental results, consistent with the simulations, show that the current ${}^{252}\mathrm{Cf}$ source (CARIBU) can be replaced by the neutron induced fission source (nuCARIBU).

Figure 1

Schematic layouts of CARIBU (a) and nuCARIBU (b). Comparison of the mass distribution for the spontaneous fission products for ${}^{252}\mathrm{Cf}$ and the neutron induced fission for ${}^{235}\mathrm{U}$ (c).

Figure 1

Schematic layouts of CARIBU (a) and nuCARIBU (b). Comparison of the mass distribution for the spontaneous fission products for ${}^{252}\mathrm{Cf}$ and the neutron induced fission for ${}^{235}\mathrm{U}$ (c).

Figure 2

Geometry for simulations.

Figure 2

Geometry for simulations.

Figure 3

Total neutron production cross section vs proton energy (left), emitted neutron energies (middle), and double differential cross sections (right) as functions of the laboratory incident proton energy and neutron emission angle for the reactions of ${}^7\mathrm{Li}(p,n){{}^7\mathrm{Be}}^g$ (upper) and ${}^7\mathrm{Li}(p,n){}^7\mathrm{Be}^{*}$ (lower). Data were taken from [17]. Here, XS stands for cross section.

Figure 3

Total neutron production cross section vs proton energy (left), emitted neutron energies (middle), and double differential cross sections (right) as functions of the laboratory incident proton energy and neutron emission angle for the reactions of ${}^7\mathrm{Li}(p,n){{}^7\mathrm{Be}}^g$ (upper) and ${}^7\mathrm{Li}(p,n){}^7\mathrm{Be}^{*}$ (lower). Data were taken from [17]. Here, XS stands for cross section.

Figure 4

Simulated neutron flux energy spectrum at the ${}^{235}\mathrm{U}$ fission target compared with the neutron-induced fission cross section.

Figure 4

Simulated neutron flux energy spectrum at the ${}^{235}\mathrm{U}$ fission target compared with the neutron-induced fission cross section.

Figure 5

Radioactivities for selected nuclei as a function of elapsed time after EOB. The bottom bar indicates the gamma measurement time beginning 17 minutes after EOB. Simulations were done with the dchain code.

Figure 5

Radioactivities for selected nuclei as a function of elapsed time after EOB. The bottom bar indicates the gamma measurement time beginning 17 minutes after EOB. Simulations were done with the dchain code.

Figure 6

Experimental setup of this experiments with 3.81 and 1.91 cm thick (${\mathrm{C}}_2{\mathrm{H}}_4{)}_n$ moderators. LiF neutron production target, and the enriched ${}^{235}\mathrm{U}$ fission foil were attached in front of and behind (${\mathrm{C}}_2{\mathrm{H}}_4{)}_n$ moderator plates. (a) Side view and real picture of the schematic view in Fig. 2. (b) Back side view showing ${}^{235}\mathrm{U}$ foil.

Figure 6

Experimental setup of this experiments with 3.81 and 1.91 cm thick (${\mathrm{C}}_2{\mathrm{H}}_4{)}_n$ moderators. LiF neutron production target, and the enriched ${}^{235}\mathrm{U}$ fission foil were attached in front of and behind (${\mathrm{C}}_2{\mathrm{H}}_4{)}_n$ moderator plates. (a) Side view and real picture of the schematic view in Fig. 2. (b) Back side view showing ${}^{235}\mathrm{U}$ foil.

Figure 7

The picture of beam on LiF target. (a) Picture of LiF target and (b) picture of fluorescent beam spot on LiF target.

Figure 7

The picture of beam on LiF target. (a) Picture of LiF target and (b) picture of fluorescent beam spot on LiF target.

Figure 8

The picture of $\gamma$-ray peak measurement with portable HPGe detector. The distance from ${}^{235}\mathrm{U}$ target to the detector was 22 cm.

Figure 8

The picture of $\gamma$-ray peak measurement with portable HPGe detector. The distance from ${}^{235}\mathrm{U}$ target to the detector was 22 cm.

Figure 9

Absolute HPGe detector efficiency at a distance of 25 cm and its uncertainty as a function of $\gamma$ energy from the mixed calibration source.

Figure 10

Comparison of $\gamma$ ray spectra from the decay of ${}^{235}\mathrm{U}$ fission products in moderator cases with thicknesses of 3.8 cm and 1.9 cm. Spectra were accumulated over a 1 h. The spectrum for the 1.9 cm case includes $\gamma$ rays from residual nuclei, as the ${}^{235}\mathrm{U}$ target was reused.

Figure 11

Comparison of fission rates between measurements and simulations. RFR indicates the ratio of fission rates between experiments and simulations.

Figure 12

(a) Fission fragment yields by mass for thermal neutron induced fission of ${}^{235}\mathrm{U}$ (nuCARIBU) and spontaneous fission of ${}^{252}\mathrm{Cf}$ (CARIBU). (b) Fission fragment distributions for nuCARIBU (upper) and CARIBU (lower).

Figure 13

(a) Geometry of nuCARIBU target assembly in the simulations. The beam comes in from the left. (b) Total neutron production cross sections for the $DD$ (dotted line) and $p-{}^7\mathrm{Li}$ (solid line) reactions as a function of incident beam energy.

Figure 14

Comparison of neutron fluxes ($E_n<10$ eV) per projectile with a same ${\mathrm{H}}_2\mathrm{O}$ moderator for the $DD$ at 0.3 MeV (a), $DD$ at 3 MeV (b), and $p-{}^7\mathrm{Li}$ at 3 MeV (c) generator options.

Figure 15

Comparison of neutron fluxes ($E_n<10$ eV) per projectile for ${\mathrm{H}}_2\mathrm{O}$ (a), ${\mathrm{D}}_2\mathrm{O}$ (b), (${\mathrm{C}}_2{\mathrm{H}}_4{)}_n$ (c), and Be (d) moderator materials.