MCNPX simulation results of beam loss-induced radiation

By using the MCNPX simulations, we first calculated the normalized neutron and gamma radiation levels in units of flux (#/cm²/s) and dose (rad/hr) for fast and slow losses with three different beams (U, O, and H). Figures 8.9 and 8.10 show the normalized neutron and gamma dose levels associated with 1 W/m slow losses of U, O, and H beams. The label 1(2) in the name of the beam species indicates that the detector1(2) is used to calculate the dose levels. The normalized dose levels are used to calculate the expected current signal in each detectors that we are interested in. The amount of dose levels detected by detector 1 and 2 is almost the same. The energy of each beam is chosen according to the values written in Table 8.1.

As illustrated in Figs. 8.9 and 8.10, the detected radiation levels decrease for heavier beams of lower energy. For very low energies (< 10 MeV/u), the radiation levels are too low to be detected.

Therefore, neutron or gamma detectors will not be used for these measurements. Instead, the destructive

Figure 8.6: Signal transfer time as a function of cable length. The LMR-400 signal cable is expected to be used in the RAON BLM system.

Figure 8.7: Geometric model of quarter-wave resonator (QWR) with specifications in SCL31 section used for MCNPX simulation. (a) QWR geometry in the x-z plane. (b) QWR geometry in the y-z plane.

methods (e.g., beam loss collector made of niobium ring) are considered for measuring the neutron and gamma radiations from very low energy beams as alternatives. In particular, we note that monitoring the Uranium beam loss in low energy section would be extremely challenging.

Next, we plot dose maps that show the spatial distribution of the beam-loss-driven radiations, which provide a visual understanding of the radiation patterns around the structures. Figure 8.11 shows the neutron and gamma dose maps outside one of the warm sections of SCL22 linac. Here, we assume that radiation is induced by losses from the 200 MeV/u uranium beam. Figures 8.11 (a) and (b) show gamma and neutron radiation patterns, which are originated from 1 W/m line (slow) loss of 200 MeV/u uranium beam. Figures. 8.11 (c) and (d) show gamma and neutron radiation patterns from 1 W point (fast) loss of the uranium beam. The fast loss point is indicated with a red point near the center of the first quadrupole, which is indicated with the blue line in Figs. 8.11 (c) and (d).

The gamma flux shown in Figs. 8.11 (a) and (c) is not uniformly distributed around the QD because

Figure 8.8: Geometric model of half-wave resonator 1 (HWR1) cavity in SCL32(1) section used for MCNPX simulation. For the fast loss case, a 1 W point source hits a point of the inner surface of the beam pipe (represented by a red dot) with 10 mrad grazing angle. Detector1 is positioned parallel to the beam propagation direction and detector2 is positioned perpendicular to the beam propagation direction.

the elements of the QD are primarily constructed from Fe, which enhances the interactions with gamma rays. By contrast, the neutron flux shown in Figs. 8.11 (b) and (d) has a rather uniform distribution because neutrons can easily penetrate the materials with a weaker interaction compared to that with gamma rays. In Fig. 8.11 (d), unlike the gamma flux distribution, the neutron flux is much higher near the end of the warm section than that at the right next to the loss point. We note that this arises because of the high penetration rate of the neutrons.

Because of the lack of a complete list of beam loss scenarios, some strategy is required to determine the number of detectors and their locations [96]. Simulation results from TRACK code indicate that the most localized losses occur in the quadrupoles, in which the beam size is largest. Therefore, as a default, we plan to place one BLM per quadrupole doublet in the warm sections. Figure 8.11 implies that we can investigate the spatial distributions of the beam-loss-induced radiation fluxes, and predict that in which point BLMs should be placed. Further study for a fine-tuning of the BLM locations will be conducted during the commissioning phase of the RAON facility. We also note that the radiation dose patterns are more or less similar to beam losses in other sections of the linac and with different beam species, even though the absolute doses are different.

Figures 8.12 and 8.13 show the uranium beam loss-induced gamma (photon) and neutron energy spectrum, respectively. The overall radiation levels increase as the beam energy increases. Because gamma (photon) radiation is a high frequency EM field radiated in the processes of stabilizing the energy states of nucleus, they have limited energy ranges. By contrast, neutrons form through nuclear reactions (e.g., neutron evaporation) in such a way that they have broader energy ranges than that of gamma rays.

We note that the y-axis values in Figs. 8.12 and 8.13 are given in arbitrary units, thus we can only gain the relative information on the radiation energy spectra. Because gamma and neutron detectors have energy dependency, these energy spectra provide an useful information in determining the proper BLM

Figure 8.9: Normalized neutron dose levels (in log scales) induced from 1 W/m slow losses of Uranium, Oxygen, and Proton beams as functions of the incident beam energy. The label 1(2) in the name of the beams indicates that the detector1(2) is used to calculate the dose levels.

systems. Furthermore, the energy spectra of beam loss-induced radiations must be distinguished from the background radiations, which are not generated from the beam loss itself.