Design study of compact medical accelerator using superconducting rf quadrupole for boron neutron capture therapy

We investigated the feasibility on the application of a superconducting radio frequency (SRF) niobium cavity to an accelerator-based neutron source for boron neutron capture therapy (BNCT). Neutron source is the key component of BNCT and adopting rf-linac based neutron source realizes a medical care system sufficient to be compact, which can be installed in a hospital and to generate intensive neutron yields that the BNCT requires. However, it is still desirable to improve the efficiency of input power on neutron yields and the achievable accelerate field gradient. SRF accelerator technology potentially allows us to enhance the performance because of its prominent lower ohmic loss and higher sustainable accelerating fields. This paper presents a first feasibility study on the application of a SRF niobium cavity to an accelerator-based neutron source for BNCT, assuming that a superconducting radio frequency quadrupole (SC-RFQ) composed of pure bulk niobium at 4.2 K accelerates the proton/deuteron beams to a beryllium or lithium target for the neutron production of BNCT via ${}^7\mathrm{Li}(\mathrm{p},\mathrm{n}){}^7\mathrm{Be}$, ${}^9\mathrm{Be}(\mathrm{p},\mathrm{n}){}^9\mathrm{B}$, or ${}^9\mathrm{Be}(\mathrm{d},\mathrm{n}){}^{10}\mathrm{B}$. The following beam parameters were used: beam energy of 2.5 MeV (for Li target)/5 MeV (for Be target), ion source current (50 keV, CW 30 mA), normalized beam emittance of 0.02 cm mrad, and resonance frequency of 325 MHz (for proton)/162.5 MHz (for deuteron). Based on these conditions, we evaluated the feasibility on the following three criteria: comparison of the cooling capacity of the refrigerator to the amount of heat, power consumption of AC, and size of the BNCT system. First, we evaluated the amount of heat generated in a cryomodule by adding the ohmic loss of SC-RFQ $Q_{\mathrm{rf}}$, beam losses in SC-RFQ $Q_{\mathrm{b}}$, heat penetration into the cryomodule $Q_{\mathrm{ext}}$, and beam losses of molecular ion beams and poor quality beams emitted from the ion-source at/near the RFQ entrance $Q_{\mathrm{emit}}$. In this study, we typically regarded $Q_{\mathrm{ext}}$ as 20 W at 4.2 K and considered a new low-energy beam transport system that can suppress $Q_{\mathrm{emit}}$ to 0. In addition, $Q_{\mathrm{rf}}$ and $Q_{\mathrm{b}}$ were numerically evaluated by beam simulation and electromagnetic calculation. The obtained results revealed that the sum of the heat amounts could be sufficiently suppressed below the typical cooling capacity of a commercially available helium refrigerator. Second, we compared the ac power consumption of BNCT between a conventional and SC-BNCT systems, which indicated that the BNCT system adopting the SRF cavity effectively reduced the ac power consumption of SC-BNCT by almost $1/4$ times. Third, the length of the SC-RFQ could be shortened by adjusting the peak surface E-field, as compared to conventional existing RFQs such as J-PARC, SNS, and IFMIF. Eventually, this study demonstrated that the application of the SRF cavity for the rf-linac-based neutron source of BNCT is feasible, and thus provides a foundation for the future development of design for next-generation BNCT systems.

Figure 1

The ohmic loss of Cu normal-conducting RFQ (NC-RFQ) as a function of the resonant frequency. The ohmic loss was evaluated by the lumped circuit model of a RFQ length and intervane voltage of 3.1 m and 80 kV, respectively, similar to those of J-PARC. The ohmic loss of NC-RFQ was calculated using Eq. (a3).

Figure 2

The ohmic loss of Nb superconducting RFQ (SC-RFQ) as a function of the resonant frequency. The ohmic loss was evaluated by the lumped circuit model with a RFQ length and intervane voltage of 3.1 m and 80 kV, respectively, similar to those of J-PARC. The ohmic loss of SC-RFQ was calculated using Eq. (a6).

Figure 3

The classification of amount of heat generated in a cryomodule [10, 11].

Figure 4

Schematic of neutron source assumed in this study.

Figure 5

Beam envelope for proton in LEBT.

Figure 6

Beam envelope for deuteron in LEBT.

Figure 7

Scatter plot for the phase space distribution of proton beams at the RFQ entrance. The top two figures correspond to the simulation result with normalized emittance of 0.02 cm mrad, whereas the bottom two figures correspond to that with 0.2 cm mrad. These scatter plots are created by the number of MC events of 10,000 of LEBT beam simulation conducted with RFQGen software and the integrated numbers of them are normalized to be 1. These histograms are divided with $100\times 100$ bins.

Figure 8

Scatter plot for the phase space distribution of deuteron beams at the RFQ entrance. The top two figures correspond to the simulation result with normalized emittance of 0.02 cm mrad, whereas the bottom two figures correspond to that with 0.2 cm mrad. These scatter plots are created by the number of MC events of 10,000 of LEBT beam simulation conducted with RFQGen software and the integrated numbers of them are normalized to be 1. These histograms are divided with $100\times 100$ bins.

Figure 9

RFQGen beam dynamics simulation for 2.5 MeV proton. The horizontal axis represents the number of cells in the RFQ. Top: x vs RFQ cells. (second from the top) y vs RFQ cells. (third from the top) the spread of synchronous phase vs RFQ cells. Bottom: energy spread vs RFQ cells.

Figure 10

Beam emittance, beam spread, phase spread, and energy spread for 2.5-MeV simulation. The top-left and top-right correspond to x-x’ and y-y’ (cm-mrad), respectively. The bottom-left denotes the beam spread (cm). The bottom–right denotes energy spread-phase spread. Theses scatter plots are created by the number of MC events of beam simulation conducted with RFQGen software and the integrated numbers of them are normalized to be 1. These histograms are divided with $100\times 100$ bins.

Figure 11

Transmission rate through RFQ compared to the beam intensity at the RFQ entrance as a function of peak surface E-field. The vertical and horizontal axes represent the transmission rate and the peak surface E-field, respectively. The red circles denote the result of 5-MeV deuteron, triangles denote the result of 5-MeV proton, and the squares denote the result of 5-MeV deuteron.

Figure 12

Heat loading owing to beam loss as a function of peak surface-E field. $Q_{\mathrm{b}}$ shown in vertical axis correspond to the heat loading due to “theoretical transmission” shown in Fig. 11.

Figure 13

Heat loading owing to rf resistance as a function of peak surface-E field.

Figure 14

RFQ length of SC cavities as a function of peak surface E-field. The vertical and horizontal axes represent the RFQ length and peak surface E-field, respectively. Moreover, the RFQ lengths of the NC cavities are plotted for reference.

Figure 15

Relative ratio of the estimation of NC-BNCT to that of SC-BNCT. Histogram and circle graph represent the cost and the area estimated by J-PARC and c-ERL at KEK, respectively.

Figure 16

Relative ratio of the estimation of NC-BNCT ($\mathrm{RFQ}+\mathrm{DTL}$) to that of SC-BNCT. Histogram and circle graph represent the cost and the area estimated by J-PARC and c-ERL at KEK, respectively.

Figure 17

Proton beam transmission rate as a function of normalized emittance. The horizontal and vertical axes represent the normalized emittance assumed in the simulation and the proton beam transmission rate through LEBT was evaluated by RFQGen simulation. The red curve depicts the fitting result with a function of $f(x)=p_0\exp (-p_{1}x)+p_2\exp (-p_{3}x)$.

Figure 18

Deuteron beam transmission rate as a function of normalized emittance. The horizontal and vertical axes represent the normalized emittance assumed in the simulation and the deuteron beam transmission rate through LEBT was evaluated by RFQGen simulation. The red curve depicts the fitting result with a function of $f(x)=p_0\exp (-p_{1}x)+p_2\exp (-p_{3}x)$.

Figure 19

Cell parameters for conceptual RFQ design of the 2.5-MeV proton accelerator proposed in this paper.

Figure 20

Cell parameters for conceptual RFQ design of the 5-MeV proton accelerator proposed in this paper.

Figure 21

Cell parameters for conceptual RFQ design of the 5-MeV deuteron accelerator proposed in this paper.

Figure 22

Comparison of 3D models for the conceptual design of SC-RFQ for BNCT. Yellow: 2.5 MeV proton, blue: 5 MeV proton, red: 5 MeV deuteron.