Design Study of Compact Medical Accelerator Using Superconducting RFQ for BNCT

Ryo Katayama1, Kensei Umemori1, Eiji Kako1, Sinichiro Michizono1, Seiya Yamaguchi1, Yasuhiro Kondo2

(1: High Energy Accelerator Organization (KEK), 2: Japan Atomic Energy Agency (JAEA) ) Simulation ResultsIntroduction

Summary• We investigated the feasibility on the application of a SRF niobium cavity to an accelerator-based neutron source for BNCT.• We considered the case that using SC-RFQ of 325 MHz composed of pure bulk Nb at 4.2 K, proton beams of 30 mA from 50 keV to 2.5 MeV are accelerated and irradiated with a Li target

for neutron production.• If the maximum surface electric field of >40 MV/m satisfied, the superconducting BNCT system become a more compact and lower power consumption system than the conventional

one, and it is concluded that the feasibility of the system can be guaranteed.

• We will present a feasibility evaluation 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.• Adopting RF-linac based neutron source realizes a medical care system

sufficient to be compact that can be installed in a hospital and to generate intensive neutron yields enough for medical treatment of BNCT.

• However, it has been still desirable to improve the efficiency of input power on neutron yields and to realize more compact system.

• SRF accelerator technology potentially allow us to enhance the performance because of its prominent lower ohmic loss and higher achievable accelerating gradient (NOTE: Kilpatrick limit does not hold for SRF cavities).

• This study describes the design study that a superconducting radio frequency quadrupole (SC-RFQ) of 325 MHz composed of pure bulk niobium at 4.2 K accelerates proton beams from 50 keV to 2.5 MeV and irradiate them into a lithium target for neutron production.

• We evaluated the feasibility on the following three criteria:① comparison of the heat amounts generated in a cryomodule with the

cooling capacity of the refrigerator.② AC power consumption of sum of RF power and refrigerator power. ③ size of the superconducting BNCT system.

Cell Parameters of SC-RFQ designed in this study are summarized in the following Table.

Method and Analysis Procedure

MOPTEV016

Qb and QRF and RFQ length evaluated in this study are shown below.

• In this study, we aim to design the superconducting BNCT system with an accelerator-based neutron source that irradiate proton beams (20 mA, 2.5 MeV) into a lithium target in order to produce an adequate number of neutrons for the medical treatment, the same as the neutron source developed by the national cancer research center in Japan.

• The categories of heat amounts generated in the cryomodule considered in this study are summarized in the following table.

• In this study, we use RFQGen software to evaluate trajectories of beams in SC-RFQ and beam loss in the transportation.

• Analysis Procedure and other simulation conditions are shown below.

Design concept of LEBT assumed in this study is shown left blow, and an example of obtained x-x’ distribution at RFQ entrance are shown in right below, where the above and bottom figures represent the obtained results at the initial beam emittance at ion source of 0.02 (original) and 0.2 cm mrad, respectively. This result show that such LEBT design enable us to effectively eliminate only beams having the poor emittance (Qemit =0). The achievable beam transmission was 76.4 % in this LEBT.

• Total sum of heat amounts generated in the cryomodule can be suppressed lower than 100 W, which is the typical cooling capacity 100 W of a commercially available 4.2 K helium refrigerator.

• Length of SC-RFQ can be shorten than that of the existing normal conducting RFQ if Ep is chosen to be > 40 MV/m.

NOTE: This study choose Qext to be 20 W

AC Power ConsumptionFinally, we will compare AC power consumptions during one day between the case of SC-BNCT (Ws) and the example of the ordinal case referred by the BNCT system at National Cancer Center of Japan (Wn). Here, we assume that BNCT is under operation per one day for 10 hours, the power efficiency of RF source is 50 %, the power efficiencies of refrigerator are 0.6 KW/W (in the medical care operation) and 1 kW (in the other duration). Red bin represents AC power consumption of RF power, blue one the refrigerator power to remove Qb and QRF, and the green one the refrigerator power to remove Qext.

Improvement Ratio

25 30 35 40 45 50 55 60

Peak Surface E-Field [MV/m]

1

2

3

4567

10

20

30

[W]

bH

eat L

oadi

ng o

f Q

25 30 35 40 45 50 55 60

Peak Surface E-Field [MV/m]

1

2

3

4567

10

20

30

[W]

RF

Hea

t Loa

ding

of Q

30 35 40 45 50 55 60

Peak Surface E-field E_p [MV/m]150

200

250

300

350

400

RFQ

Len

gth[

cm]

SC-RFQ (p, 325 MHz, 2.5 MeV)

J-PARC RFQ (p, 324 MHz. 3 MeV)

SNS RFQ (p, 402.5 MHz, 2.5 MeV)