Lithium Target for accelerator based BNCT neutron source

influence by the proton irradiation on lithium

RYO FUJII, B.S.1¶, YOSHIO IMAHORI, M.D, Ph.D.1, MASARU NAKAMURA, B.S.1

MASASHI TAKADA, Ph.D.2, SO KAMATA, Ph.D.2, TYUYOSHI HAMANO, Ph.D.2, MASAHARU HOSHI, Ph.D.3,HITOSHI SATO, M.D.4, JUN ITAMI, M.D.5, YOSHIHISA ABE, B.S.5, MASAFUMI FUSE, B.S.5

1Cancer Intelligence Care Systems., Inc., Tokyo, Japan, 2National Institute for Radiological Science, Chiba, Japan, 3Hiroshima University, Hiroshima, Japan, 4Ibaraki Prefectural University of Health Sciences, Ibaraki, Japan, 5National Cancer Center, Tokyo, Japan.

¶Corresponding author email address:

Introduction: Boron neutron capture therapy (BNCT) is a new concept therapy with different to existing radiation therapy, and it is expected to be a new cancer therapy for future.

Neutron source for BNCT depended on the reactor until now. By changing a neutron source from a nuclear reactor to accelerator, it can install this accelerator-based BNCT system into medical facility. And medical facility does not need to have a nuclear fuel, and safe.

We chose 7Li (p,n)7Be reaction as a neutron source based on accelerator.It is said that lithium thickness will become thin by irradiation of proton beam.

Therefore, we examined how lithium would be influenced by irradiating proton beam to vapor-deposited lithium actually.

Material and methods: Lithium was deposited by vacuum deposition with a thickness of 20 ~ 50μm in diameter of about 40mm on circular copper plate (130mm in diameter) of 8mm thick.

Irradiation test with proton beam was performed in an facility of NASBEE of NIRS.

Lithium thickness measurement was performed comparatively and quantitatively by using Laser thickness measurement device (accuracy: 0.1μm).

The accelerator and target side were divided with 6μm harvar fo il as not having any influence to the accelerator even if lithium would be scattered by proton beam irradiation.

Collimator with 5mm radius was installed in irradiation chamber of the Lithium target and then we examined the Lithium change at boundary while placing i right above the Lithium target and defining the borderline between irradiation area and non-irradiation area.

Proton energy and density of the beam were 2.5MeV and 60~126μA/cm2 respectively.

The target was cooled down by water with 18 degrees.

The lithium target was made by vacuum deposition in advance and was kept with blanking cover under argon gas conditions.

Lithium target was set to irradiation chamber after confirming the beam profile and its location by quartz in advance.

The proton beam irradiation was conducted for three hours continuously with the condition of cooling the havar foil by cooled helium gas.

Results: In three hours continuous irradiation by

proton beam, trace of beam irradiation was found.

But the difference of lithium thickness between irradiation area and non irradiation area was not demonstrated. The amount of neutrons production gradually decreased over time, and after 3 hours it was reduced by approximately 10%. And it had no relevancy between helium cooling and non cooling.

Figure 1: Test sample of lithium coated copper plate.

(a) before irradiation

(b) after irradiation.

Figure 2: profile of lithium surface

Conclusions: It was said that lithium would have a low melting point and weakness against proton beam irradiation. But it was suggested in this experiment that lithium demonstrate sufficiently resistance to continuous long time irradiation without lithium decomposition.

For the gradual reduction of neutron, it was considered that the reduction did not occur by lithium consumption but occurred by denaturalization of lithium nitridization.