Schematic of the neutron microbeam. The proton microbeam is incident on a lithium target, and neutrons are produced with a highly forward-peaked angular distribution.
A significant number of individuals are occupationally exposed to low doses of neutrons, mostly low-energy neutrons. These low-energy neutrons produce biological damage in a fundamentally different way from most photon or high-energy charged particle irradiations. Both the x rays and the high-energy charged particles that our users study damage DNA primarily through atomic ionization, i.e. production of electron vacancies in DNA, directly or via free radicals. By contrast, the primary DNA damage mechanism for low-energy neutrons is via direct knockout of protons in the DNA. In order to better understand possible damage response mechanisms such as the bystander effect from neutrons on biological systems, we are developing a neutron microbeam in addition to the broad beam neutron irradiator.
Our neutron microbeam design is based on the existing charged particle microbeam technology at RARAF. The principle of the neutron microbeam is to use the proton beam with a micrometer-sized diameter impinging on a very thin lithium fluoride target system. From the kinematics of the7Li(p,n)7Be reaction near the threshold of 1.881 MeV, the neutron beam is confined within a narrow, forward solid angle. Calculations show that the neutron spot using a target with a 17 µm thick gold backing foil will be less than 20 µm in diameter for cells attached to a 3.8 µm thick propylene-bottomed cell dish in contact with the target backing. The neutron flux will roughly be 2000 per second based on the current beam setup at RARAF Singleton accelerator. The dose rate will be about 200 mGy/min. By reducing the target thickness to the minimum necessary, the production of resonance gamma rays in the thin target will be limited. The principle of this neutron microbeam system has been preliminarily tested at RARAF using a collimated proton beam.
|Predicted neutron yields, microbeam diameters, neutron energies, and dose rates for different proton energies.
Selected proton energy and characteristics of the predicted neutron microbeam are highlighted.
We have produced the first ever neutron microbeam, worldwide. It is based on a proton microbeam at a specific and precise energy (1.886 MeV) impinging on a lithium target. we have now reached a ~30 µm diameter. This is sufficiently small that initial biological testing is now underway. The dots in the Figure show the neutron microbeam track profile imaged with a 6Li2CO3 coated CR-39 track-etch detector; each dot, produced by a single neutron, is a pit formed by the track of an a particle or a 3H ion emitted from the
n + 6Li →a + 3H reaction in the lithium coating. For comparison, the track-etch detector image has been superimposed on the image of a human fibroblast cell generated at the microbeam endstation.
The track-etch detector image (30 µm dia. neutron microbeam) has been superimposed on the image of a human fibroblast cell generated at the microbeam endstation.