Compact Monoenergetic Proton Generator in MeV Region Using NANOGAN

For simple applications, such as the calibration of a charged particle detector, a multi-MeV proton generator may be preferable to cyclotrons or electrostatic accelerators such as Van de Graaff. Thus, a compact proton generating system, consisting of 10Ghz ECR ion source NANOGAN and a deuteron target, was developed at the Research Center for Nuclear Physics at Osaka University. A 3He2+ beam was generated by the NANOGAN with the acceleration voltage of 20∼45 kV in an experiment that utilized the fusion reaction 3He + deuteron (d) → proton(p) + 4He. The monochromatic protons with energies of 14.67 MeV were successfully obtained at the atmosphere side of the target in the experimental setup, when a novel target base with a thin metal foil and Polyimide film window are used.


Introduction
A compact monoenergetic proton generator might be preferable to cyclotrons or electrostatic accelerators such as Van de Graaff for the purposes of detector calibration, cell irradiation or radio isotope (RI) production with energy region in MeV.The concepts of the generator should be compact and low construction cost to be widespread use.Thus, we focused on the fusion reaction 3 He+dp+ 4 He whose proton energy is 14.67 MeV.According to the reaction cross section [1], the energy requirement for 3 He is only several hundred keV, this means that the compact generator can be constructed with ion sources.

Previous studies
We already have confirmed 3 He+DP+ 4 He reaction with deuterated polyethylene (Poly-C2D2) target in previous study [2].The target has been irradiated with 3 He 2+ beam with the energy of 40 keV and current of 200 eA, and then 499 protons have been observed by plastic scintillator installed inside the vacuum chamber with 15 minutes irradiation.We also have tried to obtain protons in atmosphere through thin Al window of 20 mm aperture and 0.3 mm thickness on Al flange with deuterated polyethylene target in another previous study [3].In that study we have obtained 2204 protons with 10 minutes 3 He 2+ beam irradiation on the target with the energy of 40keVand the current of about 400 eA, however the proton energy was not monochromatic.In both of previous work, obtained proton intensity was very weak for cell irradiation or RI production.

Our goal
The goal in this study is generating protons in MeV region in atmosphere with high intensity and monochromatic energy.To achieve that, these following components are required such as high intensity ion source, high voltage acceleration for higher cross section, and thin window for less energy loss.

Components of proton generator
To achieve the high intensity monoenergetic proton generator, following components were prepared.

Compact ion source
We used PANTECHNIK's NANOGAN, the 10 GHz permanent magnet ECR ion source, for producing incident 3 He 2+ beam (Figure 1).We have modified the DC cut and insulation of stand for 50 kV acceleration and done the beam test with 4 He 2+ instead of 3 He 2+ and we have obtained CW 4 He 2+ beam of 800 eA with the 50 kV.It is noted that specification for 4 He 2+ on catalogue is 100 A at 20 kV.The reason that we choose 2+ beams instead of 1+ is that higher energy has advantage of fusion reaction cross section.

Thin Window
To obtain the generated proton in atmosphere with monochromatic energy, the window material should be as thin as possible.We used Polyimide film of 130 m as window.We also used thin Ni foil of 30 m which is placed in front of the window to protect it from beam heat load.Figure 2 shows the details about those.Estimated proton energy after going through the window and foil is about 14.0 MeV.

Deuteron target
The deuteron target is made of deuterated polyethylene.The thickness is 30~50 m after using press machine.Figure 3 shows that the targets are mounted on Ni foil described above.The 3 He 2+ beam current was measured by this Ni foil stage.There is a baffle slit with the aperture of  20 mm in front of the foil.

Figure 3. Thin deuterated polyethylene target mounted on Ni foil.
There is polyimide window behind the foil.The baffle slit would be placed in front of the target and connected to electrical ground.

Damage on window and target
We found that there are problems due to beam heat load in pre-tests. 4He 1+ beam of 20 kV and 3 mA broke Ni foil and then Polyimide film. 3He 2+ beam of 20 kV and 0.4mA also deformed target in previous study [3].So, the beam current was limited to several A this time.The cooling system will be constructed in future.

Experiment
Figure 4 shows the experimental setup.The beam 3 He 2+ extracted from NANOGAN was analyzed and transported to target via bending magnet and two quadrupole magnets.The beam current was around 2~3 eA.Conditions of acceleration voltage were 45 kV, 30 kV and 20 kV.Proton detector: Proton detector was placed at downstream of polyimide window on the end of target chamber.The detector consists with the plastic scintillator of 15mm x 15mm x 40 mm and photo multiplier.

Results and discussion
The results obtained as spectra of generated protons are shown in figure 5, and monoenergetic protons of 14.0 MeV have been seen successfully.The results are also summarized in table 1 with their experimental conditions.From the extrapolation of event rates in table 1, we can expect that over 2.5x10 4 protons / sec.would be obtained with 3 He 2+ of 800 eA at 50 kV.This corresponds to about 4 femtoA.This intensity is good enough for detector calibrations.It also might be usable for some cell irradiations.In case of RI production, it might be lack of intensity.We have checked the target after experiments and the target deformation has not been seen, but the beam unevenness has been seen as the carbon deposition on target.

Comparison with Reaction Cross Section
According to the reaction cross section curve [1], the cross sections of the fusion reaction with the energy of 40 keV, 60 keV and 90 keV were 3.2, 18 and 77 in barn respectively.It seems that the energy dependence of event rate and cross section is different.The difference might come from the beam unevenness or less reproducibility because we have monitor only beam current but not check the beam profile.The difference also might come from the large stopping power of incident 3 He 2+ in deuterated polyethylene target due to ionization reaction in several dozen keV region.Qualitatively saying, the incident 3 He 2+ easily loss their energy before fusion reaction occurs as incident energy is small.So, dense deuteron target without carbon atom might make less such energy loss and more event rate.

Figure 1 .
Figure 1.NANOGAN, the 10GHz permanent magnet ECR ion source.Modifications of DC cut and insulation of stand have been done for 50 kV acceleration

Figure 2 .
Figure 2. Conceptual diagram of thin windows and target.Polyimide film is covering the hole of 16 mm on the flange of target chamber.The distance between the film and Ni foil is 20 mm.

Figure 4 .
Figure 4. Schematic view of experimental setup.The beam extracted from NANOGAN was transported to target via bending magnet (BM) and two quadrupole magnets (Q1, Q2).Proton detector was placed at downstream of target chamber.

Figure 5 .
Figure 5. Spectra of detected protons.Horizontal axis is arbitrary unit.The spectrum a) is corresponding to the 20 kV acceleration, b) to 30 kV and c) to 45 kV.The other experimental conditions are in table 1

Table 1 .
Summary of results for each condition of acceleration voltage.