1. The improvement of the energy resolution in epi-thermal region of Bonner sphere using boric acid solution moderator
H. Ueda1, H. Tanaka2, Y. Sakurai2
1Graduate School of Engineering, Kyoto University
2Kyoto University Research Reactor Institute
(Thank you)
My name is Haruaki Ueda.
Today, I’d like to talk to you about “The improvement of the energy resolution in epi-thermal region of Bonner sphere using boric acid solution moderator”.

2. Comparison of energy resolution between two types of Bonner spheres.
Outline
Calculation of Bonner sphere response functions using polyethylene and boric acid solution as a moderator.
Comparison of energy resolution between two types of Bonner spheres.
Evaluation of the neutron spectrum at Kyoto University Reactor Heavy Water Neutron Irradiation Facility ( KUR-HWNIF )
At the beginning, I'd like to show you the contents of my presentation.
We are studying on the measurement of epi-thermal neutron spectrum using Bonner sphere.
We used boric acid solution as a moderator of Bonner sphere to narrow a peak of response function and improve energy resolution.
Firstly, I will show you the calculation results of Bonner sphere response functions using polyethylene and boric acid solution as a moderator.
And, to compare energy resolution between these two types of moderator Bonner sphere,
we simulated unfolding of neutron spectra consisting of a single peak in epi-thermal energy region.
Secondly, I will show you the unfolding results of the neutron spectra.
Finally, I will show you the estimated spectrum at KUR-HWNIF obtained from the measurement using boric acid solution moderator.

3. Introduction
RBE of neutron greatly vary depending on its energy, especially in epi-thermal region.
It is difficult to obtain the detailed information for the neutron energy spectrum in the keV region.
We are improving the energy resolution of Bonner sphere, especially in the keV region, using the boric acid solution as a moderator.
In epi-thermal neutron irradiation field for BNCT, neutron energy is over a wide range.
Because neutron effect to tissue greatly vary depending on its energy, it is necessary to evaluate the neutron dose for each neutron energy.
So, it is important to obtain the detailed information for the neutron energy spectrum before the clinical use.
But, it is difficult to obtain the detailed information for the neutron energy spectrum in the keV region by activation foils, and so on.
Bonner sphere is useful to evaluate the neutron spectrum in detail.
We are improving the energy resolution of Bonner sphere, especially in the keV region, using the boric acid solution as a moderator

4. Thermal neutron detector
Bonner Sphere1
moderator
Next, I will explain the measurement principle of Bonner sphere.
Bonner sphere consists of a spherical neutron moderator and a thermal neutron detector placed in the sphere center.
Thermal neutron detector

5. Bonner Sphere2 2. Moderation to thermal energy 1. Incidence
3. Detection
Neutron
The incident neutron on a Bonner sphere is moderated to thermal neutron and detected by a thermal neutron detector placed in the sphere center.
A small Bonner sphere is suitable for low energy neutron measurement and a large Bonner sphere is suitable for high energy neutron measurement.
Using the various sizes of Bonner spheres, we can estimate the detailed neutron spectra for the wide energy range.

6. Moderator including 10B 10B Absorbed by 10B
Thermal Neutron
10B
Boron 10 has large absorption cross section for thermal neutron, so using material including boron 10 as a moderator , thermal neutrons are hard to detect by the Bonner sphere.
And Bonner spheres using a moderator including boron 10 is relatively more sensitive to the neutrons thermalized near the sphere center than others.
This leads to a narrow response function peak and the improvement of energy resolution in epi-thermal region is expected.
Epi-thermal Neutron
Thermalized near the center and detected

7. Response function calculation
Thermal neutron detector
Manganin foil
(thickness: 200um, diameter: 2cm)
Gold foil
(thickness: 50um, diameter: 1cm)
Response function calculation
Parallel neutron beam
Moderator material
Polyethylene
Boric acid solution
( B wt%)
In this study, Bonner sphere consists of a spherical neutron moderator shell and activation foils placed in the sphere center as thermal neutron detector.
Polyethylene and boric acid solution are used as moderator material.
Generally, polyethylene is used as a moderator of Bonner sphere.
Manganin and gold are used as activation foil material.
We calculated the specific saturated activities per flux for each energy neutron as the response function of Bonner spheres.
Calculations were performed using a Monte Carlo simulation code system, PHITS.
The sphere diameters were 10, 15, 20cm
Sphere diameter
10, 15, 20 cm

8. Calculation results
The specific saturated activities per flux for each energy neutron were calculated as the response function of Bonner spheres
Activation foil: Gold(197Au)
The specific saturated activities per flux for each energy neutron were calculated as the response function of Bonner spheres
I show you the response functions of gold foil placed in the center of Bonner sphere on graphs.
D10 represents the 10cm diameter Bonner sphere.
From the graphs,
We can see the response function peaks of Bonner spheres using boric acid solution is narrower than those for polyethylene moderator and the improvement of the resolution is expected.
Moderator: polyethylene boric acid water ( B wt%)

9. Unfolding test1 Examples of Neutron spectra used in the tests
We performed unfolding tests for the comparison of energy resolution.
Saturated activities were calculated and errors were added to the data.
The data were unfolded into estimated spectrum by UMG unfolding package
for each moderator Bonner sphere.
Next, I explain the Unfolding test for the comparison of energy resolution.
We calculated saturated activity for each Bonner sphere and add random errors to the data.
Errors were normally distributed and relative standard deviation was 3%.
We got a estimated spectrum from all the data of the same moderator Bonner sphere.
Calculations of saturated activity were performed using synthetic neutron spectra consisting of a single peak in epi-thermal energy region.
Peak width was one digit.
The calculated activities were unfolded into estimated spectrum by UMG unfolding package using uniform spectrum as the default.
UMG includes two types of unfolding codes, Gravel and Maxed. We used both codes.
Examples of Neutron spectra used in the tests

10. Unfolding tests2
These graphs show the estimated spectra of the true spectrum that has the peak center at 100keV.
GRV represents the estimated spectrum by gravel.
MXD represents the estimated spectrum by maxed.
We can see that the estimated spectra using boric acid solution as a moderator reflect the true spectrum better than those for polyethylene moderator.
Moderator: polyethylene boric acid solution ( B wt%)

11. Unfolding test3
These graphs show the result of true spectrum that has the peak center at 10keV.
Moderator: polyethylene boric acid solution ( B wt%)

12. Unfolding tests4
These graphs show the estimated spectra of the true spectrum that has the peak center at 1keV.
Moderator: polyethylene boric acid solution ( B wt%)

13. Unfolding test5
These graphs show the estimated spectra of the true spectrum that has the peak center at 100eV.
As shown above, We can see that the estimated spectra using boric acid solution as a moderator reflect the true spectrum better than those for polyethylene moderator.
It is shown that we can improve energy resolution of Bonner sphere using boric acid solution as a moderator .
Moderator: polyethylene boric acid solution ( B wt%)

14. Stochastic errors : 3-5% Measurement at KUR-HWNIF Bonner sphere
Finally, I will show you the measurement for the epi-thermal neutron irradiation mode at KUR-HWNIF using boric acid solution moderator and the estimated spectrum obtained from the data.
This is the geometry of the measurement.
Bonner sphere constitute of acrylic sphere shell filled with boric acid solution or water and two foils of manganin and gold placed in the center.
Boric acid concentration was B wt%.
Sphere diamter are 10, 15, 20cm.
After the irradiations, we measured the saturated activities by a High purity germanium detector.
Stochastic errors are within 3 to 5 percent.
Stochastic errors : 3-5%

15. Influence of foil placement and solution concentration
Response functions were calculated based on beam data of previous study and the practical geometry at KUR-HWNIF.
And for the evaluation of measurement error, we calculated response functions for the variation of activation foil placement and boric acid concentration.
Unfolding simulation using these response results showed that the estimated spectrum in epi-thermal region changed by twenty and a several percentage if the foil position varied by 5mm or the boric acid concentration did by 10B 0.03wt%.
The estimated spectrum in epi-thermal region changed by twenty and a several percentage if the foil position varied by 5mm or the boric acid concentration did by 10B 0.03wt%.

16. Estimation of KUR-HWNIF spectrum
This shows the estimated neutron spectrum from calculated response functions and the measurement data by Maxed code.
Guess represents the previous study data.
The estimated spectrum agree with the previous study data within the error bars.

17. Conclusion
It was confirmed that boric acid solution moderator improves the energy resolution of the Bonner sphere in the keV region.
The estimated spectrum was in good agreement with the data from the previous study.
The estimated spectrum in epi-thermal region changed by twenty and a several percentage if the foil position varied by 5mm or the boric acid concentration did by 10B 0.03wt%.
Thank you very much for your attention.

18. Thank you very much for your attention.

19. Response Functions1 Moderator: polyethylene
Activation foil: manganin(55Mn)
Moderator: polyethylene
Activation foil: gold(197Au)

20. Response Functions2 Moderator: boric acid water(0.14wt%)
Activation foil: manganin(55Mn)
Moderator: boric acid water(0.14wt%)
Activation foil: gold(197Au)