1. Design of A New Wide-dynamic-range Neutron Spectrometer for BNCT with Liquid Moderator and Absorber S. Tamaki1, I. Murata1 1. Division of Electrical, Electronic and Information Engineering, Graduate School of Engineering, Osaka University Yamada-oka 2-1, Suita, Osaka , Japan
16th International Congress on Neutron Capture Therapy, June 14-19, 2014, Helsinki, Finland
Introduction
BNCT cases are reported only using nuclear reactors now. Recently, instead of nuclear reactor, accelerator based neutron sources (ABNS) are being developed for BNCT. However, their intensities are still weak, and patients should be hence positioned near the target. It causes difficulty to moderate neutrons sufficiently. As a result, the neutron energy spectrum is distorted depending on accelerators because high-energy neutron ratio changes depending on type of the accelerators. Therefore we have to know each energy spectrum for each ABNS precisely, especially in epithermal region. In the present paper, we carry out a new design study to measure neutron energy spectrum in thermal (< 0.5 eV), epithermal (0.5 eV ~ 10 keV) and higher energy region (> 10 keV) more accurately and precisely with a new Bonner type spectrometer.
Objectives
Numerical Test Calculation
Fold a given energy spectrum (see Fig. 2) with the evaluated response functions to prepare a count rate spectrum shown as a black line in Fig. 3.
Add a fixed statistical error to each count rate at random to simulate a real measurement (2 % case in Fig. 3).
Unfold the count rate spectrum to compare with the given energy spectrum to examine the energy spectrum reproducibility.
Develop a new neutron spectrometer based on BSS for BNCT, covering thermal, epithermal and higher energy region (BSS: Bonner Sphere Spectrometer)
In a conventional BSS, by combination of a neutron detector and moderators varying the condition such as its thickness (or density and/or material, if it is possible), neutron spectrum can be estimated by unfolding the measured values with the various moderators.
The more the number of moderators increases, the better the performance (energy resolution) becomes.
However, it is unreal to make a lot of moderator shells.
Our solution:
Using liquid moderators　and changing their thickness CONTINUOUSLY, it would be possible to evaluate neutron energy spectrum, in principle, with INFINITE NUMBER of ENERGY BINS.
In this study, we design such a unique spectrometer and carry out numerical tests to confirm an ability to measure a typical accelerator based epithermal neutron spectrum for BNCT and examine which liquid materials could give better performance.
Fig. 2. Typical accelerator based
epithermal neutron spectrum.
Fig. 3. Folded count rate with Boric Acid
response and statistical error of 2%.
Numerical Test Results
1. Boric Acid and Tri-methyl Borate indicated acceptable performance to reproduce neutron energy spectrum (see Fig. 4).
2. Comparing the true and unfolded neutron flux intensity for each energy region (integrating the results in thermal, epithermal and fast energy regions), the result with Tri-methyl borate showed a better agreement in epithermal and fast region than other materials.
Detector Model and Theory
Detector design and response evaluation
---Performed with MCNP-5 (A General Monte Carlo N-Particle Transport Code)
Calculation Model
Neutrons are moderated to change the detector response.
Liquid Moderator
(See Table 1)
Fig. 4. True energy spectrum and results by numerical test for each material (2% error case).
20 cm Φ
100 cm
Mono-energetic neutron beam
3He counter
(5 cm Φ)
Continuously change the liquid moderator’s thickness from 0 mm to 200 mm
Detector responses were evaluated for a lot of moderator combinations by calculating 3He(n,p)3H reaction rate in the detector. (see Fig. 1.)
Fig. 5. Neutron intensity ratio between true spectrum and numerical test results for each material.
Discussion and Conclusion
Fig. 1. Evaluated response function of boric acid solution.
We are developing a new neutron spectrometer with liquid moderator, the thickness of which can be changed continuously. In this study, we confirmed its availability to measure neutron spectrum in an accelerator based neutron field, in case the statistic error is less than 2 %. We will employ Tri-methyl borate as a moderator, because of its higher performance to evaluate the neutron flux intensity in epithermal and fast energy region than other examined materials. It seems to be caused by higher 10B density, in other words, larger neutron absorption cross section.
Table 1. Moderator materials examined in this study.
Material
Water
(H2O)
Heavy Water
(D2O)
Hexane
(C6H14)
Boric Acid-aq
(H3BO3-aq)
Tri-methyl Borate
( B(OCH3) 3 )
Melting Point
0 ℃
(273 K)
3.81 ℃
(277 K)
-95℃
(178 K)
-34℃
(239 K)
Boiling Point
100 ℃
(373 K)
101.4 ℃
(375 K)
69℃
(342 K)
68 ℃
(341 K)
2. Spectrum estimation
--- Solving inverse problems by Bayesian Estimation Method.
Future Work
Solving inverse problem
Y(x)=R(x,E)・𝚽(E)
𝚽(E) =R−1(x,E)・Y(x)
In future, we are going to experimentally confirm the capability of high accuracy neutron spectrum measurement, using liquid moderator evaluated in this study.
Folding
Unfolding
Y(x) : Count Rate (= experimental value) for each moderator thickness x,
which is continuously adjusted.
R(x, E) : Response function, i.e., expected count rate for neutron energy E
and each moderator thickness x.
𝚽(E) : Neutron energy spectrum
Scattering Neutron Shield
(e.g. 10B doped Poly Ethylene)
Thereafter, we will develop a proto-type neutron spectrometer with a room-return neutron shield like the figure shown in the right.
Tri-methyl Borate
detector