JAEA-Conf 2011-002 - 日本原子力研究開発機構

derive the incident neutron flux and the neutron capture yield are briefly described in the following
sections.
3.2 Incident Neutron Flux
A 10 B sample was used to determine the incident neutron flux on the sample. The 10 B(n,γ) 7 Li
reaction emits a single γ ray of 478 keV. It means that the detection efficiency for the reaction is
independent of neutron energy. Then, the neutron flux is given by the following relation:
C ( )
( ) B E
φ 0 E
n
n =
(2)
ε BYB
( En
)
where the subscript “B” means 10 B. CB(En) is the counting rate at energy En, YB(En) is the energy
dependent capture yield for the 10 B(n,γ) 7 Li reaction, εB is the detection efficiency for the 478-keV γ
ray which was calculated with the EGS5 [4]. The neutron capture yield YB(En) was obtained by the
Monte Carlo calculation with the MCNP-4C. The nuclear data used for the Monte-Carlo simulation
were taken from JENDL-4.0.
3.3 Neutron Capture Yield
The γ-ray detection efficiency of the pair of C6D6 liquid scintillators is small enough not to count
two or more γ rays per capture event. Therefore, the efficiency for detecting capture events depends on
decay modes of compound nucleus. By applying a weighting function, W(I), on the observed PH
spectrum, the detector can be treated as a total energy detector having a γ-ray detection efficiency
proportional to an incident γ-ray energy [5]. Since the sum of γ-ray energies emitted from a capture
event is independent of decay modes, the efficiency for detecting capture events is also proportional to
the excitation energy of capture state.
The weighting function, W(I), was defined as follows:
W(
I ) R(
I , Eγ
) = Eγ
(3)
I
where R(I, Eγ) is the response function defined as the probability that a γ ray with an energy of Eγ
emitted from the sample position into isotropic was detected in the I-th channel of the PH spectrum.
The response functions for discrete γ-ray energies from 0.5 to 7.0 MeV were obtained by the
calculation with the EGS5 and the experiments with standard γ-ray sources as shown in Fig. 2. The
weighting function was determined by means of a least square fitting so as to minimize the following
χ 2 :
2
JAEA-Conf 2011-002
2
2
χ = W ( I ) R(
I , Ei
) − Ei
E
(4)
i
i I
The weighting function for the pair of C6D6 liquid scintillators is shown in Fig. 3.
The neutron capture yield was obtained as follows:
S(
I ) W ( I )
I Y =
BE + En
(5)
where S(I) is the capture γ-ray PH spectrum, BE is the neutron binding energy of target nucleus, En is
the incident neutron energy. The capture γ-ray PH spectrum, S(I), was obtained by subtracting the
background(BG) from the foreground PH spectrum corresponding to each TOF region. The BG
spectrum was estimated from the measurements with an empty case.
4. Results and Discussion
(1) 151 Eu
The preliminary neutron capture cross sections of 151 Eu were obtained in the neutron energy
region from 0.03 eV to 100 keV as shown in Fig. 4. A number of experiments were reported for 151 Eu
and they are also shown in the figure for comparison. The present results give good agreement with
those experimental data, and hence we can conclude that the validity of the weighting function derived