Measurement of charged-particle emission DDX for carbon with 14-MeV
incident neutrons
Keitaro Kondo1, Isao Murata1, Kentaro Ochiai2, Hiroyuki Miyamaru1,Naoyoshi Kubota2†, Chikara Konno2 and Takeo Nishitani2
1

Division of Electrical, Electronic and Information Engineering, Graduate School of Engineering, Osaka
University, Yamada-oka 2-1, Suita, Osaka 565-0871, Japan
2

Japan Atomic Energy Agency,

Shirakata Shirane 2-4, Tokai-mura, Naka-gun, Ibaraki, 319-1195, Japan†Present address: Materials Characterization Laboratory, Nippon Steel Corporatione-mail: ktaro@ef.eie.eng.osaka-u.ac.jp

A preliminary measurement of charged-particle emission double differential
cross-section (DDX) for carbon with 14-MeV incident neutrons was carried out. In themeasurement, a superior S/N ratio, fine energy and angular resolution were realized with apencil-beam neutron source and a counter telescope consisting of a pair of silicon surface
barrier detectors, ΔE and E. Minimum detection energy of 1.0 MeV for α-particles wasachieved by utilizing an anticoincidence spectrum of the ΔE detector. The agreement of ourmeasurement with a previous data measured by Haight et al. was fairly well in the higher
energy part of DDX, while a discrepancy was observed below 3 MeV. In order to investigate
the mechanism of the 12C(n,n’+3α) reaction, we tried to calculate energy distributions ofemitted particles by the Monte Carlo method considering reaction kinematics of a lot ofchannels which contribute to the reaction. As a results, the contribution of the 9Be*(4.7 MeV)
channel was suggested and the estimated branching ratio for the 12C(n,α)9Be*(≥2.43MeV)
channels was more than 30%.

1. Introduction
In fusion reactor development, double-differential cross-section (DDX) for
charged-particle emission reaction induced with 14-MeV neutrons is needed to calculate
nuclear heating and fundamental values to evaluate material damages, i.e. primary knock-on
atom (PKA) spectra, amount of gas production and displacement per atom (DPA)
cross-sections. The particularly important charged-particle emission DDX is of nuclides
contained in the first wall and blanket materials highly exposed to 14-MeV incident neutrons.
We recently developed an improved measurement system for secondary emitted charged
particles using a pencil-beam neutron source furnished in the Fusion Neutronics Source (FNS)in Japan Atomic Energy Agency (JAEA) [1]. Systematic measurements are being carriedout for light nuclei of which the measurement has not yet been performed sufficiently so far[2]. In this paper, a preliminary result of measurement for carbon is presented. Carbon is
one of the important nuclides for organic materials. Regarding the fusion reactor

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development, carbon is proposed for an alternative first wall and contained in SiC, which
would be an advanced material for various devices. Detailed measurement is also importantfrom an aspect of nuclear physics because of the complex mechanism of the 12C(n,n’+3α)
reaction. We tried to reproduce energy spectra of emitted particles from the reaction byMonte Carlo calculations considering reaction kinematics. The reaction mechanism wasinvestigated through the calculation and analysis.

2. Experimental
2.1 Charged-particle spectrometer using a pencil-beam neutron source
All the present measurements of DDX were carried out with the pencil-beam DT
neutron source available at FNS/JAEA. A schematic view of the facility and theexperimental setup is shown in Figure 1. In the facility, a deuteron beam of 350 keV and20mA at the maximum bombards a large tritium target. Generated DT neutrons arecollimated by a 2 m thick shielding structure with a narrow hole of 2 cm in diameter. The
mean neutron energy is 14.2 MeV. A vacuum chamber was set at the outlet of the neutron
beam, and a sample material was fixed at the center of the chamber. The sample material
used in the present study is a self-supported carbon foil of 5 μm thickness (1033 μg/cm2). A
counter-telescope system with a pair of silicon surface barrier detectors, one for ΔE (thicknessof 9.6 μm) and the other for E (thickness of 760 μm), was employed in order to distinguishkinds of emitted charged particles. The minimum detectable energy of the telescope, whichdepends on the thickness of the ΔE detector, is 2.5 MeV for α-particles. In order to extend
the detectable energy range for α-particles as much as possible, we attempted to use ananticoincidence spectrum of the ΔE detector. When the ΔE detector of 9.6 μm thickness isused, the threshold energy beyond which the ΔE detector can be penetrated is around 700 keV
for protons and around 1.0 MeV for tritons. The anticoincidence spectrum above thosethreshold energies for protons and tritons hence originates only from α-particles or particles
heavier than α-particles. In the present measurement, recoiling carbon and 9Be particles
emitted via the 12C(n,α0)9Be reaction cannot be negligible and their contributions werecalculated and subtracted. As a result, the measurement of α-particles with a minimum
energy of around 1 MeV was successfully realized.
2.2 Data analysis
In order to obtain an actual energy spectrum of emitted charged particles, themeasured spectrum must be corrected for energy loss in the sample. A relationship between
the actual spectrum and the measured spectrum was calculated by the Monte Carlo code
SRIM-2003 [3] combined with the processing codes we made. Then the spectrum unfolding
was carried out with our original code based on the spectrum type Bayes estimation method[4] to obtain the actual spectrum. For a standard cross section, 122.0 mb for the 27Al(n,α)
reaction evaluated in JENDL-3.3 [5] was used.

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Fe

Polyethylene CdPb

Pre-collimator
Detectors

2cm-φ neutron beam

238

U Fission Chamber

SampleTritium target
Scattering Angle
200cm
150cm

Vacuum Chamber

90cm+D beamConcrete

Figure 1 Experimental arrangement.

3. Results and discussion
3.1 Measured double-differential cross-section for α-particles
Up to now, DDX for α-particles has been obtained for only 30 degrees of emission
angle in our measurement and further measurements are still in progress. Figure 2 shows
the obtained DDX and the previously measured DDX by R. C. Haight et al. [6] at theemission angle of 30 deg.. In our DDX, the contribution from the 12C(n,α)9Be was clearly
identified. The obvious structures according to 9Be(Ground State) and 9Be*(2.43 MeV) appeared.
Also the contribution of 9Be*(4.7 MeV) might exist. Between the both measurements, theagreement of the higher energy part of DDX is fairly well, while a slight discrepancy wasobserved below 3 MeV.
Ground State100

4.7MeV

DDX [mb/sr/MeV]

2.43-3.05MeV101natC(n,xα) at 30 deg.
Present exp.
R.C.Haight et al.

0.10246810α-particle energy [MeV]

Figure 2 Our obtained DDX and previously measured DDX at the emission angle of 30 deg.

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3.2 Monte Carlo calculation and analysis
The mechanism of the 12C(n,n’+3α) reaction is complex, because a lot of reaction
channels can contribute to formation of 4-body final state of n’+3α. In order to investigate
the reaction mechanism, we tried to calculate energy distribution of emitted particles for eachchannel which contributes to the reaction and determine the branching ratio to the channels so
as to reproduce the experimental result well. This analysis is also useful to examine thecause of the large discrepancy in Fig. 2 in the lower energy part of DDX, which would make alarge impact on the evaluation of the total α-particle production cross-section. The emitted
energy distributions both for α-particles and neutrons were calculated by the Monte Carlo
method based on reaction kinematics. The calculation scheme was entirely adopted fromRef. [7]. The contributed channels for the 12C(n, n’+3α) reaction were considerably
identified by Antolković et al., who carried out a kinematical analysis for the reaction using
nuclear emulsions [8]. In the present calculation, simply 2-body sequential decays, whichreach to the final states of n’+3α, via excited states of 12C and 9Be were considered based on
their analysis. The decay schemes are as follows:n + 12C Æ n’ + 12C* Æ n’ + [α + 8Be*(GS or 3MeV)] Æ n’ + [α + (2α)]
n + 12C Æ α + 9Be* Æ α + [n’ + 8Be*(GS or 3MeV)] Æ α + [n’ + (2α)]
n + 12C Æ α + 9Be* Æ α + [α + 5He*] Æ α + [α + (n’ + α)]

(1)
(2)
(3)

Decay modes of the intermediate nuclei, 8Be* and 5He*, were adopted according to literatures[8, 9, 10]. For all the intermediate states, the density-of-states function was given by aBreit-Wigner distribution with constant level widths [9, 10]. The angular distributions forthe inelastic scattering of neutrons were extracted from the neutron emission DDX measured
by Takahashi et al. [11] for the excited states of 12C at 7.65 and 9.64 MeV. For the otherexcited states of 12C, the isotropic distribution in center-of-mass system was assumed. In
other 2-body decays, also the isotropic distribution in center-of-mass system was assumed.
The calculated spectra were fitted into our obtained DDX for α-particles and DDX
for neutrons measured by Takahashi et al. [11], and the branching ratio for the contributedchannels was estimated. Figure 3 shows the best fitted result of the DDX both for emitted
α-particles and neutrons at the emission angle of 30 deg.. The continuum in lower energy ofthe neutron DDX is reproduced fairly well. In the present estimation, a large contribution of
the 9Be*(4.7 MeV) channel plays an important role, although its validity should be confirmed bysome theoretical analyses. The estimated branching ratio for the 12C(n,α)9Be*(≥2.43MeV)
channels was more than 30%. Such a large contribution of the 9Be* channels might suggest
importance of the α-particle knock-on or stripping process. This supposition will beconfirmed by further detailed measurement of the angular distribution of emitted α-particles.
The measurement is also needed in order to estimate the total α-production cross-section.
From the present analysis, it was found that the lower energy part of our measured
DDX would be reasonable when the assumed reaction channels contribute to the reaction.

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To examine the cause of the discrepancy with the Haight's result precisely, furthermeasurements for other angles than 30 deg. are indispensable.

9.64MeV

100100Measurement
Calculation (SUM)
12C*(7.65)
12C*(9.64)
12C*(10.3)
12C*(10.84)
12C*(11.83)
9Be*(2.43-3.05)
9Be*(4.7)

4.7 MeV
2.43 MeV

DDX [mb/sr/MeV]

DDX [mb/sr/MeV]

7.65 MeV
10.28 MeV

1010.1

101

0.1

02

46

Energy of emitted neutrons [MeV]

802

46

Energy of emitted α-particles [MeV]

Figure 3 Best fitted result of the DDX calculation for both emitted α-particles and neutrons
at the emission angles of 30 deg.

4. Conclusion
Measurement of the α-particle emission double differential cross-section (DDX) for
carbon with 14-MeV incident neutrons is being carried out and a preliminary result wasdescribed in this paper. The agreement of our obtained DDX at the emission angle of 30 deg.
with a previous data measured by Haight et al. was fairly well in the higher energy part, whilea slight discrepancy was observed below 3 MeV. In order to investigate the mechanism of
the 12C(n,n’+3α) reaction, we tried to calculate the energy spectra of emitted particles by theMonte Carlo method considering reaction kinematics of a lot of channels which contribute tothe reaction. As a results, the contribution of 9Be*(4.7 MeV) was suggested and it was foundthat rather large contribution of the 12C(n,α)9Be*(≥2.43MeV) channels had to be assumed toreproduce the experimental results well. The assumed ratio to the total α-particle production
was more than 30%. Such a large contribution of the 9Be* channels might suggest
importance of the direct reaction process. More detailed measurement and analysis of theangular distribution of emitted α-particles are needed to reveal the mechanism of the12C(n,n’+3α) reaction. Further measurement is also needed in order to estimate the total
α-production cross-section.

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Acknowledgements
The authors wish to acknowledge the FNS staff for their excellent operation of theFNS accelerator: Messrs. C. Kutsukake, S. Tanaka, Y. Abe, M. Seki, Y. Oginuma and M.
Kawabe. The support and encouragement provided by K. Okada, K. Takakura, S. Takagiand S. Shido are also greatly appreciated.

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