J. Plasma Fusion Res. SERIES, Vol.5 (2002) 179-183

Gharacterization of Gompact Accelerator DD Neutron
Source for in situ Calibration Experiment
on Neutron Measurement at LHD
SHIBATA Yasunari, IGUCHI Tetsuo, SASAO Mamiko', OGATA Tomohiro, UMEMURA Norihiro,
ASAI Keisuke and KAWARABAYASHI Jun
Nagoya University, Nagoya 464-8603, JapanI National Institutefor Fusion Science, Toki, 509-5292, Japan(Received: 14 December 2001 / Accepted: 6 July 2002)

Abstract
A compact Cockcroft-Walton type accelerator DD neutron source has been developed for in situ
calibration experiments on neutron measurements at LHD. The equipment mainly consists of three parts;
a deuterium (D) reservoir/ion source, a self-loaded deuterium target and a 100 kV high voltage power
supply, all of which are contained in a compact cylindrical stainless steel (SUS) tube of 70 mm indiameter and 780 mm in length. About one hour steady operation was performed under the acceleration
voltage of 80 keV and the ion beam current of -60 pA, corresponding to the DD neutron yield of around
105 n/s. The neutron emission profile and energy spectrum were measured with an NE2l3 scintillator anda 3He gas proportional counter.
Preliminary neutronic calculations with a Monte Carlo neutron transport code 'MCNP' were also
executed for simulating the in situ calibration experiment for neutron detectors that will be installed on

LHD.
Through the experiments and the calculations, it is shown that the present DD neutron source isvalid for in situ calibration on threshold type detectors used for neutron emission profile monitoring andneutron spectrometry at DD plasma experiments.

Keywords:
fusion neutron, neutron measurement, compact accelerator, DD neutron source, neutron source characterization, in situ callbration experiment

1. lntroduction

standard neutron sources is essential to check and/or
keep their accuracy, when these neutron measurement
systems are installed on a fusion device. Up to now,
utilization of a 252Cf neutron source is conventionally

Fusion neutron measurements will be one of themost important diagnostic tools for DD/DT plasma
experiments in existent fusion devices as well as forcontrol of burning plasmas in future fusion reactors like
ITER [1]. The measurement techniques have beendeveloped and refined on total neutron yields for fusion

preferred for this purpose. However, there are someproblems on its radiation safety handling, shape of thesource spectrum, etc. [3-5].

power, neutron emission profiles for plasma ion
distribution and neutron energy spectra for ion

To improve the in situ calibration technique forneutron measurement mainly at LHD [6], we havetemperature [21. An in situ callbration experiment with@2OO2 by The Japan Society of Plasma

Corresponding author's e-mail: h992402d@ mbox.media.nagoya-u.acjp

t79

Science and Nuclear Fusion ResearchShibata Y. et al., Characterization of Compact Accelerator DD Neutron Source for in situ Calibratton Experimentdeveloped a compact accelerator DD neutron source,which can change neutron source strength and has aquasi mono-energetic spectrum of which peak is aroundthe 2.5 MeV similar to that of DD plasma neutrons.
In this paper, the results of performance tests of theneutron source are presented on the neutron yield, theneutron emission profile and the neutron spectrum,including preliminary consideration on its applicability
to LHD through the neutronic calculations.

2. Outline of Neutron Source
The compact accelerator DD neutron source wehave developed is a Cockcroft-Walton type accelerator[7,8]. The equipment mainly consists of three parts; aFig. 1 Structure of a part of accelerator tube and ar-rangement of experiment.

deuterium (D) reservoir /ion source, a self-loaded
deuterium target and a 100 kV high voltage power
supply, all of which are contained in a compactcylindrical stainless steel (SUS) tube of 70 mm inlxtoal

Io ion beam cuent 40[pA]

I

diameter and 780 mm in length. Figure I illustrates apart of the accelerator tube, which is connected with apower supply tube of 600 mm in length. The deuterium(D) ions are supplied to an acceleration space from thePenning type ion source with the reservoir consisting ofaZi-Y-Fe alloy getter and a molybdenum heater, and;calculation

0.

5EoI
looa!

are self-loaded into a Titanium target of 6 mm indiameter and 5 ,um in thickness to generate neutrons

Deuterim ion energy [keV]

through DD reactions.

Fig. 2 Dependence of DD neutron yield on acceleration

We have experienced about one hour steadyvoltage.

operation and confirmed that the ion beam current of 60
pA is available under the acceleration voltage of 80 kV.

from the NE2l3 pulse height data based on a differential
method [9,10], and complementarily checked with thepulse height measured with the 3He gas proportional

3. Experiments for Neutron Source
Gharacterization

To experimentally make clear the basic
characteristics of the present DD neutron source, wehave measured the neutron yields, the neutron emission
profiles and the neutron spectra under several operation
conditions, such as an acceleration voltage and an ion
beam current. Figure I shows the experimentalcounter.

4. Results and Discussion
4.1 Neutron yield

Inc., RS-P4-0840-218) with a polyethylene moderator
was used, of which absolute sensitivity had been

Figure 2 gives an example of the measured resultsof the dependence of DD neutron yield on theacceleration voltage (or deuteron energy), while typicalresults on the relation between the source strength andthe ion beam current are shown in Fig. 3. It has been
confirmed that the maximum neutron yield is around 2.0
x lOa n/s under steady operation of about 1000 s andcalibrated with a standard 252Cf neutron source. An
NE213 liquid organic scintillation counter (OHYOKOKEN KOGYO CO.LTD., 3-2477) with a neutrongamma ray discrimination circuit was adopted tomeasure the neutron emission profile and energy spectra
around the target. The neutron spectra were derived

around 8.0 x 104 n/s under transient operation of about
100 s or shorter, at the acceleration voltage of 80 kV
and the ion beam current of 60 pA. The result of steady
operation correspond to the source strength of about 1/20 less than that of 100 ltCi 1= 4.3 x lOs n/s) sealed
252Cf source. However, it would be possible to enhancearrangement.

For the neutron yield measurement, a high sensitive
3He gas proportional counter (GE REUTER-STOKES,180

Shibata Y. et al., Chancterization of Compact Accelerator DD Neutron Source for in situ Callbration Experiment

I xt041

2'00

102

acceleration voltage: 70 [kV]ion beam cunent: 50 [pA]

acceleration voltage. 60[kv],1 80[kv]10t;uDf; r.oo'*o 100otaI

Toa0L
2030

z 10-'tl40

Beam current [pA]

5060t o-'o'3060

90t20

150

180Angle [degree]

Fig. 3 Relation between source strength and ion beam

current.

Fig. 4 Neutron emission profile around the source.

the neutron source strength up to 10o n/s by installing ahigh density deuterium pre-loaded target and keeping
the acceleration voltage at 90 kV.

accelemtion voltage: 75 [kV]ion beam cuent: 46 [A]*i

4.2 Neutron emission profile
Figure 4 shows some results of the neutronemission profile around the source target in directions

qfrom 0 to 180 degrees to the beam axis on the horizontal
plane, where the neutrons with energy higher than 1.5
MeV are adopled. In this operation, at the acceleration
voltage of 70 kV and the ion beam current of 50 pA.
The DD neutron source has a roughly isotropic emission

Hoz

1.5

2Energy [MeV]

profile. This emission is signified by kinematic
collision. However, these results are from initial data, so

2.5

3Fig.5 Neutron spectra obtained from NE213 data unfolding.

we should get rnore data and consider again.

4.3 Neutron energy spectrum
Figure 5 shows a typical example of neutron
spectra that depend on the emission angle. Theseneutron spectra were obtained with unfolding of theN8213 data. l'he neutron spectrum at each emission

Fig. 6, it should be noted that the DD neutron peak
appears on the energy added to 0.764 MeV because thepulse height signals are proportional to the total energy+ 0.5 MeV, of which peak energy changes withdeposited in the counter gas through 3He(n,p)t
exothermic reaction (Q-value = 0.764 MeV), and thesharp peak around 0.764 MeV is caused by thermal

kinematics of DD reaction. However, some peaks ofneutrons.

angle is found to be quasi mono-energetic of around 2.5

lower energy region are caused by scattered components
and the data unfolding with the differential method.
We have made the detailed analysis on the neutron
peaks at the emission angle of 90 degrees, together with

5. Applicability to in situ Galibration
Experiment at LHD
To check an applicability of the present DD
neutron source to in situ calibration experiments at

supplemental measurements with the 3He gas

LHD, we made a preliminary model of the MCNP I l]
calculation for LHD, which mainly consists of theproportional counter. Figure 6 shows the neutron spectra

obtained with the NE213 detector and the 3He gas
proportional c()unter, at the acceleration voltage of 75
kV and the ion beam current of 46 1tA. In the pulse
height spectrum of the 3He gas proportional counter invacuum vessel, simplified helical coils and a part of theexperimental building, as shown in Fig. 7. The
responses of neutron detectors placed outside the device

l8t

Shibata Y. et al., Charucterization of Compact Accelerator DD Neutron Source for in situ Calibration Experiment

the result of fast neutron detector. From anothercalculation done in poloidal cross section geometry ofN8213 organic liquid scintillator

the 4.5-L port of LHD, where a multi-channel neuhon
collimator made of polyethylene will be installed under
the floor for neutron emission profile measurement. It isalso shown that the measured neutron spectra of thexpresent DD neutron source would be a good"n gu. .i'*t",'H.

zolccllaation-rcltage 75 {kVlapproximation for the typical neutron spectra at the endof the neutron collimator. This means that the present
DD neutron source is especially useful for the in situ

ionbeamcunent46 [pA]

calibration on neutron emission profile monitors andneutron spectrometers for DD plasma experiments.

6. Summary

Neuton energy [MeV]

We have developed and characterized the compact
accelerator DD neutron source to improve the in situ
calibration experiment on neutron measurement at
magnetic confinement fusion devices like LHD. In thesystem start-up within 100 hours operation, main resultson the neutron source characterization are summarized

Fig.6 Results of neutron spectra obtained from NE213
detector and 3He gas counter.

C?ffihie10-6

l1

10-7t
alro{

as follows;(l) The maximum neutron yield is around 2.0 x 104 n/sa]ttt...
tl.

.il:r

rttll

r:^E 10"Q10-10lunder steady operation over 1000 s and around 8.0
x lOa n/s under transient operation of about 100 s

Itlor shofier.

(2) The neutron emission profile is roughly isotropic.

aa)IHowever because of these are initial data, it isnecessary for us to be more measurements and252Cf

solre flst rcstrse reutrot detector
PP sowe flat resporc neumn ddector

a '"'Cf some feI neutun deEctor
DD sowe fast neutron detector

consideration.

(3) The neutron energy spectrum is quasi monoAngle between detector and souce: 0 [degree]

energetic around 2.5 + 0.5 MeV.
Comparing with a conventional 252Cf neutron source, itis necessary to improve the source strength and the operational stability of the present DD neutron source.

Fig. 7 Simulation result on toroidal angle dependence ofneutron detector responses when DD and 252Cf
neutron point source are rotated in the toroidal
direciton.

However, from the viewpoint of neutron spectral shapeand radiation safety handling, the DD neutron source
would be quite useful for in situ calibration on thresholdwere calculated when the mono-energetic (2.45 MeV)
DD source ot 252Cf neutron point source were rotated in
the toroidal direction on the center axis inside thevacuum vessel. Two kinds ofdetectors were considered;
one is a flat response neutron detector mainly used forneutron yield monitoring such as a 23sU fission chamber,

type detectors used for neutron emission profile monitoring and neutron spectrometry at DD plasma experiments.

As further works, we are making more detailanalysis of the neutron source characteristic under well

the other is a fast (or the threshold type) neutron

conditioned and continuous or turned operation,detector used as a neutron emission profile monitor anda neutron spectrometer such as a scintillator. Figure 7

including great efforts to enhance the neutron sourceshows the simulation results on toroidal angle

target and keeping as high as acceleration voltage as

dependence of these neutron detector responses. For thepossible.

strength by installing high density pre-loaded deuterium

flat response neutron detector, almost no difference isfound between the responses for the DD and the 2s2cf
neutron sources, while significant difference is seen inAcknowledgements
The authors wish to express their thanks to Mr. K.

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Shibata Y. et al., Characterization of Compact Accelerator DD Neutron Source for in situ Calibration Experiment

Nishimura of CSD, Mr. Y. Miyake of Taiyo Material,

t4l B.Wolle, G. Beikert and F. Gadelmeier, Rev. Sci.

Dr. T. Nishitani of Naka institute, JAERI, Drs. A.
Uritani and C. Pyeon of AIST, Mr. K. Watanabe ofInstrum. 424, 561 (1999).

[5] Y. Oyama and K. Sekiyama and H. Maekawa,

Nagoya University, for their encouragement and helpful

Fusion Technol. 26,1098 (1994).

advices.

[6] O. Motojima et al., Nucl. Fusion 40,599 (2000).
l7l J.D. Cockcroft and E.T.S. Walton, Proc. R. Soc.
London, A136, 619 (1932).
J.D.
Cockcroft and E.T.S. Walton, Proc. R. Soc.
[8]London, 4137, 229 (1932).

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