Journal of Nuclear and Radiochemical Sciences, Vol. 4, No. 2, pp. R15-R24, 2003

Reviews
Recent Development of Advanced Neutron Detection Technology
Takashi Nakamura*
Cyclotron and Radioisotope Center, Tohoku University
Aoba, Aramaki, Aoba-ku, Sendai, Japan 980-8578
Received: August, 19, 2003; In Final Form: August 21, 2003
This review paper gives the description on the recent development of various new sophisticated neutron spectrometers. For neutrons of energies beyond 100 MeV, the self-TOF detector using the NE102A plastic scintillators,
large-scale NE213 organic liquid scintillator, and spallation detectors of C and Bi have recently been developedby our group, especially for use in neutron target and shielding experiments. Various neutron spectrometers havealso been developed for use in the charged particle and neutron mixed field which is realized in space environment. The following spectrometers are briefly described: 1) Phoswich detector which combines the NE115 plastic
scintillator and the NE213 scintillator, 2) Anti-coincidence detector system using ∆E and E counters, and 3)
Bonner sphere (multi-moderator detector) mounted a 3He counter or a pair of 6Li and 7Li glass scintillators.

I. Introduction
Neutron measurement is in general more difficult thanphoton and charged-particle measurements for the following
reasons:
1) Neutrons interact with only nucleus and the reaction crosssections vary largely with each nucleus and with incident
neutron energy, which indicates that the detector responseto neutrons will be very complicated.
2) Neutron energies existing in various radiation environments
extend from thermal energy to GeV energy, which requiresthat the neutron detection must be done over 10 decades ofenergy region.
3) Neutrons are always accompanied by photons and in spaceenvironment by charged particles, which means that theneutron detection necessitates the event discrimination
technique between neutron events and other events.
For neutron measurements in nuclear facilities, the multimoderator spectrometer, so-called Bonner Ball, the organic
liquid scintillator, and the activation detector have been widelyused as spectrometers. But for neutrons of energy above 100
MeV, there have been no suitable spectrometers with reasonable resolutions. Our group recently developed the self-TOF
detector using the NE102A plastic scintillators, large-scale
NE213 organic liquid scintillator, and spallation detectors of C
and Bi for measuring these high-energy neutrons, whichbecome very important in the radiation environment
surrounding the high-energy and high-intensity accelerators
because of their strong penetrability.
Inside a spacecraft, there are high-energy protons and heavyions, and the secondary neutrons are generated by the nuclear
reactions between these particles and the material of the spacecraft. Although these secondary neutrons are estimated tocontribute to 5 to 30% of the dose equivalent of intravehicularcrews,1, 2 the information on these neutrons is still very poor,
because it is very difficult to discriminate neutron events fromdominant charged particle events in the mixed field of neutronsand charged particles in space environment. In the past only
two neutron measurements have been done in spacecraft by
Russia and USA.1, 3 Keith et al.1 measured a neutron spectrum
using a passive Bonner Ball detector (Au Foil). Labakov et*Corresponding author. E-mail: nakamura@cyric.tohoku.ac.jp.
FAX: +81-22-217-7809.

al.3 measured neutron flux using a Phoswitch-type neutron
detector to eliminate charged particles.
Recently, various neutron spectrometers to be used in spaceenvironment have been developed and are now being developed with the discrimination technique between charged
particle and neutron. Here the outlines of these newly developed neutron spectrometers are described: 1) Phoswich
detector which combines the NE115 plastic scintillator and theNE213 scintillator, 2) Anti-coincidence detector system using∆E and E counters, and 3) Bonner sphere (multi-moderator
detector) mounted a 3He counter or a pair of 6Li and 7Li glass
scintillators.
II. High-Energy Neutron Detectors
II-1. Large NE213 Organic Liquid Scintillator. An
NE213 (or BC501A) organic liquid scintillator coupled with aphoto-multiplier is widely used for high energy neutron spectrometry with the unfolding method in target and shielding
experiments because of good quality of neutron and photon
discrimination, and energy resolution.
The neutron and photon events are usually separated byusing two-dimensional graphical plots of slow and totalcomponent pulse-heights (Figure 1). When charged particles
produced by neutron reactions (mainly recoil protons by theH(n,n)p elastic collision) escape from the NE213 scintillator
without complete energy loss, the pulse shapes from highenergy neutron events are close to those from gamma-ray
events, that is called as a wall effect, and it is impossible todiscriminate only the photon events as shown in Figure 1.
Both events caused by photons and by escaping charged particles are removed. Therefore, the response functions
mentioned below exclude the light outputs caused by theescaping charged particles.
The physical characteristics of the NE213 scintillator, suchas response function, detection efficiency and light output
yield, are well known in detail for neutrons below 20 MeV
through many studies. For higher energy neutron measurement, a large-size NE213 detector, 12.7 cm (5 inch) diam by
12.7 cm (5 inch) long, coupled with a photomultiplier(Hamamatsu R4144) has been used in our group and a larger
25.4 cm (10 inch) diam by 25.4 cm (10 inch) long NE213 is
now in preliminary use. With regard to the response functionsof the 12.7 cm diam by 12.7 cm long NE213 detector for© 2003 The Japan Society of Nuclear and Radiochemical Sciences
Published on Web 12/09/2003

R16 J. Nucl. Radiochem. Sci., Vol. 4, No. 2, 2003

Nakamura

Figure 3. Measured response functions of the NE213 scintillator for
180−200, 200−220, 220−260, 260−320, 320−400, 400−550, and 550−
800 MeV neutrons.6

Figure 1. Two-dimensional pulse height scatter plots of the slow
component versus total component to identify the particles producedin the NE213 scintillator. The discrimination boundary is indicated by
the broken line. The symbol e is the electron produced by photons
and p, d, and α, are the proton, deuteron and alpha particle producedby neutrons, respectively.6

Figure 4. Measured response functions of the NE213 scintillator for
400−550, 205−210, and 130−135 MeV neutrons6 compared with the
CECIL calculation7 and Nakao’s data.5

Figure 2. Measured response functions of the NE213 scintillator for
50−60, 60−70, 70−80, 80−100, 100−120, 120−140, 140−160, and160−180 MeV neutrons.6

neutrons above 20 MeV, Nakao et al.4, 5 measured them in theenergy range up to 206 MeV using the 7Li(p,n) quasi-monoenergetic and white neutron sources at the ring cyclotron facility(RRC) of Institute of Physical and Chemical Research(RIKEN), Japan and Sasaki et al.6 extended them up to 800
MeV using the white neutron source from thick carbon target
bombarded by 400 MeV/nucleon carbon and 800 MeV/nucleon
silicon ions at the heavy ion medical accelerator facility(HIMAC) of National Institute of Radiological Sciences(NIRS), Japan.
The response functions to neutrons with energies between
50 and 800 MeV measured by Sasaki et al. are illustrated in
Figures 2 and 3. The alphabets which appear at the left shoulders of each spectrum from “A” to “O” correspond to theneutron energy boundaries listed in the figures. It has been
hitherto said that the response function does not change above
a few hundreds MeV region where the charged particles
produced by neutron reactions escape from the scintillator, andindeed, the whole shape and the upper edge of the response
functions are very similar to each other beyond 200 MeV
neutrons. But as shown in these graphs, a small change of the
response function and a further expansion of the upper edge tohigher light output can be clearly seen up to 800 MeV bychoosing a wide neutron energy interval beyond 200 MeV.

The event which induces such large light output might bebrought by the multiple collisions through the C(n, x) reactionin the NE213 scintillator, and the resultant charged particles,
such as Be, Li and alpha particle, have higher energy transferfrom the incident neutron with increasing neutron energy.
The measured response functions to 130–135 MeV,
205–210 MeV and 400–550 MeV neutrons are compared withthe results calculated by the CECIL code7 in Figure 4. In thecases of 130–135 MeV and 205–210 MeV neutrons, two
experimental results obtained by Sasaki et al.6 and by Nakao etal.5 are also compared and give very good agreement. The
contribution to light output from the charged particles escaping
from the detector is excluded in the CECIL calculation. For
130–135 MeV and 205–210 MeV neutrons, these experimental
results and the CECIL calculations are in good agreement in
absolute values, although a slight disagreement can be seen in
the low light output region. For 400–450 MeV neutrons, alarge discrepancy in the high energy region can be seen. The
difference of the upper end of light outputs between Sasaki’s
data and the CECIL calculation gradually increases with theneutron energy. This discrepancy is introduced by the inaccurate light output approximation used in the CECIL code.
This different increasing tendency of the light output with
the incident neutron energies is clearly observed by comparison of the light output having the response value of 10 −3(counts MeVee−1 (n cm−2)−1, ee: electron-equivalent) between
the experiment and the CECIL calculation as shown in Figure
5. The light outputs which were experimentally obtained
monotonously increase with the neutron energy while somefluctuation can be observed in the CECIL calculations. Both
light outputs are in good agreement below 100 MeV, but the
CECIL results indicate an overestimation from 100 to 200

Recent Development of Advanced Neutron Detection Technology

J. Nucl. Radiochem. Sci., Vol. 4, No. 1, 2003 R17

Figure 5. Comparison of the increasing tendency of the light output
having the response value of 10−3 (counts MeVee−1 (n cm−2)−1) for each
incident neutron energy between the experiment and the CECIL calculation.6

Figure 7. Measured 209 Bi(n, xn) 210−x Bi (x =3−12) reaction crosssections compared with the ENDF/B-VI high energy library data.9

mb above about 20 MeV as shown in Figure 6. The half life of
C is about 20 min, which makes a shortly-activated goodneutron flux monitor for an energy above 20 MeV.
Bismuth detectors have been conventionally used for highenergy neutron detection through the fission reaction, but ourgroup found that the spallation reactions of 209Bi(n, xn) 210−xBi(x =3−12) are more useful for high-energy neutron spectrometry. The cross sections were also measured by Kim et al.9 inthe energy range of 20 to 150 MeV and the obtained data are ingood agreement with the ENDF/B-VI high-energy librarydata13 calculated by Fukahori with the ALICE code,14 as can beseen in Figure 7. Their threshold energies regularly increase
from 14 MeV of 209Bi(n, 3n) to 88 MeV of 209Bi(n, 12n) withan interval of 8 MeV corresponding to the binding energy per
nucleon. The half lives of 200Bi to 206Bi are between 36.4 min
and 15.31 days. Table 1 shows the physical characteristics of
12
C(n, 2n) 11C and 209Bi(n, xn) 210−x Bi reactions.
The reaction rate, R, is connected to neutron cross section
σ(E) as follows:
11

Figure 6. Measured 12C(n,2n)11C reaction cross section.9

MeV, an underestimation from 200 to 350 MeV and a large
overestimation above 350 MeV. These discrepancies are introduced by the hypothesis of reaction cross sections being kept
constant above 200 MeV. The CECIL code treats insufficientnumber of reaction channels and considers the light output
only for protons and alpha particles. Therefore, it is notcapable to simulate the pion production processes: e.g. np →dπ0 and np → dπ− π+.
In order to measure high-energy neutrons with higher efficiency and better energy resolution, a bigger 25.4 cm diam by
25.4 cm long NE213 scintillator is now in preparation for theexperimental use. The response functions of this detector havealso been measured preliminarily with HIMAC.8
II-2. Spallation Detectors of C and Bi. The spallation
reactions of C and Bi induced by neutrons were used to detect
high energy neutrons. The activation detectors are useful without a pulse pile-up problem in a burst-pulse field, but havegenerally low detection efficiencies. Large-volume activation
detectors are therefore adopted in order to obtain high detection efficiencies. After neutron irradiations, the gamma raysfrom the activation detectors are measured with a high purity
germanium (HPGe) detector.
The 12C(n, 2n)11C reaction has a threshold energy of about
20 MeV and the cross section has recently been measured byKim et al.9 up to 150 MeV by using the p-Li quasi-monoenergetic neutron fields at the SF cyclotron facility at the Institutefor Nuclear Study (INS), University of Tokyo, 10 TIARA(Takasaki Institute of Advanced Radiation Research) AVF
cyclotron facility11 and the RIKEN ring cyclotron facility.12
This reaction has an almost constant cross section of about 20

Emax

R = ∫ σ (E) φ (E)dE

(1)

Eth

where φ (E) = neutron fluence [n cm−2 MeV−1],
E th = threshold energy [MeV],
E max = maximum energy of the monoenergetic peakneutrons [MeV].
II-3. Self-TOF Detector. Sasaki et al.15, 16 developed theself-TOF detector for high energy neutron spectrometry behinda shield. The self-TOF detector consists of radiator detectors,
a start counter and a stop counter of nine segments. A
schematic drawing of the detector is given in Figure 8. The
radiator, called RAD, is a stack of 20 thin plastic scintillators(NE102A) of 10 cm × 10 cm × 0.6 cm. Each plastic scintillator is viewed by a 0.95 cm diam photo-multiplier (Hamamatsu
R1635) through a light guide. The start counter, called
START, is a plastic scintillator (NE102A) of 10 cm × 10 cm ×
0.5 cm viewed by two 5.08 cm diam photo-multipliers(Hamamatsu R2083) from both sides through light guides.

R18 J. Nucl. Radiochem. Sci., Vol. 4, No. 2, 2003

Nakamura

TABLE 1: Physical Characteristics of 12C(n,2n)11C and 209Bi(n,xn)210–x Bi Reactions9
SampleProduction
Nuclei

Nuclear Reaction

Q Value/MeV

ThresholdEnergy/MeV

Half-Life(T1/2)

Emission
Gamma Ray/keV

Emission
Rate/%

12

C

11

12

C(n,2n)11C-18.72

20.4

20.4

min

511

199.5

209

207

209

207

Bi(n,3n) Bi-14.35

14.42

38

y

206

209

Bi(n,4n)206Bi-22.4422.55

6.243 d205

209

Bi(n,5n)205Bi-29.4829.62

15.31 d204

209

Bi(n,6n)204Bi-37.9438.13

11.30 h203

209

Bi(n,7n)203Bi-45.1545.37

11.76 h202

209

Bi(n,8n)202Bi-53.9854.24

1.670 h201

209

Bi(n,9n)201Bi-61.3961.69

1.770 h200

209

Bi(n,10n)200Bi-70.5670.89

36.40 min

199

209

Bi(n,11n)199Bi

-78.1

78.47

24.7

min

198

209

Bi(n,12n)198Bi-87.52

87.94

11.1

min

569.7
1063.6
803.1
881.1
703.4
1763.4
899.2
983.9
820.5
825.2
422.1
960.6
628.2
935.7
462.2
1026.2
425
841
562.4
1063.4

97.8
74.9
98.9
66.2
31
32.4
99.2
58.4
29.7
14.6
83.8
99.4
24.5
11.3
98
100
22
11
79
100

Bi

C
Bi
Bi
Bi
Bi
Bi
Bi
Bi
Bi
Bi
Bi

Figure 9. Experimental setup at the HIMAC.16
Figure 8. Schematic drawing of the self-TOF detector.15, 16

The stop counter, called STOP, is designed to cover an area of
60 cm × 60 cm and is segmented into nine plastic scintillators(NE102A) of 20 cm × 20 cm × 2 cm, each of which is viewed
by a 12.7 cm diam photo-multiplier (Hamamatsu R1259)
through a light guide. The distance between the start and stop
counters is adjustable, and is chosen to be 1.2 m for a usual
measurement.
An in-coming neutron produces charged particles in twenty
radiators, and then the charged particles emitted in the forward
direction reach any one of nine stop counters through the start
counter. The energy of the charged particle is determined byusing the TOF (Time-of-Flight) method between the start andstop counters. In this detector, only proton events from H(n,p)
and C(n,p) reactions are selected to obtain the detector
response function. The neutron energy spectrum can beobtained from the measured proton energy spectrum using an
unfolding method with the response functions.
The response functions of the self-TOF detector for highenergy neutrons up to 800 MeV were measured at HIMAC of
NIRS. Figure 9 shows the experimental setup at HIMAC. The
neutrons were produced by bombarding 400 MeV/nucleon 12C
ion and 800 MeV/nucleon 28Si ion beams with an RF structureon thick (stopping-length) carbon target. The target size was
10 cm × 10 cm and 20 cm thick for 400 MeV/nucleon 12C, and10 cm × 10 cm and 23 cm thick for 800 MeV/nucleon 28Si ion.
The thin NE102A plastic scintillator as a beam pickup
detector, called PICK, (30 mm diam and 0.5 mm thick) was
placed just behind the end window of beam line. This PICK

detector was used to count the number of projectiles and also
as a start signal for conventional neutron TOF measurement.
The carbon target was placed just behind the PICK detector.
The self-TOF detector was placed 516.5 cm downstream
from the target front face. It is already known that this radiation field has a lot of charged particles generated by fragmentation reactions, 1 7 especially in the forward direction.
Accordingly the detector was placed at 15 degrees to the beamline to avoid a large amount of charged fragment particles.
Furthermore, the iron collimator of 60 cm × 60 cm and 40 cm
thick with a hole of 10 cm × 10 cm was set in front of the selfTOF detector to decrease the accidental signals which were
induced by the incidence of charged fragment particles on the
STOP counters, and also to inject neutrons almost normally
into the detector. The veto counter, called VETO, (150 mm ×
150 mm and 5 mm thick NE102A plastic scintillator), wasplaced in front of the radiator to reject charged particle eventsfrom neutron events.
The data were analyzed as follows: (i)Select only the proton
events generated in the radiators by using the light output dataof the veto counter and the most downstream radiator,(ii)Select only the proton event from the two dimensional plotof time-of-flight versus light output of stop counter, and(iii)Convert TOF spectrum into proton energy spectrum.
When all of the events selected are the proton events, theTOF, Tp(x) appeared at the channel x is given by the following
equation,Tp(x) = (x − x0) · ∆T

(2)

Recent Development of Advanced Neutron Detection Technology

J. Nucl. Radiochem. Sci., Vol. 4, No. 1, 2003 R19

where x0 is the channel which corresponds to the time when aproton passes through the start counter, ∆ T is the time perchannel in ns which is determined by a time calibrator module.
Then, the proton energy, Ep(x), is determined by the following
equation using the relativistic kinematics,

about 100 MeV.
This detector can give the neutron energy spectrum by using
the FERDO-U19 or FORIST20 unfolding code combined withthe measured response functions, and is useful for high energy
neutron spectrometry because of the almost constant efficiency
for neutrons above 100 MeV.

1−1)
1− β 2

(3)

V (x)
L
β = pc =c · Tp(x)(4)

Ep(x) = M 0 c2 (where M0 is the rest mass of a proton, Vp is the proton velocity,
c is the light velocity and L is the flight path of 1.2 m.
The correction for proton energy loss in the radiator detectors is not necessary to be considered because the obtained
proton energy spectra are used as response functions of thewhole detector system. The incident neutron energy from thesource is determined by the TOF between the beam pickup
detector and the start counter. By repeating the analysis
mentioned above with changing the ROI of the neutron energy,
the neutron response function matrices can be obtained
between 60 MeV and 800 MeV.
The measured responses were compared with those calculated using the LCS (LAHET Code System).18 As examples,
Figures 10 and 11 show the responses for 100 to 120 MeV
neutrons and for 450 to 500 MeV neutrons, respectively, withthe results calculated by the LCS. The LCS results considerably overestimate the measured results for higher neutronenergy. By using the measured response matrices, the neutron
detection efficiency of the detector was obtained as shown in
Figure 12 (solid line) by comparing with the detection efficiency for 10 MeVee threshold of the NE213 organic liquid
scintillator5 calculated by the CECIL code (broken line).7 Thisdetector has a good efficiency for high-energy neutrons above

Figure 10. Comparison of measured and calculated response functions for 100 to 120 MeV neutrons.16

Figure 11. Comparison of measured and calculated response functions for 450 to 500 MeV neutrons.16

III. Neutron Detectors for Use in a Mixed Field of Charged
Particles and Neutrons
III-1. Phoswich Detector of NE115 and NE213. In thephoswich detector developed by Takada et al.,21 the two scintillators are optically coupled to a single photomultiplier tube
(Hamamatsu H1949). A cross-sectional view is shown in
Figure13. The liquid scintillator is surrounded by a thin slow
plastic scintillator (NE115, 5 mm thick) with a low sensitivity
to neutral particles. The inner detector is a liquid organic scintillator (NE213, 58.5 mm diam and 58 mm length which corresponds to the range of 70 MeV proton) contained in the glass
cell and has much greater sensitivity to neutrons. Charged particles are detected by both scintillators. The light output pulsein the NE115 from charged particles has a long characteristic
time constant of about 225 ns, whereas the light output pulse in
the NE213 by recoil protons from energetic neutrons has atime constant of about 30 ns and that by Compton electrons
from gamma rays is about 3.7 ns. These differences in thelight-decay time constant make it possible to separate pulses of
the three different particle species.
Protons, neutrons and gamma rays are detected separately
by use of a pulse-shape discrimination technique based on thestandard CAMAC charge integration ADCs. The charge integration of the signal is carried out during the time period specified by a gate pulse (total-gate, slow-gate). The total and slow
components can be analyzed by a total gate pulse adjusted at

Figure 12. Neutron detection efficiencies of the self-TOF detector(solid line) and NE213 organic liquid scintillator (broken line).16

Figure 13. Diagram of the phoswich detector.21, 22

R20 J. Nucl. Radiochem. Sci., Vol. 4, No. 2, 2003
the peak of the signal and by a wider delayed slow gate set atthe long tail of the signal, respectively.
Experiments were carried out at two cyclotron facilities: thering cyclotron facility at RIKEN and the AVF cyclotron
facility at NIRS. They produced mixed fields of neutrons andprotons by bombarding 135 MeV protons on a 10 mm thick Litarget at RIKEN and by bombarding 70 MeV protons on a 2
mm thick Be target at NIRS. The detector was placed at 5 −10
m downstream from the target at 30−45 degrees to the proton
beam axis as shown in Figure14. A thin plastic scintillation
detector (NE102A) was positioned in front of the phoswich
detector. This thin detector was used to discriminate between
neutrons and protons for confirming the particle identification
ability of the phoswich detector.
A two-dimensional spectrum of the light output of the slow
component versus the total component measured with thephoswich detector at NIRS is presented in Figure 15 as an
example. In this figure, there are seven distinct groups, labeled“A” through “G.” The component “A” comes from the electrons scattered by gamma rays. The four components “B”
through “E” containing small fractions of slow components are
due to neutrons, where the components “B” and “C” correspond to recoiled protons produced by elastic collisions ofneutrons with hydrogen in the liquid scintillator. The component “D” represents deuterons attributed to the 12C(n,d) reaction, and the component “E” represents alpha particles
produced in the 12C(n,α) and 12C(n,n’3α) reactions. Somefractions of component “B” is due to protons that lost someenergy in the NE213 and then escaped from the NE213. The
component “C” corresponds to protons that deposited all of
their energy in the NE213. The components “F” and “G”

Nakamura
doubtless contain external protons reaching the NE213 throughthe NE115 scintillator and the glass cell. The events where theexternal protons stopped in the NE115 are not seen in thisfigure because of the detection logic.
All events in the two-dimensional plot of Figure 15 were
projected onto the total component axis to obtain individual
spectra of gamma-ray, neutron and proton events, as shown in
Figure 16. We can see in these spectra that the discrimination
between gamma rays, neutrons and protons is excellent, andthat the proton peaks are clearly identifiable. This proton peakcorresponds to the elastically scattered one toward the detector
from the target having about 58 MeV energy.
The response functions of this type phoswich detector oflarger size (15 mm thick NE115 and 133 mm diam by 133 mm
long NE213) were also measured for neutrons up to 130 MeV
at RIKEN using a white neutron source of 135 MeV proton on
thick Be and C targets, coupled with the neutron TOF technique. By using the measured response functions, the neutronenergy spectrum can be obtained separately from the gammaray and proton energy spectra by unfolding with the FERDO-U
code,19 as shown in Figure 17.22 The proton has a monoenergetic peak at around 57 MeV which corresponds to the elastically scattered proton energy of 58 MeV. The neutron
spectrum has a continuous energy distribution extended toabout 70 MeV.
III-2. Anti-coincidence System of ∆E and E Counters.
The ∆ E and E counter telescope with the anti-coincidence
detection system is widely used for particle identification. We
used this detection system to measure neutron energy spectra

Figure 14. Schematic view of the experimental set up in a protonmixed field at NIRS and RIKEN.21, 22

Figure 16. The light output spectra projected on the total component
axis obtained by discriminating between gamma-ray, neutron andproton events in Figure 15.21, 22

Figure 15. A two-dimensional pulse height spectrum of the slow
component versus the total component measured with the phoswich
detector at the NIRS. The component “A” shows gamma-ray events,
the components “B” through “E” show neutron events and the components “F” and “G” show proton events.21, 22

Figure 17. Neutron, proton and gamma-ray energy spectra 21, 22
obtained by the FERDO-U unfolding code.19

Recent Development of Advanced Neutron Detection Technology(energy response function) obtained for the various thicknessesof polyethylene moderator. This Bonner sphere is the same
one as that developed by Uwamino et al.,25 but adjusted for use
in space. Figure 20 shows the cross sectional view of sensors.
Sensor 2 and sensor 3 are covered with 1mm-thick gadolinium
to block thermal neutrons.
The response functions of the Bonner ball were calculated in
the neutron energy range of 0.025 eV to 100 MeV by using the
MCNP-4B Monte Carlo code26 with several cross section datalibraries, and the calculation accuracy is checked by the monoenergetic neutron measurements at the Fast Neutron
Laboratory (FNL) of Tohoku University. The neutron energy
spectrum can be obtained by unfolding with the SAND-2
code27 under a certain initial guess spectrum.
The Bonner ball detector was mounted in the SPACEHAB
module in the space shuttle cargo bay, and the neutron energy
spectrum in the shuttle was measured on January 24 to 28,
1998 in S/MM-08 (STS-89). Measured data were monitored
through downlink at the NASA Johnson Space Center Mission
Control Center and transmitted for analysis with orbital data to
Tsukuba Space Center, NASDA. The output pulse-height
distribution obtained from each Bonner ball presented a sharp
thermal neutron peak, and neutrons could be clearly distinguished from protons and other particles. Figure 21 compares
the predicted pulse-height distribution (worst case) and themeasured output pulse-height distribution. Figure 22 gives thethree data of the neutron energy spectra obtained at the equatorial region, polar region and SAA (South Atlantic Anomaly)
Polyethylene sphere
3

Covered by gadolinium

H proportional counter

230 φ

150 φ

110 φ

81 φ

51 φ

Sensor 6

Sensor 5

Sensor 4

Sensor 3

Sensor 2

51 φ [mm]Sensor 1

Figure 20. Cross sectional view of each sensor.24
Thermal neutron peak

300

Thermal neutron peak (765 keV)
countproduced from the high energy heavy ions at HIMAC.
The NE213 liquid scintillator (12.7 cm diam by 12.7 cm
thick) coupled with the R4144 photomultiplier connected toE1458 base (Hamamatsu) were used for the neutron detector(E counter). The NE102A plastic scintillator (15 cm by 15 cm
square and 0.5 cm thick), coupled with the H1949 photomultiplier and base (Hamamatsu), were used for a ∆E counter andwere placed in front of the E counter to discriminate charged
particles from uncharged particles, neutrons and photons. The
total and slow gate timings for the neutron-photon discrimination with respect to the analog signals were shown in Figure
18.
To eliminate charged particles, we used two-dimensional∆E-E graphical plots (Figure 19). As neutrons and gamma
rays do not scintillate the ∆E counter, the neutron and gammaray events could be selected from the charged particle events
as seen in Figure 19. After this discrimination, the neutron andgamma-ray events were separated by using two-dimensional,
total-slow pulse-height graphical plots (Figure 19). In thisdiscrimination, the pulse shapes from high energy neutron
events in which recoil protons escape from the E counter areclose to those from gamma-ray events, and these events were
eliminated from the neutron events.
The neutron energy spectrum can also be obtained byunfolding with the FERDO-U code coupled with the pre-determined response functions. In space, the charged particles andneutrons enter into the detector from all directions. Then the∆E counter must fully cover the E counter for neutron identification. As a result, the ∆ E-E counter telescope inevitably
becomes rather big.
III-3. Bonner Sphere with 3 He Counter. The Bonner
sphere (Bonner ball) fabricated by the National Space
Development Agency (NASDA) and Mitsubishi ResearchInstitute Inc. (MRI) consists of six 3He proportional counters
covered with a neutron moderator of different thickness.24 The
3
He proportional counters at the center of the six sensors are
identical, 5.08 cm diam spheres and filled with 5 atm 3He gas.
Each counter is covered by a polyethylene moderator of different thickness (1.5 cm, 3 cm, 5 cm, and 9 cm), and neutronenergy spectra can be obtained through the unfolding technique using the difference in the neutron sensitivity function

J. Nucl. Radiochem. Sci., Vol. 4, No. 1, 2003 R21

count

23

200

100

Proton Pulse
0

Pulse-height CH

10

20

30

40

50

60

CH

Figure 21. Output pulse height distribution. (a) Predicted pulseheight distribution. (b) Measured output pulse-height.24

Figure 18. The gate timing of the total and slow pulse heights.

Figure 19. Two-dimensional plots of E and ∆E counters for (a) the
pulse heights and (b) the total and slow pulse heights.23

Figure 22. Neutron energy spectra inside shuttle at three regions.24
SAA means the South Atlantic Anomaly.

R22 J. Nucl. Radiochem. Sci., Vol. 4, No. 2, 2003
region under the initial guess spectrum that the neutron energy
spectrum φ (E) is proportional to l/E.24 These three spectra areclose together in their shapes, but the magnitude at SAA isabout 100 and 1000 times higher than the polar region and theequatorial region, respectively.
III-4. Bonner Sphere with 6Li and 7Li Glass Scintillators.
Two types of the Bonner sphere with a pair of 6Li and 7Li glass
scintillators were developed. One type uses many pairs of 6Li
and 7Li glass scintillators inserted into one polyethylene moderator which has been developed by Yamaguchi et al.28 and theother uses one pair of 6Li and 7Li glass scintillator mounted
into several polyethylene moderators of different thickness
which has been developed by Taniguchi et al.29
An illustration of the former detector is shown in Figure 23.
The detector consists of a spherical polyethylene moderator
with a diameter of 30 cm and 19 pairs of 6Li- and 7Li-glass
scintillators (10 mm diam with 2 mm thickness) arranged
along the orthogonal x, y, and z axes of the sphere. Each scintillator is coupled to a thin photomultiplier tube (Hamamatsu
R5600P) using an acrylic light-guide with silicon grease. The
scintillator, the light-guide and the photomultiplier are covered
with a black light-shield case which are made of a similarmaterial to polyethylene and do not obstruct the thermal
neutron flux in the spherical moderator.
The different response between the 6Li and 7Li glass scintillators enables to distinguish neutrons from other radiation.
The 6 Li-glass scintillators are sensitive to both thermal
neutrons and the other radiation. The 7Li glass scintillators are
sensitive only to radiation except for thermal neutrons. The
thermal neutron flux at the scintillator pair can be obtained bysubtracting the number of events detected by the 7 Li-glass
scintillator from that by 6Li-glass one. The dose equivalentand the energy spectrum can be obtained with a proper
unfolding process. The present counter allows to obtain theinformation with only one measurement by using the position
information of the thermal neutron flux in the sphere.
Three monoenergetic neutron beams with energies of 165
keV, 565 keV and 5 MeV were generated by a CockcroftWalton accelerator at the Electro-Technical Laboratory.28
Figure 24 presents the response of the detector obtained by theaxial scintillator pairs at the three angles for the respective
monoenergetic neutron. The points stand for the number of theneutron detection events obtained by each scintillator pair.
The line shows the thermal neutron flux distribution expectedfrom the experimental points. First, comparing the responsesof the x-axial scintillator pairs at an angle of 0˚ obtained at
different energies of the beams, the thermal neutron flux distribution is extended to the center of the spherical moderator withincrease of the neutron energy. The responses of the y- and z-

Nakamura
axial scintillator pairs show a good symmetry about the centerof the sphere. The number of the neutron detection events
increases with neutron energy similar to that of the x-axial
scintillator pairs. These responses of the detector express thecharacteristics of the neutron mean free pass in polyethylene.
Second, comparing the responses of the x-axial scintillator
pairs at different angles, the response varied reasonably withthe change of the incident angle. The detector shows an excellent response against the different energy and the different
direction of the neutrons.
The results demonstrate that the present detector allows to
obtain much information about incident neutrons. This abilityof the detector, however, was checked only for the monoenergetic neutron beam and the point-like neutron source, notchecked in isotropic and proton existing fields. For practicaluse of the detector in the space environment, characterizationof the detector in such fields is necessary.
The latter detector uses a pair of 2.54 cm diam × 2.54 cm
long NE912 (7.7 wt% of 95% 6Li-enriched lithium doped) andNE913 (8.3 wt% of 99.99% 7Li-enriched lithium doped) glass
scintillators. Each of these two scintillators is coupled with aphoto-multiplier (Hamamatsu R1924) mounted out of themoderator through the acrylic light-guide. The sphericalmoderators are made of polyethylene and their thicknesses are
1.5, 3.0, 5.0, and 9.0 cm. A schematic view of the spectrometer is shown in Figure 25. The response functions to neutrons
from 10 −9 MeV up to 200 MeV were calculated 29 by the
MCNPX Monte Carlo calculation 30 with the ENDF/B-VI
neutron cross section data library.13

Black light-shield
Photomultiplier tube

Acrylic light-guide
Scintillator pairs
Polyethylene sphere

Figure 23. A schematic drawing of the Bonner sphere.28

Figure 24. Experimentally obtained response of the axial scintillator
pairs for monoenergetic neutron beams of (a) 165 keV, (b) 565 keV
and (c) 5 MeV from angles of 0˚, 22.8˚ and 45.6˚.28

Recent Development of Advanced Neutron Detection Technology

J. Nucl. Radiochem. Sci., Vol. 4, No. 1, 2003 R23

Figure 25. Schematic view of the multi-moderator spectrometer witha pair of lithium glass scintillators.29
Figure 27. The unfolded spectrum with the spectrum calculated by
the MCNPX code.29

IV. Summary

Figure 26. An example of the pulse height spectra of 6Li and 7Li glass
scintillators.29

The neutron spectrum measurement was performed in the
proton and neutron mixed field using the AVF cyclotron at
NIRS. The protons accelerated up to 70 MeV were irradiatedonto a 2 mm thick beryllium target. The detector was placed at
45 degrees to the proton beam axis 350 cm behind the target.
The protons passed through the target were stopped in analuminum beam dump and the polyethylene blocks were put
between the beam dump and the detector for neutron shielding
from the dump. Figure 26 shows an example of the pulse
height spectra of 6Li and 7Li glass scintillators mounted in 5
cm thick polyethylene. The 6Li glass scintillator gives a large
peak due to thermal neutrons through the 6Li(n,α) reaction,
while no peak can be found in the 7Li glass scintillator. The
thermal neutron events can easily be obtained from these twopulse height spectra. The counts measured by using fivedifferent moderators were unfolded with the SAND-2 code to
get the neutron spectrum in this field. In this unfolding the l /E
spectrum described before was also used as an initial guess
spectrum. The unfolded spectrum is shown in Figure 27
together with the spectrum calculated by the MCNPX code forcomparison. The spectrum measured with the detector gives
rather good agreement with the calculation both in spectral
shape and in absolute values.

For neutrons of energies beyond 100 MeV, the self-TOF
detector using the NE102A plastic scintillators, large-scale
NE213 organic liquid scintillator, and spallation detectors of C
and Bi have been recently developed by our group, especiallyfor use in neutron target and shielding experiments.
Various neutron spectrometers have also been developed foruse in the charged particle and neutron mixed field to be realized in space environment. The following spectrometers arebriefly described: 1) Phoswich detector which combines theNE115 plastic scintillator and the NE213 scintillator, 2) Anticoincidence detector system using ∆E and E counters, and 3)
Bonner sphere (multi-moderator detector) mounted a 3 He
counter or a pair of 6Li and 7Li glass scintillators.
Our group has also developed a new-type Bonner sphereusing a combination of polyethylene and boron moderators formeasuring the neutron energy spectrum in the epithermalenergy region of 1 eV to 100 keV.31 This detector will be used
for neutron measurement in the accelerator-based boron
neutron capture therapy.
These newly-developed detectors are now used in various
applications. Further development of other advanced neutron
spectrometers is now strongly required for increasing use ofneutrons in various fields.
REFERENCES(1) J. E. Keith, G.D. Badhwar, and D. J. Lindstrom, Nucl.
Tracks. Radiat. Meas. 20, 41 (1992).
(2) V. Dudkin, Y. V. Potapov, A. B. Akopova, L. V.
Melkumyan, E. V. Benton, and A. L. Frank, Nucl. Tracks.
Radiat. Meas. 17, 87 (1990).
(3) A. P. Labakov, V. I. Lyagushin, M. I. Panasyuk, V. M.
Petrov, and P. I. Shavrin, Nucl. Tracks. Radiat. Meas. 20,
55 (1992).
(4) N. Nakao, T. Nakamura, M. Baba, Y. Uwamino, N.
Nakanishi, H. Nakashima, and Sh. Tanaka, Nucl. Instr.
and Meth. A362, 454 (1995).
(5) N. Nakao, T. Kurosawa, T. Nakamura, and Y. Uwamino,
Nucl. Instr. and Meth. A463, 275 (2001).
(6) M. Sasaki, N. Nakao, T. Nakamura, T. Shibata, and A.
Fukumura, Nucl. Instr. and Meth. A480, 440 (2002).
(7) R. A. Cecil, B. D. Anderson, and R. Madey, Nucl. Instr.
and Meth. 161, 439 (1979).

R24 J. Nucl. Radiochem. Sci., Vol. 4, No. 2, 2003
(8) T. Moriya, Master Thesis of Kyoto University, March
2003, (in Japanese).
(9) E. Kim, T. Nakamura, A.Konno, Y. Uwamino, N.
Nakanishi, M. Imamura, N. Nakao, T. Shibata, and Su.
Tanaka, Nucl. Sci. Eng. 129, 209 (1998).
(10) Y. Uno, Y. Uwamino, T. S. Soewarsono, and T. Nakamura,
Nucl. Sci. Eng. 122, 247 (1996).
(11) M. Baba, Y. Nauchi, T. Iwasaki, T. Kiyosumi, M. Yoshioka,
S. Matsuyama, N. Hirakawa, T. Nakamura, Su. Tanaka, S.
Meigo, H. Nakashima, Sh. Tanaka, and N. Nakao, Nucl.
Instr. and Meth. A428, 454 (1999).
(12) N. Nakao, Y. Uwamino, T. Nakamura, T. Shibata, N.
Nakanishi, M. Takada, E. Kim, and T. Kurosawa, Nucl.
Instr. and Meth. A420, 218 (1999).
(13) Evaluated Nuclear Data File ENDF/B-VI (1990) National
Neutron Cross Section Center, Brookhaven National
Laboratory; P. F. Rose, ENDF-201, ENDF/B-VI Summary
Documentation, BNL-NCS-17541, 4th Edition (1999).
(14) T. Fukahori, Proc. the 1991 Symposium on Nuclear Data,
Tokai, Japan, JAERI-M 91-032, p106 (1991).
(15) M. Sasaki, M. Nakao, T. Shibata, N. Nakao, and T.
Nakamura, Nucl. Instr. and Meth. A446, 545 (2000).
(16) M. Sasaki, N. Nakao, T. Nunomiya, T. Nakamura, T.
Shibata, and A. Fukumura, J. Nucl. Sci. Technol. 38, 8(2001).
(17) T. Kurosawa, T. Nakamura, N. Nakao, T. Shibata, Y.
Uwamino, and A. Fukumura, Nucl. Instr. and Meth. A430,
400 (1999).
(18) R. E. Prael and H. Lichtenstein, User Guide to LCS: The
LAHET Code System, MS B226, Los Alamos National
Laboratory (1989).
(19) K. Shin, Y. Uwamino, and T. Hyodo, Nucl. Technol. 53,

Nakamura78 (1981).
(20) R. H. Johnson, FORIST: Neutron Spectrum Unfolding
Code-Iterative Smoothing Technique, PSR-92, ORNL/RSIC, Oak Ridge National Laboratory (1976).
(21) M. Takada, S. Taniguchi, T. Nakamura, and K. Fujitaka,IEEE Trans. on Nucl. Sci. 45, 888 (1998).
(22) M. Takada, S. Taniguchi, T. Nakamura, N. Nakao, Y.
Uwamino, T. Shibata, and K. Fujitaka, Nucl. Instr. andMeth. A465, 498 (2001).
(23) T. Kurosawa, T. Nakamura, N. Nakao, T. Shibata, Y.
Uwamino, and A. Fukumura, Nucl. Instr. and Meth. A430,
400 (1999).
(24) H. Matsumoto, T. Goka, K. Koga, S. Iwai, T. Uehara, O.
Sato, and S. Takagi, Radiat. Meas. 33, 321 (2001).
(25) Y. Uwamino, T. Nakamura, and A. Hara, Nucl. Instr. andMeth. A239, 299 (1985).
(26) F. Judith, MCNP-A General Monte Carlo N-Particle
Transport Code Version 4B, LA-12625-M, Los Alamos
National Laboratory (1997).
(27) W. N. McElroy, S. Berg, T. Crockett, and R. G. Hawkins,
A Computer-Automated Iterative Method for Neutron
Spectra Determination by Foil Activation, AFWL-TR-6741, Air Force Weapons Laboratory (1967).
(28) S. Yamaguchi, A. Uritani, H. Sakai, C. Mori, T. Iguchi, H.
Toyokawa, N. Takeda, and K. Kudo, Nucl. Instr. andMeth. A422, 600 (1999).
(29) S. Taniguchi, M. Takada, and T. Nakamura, Nucl. Instr.
and Meth. A460, 368 (2001).
(30) L. S. Waters, ed. MCNPX USER’S MANUAL, TPOE83-G-UG-X-00001 (1991).
(31) S. Yonai, T. Itaga, T. Nakamura, and M. Baba, J. Nucl.
Sci. Technol. (in press).