Problems of remote detection of chemical explosives and fissile materials using neutron-activation diagnostics method
This paper presents results of computer simulation of nuclear processes (using Geant4 9.0 package) for neutrons passing through a shell-free explosive model and through a model of military load containing some fissile material. Spectral distributions of γ-quanta were calculated for the neutron with...
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irk-123456789-804872015-04-19T03:02:35Z Problems of remote detection of chemical explosives and fissile materials using neutron-activation diagnostics method Dubina, V.N. Chornyj, A.V. Chornyj, V.V. Ядерно-физические методы и обработка данных This paper presents results of computer simulation of nuclear processes (using Geant4 9.0 package) for neutrons passing through a shell-free explosive model and through a model of military load containing some fissile material. Spectral distributions of γ-quanta were calculated for the neutron with fixed energy at different points of time after irradiation. The programs were designed using C++ and function under execution control of OS Red Hat LINUX 6.2 FEDORA. Представлены результаты компьютерного моделирования с использованием пакета Geant4-09 ядерно-физических процессов прохождения нейтронов через безоболочечную модель взрывчатого вещества, а также через модель боезаряда с делящимся веществом. Рассчитаны спектральные распределения γ-квантов для фиксированной энергии нейтронов в различные моменты времени после облучения.. Программы разработаны на языке C++ и работают под управлением OS Red Hat LINUX 6.2 FEDORA Представлено результати комп'ютерного моделювання з використанням пакету Geant4-09 ядерно-фiзичних процесiв проходження нейтронiв через модель вибухової речовини без оболонки, а також через модель боєзаряду з подiльчим матерiалом. Розрахованi спектральнi розподiли γ-квантiв для фiксованої енергiї нейтронiв у рiзнi моменти часу пiсля опромiнення. Програми розробленi на мовi C + + i працюють пiд управлiнням OS Red Hat LINUX 6.2 FEDORA. 2014 Article Problems of remote detection of chemical explosives and fissile materials using neutron-activation diagnostics method / V.N. Dubina, A.V. Chornyj, V.V. Chornyj // Вопросы атомной науки и техники. — 2014. — № 5. — С. 69-75. — Бібліогр.: 13 назв. — англ. 1562-6016 PACS: 28.50.Ma, 61.20.ja http://dspace.nbuv.gov.ua/handle/123456789/80487 en Вопросы атомной науки и техники Національний науковий центр «Харківський фізико-технічний інститут» НАН України |
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Ядерно-физические методы и обработка данных Ядерно-физические методы и обработка данных |
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Ядерно-физические методы и обработка данных Ядерно-физические методы и обработка данных Dubina, V.N. Chornyj, A.V. Chornyj, V.V. Problems of remote detection of chemical explosives and fissile materials using neutron-activation diagnostics method Вопросы атомной науки и техники |
| description |
This paper presents results of computer simulation of nuclear processes (using Geant4 9.0 package) for neutrons passing through a shell-free explosive model and through a model of military load containing some fissile material. Spectral distributions of γ-quanta were calculated for the neutron with fixed energy at different points of time after irradiation. The programs were designed using C++ and function under execution control of OS Red Hat LINUX 6.2 FEDORA. |
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Article |
| author |
Dubina, V.N. Chornyj, A.V. Chornyj, V.V. |
| author_facet |
Dubina, V.N. Chornyj, A.V. Chornyj, V.V. |
| author_sort |
Dubina, V.N. |
| title |
Problems of remote detection of chemical explosives and fissile materials using neutron-activation diagnostics method |
| title_short |
Problems of remote detection of chemical explosives and fissile materials using neutron-activation diagnostics method |
| title_full |
Problems of remote detection of chemical explosives and fissile materials using neutron-activation diagnostics method |
| title_fullStr |
Problems of remote detection of chemical explosives and fissile materials using neutron-activation diagnostics method |
| title_full_unstemmed |
Problems of remote detection of chemical explosives and fissile materials using neutron-activation diagnostics method |
| title_sort |
problems of remote detection of chemical explosives and fissile materials using neutron-activation diagnostics method |
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Національний науковий центр «Харківський фізико-технічний інститут» НАН України |
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2014 |
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Ядерно-физические методы и обработка данных |
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http://dspace.nbuv.gov.ua/handle/123456789/80487 |
| citation_txt |
Problems of remote detection of chemical explosives and fissile materials using neutron-activation diagnostics method / V.N. Dubina, A.V. Chornyj, V.V. Chornyj // Вопросы атомной науки и техники. — 2014. — № 5. — С. 69-75. — Бібліогр.: 13 назв. — англ. |
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Вопросы атомной науки и техники |
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2025-07-06T04:29:58Z |
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| fulltext |
PROBLEMS OF REMOTE DETECTION OF CHEMICAL
EXPLOSIVES AND FISSILE MATERIALS USING
NEUTRON-ACTIVATION DIAGNOSTICS METHOD
V.N.Dubina, A.V.Chornyj, V.V.Chornyj∗
V.N. Karazin Kharkov National University, 61022, Kharkov, Ukraine
(Received June 27, 2014)
This paper presents results of computer simulation of nuclear processes (using Geant4 9.0 package) for neutrons
passing through a shell-free explosive model and through a model of military load containing some fissile material.
Spectral distributions of γ-quanta were calculated for the neutron with fixed energy at different points of time after
irradiation. The programs were designed using C++ and function under execution control of OS Red Hat LINUX 6.2
FEDORA.
PACS: 28.50.Ma, 61.20.ja
1. INTRODUCTION
Detection of secretly transported explosives is one
of the major components of terrorism control. In
connection with growth of the number of countries
possessing nuclear weapon, there is a threat of nu-
clear terrorism and, naturally, a problem of inspect-
ing possible secret transportation of nuclear weapon
fissile materials. Experts in many laboratories over
the world have been working for the period of more
than two dozen years already on development of en-
gineering tools for detection of standard chemical ex-
plosives, as well as fissile materials transported in
the luggage of passengers travelling by various types
of transport, especially by air. In spite of certain
progress in development of the detection engineering
tools it is still early to state that the problem of quick
and faultless detection of explosives has been solved.
At first, this problem is caused by a large amount
of luggage transported all over the world; the air-
line traffic industry alone services more than billion
passengers a year. Hence, to prevent getting an ex-
plosive aboard a plane, the detection system should
function quickly, automatically, with high probabil-
ity of identification and very low level of false oper-
ations. Secondly, the explosive may be masked with
any materials containing the same set of chemical el-
ements, for example, various types of plastics, as well
as with some special kinds of masking. Therefore,
the detection system should be capable to detect and
unmistakably recognize the unique characteristics in-
herent only in the explosive. Similar problems might
also occur when detecting fissile materials. Chem-
ical explosive consists, generally, of atoms of nitro-
gen and oxygen in the condensed state. Detection of
nitrogen high concentration would signal about pos-
sible presence of explosives, and high concentration
of nitrogen and oxygen with a high level of proba-
bility points to the presence of a bomb [1]. More
than a dozen of various nuclear-physical techniques
have been offered for scanning luggage with the pur-
pose of detecting any hidden explosives by measur-
ing distribution of these elements. These techniques
are based on physical principles, well-known since
the 50th - 60th of the last century, however to im-
plement these principles the most advanced techno-
logical achievements are required today. The pur-
pose of this work is to further research feasibilities
of remote detection of fissile materials and chemi-
cal explosives using method of sounding them with
high power pulses of neutrons having different ener-
gies, initiated by authors in [2]. As it was stated
in http://www.jurnal.org/articles/2008/enerj4.html,
passive methods show rather unreliable result, in par-
ticular, because of possible usage of some kind of pro-
tection (this article is also devoted to this case); as
to conventional explosives, just none of any other ef-
fective possibilities to detect them is available now.
Pulsed ion sources would enable creating rather pow-
erful neutron sources for irradiation of remote ob-
jects. Preliminary calculations [3] showed a number
of essential advantages in usage of sub-MeV neutrons
for reliable detection of fissile substance at rather
long distances. However, simulation of actual mil-
itary loads having shell structure, showed essential
problems in the process of implementation of such
projects. In the process of the universal activation
method development [9] the possibilities to detect
fissile materials as well as chemical explosives, us-
ing neutrons from T (d, n)4He-type reaction were an-
alyzed. Energies of about 14MeV allow researching
∗Corresponding author E-mail address: chorny@pht.univer.kharkov.ua
ISSN 1562-6016. PROBLEMS OF ATOMIC SCIENCE AND TECHNOLOGY, 2014, N5 (93).
Series: Nuclear Physics Investigations (63), p.69-75.
69
a substance gamma response in a broad range of en-
ergies what is especially important for obtaining reli-
able information about presence of explosives gener-
ally used during acts of terror. Taking into account
that the technique of nuclear radiation detection has
been continually improved (spectral selectivity, time
resolution, sensitivity, etc.), special attention in this
paper is given to progress of the object response in
time after irradiation it by a short neutron impulse.
In combination with quickly developing computer
possibilities as to the information flow processing,
the obtained data, according to the authors’ opinion,
would allow creating more perfect recognition algo-
rithms in systems designed for detection of explosives
and fissile materials.
2. NEUTRON SOUNDING OF A
MILITARY LOAD FILLED WITH A
FISSILE MATERIAL
Usage of sub-MeV neutrons for irradiation of 235U
and 239Pu nuclides that are principal components of
nuclear weapon, allows detecting them against the
background of other masking materials, such as 238U
or W which also might be used as elements for mili-
tary load construction. As far as 238U and tungsten
could not undergo fission by sub-MeV neutrons, so,
occurrence of the fission neutron response in a sig-
nal would signalize about presence of true weapons
grade uranium or plutonium. Up-to-date technolo-
gies of high power pulsed beams of light ions allowed
to create compact accelerators generating ion flows of
more than 3.12 · 1016 ions per pulse whose duration
is several tens of nanoseconds what allows generat-
ing neutron flows of 1012 per pulse. When using a
low-threshold nuclear reaction 12C(d, n)13N with en-
ergy threshold of neutron generation Ed = 324 keV ,
or more effective reactions T (p, n)3He, 7Li(p, n)7Be
with energy thresholds of Ep = 1.018MeV , Ep =
2.25MeV accordingly, it is possible to develop sub-
MeV-neutron generators with variable energy. High-
efficiency T (d, n)4He reaction is used to generate
mono-energetic neutrons with energy of 14MeV .
High-energy gamma rays of bremsstrahlung (more
than 6MeV , photofission threshold (γ, f)) generated
by electron beams, are also capable to cause fission
and to get a stable signal from a spherical object with
radius of 23 cm at a distance of up to 50m. How-
ever reaction (γ, f) deals with large cross-sections
not only for 235U , but for other transuranic elements
too, including 238U . I.e., adding of a signal from 235U
at big enough mass ratios of 238U/235U , just as for
fast neutrons, could be insignificant. An advantage of
gamma radiation as compared to the neutrons is its
high coefficient of passage through the shell surround-
ing the fissile material, for example, the hydrogenous
layer of the explosive.
To calculate the warhead response signal
to the probing neutron pulse the hypotheti-
cal warhead model from [5] was used (Fig.1).
Fig.1. Warhead model. The model mass is of about
182 kg. WgU or WgPu – weapons grade 235U or
239Pu (with the degree of enrichment more than
93, 3%)
The model is presented by a series of concentric
spherical shells with some fissile material inside sur-
rounded by Beryllium reflector, a large case (tam-
per) of depleted uranium or tungsten, a layer of
high-efficiency explosive and an outer case of alu-
minum. Neutron beams with energy of 0.5MeV and
14MeV were used in calculations; beam diameters
were accepted to be equal to the exterior diameter
of the warhead model. At the same time the com-
position of radiation emitted to the front and back
hemispheres after interaction between the incident
neutrons and the irradiated object were analyzed.
When the fissile substance enclosed in 10 cm hy-
drogenous explosive (H2CN2O2) is irradiated by neu-
trons, a significant portion of the irradiating neutron
flow (Figs.3, 4) is moderated to the energy lower than
1 keV at which the 235U fission cross-section increases
to more than 10 barn. Gamma-radiation accompany-
ing neutron fission and capture by a fissile material is
essentially absorbed by the fissile material itself and
by surrounding heavy cases. The greatest yield of
gamma radiation is along the channel (n, f).
Fig.2,a. presents absorption of gamma signal
(n, f), mainly by the fissile 235U itself and the shells
of 238U orW . The peak in the energy range of 94 keV
within the emitted radiation spectrum corresponds to
the uranium K-line generated in the process of ab-
sorption of higher energy gamma rays. Gamma radia-
tion yield is less by the factor of ten along the channel
(n, γ). Fig.2,b presents this signal self-absorption in
235U .
When the war head model is irradiated the neu-
tron flow spectrum undergoes essential transforma-
tion, which, first of all, is due to neutrons penetra-
tion of the explosive material (up to 20 cm). Fig.3a
presents spectral distribution of neutrons emitted
from the model to the front hemisphere after the
model was irradiated by 14MeV neutrons.
The model neutron yield forward makes 27% of
the incident flow, while 8% of neutrons passes without
interaction which may be accounted for by tangential
passage of these neutrons in regard to the spherical
object), 5% of neutrons reduce their energy to the
value below 1 keV due to their elastic scattering.
70
a
b
Fig.2. Absorption of fission gamma rays (a) and
capture gamma rays (b) by elements of the war head
structure. The dotted line here is for the generated
radiation, and solid line for the radiation emitted
from the model
Fig.3. Spectral distribution of the neutrons emitted
forward after passing through the test specimen (
dotted line) and the neutron reflected signal (solid
line) at irradiation energy En = 14MeV
In the diagram maximal and minimal energies
show approximately identical marked peaks, in the
middle of the energy range the neutrons are dis-
tributed evenly enough what points to deceleration
occurring in the hydrogenous medium. The most
promising as to the detection problem, is the anal-
ysis of the neutron reflected signal. The neutron
spectrum directed to the background hemisphere
when the model is irradiated by 14MeV neutrons is
also shown in Fig.3. The figure shows that 14MeV
neutrons are virtually not reflected by the model,
and neutrons, strongly decelerated in the explo-
sive, as well as fission neutrons emit backward. If
the reflected signal makes 50% of the incident flow
(together with the fission neutrons emitted back-
ward), then about 20% of neutrons in this signal
have energy less than 1 keV . Rather a different
pattern is watched when the model is irradiated
by sub-MeV neutrons (500 keV ). The reflected sig-
nal for the sub-MeV neutrons is shown in Fig.4.
Fig.4. Spectral distribution of sub-MeV neutrons
(500 keV ), emitted through the model (solid line).
The reflected signal of sub-MeV neutrons (dotted
line)
About 20% of neutrons are reflected practically with-
out interaction with the model. The number of neu-
trons moderated to the energy of less than 1 keV , is
also about 20%. Distribution of the neutrons which
have passed through the model, is similar to that
of the reflected neutrons (Fig.4). Nearly 50% of
the signal are the neutrons that passed through the
model practically without interaction (elastic scatter-
ing). Most likely, major portion of them passed over
periphery of the spherical model, 25% of them are
neutrons moderated to the energy of less than 1 keV .
However, the total amount of the neutrons emitted
forward is not more than 3% of the incident flow.
To detect the military load on the basis of fission
neutrons for certain it is required that the primary
neutrons not only effectively hit the area under re-
search (the area containing 235U), but also that fast
(”super-MeV ”) fission neutrons of 235U effectively
emitted from the model. Special calculations showed
the following effectiveness of fast neutrons passage
through 10 cm of a shell containing some hydroge-
nous substance.
Penetration effectiveness of neutrons with various
energies through a 10-cm explosive target
En 100 keV 1MeV 2MeV 14MeV
K = 0.15 0.29 0.37 0.66
N/N0 (>700 (>1 (>1
keV) MeV) MeV)
Only 15.4% of the sub-MeV neutrons passes
through the explosive and 30% of them are moder-
ated to thermal ones (see Table). 30% of neutrons
with primary energy of 1MeV would conserve en-
ergy in the range of over 700 keV ; accordingly 37%
71
of the 2MeV neutrons leave the moderator with en-
ergy of over 1MeV . The Table shows as well high
penetration efficiency of 14MeV neutrons with low
reflection coefficient (see Fig.3).
Calculations for the warhead model were carried
out by comparison of algorithm of signals in the pres-
ence of 235U and at in its absence. Both gamma re-
sponse signals and the reflected neutron signal in the
energy range of more than 100 keV were analyzed.
When the model with tamper of 238U was irradi-
ated by 14MeV neutrons, the signal increased by
16%, and by 8% when the model was with tamper of
W . Relative amplification of the signal is accounted
for the fact that in the spectrum of 235U fission the
neutrons with energy higher than 106 eV are present
which cause additional fission in 238U . In case of ir-
radiation by sub-MeV neutrons the signal increased
by 7% when the model with 238U shell was irradi-
ated, and by 1.3% when the model with a tungsten
shell was irradiated. The relative effect of the sig-
nal amplification caused by fast neutrons fission in
the 238U shell is more evident. However in case with
a tungsten shell it is more difficult to note change
of the output. Nevertheless, the signal of fast neu-
trons (above 1MeV ) even in case with a tungsten
shell which makes only 0.5% of general signal of the
reflected neutrons can be fixed at a distance of up to
10 m using a 100×100 cm detector when the model is
irradiated by a pulsed source of neutrons with a flow
of 1011/pulse.
When a variety of materials with big (n, γ) cross-
section, are present in an object, in particular, such
as tungsten, to analyze the gamma signal spectrum
is rather difficult since the value of gamma signal is
determined by the value of the object under irradia-
tion area and mass. Research of the active detection
technique for the fissile material placed in the shell
simulating a nuclear warhead, indicates that analysis
of the integral signal of the reflected neutrons (the sig-
nal containing fission neutrons whose amount could
essentially exceed the flow of incident neutrons due
to their multiplication) is most preferable. But for
this purpose it is necessary to know precisely enough
the value of the expected reflected signal. As a rule,
it does not always work. Irradiation of an object by
fast neutrons or gamma rays excludes any possibil-
ity to identify the object through spectral analysis
of the neutron reflected signal. But irradiation of
an object by sub-MeV neutrons allows to explicitly
identify presence of a threshold-free fissile material,
for that it would be quite enough to register at least
one fast neutron. This is feasible for the up-to-date
diagnostics.
3. USE OF 14MeV NEUTRON PULSED
SOURCES FOR DETECTION OF
CHEMICAL EXPLOSIVES
Passive detection methods are absolutely unsuitable
for detection of chemical explosives unlike of the
fissile materials, therefore, a wide range of nuclear-
activation methods for detection of appropriate com-
ponents of explosives are developed. On the other
hand, activation methods allow defining chemical
composition of materials, including both fissile ma-
terials, and the radiation-passive ones. For example,
some general-purpose portals for detection of explo-
sive and fissile materials were developed on the basis
of usage of a source of neutrons generated during ir-
radiation of a beryllium target by a deyton beam [10]
and they are widely used nowadays. The activation
methods utilize not only neutrons, but also high-
energy (up to 50MeV ) gamma-ray beams produced
on electron accelerators with appropriate energy [11].
However, for the purpose of detection the neutrons of
wide energy range are mostly used, and, accordingly,
with using different channels of nuclear reactions [1].
Usage of the high-energy neutron sources allows es-
sential increase of the detection depth of the hidden
explosives. For diagnostics aims this method allows
both using threshold gamma radiation of inelastically
scattered neutrons (reaction (n, n′)), and effective
using capture gamma-rays of the moderated (to ther-
mal energies) neutrons (reaction (n, γ)). Usage of the
high-energy neutrons for explosive detection allows
generating gamma-quantum lines specific for each
component of the explosive. Simulation by applying
Geant4 9.0 package [4] allowed to identify the follow-
ing characteristic lines generated along the channel
(n, n′) within the first moments of irradiation: for
nitrogen – 2.30MeV , for carbon – 4.43MeV , for oxy-
gen – 6.14MeV , 6.92MeV , 7.128MeV [6] (Fig.5).
Fig.5. Characteristic lines of the explosive basic
elements in t=1.5 and 10 nanoseconds after irradia-
tion
As is evident from Fig.5, all main lines necessary
for identification of explosives can be simulated by
the program quite adequately. The maximum signal
would be registered within the first 5 nanoseconds af-
ter irradiation due to elastic scattering of the incident
neutrons in the material, especially on the hydrogen
atoms which are inherent chemical components of
explosives as well as of many types of plastics (see
Figs.5,6).
Further moderation of neutrons in the explosives
results in increase of (n, γ) reaction cross-sections,
but at the same time one may observe neutrons escap-
ing the explosive volume which results in very slow
decrease of γ-quanta radiation intensity lasting about
100µs after beginning of the irradiation when large
masses of explosives are irradiated (Fig.7, curve 2).
72
Fig.6. Dependence of the carbon characteristic line
on time in (n, n′) reaction, Eγ = 4.43MeV
Fig.7. Dependence of generation of gamma rays
with energy of 2.23MeV ( 1H(n, γ)2H reaction) on
the moderation time in an explosive with m = 1.9 kg
(1), m = 322 kg (2)
In case when smaller masses are irradiated the time
of gamma release virtually does not change, but in-
tensity would drop more quickly because of faster
escape of neutrons from the explosive volume (see
Fig.7, curve 1).
Calculations of distribution of the capture gamma
ray characteristic lines in time showed conservation
of the hydrogen line during all the points of time
under consideration (Fig.9,a) what is accounted for
by effective moderation of neutrons in hydrogenous
materials to 1 keV during 100ns [7]. Fig.8 presents
time dependence of fluorescence intensity of nitro-
gen capture lines Eγ = 5.269MeV and 10.829MeV .
These curves show a pronounced peak during the first
10ns which is determined by the time of fast neu-
trons escape from the volume under consideration.
During interaction of a neutron along the channel
(n, n′), the neutron loses most of its energy, com-
parable with energy of the generated γ-ray, which
results in more probable reaction of neutron capture
(n, γ) and less probable reaction of elastic scatter-
ing. Further dip of the curve during 10...100ns is
connected with escape of slow neutrons from the vol-
ume due to their elastic scattering. And only after
the period of time of about 100ns, when the neutrons
energy drops to the value of the order less than uni-
ties of keV and still less, the explosives capture lines
become more clearly observed (see Fig.9). However
the peak value of these lines is much less than that
of gamma radiation induced by (n, n′) reaction. In
the process of simulation the value of radiation emit-
ted from the cylinder with dimensions R = 30 cm,
h = 60 cm, m = 322 kg was calculated, (such dimen-
sions were chosen in order to obtain reliable resolu-
tion of characteristic lines due to the great statistics).
The signal induced by the thermal neutrons capture
the period of time from 100ns to 500µs after irra-
diation) was of the order of 10% of the total yield
of gamma rays emitted from a target. But when a
target with m = 1.9 kg was irradiated the contribu-
tion of the gamma quanta from the slow neutrons
was already less than 1%. However even at sharp
decrease of the capture radiation component all the
characteristic lines (see Fig.9) were clearly identified.
Fig.8. Dependence of generation of nitrogen
lines gamma radiation with energy of 5.269 and
10.829MeV ((n, γ) reaction) on the neutron mod-
eration time in an explosive with m = 322 kg, and
in nitrogen substance having the same mass
As it was mentioned above a characteristic fea-
ture in the spectrum of the capture gamma ray
in any of the hydrogenous materials including ex-
plosives, is a high peak value of the hydrogen line
Eγ = 2.23MeV , the peak value of all the rest lines
is essentially smaller (see Fig.9). Presence of a hy-
drogen line, at essentially smaller peak values of
other lines in spectrum of the explosives capture
radiation and at small intensities of characteristic
capture radiation could be used as the reference
signal when adjusting some kind of diagnostic equip-
ment and, accordingly, might increase reliability of
the capture lines detection within certain ranges
of spectral distribution. Besides the hydrogen line
also nitrogen capture lines are clearly observed in
the capture spectrum of the explosive gamma ra-
diation (compare Fig.9,a and Fig.9,b). Simulation
with usage of Geant4 9.0 package allowed to identify
the following nitrogen capture lines: 1.678MeV ,
73
1.884MeV , 3.677MeV , 4.509MeV , 5.269MeV ,
5.298MeV , 5.533MeV , 5.562MeV , 6.322MeV ,
7.299MeV , 8.310MeV , 9.149MeV and 10.829MeV
[6]. It was expected to find out also characteris-
tic lines of oxygen: 1.087MeV , 2.184MeV , and
3.271MeV . However because of negligibly small
cross-sections of (n, γ) reaction for carbon and oxy-
gen against cross-sections of nitrogen [8] these lines
could not be detected. Thus, for 1 keV neutron
energy the capture cross-section for nitrogen is 20
times larger than that for carbon (that is the rea-
son for carbon to be a good moderating material)
and more than 300 times larger than cross-section
for oxygen [7,8]. Therefore, lines of nitrogen and
hydrogen dominate in the capture radiation of explo-
sives. As a rule, the presence of nitrogen in the pro-
cess of irradiation by thermal neutrons is identified
by the 10.8MeV line of γ-radiation [1] (see Fig.9).
a
b
Fig.9. Spectral distribution of the capture radiation
gamma-rays after the period of time of 10µs: a -
explosives, b - pure nitrogen under irradiation by
14MeV neutrons. 1 - a hydrogen line of 2.28MeV ,
2 - a nitrogen line of 5.269MeV , 3 - a nitrogen line
of 10.84MeV
The luminescence time in materials with high con-
tent of nitrogen (pure nitrogen with density inher-
ent to explosives was calculated) would not exceed
200µs after a pulse of irradiation (see Fig.8). In the
carbonaceous substances (including explosives) the
nitrogen lines fluorescence time essentially increases
and reaches 500µs (see Fig.8) which is accounted for
by low capture cross-section of carbon (lifetime of a
neutron in graphite is known to be 12.9ms [6]).When
interacting with carbon, the neutron is elastically
scattered, and remains in volume for a longer time.
Thus, time characteristics of the nitrogen lines cap-
ture radiation allow obtaining information on the
nature of the material containing nitrogen.
4. CONCLUSIONS
When using sub-MeV neutrons for detection of fis-
sile material by active method, in spite of such ob-
vious advantages as: possibility to diagnose neutrons
with energy higher than that of neutrons from the ir-
radiation source; possibility to diagnose increase of
gamma response in comparison with that of non-
fissile materials [3], it is required to solve a num-
ber of problems connected with primary irradiation.
Among them are the reflected signal, and irradia-
tion by a ”wide-angle beam” when the background
signal is a strong noise due to the large volume un-
der irradiation in comparison with the volume of a
potentially dangerous object. To solve these prob-
lems it is necessary to conduct very serious computer
analysis, to provide shielding of the diagnostic equip-
ment, etc. The presented comparative data on irradi-
ation by a 14MeV neutron source show such advan-
tages as: more expressed signal of gamma response,
greater neutron multiplication factor, and possibility
to draw a single-valued conclusion by the number of
the reflected neutrons. A further advantage of us-
ing a high-energy irradiation source is the possibility
to obtain information from the potentially danger-
ous object that is ”placed deeper”, and it is easier
to break a conventional shielding against neutrons,
usually it is a hydrogenous shell. However in the
process of fissile substance detection the capture ra-
diation of the materials surrounding the fissile sub-
stance presents some difficulties in gamma response
estimation. Fissile material identification by the neu-
tron flow, when large area is irradiated, also makes
some difficulties for analyzing increase of the neutron
flow due to the neutron multiplication. Presence in
the neutron response of the whole range of neutron
energies, that is, from thermal energies to the ener-
gies of probing neutrons, impedes detecting a fission
material by neutron response spectrum. Unlike the
fissile material, the capture gamma radiation allows
definitely to identify presence of a conventional ex-
plosive. As it was mentioned above, in this case only
the source of high-energy neutrons allows to definitely
identify presence of the main characteristic compo-
nents of a conventional explosive. The task to reduce
the number of false operations is the main problem
here. Therefore, undoubtedly, nitrogen diagnostics
alone is insufficient. A definite ratio of carbon and
oxygen lines would allow appreciable reduction of the
number of false operations. The pulsed irradiation by
high-energy neutrons would allow measuring spectral
distribution of characteristic lines of the explosive ba-
sic elements (see Fig.5.) during the first points of
time (before deceleration of the total neutron flow),
and tracing in time characteristic lines of the cap-
ture gamma radiation by a signal from the moder-
ated neutrons (see Figs.7,8). The multiple-detector
detection system with a short-pulse source of neu-
trons would allow to distinguish between the cap-
74
ture radiation and the radiation induced by neutron
inelastic scattering (system with time resolution of
spectra). When analyzing the capture radiation, it
is possible to obtain the hydrogen/nitrogen ratio by
weight, and analyzing the deceleration time of the
capture radiation fluorescence it is possible to obtain
information concerning the amount of carbon or the
hydrogen/nitrogen ratio by weight. These problems
require further researches. Comparison of the results
concerning the material weight composition obtained
when analyzing the gamma radiation fast component
with the results obtained when analyzing the capture
radiation, would allow considerable improvement of
reliability and validity of the explosives detection.
References
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