METHOD AND APPARATUS FOR DETECTING AND IDENTIFYING FISSIONABLE MATERIAL

PROCEDE ET APPAREIL POUR DETECTER ET IDENTIFIER DES MATIERES FISSIBLES

English Abstract

The invention relates to a method and apparatus for detecting and identifying
fissionable materials from among other transported goods, in which method the
material (4) under inspection is irradiated with bremmstrahlung (2) generated
by a linear particle accelerator (1). According to the invention, a delayed
neutrons flux emitted from the material (4) under inspection by means of
bremmstrahlung (2) is detected with at least one neutron detector (7) after
interrupting the bremmstrahlung (2), and if essential changes occur in the
neutron flux, the position of the material (4) in relation to the detector is
determined. After inspecting the whole bulk of goods, the material (4) is
returned to the position determined according to the change in the neutron
flux in relation to the detector (7), and in this determined position the
material (4) is again irradiated with bremmstrahlung (2) in order to identify
the material generating the neutron flux, and at least one neutron detector
(7) is used for detecting the delayed neutrons emitted from the fissionable
material, in order to identify the fissionable material (6) in question.

French Abstract

L'invention concerne un procédé et un appareil pour détecter et identifier des matières fissibles parmi d'autres marchandises transportées. Dans ce procédé, la matière inspectée (4) est irradiée avec un rayonnement de type bremsstrahlung (2) généré par un accélérateur linéaire (1) de particules. Selon l'invention, un flux de neutrons retardés émis par la matière (4) exposée au rayonnement de type bremsstrahlung (2) est détectée avec au moins un détecteur de neutrons (7) après interruption du rayonnement de type bremsstrahlung (2). En cas de changement important dans le flux de neutrons, on détermine la position de la matière (4) par rapport au détecteur. Après l'inspection de l'ensemble des marchandises, la matière (4) est retournée dans sa position déterminée selon le changement du flux de neutrons, par rapport au détecteur (7). Dans cette position déterminée, la matière (4) est à nouveau soumise au rayonnement du type bremsstrahlung (2) pour identifier la matière générant le flux de neutrons et au moins un détecteur de neutrons (7) est utilisé pour détecter les neutrons retardés émis par la matière fissible (6), afin de l'identifier.

Claims

CLAIMS
1. A method for detecting and identifying fissionable materials among other
transported goods, in which method the material (4) to be inspected is irradiated
with bremmstrahlung (2) generated by a linear particle accelerator (1), and with at
least one neutron detector (7) and by means of the bremmstrahlung (2), a delayedneutrons flux emitted from the material (4) under inspection is detected, and the
bremmstrahlung (2) is interrupted for the duration of the measuring of the delayed
neutrons flux,
characterized in that
1) when essential changes occur in the neutrons flux, the position of the material
(4) in relation to the detector (7) is determined,
2) after the inspection of the whole bulk of material, the material (4) is returned to
the position determined according to the change in the neutron flux in relation to
the detector (7),
3) in this determined position, the material (4) is again irradiated with
bremmstrahlung (2) in order to identify the material generating the neutrons flux,
4) the delayed neutrons emitted from the fissionable material are detected with at
least one neutron detector (7) in order to identify the fissionable material (6).
2. A method according to claim 1, characterized in that the energy level of the
bremmstrahlung (2) is within the range of 7 - 10 MeV.
3. A method according to claim 1 or 2, characterized in that the bremmstrahlung
(2) is generated by a pulse-operated particle accelerator (1).
4. A method according to claim 1, 2 or 3, characterized in that the delayed
neutrons flux is detected with a helium-3 filled detector (7).
5. A method according to any of the preceding claims 1 - 4, characterized in that
the material (4) is irradiated with bremmstrahlung (2) in order to identify the

7
material for the duration of at least 30 seconds.
6. A method according to any of the preceding claims 1 - 5, characterized in that
the operation of the registering unit (17) of the neutron detectors is controlled by
the control system (16) of the particle accelerator.
7. A method according to claim 6, characterized in that in the detection of the
delayed neutrons flux after interrupting the bremmstrahlung (2), there is used atime window with a duration within the range of 150 - 200 microseconds.
8. An apparatus for realizing the method according to claim 1, said apparatus
comprising a linear particle accelerator (1), a collimator (3) and means for moving
the material (4) under inspection to the zone of the bremmstrahlung emitted fromthe linear particle accelerator, as well as means (7) for detecting the radiation
emitted from the material under inspection, characterized in that the collimator (3)
is installed in between the neutron detectors (7) and the particle accelerator (1)
immersed in a medium (8) in order to prevent a primary irradiation of the neutron
detectors (7) with bremmstrahlung (2).
9. An apparatus according to claim 8, characterized in that the collimator (3) is
made of steel.
10. An apparatus according to claim 8 or 9, characterized in that the neutron
detector (7) is immersed in a medium (8) containing hydrogen.
11. An apparatus according to claim 8, 9 or 10, characterized in that the medium(8) is water.
12. An apparatus according to any of the preceding claims 8 - 11, characterized
in that the neutron detector (7) is filled with helium 3.

Description

i CA 02232039 1998-03-13
W O 97/11388 PCTAFI9~/0~180
METHOD AND APPARATUS FOR DETECTING AND IDENTIFYING
FISSIONABLE MATERIAL
~ The present invention relates to a method and apparatus for detecting and
indentifying fissionable material, in which method the intensity of the neutron
radiation emitted by the fissionable material is measured by means of a neutron
detector.
Fissionable materials, such as plutonium, uranium and thorium, can be used in a
fission reaction, where the heavy nucleus of an atom is split into two or more parts,
generally by bombarding the nucleus with neutrons. In general, fission takes place
in connection with a neutrons or gamma rays emission. An uncontrolled and illegal
distribution of fissionable materials is extremely dangerous, because an incorrect
use of the materials can, owing to the immense powers contained in the fission
reaction, lead to remarkable destruction in the surroundings. An illegal
transportation of hssionable material from one country to another is usually carried
out in large containers, where small amounts of fissionable material are easily
hidden. Such containers are generally inspected visually, in which case small
amounts of fissionable material are difficult to detect.
In order to automatically inspect containers and detect objects hidden therein,
there are developed methods utilizing bremmstrahlung generated by a linear
particle accelerator, such as an electron accelerator, for surveying the contents of
a container, and further for moving in between a target and a target inspection
device in order to form an image of the objects contained in the container.
The object of the present invention is to eliminate some of the drawbacks of theprior art and to realize a method and apparatus suited for the detection and
identification of fissionable materials, such as plutonium, uranium and thorium,utilizing bremmstrahlung generated by a linear particle accelerator, and a delayed
neutrons emission from the hssionable material created by said bremmstrahlung.

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The essential novel features of the invention are apparent from the appended
patent claims.
According to the invention, in order to detect and identify fissionable material, there
is used a high-energy b,e"""~ hlung source operated within the range of 7 - 10
MeV; the radiation created thereby is directed to the target to be inspected, such
as a transport container. When the bre,r,r":,L,~hlung hits fissionable material,between the radiation and the material there occur interactive reactions of the
photon and the nucleus, followed by a neutrons emission. As interactive reactions
of the photon and the nucleus, there occur photoneutron and photofission
reactions, which are characterized by a prompt neutrons emission. In this promptneutrons emission, per one interactive reaction there is emitted one neutron from
the photoneutron reaction, and 2.5 neutrons from the photofission reaction.
Photofission further results in the creation of radioactive fission fragments, part of
which contain beta-decay energy that exceeds the neutron binding energy in the
nucleus. The beta-decay energy further leads to the emission of delayed neutrons.
The fission fragment emitting delayed neutrons is called a precursor. Fissionable
materials have the lowest threshold energy of photonuclear reactions with neutron
emissions. The ranges of the photonuclear reactions and photofission reactions
for the fissionable material are large; for instance with the energy E = 8 MeV, the
value is about 100 and 30 millibarn respectively.
According to the invention, the delayed neutrons emission emitted from the
fissionable material and passing through the bremmstrahlung irradiation zone is
detected with at least one neutron detector. The neutron detector used accordingto the invention is advantageously for instance a helium-3 filled or a borium-10filled neutron detector or a neutron-sensitive scintillation detector. Advantageously
the neutron detector is arranged, by means of a collimator, in a shadowed position
so that a primary irradiation of the detector with bremmstrahlung is essentiallyprevented. The neutron detector particularly detects delayed neutrons, because
delayed neutrons are natural only in fissionable materials. The yield of delayed

¦ CA 02232039 1998-03-13
W O 97/11388 PCTAFI96/00480
neutrons is about 1 % of the yield of prompt neutrons, but the sensitivity of
detection is high enough due to a much longer exposition and a practically absent
neutrons background.
A linear particle accelerator used according to the invention is advantageously
operated in pulse mode. Thus a bl~n,n,~ hlung burst can be made to appear for
instance within the frequency range of 50 - 500 Hz, while the duration of the
bremmstrahlung is about 1.5 microseconds. Now the interval between two
successive bursts is 2.000 - 20.000 microseconds. Irrespective of the installation
of the neutron detectors in the shadow of a collimator in order to be protected from
bre, ~ ~m~l~ ahlung, the bremmstrahlung bursts, scattered in the accelerator shielding
chamber, produce ionization in the neutron detectors. As a result, high-amplitude
pulses with a duration of about 10 microseconds appear in the detection system
output. During high-amplitude pulses, neutron detection is impossible. Thereforethere is used, according to the invention, a time selection unit that blocks theoperation of the detection system and reopens the time window when the
bremmstrahlung burst pulse is finished. The duration of the time window is within
the range of 150 - 200 microseconds.
According to the invention the whole bulk of material to be inspected, such as atransport container, is irradiated with brer"n,:jl,ahlung. If an essential change is
detected in the flux of delayed neutrons, the location of the material corresponding
to this change is determined. After irradiating the whole bulk of material, the
material is returned to the location corresponding to the neutron change, and this
location is re-irradiated with bre",l"~ hlung in order to identify the material in
question, essentially for a long period of time, said period being 30 - 60 seconds.
For the identification of fissionable material, the bl~:n " "sl, ~hlung is first interrupted,
and in this case the flux of delayed neutrons is measured for at least one minute
after interrupting the bremmstrahlung.
The invention is explained in more detail below, with reference to the appended

CA 02232039 1998-03-13
W O 97/11388 PCT~FI96/00480
drawing, where
figure 1 illu~llales a preferred embodiment of the invention, seen in a partial side-
view cross-section,
figure 2 is a block diagram of the operation of the time selection unit of the neutron
detection system connected to the embodiment of figure 1, and
figure 3 illustrates the principle of operation of the time seiection unit of figure 2 by
means of time-area coordinates.
According to figure 1, a linear particle accelerator 1 generates a bre"~m~ hlungbeam 2, passing through a chink provided in a collimator 3, and irradiates a
container 4. A point scintillation detector 5, together with an image processor, is
used for creating an image of the contents of the container 4. If the container 4
contains fissionable material 6, the fissionable material 6 begins to generate aprompt neutrons emission, when the fissionable material falls within the irradiation
zone of the bremmstrahlung 2. The neutrons are detected with helium-filled
detectors 7, the molar mass of helium being 3. The detectors 7 are immersed in
a hydrogen-containing medium 8, for example water or paraffin, in order to soften
the spectrum created by the neutrons, which ensures a high detection efficiency.The collimator 3 is installed in between the neutron detector 7 and the particleaccelerator 1, so that a primary irradiation of the detector with brell""sll~hlung 2
is prevented. Advantageously the collimator 3 is made of steel in order to ensure
a sufficient threshold energy for the neutrons emission.
During the inspection, the container 4 moves within the scanning bremmstrahlung
field of the particle accelerator 1. In an ordinary case, the neutron detectors 7
register a certain amount of background neutrons emission. If, during the motionof the container, the neutron flux is suddenly increased, a special inspection unit
marks the exact spot within the container 4 where the suspicious object is located.
Now, after the inspection of the whole container 4, the container 4 is returned to
the marked spot, and the material that emits photoneutrons is identified. For
identification, the suspicious object 6 is irradiated with brerllnlsll~hlung for 30 - 60

CA 02232039 l998-03-l3
W O 97/11388 PCTAFI9C/'~3580
seconds. If the object 6 contains fissionable materiai, the amount of fission
fragments, accumulating close to the saturation level during the irradiation, c~uses
the delayed neutrons to be detected at 1 - 2 minutes after the accelerator 1 is
~ switched off. If the delayed neutrons flux is negligible, the suspicious object
contains an element with a low photoneutron reaction threshold energy, i.e. an
element that is not fissionable material.
In figure 2, the neutron detector 7 is supplied from a high-voltage unit 12. Thepulses received from the detector 7 pass through an amplifier 13 and a
discriminator 14, whereafter the pulses enter the time selection unit 15. The time
selection unit 15 is controlled by the particle accelerator control system 16. The
pulses that arrive while the time window is open are registered by the counter 17.
Figure 3 illusl,ales a bl~:",r"~l,ahlung burst as a function of time. At a point of time
to~ a brellll,l~ hlung burst is generated, causing a pulse 21. The curve 22
represents the time distribution of the prompt neutrons flux. Within the period t1 -
t2, i.e. during the time window, the time selection unit according to figure 2 is open
for the passing pulses 23, which cause a neutrons capture in the detector.

Representative Drawing