60
P. Lecoq
where N phe is the number of photoelectrons readout from the crystal. Clearly, there
is a premium for a high photon rate in the leading edge of the scintillation pulse, a
high light yield as well as a short rise and decay times for improving the CTR.
3.1.3.5 Safety Systems and Homeland Security
Scintillators are used in three main types of equipment related to safety and
homeland security: express control of luggage and passengers, search for explosive
materials and remote detection of fissile materials.
Luggage inspection requires the highest possible throughput to quickly identify
a suspect luggage in a few cubic meter large container moving across the inspection
device. The spatial resolution is determined by the need to quickly localize and
identify the suspect object in a large container. Fast scintillation kinetics with no
afterglow is therefore the most important parameter.
For the remote detection of explosives the most attractive methods are based on
the detection of natural or induced characteristic neutron and γ-rays under activation
by a neutron source, either with fast neutrons from the 252 Cf radioisotope or fastthermal neutrons from a pulsed electronic neutron generator. Neutrons initiate
nuclear reactions in some elements, some of them producing characteristic γrays. Plastic explosives for instance are generally rich in nitrogen. The nitrogen
(n,γ) reaction has a cross section of 75 mb and produces a characteristic γ-ray of
10.83 MeV.
For such applications, the most important scintillation crystal parameters are:
high stopping power to improve the detector sensitivity; high light yield to improve
the detector energy selectivity; fast scintillation decay time to allow time-of-flight
analysis with pulsed neutron generators to increase the signal to noise ratio. Good
stability of the scintillator parameters under ionizing and neutron irradiation allows
the use of strong activation sources for a better sensitivity.
Remote detection and fissile materials warhead inspection has been for a long
time restricted to the detection of neutrons, as the γ-channel would have easily
revealed secret characteristics of the nuclear device. This has changed recently and
opens new possibilities to detect the radiation emitted by Nuclear Explosive Devices
(NED) based on enriched uranium or plutonium. The most useful energy range to
detect fissile material is E γ ≥ 3 MeV because of (1) the absence in this range of
natural radioactive sources and therefore an acceptable signal to background ratio;
(2) the high penetration power of these energetic γ-quanta making the deliberate
concealment of the intrinsic NED radiation more difficult.
Here, the most important parameters are sensitivity to allow detection at large
distance (at least several meters) and good background rejection. High stopping
power (and therefore high density) is mandatory. However, the crystals should be
made from materials with very low natural radioactivity, which restricts the choice
of heavy materials to the ones with no unstable isotopes. As the counting rates
are usually low, there is no need for ultra-fast scintillators. A phoswich geometry
based on two different crystals on top of each other can be an attractive solution for
P. Lecoq
where N phe is the number of photoelectrons readout from the crystal. Clearly, there
is a premium for a high photon rate in the leading edge of the scintillation pulse, a
high light yield as well as a short rise and decay times for improving the CTR.
3.1.3.5 Safety Systems and Homeland Security
Scintillators are used in three main types of equipment related to safety and
homeland security: express control of luggage and passengers, search for explosive
materials and remote detection of fissile materials.
Luggage inspection requires the highest possible throughput to quickly identify
a suspect luggage in a few cubic meter large container moving across the inspection
device. The spatial resolution is determined by the need to quickly localize and
identify the suspect object in a large container. Fast scintillation kinetics with no
afterglow is therefore the most important parameter.
For the remote detection of explosives the most attractive methods are based on
the detection of natural or induced characteristic neutron and γ-rays under activation
by a neutron source, either with fast neutrons from the 252 Cf radioisotope or fastthermal neutrons from a pulsed electronic neutron generator. Neutrons initiate
nuclear reactions in some elements, some of them producing characteristic γrays. Plastic explosives for instance are generally rich in nitrogen. The nitrogen
(n,γ) reaction has a cross section of 75 mb and produces a characteristic γ-ray of
10.83 MeV.
For such applications, the most important scintillation crystal parameters are:
high stopping power to improve the detector sensitivity; high light yield to improve
the detector energy selectivity; fast scintillation decay time to allow time-of-flight
analysis with pulsed neutron generators to increase the signal to noise ratio. Good
stability of the scintillator parameters under ionizing and neutron irradiation allows
the use of strong activation sources for a better sensitivity.
Remote detection and fissile materials warhead inspection has been for a long
time restricted to the detection of neutrons, as the γ-channel would have easily
revealed secret characteristics of the nuclear device. This has changed recently and
opens new possibilities to detect the radiation emitted by Nuclear Explosive Devices
(NED) based on enriched uranium or plutonium. The most useful energy range to
detect fissile material is E γ ≥ 3 MeV because of (1) the absence in this range of
natural radioactive sources and therefore an acceptable signal to background ratio;
(2) the high penetration power of these energetic γ-quanta making the deliberate
concealment of the intrinsic NED radiation more difficult.
Here, the most important parameters are sensitivity to allow detection at large
distance (at least several meters) and good background rejection. High stopping
power (and therefore high density) is mandatory. However, the crystals should be
made from materials with very low natural radioactivity, which restricts the choice
of heavy materials to the ones with no unstable isotopes. As the counting rates
are usually low, there is no need for ultra-fast scintillators. A phoswich geometry
based on two different crystals on top of each other can be an attractive solution for