Associated-particle Technique Research Articles

Sea container inspection with tagged neutrons

Neutron inspection of sea-going cargo containers has been widely studied in the past 20 yr to non-intrusively detect terrorist threats, like explosives or Special Nuclear Materials (SNM), and illicit goods, like narcotics or smuggling materials. Fast 14 MeV neutrons are produced by a portable generator with the t(d, n)α fusion reaction, and tagged in both direction and time thanks to the alpha particle detection. This Associated Particle Technique (APT) allows focusing inspection on specific areas of interest in the containers, previously identified as containing suspicious items with X-ray radiographic scanners or radiation portal monitors. We describe the principle of APT for non-nuclear material identification, and for nuclear material detection, then we provide illustrations of the performances for 10 min inspections with significant quantities (kilograms) of explosives, illicit drugs, or SNM, in different cargo cover loads (e.g. metallic, organic, or ceramic matrices).

Associated particle technique in single-sided geometry for detection of explosives

Associated particle technique (APT) for detection of explosives is well established but has been implemented mostly for fixed portal systems. In certain situations, a portable system is required where the suspect object cannot be moved from site. This paper discusses the development of a portable APT system in single-sided geometry which can be transported to site and requires only one-sided access to the object. The system comprised D-T neutron source and bismuth germanate (BGO) detectors fixed on a portable module. Different aspects of the system have been discussed such as background contribution, time selection, and elemental signatures. The system was used to detect benign samples and explosive simulants under laboratory condition. The elemental ratios obtained by analyzing the gamma spectra show good match with the theoretical ratios.

Détection neutronique de matières illicites avec la technique de la particule associée

The Associated Particle Technique (APT) is a fast neutron interrogation method allowing elemental characterization of bulk materials in 3D. It can be used as a 2nd level inspection complementary to X-ray imaging, which is used as 1st level inspection to detect a suspicious item in a luggage or container, to confirm and identify the nature of the threat. APT can thus detect explosives, illicit drugs chemical agents or nuclear materials at different levels of the security chain (public areas, seaports, boarders...).

Passive and Active Correlation Techniques for the Detection of Nuclear Materials

In the frame of the French trans-governmental R&D program against CBRN-E threats, CEA (French Alternative Energies and Atomic Energy Commission) is studying the detection of Special Nuclear Materials (SNM) by neutron interrogation with the Associated Particle Technique (APT). Coincidences including at least 3 fission neutrons or gamma rays induced by tagged neutrons are used to detect and distinguish SNM from benign materials in which lower multiplicity events of 1 or 2 particles are produced by (n,2n) or (n,n' ) reactions. Coincidences are detected by fast plastic scintillators and correlated with tagged neutrons to improve the signal-to-noise ratio. Dedicated data acquisition electronics (DAQ) have been developed with independent FPGA cards associated with each detector, so that the acquisition window can be opened by any of the plastic scintillators. DAQ tests in passive mode are presented, in which acquisition is triggered by the sum signal of all detectors. The system time and energy calibration and resolution are reported, as well as the qualification of numerical simulations thanks to experimental data acquired with simple setups using a source. Numerical studies for the design and performance of cargo container inspection are also performed with the MCNP-PoliMi computer code and the ROOT data analysis package. SNM detection in iron matrix is quite straightforward, but in organic matrix, data processing will need to combine more information to evidence SNM.

Progress in obtaining an absolute calibration of a total deuterium–tritium neutron yield diagnostic based on copper activation

The 350-keV Cockroft-Walton accelerator at Sandia National laboratory's Ion Beam facility is being used to calibrate absolutely a total DT neutron yield diagnostic based on the (63)Cu(n,2n)(62)Cu(β+) reaction. These investigations have led to first-order uncertainties approaching 5% or better. The experiments employ the associated-particle technique. Deuterons at 175 keV impinge a 2.6 μm thick erbium tritide target producing 14.1 MeV neutrons from the T(d,n)(4)He reaction. The alpha particles emitted are measured at two angles relative to the beam direction and used to infer the neutron flux on a copper sample. The induced (62)Cu activity is then measured and related to the neutron flux. This method is known as the F-factor technique. Description of the associated-particle method, copper sample geometries employed, and the present estimates of the uncertainties to the F-factor obtained are given.

Elemental Characterization of LL-MA Radioactive Waste Packages With the Associated Particle Technique

The French Alternative Energies and Atomic Energy Commission (CEA) and National Radioactive Waste Management Agency (ANDRA) are conducting an R&D program to improve the characterization of long-lived and medium-activity (LL-MA) radioactive waste packages with analytical methods and with non-destructive nuclear measurements. This paper discusses fast neutron interrogation with the associated particle technique (APT), which provides 3D information about the waste material composition. The characterization of volume elements filled with iron, water, aluminium and PVC in bituminized and fibre concrete LL-MA waste packages has been investigated with MCNP and MODAR data analysis software . APT provides useful information about major elements present in the volumes of interest. However, neutron scattering (especially on hydrogen nuclei) spreads the tagged neutron beam out of the targeted volume towards surrounding materials, reducing spatial selectivity. Simulation shows that targets smaller than 1 L can be characterized up to the half-radius of a 225 L bituminized drum, the matrix of which is very rich in hydrogen. Deeper characterization in concrete is possible but limited by counting statistics due to photon attenuation in this dense matrix and, unless large inspection volumes are considered, by the lack of spatial selectivity of the tagged neutron beam due to neutron scattering.

Material characterization in cemented radioactive waste with the associated particle technique

The elemental characterization of materials constituting radioactive waste is of great importance for the management of storage and repository facilities. To complement the information brought by gamma or X-ray imaging, the performance of a fast neutron interrogation system based on the associated particle technique (APT) has been investigated by using MCNP simulations and by performing proof-of-principle experiments. APT provides a 3D localisation of the emission of fast neutron induced gamma rays, whose spectroscopic analysis allows to identify the elements present in specific volumes of interest in the waste package. Monte Carlo calculations show that it is possible to identify materials enclosed behind the thick outer envelop of a ≈1 m 3 cemented waste drum, provided the excited nuclei emit gamma rays with a sufficient energy to limit photon attenuation. Neutron attenuation and scattering are also predominant effects that reduce the sensitivity and spatial selectivity of APT, but it is still possible to localise items in the waste by neutron time-of-flight and gamma-ray spectroscopy. Experimental tests confirm that the elemental characterization is possible across thick mortar slabs.

Elastic scattering of neutrons on natPb at forward angles

Angular distributions were measured for neutron elastically scattered by natPb at 2.9, 3.0, 3.1, 3.2 and 3.4MeV at very forward angles. The mono-energetic neutron flux was produced at the tandem accelerator facility of the National Institute of Nuclear Research (ININ), Mexico, using the Associated Particle Technique (APT) in the D(d, n)3He reaction. Optical model predictions with best potential systematic show a fairly good agreement with our data, and also the need for a better understanding of the neutron scattering process at very small angles from the theoretical point of view.

Accurate determination of the fast-neutron detection efficiency for organic scintillators using a collimated neutron beam

A practical method is described for an accurate determination of the fast-neutron detection efficiency of organic scintillators. The procedure is based on the associated-particle technique, using n–p scattering with a collimated, continuous-energy neutron beam. The central efficiency of two NE213 detectors has been measured for energies between 2 and 11 MeV with an accuracy of 0.9% from which, by use of a Monte-Carlo program, the average efficiency was determined to ±1.4%.

Absolute neutron detection efficiency calibration of a stilbene organic scintillator in the energy range 2 to 20 MeV with the associated particle technique

The associated particle technique (APT) has been used with neutrons from D(d, n) 3He and T(d, n) 4He reactions to measure the absolute neutron detection efficiency of a stilbene detector (3.81 cm diameter, 1.27 cm thick) in the energy range 2–20 MeV as a function of the light collection threshold. The measurements have been carried out using thin TiT and home made self-supporting deuterated polyethylene targets. The APT facility of the INFN-Laboratori Nazionali di Legnaro, the efficiency measurements and their comparison with both analytical and Monte Carlo (SANREMO code) calculations are described. The good agreement between the experimental data and the SANREMO code predictions allows extension of the efficiency evaluation throughout the whole energy range 2–20 MeV with an uncertainty of 5–10%. The experimental uncertainty in efficiency values is about 2%.

Design and calibration of an absolute flux detector for 1–15 MeV neutrons

An absolute neutron flux monitor having fast timing and a calculable response has been developed for use in a collimated beam of 1–15 MeV neutrons. The detector consists of dual thin plastic scintillators in which the proton recoil spectrum distortion caused by the escape of protons from the first scintillator is eliminated experimentally. The absolute detector efficiency was measured at 2.45 and 1.40 MeV neutron energies using the associated-particle technique at the NBS Positive-Ion Van de Graaff facility. The efficiency and pulse height distributions were calculated using a Monte Carlo based program in order to extent the efficiency throughout the 1–15 MeV interval. The uncertainty in the efficiency is 1–2% (standard deviation).

Absolute Measurement of the Neutron Sensitivity of a ZP1320 Geiger-muller Counter Using the Associated-particle Technique

Because of their low neutron sensitivity Geiger-Müller counters are often used in mixed-field dosimetry to determine the photon dose fraction. The associated-particle technique has been used to determine absolutely the neutron sensitivity of an energy-compensated ZP1320 Geiger-Müller counter at 3 MeV. The measurement is consistent with previous measurements using this type of counter.

Absolute Measurement of the Uranium-235 Fission Cross Section from 0.2 to 1.2 MeV

The absolute 235U neutron-induced fission cross section has been measured at the U.S. National Bureau of Standards (NBS) 3-MV Van de Graaff Laboratory from 0.2- to 1.2-MeV neutron energy. The mass of the 235U contained in a large volume multiplated fission ionization chamber was measured relative to the NBS fissionable isotope mass standards. Pulsed beam time-of-flight techniques were used with neutrons from the 7Li(p, n)7 Be reaction while the neutron flux was monitored with a large plastic scintillator whose efficiency was both calculated and measured with the associated-particle technique. The cross sections, which were measured with a typical uncertainty of 2.3%, are ∼2% lower than the ENDF/B-V evaluation.

Measurement of the235U Neutron-Induced Fission Cross Section at 14.1 MeV

The 235U neutron-induced fission cross section was measured at a neutron energy of 14.1 MeV using the time-correlated associated-particle technique with the 3H(d,α) n reaction at the National Bureau of Standards 3-MV Van de Graaff Laboratory. The areal density and total mass of the 235U deposits were measured relative to the standard 235U reference deposit (Los Alamos National Laboratory Spare Number 1) using thermal-neutron-induced fission counting. The total mass was also determined from the alpha-particle decay rate. The measured 235U cross section at 14.1 ± 0.1 MeV is 2.080 ± 0.030 b where the uncertainty is one standard deviation. This value agrees within 1% with other recent measurements using this technique and with the ENDF/B-V evaluation.

Experimental and theoretical neutron cross sections at 14 MeV

Activation techniques have been used to measure the neutron cross sections for 27Al(n,p)27Mg, 58Ni(n,2n)57Ni, 93Nb(n,2n)92mNb, 93Nb(n, alpha )90mY and 197Au(n,2n)196mAu at a neutron energy of 14.2+or-0.1 MeV. The absolute cross sections were measured using the associated-particle technique to determine the neutron flux from the 3H(d,n)4He reaction and a Ge(Li) gamma-ray spectrometer to determine the activity of the neutron-irradiated samples. These experimental cross sections are compared with theoretical calculations based on the statistical model with corrections for pre-equilibrium and direct-reaction effects.

Source Imaging Using Neutron Pinhole Cameras Based on Position-Sensitive Proportional Counters

A pinhole camera technique has been used to measure the variation in neutron emission intensity over the area of the neutron-producing target of the National Bureau of Standards Electron Linac. The method uses a one-dimensional position-sensitive proportional counter (PSPC) with an intrinsic spatial resolution of 1.0 mm. The pinhole is made in a thick sheet of cadmium and neutron energy (<0.3-eV) selection is achieved by time-of-flight. In a completely separate experiment, the neutron cone obtained from the (d,t) reaction using the associated-particle technique was imaged by a two-dimensional PSPC. This second measurement demonstrated the use of the two-dimensional detector for imaging high-energy (14-MeV) neutrons.

Absolute monitoring of DD and DT neutron fluences using the associated-particle technique

An associated-particle system was constructed for use with a Texas Nuclear neutron generator. Associated-particle and neutron energy spectra were measured simultaneously using this system and an NE-213 proton recoil spectrometer, respectively. The associated-particle system proved to be not only an accurate monitor of DT neutron fluence, but also an accurate monitor of DD contamination in the DT spectrum. The DD and DT neutron fluences calculated from the measured associated-particle counting rates showed the best agreement with the measured neutron fluences when the laboratory distributions were assumed to be isotropic.

Ein schneller impulshöhendiskriminator mit kompensation der anstiegszeit und des schwelleneffektes

Variations in the amplitude of pulses give rise to time dispersion in a subsequent pulse-height discriminator. This is due to two factors, namely the finite pulse rise-time and the nonideal behaviour of the discriminator. A pulse-height discriminator is described which electronically effects a compensation for the time dispersion. It is designed for pulses which have a common rise-time T R between 0.5 and 5 ns. The discriminator threshold can be set to any desired height from 1.6 V to over 5 V. The threshold width is below 50 mV. For pulses with T R = 5 ns, the maximum time dispersion lies below ±50 ps for amplitude variations between 0 and 15 V and frequency variations between 0 and 100 kHz. The discriminator was designed for a fast neutron time-of-flight spectrometer, with a view to reducing the line broadening caused by the continuous pulse-height spectrum of the neutron detector. The spectrometer, based upon the associated-particle technique, is briefly described in sect. 7. The full width at half height of the coincident neutron spectrum is smaller than 0.7 ns for a scintillator thickness d = 1 cm, and is virtually independent of the threshold energy.

Elastic Neutron Scattering from B, C, K, and Ca at 14.0 MeV

Differential cross sections of 14.0-MeV neutrons scattered elastically from B, C, K, and Ca have been measured with the Ohio University time-of-flight spectrometer. The associated-particle technique was used. A time resolution of 1.3 nsec was sufficient to guarantee discrimination against inelastic events at a flight path of 1.68 m. The data were corrected for finite geometry and double scattering. An automatic sixparameter search routine was used to calculate an optical-model fit to the data. Improved fits were obtained for the B, C, and Ca distributions. Results for the previously unmeasured K differential cross section are presented.