Neutron detection with

Slow Neutron Detection with Li-6 Phosphate ( Li P04) Nanocrystals as Neutron Absorbers... 

At T < tunneling occurs not only in irreversible chemical reactions, but also in spectroscopic splittings. Tunneling eliminates degeneracy and gives rise to tunneling multiplets, which can be detected with various spectroscopic techniques, from inelastic neutron scattering to optical and microwave spectroscopy. The most illustrative examples of this sort are the inversion of the... 

At PicArsn (Ref 19), the fast neutron activation approach for detection of expls in suitcases was extended to the activation of both nitrogen and oxygen using two 7-ray detector stations in sequence. After 14 MeV neutron irradiation, the baggage is first monitored for 6.1 MeV 7-rays from the l60(n,p),6N reaction (7.5 sec half-life), followed by measurement of the 10 min 13N. Because expls are also rich in oxygen and have characteristic ratios of N/O, it was felt that this approach would increase the probability of detection with a corresponding decrease in the false alarm rate... 

Fission chambers use neutron-induced fission to detect neutrons. The chamber is usually similar in construction to that of an ionization chamber, except that the coating material is highly enriched U235. The neutrons interact with the U235, causing fission. One of the two fission fragments enters the chamber, while the other fission fragment embeds itself in the chamber wall. 

With analytical methods such as x-ray fluorescence (XRF), proton-induced x-ray emission (PIXE), and instrumental neutron activation analysis (INAA), many metals can be simultaneously analyzed without destroying the sample matrix. Of these, XRF and PEXE have good sensitivity and are frequently used to analyze nickel in environmental samples containing low levels of nickel such as rain, snow, and air (Hansson et al. 1988 Landsberger et al. 1983 Schroeder et al. 1987 Wiersema et al. 1984). The Texas Air Control Board, which uses XRF in its network of air monitors, reported a mean minimum detectable value of 6 ng nickel/m (Wiersema et al. 1984). A detection limit of 30 ng/L was obtained using PIXE with a nonselective preconcentration step (Hansson et al. 1988). In these techniques, the sample (e.g., air particulates collected on a filter) is irradiated with a source of x-ray photons or protons. The excited atoms emit their own characteristic energy spectrum, which is detected with an x-ray detector and multichannel analyzer. INAA and neutron activation analysis (NAA) with prior nickel separation and concentration have poor sensitivity and are rarely used (Schroeder et al. 1987 Stoeppler 1984). 

For a description of the electron-counting procedure as applied to metal clusters, see Ref 37.) The paramagnetism of the nickel cluster, in principle, could be detected directly by neutron diffraction with a polarized beam and an external magnetic field. However, such measurements were not undertaken, and the effects of paramagnetism on the observed diffraction intensities, that are small in the present experiment, were ignored. 

The activation [672] of Lil with Eu2+ and the use of an activated Lil phosphor as a scintillation detector for slow neutron detection [673] has been investigated. Blue, fluorescent Lil (0.03 mole % Eu) phosphor was found to be the most useful [673] phosphor because of its ease to growth, relatively high light output, chemical stability and good match with spectral characteristics of the 6260 type photomultiplier. Lil (Eu), however, does have an interfering y radiation sensitivity. Fast neutron scintillation spectra of Li6(w, a)H3 in Eu doped Lil crystals has also been investigated [674]. 

Industrial utilization of neptunium has been very limited. The isotope 1 Np has been used as a component in neutron detection instruments. Neptunium is present in significant quantities in spent nuclear reactor fuel and poses a threat to the environment. A group of scientists at the U.S. Geological Survey (Denver, Colorado) has studied the chemical speciation of neptunium (and americium) in ground waters associated with rock types that have been proposed as possible hosts for nuclear waste repositories. See Cleveland reference. 

The most serious problem associated with the use of neutron scattering for nuclear spectroscopy comes from the fact that the resolution for neutron detection is typically rather poor, and the sensitivity to small transition probabilities is also poor when neutron detection is being employed. These difficulties can be alleviated by observing the y rays which de-excite the excited levels rather than the inelastically scattered neutrons. 

In both neutron and y-ray detection, the shielding of the detector is extremely important. Especially in the neutron detection measurements, the long target-to-detector distance (2-4 in) which is required to obtain velocity resolution via the neutron TOF technique means many more neutrons are produced than are actually scattered from the sample and then detected. These extraneous neutrons create a disastrous background unless the detector is adequately shielded. We have accomplished this with a large cylindrical shield which contains a lead cavity surrounded by Li2C03 loaded paraffin. The entrance collimator has steel and lead liners the main detector shield weighs about 2000 kg (see B in Fig. 1). 

The fact that neutrons can be detected with reasonably high efficiency and with minimal interferences from other radiations permits the practical determination of fissionable species such as isotopes of uranium and thorium by delayed neutron counting. The known delayed neutron emitter precursors are all short lived and the irradiated samples are counted with 10BF3-filled proportional counters immediately after irradiation without any separation chemistry. 

A. Buffler, Contraband detection with fast neutrons, Radiat. Phys. Chem., 71(3-4) (2004) 853-861. 

Fig. 6. Large neutron counter with 3He counting tubes for the detection of neutron bursts emitted in the spontaneous fission of superheavy nuclei. Reproduced from R.L. Macklin et al. [43], Copyright (2002), with permission from Elsevier Science.

Neutron activation analysis has low sensitivity for the detection of Mg. The best results were obtained in our hands by irradiating samples containing 200-300 yg natural magnesium (20-35 yg Mg) for one minute at a neutron flux of 10 cm sec followed by detection with a HP-6e detector. Since it is usually necessary to run activations in triplicate and to carry out additional analyses for total magnesium by an Independent method such as atomic absorption NAA is not suitable for specimens containing magnesium at low concentrations. It was found suitable for the analyses of fecal samples, marginal for urine and unsuitable for plasma (15,21). 

Many types of plastic scintillators are commercially available and find applications in fast timing, charged particle or neutron detection, as well as in cases where the rugged nature of the plastic (compared to Nal), or very large detector sizes, are appropriate. Sub-nanosecond rise times are achieved with plastic detectors coupled to fast photomultiplier tubes, and these assemblies are ideal for fast timing work. 

Nuclear track detectors can also be used to indirectly detect fast neutrons. Fast neutrons interact with the base material of a special film and cause recoil protons to be released. These protons then cause damage tracks in the film which can be made visible and counted as described above. The number of tracks can be used to determine the neutron dose. 

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